tag:blogger.com,1999:blog-1356118047479027272024-02-07T17:54:23.998-08:00human anatomy and physiologyinternet fast worldhttp://www.blogger.com/profile/13869077830569899582noreply@blogger.comBlogger8125tag:blogger.com,1999:blog-135611804747902727.post-67054061172467675132010-06-27T11:07:00.001-07:002010-06-27T11:23:43.479-07:00images<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgPrJT1sp_TwWdT2VbsJcLkCJrPAoI2iHmJmGLnPhyphenhyphen35Q2yWPRek4LDVdAWiWW6hS2o3jI_bmdh3vm0_IkEo1oxU-Ts7rGrJifJ8guRSilI_1KuUt-_UHb2Y3WVI28oR5OBCBiovG3kOqM/s1600/Gray_AnteriorSkull_190_000.jpg"><img style="display: block; margin: 0px auto 10px; text-align: center; cursor: pointer; width: 487px; height: 501px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgPrJT1sp_TwWdT2VbsJcLkCJrPAoI2iHmJmGLnPhyphenhyphen35Q2yWPRek4LDVdAWiWW6hS2o3jI_bmdh3vm0_IkEo1oxU-Ts7rGrJifJ8guRSilI_1KuUt-_UHb2Y3WVI28oR5OBCBiovG3kOqM/s320/Gray_AnteriorSkull_190_000.jpg" alt="" id="BLOGGER_PHOTO_ID_5487520030591762610" border="0" /></a><br /><img src="file:///C:/Users/nevil/AppData/Local/Temp/moz-screenshot-1.png" alt="" /><img src="file:///C:/Users/nevil/AppData/Local/Temp/moz-screenshot-2.png" alt="" /><img src="file:///C:/Users/nevil/AppData/Local/Temp/moz-screenshot-3.png" alt="" />internet fast worldhttp://www.blogger.com/profile/13869077830569899582noreply@blogger.com1tag:blogger.com,1999:blog-135611804747902727.post-12690854510082255782010-06-27T10:38:00.000-07:002010-06-27T11:00:51.363-07:00<span style="font-size:180%;"><br /><br /><span style="color: rgb(204, 0, 0);">human anatomy and physiology</span><br /></span><span style="font-weight: bold; color: rgb(0, 51, 0);font-size:180%;" >baisc</span><span style="font-weight: bold; color: rgb(0, 51, 0);font-size:180%;" > </span><br /><br />LEVELS OF ORGANIZATION<br />The human body is organized into structural and<br />functional levels of increasing complexity. Each higher<br />level incorporates the structures and functions of the<br />previous level, as you will see. We will begin with the<br />simplest level, which is the chemical level, and proceed<br />to cells, tissues, organs, and organ systems. All of<br />the levels of organization are depicted in Fig. 1–1.<br />CHEMICALS<br />The chemicals that make up the body may be divided<br />into two major categories: inorganic and organic.<br />Inorganic chemicals are usually simple molecules<br />made of one or two elements other than carbon (with<br />a few exceptions). Examples of inorganic chemicals are<br />water (H2O); oxygen (O2); one of the exceptions, carbon<br />dioxide (CO2); and minerals such as iron (Fe), calcium<br />(Ca), and sodium (Na). Organic chemicals are<br />often very complex and always contain the elements<br />carbon and hydrogen. In this category of organic<br />chemicals are carbohydrates, fats, proteins, and<br />nucleic acids. The chemical organization of the body<br />is the subject of Chapter 2.<br />CELLS<br />The smallest living units of structure and function are<br />cells. There are many different types of human cells,<br />though they all have certain similarities. Each type of<br />cell is made of chemicals and carries out specific<br />chemical reactions. Cell structure and function are<br />discussed in Chapter 3.<br />TISSUES<br />A tissue is a group of cells with similar structure and<br />function. There are four groups of tissues:<br />Epithelial tissues—cover or line body surfaces; some<br />are capable of producing secretions with specific<br />functions. The outer layer of the skin and sweat<br />glands are examples of epithelial tissues. Internal<br />epithelial tissues include the walls of capillaries<br />(squamous epithelium) and the kidney tubules<br />(cuboidal epithelium), as shown in Fig. 1–1.<br />4 Organization and General Plan of the Body<br />1. Chemical Level<br />2. Cellular Level<br />3. Tissue Level<br />4. Organ Level<br />5. Organ System<br />Level<br />6. Organism Level<br />Cuboidal epithelium<br />Squamous epithelium<br />Smooth muscle<br />Kidney<br />Urinary<br />bladder<br />Urinary<br />system<br />Figure 1–1. Levels of structural organization of the human body, depicted from the<br />simplest (chemical) to the most complex (organism). The organ system shown here is the<br />urinary system.<br />QUESTION: What other organ system seems to work directly with the urinary system?<br />5<br />Connective tissues—connect and support parts of<br />the body; some transport or store materials. Blood,<br />bone, cartilage, and adipose tissue are examples of<br />this group.<br />Muscle tissues—specialized for contraction, which<br />brings about movement. Our skeletal muscles and<br />the heart are examples of muscle tissue. In Fig. 1–1,<br />you see smooth muscle tissue, which is found in<br />organs such as the urinary bladder and stomach.<br />Nerve tissue—specialized to generate and transmit<br />electrochemical impulses that regulate body functions.<br />The brain and optic nerves are examples of<br />nerve tissue.<br />The types of tissues in these four groups, as well as<br />their specific functions, are the subject of Chapter 4.<br />ORGANS<br />An organ is a group of tissues precisely arranged so as<br />to accomplish specific functions. Examples of organs<br />are the kidneys, individual bones, the liver, lungs,<br />and stomach. The kidneys contain several kinds of<br />epithelial, or surface tissues, for their work of absorption.<br />The stomach is lined with epithelial tissue that<br />secretes gastric juice for digestion. Smooth muscle<br />tissue in the wall of the stomach contracts to mix<br />food with gastric juice and propel it to the small intestine.<br />Nerve tissue carries impulses that increase or<br />decrease the contractions of the stomach (see Box 1–1:<br />Replacing Tissues and Organs).<br />ORGAN SYSTEMS<br />An organ system is a group of organs that all contribute<br />to a particular function. Examples are the urinary<br />system, digestive system, and respiratory system.<br />In Fig. 1–1 you see the urinary system, which consists<br />of the kidneys, ureters, urinary bladder, and urethra.<br />These organs all contribute to the formation and<br />elimination of urine.<br />As a starting point, Table 1–1 lists the organ systems<br />of the human body with their general functions,<br />and some representative organs, and Fig. 1–2 depicts<br />6 Organization and General Plan of the Body<br />BOX 1–1 REPLACING TISSUES AND ORGANS<br />eventually be used to cover a large surface. Other<br />cells grown in culture include cartilage, bone, pancreas,<br />and liver. Much research is being done on<br />liver implants (not transplants), clusters of functional<br />liver cells grown in a lab. Such implants<br />would reduce or eliminate the need for human<br />donors. Tissue engineering is also being used to create<br />arteries and urinary bladders.<br />Many artificial replacement parts have also been<br />developed. These are made of plastic or metal and<br />are not rejected as foreign by the recipient’s<br />immune system. Damaged heart valves, for example,<br />may be replaced by artificial ones, and sections<br />of arteries may be replaced by tubular grafts made<br />of synthetic materials. Artificial joints are available<br />for every joint in the body, as is artificial bone for<br />reconstructive surgery. Cochlear implants are tiny<br />instruments that convert sound waves to electrical<br />impulses the brain can learn to interpret, and have<br />provided some sense of hearing for people with certain<br />types of deafness. Work is also progressing on<br />the use of a featherweight computer chip as an artificial<br />retina, on devices that help damaged hearts<br />pump blood more efficiently, and on small, selfcontained<br />artificial hearts.<br />Blood transfusions are probably the most familiar<br />and frequent form of “replacement parts” for people.<br />Blood is a tissue, and when properly typed and<br />cross-matched (blood types will be discussed in<br />Chapter 11) may safely be given to someone with<br />the same or a compatible blood type.<br />Organs, however, are much more complex structures.<br />When a patient receives an organ transplant,<br />there is always the possibility of rejection (destruction)<br />of the organ by the recipient’s immune system<br />(Chapter 14). With the discovery and use of<br />more effective immune-suppressing medications,<br />however, the success rate for many types of organ<br />transplants has increased. Organs that may be transplanted<br />include corneas, kidneys, the heart, the<br />liver, and the lungs.<br />The skin is also an organ, but skin transplanted<br />from another person will not survive very long.<br />Several kinds of artificial skin are now available to<br />temporarily cover large areas of damaged skin.<br />Patients with severe burns, for example, will eventually<br />need skin grafts from their own unburned<br />skin to form permanent new skin over the burn<br />sites. It is possible to “grow” a patient’s skin in laboratory<br />culture, so that a small patch of skin may<br />all of the organ systems. Some organs are part of two<br />organ systems; the pancreas, for example, is both a<br />digestive and an endocrine organ, and the diaphragm<br />is part of both the muscular and respiratory systems.<br />All of the organ systems make up an individual person.<br />The balance of this text discusses each system in more<br />detail.<br />METABOLISM AND HOMEOSTASIS<br />Metabolism is a collective noun; it is all of the chemical<br />reactions and physical processes that take place<br />within the body. Metabolism includes growing, repairing,<br />reacting, and reproducing—all the characteristics<br />of life. The pumping of the heart, the digestion of<br />food in the stomach, the diffusion of gases in the lungs<br />and tissues, and the production of energy in each cell<br />of the body are just a few of the thousands of aspects<br />of metabolism. Metabolism comes from a Greek word<br />meaning “change,” and the body is always changing in<br />visible ways (walking down the street), microscopic<br />ways (cells dividing in the skin to produce new epidermis),<br />and submicroscopic or molecular ways (RNA<br />and enzymes constructing new proteins). A related<br />concept, metabolic rate, is most often used to mean<br />the speed at which the body produces energy and heat,<br />or, put another way, energy production per unit of<br />time, such as 24 hours. Metabolic rate, therefore, is<br />one aspect of metabolism.<br /><br /><span style="color: rgb(51, 51, 255); font-weight: bold;font-size:180%;" >introduction about this blog</span><span style="color: rgb(255, 0, 0);font-size:130%;" ><br /><br />onlineanatomyandphysiology.blogspot.com</span><br /><br />As the science and arts of medicine and health care become increasingly complex,<br />so too does the education of those who pursue careers in nursing and other healthrelated<br />fields. Human anatomy and physiology is often a first course in many education<br />programs, and is the basis for so many of the more specialized courses. Teachers<br />of introductory anatomy and physiology thus take on a special challenge: We must<br />distill and express the complexities of human structure and function in a simple way,<br />without losing the essence and meaning of the material. That is the goal of this textbook:<br />to make this material readily accessible to students with diverse backgrounds<br />and varying levels of educational preparation.<br />No prior knowledge of biology or chemistry is assumed, and even the most fundamental<br />terms are defined thoroughly. Essential aspects of anatomy are presented<br />clearly and reinforced with excellent illustrations. Essential aspects of physiology are<br />discussed simply, yet with accuracy and precision. Again, the illustrations complement<br />the text material and foster comprehension on the part of the student. These illustrations<br />were prepared especially for students for whom this is a first course in anatomy<br />and physiology. As you will see, these are images in which detail is readily apparent.<br />All important parts have been labeled, but the student is not overwhelmed with<br />unnecessary labels. Illustrations of physiology lead the student step-by-step.<br />Wherever appropriate, the legends refer students to the text for further description or<br />explanation. Each illustration also has a question for the student; the illustration questions<br />in a chapter form an ongoing self-test. (The answers are given in Appendix G.)<br />The text has three unifying themes: the relationship between physiology and<br />anatomy, the interrelations among the organ systems, and the relationship of each<br />organ system to homeostasis. Although each type of cell, tissue, organ, or organ system<br />is discussed simply and thoroughly in itself, applicable connections are made to<br />other aspects of the body or to the functioning of the body as a whole. Our goal is to<br />provide your students with the essentials of anatomy and physiology, and in doing so,<br />to help give them a solid foundation for their future work, and an appreciation for the<br />incredible living organism that is the human body.<br />The sequence of chapters is a very traditional one. Cross-references are used to<br />remind students of what they have learned from previous chapters. Nevertheless, the<br />textbook is very flexible, and, following the introductory four chapters, the organ systems<br />may be covered in almost any order, depending on the needs of your course.<br />Each chapter is organized from the simple to the more complex, with the anatomy<br />followed by the physiology. The Instructor’s Guide presents modifications of the topic<br />sequences that may be used, again depending on the needs of your course. Certain<br />more advanced topics may be omitted from each chapter without losing the meaning<br />or flow of the rest of the material, and these are indicated, for each chapter, in the<br />Instructor’s Guide.<br />Clinical applications are set apart from the text in boxed inserts. These are often<br />aspects of pathophysiology that are related to the normal anatomy or physiology in the<br />text discussion. Each box presents one particular topic and is referenced at the appropriate<br />point in the text. This material is intended to be an integral part of the chapter<br />but is set apart for ease of reference and to enable you to include or omit as many of<br />these topics as you wish. The use of these boxes also enables students to read the text<br />material without interruption and then to focus on specific aspects of pathophysiology.<br />A comprehensive list of the boxes appears inside the book’s front and back covers, and<br />another list at the beginning of each chapter cites the boxes within that chapter.<br />Tables are utilized as summaries of structure and function, to present a sequence of<br />events, or additional material that you may choose to include. Each table is referenced<br />in the text and is intended to facilitate your teaching and to help your students learn.<br />New terms appear in bold type within the text, and all such terms are fully defined<br />in an extensive glossary, with phonetic pronunciations. Bold type may also be used for<br />emphasis whenever one of these terms is used again in a later chapter.<br />Each chapter begins with a chapter outline and student objectives to prepare the<br />student for the chapter itself. New terminology and related clinical terms are also<br />listed, with phonetic pronunciations. Each of these terms is fully defined in the glossary,<br />with cross-references back to the chapter in which the term is introduced.<br />At the end of each chapter are a study outline and review questions. The study outline<br />includes all of the essentials of the chapter in a concise outline form. The review<br />questions may be assigned as homework, or used by the students as a review or selftest.<br />Following each question is a page reference in parentheses. This reference cites<br />the page(s) in the chapter on which the content needed to answer the question correctly<br />can be found. The answers themselves are included in the Instructor’s Guide.<br />The questions in the sections titled For Further Thought may be used in a variety of<br />ways, and the answers are in the Instructor’s Guide.<br />An important supplementary learning tool for your students is available in the form<br />of a Student Workbook that accompanies this text. For each chapter in the textbook, the<br />workbook offers fill-in and matching-column questions, figure-labeling and figurecoloring<br />exercises, and crossword puzzles based on the chapter’s vocabulary list. Also<br />included are two comprehensive, multiple-choice chapter tests to provide a thorough<br />review. All answers are provided at the end of the workbook.<br />Ancillary materials for the teacher using this text are all on a CD-ROM: a complete<br />Instructor’s Guide, two computerized test banks, and an Image Ancillary presentation<br />of the text illustrations. The Instructor’s Guide contains notes on each chapter’s<br />organization and content (useful for modifying the book to your specific teaching<br />needs), topics for class discussion, answers to the chapter review questions from the<br />textbook, and detailed answers to the For Further Thought questions. The multiplechoice<br />test bank contains more than 2600 questions that have been organized in<br />relation to the chapter review questions, and further explanation may be found in the<br />Instructor’s Guide. The fill-in test bank contains more than 2100 questions organized<br />by textbook chapter. The Image Ancillary presentation contains many of the illustrations<br />from the textbook, with suggested points for use in classroom lectures.<br />Suggestions and comments from colleagues are always valuable, and yours would<br />be greatly appreciated.<br />Any suggestions that you can provide to help us achieve that goal are most welcome,internet fast worldhttp://www.blogger.com/profile/13869077830569899582noreply@blogger.com0tag:blogger.com,1999:blog-135611804747902727.post-2548506790586765102010-06-27T07:56:00.000-07:002010-06-27T08:08:01.194-07:00lymphCHAPTER 14<br />Chapter Outline<br />Lymph<br />Lymph Vessels<br />Lymphatic Tissue<br />Lymph Nodes and Nodules<br />Spleen<br />Thymus<br />Immunity<br />Innate Immunity<br />Barriers<br />Defensive cells<br />Chemical defenses<br />Adaptive Immunity<br />Cell-Mediated Immunity<br />Antibody-Mediated Immunity<br />Antibody Responses<br />Types of Immunity<br />Aging and the Lymphatic System<br />BOX 14–1 HODGKIN’S DISEASE<br />BOX 14–2 AIDS<br />BOX 14–3 DIAGNOSTIC TESTS<br />BOX 14–4 VACCINES<br />BOX 14–5 ALLERGIES<br />BOX 14–6 VACCINES THAT HAVE CHANGED OUR<br />LIVES<br />Student Objectives<br />• Describe the functions of the lymphatic system.<br />• Describe how lymph is formed.<br />• Describe the system of lymph vessels, and explain<br />how lymph is returned to the blood.<br />• State the locations and functions of the lymph<br />nodes and nodules.<br />• State the location and functions of the spleen and<br />thymus.<br />• Explain what is meant by immunity.<br />• Describe the aspects of innate immunity.<br />• Describe adaptive immunity: cell-mediated and<br />antibody-mediated.<br />• Describe the responses to a first and second exposure<br />to a pathogen.<br />• Explain the difference between genetic immunity<br />and acquired immunity.<br />• Explain the difference between passive acquired<br />immunity and active acquired immunity.<br />• Explain how vaccines work.<br />The Lymphatic System<br />and Immunity<br />321<br />New Terminology<br />Acquired immunity (uh-KWHY-erd)<br />Active immunity (AK-tiv)<br />Antibody-mediated immunity (AN-ti-BAH-dee<br />ME-dee-ay-ted)<br />Antigen (AN-ti-jen)<br />B cells (B SELLS)<br />Cell-mediated immunity (SELL ME-dee-ay-ted)<br />Complement (KOM-ple-ment)<br />Cytokines (SIGH-toh-kines)<br />Genetic immunity (je-NET-ik)<br />Humoral immunity (HYOO-mohr-uhl)<br />Interferon (in-ter-FEER-on)<br />Lymph (LIMF)<br />Lymph nodes (LIMF NOHDS)<br />Lymph nodules (LIMF NAHD-yools)<br />Opsonization (OP-sah-ni-ZAY-shun)<br />Passive immunity (PASS-iv)<br />Plasma cell (PLAZ-mah SELL)<br />Spleen (SPLEEN)<br />T cells (T SELLS)<br />Thymus (THIGH-mus)<br />Tonsils (TAHN-sills)<br />Related Clinical Terminology<br />AIDS (AYDS)<br />Allergy (AL-er-jee)<br />Antibody titer (AN-ti-BAH-dee TIGH-ter)<br />Attenuated (uh-TEN-yoo-AY-ted)<br />Complement fixation test (KOM-ple-ment<br />fik-SAY-shun)<br />Fluorescent antibody test (floor-ESS-ent)<br />Hodgkin’s disease (HODJ-kinz)<br />Tonsillectomy (TAHN-si-LEK-toh-mee)<br />Toxoid (TOK-soyd)<br />Vaccine (vak-SEEN)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />Achild falls and scrapes her knee. Is this likely to<br />be a life-threatening injury? Probably not, even<br />though the breaks in the skin have permitted the entry<br />of thousands or even millions of bacteria. Those bacteria,<br />however, will be quickly destroyed by the cells<br />and organs of the lymphatic system.<br />Although the lymphatic system may be considered<br />part of the circulatory system, we will consider it separately<br />because its functions are so different from<br />those of the heart and blood vessels. Keep in mind,<br />however, that all of these functions are interdependent.<br />The lymphatic system is responsible for returning<br />tissue fluid to the blood and for protecting the body<br />against foreign material. The parts of the lymphatic<br />system are the lymph, the system of lymph vessels, and<br />lymphatic tissue, which includes lymph nodes and<br />nodules, the spleen, and the thymus gland.<br />LYMPH<br />Lymph is the name for tissue fluid that enters lymph<br />capillaries. As you may recall from Chapter 13, filtration<br />in capillaries creates tissue fluid from blood<br />plasma, most of which returns almost immediately to<br />the blood in the capillaries by osmosis. Some tissue<br />fluid, however, remains in interstitial spaces and must<br />be returned to the blood by way of the lymphatic vessels.<br />Without this return, blood volume and blood<br />pressure would very soon decrease. The relationship<br />of the lymphatic vessels to the cardiovascular system is<br />depicted in Fig. 14–1.<br />LYMPH VESSELS<br />The system of lymph vessels begins as dead-end<br />lymph capillaries found in most tissue spaces (Fig.<br />14–2). Lymph capillaries are very permeable and collect<br />tissue fluid and proteins. Lacteals are specialized<br />lymph capillaries in the villi of the small intestine; they<br />absorb the fat-soluble end products of digestion, such<br />as fatty acids and vitamins A, D, E, and K.<br />Lymph capillaries unite to form larger lymph vessels,<br />whose structure is very much like that of veins.<br />There is no pump for lymph (as the heart is the pump<br />for blood), but the lymph is kept moving within lymph<br />vessels by the same mechanisms that promote venous<br />return. The smooth muscle layer of the larger lymph<br />vessels constricts, and the one-way valves (just like<br />those of veins) prevent backflow of lymph. Lymph vessels<br />in the extremities, especially the legs, are compressed<br />by the skeletal muscles that surround them;<br />this is the skeletal muscle pump. The respiratory<br />pump alternately expands and compresses the lymph<br />vessels in the chest cavity and keeps the lymph moving.<br />Where is the lymph going? Back to the blood to<br />become plasma again. Refer to Fig. 14–3 as you read<br />the following. The lymph vessels from the lower body<br />unite in front of the lumbar vertebrae to form a vessel<br />called the cisterna chyli, which continues upward in<br />front of the backbone as the thoracic duct. Lymph<br />vessels from the upper left quadrant of the body join<br />the thoracic duct, which empties lymph into the left<br />subclavian vein. Lymph vessels from the upper right<br />quadrant of the body unite to form the right lymphatic<br />duct, which empties lymph into the right subclavian<br />vein. Flaps in both subclavian veins permit the entry of<br />lymph but prevent blood from flowing into the lymph<br />vessels.<br />LYMPHATIC TISSUE<br />Lymphatic tissue consists mainly of lymphocytes in a<br />mesh-like framework of connective tissue. Recall that<br />most lymphocytes are produced from stem cells in the<br />red bone marrow, then migrate to the lymph nodes<br />and nodules, to the spleen, and to the thymus. In these<br />structures, lymphocytes become activated and proliferate<br />in response to infection (this is a function of all<br />lymphatic tissue). The thymus has stem cells that produce<br />a significant portion of the T lymphocytes.<br />LYMPH NODES AND NODULES<br />Lymph nodes and nodules are masses of lymphatic<br />tissue. Nodes and nodules differ with respect to size<br />and location. Nodes are usually larger, 10 to 20 mm in<br />length, and are encapsulated; nodules range from a<br />fraction of a millimeter to several millimeters in length<br />and do not have capsules.<br />Lymph nodes are found in groups along the pathways<br />of lymph vessels, and lymph flows through these<br />nodes on its way to the subclavian veins. Lymph enters<br />a node through several afferent lymph vessels and<br />322 The Lymphatic System and Immunity<br />leaves through one or two efferent vessels (Fig. 14–4).<br />As lymph passes through a lymph node, bacteria and<br />other foreign materials are phagocytized by fixed (stationary)<br />macrophages. Plasma cells develop from<br />lymphocytes exposed to pathogens in the lymph and<br />produce antibodies. These antibodies will eventually<br />reach the blood and circulate throughout the body.<br />There are many groups of lymph nodes along all<br />the lymph vessels throughout the body, but three<br />paired groups deserve mention because of their strategic<br />locations. These are the cervical, axillary, and<br />inguinal lymph nodes (see Fig. 14–3). Notice that<br />these are at the junctions of the head and extremities<br />with the trunk of the body. Breaks in the skin, with<br />entry of pathogens, are much more likely to occur in<br />the arms or legs or head rather than in the trunk. If<br />these pathogens get to the lymph, they will be<br />destroyed by the lymph nodes before they get to the<br />trunk, before the lymph is returned to the blood in the<br />subclavian veins.<br />The Lymphatic System and Immunity 323<br />Subclavian vein<br />Lymphatic<br />vessel<br />Valve<br />Heart<br />Lymph<br />flow<br />Lymph node<br />Blood<br />flow<br />Lymph<br />capillaries<br />Blood<br />capillaries<br />Figure 14–1. Relationship of lymphatic<br />vessels to the cardiovascular<br />system. Lymph capillaries collect tissue<br />fluid, which is returned to the<br />blood. The arrows indicate the direction<br />of flow of the blood and lymph.<br />QUESTION: To which large veins is<br />lymph returned, and why is this<br />return important?<br />You may be familiar with the expression “swollen<br />glands,” as when a child has a strep throat (an inflammation<br />of the pharynx caused by Streptococcus bacteria).<br />These “glands” are the cervical lymph nodes that have<br />enlarged as their macrophages attempt to destroy the<br />bacteria in the lymph from the pharynx (see Box 14–1:<br />Hodgkin’s Disease).<br />Lymph nodules are small masses of lymphatic tissue<br />found just beneath the epithelium of all mucous<br />membranes. The body systems lined with mucous<br />membranes are those that have openings to the environment:<br />the respiratory, digestive, urinary, and reproductive<br />tracts. You can probably see that these are also<br />strategic locations for lymph nodules, because any natural<br />body opening is a possible portal of entry for<br />pathogens. For example, if bacteria in inhaled air get<br />through the epithelium of the trachea, lymph nodules<br />with their macrophages are in position to destroy these<br />bacteria before they get to the blood.<br />Some of the lymph nodules have specific names.<br />Those of the small intestine are called Peyer’s<br />patches, and those of the pharynx are called tonsils.<br />The palatine tonsils are on the lateral walls of the<br />pharynx, the adenoid (pharyngeal tonsil) is on the posterior<br />wall, and the lingual tonsils are on the base of<br />the tongue. The tonsils, therefore, form a ring of lymphatic<br />tissue around the pharynx, which is a common<br />pathway for food and air and for the pathogens they<br />contain. A tonsillectomy is the surgical removal of<br />the palatine tonsils and the adenoid and may be performed<br />if the tonsils are chronically inflamed and<br />swollen, as may happen in children. As mentioned earlier,<br />the body has redundant structures to help ensure<br />survival if one structure is lost or seriously impaired.<br />Thus, there are many other lymph nodules in the<br />pharynx to take over the function of the surgically<br />removed tonsils.<br />SPLEEN<br />The spleen is located in the upper left quadrant of the<br />abdominal cavity, just below the diaphragm, behind<br />the stomach. The lower rib cage protects the spleen<br />from physical trauma (see Fig. 14–3).<br />In the fetus, the spleen produces red blood cells, a<br />function assumed by the red bone marrow after birth.<br />After birth the spleen is very much like a large lymph<br />node, except that its functions affect the blood that<br />flows through it rather than lymph.<br />The functions of the spleen after birth are:<br />1. Contains plasma cells that produce antibodies to<br />foreign antigens.<br />2. Contains fixed macrophages (RE cells) that phagocytize<br />pathogens or other foreign material in the<br />blood. The macrophages of the spleen also phagocytize<br />old red blood cells and form bilirubin. By<br />way of portal circulation, the bilirubin is sent to the<br />liver for excretion in bile.<br />3. Stores platelets and destroys them when they are<br />no longer useful.<br />The spleen is not considered a vital organ, because<br />other organs compensate for its functions if the spleen<br />must be removed. The liver and red bone marrow<br />will remove old red blood cells and platelets from circulation.<br />The many lymph nodes and nodules will<br />phagocytize pathogens (as will the liver) and have lymphocytes<br />to be activated and plasma cells to produce<br />antibodies. Despite this redundancy, a person without<br />a spleen is somewhat more susceptible to certain bacterial<br />infections such as pneumonia and meningitis.<br />THYMUS<br />The thymus is located inferior to the thyroid gland.<br />In the fetus and infant, the thymus is large and extends<br />under the sternum (Fig. 14–5). With increasing age,<br />the thymus shrinks, and relatively little thymus tissue<br />is found in adults, though it is still active.<br />324 The Lymphatic System and Immunity<br />Tissue fluid<br />Cells<br />Lymph<br />capillary<br />Venule<br />Blood capillary<br />Arteriole<br />Figure 14–2. Dead-end lymph capillaries found in tissue<br />spaces. Arrows indicate the movement of plasma,<br />lymph, and tissue fluid.<br />QUESTION: Just before water enters lymph capillaries,<br />what name does it have?<br />The Lymphatic System and Immunity 325<br />Cervical nodes<br />Submaxillary nodes<br />Left subclavian vein<br />Thoracic duct<br />Axillary nodes<br />Cisterna chyli<br />Mesenteric nodes<br />Inguinal nodes<br />Right lymphatic duct<br />Mammary plexus<br />Cubital nodes<br />Iliac nodes<br />Popliteal nodes<br />Spleen<br />Figure 14–3. System of lymph vessels and the major groups of lymph nodes. Lymph is<br />returned to the blood in the right and left subclavian veins.<br />QUESTION: Where are the major paired groups of lymph nodes located?<br />The stem cells of the thymus produce T lymphocytes<br />or T cells; their functions are discussed in the<br />next section. Thymic hormones are necessary for what<br />may be called “immunological competence.” To be<br />competent means to be able to do something well.<br />The thymic hormones enable the T cells to participate<br />in the recognition of foreign antigens and to provide<br />immunity. This capability of T cells is established<br />early in life and then is perpetuated by the lymphocytes<br />themselves. The newborn’s immune system is<br />not yet fully mature, and infants are more susceptible<br />to certain infections than are older children and<br />adults. Usually by the age of 2 years, the immune system<br />matures and becomes fully functional. This is why<br />some vaccines, such as the measles vaccine, are not<br />recommended for infants younger than 15 to 18<br />months of age. Their immune systems are not mature<br />enough to respond strongly to the vaccine, and the<br />326 The Lymphatic System and Immunity<br />Afferent lymphatic vessel<br />Capsule<br />Cortex<br />Nodal vein<br />Nodal artery<br />Hilus<br />Valve<br />Efferent lymphatic<br />vessel<br />Bacteria<br />Lymphocytes<br />Neutrophil<br />Plasma cell<br />Macrophage<br />Antibody molecule<br />(enlarged)<br />Antigen<br />(enlarged)<br />A<br />B<br />Figure 14–4. Lymph node. (A) Section through a lymph node, showing the flow of<br />lymph. (B) Microscopic detail of bacteria being destroyed within the lymph node.<br />QUESTION: What is the function of the plasma cells in a lymph node?<br />protection provided by the vaccine may be incomplete.<br />IMMUNITY<br />Immunity may be defined as the ability to destroy<br />pathogens or other foreign material and to prevent<br />further cases of certain infectious diseases. This ability<br />is of vital importance because the body is exposed to<br />pathogens from the moment of birth.<br />Antigens are chemical markers that identify cells.<br />Human cells have their own antigens that identify all<br />the cells in an individual as “self ” (recall the HLA<br />types mentioned in Chapter 11). When antigens are<br />foreign, or “non-self,” they may be recognized as such<br />and destroyed. Bacteria, viruses, fungi, and protozoa<br />are all foreign antigens that activate immune responses,<br />as are cell products such as bacterial toxins.<br />Malignant cells, which may be formed within the<br />body as a result of mutations of normal cells, are also<br />recognized as foreign and are usually destroyed before<br />they can establish themselves and cause cancer. Unfortunately,<br />organ transplants are also foreign tissue, and<br />the immune system may reject (destroy) a transplanted<br />kidney or heart. Sometimes the immune system mistakenly<br />reacts to part of the body itself and causes<br />an autoimmune disease; several of these were mentioned<br />in previous chapters. Most often, however, the<br />immune mechanisms function to protect the body<br />from the microorganisms around us and within us.<br />Immunity has two main components: innate immunity<br />and adaptive immunity. Before we describe each<br />component, a brief general comparison may be helpful.<br />Innate immunity may be called nonspecific, does<br />not create memory, and its responses are always the<br />same regardless of the target. Adaptive immunity is<br />very specific as to its target, may involve antibodies,<br />does create memory, and may become more efficient.<br />Both kinds of immunity work together to prevent<br />damage and disease.<br />INNATE IMMUNITY<br />Innate immunity has several aspects: anatomic and<br />physiological barriers, phagocytic and other defensive<br />cells, and chemical secretions and reactions, including<br />inflammation. These are not separate and distinct;<br />rather there is a great deal of overlap among them, as<br />you will see. The innate immune responses are always<br />the same, and their degree of efficiency does not<br />increase with repeated exposure.<br />Barriers<br />The stratum corneum of the epidermis of the skin is<br />non-living, and when unbroken is an excellent barrier<br />to pathogens of all kinds. The fatty acids in sebum<br />help limit the growth of bacteria on the skin. The living<br />cells of the epidermis produce defensins, which are<br />antimicrobial chemicals. The mucous membranes of<br />the respiratory, digestive, urinary, and reproductive<br />tracts are living tissue, yet still a good barrier. The<br />The Lymphatic System and Immunity 327<br />BOX 14–1 HODGKIN’S DISEASE<br />Hodgkin’s disease is a malignant disorder of<br />the lymph nodes; the cause is not known. The<br />first symptom is usually a swollen but painless<br />lymph node, often in the cervical region. The<br />individual is prompted to seek medical attention<br />because of other symptoms: chronic fever,<br />fatigue, and weight loss. The diagnosis involves<br />biopsy of the lymph node and the finding of<br />characteristic cells.<br />Treatment of Hodgkin’s disease requires<br />chemotherapy, radiation, or both. With early<br />diagnosis and proper treatment, this malignancy<br />is very often curable.<br />Trachea<br />Clavicle<br />First<br />rib<br />Thymus<br />gland<br />Figure 14–5. Location of the thymus in a young child.<br />QUESTION: Which blood cells mature in the thymus?<br />ciliated epithelium of the upper respiratory tract is an<br />especially effective barrier. Dust and pathogens are<br />trapped on the mucus, the cilia sweep the mucus to the<br />pharynx, and it is swallowed. The hydrochloric acid of<br />the gastric juice destroys most pathogens that enter<br />the stomach, either in mucus or with food and drink.<br />Lysozyme, an enzyme found in saliva and tears,<br />inhibits the growth of bacteria in the oral cavity and<br />on the warm, wet surface of the eye. The subcutaneous<br />tissue contains many white blood cells (WBCs),<br />as does the areolar connective tissue below the epithelium<br />of mucous membranes.<br />Defensive Cells<br />Recall from Chapter 11 that many of our defensive<br />cells are white blood cells. Macrophages, both fixed<br />and wandering, have receptors for the pathogens<br />humans are likely to encounter (this probably reflects<br />millions of years of coexistence) and are very efficient<br />phagocytes. Other cells capable of phagocytosis of<br />pathogens or other foreign antigens are the neutrophils<br />and, to a lesser extent, the eosinophils. Phagocytic<br />cells use intracellular enzymes and chemicals<br />such as hydrogen peroxide (H2O2) to destroy ingested<br />pathogens.<br />The Langerhans cells of the skin, and other dendritic<br />cells throughout the body, also phagocytize foreign<br />material, not merely to destroy it, but to take it to<br />a lymph node where the lymphocytes of adaptive<br />immune mechanisms are then activated. The macrophages<br />are also involved in activating these lymphocytes.<br />This is a very important link between the two<br />components of immunity.<br />Natural killer cells (NK cells) circulate in the<br />blood but are also found in the red bone marrow,<br />spleen, and lymph nodes. They are a small portion<br />(about 10%) of the total lymphocytes, but are able to<br />destroy many kinds of pathogens and tumor cells. NK<br />cells make direct contact with foreign cells, and kill<br />them by rupturing their cell membranes (with chemicals<br />called perforins) or by inflicting some other kind<br />of chemical damage.<br />Basophils and mast cells (a type of connective tissue<br />cell) are also defensive cells that are found throughout<br />areolar connective tissue. They produce histamine and<br />leukotrienes. Histamine causes vasodilation and makes<br />capillaries more permeable; these are aspects of<br />inflammation. Leukotrienes also increase capillary<br />permeability and attract phagocytic cells to the area.<br />Chemical Defenses<br />Chemicals that help the body resist infection include<br />the interferons, complement, and the chemicals<br />involved in inflammation. The interferons (alpha-,<br />beta-, and gamma-interferons) are proteins produced<br />by cells infected with viruses and by T cells. Viruses<br />must be inside a living cell to reproduce, and although<br />interferon cannot prevent the entry of viruses into<br />cells, it does block their reproduction. When viral<br />reproduction is blocked, the viruses cannot infect new<br />cells and cause disease. Interferon is probably a factor<br />in the self-limiting nature of many viral diseases (and<br />is used in the treatment of some diseases, such as hepatitis<br />C).<br />Complement is a group of more than 20 plasma<br />proteins that circulate in the blood until activated.<br />They are involved in the lysis of cellular antigens and<br />the labeling of noncellular antigens. Some stimulate<br />the release of histamine in inflammation; others<br />attract WBCs to the site.<br />Inflammation is a general response to damage of<br />any kind: microbial, chemical, or physical. Basophils<br />and mast cells release histamine and leukotrienes,<br />which affect blood vessels as previously described.<br />Vasodilation increases blood flow to the damaged area,<br />and capillaries become more permeable; tissue fluid<br />and WBCs collect at the site. The purpose of inflammation<br />is to try to contain the damage, keep it from<br />spreading, eliminate the cause, and permit repair of the<br />tissue to begin. From even this brief description you<br />can see why the four signs of inflammation are redness,<br />heat, swelling, and pain: redness from greater blood<br />flow, heat from the blood and greater metabolic activity,<br />swelling from the accumulation of tissue fluid, and<br />pain from the damage itself and perhaps the swelling.<br />As mentioned in Chapter 10, inflammation is a positive<br />feedback mechanism that may become a vicious<br />cycle of damage and more damage. The hormone cortisol<br />is one brake that prevents this, and at least one of<br />the complement proteins has this function as well.<br />There are probably other chemical signals (in general<br />called cytokines and chemokines) that help limit<br />inflammation to an extent that is useful.<br />In summary, innate immunity is nonspecific, is<br />always the same, does not create memory, and does<br />not become more efficient upon repeated exposures.<br />Some cells of innate immune mechanisms also activate<br />the adaptive immune mechanisms. The aspects of<br />innate immunity are shown in Fig. 14–6.<br />328 The Lymphatic System and Immunity<br />The Lymphatic System and Immunity 329<br />•<br />Integumentary system<br />Mucous membranes and lysozyme<br />Langerhans cells Phagocytes<br />Natural killer cells<br />Basophils and mast cells<br />Interferon Complement<br />Inflammation<br />T cell<br />Blocks viral<br />reproduction<br />Lyses cells,<br />attracts WBC's<br />Histamine and leukotrienes<br />Vasodilation •<br />Increased<br />capillary<br />permeability •<br />Tissue fluid<br />and WBCs •<br />Histamine and leukotrienes<br />Activate lymphocytes<br />Perforins<br />•<br />Macrophage<br />•<br />•<br />Bacteria<br />•<br />Neutrophils<br />•<br />•<br />Antigen<br />•<br />•<br />Subcutaneous tissue<br />and WBC's<br />Defensins Langerhans cells<br />Stratum corneum<br />• •<br />•<br />Hydrochloric<br />acid<br />Ciliated epithelium<br />Lacrimal gland<br />•<br />Salivary glands<br />•<br />• •<br />A Barriers<br />B Cells<br />C Chemicals<br />Figure 14–6. Innate immunity. (A) Barriers. (B) Defensive cells. (C) Chemical defenses.<br />See text for description.<br />QUESTION: The three aspects of innate immunity are interconnected; describe two of<br />these connections.<br />ADAPTIVE IMMUNITY<br />To adapt means to become suitable, and adaptive<br />immunity can become “suitable” for and respond to<br />almost any foreign antigen. Adaptive immunity is specific<br />and is carried out by lymphocytes and macrophages.<br />The majority of lymphocytes are the T lymphocytes<br />and B lymphocytes, or, more simply, T cells and<br />B cells. In the embryo, T cells are produced in the<br />bone marrow and thymus. They must pass through<br />the thymus, where the thymic hormones bring about<br />their maturation. The T cells then migrate to the<br />spleen, lymph nodes, and lymph nodules, where they<br />are found after birth.<br />Produced in the embryonic bone marrow, B cells<br />migrate directly to the spleen and lymph nodes and<br />nodules. When activated during an immune response,<br />some B cells will divide many times and become<br />plasma cells that produce antibodies to a specific foreign<br />antigen.<br />The mechanisms of immunity that involve T cells<br />and B cells are specific, meaning that one foreign antigen<br />is the target each time a mechanism is activated. A<br />macrophage has receptor sites for foreign chemicals<br />such as those of bacterial cell walls or flagella, and may<br />phagocytize just about any foreign material it comes<br />across (as will the Langerhans or dendritic cells). T<br />cells and B cells, however, become very specific, as you<br />will see.<br />The first step in the destruction of a pathogen or<br />foreign cell is the recognition of its antigens as foreign.<br />Both T cells and B cells are capable of this, but<br />the immune mechanisms are activated especially well<br />when this recognition is accomplished by macrophages<br />and a specialized group of T lymphocytes<br />called helper T cells (also called CD4 T cells). The<br />foreign antigen is first phagocytized by a macrophage,<br />and parts of it are “presented” on the macrophage’s<br />cell membrane. Also on the macrophage membrane<br />are “self” antigens that are representative of the<br />antigens found on all of the cells of the individual.<br />Therefore, the helper T cell that encounters this<br />macrophage is presented not only with the foreign<br />antigen but also with “self” antigens for comparison.<br />The helper T cell becomes sensitized to and specific<br />for the foreign antigen, the one that does not belong<br />in the body (see Box 14–2: AIDS).<br />The recognition of an antigen as foreign initiates<br />one or both of the mechanisms of adaptive immunity.<br />These are cell-mediated immunity (sometimes<br />called simply cellular immunity), in which T cells and<br />macrophages participate, and antibody-mediated<br />immunity (or humoral immunity), which involves T<br />cells, B cells, and macrophages.<br />Cell-Mediated Immunity<br />This mechanism of immunity does not result in the<br />production of antibodies, but it is effective against<br />intracellular pathogens (such as viruses), fungi,<br />malignant cells, and grafts of foreign tissue. As mentioned<br />earlier, the first step is the recognition of the<br />foreign antigen by macrophages and helper T cells,<br />which become activated and are specific. (You may<br />find it helpful to refer to Fig. 14–7 as you read the following.)<br />These activated T cells, which are antigen specific,<br />divide many times to form memory T cells and cytotoxic<br />(killer) T cells (also called CD8 T cells). The<br />memory T cells will remember the specific foreign<br />antigen and become active if it enters the body again.<br />Cytotoxic T cells are able to chemically destroy foreign<br />antigens by disrupting cell membranes. This<br />is how cytotoxic T cells destroy cells infected with<br />viruses and prevent the viruses from reproducing.<br />These T cells also produce cytokines, which are chemicals<br />that attract macrophages to the area and activate<br />them to phagocytize the foreign antigen and cellular<br />debris.<br />It was once believed that another subset of T cells<br />served to stop the immune response, but this may not<br />be so. It seems probable that the CD4 and CD8 T<br />cells also produce feedback chemicals to limit the<br />immune response once the foreign antigen has been<br />destroyed. The memory T cells, however, will quickly<br />initiate the cell-mediated immune response should<br />there be a future exposure to the antigen.<br />Antibody-Mediated Immunity<br />This mechanism of immunity does involve the production<br />of antibodies and is also diagrammed in Fig.<br />14–7. Again, the first step is the recognition of the foreign<br />antigen, this time by B cells as well as by<br />macrophages and helper T cells. The sensitized helper<br />T cell presents the foreign antigen to B cells, which<br />provides a strong stimulus for the activation of B cells<br />specific for this antigen. The activated B cells begin to<br />divide many times, and two types of cells are formed.<br />Some of the new B cells produced are memory B<br />cells, which will remember the specific antigen and<br />330 The Lymphatic System and Immunity<br />The Lymphatic System and Immunity 331<br />331<br />Produces cytokines to<br />attract macrophages<br />Chemically destroys<br />foreign cells<br />Memory T cell<br />Helper T cell Cytotoxic T cell<br />Macrophage<br />•<br />Foreign antigen<br />•<br />•<br />Self antigens<br />•<br />•<br />Receptor sites<br />•<br />•<br />Self antigens<br />•<br />•<br />Receptor sites<br />Macrophage<br />Helper T cells Memory B cell<br />B cell Plasma cell<br />Antibodies<br />Opsonization Antigen-antibody complex Complement fixation<br />lysis of<br />cellular antigen<br />Macrophage<br />A Cell-mediated<br />B Antibody-mediated<br />Figure 14–7. Adaptive immunity. (A) Cell-mediated immunity. (B) Antibody-mediated<br />immunity. See text for description.<br />QUESTION: Adaptive immunity has memory; which cells provide this? What kind of memory<br />is it?<br />332 The Lymphatic System and Immunity<br />initiate a rapid response upon a second exposure.<br />Other B cells become plasma cells that produce antibodies<br />specific for this one foreign antigen.<br />Antibodies, also called immune globulins (Ig) or<br />gamma globulins, are proteins shaped somewhat like<br />the letter Y. Antibodies do not themselves destroy foreign<br />antigens, but rather become attached to such antigens<br />to “label” them for destruction. Each antibody<br />BOX 14–2 AIDS<br />placental transmission of the virus from mother to<br />fetus.<br />In the United States, most of the cases of AIDS<br />during the 1980s were in homosexual men and IV<br />drug users who shared syringes contaminated with<br />their blood. By the 1990s, however, it was clear that<br />AIDS was becoming more of a heterosexually transmitted<br />disease, with rapidly increasing case rates<br />among women and teenagers. In much of the rest<br />of the world, especially Africa and Asia, the transmission<br />of AIDS has always been primarily by heterosexual<br />contact, with equal numbers of women<br />and men infected. In many of these countries AIDS<br />is an enormous public health problem, and the<br />annual number of new cases is still rising.<br />At present we have no medications that will<br />eradicate HIV, although certain combinations of<br />drugs effectively suppress the virus in some people.<br />For these people, AIDS may become a chronic but<br />not fatal disease. Unfortunately, the medications do<br />not work for everyone, and they are very expensive,<br />beyond the means of most of the world’s AIDS<br />patients.<br />Development of an AIDS vaccine has not yet<br />been successful, although dozens of vaccines are<br />undergoing clinical trials. A vaccine stimulates antibody<br />production to a specific pathogen, but everyone<br />who has died of AIDS had antibodies to HIV.<br />Those antibodies were not protective because HIV is<br />a mutating virus; it constantly changes itself, making<br />previously produced antibodies ineffective. An<br />AIDS vaccine may not be entirely effective, may not<br />have the 80% to 90% protection rate we have<br />come to expect from vaccines.<br />If we cannot cure AIDS and we cannot yet prevent<br />it by vaccination, what recourse is left?<br />Education. Everyone should know how AIDS is<br />spread. The obvious reason is to be able to avoid<br />the high-risk behaviors that make acquiring HIV<br />more likely. Yet another reason, however, is that<br />everyone should know that they need not fear<br />casual contact with people with AIDS. Healthcare<br />personnel have a special responsibility, not only to<br />educate themselves, but to provide education<br />about AIDS for their patients and the families of<br />their patients.<br />In 1981, young homosexual men in New York and<br />California were diagnosed with Kaposi’s sarcoma<br />and Pneumocystis carinii pneumonia. At that time,<br />Kaposi’s sarcoma was known as a rare, slowly growing<br />malignancy in elderly men. Pneumocystis pneumonia<br />was almost unheard of; P. carinii (now P.<br />jiroveci) is a pathogen that does not cause disease in<br />healthy people. That in itself was a clue. These<br />young men were not healthy; their immune systems<br />were not functioning normally. As the number<br />of patients increased rapidly, the disease was given<br />a name (acquired immunodeficiency syndrome—<br />AIDS) and the pathogen was found. Human<br />immunodeficiency virus (HIV) is a retrovirus that<br />infects helper T cells, macrophages, and other<br />human cells. Once infected, the human cells contain<br />HIV genes for the rest of their lives. Without sufficient<br />helper T cells, the immune system is seriously<br />impaired. Foreign antigens are not recognized, B<br />cells are not activated, and killer T cells are not<br />stimulated to proliferate.<br />The person with AIDS is susceptible to opportunistic<br />infections, that is, those infections caused<br />by fungi and protozoa that would not affect average<br />healthy adults. Some of these infections may be<br />treated with medications and even temporarily<br />cured, but the immune system cannot prevent the<br />next infection, or the next. As of this writing, AIDS<br />is considered an incurable disease, although with<br />proper medical treatment, some people with AIDS<br />may live for many years.<br />Where did this virus come from? The latest<br />research suggests that HIV evolved from a harmless<br />chimpanzee virus in Africa sometime during the<br />1930s. Spread of the virus was very slow at first,<br />and only when air travel became commonplace did<br />the virus spread worldwide.<br />The incubation period of AIDS is highly variable,<br />ranging from a few months to several years.<br />An infected person may unknowingly spread HIV to<br />others before any symptoms appear. It should<br />be emphasized that AIDS, although communicable,<br />is not a contagious disease. It is not spread<br />by casual contact as is measles or the common<br />cold. Transmission of AIDS occurs through sexual<br />contact, by contact with infected blood, or by<br />produced is specific for only one antigen. Because<br />there are so many different pathogens, you might<br />think that the immune system would have to be capable<br />of producing many different antibodies, and in fact<br />this is so. It is estimated that millions of different antigen-<br />specific antibodies can be produced, should there<br />be a need for them. The structure of antibodies is<br />shown in Fig. 14–8, and the five classes of antibodies<br />are described in Table 14–1.<br />The antibodies produced will bond to the antigen,<br />forming an antigen–antibody complex. This complex<br />results in opsonization, which means that the antigen<br />The Lymphatic System and Immunity 333<br />Table 14–1 CLASSES OF ANTIBODIES<br />Name Location Functions<br />IgG<br />IgA<br />IgM<br />IgD<br />IgE<br />Blood<br />Extracellular fluid<br />External secretions (tears,<br />saliva, etc.)<br />Blood<br />B lymphocytes<br />Mast cells or basophils<br />• Crosses the placenta to provide passive immunity for newborns<br />• Provides long-term immunity following recovery or a vaccine<br />• Present in breast milk to provide passive immunity for breast-fed infants<br />• Found in secretions of all mucous membranes<br />• Produced first by the maturing immune system of infants<br />• Produced first during an infection (IgG production follows)<br />• Part of the ABO blood group<br />• Receptors on B lymphocytes<br />• Important in allergic reactions (mast cells release histamine)<br />A<br />Antigen binding<br />site<br />Complement<br />binding site<br />Macrophage<br />binding site<br />B<br />IgG IgD IgE<br />IgA<br />IgM<br />Bacteria Virus Toxin<br />Agglutination<br />C<br />Neutralization<br />Disulfide<br />bonds<br />Figure 14–8. Antibodies.<br />(A) Structure of one IgG molecule.<br />Notice how the many<br />disulfide bonds maintain the<br />shape of the molecule.<br />(B) Structure of the five classes<br />of antibodies. (C) Antibody<br />activity: Agglutination of bacteria<br />and neutralization of<br />viruses or toxins.<br />QUESTION: In part C, why<br />does neutralization inactivate<br />a bacterial toxin?<br />is now “labeled” for phagocytosis by macrophages or<br />neutrophils. The antigen–antibody complex also stimulates<br />the process of complement fixation (see Box<br />14–3: Diagnostic Tests).<br />Some of the circulating complement proteins are<br />activated, or fixed, by an antigen–antibody complex.<br />Complement fixation may be complete or partial. If<br />the foreign antigen is cellular, the complement proteins<br />bond to the antigen–antibody complex, then to<br />one another, forming an enzymatic ring that punches<br />a hole in the cell to bring about the death of the cell.<br />This is complete (or entire) complement fixation and<br />is what happens to bacterial cells (it is also the cause of<br />hemolysis in a transfusion reaction).<br />If the foreign antigen is not a cell—let’s say it’s a<br />virus for example—partial complement fixation takes<br />place, in which some of the complement proteins bond<br />to the antigen–antibody complex. This is a chemotactic<br />factor. Chemotaxis means “chemical movement” and<br />is actually another label that attracts macrophages to<br />engulf and destroy the foreign antigen.<br />In summary, adaptive immunity is very specific,<br />does create memory, and because it does, often becomes<br />more efficient with repeated exposures.<br />Antibody Responses<br />The first exposure to a foreign antigen does stimulate<br />antibody production, but antibodies are produced<br />slowly and in small amounts (see Fig. 14–9). Let us<br />take as a specific example the measles virus. On a person’s<br />first exposure to this virus, antibody production is<br />usually too slow to prevent the disease itself, and the<br />person will have clinical measles. Most people who get<br />measles recover, and upon recovery have antibodies<br />and memory cells that are specific for the measles<br />virus.<br />On a second exposure to this virus, the memory<br />cells initiate rapid production of large amounts of antibodies,<br />enough to prevent a second case of measles.<br />This is the reason why we develop immunity to certain<br />diseases, and this is also the basis for the protection<br />given by vaccines (see Box 14–4: Vaccines).<br />As mentioned previously, antibodies label pathogens<br />or other foreign antigens for phagocytosis or<br />complement fixation. More specifically, antibodies<br />cause agglutination or neutralization of pathogens<br />before their eventual destruction. Agglutination<br />means “clumping,” and this is what happens when<br />antibodies bind to bacterial cells. The bacteria that are<br />clumped together by attached antibodies are more easily<br />phagocytized by macrophages (see Fig. 14–8).<br />The activity of viruses may be neutralized by antibodies.<br />A virus must get inside a living cell in order to<br />reproduce itself. However, a virus with antibodies<br />attached to it is unable to enter a cell, cannot reproduce,<br />and will soon be phagocytized. Bacterial toxins<br />may also be neutralized by attached antibodies. The<br />antibodies change the shape of the toxin, prevent it<br />from exerting its harmful effects, and promote its<br />phagocytosis by macrophages.<br />Allergies are also the result of antibody activity<br />(see box 14–5: Allergies).<br />TYPES OF IMMUNITY<br />If we consider the source of immunity, that is, where<br />it comes from, we can begin with two major categories:<br />genetic immunity and acquired immunity.<br />Genetic immunity is conferred by our DNA, and<br />acquired immunity is developed or acquired by natural<br />or artificial means.<br />Genetic immunity does not involve antibodies or<br />the immune system; it is the result of our genetic<br />makeup. What this means is that some pathogens<br />334 The Lymphatic System and Immunity<br />BOX 14–3 DIAGNOSTIC TESTS<br />Several important laboratory tests involve antibodies<br />and may be very useful to confirm a diagnosis.<br />Complement fixation test—determines the<br />presence of a particular antibody in the patient’s<br />blood, but does not indicate when the infection<br />occurred.<br />Antibody titer—determines the level or<br />amount of a specific antibody in the patient’s<br />blood. If another titer is done 1 to several weeks<br />later, an increase in the antibody level shows the<br />infection to be current.<br />Fluorescent antibody test—uses antibodies<br />tagged with fluorescent dyes, which are added<br />to a clinical specimen such as blood, sputum, or<br />a biopsy of tissue. If the suspected pathogen is<br />present, the fluorescent antibodies will bond to it<br />and the antigen–antibody complex will “glow”<br />when examined with a fluorescent microscope.<br />Tests such as these are used in conjunction<br />with patient history and symptoms to arrive at a<br />diagnosis.<br />The Lymphatic System and Immunity 335<br />Primary and secondary antibody responses<br />Antibody level<br />First<br />exposure<br />to<br />antigen<br />Second<br />exposure<br />to<br />antigen<br />10 days 20 days some months some years 10 days 20 days<br />Time after exposure<br />IgG<br />IgG<br />IgM IgM<br />Figure 14–9. Antibody responses to first and subsequent exposures to a pathogen. See<br />text for description.<br />QUESTION: State the two differences in IgG production after a first and a second exposure<br />to the same antigen.<br />BOX 14–4 VACCINES<br />the inactivated toxins of these bacteria. Vaccines for<br />pneumococcal pneumonia and meningitis contain<br />bacterial capsules. These vaccines cannot cause disease<br />because the capsules are non-toxic and nonliving;<br />there is nothing that can reproduce.<br />Influenza and rabies vaccines contain killed viruses.<br />Measles and the oral polio vaccines contain attenuated<br />(weakened) viruses.<br />Although attenuated pathogens are usually<br />strongly antigenic and stimulate a protective<br />immune response, there is a very small chance that<br />the pathogen may regain its virulence and cause<br />the disease. The live-virus oral polio vaccine (still<br />being used in the quest to eliminate polio throughout<br />the world) has a risk of 1 in 500,000 of causing<br />polio. The killed-virus injectable polio vaccine has<br />no such risk.<br />The purpose of vaccines is to prevent disease. A vaccine<br />contains an antigen that the immune system<br />will respond to, just as it would to the actual<br />pathogen. The types of vaccine antigens are a killed<br />or weakened (attenuated) pathogen, part of a<br />pathogen such as a bacterial capsule, or an inactivated<br />bacterial toxin called a toxoid.<br />Because the vaccine itself does not cause disease<br />(with very rare exceptions), the fact that antibody<br />production to it is slow is not detrimental to the<br />person. The vaccine takes the place of the first<br />exposure to the pathogen and stimulates production<br />of antibodies and memory cells. On exposure<br />to the pathogen itself, the memory cells initiate<br />rapid production of large amounts of antibody,<br />enough to prevent disease.<br />We now have vaccines for many diseases. The<br />tetanus and diphtheria vaccines contain toxoids,<br />cause disease in certain host species but not in others.<br />Dogs and cats, for example, have genetic immunity<br />to the measles virus, which is a pathogen only for people.<br />Mouse leukemia viruses affect only mice, not<br />people; we have genetic immunity to them. This is<br />not because we have antibodies against these mouse<br />viruses, but rather that we have genes that are the<br />codes for proteins that make it impossible for such<br />pathogens to reproduce in our cells and tissues.<br />Monkeys have similar protective genes and proteins<br />for the human AIDS virus; HIV does not cause disease<br />in these monkeys. Because this is a genetic characteristic<br />programmed in DNA, genetic immunity always<br />lasts a lifetime.<br />Acquired immunity does involve antibodies.<br />Passive immunity means that the antibodies are from<br />another source, whereas active immunity means that<br />the individual produces his or her own antibodies.<br />One type of naturally acquired passive immunity is<br />the placental transmission of antibodies (IgG) from<br />maternal blood to fetal circulation. The baby will then<br />be born temporarily immune to the diseases the<br />mother is immune to. Such passive immunity may be<br />prolonged by breast-feeding, because breast milk also<br />contains maternal antibodies (IgA).<br />Artificially acquired passive immunity is obtained<br />by the injection of immune globulins (gamma globulins<br />or preformed antibodies) after presumed exposure<br />to a particular pathogen. Such immune globulins are<br />available for German measles, hepatitis A and B,<br />tetanus and botulism (anti-toxins), and rabies. These<br />are not vaccines; they do not stimulate immune mechanisms,<br />but rather provide immediate antibody protection.<br />Passive immunity is always temporary, lasting<br />a few weeks to a few months, because antibodies from<br />another source eventually break down.<br />Active immunity is the production of one’s own<br />antibodies and may be stimulated by natural or artificial<br />means. Naturally acquired active immunity means<br />that a person has recovered from a disease and now<br />has antibodies and memory cells specific for that<br />pathogen. Artificially acquired active immunity is the<br />result of a vaccine that has stimulated production of<br />antibodies and memory cells (see Box 14–6: Vaccines<br />That Have Changed Our Lives). No general statement<br />can be made about the duration of active immunity.<br />Recovering from plague, for example, confers<br />lifelong immunity, but the plague vaccine does not.<br />Duration of active immunity, therefore, varies with<br />the particular disease or vaccine.<br />The types of immunity are summarized in Table<br />14–2.<br />336 The Lymphatic System and Immunity<br />BOX 14–5 ALLERGIES<br />In an allergic reaction, the effects of inflammatory<br />chemicals create symptoms such as watery<br />eyes and runny nose (hay fever) or the more serious<br />wheezing and difficult breathing that characterize<br />asthma. Several medications are available to counteract<br />these effects (see Chapter 15 for a description<br />of asthma).<br />Anaphylactic shock is an extreme allergic<br />response that may be elicited by exposure to penicillin<br />or insect venoms. On the first exposure, the<br />person becomes highly sensitized to the foreign<br />antigen. On the second exposure, histamine is<br />released from mast cells throughout the body and<br />causes a drastic decrease in blood volume. The<br />resulting drop in blood pressure may be fatal in<br />only a few minutes. People who know they are<br />allergic to bee stings, for example, may obtain a<br />self-contained syringe of epinephrine to carry with<br />them. Epinephrine can delay the progression of<br />anaphylactic shock long enough for the person to<br />seek medical attention.<br />An allergy is a hypersensitivity to a particular foreign<br />antigen, called an allergen. Allergens include<br />plant pollens, foods, chemicals in cosmetics, antibiotics<br />such as penicillin, dust, and mold spores. Such<br />allergens are not themselves harmful. Most people,<br />for example, can inhale pollen, eat peanuts, or take<br />penicillin with no ill effects.<br />Hypersensitivity means that the immune system<br />overresponds to the allergen, and produces tissue<br />damage by doing so. Allergic responses are characterized<br />by the production of IgE antibodies, which<br />bond to mast cells. Mast cells are specialized connective<br />tissue cells and are numerous in the connective<br />tissue of the skin and mucous membranes.<br />Chemicals in mast cells include histamine and<br />leukotrienes, which are released by the bonding of<br />IgE antibodies or when tissue damage occurs.<br />These chemicals contribute to the process of<br />inflammation by increasing the permeability of capillaries<br />and venules. Tissue fluid collects and more<br />WBCs are brought to the damaged area.<br />AGING AND THE<br />LYMPHATIC SYSTEM<br />The aging of the lymphatic system is apparent in the<br />decreased efficiency of immune responses. Elderly<br />people are more likely than younger ones to develop<br />shingles, when an aging immune system cannot keep<br />the chickenpox virus dormant. They are also more<br />susceptible to infections such as influenza and to what<br />are called secondary infections, such as pneumonia<br />following a case of the flu. Vaccines for both of these<br />are available, and elderly people should be encouraged<br />to get them. Elderly people should also be sure to get<br />a tetanus-diphtheria booster every 10 years.<br />Autoimmune disorders are also more common<br />among older people; the immune system mistakenly<br />perceives a body tissue as foreign and initiates its destruction.<br />Rheumatoid arthritis and myasthenia gravis<br />are examples of autoimmune diseases. The incidence<br />of cancer is also higher. Malignant cells that once<br />might have been quickly destroyed remain alive and<br />proliferate.<br />The Lymphatic System and Immunity 337<br />BOX 14–6 VACCINES THAT HAVE CHANGED OUR LIVES<br />they are no longer possible reservoirs or sources of<br />the pathogen for others, and the spread of disease<br />may be greatly limited.<br />Other diseases that have been controlled by the<br />use of vaccines are tetanus, mumps, influenza,<br />measles, and German measles. Whooping cough<br />had been controlled until recently, when the vaccination<br />rate decreased; the annual number of cases<br />in the United States has more than doubled. The<br />vaccine for hepatitis B has significantly decreased<br />the number of cases of this disease among healthcare<br />workers, and the vaccine is recommended for<br />all children. People who have been exposed to<br />rabies, which is virtually always fatal, can be protected<br />by a safe vaccine.<br />Without such vaccines our lives would be very<br />different. Infant mortality or death in childhood<br />would be much more frequent, and all of us would<br />have to be much more aware of infectious diseases.<br />In many parts of the world this is still true; many of<br />the developing countries in Africa and Asia still cannot<br />afford extensive vaccination programs for their<br />children. Many of the diseases mentioned here,<br />which we may rarely think of, are still a very significant<br />part of the lives of millions of people.<br />In 1797, Edward Jenner (in England) published his<br />results on the use of the cowpox virus called vaccinia<br />as the first vaccine for smallpox, a closely<br />related virus. (He was unaware of the actual pathogens,<br />because viruses had not yet been discovered,<br />but he had noticed that milkmaids who got cowpox<br />rarely got smallpox.) In 1980, the World Health<br />Organization declared that smallpox had been<br />eradicated throughout the world. A disease that<br />had killed or disfigured millions of people throughout<br />recorded history is now considered part of history<br />(except for the possible use of the virus as a<br />biological weapon).<br />In the 19th century in the northern United<br />States, thousands of children died of diphtheria<br />every winter. Today there are fewer than 10 cases<br />of diphtheria each year in the entire country. In<br />the early 1950s, 50,000 cases of paralytic polio<br />were reported in the United States each year.<br />Today, wild-type polio virus is not found in North<br />America.<br />Smallpox, diphtheria, and polio are no longer<br />the terrible diseases they once were, and this is<br />because of the development and widespread use of<br />vaccines. When people are protected by a vaccine,<br />Table 14–2 TYPES OF IMMUNITY<br />Type Description<br />Genetic<br />Acquired<br />Passive<br />NATURAL<br />ARTIFICIAL<br />Active<br />NATURAL<br />ARTIFICIAL<br />• Does not involve antibodies; is<br />programmed in DNA<br />• Some pathogens affect certain<br />host species but not others<br />• Does involve antibodies<br />• Antibodies from another source<br />• Placental transmission of antibodies<br />from mother to fetus<br />• Transmission of antibodies in<br />breast milk<br />• Injection of preformed antibodies<br />(gamma globulins or immune<br />globulins) after presumed exposure<br />• Production of one’s own antibodies<br />• Recovery from a disease, with production<br />of antibodies and memory<br />cells<br />• A vaccine stimulates production of<br />antibodies and memory cells<br />SUMMARY<br />The preceding discussions of immunity will give you a<br />small idea of the complexity of the body’s defense system.<br />However, there is still much more to be learned,<br />especially about the effects of the nervous system and<br />endocrine system on immunity. For example, it is<br />known that people under great stress have immune<br />systems that may not function as they did when stress<br />was absent.<br />At present, much research is being done in this<br />field. The goal is not to eliminate all disease, for that<br />would not be possible. Rather, the aim is to enable<br />people to live healthier lives by preventing certain<br />diseases.<br />338 The Lymphatic System and Immunity<br />STUDY OUTLINE<br />Functions of the Lymphatic System<br />1. To return tissue fluid to the blood to maintain<br />blood volume (see Fig. 14–1).<br />2. To protect the body against pathogens and other<br />foreign material.<br />Parts of the Lymphatic System<br />1. Lymph and lymph vessels.<br />2. Lymphatic tissue: lymph nodes and nodules,<br />spleen, and thymus; lymphocytes mature and proliferate.<br />Lymph—the tissue fluid that enters lymph<br />capillaries<br />1. Similar to plasma, but more WBCs are present,<br />and has less protein.<br />2. Must be returned to the blood to maintain blood<br />volume and blood pressure.<br />Lymph Vessels<br />1. Dead-end lymph capillaries are found in most tissue<br />spaces; collect tissue fluid and proteins (see Fig.<br />14–2).<br />2. The structure of larger lymph vessels is like that of<br />veins; valves prevent the backflow of lymph.<br />3. Lymph is kept moving in lymph vessels by:<br />• constriction of the lymph vessels<br />• the skeletal muscle pump<br />• the respiratory pump<br />4. Lymph from the lower body and upper left quadrant<br />enters the thoracic duct and is returned to the<br />blood in the left subclavian vein (see Fig. 14–3).<br />5. Lymph from the upper right quadrant enters the<br />right lymphatic duct and is returned to the blood in<br />the right subclavian vein.<br />Lymph Nodes—encapsulated masses of lymphatic<br />tissue<br />1. Found in groups along the pathways of lymph vessels.<br />2. As lymph flows through the nodes:<br />• foreign material is phagocytized by fixed macrophages<br />• lymphocytes are activated and fixed plasma cells<br />produce antibodies to foreign antigens (see Fig.<br />14–4)<br />3. The major paired groups of lymph nodes are the<br />cervical, axillary, and inguinal groups. These are<br />at the junctions of the head and extremities with<br />the trunk; remove pathogens from the lymph from<br />the extremities before the lymph is returned to the<br />blood.<br />Lymph Nodules—small unencapsulated<br />masses of lymphatic tissue<br />1. Found beneath the epithelium of all mucous membranes,<br />that is, the tracts that have natural openings<br />to the environment.<br />2. Destroy pathogens that penetrate the epithelium of<br />the respiratory, digestive, urinary, or reproductive<br />tracts.<br />3. Tonsils are the lymph nodules of the pharynx;<br />Peyer’s patches are those of the small intestine.<br />Spleen—located in the upper left abdominal<br />quadrant behind the stomach<br />1. The fetal spleen produces RBCs.<br />2. Functions after birth:<br />• contains lymphocytes to be activated and fixed<br />plasma cells that produce antibodies<br />• contains fixed macrophages (RE cells) that<br />phagocytize pathogens and old RBCs; bilirubin<br />is formed and sent to the liver for excretion in<br />bile<br />• stores platelets and destroys damaged platelets<br />Thymus—inferior to the thyroid gland; in<br />the fetus and infant the thymus is large (see<br />Fig. 14–5); with age the thymus shrinks<br />1. Produces T lymphocytes (T cells).<br />2. Produces thymic hormones that make T cells<br />immunologically competent, that is, able to recognize<br />foreign antigens and provide immunity.<br />Immunity—the ability to destroy foreign<br />antigens and prevent future cases of certain<br />infectious diseases<br />1. Antigens are chemical markers that identify cells.<br />Human cells have “self” antigens—the HLA types.<br />2. Foreign antigens stimulate antibody production<br />or other immune responses, and include bacteria,<br />viruses, fungi, protozoa, and malignant cells.<br />Innate Immunity (see Fig. 14–6)<br />1. Is nonspecific, responses are always the same, does<br />not create memory, and does not become more<br />efficient. Consists of barriers, defensive cells, and<br />chemical defenses.<br />2. Barriers<br />• Unbroken stratum corneum and sebum; living<br />epidermal cells secrete defensins<br />• Subcutaneous tissue with WBCs<br />• Mucous membranes and areolar CT with WBCs;<br />upper respiratory epithelium is ciliated<br />• HCl in gastric juice<br />• Lysozyme in saliva and tears<br />3. Defensive cells<br />• Phagocytes—macrophages, neutrophils, eosinophils;<br />macrophages also activate the lymphocytes<br />of adaptive immunity<br />• Langerhans cells and other dendritic cells—activate<br />lymphocytes<br />• Natural killer cells—destroy foreign cells by rupturing<br />their cell membranes<br />• Basophils and mast cells—produce histamine and<br />leukotrienes (inflammation)<br />4. Chemical defenses<br />• Interferon blocks viral reproduction<br />• Complement proteins lyse foreign cells, attract<br />WBCs, and contribute to inflammation<br />• Inflammation—the response to any kind of damage;<br />vasodilation and increased capillary permeability<br />bring tissue fluid and WBCs to the area.<br />Purpose: to contain the damage, eliminate the<br />cause, and make tissue repair possible.<br />Signs: redness, heat, swelling, and pain<br />Adaptive Immunity (see Fig. 14–7)<br />1. Is very specific, may involve antibodies, does create<br />memory, and responses become more efficient.<br />Consists of cell-mediated and antibody-mediated<br />immunity; is carried out by T cells, B cells, and<br />macrophages.<br />2. T lymphocytes (T cells)—in the embryo are produced<br />in the thymus and RBM; they require the<br />hormones of the thymus for maturation; migrate to<br />the spleen, lymph nodes, and nodules.<br />3. B lymphocytes (B cells)—in the embryo are produced<br />in the RBM; migrate to the spleen, lymph<br />nodes, and nodules.<br />4. The antigen must first be recognized as foreign;<br />this is accomplished by B cells or by helper T cells<br />that compare the foreign antigen to “self” antigens<br />present on macrophages.<br />5. Helper T cells strongly initiate one or both of the<br />immune mechanisms: cell-mediated immunity and<br />antibody-mediated immunity.<br />Cell-Mediated (cellular) Immunity (see Fig.<br />14–7)<br />1. Does not involve antibodies; is effective against<br />intracellular pathogens, malignant cells, and grafts<br />of foreign tissue.<br />2. Helper T cells recognize the foreign antigen, are<br />antigen specific, and begin to divide to form different<br />groups of T cells.<br />3. Memory T cells will remember the specific foreign<br />antigen.<br />4. Cytotoxic (killer) T cells chemically destroy foreign<br />cells and produce cytokines to attract macrophages.<br />Antibody-Mediated (Humoral) Immunity<br />(see Fig. 14–7)<br />1. Does involve antibody production; is effective<br />against pathogens and foreign cells.<br />The Lymphatic System and Immunity 339<br />2. B cells and helper T cells recognize the foreign<br />antigen; the B cells are antigen specific and begin<br />to divide.<br />3. Memory B cells will remember the specific foreign<br />antigen.<br />4. Other B cells become plasma cells that produce<br />antigen-specific antibodies.<br />5. An antigen–antibody complex is formed, which<br />attracts macrophages (opsonization).<br />6. Complement fixation is stimulated by antigen–<br />antibody complexes. The complement proteins<br />bind to the antigen–antibody complex and lyse cellular<br />antigens or enhance the phagocytosis of noncellular<br />antigens.<br />Antibodies—immune globulins (Ig) or<br />gamma globulins (see Table 14–1 and<br />Fig. 14–8)<br />1. Proteins produced by plasma cells in response to<br />foreign antigens.<br />2. Each antibody is specific for only one foreign antigen.<br />3. Bond to the foreign antigen to label it for phagocytosis<br />(opsonization).<br />Antibody Responses and Functions (see Fig.<br />14–9)<br />1. On the first exposure to a foreign antigen, antibodies<br />are produced slowly and in small amounts, and<br />the person may develop clinical disease.<br />2. On the second exposure, the memory cells initiate<br />rapid production of large amounts of antibodies,<br />and a second case of the disease may be prevented.<br />This is the basis for the protection given by vaccines,<br />which take the place of the first exposure.<br />3. Antibodies cause agglutination (clumping) of bacterial<br />cells; clumped cells are easier for macrophages<br />to phagocytize (see Fig. 14–8).<br />4. Antibodies neutralize viruses by bonding to them<br />and preventing their entry into cells.<br />5. Antibodies neutralize bacterial toxins by bonding<br />to them and changing their shape.<br />Types of Immunity (see Table 14–2)<br />340 The Lymphatic System and Immunity<br />REVIEW QUESTIONS<br />1. Explain the relationships among plasma, tissue<br />fluid, and lymph, in terms of movement of water<br />throughout the body. (p. 322)<br />2. Describe the system of lymph vessels. Explain how<br />lymph is kept moving in these vessels. Into which<br />veins is lymph emptied? (p. 322)<br />3. State the locations of the major groups of lymph<br />nodes, and explain their functions. (pp. 322–323)<br />4. State the locations of lymph nodules, and explain<br />their functions. (pp. 324)<br />5. Describe the location of the spleen and explain its<br />functions. If the spleen is removed, what organs<br />will compensate for its functions? (p. 324)<br />6. Explain the function of the thymus, and state when<br />(age). this function is most important. (pp. 324,<br />326)<br />7. Name the different kinds of foreign antigens to<br />which the immune system responds, and state three<br />general differences between innate immunity and<br />adaptive immunity. (p. 327)<br />8. Innate immunity includes barriers, defensive<br />cells, and chemicals; give two examples of each.<br />(p. 328)<br />9. Explain how a foreign antigen is recognized as<br />foreign. Which mechanism of adaptive immunity<br />involves antibody production? Explain what<br />opsonization means. (pp. 330, 333)<br />10. State the functions of helper T cells, cytotoxic<br />T cells, and memory T cells. Plasma cells<br />differentiate from which type of lymphocyte?<br />State the function of plasma cells. What other<br />type of cell comes from B lymphocytes? (pp. 330,<br />332)<br />11. What is the stimulus for complement fixation?<br />How does this process destroy cellular antigens<br />and non-cellular antigens? (pp. 334)<br />12. Explain the antibody reactions of agglutination<br />and neutralization. (p. 334)<br />13. Explain how a vaccine provides protective immunity<br />in terms of first and second exposures to a<br />pathogen. (p. 334)<br />14. Explain the difference between the following: (pp.<br />336–337)<br />a. Genetic immunity and acquired immunity<br />b. Passive acquired immunity and active acquired<br />immunity<br />c. Natural and artificial passive acquired immunity<br />d. Natural and artificial active acquired immunity<br />The Lymphatic System and Immunity 341<br />FOR FURTHER THOUGHT<br />1. Bubonic plague, also called black plague, is a serious<br />disease caused by a bacterium and spread by<br />fleas from rats or rodents to people. It got its<br />“black” name from “buboes,” dark swellings found<br />in the groin or armpit of people with plague.<br />Explain what buboes are, and why they were usually<br />found in the groin and armpit.<br />2. In Rh disease of the newborn, maternal antibodies<br />enter fetal circulation and destroy the red blood<br />cells of the fetus. A mother with type O blood has<br />anti-A and anti-B antibodies, but may have a dozen<br />type A children without any problem at all. Explain<br />why. (Look at Table 14–1 and Fig. 14–8.)<br />3. Most vaccines are given by injection. The oral<br />polio vaccine (OPV), however, is not; it is given by<br />mouth. Remembering that the purpose of a vaccine<br />is to expose the individual to the pathogen, what<br />does this tell you about the polio viruses (there are<br />three) and their usual site of infection?<br />4. Everyone should have a tetanus booster shot every<br />10 years. That is what we often call a “tetanus<br />shot.” Someone who sustains a soil-contaminated<br />injury should also receive a tetanus booster (if none<br />in the past 10 years). But someone who has symptoms<br />of tetanus should get TIG, tetanus immune<br />globulin. Explain the difference, and why TIG is so<br />important.<br />5. People with AIDS are susceptible to many other<br />diseases. Which of these would be least likely:<br />pneumonia, rheumatoid arthritis, yeast infection of<br />the mouth, or protozoan infection of the intestines?<br />Explain your answer.<br />342<br />CHAPTER 15<br />Chapter Outline<br />Divisions of the Respiratory System<br />Nose and Nasal Cavities<br />Pharynx<br />Larynx<br />Trachea and Bronchial Tree<br />Lungs and Pleural Membranes<br />Alveoli<br />Mechanism of Breathing<br />Inhalation<br />Exhalation<br />Pulmonary Volumes<br />Exchange of Gases<br />Diffusion of Gases—Partial Pressures<br />Transport of Gases in the Blood<br />Regulation of Respiration<br />Nervous Regulation<br />Chemical Regulation<br />Respiration and Acid–Base Balance<br />Respiratory Acidosis and Alkalosis<br />Respiratory Compensation<br />Aging and the Respiratory System<br />BOX 15–1 ASTHMA<br />BOX 15–2 HYALINE MEMBRANE DISEASE<br />BOX 15–3 PNEUMOTHORAX<br />BOX 15–4 EMPHYSEMA<br />BOX 15–5 THE HEIMLICH MANEUVER<br />BOX 15–6 PULMONARY EDEMA<br />BOX 15–7 PNEUMONIA<br />BOX 15–8 CARBON MONOXIDE<br />Student Objectives<br />• State the general function of the respiratory system.<br />• Describe the structure and functions of the nasal<br />cavities and pharynx.<br />• Describe the structure of the larynx and explain<br />the speaking mechanism.<br />• Describe the structure and functions of the trachea<br />and bronchial tree.<br />• State the locations of the pleural membranes, and<br />explain the functions of serous fluid.<br />• Describe the structure of the alveoli and pulmonary<br />capillaries, and explain the importance of<br />surfactant.<br />• Name and describe the important air pressures<br />involved in breathing.<br />• Describe normal inhalation and exhalation and<br />forced exhalation.<br />• Name the pulmonary volumes and define each.<br />• Explain the diffusion of gases in external respiration<br />and internal respiration.<br />• Describe how oxygen and carbon dioxide are<br />transported in the blood.<br />• Explain the nervous and chemical mechanisms<br />that regulate respiration.<br />• Explain how respiration affects the pH of body<br />fluids.<br />The Respiratory System<br />343<br />New Terminology<br />Alveoli (al-VEE-oh-lye)<br />Bronchial tree (BRONG-kee-uhl TREE)<br />Epiglottis (ep-i-GLAH-tis)<br />Glottis (GLAH-tis)<br />Intrapleural pressure (IN-trah-PLOOR-uhl PRESshur)<br />Intrapulmonic pressure (IN-trah-pull-MAHN-ik<br />PRES-shur)<br />Larynx (LA-rinks)<br />Partial pressure (PAR-shul PRES-shur)<br />Phrenic nerves (FREN-ik NURVZ)<br />Pulmonary surfactant (PULL-muh-ner-ee sir-FAKtent)<br />Residual air (ree-ZID-yoo-al AYRE)<br />Respiratory acidosis (RES-pi-rah-TOR-ee ass-i-<br />DOH-sis)<br />Respiratory alkalosis (RES-pi-rah-TOR-ee al-kah-<br />LOH-sis)<br />Soft palate (SAWFT PAL-uht)<br />Tidal volume (TIGH-duhl VAHL-yoom)<br />Ventilation (VEN-ti-LAY-shun)<br />Vital capacity (VY-tuhl kuh-PASS-i-tee)<br />Related Clinical Terminology<br />Cyanosis (SIGH-uh-NOH-sis)<br />Dyspnea (DISP-nee-ah)<br />Emphysema (EM-fi-SEE-mah)<br />Heimlich maneuver (HIGHM-lik ma-NEW-ver)<br />Hyaline membrane disease (HIGH-e-lin MEMbrain<br />dis-EEZ)<br />Pneumonia (new-MOH-nee-ah)<br />Pneumothorax (NEW-moh-THAW-raks)<br />Pulmonary edema (PULL-muh-ner-ee<br />uh-DEE-muh).<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />Sometimes a person will describe a habit as being<br />“as natural as breathing.” Indeed, what could be more<br />natural? We rarely think about breathing, and it isn’t<br />something we look forward to, as we might look forward<br />to a good dinner. We just breathe, usually at the<br />rate of 12 to 20 times per minute, and faster when<br />necessary (such as during exercise). You may have<br />heard of trained singers “learning how to breathe,”<br />but they are really learning how to make their breathing<br />more efficient.<br />Most of the respiratory system is concerned with<br />what we think of as breathing: moving air into and out<br />of the lungs. The lungs are the site of the exchanges of<br />oxygen and carbon dioxide between the air and the<br />blood. Both of these exchanges are important. All<br />of our cells must obtain oxygen to carry out cell respiration<br />to produce ATP. Just as crucial is the elimination<br />of the CO2 produced as a waste product of cell<br />respiration, and, as you already know, the proper functioning<br />of the circulatory system is essential for the<br />transport of these gases in the blood.<br />DIVISIONS OF THE<br />RESPIRATORY SYSTEM<br />The respiratory system may be divided into the upper<br />respiratory tract and the lower respiratory tract. The<br />upper respiratory tract consists of the parts outside<br />the chest cavity: the air passages of the nose, nasal cavities,<br />pharynx, larynx, and upper trachea. The lower<br />respiratory tract consists of the parts found within<br />the chest cavity: the lower trachea and the lungs themselves,<br />which include the bronchial tubes and alveoli.<br />Also part of the respiratory system are the pleural<br />membranes and the respiratory muscles that form the<br />chest cavity: the diaphragm and intercostal muscles.<br />Have you recognized some familiar organs and<br />structures thus far? There will be more, because this<br />chapter includes material from all of the previous<br />chapters. Even though we are discussing the body<br />system by system, the respiratory system is an excellent<br />example of the interdependent functioning of all<br />the body systems.<br />NOSE AND NASAL CAVITIES<br />Air enters and leaves the respiratory system through<br />the nose, which is made of bone and cartilage covered<br />with skin. Just inside the nostrils are hairs, which help<br />block the entry of dust.<br />The two nasal cavities are within the skull, separated<br />by the nasal septum, which is a bony plate made<br />of the ethmoid bone and vomer. The nasal mucosa<br />(lining) is ciliated epithelium, with goblet cells that<br />produce mucus. Three shelf-like or scroll-like bones<br />called conchae project from the lateral wall of each<br />nasal cavity (Figs. 15–1 and 6–6). Just as shelves in a<br />cabinet provide more flat space for storage, the conchae<br />increase the surface area of the nasal mucosa. As<br />air passes through the nasal cavities it is warmed and<br />humidified, so that air that reaches the lungs is warm<br />and moist. Bacteria and particles of air pollution are<br />trapped on the mucus; the cilia continuously sweep<br />the mucus toward the pharynx. Most of this mucus is<br />eventually swallowed, and most bacteria present will<br />be destroyed by the hydrochloric acid in the gastric<br />juice.<br />In the upper nasal cavities are the olfactory receptors,<br />which detect vaporized chemicals that have been<br />inhaled. The olfactory nerves pass through the ethmoid<br />bone to the brain.<br />You may also recall our earlier discussion of the<br />paranasal sinuses, air cavities in the maxillae, frontal,<br />sphenoid, and ethmoid bones (see Figs. 15–1 and 6–9).<br />These sinuses are lined with ciliated epithelium, and<br />the mucus produced drains into the nasal cavities. The<br />functions of the paranasal sinuses are to lighten the<br />skull and provide resonance (more vibrating air) for<br />the voice.<br />PHARYNX<br />The pharynx is a muscular tube posterior to the nasal<br />and oral cavities and anterior to the cervical vertebrae.<br />For descriptive purposes, the pharynx may be divided<br />into three parts: the nasopharynx, oropharynx, and<br />laryngopharynx (see Fig. 15–1).<br />The uppermost portion is the nasopharynx, which<br />is behind the nasal cavities. The soft palate is elevated<br />during swallowing to block the nasopharynx and prevent<br />food or saliva from going up rather than down.<br />The uvula is the part of the soft palate you can see at<br />the back of the throat. On the posterior wall of the<br />nasopharynx is the adenoid or pharyngeal tonsil, a<br />lymph nodule that contains macrophages. Opening<br />into the nasopharynx are the two eustachian tubes,<br />which extend to the middle ear cavities. The purpose<br />344 The Respiratory System<br />of the eustachian tubes is to permit air to enter or<br />leave the middle ears, allowing the eardrums to vibrate<br />properly.<br />The nasopharynx is a passageway for air only, but<br />the remainder of the pharynx serves as both an air and<br />food passageway, although not for both at the same<br />time. The oropharynx is behind the mouth; its<br />mucosa is stratified squamous epithelium, continuous<br />with that of the oral cavity. On its lateral walls are the<br />palatine tonsils, also lymph nodules. Together with<br />The Respiratory System 345<br />Frontal sinus<br />Ethmoid bone Olfactory receptors<br />Conchae<br />Superior<br />Middle<br />Inferior<br />Nostril<br />Hard<br />palate<br />Maxilla<br />Palatine<br />bone<br />Sphenoid sinus<br />Opening of<br />eustachian tube<br />Pharyngeal tonsil<br />Nasopharynx<br />Soft palate<br />Uvula<br />Palatine tonsil<br />Oropharynx<br />Lingual tonsil<br />Epiglottis<br />Laryngopharynx<br />Esophagus<br />Hyoid bone<br />Larynx<br />Thyroid<br />cartilage<br />Cricoid<br />cartilage<br />Trachea<br />Figure 15–1. Midsagittal section of the head and neck showing the structures of the<br />upper respiratory tract.<br />QUESTION: Describe the shape of the conchae by using a familiar object. What is the function<br />of the conchae?<br />the adenoid and the lingual tonsils on the base of the<br />tongue, they form a ring of lymphatic tissue around<br />the pharynx to destroy pathogens that penetrate the<br />mucosa.<br />The laryngopharynx is the most inferior portion<br />of the pharynx. It opens anteriorly into the larynx and<br />posteriorly into the esophagus. Contraction of the<br />muscular wall of the oropharynx and laryngopharynx<br />is part of the swallowing reflex.<br />LARYNX<br />The larynx is often called the voice box, a name that<br />indicates one of its functions, which is speaking. The<br />other function of the larynx is to be an air passageway<br />between the pharynx and the trachea. Air passages<br />must be kept open at all times, and so the larynx is<br />made of nine pieces of cartilage connected by ligaments.<br />Cartilage is a firm yet flexible tissue that prevents<br />collapse of the larynx. In comparison, the<br />esophagus is a collapsed tube except when food is passing<br />through it.<br />The largest cartilage of the larynx is the thyroid<br />cartilage (Fig. 15–2), which you can feel on the anterior<br />surface of your neck. The epiglottis is the uppermost<br />cartilage. During swallowing, the larynx is<br />elevated, and the epiglottis closes over the top, rather<br />like a trap door or hinged lid, to prevent the entry of<br />saliva or food into the larynx.<br />The mucosa of the larynx is ciliated epithelium,<br />except for the vocal cords (stratified squamous epithelium).<br />The cilia of the mucosa sweep upward to<br />remove mucus and trapped dust and microorganisms.<br />The vocal cords (or vocal folds) are on either side<br />of the glottis, the opening between them. During<br />breathing, the vocal cords are held at the sides of the<br />glottis, so that air passes freely into and out of the trachea<br />(Fig. 15–3). During speaking, the intrinsic muscles<br />of the larynx pull the vocal cords across the glottis,<br />and exhaled air vibrates the vocal cords to produce<br />sounds that can be turned into speech. It is also physically<br />possible to speak while inhaling, but this is not<br />what we are used to. The cranial nerves that are motor<br />nerves to the larynx for speaking are the vagus and<br />accessory nerves. You may also recall that for most<br />people, the speech areas are in the left cerebral hemisphere.<br />TRACHEA AND BRONCHIAL TREE<br />The trachea is about 4 to 5 inches (10 to 13 cm) long<br />and extends from the larynx to the primary bronchi.<br />The wall of the trachea contains 16 to 20 C-shaped<br />pieces of cartilage, which keep the trachea open. The<br />gaps in these incomplete cartilage rings are posterior,<br />to permit the expansion of the esophagus when food is<br />swallowed. The mucosa of the trachea is ciliated<br />epithelium with goblet cells. As in the larynx, the cilia<br />sweep upward toward the pharynx.<br />The right and left primary bronchi (Fig. 15–4) are<br />the branches of the trachea that enter the lungs. Their<br />structure is just like that of the trachea, with C-shaped<br />cartilages and ciliated epithelium. Within the lungs,<br />each primary bronchus branches into secondary<br />bronchi leading to the lobes of each lung (three right,<br />two left). The further branching of the bronchial tubes<br />is often called the bronchial tree. Imagine the trachea<br />as the trunk of an upside-down tree with extensive<br />branches that become smaller and smaller; these<br />smaller branches are the bronchioles. No cartilage is<br />present in the walls of the bronchioles; this becomes<br />clinically important in asthma (see Box 15–1: Asthma).<br />The smallest bronchioles terminate in clusters of alveoli,<br />the air sacs of the lungs.<br />LUNGS AND PLEURAL MEMBRANES<br />The lungs are located on either side of the heart in<br />the chest cavity and are encircled and protected by the<br />346 The Respiratory System<br />Epiglottis<br />Hyoid<br />bone<br />Thyroid<br />cartilage<br />Vocal<br />cords<br />Cricoid<br />cartilage<br />Tracheal<br />cartilages<br />A B<br />Figure 15–2. Larynx. (A) Anterior view. (B) Midsagittal<br />section through the larynx, viewed from the left side.<br />QUESTION: What is the function of the epiglottis?<br />rib cage. The base of each lung rests on the diaphragm<br />below; the apex (superior tip) is at the level of the clavicle.<br />On the medial surface of each lung is an indentation<br />called the hilus, where the primary bronchus and<br />the pulmonary artery and veins enter the lung.<br />The pleural membranes are the serous membranes<br />of the thoracic cavity. The parietal pleura lines the<br />chest wall, and the visceral pleura is on the surface of<br />the lungs. Between the pleural membranes is serous<br />fluid, which prevents friction and keeps the two membranes<br />together during breathing.<br />Alveoli<br />The functional units of the lungs are the air sacs called<br />alveoli. The flat alveolar type I cells that form most of<br />the alveolar walls are simple squamous epithelium. In<br />the spaces between clusters of alveoli is elastic connective<br />tissue, which is important for exhalation.<br />Within the alveoli are macrophages that phagocytize<br />pathogens or other foreign material that may not have<br />been swept out by the ciliated epithelium of the<br />bronchial tree. There are millions of alveoli in each<br />lung, and their total surface area is estimated to be 700<br />to 800 square feet (picture a sidewalk two and a half<br />feet wide that is as long as an American football field,<br />or a rectangle 25 feet by 30 feet). Each alveolus is surrounded<br />by a network of pulmonary capillaries (see<br />Fig 15–4). Recall that capillaries are also made of simple<br />squamous epithelium, so there are only two cells<br />between the air in the alveoli and the blood in the pulmonary<br />capillaries, which permits efficient diffusion of<br />gases (Fig. 15–5).<br />Each alveolus is lined with a thin layer of tissue<br />fluid, which is essential for the diffusion of gases, because<br />a gas must dissolve in a liquid in order to enter<br />or leave a cell (the earthworm principle—an earthworm<br />breathes through its moist skin, and will suffocate<br />if its skin dries out). Although this tissue fluid is<br />necessary, it creates a potential problem in that it<br />would make the walls of an alveolus stick together<br />internally. Imagine a plastic bag that is wet inside; its<br />walls would stick together because of the surface tension<br />of the water. This is just what would happen in<br />alveoli, and inflation would be very difficult.<br />This problem is overcome by pulmonary surfactant,<br />a lipoprotein secreted by alveolar type II cells,<br />also called septal cells. Surfactant mixes with the tissue<br />fluid within the alveoli and decreases its surface tension,<br />permitting inflation of the alveoli (see Box 15–2:<br />Hyaline Membrane Disease). Normal inflation of the<br />alveoli in turn permits the exchange of gases, but<br />before we discuss this process, we will first see how air<br />gets into and out of the lungs.<br />MECHANISM OF BREATHING<br />Ventilation is the term for the movement of air to and<br />from the alveoli. The two aspects of ventilation are<br />inhalation and exhalation, which are brought about by<br />The Respiratory System 347<br />Epiglottis<br />Vocal cord<br />A B<br />Trachea<br />Glottis<br />Figure 15–3. Vocal cords and glottis, superior view. (A) Position of the vocal cords during<br />breathing. (B) Position of the vocal cords during speaking.<br />QUESTION: What makes the vocal cords vibrate?<br />the nervous system and the respiratory muscles. The<br />respiratory centers are located in the medulla and<br />pons. Their specific functions will be covered in a later<br />section, but it is the medulla that generates impulses<br />to the respiratory muscles.<br />These muscles are the diaphragm and the external<br />and internal intercostal muscles (Fig. 15–6). The<br />diaphragm is a dome-shaped muscle below the lungs;<br />when it contracts, the diaphragm flattens and moves<br />downward. The intercostal muscles are found between<br />the ribs. The external intercostal muscles pull the<br />ribs upward and outward, and the internal intercostal<br />muscles pull the ribs downward and inward.<br />Ventilation is the result of the respiratory muscles producing<br />changes in the pressure within the alveoli and<br />bronchial tree.<br />348 The Respiratory System<br />Frontal sinuses<br />Sphenoidal sinuses<br />Nasal cavity<br />Nasopharynx<br />Soft palate<br />Epiglottis<br />Larynx and vocal folds<br />Trachea<br />Superior lobe<br />Right lung<br />Right primary<br />bronchus<br />Inferior lobe<br />Mediastinum<br />Cardiac notch<br />Pleural space<br />Pleural membranes<br />Inferior lobe<br />Bronchioles<br />Superior lobe<br />Left primary bronchus<br />Left lung<br />Venule<br />Alveolus<br />Alveolar duct<br />Arteriole<br />Pulmonary capillaries<br />B<br />A<br />Middle lobe<br />Diaphragm<br />Figure 15–4. Respiratory system. (A) Anterior view of the upper and lower respiratory<br />tracts. (B) Microscopic view of alveoli and pulmonary capillaries. (The colors represent the<br />vessels, not the oxygen content of the blood within the vessels.)<br />QUESTION: What are the first branches of the trachea, and how do they resemble the trachea<br />in structure?<br />With respect to breathing, three types of pressure<br />are important:<br />1. Atmospheric pressure—the pressure of the air<br />around us. At sea level, atmospheric pressure is 760<br />mmHg. At higher altitudes, of course, atmospheric<br />pressure is lower.<br />2. Intrapleural pressure—the pressure within the<br />potential pleural space between the parietal pleura<br />and visceral pleura. This is a potential rather than a<br />real space. A thin layer of serous fluid causes the<br />two pleural membranes to adhere to one another.<br />Intrapleural pressure is always slightly below<br />atmospheric pressure (about 756 mmHg), and is<br />called a negative pressure. The elastic lungs are<br />always tending to collapse and pull the visceral<br />pleura away from the parietal pleura. The serous<br />fluid, however, prevents actual separation of the<br />pleural membranes (see Box 15–3: Pneumothorax).<br />3. Intrapulmonic pressure—the pressure within the<br />bronchial tree and alveoli. This pressure fluctuates<br />below and above atmospheric pressure during each<br />cycle of breathing.<br />INHALATION<br />Inhalation, also called inspiration, is a precise<br />sequence of events that may be described as follows:<br />Motor impulses from the medulla travel along the<br />phrenic nerves to the diaphragm and along the intercostal<br />nerves to the external intercostal muscles. The<br />diaphragm contracts, moves downward, and expands<br />the chest cavity from top to bottom. The external<br />intercostal muscles pull the ribs up and out, which<br />expands the chest cavity from side to side and front to<br />back.<br />As the chest cavity is expanded, the parietal pleura<br />expands with it. Intrapleural pressure becomes even<br />more negative as a sort of suction is created between<br />the pleural membranes. The adhesion created by the<br />serous fluid, however, permits the visceral pleura to be<br />expanded too, and this expands the lungs as well.<br />As the lungs expand, intrapulmonic pressure falls<br />below atmospheric pressure, and air enters the nose<br />and travels through the respiratory passages to the<br />alveoli. Entry of air continues until intrapulmonic<br />pressure is equal to atmospheric pressure; this is a normal<br />inhalation. Of course, inhalation can be continued<br />beyond normal, that is, a deep breath. This requires a<br />more forceful contraction of the respiratory muscles<br />to further expand the lungs, permitting the entry of<br />more air.<br />EXHALATION<br />Exhalation may also be called expiration and begins<br />when motor impulses from the medulla decrease and<br />the diaphragm and external intercostal muscles relax.<br />As the chest cavity becomes smaller, the lungs are<br />compressed, and their elastic connective tissue, which<br />was stretched during inhalation, recoils and also com-<br />The Respiratory System 349<br />BOX 15–1 ASTHMA<br />emphysema. When obstructed bronchioles prevent<br />ventilation of alveoli, the walls of the alveoli begin<br />to deteriorate and break down, leaving large cavities<br />that do not provide much surface area for gas<br />exchange.<br />One possible way to prevent such serious lung<br />damage is to prevent asthma attacks with a medication<br />that blocks the release of IgE antibodies. An<br />allergy is an immune overreaction, and blocking<br />such a reaction would prevent the damaging effects<br />of inflammation. In the United States the incidence<br />of asthma is increasing among children; this may be<br />a result of higher levels of air pollution, though<br />genetic and immunologic factors may contribute as<br />well.<br />Asthma is usually triggered by an infection or allergic<br />reaction that affects the smooth muscle and<br />glands of the bronchioles. Allergens include foods<br />and inhaled substances such as dust and pollen.<br />Wheezing and dyspnea (difficult breathing) characterize<br />an asthma attack, which may range from<br />mild to fatal.<br />As part of the allergic response, the smooth muscle<br />of the bronchioles constricts. Because there is no<br />cartilage present in their walls, the bronchioles may<br />close completely. The secretion of mucus increases,<br />perhaps markedly, so the already constricted bronchioles<br />may become clogged or completely<br />obstructed with mucus.<br />Chronic asthma is a predisposing factor for<br />350 The Respiratory System<br />Figure 15–5. (A) Alveolar structure showing type I and type II cells, and alveolar<br />macrophages. The respiratory membrane: the structures and substances through which<br />gases must pass as they diffuse from air to blood (oxygen) or from blood to air (CO2).<br />(B) Sections of human lungs embedded in plastic. On the left is a normal adult lung; on<br />the right is a smoker’s lung. (Photograph by Dan Kaufman.)<br />QUESTION: Which cells shown here are part of the respiratory membrane? Which cells are<br />not, and what are their functions?<br />Elastin fibers<br />Macrophage<br />Type I cell<br />Type II<br />surfactant<br />cell<br />Surfactant<br />and tissue fluid<br />Alveolar epithelium<br />Capillary endothelium<br />Basement membrane<br />of capillary endothelium<br />Capillary<br />Red blood cells<br />Interstitial<br />space<br />Exhalation<br />Inhalation<br />Respiration<br />Oxygen (O )<br />Carbon dioxide<br />(CO )<br />2<br />2<br />Primary<br />bronchi<br />B<br />A<br />Alveolus<br />Respiratory<br />membrane<br />presses the alveoli. As intrapulmonic pressure rises<br />above atmospheric pressure, air is forced out of the<br />lungs until the two pressures are again equal.<br />Notice that inhalation is an active process that<br />requires muscle contraction, but normal exhalation is<br />a passive process, depending to a great extent on the<br />normal elasticity of healthy lungs. In other words,<br />under normal circumstances we must expend energy<br />to inhale but not to exhale (see Box 15–4: Emphysema).<br />We can, however, go beyond a normal exhalation<br />and expel more air, such as when talking, singing, or<br />blowing up a balloon. Such a forced exhalation is an<br />active process that requires contraction of other muscles.<br />Contraction of the internal intercostal muscles<br />pulls the ribs down and in and squeezes even more air<br />out of the lungs. Contraction of abdominal muscles,<br />such as the rectus abdominis, compresses the abdominal<br />organs and pushes the diaphragm upward, which<br />also forces more air out of the lungs (see Box 15–5:<br />The Heimlich Maneuver).<br />PULMONARY VOLUMES<br />The capacity of the lungs varies with the size and age<br />of the person. Taller people have larger lungs than do<br />shorter people. Also, as we get older our lung capacity<br />diminishes as lungs lose their elasticity and the respiratory<br />muscles become less efficient. For the following<br />pulmonary volumes, the values given are those for<br />healthy young adults. These are also shown in Fig.<br />15–7.<br />The Respiratory System 351<br />External<br />intercostal<br />muscles<br />Sternum<br />Diaphragm<br />Lung Ribs<br />Trachea<br />Ribs<br />Inhalation Exhalation<br />A B<br />Figure 15–6. Actions of the respiratory<br />muscles. (A) Inhalation:<br />diaphragm contracts downward;<br />external intercostal muscles pull rib<br />cage upward and outward; lungs<br />are expanded. (B) Normal exhalation:<br />diaphragm relaxes upward;<br />rib cage falls down and in as external<br />intercostal muscles relax; lungs<br />are compressed.<br />QUESTION: Why is a normal exhalation<br />a passive process?<br />BOX 15–2 HYALINE MEMBRANE DISEASE<br />lapse after each breath rather than remain inflated.<br />Each breath, therefore, is difficult, and the newborn<br />must expend a great deal of energy just to breathe.<br />Premature infants may require respiratory assistance<br />until their lungs are mature enough to produce<br />surfactant. Use of a synthetic surfactant has<br />significantly helped some infants, and because they<br />can breathe more normally, their dependence on<br />respirators is minimized. Still undergoing evaluation<br />are the effects of the long-term use of this surfactant<br />in the most premature babies, who may<br />require it for much longer periods of time.<br />Hyaline membrane disease is also called respiratory<br />distress syndrome (RDS) of the newborn, and<br />most often affects premature infants whose lungs<br />have not yet produced sufficient quantities of pulmonary<br />surfactant.<br />The first few breaths of a newborn inflate most of<br />the previously collapsed lungs, and the presence of<br />surfactant permits the alveoli to remain open. The<br />following breaths become much easier, and normal<br />breathing is established.<br />Without surfactant, the surface tension of the tissue<br />fluid lining the alveoli causes the air sacs to col-<br />BOX 15–3 PNEUMOTHORAX<br />trauma, may result from rupture of weakened alveoli<br />on the lung surface. Pulmonary diseases such as<br />emphysema may weaken alveoli.<br />Puncture wounds of the chest wall also allow air<br />into the pleural space, with resulting collapse of a<br />lung. In severe cases, large amounts of air push the<br />heart, great vessels, trachea, and esophagus toward<br />the opposite side (mediastinal shift), putting pressure<br />on the other lung and making breathing difficult.<br />This is called tension pneumothorax, and<br />requires rapid medical intervention to remove the<br />trapped air.<br />Pneumothorax is the presence of air in the pleural<br />space, which causes collapse of the lung on that<br />side. Recall that the pleural space is only a potential<br />space because the serous fluid keeps the pleural<br />membranes adhering to one another, and the<br />intrapleural pressure is always slightly below atmospheric<br />pressure. Should air at atmospheric pressure<br />enter the pleural cavity, the suddenly higher pressure<br />outside the lung will contribute to its collapse<br />(the other factor is the normal elasticity of the<br />lungs).<br />A spontaneous pneumothorax, without apparent<br />BOX 15–4 EMPHYSEMA<br />In progressive emphysema, damaged lung tissue<br />is replaced by fibrous connective tissue (scar tissue),<br />which further limits the diffusion of gases. Blood<br />oxygen level decreases, and blood carbon dioxide<br />level increases. Accumulating carbon dioxide<br />decreases the pH of body fluids; this is a respiratory<br />acidosis.<br />One of the most characteristic signs of emphysema<br />is that the affected person must make an<br />effort to exhale. The loss of lung elasticity makes<br />normal exhalation an active process, rather than the<br />passive process it usually is. The person must<br />expend energy to exhale in order to make room in<br />the lungs for inhaled air. This extra “work” required<br />for exhalation may be exhausting for the person<br />and contribute to the debilitating nature of emphysema.<br />Emphysema, a form of chronic obstructive pulmonary<br />disease (COPD), is a degenerative disease<br />in which the alveoli lose their elasticity and cannot<br />recoil. Perhaps the most common (and avoidable)<br />cause is cigarette smoking; other causes are longterm<br />exposure to severe air pollution or industrial<br />dusts, or chronic asthma. Inhaled irritants damage<br />the alveolar walls and cause deterioration of the<br />elastic connective tissue surrounding the alveoli.<br />Macrophages migrate to the damaged areas and<br />seem to produce an enzyme that contributes to the<br />destruction of the protein elastin. This is an instance<br />of a useful body response (for cleaning up damaged<br />tissue) becoming damaging when it is excessive. As<br />the alveoli break down, larger air cavities are created<br />that are not efficient in gas exchange (see Box<br />Fig. 15–A).<br />A Normal Lung B Emphysema<br />Box Figure 15–A (A) Lung tissue with normal alveoli. (B) Lung tissue in emphysema.<br />352<br />1. Tidal volume—the amount of air involved in one<br />normal inhalation and exhalation. The average tidal<br />volume is 500 mL, but many people often have<br />lower tidal volumes because of shallow breathing.<br />2. Minute respiratory volume (MRV)—the amount<br />of air inhaled and exhaled in 1 minute. MRV is calculated<br />by multiplying tidal volume by the number<br />of respirations per minute (average range: 12 to 20<br />per minute). If tidal volume is 500 mL and the respiratory<br />rate is 12 breaths per minute, the MRV is<br />6000 mL, or 6 liters of air per minute, which is<br />average. Shallow breathing usually indicates a<br />smaller than average tidal volume, and would thus<br />require more respirations per minute to obtain the<br />necessary MRV.<br />3. Inspiratory reserve—the amount of air, beyond<br />tidal volume, that can be taken in with the deepest<br />possible inhalation. Normal inspiratory reserve<br />ranges from 2000 to 3000 mL.<br />4. Expiratory reserve—the amount of air, beyond<br />tidal volume, that can be expelled with the most<br />forceful exhalation. Normal expiratory reserve<br />ranges from 1000 to 1500 mL.<br />5. Vital capacity—the sum of tidal volume, inspiratory<br />reserve, and expiratory reserve. Stated another<br />way, vital capacity is the amount of air involved in<br />the deepest inhalation followed by the most forceful<br />exhalation. Average range of vital capacity is<br />3500 to 5000 mL.<br />6. Residual air—the amount of air that remains in<br />the lungs after the most forceful exhalation; the<br />average range is 1000 to 1500 mL. Residual air is<br />important to ensure that there is some air in the<br />lungs at all times, so that exchange of gases is a continuous<br />process, even between breaths.<br />Some of the volumes just described can be determined<br />with instruments called spirometers, which<br />measure movement of air. Trained singers and musicians<br />who play wind instruments often have vital<br />capacities much larger than would be expected for<br />their height and age, because their respiratory muscles<br />have become more efficient with “practice.” The same<br />is true for athletes who exercise regularly. A person<br />with emphysema, however, must “work” to exhale,<br />and vital capacity and expiratory reserve volume are<br />often much lower than average.<br />Another kind of pulmonary volume is alveolar<br />The Respiratory System 353<br />BOX 15–5 THE HEIMLICH MANEUVER<br />The Heimlich maneuver has received much welldeserved<br />publicity, and indeed it is a life-saving<br />technique.<br />If a person is choking on a foreign object (such<br />as food) lodged in the pharynx or larynx, the air in<br />the lungs may be utilized to remove the object. The<br />physiology of this technique is illustrated in the<br />accompanying figure.<br />The person performing the maneuver stands<br />behind the choking victim and puts both arms<br />around the victim’s waist. One hand forms a fist<br />that is placed between the victim’s navel and rib<br />cage (below the diaphragm), and the other hand<br />covers the fist. It is important to place hands correctly,<br />in order to avoid breaking the victim’s ribs.<br />With both hands, a quick, forceful upward thrust is<br />made and repeated if necessary. This forces the<br />diaphragm upward to compress the lungs and force<br />air out. The forcefully expelled air is often sufficient<br />to dislodge the foreign object.<br />Foreign object<br />Lung<br />Diaphragm<br />Box Figure 15–B The Heimlich maneuver.<br />ventilation, which is the amount of air that actually<br />reaches the alveoli and participates in gas exchange.<br />An average tidal volume is 500 mL, of which 350 to<br />400 mL is in the alveoli at the end of an inhalation.<br />The remaining 100 to 150 mL of air is anatomic dead<br />space, the air still within the respiratory passages.<br />Despite the rather grim name, anatomic dead space is<br />normal; everyone has it.<br />Physiological dead space is not normal, and is the<br />volume of non-functioning alveoli that decrease gas<br />exchange. Causes of increased physiological dead space<br />include bronchitis, pneumonia, tuberculosis, emphysema,<br />asthma, pulmonary edema, and a collapsed lung.<br />The compliance of the thoracic wall and the lungs,<br />that is, their normal expansibility, is necessary for sufficient<br />alveolar ventilation. Thoracic compliance may<br />be decreased by fractured ribs, scoliosis, pleurisy, or<br />ascites. Lung compliance will be decreased by any<br />condition that increases physiologic dead space. Normal<br />compliance thus promotes sufficient gas exchange<br />in the alveoli.<br />EXCHANGE OF GASES<br />There are two sites of exchange of oxygen and carbon<br />dioxide: the lungs and the tissues of the body. The<br />exchange of gases between the air in the alveoli and<br />the blood in the pulmonary capillaries is called external<br />respiration. This term may be a bit confusing at<br />first, because we often think of “external” as being<br />outside the body. In this case, however, “external”<br />means the exchange that involves air from the external<br />environment, though the exchange takes place within<br />the lungs. Internal respiration is the exchange of<br />gases between the blood in the systemic capillaries and<br />the tissue fluid (cells) of the body.<br />The air we inhale (the earth’s atmosphere) is<br />approximately 21% oxygen and 0.04% carbon dioxide.<br />Although most (78%) of the atmosphere is nitrogen,<br />this gas is not physiologically available to us, and<br />we simply exhale it. This exhaled air also contains<br />about 16% oxygen and 4.5% carbon dioxide, so it is<br />apparent that some oxygen is retained within the body<br />and the carbon dioxide produced by cells is exhaled.<br />DIFFUSION OF GASES—<br />PARTIAL PRESSURES<br />Within the body, a gas will diffuse from an area of<br />greater concentration to an area of lesser concentration.<br />The concentration of each gas in a particular site<br />(alveolar air, pulmonary blood, and so on) is expressed<br />in a value called partial pressure. The partial pressure<br />354 The Respiratory System<br />Figure 15–7. Pulmonary<br />volumes. See text for description.<br />QUESTION: Which volumes<br />make up vital capacity? Which<br />volume cannot be measured<br />with a spirometer?<br />Liters<br />6<br />5<br />4<br />3<br />2.5<br />2<br />1<br />0<br />Inspiratory<br />reserve<br />Expiratory<br />reserve<br />Residual volume<br />Tidal volume<br />(normal breath)<br />Total<br />lung<br />capacity<br />Vital<br />capacity<br />Time<br />of a gas, measured in mmHg, is the pressure it exerts<br />within a mixture of gases, whether the mixture is actually<br />in a gaseous state or is in a liquid such as blood.<br />The partial pressures of oxygen and carbon dioxide in<br />the atmosphere and in the sites of exchange in the<br />body are listed in Table 15–1. The abbreviation for<br />partial pressure is “P,” which is used, for example, on<br />hospital lab slips for blood gases and will be used here.<br />The partial pressures of oxygen and carbon dioxide<br />at the sites of external respiration (lungs) and internal<br />respiration (body) are shown in Fig. 15–8. Because<br />partial pressure reflects concentration, a gas will diffuse<br />from an area of higher partial pressure to an area<br />of lower partial pressure.<br />The air in the alveoli has a high PO2 and a low PCO2.<br />The blood in the pulmonary capillaries, which has just<br />come from the body, has a low PO2 and a high PCO2.<br />Therefore, in external respiration, oxygen diffuses<br />from the air in the alveoli to the blood, and carbon<br />dioxide diffuses from the blood to the air in the alveoli.<br />The blood that returns to the heart now has a high PO2<br />and a low PCO2 and is pumped by the left ventricle into<br />systemic circulation.<br />The arterial blood that reaches systemic capillaries<br />has a high PO2 and a low PCO2. The body cells and tissue<br />fluid have a low PO2 and a high PCO2 because cells<br />continuously use oxygen in cell respiration (energy<br />production) and produce carbon dioxide in this<br />process. Therefore, in internal respiration, oxygen diffuses<br />from the blood to tissue fluid (cells), and carbon<br />dioxide diffuses from tissue fluid to the blood. The<br />blood that enters systemic veins to return to the heart<br />now has a low PO2 and a high PCO2 and is pumped by<br />the right ventricle to the lungs to participate in external<br />respiration.<br />Disorders of gas exchange often involve the lungs,<br />that is, external respiration (see Box 15–6: Pulmonary<br />Edema and Box 15–7: Pneumonia).<br />TRANSPORT OF GASES<br />IN THE BLOOD<br />Although some oxygen is dissolved in blood plasma<br />and does create the PO2 values, it is only about 1.5%<br />of the total oxygen transported, not enough to sustain<br />life. As you already know, most oxygen is carried in the<br />blood bonded to the hemoglobin in red blood cells<br />(RBCs). The mineral iron is part of hemoglobin and<br />gives this protein its oxygen-carrying ability.<br />The Respiratory System 355<br />Table 15–1 PARTIAL PRESSURES AND OXYGEN SATURATION<br />Site PO2 (mmHg) PCO2 (mmHg) Hemoglobin Saturation (SaO2)<br />Atmosphere 160 0.15 —<br />Alveolar air 104 40 —<br />Systemic venous blood 40 45 70–75%<br />(to pulmonary arteries)<br />Systemic arterial blood 100 40 95–100%<br />(from pulmonary veins)<br />Tissue fluid 40 50 —<br />Partial pressure is calculated as follows:<br />% of the gas in the mixture total pressure PGAS<br />Example: O2 in the atmosphere<br />21% 760 mmHg 160 mmHg (PO2)<br />Example: CO2 in the atmosphere<br />0.04% 760 mmHg 0.15 mmHg (PCO2)<br />Notice that alveolar partial pressures are not exactly those of the atmosphere. Alveolar air contains significant amounts of<br />water vapor and the CO2 diffusing in from the blood. Oxygen also diffuses readily from the alveoli into the pulmonary capillaries.<br />Therefore, alveolar PO2 is lower than atmospheric PO2, and alveolar PCO2 is significantly higher than atmospheric PCO2.<br />The oxygen–hemoglobin bond is formed in the<br />lungs where PO2 is high. This bond, however, is relatively<br />unstable, and when blood passes through tissues<br />with a low PO2, the bond breaks, and oxygen is<br />released to the tissues. The lower the oxygen concentration<br />in a tissue, the more oxygen the hemoglobin<br />will release. This ensures that active tissues, such as<br />exercising muscles, receive as much oxygen as possible<br />to continue cell respiration. Other factors that increase<br />the release of oxygen from hemoglobin are a high<br />PCO2 (actually a lower pH) and a high temperature,<br />both of which are also characteristic of active tissues.<br />Another measure of blood oxygen is the percent of<br />oxygen saturation of hemoglobin (SaO2). The higher<br />the PO2, the higher the SaO2, and as PO2 decreases, so<br />does SaO2, though not as rapidly. A PO2 of 100 is an<br />SaO2 of about 97% , as is found in systemic arteries. A<br />PO2 of 40, as is found in systemic veins, is an SaO2 of<br />about 75%. Notice that venous blood still has quite a<br />bit of oxygen. Had this blood flowed through a very<br />active tissue, more of its oxygen would have been<br />released from hemoglobin. This venous reserve of<br />oxygen provides active tissues with the oxygen they<br />need (see also Box 15–8: Carbon Monoxide).<br />356 The Respiratory System<br />Pulmonary<br />capillaries<br />Alveoli<br />Po 40 2<br />Po 105 2<br />Po 40 2<br />Po 40 2<br />Po 100 2<br />Po 100 2<br />Pco 40 2<br />Pco 45 2<br />Pco 45 2<br />Pco 50 2<br />Pco 40 2<br />Pco 40 2<br />Pulmonary<br />artery<br />External<br />respiration<br />Pulmonary<br />veins<br />Aorta<br />Veins Arteries<br />Venae<br />cavae<br />Right<br />heart<br />Left<br />heart<br />CO2 to<br />alveoli<br />CO2<br />to blood<br />O2 to<br />blood<br />O2 to tissue<br />Internal<br />respiration<br />Systemic<br />capillaries<br />Figure 15–8. External respiration<br />in the lungs and internal<br />respiration in the body. The<br />partial pressures of oxygen and<br />carbon dioxide are shown at<br />each site.<br />QUESTION: In external respiration,<br />describe the movement<br />of oxygen. In internal respiration,<br />describe the movement<br />of carbon dioxide.<br />The Respiratory System 357<br />BOX 15–8 CARBON MONOXIDE<br />light skin as cyanosis, a bluish cast to the skin, lips,<br />and nail beds. This is because hemoglobin is dark<br />red unless something (usually oxygen) is bonded to<br />it. When hemoglobin bonds to CO, however, it<br />becomes a bright, cherry red. This color may be<br />seen in light skin and may be very misleading; the<br />person with CO poisoning is in a severely hypoxic<br />state.<br />Although CO is found in cigarette smoke, it is<br />present in such minute quantities that it is not<br />lethal. Heavy smokers, however, may be in a mild<br />but chronic hypoxic state because much of their<br />hemoglobin is firmly bonded to CO. As a compensation,<br />RBC production may increase, and a heavy<br />smoker may have a hematocrit over 50%.<br />Carbon monoxide (CO) is a colorless, odorless gas<br />that is produced during the combustion of fuels<br />such as gasoline, coal, oil, and wood. As you know,<br />CO is a poison that may cause death if inhaled in<br />more than very small quantities or for more than a<br />short period of time.<br />The reason CO is so toxic is that it forms a very<br />strong and stable bond with the hemoglobin in<br />RBCs (carboxyhemoglobin). Hemoglobin with CO<br />bonded to it cannot bond to and transport oxygen.<br />The effect of CO, therefore, is to drastically decrease<br />the amount of oxygen carried in the blood. As little<br />as 0.1% CO in inhaled air can saturate half the total<br />hemoglobin with CO.<br />Lack of oxygen is often apparent in people with<br />BOX 15–6 PULMONARY EDEMA<br />capillaries. As blood pressure increases in the pulmonary<br />capillaries, filtration creates tissue fluid that<br />collects in the alveoli.<br />Fluid-filled alveoli are no longer sites of efficient<br />gas exchange, and the resulting hypoxia leads to<br />the symptoms of dyspnea and increased respiratory<br />rate. The most effective treatment is that which<br />restores the pumping ability of the heart to normal.<br />Pulmonary edema is the accumulation of fluid in<br />the alveoli. This is often a consequence of congestive<br />heart failure in which the left side of the heart<br />(or the entire heart) is not pumping efficiently. If the<br />left ventricle does not pump strongly, the chamber<br />does not empty as it should and cannot receive all<br />the blood flowing in from the left atrium. Blood<br />flow, therefore, is “congested,” and blood backs up<br />in the pulmonary veins and then in the pulmonary<br />BOX 15–7 PNEUMONIA<br />that accumulates in the air sacs. Many neutrophils<br />migrate to the site of infection and attempt to<br />phagocytize the bacteria. The alveoli become filled<br />with fluid, bacteria, and neutrophils (this is called<br />consolidation); this decreases the exchange of<br />gases.<br />Pneumovax is a vaccine for this type of pneumonia.<br />It contains only the capsules of S. pneumoniae<br />and cannot cause the disease. The vaccine is recommended<br />for people over the age of 60 years,<br />and for those with chronic pulmonary disorders or<br />any debilitating disease. It has also been approved<br />for administration to infants.<br />Pneumonia is a bacterial infection of the lungs.<br />Although many bacteria can cause pneumonia,<br />the most common one is probably Streptococcus<br />pneumoniae. This species is estimated to cause at<br />least 500,000 cases of pneumonia every year in the<br />United States, with 50,000 deaths.<br />S. pneumoniae is a transient inhabitant of the<br />upper respiratory tract, but in otherwise healthy<br />people, the ciliated epithelium and the immune system<br />prevent infection. Most cases of pneumonia<br />occur in elderly people following a primary infection<br />such as influenza.<br />When the bacteria are able to establish themselves<br />in the alveoli, the alveolar cells secrete fluid<br />Carbon dioxide transport is a little more complicated.<br />Some carbon dioxide is dissolved in the plasma,<br />and some is carried by hemoglobin (carbaminohemoglobin),<br />but these account for only about 20% of total<br />CO2 transport. Most carbon dioxide is carried in the<br />plasma in the form of bicarbonate ions (HCO3<br />–). Let<br />us look at the reactions that transform CO2 into a<br />bicarbonate ion.<br />When carbon dioxide enters the blood, most diffuses<br />into red blood cells, which contain the enzyme<br />carbonic anhydrase. This enzyme (which contains<br />zinc) catalyzes the reaction of carbon dioxide and<br />water to form carbonic acid:<br />CO2 + H2O → H2CO3<br />The carbonic acid then dissociates:<br />H2CO3 → H+ + HCO3<br />–<br />The bicarbonate ions diffuse out of the red blood<br />cells into the plasma, leaving the hydrogen ions (H+)<br />in the red blood cells. The many H+ ions would tend<br />to make the red blood cells too acidic, but hemoglobin<br />acts as a buffer to prevent acidosis. To maintain an<br />ionic equilibrium, chloride ions (Cl–) from the plasma<br />enter the red blood cells; this is called the chloride<br />shift. Where is the CO2? It is in the plasma as part of<br />HCO3<br />– ions. When the blood reaches the lungs, an<br />area of lower PCO2, these reactions are reversed, and<br />CO2 is re-formed and diffuses into the alveoli to be<br />exhaled.<br />REGULATION OF RESPIRATION<br />Two types of mechanisms regulate breathing: nervous<br />mechanisms and chemical mechanisms. Because any<br />changes in the rate or depth of breathing are ultimately<br />brought about by nerve impulses, we will consider<br />nervous mechanisms first.<br />NERVOUS REGULATION<br />The respiratory centers are located in the medulla<br />and pons, which are parts of the brain stem (see Fig.<br />15–9). Within the medulla are the inspiration center<br />and expiration center.<br />The inspiration center automatically generates<br />impulses in rhythmic spurts. These impulses travel<br />along nerves to the respiratory muscles to stimulate<br />their contraction. The result is inhalation. As the<br />lungs inflate, baroreceptors in lung tissue detect this<br />stretching and generate sensory impulses to the<br />medulla; these impulses begin to depress the inspiration<br />center. This is called the Hering-Breuer inflation<br />reflex, which also helps prevent overinflation of the<br />lungs.<br />As the inspiration center is depressed, the result is<br />a decrease in impulses to the respiratory muscles,<br />which relax to bring about exhalation. Then the inspiration<br />center becomes active again to begin another<br />cycle of breathing. When there is a need for more<br />forceful exhalations, such as during exercise, the inspiration<br />center activates the expiration center, which<br />generates impulses to the internal intercostal and<br />abdominal muscles.<br />The two respiratory centers in the pons work with<br />the inspiration center to produce a normal rhythm of<br />breathing. The apneustic center prolongs inhalation,<br />and is then interrupted by impulses from the pneumotaxic<br />center, which contributes to exhalation. In<br />normal breathing, inhalation lasts 1 to 2 seconds, followed<br />by a slightly longer (2 to 3 seconds) exhalation,<br />producing the normal respiratory rate range of 12 to<br />20 breaths per minute.<br />What has just been described is normal breathing,<br />but variations are possible and quite common. Emotions<br />often affect respiration; a sudden fright may bring<br />about a gasp or a scream, and anger usually increases<br />the respiratory rate. In these situations, impulses from<br />the hypothalamus modify the output from the<br />medulla. The cerebral cortex enables us to voluntarily<br />change our breathing rate or rhythm to talk, sing,<br />breathe faster or slower, or even to stop breathing for<br />1 or 2 minutes. Such changes cannot be continued<br />indefinitely, however, and the medulla will eventually<br />resume control.<br />Coughing and sneezing are reflexes that remove<br />irritants from the respiratory passages; the medulla<br />contains the centers for both of these reflexes. Sneezing<br />is stimulated by an irritation of the nasal mucosa,<br />and coughing is stimulated by irritation of the mucosa<br />of the pharynx, larynx, or trachea. The reflex action is<br />essentially the same for both: An inhalation is followed<br />by exhalation beginning with the glottis closed to<br />build up pressure. Then the glottis opens suddenly,<br />and the exhalation is explosive. A cough directs the<br />exhalation out the mouth, while a sneeze directs the<br />exhalation out the nose.<br />358 The Respiratory System<br />Hiccups, also a reflex, are spasms of the diaphragm.<br />The result is a quick inhalation that is stopped when<br />the glottis snaps shut, causing the “hic” sound. The<br />stimulus may be irritation of the phrenic nerves<br />or nerves of the stomach. Excessive alcohol is an irritant<br />that can cause hiccups. Some causes are simply<br />unknown.<br />Yet another respiratory reflex is yawning. Most of<br />us yawn when we are tired, but the stimulus for and<br />purpose of yawning are not known with certainty.<br />There are several possibilities, such as lack of oxygen<br />or accumulation of carbon dioxide, but we really do<br />not know. Nor do we know why yawning is contagious,<br />but seeing someone yawn is almost sure to elicit<br />a yawn of one’s own. You may even have yawned while<br />reading this paragraph about yawning.<br />CHEMICAL REGULATION<br />Chemical regulation refers to the effect on breathing<br />of blood pH and blood levels of oxygen and carbon<br />dioxide. This is shown in Fig. 15–10. Chemorecep-<br />The Respiratory System 359<br />Hypothalamus<br />Pneumotaxic<br />center<br />Apneustic<br />center<br />Stimulatory<br />Inhibitory<br />Cerebral cortex<br />External intercostal muscles<br />Baroreceptors in<br />lungs<br />Inspiration center Diaphragm<br />Expiration center<br />Medulla<br />Pons<br />A<br />B<br />C<br />Figure 15–9. Nervous regulation of respiration. (A) Midsagittal section of brain.<br />(B) Respiratory centers in medulla and pons. (C) Respiratory muscles. See text for description.<br />QUESTION: Which center directly stimulates inhalation? How can you tell from this<br />picture?<br />tors that detect changes in blood gases and pH are<br />located in the carotid and aortic bodies and in the<br />medulla itself.<br />A decrease in the blood level of oxygen (hypoxia) is<br />detected by the chemoreceptors in the carotid and<br />aortic bodies. The sensory impulses generated by<br />these receptors travel along the glossopharyngeal and<br />vagus nerves to the medulla, which responds by<br />increasing respiratory rate or depth (or both). This<br />response will bring more air into the lungs so that<br />more oxygen can diffuse into the blood to correct the<br />hypoxic state.<br />Carbon dioxide becomes a problem when it is present<br />in excess in the blood, because excess CO2 (hypercapnia)<br />lowers the pH when it reacts with water to<br />form carbonic acid (a source of H+ ions). That is,<br />excess CO2 makes the blood or other body fluids less<br />alkaline (or more acidic). The medulla contains<br />chemoreceptors that are very sensitive to changes<br />in pH, especially decreases. If accumulating CO2 lowers<br />blood pH, the medulla responds by increasing respiration.<br />This is not for the purpose of inhaling, but<br />rather to exhale more CO2 to raise the pH back to<br />normal.<br />Of the two respiratory gases, which is the more<br />important as a regulator of respiration? Our guess<br />might be oxygen, because it is essential for energy production<br />in cell respiration. However, the respiratory<br />system can maintain a normal blood level of oxygen<br />even if breathing decreases to half the normal rate or<br />stops for a few moments. Recall that exhaled air is<br />16% oxygen. This oxygen did not enter the blood but<br />was available to do so if needed. Also, the residual air<br />in the lungs supplies oxygen to the blood even if<br />breathing rate slows.<br />Therefore, carbon dioxide must be the major regulator<br />of respiration, and the reason is that carbon dioxide<br />affects the pH of the blood. As was just mentioned,<br />an excess of CO2 causes the blood pH to decrease, a<br />process that must not be allowed to continue.<br />Therefore, any increase in the blood CO2 level is<br />quickly compensated for by increased breathing to<br />exhale more CO2. If, for example, you hold your<br />breath, what is it that makes you breathe again? Have<br />you run out of oxygen? Probably not, for the reasons<br />mentioned. What has happened is that accumulating<br />CO2 has lowered blood pH enough to stimulate the<br />medulla to start the breathing cycle again.<br />In some situations, oxygen does become the major<br />regulator of respiration. People with severe, chronic<br />pulmonary diseases such as emphysema have decreased<br />exchange of both oxygen and carbon dioxide<br />in the lungs. The decrease in pH caused by accumulating<br />CO2 is corrected by the kidneys, but the blood<br />oxygen level keeps decreasing. Eventually, the oxygen<br />level may fall so low that it does provide a very strong<br />stimulus to increase the rate and depth of respiration.<br />RESPIRATION AND<br />ACID–BASE BALANCE<br />As you have just seen, respiration affects the pH of<br />body fluids because it regulates the amount of carbon<br />360 The Respiratory System<br />Chemoreceptors<br />in carotid and<br />aortic bodies<br />Chemoreceptors<br />in medulla<br />O2<br />CO2 or<br />pH<br />Medulla;<br />inspiration<br />center<br />Increase rate<br />and depth of<br />respiration<br />More O2 available<br />to enter blood<br />More CO2 exhaled<br />pH<br />Figure 15–10. Chemical regulation of respiration. See text for description.<br />QUESTION: The body’s response to two very different changes (less O2 or more CO2)<br />is the same. Explain why.<br />dioxide in these fluids. Remember that CO2 reacts<br />with water to form carbonic acid (H2CO3), which ionizes<br />into H+ ions and HCO3<br />– ions. The more hydrogen<br />ions present in a body fluid, the lower the pH, and<br />the fewer hydrogen ions present, the higher the pH.<br />The respiratory system may be the cause of a pH<br />imbalance, or it may help correct a pH imbalance created<br />by some other cause.<br />RESPIRATORY ACIDOSIS<br />AND ALKALOSIS<br />Respiratory acidosis occurs when the rate or efficiency<br />of respiration decreases, permitting carbon<br />dioxide to accumulate in body fluids. The excess CO2<br />results in the formation of more H+ ions, which<br />decrease the pH. Holding one’s breath can bring about<br />a mild respiratory acidosis, which will soon stimulate<br />the medulla to initiate breathing again. More serious<br />causes of respiratory acidosis are pulmonary diseases<br />such as pneumonia and emphysema, or severe asthma.<br />Each of these impairs gas exchange and allows excess<br />CO2 to remain in body fluids.<br />Respiratory alkalosis occurs when the rate of respiration<br />increases, and CO2 is very rapidly exhaled.<br />Less CO2 decreases H+ ion formation, which increases<br />the pH. Breathing faster for a few minutes can bring<br />about a mild state of respiratory alkalosis. Babies who<br />cry for extended periods (crying is a noisy exhalation)<br />put themselves in this condition. In general, however,<br />respiratory alkalosis is not a common occurrence.<br />Severe physical trauma and shock, or certain states<br />of mental or emotional anxiety, may be accompanied<br />by hyperventilation and also result in respiratory<br />alkalosis. In addition, traveling to a higher altitude<br />(less oxygen in the atmosphere) may cause a temporary<br />increase in breathing rate before compensation<br />occurs (increased rate of RBC production—see<br />Chapter 11).<br />RESPIRATORY COMPENSATION<br />If a pH imbalance is caused by something other than a<br />change in respiration, it is called metabolic acidosis or<br />alkalosis. In either case, the change in pH stimulates a<br />change in respiration that may help restore the pH of<br />body fluids to normal.<br />Metabolic acidosis may be caused by untreated<br />diabetes mellitus (ketoacidosis), kidney disease, or<br />severe diarrhea. In such situations, the H+ ion concentration<br />of body fluids is increased. Respiratory compensation<br />involves an increase in the rate and depth of<br />respiration to exhale more CO2 to decrease H+ ion<br />formation, which will raise the pH toward the normal<br />range.<br />Metabolic alkalosis is not a common occurrence<br />but may be caused by ingestion of excessive amounts<br />of alkaline medications such as those used to relieve<br />gastric disturbances. Another possible cause is vomiting<br />of stomach contents only. In such situations, the<br />H+ ion concentration of body fluids is decreased.<br />Respiratory compensation involves a decrease in respiration<br />to retain CO2 in the body to increase H+ ion<br />formation, which will lower the pH toward the normal<br />range.<br />Respiratory compensation for an ongoing metabolic<br />pH imbalance cannot be complete, because there<br />are limits to the amounts of CO2 that may be exhaled<br />or retained. At most, respiratory compensation is only<br />about 75% effective. A complete discussion of acid–<br />base balance is found in Chapter 19.<br />AGING AND THE<br />RESPIRATORY SYSTEM<br />Perhaps the most important way to help your respiratory<br />system age gracefully is not to smoke. In the<br />absence of chemical assault, respiratory function does<br />diminish but usually remains adequate. The respiratory<br />muscles, like all skeletal muscles, weaken with<br />age. Lung tissue loses its elasticity and alveoli are<br />lost as their walls deteriorate. All of this results in decreased<br />ventilation and lung capacity, but the remaining<br />capacity is usually sufficient for ordinary activities.<br />The cilia of the respiratory mucosa deteriorate with<br />age, and the alveolar macrophages are not as efficient,<br />which make elderly people more prone to pneumonia,<br />a serious pulmonary infection.<br />Chronic alveolar hypoxia from diseases such as<br />emphysema or chronic bronchitis may lead to pulmonary<br />hypertension, which in turn overworks the<br />right ventricle of the heart. Systemic hypertension<br />often weakens the left ventricle of the heart, leading to<br />congestive heart failure and pulmonary edema, in<br />which excess tissue fluid collects in the alveoli and<br />decreases gas exchange. Though true at any age, the<br />interdependence of the respiratory and circulatory<br />systems is particularly apparent in elderly people.<br />The Respiratory System 361<br />SUMMARY<br />As you have learned, respiration is much more than<br />the simple mechanical actions of breathing. Inhalation<br />provides the body with the oxygen that is necessary for<br />the production of ATP in the process of cell respiration.<br />Exhalation removes the CO2 that is a product of<br />cell respiration. Breathing also regulates the level of<br />CO2 within the body, and this contributes to the<br />maintenance of the acid–base balance of body fluids.<br />Although the respiratory gases do not form structural<br />components of the body, their contributions to the<br />chemical level of organization are essential to the<br />functioning of the body at every level.<br />362 The Respiratory System<br />STUDY OUTLINE<br />The respiratory system moves air into and<br />out of the lungs, which are the site of<br />exchange for O2 and CO2 between the air<br />and the blood. The functioning of the respiratory<br />system depends directly on the<br />proper functioning of the circulatory system.<br />1. The upper respiratory tract consists of those parts<br />outside the chest cavity.<br />2. The lower respiratory tract consists of those parts<br />within the chest cavity.<br />Nose—made of bone and cartilage covered<br />with skin<br />1. Hairs inside the nostrils block the entry of dust.<br />Nasal Cavities—within the skull; separated<br />by the nasal septum (see Fig. 15–1)<br />1. Nasal mucosa is ciliated epithelium with goblet<br />cells; surface area is increased by the conchae.<br />2. Nasal mucosa warms and moistens the incoming<br />air; dust and microorganisms are trapped on mucus<br />and swept by the cilia to the pharynx.<br />3. Olfactory receptors respond to vapors in inhaled<br />air.<br />4. Paranasal sinuses in the maxillae, frontal, sphenoid,<br />and ethmoid bones open into the nasal cavities:<br />functions are to lighten the skull and provide resonance<br />for the voice.<br />Pharynx—posterior to nasal and oral cavities<br />(see Fig. 15–1)<br />1. Nasopharynx—above the level of the soft palate,<br />which blocks it during swallowing; a passageway<br />for air only. The eustachian tubes from the middle<br />ears open into it. The adenoid is a lymph nodule on<br />the posterior wall.<br />2. Oropharynx—behind the mouth; a passageway for<br />both air and food. Palatine tonsils are on the lateral<br />walls.<br />3. Laryngopharynx—a passageway for both air and<br />food; opens anteriorly into the larynx and posteriorly<br />into the esophagus.<br />Larynx—the voice box and the airway<br />between the pharynx and trachea (see Fig.<br />15–2)<br />1. Made of nine cartilages; the thyroid cartilage is the<br />largest and most anterior.<br />2. The epiglottis is the uppermost cartilage; covers<br />the larynx during swallowing.<br />3. The vocal cords are lateral to the glottis, the opening<br />for air (see Fig. 15–3).<br />4. During speaking, the vocal cords are pulled across<br />the glottis and vibrated by exhaled air, producing<br />sounds that may be turned into speech.<br />5. The cranial nerves for speaking are the vagus and<br />accessory.<br />Trachea—extends from the larynx to the primary<br />bronchi (see Fig. 15–4)<br />1. Sixteen to 20 C-shaped cartilages in the tracheal<br />wall keep the trachea open.<br />2. Mucosa is ciliated epithelium with goblet cells; cilia<br />sweep mucus, trapped dust, and microorganisms<br />upward to the pharynx.<br />Bronchial Tree—extends from the trachea to<br />the alveoli (see Fig. 15–4)<br />1. The right and left primary bronchi are branches of<br />the trachea; one to each lung; same structure as the<br />trachea.<br />2. Secondary bronchi: to the lobes of each lung (three<br />right, two left)<br />3. Bronchioles—no cartilage in their walls.<br />Pleural Membranes—serous membranes of<br />the thoracic cavity<br />1. Parietal pleura lines the chest wall.<br />2. Visceral pleura covers the lungs.<br />3. Serous fluid between the two layers prevents friction<br />and keeps the membranes together during<br />breathing.<br />Lungs—on either side of the heart in the<br />chest cavity; extend from the diaphragm<br />below up to the level of the clavicles<br />1. The rib cage protects the lungs from mechanical<br />injury.<br />2. Hilus—indentation on the medial side: primary<br />bronchus and pulmonary artery and veins enter<br />(also bronchial vessels).<br />Alveoli—the sites of gas exchange in the<br />lungs<br />1. Made of alveolar type I cells, simple squamous<br />epithelium; thin to permit diffusion of gases.<br />2. Surrounded by pulmonary capillaries, which are<br />also made of simple squamous epithelium (see Fig.<br />15–4).<br />3. Elastic connective tissue between alveoli is important<br />for normal exhalation.<br />4. A thin layer of tissue fluid lines each alveolus; essential<br />to permit diffusion of gases (see Fig. 15–5).<br />5. Alveolar type II cells produce pulmonary surfactant<br />that mixes with the tissue fluid lining to decrease<br />surface tension to permit inflation of the alveoli.<br />6. Alveolar macrophages phagocytize foreign material.<br />Mechanism of Breathing<br />1. Ventilation is the movement of air into and out of<br />the lungs: inhalation and exhalation.<br />2. Respiratory centers are in the medulla and pons.<br />3. Respiratory muscles are the diaphragm and external<br />and internal intercostal muscles (see Fig. 15–6).<br />• Atmospheric pressure is air pressure: 760 mmHg<br />at sea level.<br />• Intrapleural pressure is within the potential pleural<br />space; always slightly below atmospheric<br />pressure (“negative”).<br />• Intrapulmonic pressure is within the bronchial<br />tree and alveoli; fluctuates during breathing.<br />Inhalation (inspiration)<br />1. Motor impulses from medulla travel along phrenic<br />nerves to diaphragm, which contracts and moves<br />down. Impulses are sent along intercostal nerves to<br />external intercostal muscles, which pull ribs up and<br />out.<br />2. The chest cavity is expanded and expands the parietal<br />pleura.<br />3. The visceral pleura adheres to the parietal pleura<br />and is also expanded and in turn expands the lungs.<br />4. Intrapulmonic pressure decreases, and air rushes<br />into the lungs.<br />Exhalation (expiration)<br />1. Motor impulses from the medulla decrease, and the<br />diaphragm and external intercostal muscles relax.<br />2. The chest cavity becomes smaller and compresses<br />the lungs.<br />3. The elastic lungs recoil and further compress the<br />alveoli.<br />4. Intrapulmonic pressure increases, and air is forced<br />out of the lungs. Normal exhalation is passive.<br />5. Forced exhalation: contraction of the internal<br />intercostal muscles pulls the ribs down and in; contraction<br />of the abdominal muscles forces the<br />diaphragm upward.<br />Pulmonary Volumes (see Fig. 15–7)<br />1. Tidal volume—the amount of air in one normal<br />inhalation and exhalation.<br />2. Minute respiratory volume—the amount of air<br />inhaled and exhaled in 1 minute.<br />3. Inspiratory reserve—the amount of air beyond<br />tidal in a maximal inhalation.<br />4. Expiratory reserve—the amount of air beyond tidal<br />in the most forceful exhalation.<br />5. Vital capacity—the sum of tidal volume, inspiratory<br />and expiratory reserves.<br />6. Residual volume—the amount of air that remains<br />in the lungs after the most forceful exhalation; provides<br />for continuous exchange of gases.<br />7. Alveolar ventilation—air that reaches the alveoli<br />for gas exchange; depends on normal thoracic and<br />lung compliance.<br />• Anatomic dead space—air still in the respiratory<br />passages at the end of inhalation (is normal).<br />The Respiratory System 363<br />• Physiological dead space—the volume of nonfunctional<br />alveoli; decreases compliance.<br />Exchange of Gases<br />1. External respiration is the exchange of gases<br />between the air in the alveoli and the blood in the<br />pulmonary capillaries.<br />2. Internal respiration is the exchange of gases<br />between blood in the systemic capillaries and tissue<br />fluid (cells).<br />3. Inhaled air (atmosphere) is 21% O2 and 0.04%<br />CO2. Exhaled air is 16% O2 and 4.5% CO2.<br />4. Diffusion of O2 and CO2 in the body occurs<br />because of pressure gradients (see Table 15–1). A<br />gas will diffuse from an area of higher partial pressure<br />to an area of lower partial pressure.<br />5. External respiration: PO2 in the alveoli is high, and<br />PO2in the pulmonary capillaries is low, so O2 diffuses<br />from the air to the blood. PCO2 in the alveoli<br />is low, and PCO2 in the pulmonary capillaries is<br />high, so CO2 diffuses from the blood to the air and<br />is exhaled (see Fig. 15–8).<br />6. Internal respiration: PO2 in the systemic capillaries<br />is high, and PO2 in the tissue fluid is low, so O2 diffuses<br />from the blood to the tissue fluid and cells.<br />PCO2 in the systemic capillaries is low, and PCO2 in<br />the tissue fluid is high, so CO2 diffuses from the tissue<br />fluid to the blood (see Fig. 15–8).<br />Transport of Gases in the Blood<br />1. Oxygen is carried by the iron of hemoglobin (Hb)<br />in the RBCs. The O2–Hb bond is formed in the<br />lungs where the PO2 is high.<br />2. In tissues, Hb releases much of its O2; the important<br />factors are low PO2 in tissues, high PCO2 in tissues,<br />and a high temperature in tissues.<br />3. Oxygen saturation of hemoglobin (SaO2) is 95% to<br />97% in systemic arteries and averages 70% to 75%<br />in systemic veins.<br />4. Most CO2 is carried as HCO3<br />– ions in blood<br />plasma. CO2 enters the RBCs and reacts with H2O<br />to form carbonic acid (H2CO3). Carbonic anhydrase<br />is the enzyme that catalyzes this reaction.<br />H2CO3 dissociates to H+ ions and HCO3<br />– ions.<br />The HCO3<br />– ions leave the RBCs and enter the<br />plasma; Hb buffers the H+ ions that remain in the<br />RBCs. Cl– ions from the plasma enter the RBCs to<br />maintain ionic equilibrium (the chloride shift).<br />5. When blood reaches the lungs, CO2 is re-formed,<br />diffuses into the alveoli, and is exhaled.<br />Nervous Regulation of Respiration<br />(see Fig. 15–9)<br />1. The medulla contains the inspiration center and<br />expiration center.<br />2. Impulses from the inspiration center to the respiratory<br />muscles cause their contraction; the chest cavity<br />is expanded.<br />3. Baroreceptors in lung tissue detect stretching and<br />send impulses to the medulla to depress the inspiration<br />center. This is the Hering-Breuer inflation<br />reflex, which also prevents overinflation of the<br />lungs.<br />4. The expiration center is stimulated by the inspiration<br />center when forceful exhalations are needed.<br />5. In the pons: the apneustic center prolongs inhalation,<br />and the pneumotaxic center helps bring about<br />exhalation. These centers work with the inspiration<br />center in the medulla to produce a normal breathing<br />rhythm.<br />6. The hypothalamus influences changes in breathing<br />in emotional situations. The cerebral cortex permits<br />voluntary changes in breathing.<br />7. Coughing and sneezing remove irritants from the<br />upper respiratory tract; the centers for these<br />reflexes are in the medulla.<br />Chemical Regulation of Respiration<br />(see Fig. 15–10)<br />1. Decreased blood O2 is detected by chemoreceptors<br />in the carotid body and aortic body. Response:<br />increased respiration to take more air into the<br />lungs.<br />2. Increased blood CO2 level is detected by chemoreceptors<br />in the medulla. Response: increased respiration<br />to exhale more CO2.<br />3. CO2 is the major regulator of respiration because<br />excess CO2 decreases the pH of body fluids (CO2 +<br />H2O → H2CO3 → H+ + HCO3<br />–). Excess H+ ions<br />lower pH.<br />4. Oxygen becomes a major regulator of respiration<br />when blood level is very low, as may occur with<br />severe, chronic pulmonary disease.<br />Respiration and Acid–Base Balance<br />1. Respiratory acidosis: a decrease in the rate or efficiency<br />of respiration permits excess CO2 to accumulate<br />in body fluids, resulting in the formation of<br />excess H+ ions, which lower pH. Occurs in severe<br />pulmonary disease.<br />2. Respiratory alkalosis: an increase in the rate of res-<br />364 The Respiratory System<br />The Respiratory System 365<br />REVIEW QUESTIONS<br />1. State the three functions of the nasal mucosa.<br />(p. 344)<br />2. Name the three parts of the pharynx; state whether<br />each is an air passage only or an air and food passage.<br />(pp. 344–346)<br />3. Name the tissue that lines the larynx and trachea,<br />and describe its function. State the function of the<br />cartilage of the larynx and trachea. (p. 346)<br />4. Name the pleural membranes, state the location<br />of each, and describe the functions of serous fluid.<br />(pp. 347)<br />5. Name the tissue of which the alveoli and pulmonary<br />capillaries are made, and explain the<br />importance of this tissue in these locations. Explain<br />the function of pulmonary surfactant. (p. 347)<br />6. Name the respiratory muscles, and describe how<br />they are involved in normal inhalation and exhalation.<br />Define these pressures and relate them to a<br />cycle of breathing: atmospheric pressure, intrapulmonic<br />pressure. (pp. 348–349)<br />7. Describe external respiration in terms of partial<br />pressures of oxygen and carbon dioxide. (p. 355)<br />8. Describe internal respiration in terms of partial<br />pressures of oxygen and carbon dioxide. (p. 355)<br />9. Name the cell, protein, and mineral that transport<br />oxygen in the blood. State the three factors<br />that increase the release of oxygen in tissues.<br />(pp. 355–356)<br />10. Most carbon dioxide is transported in what part of<br />the blood, and in what form? Explain the function<br />of hemoglobin with respect to carbon dioxide<br />transport. (p. 358)<br />11. Name the respiratory centers in the medulla and<br />pons, and explain how each is involved in a<br />breathing cycle. (p. 358)<br />12. State the location of chemoreceptors affected by a<br />low blood oxygen level; describe the body’s<br />response to hypoxia and its purpose. State the<br />location of chemoreceptors affected by a high<br />blood CO2 level; describe the body’s response and<br />its purpose. (p. 360)<br />13. For respiratory acidosis and alkalosis: state a cause<br />and explain what happens to the pH of body fluids.<br />(p. 361)<br />14. Explain how the respiratory system may compensate<br />for metabolic acidosis or alkalosis. For an<br />ongoing pH imbalance, what is the limit of respiratory<br />compensation? (pp. 361)<br />FOR FURTHER THOUGHT<br />1. The success of an organ transplant depends on<br />many factors. What factor would diminish the<br />chance of success of a lung transplant, but is not a<br />factor at all in a heart transplant?<br />2. Name four types of tissues that contribute to the<br />functioning of the lungs, and describe the physical<br />characteristics of each that are important. Name<br />two types of cells that are also important to the<br />functioning of the lungs.<br />3. As recently as 45 years ago (the early 1960s)<br />it was believed that mouth-to-mouth resuscitation<br />was not really helpful to another person.<br />What mistaken belief about the air we exhale<br />contributed to that thinking, and what are the<br />facts?<br />4. You are making a list of vital organs, organs we cannot<br />live without. Should you include the larynx on<br />your list? Explain why or why not.<br />piration increases the CO2 exhaled, which decreases<br />the formation of H+ ions and raises pH.<br />Occurs during hyperventilation or when first at a<br />high altitude.<br />3. Respiratory compensation for metabolic acidosis:<br />increased respiration to exhale CO2 to decrease H+<br />ion formation to raise pH to normal.<br />4. Respiratory compensation for metabolic alkalosis:<br />decreased respiration to retain CO2 to increase H+<br />ion formation to lower pH to normal.<br />5. At a construction site, a hole caved in and buried a<br />workman up to his shoulders in wet sand. The foreman<br />told the trapped man that a crane would be<br />there in 20 minutes to pull him out, but another<br />worker said they couldn’t wait, and had to dig now.<br />He was right. Why is this a life-threatening emergency?<br />6. A patient’s blood pH is 7.34, and his respirations<br />are 32 per minute. What acid–base situation is this<br />patient in? State your reasoning step-by-step.<br />7. Mrs. D is in the emergency room because of severe<br />abdominal pain that may be appendicitis. Her<br />blood pH is 7.47 and her respirations are 34 per<br />minute. What acid–base situation is Mrs. D in?<br />State your reasoning step-by-step.<br />366 The Respiratory System<br />CHAPTER 16<br />The Digestive System<br />367<br />368<br />CHAPTER 16<br />Chapter Outline<br />Divisions of the Digestive System<br />Types of Digestion<br />End Products of Digestion<br />Oral Cavity<br />Teeth<br />Tongue<br />Salivary Glands<br />Pharynx<br />Esophagus<br />Structural Layers of the Alimentary Tube<br />Mucosa<br />Submucosa<br />External Muscle Layer<br />Serosa<br />Stomach<br />Small Intestine<br />Liver<br />Gallbladder<br />Pancreas<br />Completion of Digestion and Absorption<br />Small Intestine<br />Absorption<br />Large Intestine<br />Elimination of Feces<br />Other Functions of the Liver<br />Aging and the Digestive System<br />BOX 16–1 DISORDERS OF THE STOMACH<br />BOX 16–2 GALLSTONES<br />BOX 16–3 DISORDERS OF THE INTESTINES<br />BOX 16–4 INFANT BOTULISM<br />BOX 16–5 FIBER<br />BOX 16–6 HEPATITIS<br />Student Objectives<br />• Describe the general functions of the digestive<br />system, and name its major divisions.<br />• Explain the difference between mechanical and<br />chemical digestion, and name the end products of<br />digestion.<br />• Describe the structure and functions of the teeth<br />and tongue.<br />• Explain the functions of saliva.<br />• Describe the location and function of the pharynx<br />and esophagus.<br />• Describe the structure and function of each of the<br />four layers of the alimentary tube.<br />• Describe the location, structure, and function of<br />the stomach, small intestine, liver, gallbladder, and<br />pancreas.<br />• Describe absorption in the small intestine.<br />• Describe the location and functions of the large<br />intestine.<br />• Explain the functions of the normal flora of the<br />colon.<br />• Describe the functions of the liver.<br />The Digestive System<br />369<br />New Terminology<br />Alimentary tube (AL-i-MEN-tah-ree TOOB)<br />Chemical digestion (KEM-i-kuhl dye-JES-chun)<br />Common bile duct (KOM-mon BYL DUKT)<br />Defecation reflex (DEF-e-KAY-shun)<br />Duodenum (dew-AH-den-um)<br />Emulsify (e-MULL-si-fye)<br />Enamel (e-NAM-uhl)<br />Essential amino acids (e-SEN-shul ah-ME-noh<br />ASS-ids)<br />External anal sphincter (eks-TER-nuhl AY-nuhl<br />SFINK-ter)<br />Ileocecal valve (ILL-ee-oh-SEE-kuhl VALV)<br />Internal anal sphincter (in-TER-nuhl AY-nuhl<br />SFINK-ter)<br />Lower esophageal sphincter (e-SOF-uh-JEE-uhl<br />SFINK-ter)<br />Mechanical digestion (muh-KAN-i-kuhl dye-JESchun)<br />Non-essential amino acids (NON-e-SEN-shul ah-<br />ME-noh ASS-ids)<br />Normal flora (NOR-muhl FLOOR-ah)<br />Periodontal membrane (PER-ee-oh-DON-tal)<br />Pyloric sphincter (pye-LOR-ik SFINK-ter)<br />Rugae (ROO-gay)<br />Villi (VILL-eye)<br />Related Clinical Terminology<br />Appendicitis (uh-PEN-di-SIGH-tis)<br />Diverticulitis (DYE-ver-TIK-yoo-LYE-tis)<br />Gastric ulcer (GAS-trik UL-ser)<br />Hepatitis (HEP-uh-TIGH-tis)<br />Lactose intolerance (LAK-tohs in-TAHL-er-ense)<br />Lithotripsy (LITH-oh-TRIP-see)<br />Paralytic ileus (PAR-uh-LIT-ik ILL-ee-us)<br />Peritonitis (per-i-toh-NIGH-tis)<br />Pyloric stenosis (pye-LOR-ik ste-NOH-sis)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />Ahurried breakfast when you are late for work or<br />school . . . Thanksgiving dinner . . . going on a diet to<br />lose 5 pounds . . . what do these experiences all have in<br />common? Food. We may take food for granted, celebrate<br />with it, or wish we wouldn’t eat quite so much of<br />it. Although food is not as immediate a need for<br />human beings as is oxygen, it is a very important part<br />of our lives. Food provides the raw materials or nutrients<br />that cells use to reproduce and to build new tissue.<br />The energy needed for cell reproduction and<br />tissue building is released from food in the process of<br />cell respiration. In fact, a supply of nutrients from regular<br />food intake is so important that the body can even<br />store any excess for use later. Those “extra 5 pounds”<br />are often stored fat in adipose tissue.<br />The food we eat, however, is not in a form that our<br />body cells can use. A turkey sandwich, for example,<br />consists of complex proteins, fats, and carbohydrates.<br />The function of the digestive system is to change<br />these complex organic nutrient molecules into simple<br />organic and inorganic molecules that can then be<br />absorbed into the blood or lymph to be transported to<br />cells. In this chapter we will discuss the organs of<br />digestion and the contribution each makes to digestion<br />and absorption.<br />DIVISIONS OF THE<br />DIGESTIVE SYSTEM<br />The two divisions of the digestive system are the alimentary<br />tube and the accessory organs (Fig. 16–1).<br />The alimentary tube extends from the mouth to the<br />anus. It consists of the oral cavity, pharynx, esophagus,<br />stomach, small intestine, and large intestine.<br />Digestion takes place within the oral cavity, stomach,<br />and small intestine; most absorption of nutrients takes<br />place in the small intestine. Undigestible material, primarily<br />cellulose, is eliminated by the large intestine<br />(also called the colon).<br />The accessory organs of digestion are the teeth,<br />tongue, salivary glands, liver, gallbladder, and pancreas.<br />Digestion does not take place within these organs, but<br />each contributes something to the digestive process.<br />TYPES OF DIGESTION<br />The food we eat is broken down in two complementary<br />processes: mechanical digestion and chemical<br />digestion. Mechanical digestion is the physical breaking<br />up of food into smaller pieces. Chewing is an<br />example of this. As food is broken up, more of its surface<br />area is exposed for the action of digestive enzymes.<br />Enzymes are discussed in Chapter 2. The work of the<br />digestive enzymes is the chemical digestion of broken-<br />up food particles, in which complex chemical molecules<br />are changed into much simpler chemicals that<br />the body can utilize. Such enzymes are specific with<br />respect to the fat, protein, or carbohydrate food molecules<br />each can digest. For example, protein-digesting<br />enzymes work only on proteins, not on carbohydrates<br />or fats. Each enzyme is produced by a particular digestive<br />organ and functions at a specific site. However, the<br />enzyme’s site of action may or may not be its site of<br />production. These digestive enzymes and their functions<br />are discussed in later sections.<br />END PRODUCTS OF DIGESTION<br />Before we describe the organs of digestion, let us see<br />where the process of digestion will take us, or rather,<br />will take our food. The three types of complex organic<br />molecules found in food are carbohydrates, proteins,<br />and fats. Each of these complex molecules is digested<br />to a much more simple substance that the body can<br />then use. Carbohydrates, such as starches and disaccharides,<br />are digested to monosaccharides such as glucose,<br />fructose, and galactose. Proteins are digested to<br />amino acids, and fats are digested to fatty acids and<br />glycerol. Also part of food, and released during digestion,<br />are vitamins, minerals, and water.<br />We will now return to the beginning of the alimentary<br />tube and consider the digestive organs and the<br />process of digestion.<br />ORAL CAVITY<br />Food enters the oral cavity (or buccal cavity) by way<br />of the mouth. The boundaries of the oral cavity are<br />the hard and soft palates superiorly; the cheeks laterally;<br />and the floor of the mouth inferiorly. Within the<br />oral cavity are the teeth and tongue and the openings<br />of the ducts of the salivary glands.<br />TEETH<br />The function of the teeth is, of course, chewing. This<br />is the process that mechanically breaks food into<br />smaller pieces and mixes it with saliva. An individual<br />370 The Digestive System<br />The Digestive System 371<br />Tongue<br />Teeth<br />Parotid gland<br />Sublingual gland<br />Submandibular gland<br />Esophagus<br />Liver Left lobe<br />Stomach (cut)<br />Right lobe<br />Gallbladder<br />Bile duct<br />Transverse colon<br />(cut)<br />Ascending colon<br />Cecum<br />Vermiform appendix<br />Spleen<br />Duodenum<br />Pancreas<br />Descending colon<br />Small intestine<br />Rectum<br />Anal canal<br />Pharynx<br />Figure 16–1. The digestive organs<br />shown in anterior view of the trunk<br />and left lateral view of the head. (The<br />spleen is not a digestive organ but is<br />included to show its location relative<br />to the stomach, pancreas, and colon.)<br />QUESTION: In which parts of the<br />digestive system does digestion actually<br />take place?<br />develops two sets of teeth: deciduous and permanent.<br />The deciduous teeth begin to erupt through the<br />gums at about 6 months of age, and the set of 20 teeth<br />is usually complete by the age of 2 years. These teeth<br />are gradually lost throughout childhood and replaced<br />by the permanent teeth, the first of which are molars<br />that emerge around the age of 6 years. A complete set<br />of permanent teeth consists of 32 teeth; the types of<br />teeth are incisors, canines, premolars, and molars. The<br />wisdom teeth are the third molars on either side of<br />each jawbone. In some people, the wisdom teeth may<br />not emerge from the jawbone because there is no<br />room for them along the gum line. These wisdom<br />teeth are said to be impacted and may put pressure on<br />the roots of the second molars. In such cases, extraction<br />of a wisdom tooth may be necessary to prevent<br />damage to other teeth.<br />The structure of a tooth is shown in Fig. 16–2. The<br />crown is visible above the gum (gingiva). The root is<br />enclosed in a socket in the mandible or maxillae. The<br />periodontal membrane lines the socket and produces<br />a bone-like cement that anchors the tooth. The outermost<br />layer of the crown is enamel, which is made by<br />cells called ameloblasts. Enamel provides a hard chewing<br />surface and is more resistant to decay than are<br />other parts of the tooth. Within the enamel is dentin,<br />which is very similar to bone and is produced by cells<br />called odontoblasts. Dentin also forms the roots of a<br />tooth. The innermost portion of a tooth is the pulp<br />cavity, which contains blood vessels and nerve endings<br />of the trigeminal nerve (5th cranial). Erosion of<br />the enamel and dentin layers by bacterial acids (dental<br />caries or cavities) may result in bacterial invasion of<br />the pulp cavity and a very painful toothache.<br />TONGUE<br />The tongue is made of skeletal muscle that is innervated<br />by the hypoglossal nerves (12th cranial). On the<br />upper surface of the tongue are small projections<br />called papillae, many of which contain taste buds (see<br />also Chapter 9). The sensory nerves for taste are also<br />cranial nerves: the facial (7th) and glossopharyngeal<br />(9th). As you know, the sense of taste is important<br />because it makes eating enjoyable, but the tongue has<br />other functions as well.<br />Chewing is efficient because of the action of the<br />tongue in keeping the food between the teeth and<br />mixing it with saliva. Elevation of the tongue is the<br />first step in swallowing. This is a voluntary action, in<br />which the tongue contracts and meets the resistance of<br />the hard palate. The mass of food, called a bolus, is<br />thus pushed backward toward the pharynx. The<br />remainder of swallowing is a reflex, which is described<br />in the section on the pharynx.<br />SALIVARY GLANDS<br />The digestive secretion in the oral cavity is saliva,<br />produced by three pairs of salivary glands, which are<br />shown in Fig. 16–3. The parotid glands are just<br />below and in front of the ears. The submandibular<br />(also called submaxillary) glands are at the posterior<br />corners of the mandible, and the sublingual glands<br />are below the floor of the mouth. Each gland has at<br />least one duct that takes saliva to the oral cavity.<br />Secretion of saliva is continuous, but the amount<br />varies in different situations. The presence of food<br />(or anything else) in the mouth increases saliva secretion.<br />This is a parasympathetic response mediated<br />by the facial and glossopharyngeal nerves. The sight<br />or smell of food also increases secretion of saliva.<br />Sympathetic stimulation in stress situations decreases<br />secretion, making the mouth dry and swallowing<br />difficult.<br />Saliva is mostly water, which is important to dissolve<br />food for tasting and to moisten food for swallowing.<br />The digestive enzyme in saliva is salivary<br />amylase, which breaks down starch molecules to<br />shorter chains of glucose molecules, or to maltose, a<br />disaccharide. Most of us, however, do not chew our<br />food long enough for the action of salivary amylase to<br />be truly effective. As you will see, another amylase<br />from the pancreas is also available to digest starch.<br />Table 16–1 summarizes the functions of digestive<br />secretions.<br />Saliva is made from blood plasma and thus contains<br />many of the chemicals that are found in plasma.<br />Considerable research is focused on detecting in saliva<br />chemical markers for diseases such as cancer, with the<br />372 The Digestive System<br />Enamel<br />Dentin<br />Pulp<br />cavity<br />Gingiva<br />(gum)<br />Cementum<br />Periodontal<br />membrane<br />Blood vessels<br />Crown<br />Neck<br />Root<br />Nerve<br />Figure 16–2. Tooth structure. Longitudinal section of a<br />tooth showing internal structure.<br />QUESTION: Which parts of a tooth are living? How do<br />you know?<br />goal of using saliva rather than blood for diagnostic<br />tests.<br />PHARYNX<br />As described in the preceding chapter, the oropharynx<br />and laryngopharynx are food passageways connecting<br />the oral cavity to the esophagus. No digestion takes<br />place in the pharynx. Its only related function is swallowing,<br />the mechanical movement of food. When the<br />bolus of food is pushed backward by the tongue, the<br />constrictor muscles of the pharynx contract as part of<br />the swallowing reflex. The reflex center for swallowing<br />is in the medulla, which coordinates the many<br />actions that take place: constriction of the pharynx,<br />cessation of breathing, elevation of the soft palate to<br />block the nasopharynx, elevation of the larynx and closure<br />of the epiglottis, and peristalsis of the esophagus.<br />As you can see, swallowing is rather complicated, but<br />because it is a reflex we don’t have to think about making<br />it happen correctly. Talking or laughing while eating,<br />however, may interfere with the reflex and cause<br />food to go into the “wrong pipe,” the larynx. When<br />that happens, the cough reflex is usually effective in<br />clearing the airway.<br />ESOPHAGUS<br />The esophagus is a muscular tube that takes food<br />from the pharynx to the stomach; no digestion takes<br />place here. Peristalsis of the esophagus propels food in<br />one direction and ensures that food gets to the stomach<br />even if the body is horizontal or upside down. At<br />the junction with the stomach, the lumen (cavity) of<br />the esophagus is surrounded by the lower esophageal<br />sphincter (LES or cardiac sphincter), a circular<br />smooth muscle. The LES relaxes to permit food to<br />enter the stomach, then contracts to prevent the<br />backup of stomach contents. If the LES does not close<br />completely, gastric juice may splash up into the esophagus;<br />this is a painful condition we call heartburn, or<br />gastroesophageal reflux disease (GERD). Most people<br />experience heartburn once in a while, and it is merely<br />uncomfortable, but chronic GERD is more serious.<br />The lining of the esophagus cannot withstand the corrosive<br />action of gastric acid and will be damaged, perhaps<br />resulting in bleeding or even perforation.<br />Medications are available to treat this condition.<br />STRUCTURAL LAYERS<br />OF THE ALIMENTARY TUBE<br />Before we continue with our discussion of the organs<br />of digestion, we will first examine the typical structure<br />of the alimentary tube. When viewed in cross-section,<br />the alimentary tube has four layers (Fig. 16–4): the<br />mucosa, submucosa, external muscle layer, and serosa.<br />Each layer has a specific structure, and its functions<br />contribute to the functioning of the organs of which it<br />is a part.<br />MUCOSA<br />The mucosa, or lining, of the alimentary tube is made<br />of epithelial tissue, areolar connective tissue, and two<br />The Digestive System 373<br />Parotid duct<br />Sublingual<br />ducts<br />Parotid<br />gland<br />Submandibular<br />gland<br />Sublingual gland<br />Submandibular duct<br />Figure 16–3. The salivary glands shown in left lateral<br />view.<br />QUESTION: Why are these exocrine glands? What is<br />saliva made from?<br />374 The Digestive System<br />Table 16–1 THE PROCESS OF DIGESTION<br />Enzyme or<br />Organ Other Secretion Function Site of Action<br />Salivary glands<br />Stomach<br />Liver<br />Pancreas<br />Small intestine<br />Amylase<br />Amylase<br />Sucrase, Maltase, Lactase<br />Lipase<br />Bile<br />Pepsin<br />Trypsin<br />Peptidases<br />Proteins<br />Fats<br />Salivary Carbohydrates<br />glands<br />Liver<br />Stomach<br />Pancreas<br />Small<br />intestine<br />Amylase<br />Pepsin<br />HCI<br />Bile salts<br />Amylase<br />Trypsin<br />Lipase<br />Peptidases<br />Sucrase<br />Maltase<br />Lactase<br />• Converts starch to maltose<br />• Converts proteins to polypeptides<br />• Changes pepsinogen to pepsin; maintains pH<br />1–2; destroys pathogens<br />• Emulsify fats<br />• Converts starch to maltose<br />• Converts polypeptides to peptides<br />• Converts emulsified fats to fatty acids and glycerol<br />• Convert peptides to amino acids<br />• Converts sucrose to glucose and fructose<br />• Converts maltose to glucose (2)<br />• Converts lactose to glucose and galactose<br />Oral cavity<br />Stomach<br />Stomach<br />Small intestine<br />Small intestine<br />Small intestine<br />Small intestine<br />Small intestine<br />Small intestine<br />Small intestine<br />Small intestine<br />Table Figure 16–A Functions of digestive secretions.<br />QUESTION: Proteins are digested by secretions from which organs? How did you decide?<br />375<br />Small intestine<br />Mucosa<br />Submucosa<br />External<br />muscle<br />layer<br />Serosa<br />Epithelium<br />Lacteal<br />Capillary<br />network<br />Lymph<br />nodule<br />Lymphatic<br />vessel<br />Smooth<br />muscle<br />Venule<br />Arteriole<br />Meissner's<br />plexus<br />Circular<br />smooth muscle<br />Longitudinal<br />smooth muscle<br />Auerbach's<br />plexus<br />Parietal<br />peritoneum<br />Transverse<br />colon<br />Greater<br />omentum<br />Uterus<br />Bladder<br />Diaphragm<br />Liver<br />Pancreas<br />Stomach<br />Duodenum<br />Mesentery<br />Small intestine<br />Sigmoid colon<br />Rectum<br />A<br />B<br />Figure 16–4. (A) The four layers of the wall of the<br />alimentary tube. A small part of the wall of the small<br />intestine has been magnified to show the four layers<br />typical of the alimentary tube. (B) Sagittal section<br />through the abdomen showing the relationship of the<br />peritoneum and mesentery to the abdominal organs.<br />QUESTION: What is the function of the external muscle<br />layer?<br />thin layers of smooth muscle. In the esophagus the<br />epithelium is stratified squamous epithelium; in the<br />stomach and intestines it is simple columnar epithelium.<br />The epithelium secretes mucus, which lubricates<br />the passage of food, and also secretes the digestive<br />enzymes of the stomach and small intestine. Just<br />below the epithelium, within the areolar connective<br />tissue, are lymph nodules that contain lymphocytes to<br />produce antibodies, and macrophages to phagocytize<br />bacteria or other foreign material that get through the<br />epithelium. The thin layers of smooth muscle create<br />folds in the mucosa, and ripples, so that all of the<br />epithelial cells are in touch with the contents of the<br />organ. In the stomach and small intestine this is<br />important for absorption.<br />SUBMUCOSA<br />The submucosa is made of areolar connective tissue<br />with many blood vessels and lymphatic vessels. Many<br />millions of nerve fibers are also present, part of what<br />is called the enteric nervous system, or the “brain of<br />the gut,” which extends the entire length of the alimentary<br />tube. The nerve networks in the submucosa<br />are called Meissner’s plexus (or submucosal plexus),<br />and they innervate the mucosa to regulate secretions.<br />Parasympathetic impulses increase secretions, whereas<br />sympathetic impulses decrease secretions. Sensory<br />neurons are also present to the smooth muscle (a<br />stretched or cramping gut is painful), as are motor<br />neurons to blood vessels, to regulate vessel diameter<br />and blood flow.<br />EXTERNAL MUSCLE LAYER<br />The external muscle layer typically contains two layers<br />of smooth muscle: an inner, circular layer and an outer,<br />longitudinal layer. Variations from the typical do<br />occur, however. In the esophagus, this layer is striated<br />muscle in the upper third, which gradually changes to<br />smooth muscle in the lower portions. The stomach has<br />three layers of smooth muscle, rather than two.<br />Contractions of this muscle layer help break up<br />food and mix it with digestive juices. The one-way<br />contractions of peristalsis move the food toward the<br />anus. Auerbach’s plexus (or myenteric plexus) is the<br />portion of the enteric nervous system in this layer, and<br />some of its millions of neurons are autonomic. Sympathetic<br />impulses decrease contractions and peristalsis,<br />whereas parasympathetic impulses increase contractions<br />and peristalsis, promoting normal digestion. The<br />parasympathetic nerves are the vagus (10th cranial)<br />nerves; they truly live up to the meaning of vagus,<br />which is “wanderer.”<br />SEROSA<br />Above the diaphragm, for the esophagus, the serosa,<br />the outermost layer, is fibrous connective tissue. Below<br />the diaphragm, the serosa is the mesentery or visceral<br />peritoneum, a serous membrane. Lining the abdominal<br />cavity is the parietal peritoneum, usually simply<br />called the peritoneum. The peritoneum-mesentery is<br />actually one continuous membrane (see Fig. 16–4).<br />The serous fluid between the peritoneum and mesentery<br />prevents friction when the alimentary tube contracts<br />and the organs slide against one another.<br />The preceding descriptions are typical of the layers<br />of the alimentary tube. As noted, variations are possible,<br />and any important differences are mentioned in<br />the sections that follow on specific organs.<br />STOMACH<br />The stomach is located in the upper left quadrant of<br />the abdominal cavity, to the left of the liver and in<br />front of the spleen. Although part of the alimentary<br />tube, the stomach is not a tube, but rather a sac that<br />extends from the esophagus to the small intestine.<br />Because it is a sac, the stomach is a reservoir for food,<br />so that digestion proceeds gradually and we do not<br />have to eat constantly. Both mechanical and chemical<br />digestion take place in the stomach.<br />The parts of the stomach are shown in Fig. 16–5.<br />The cardiac orifice is the opening of the esophagus,<br />and the fundus is the portion above the level of this<br />opening. The body of the stomach is the large central<br />portion, bounded laterally by the greater curvature and<br />medially by the lesser curvature. The pylorus is adjacent<br />to the duodenum of the small intestine, and the<br />pyloric sphincter surrounds the junction of the two<br />organs. The fundus and body are mainly storage areas,<br />whereas most digestion takes place in the pylorus.<br />When the stomach is empty, the mucosa appears<br />wrinkled or folded. These folds are called rugae; they<br />flatten out as the stomach is filled and permit expansion<br />of the lining without tearing it. The gastric pits<br />are the glands of the stomach and consist of several<br />376 The Digestive System<br />types of cells; their collective secretions are called gastric<br />juice. Mucous cells secrete mucus, which coats<br />the stomach lining and helps prevent erosion by the<br />gastric juice. Chief cells secrete pepsinogen, an inactive<br />form of the enzyme pepsin. Parietal cells produce<br />hydrochloric acid (HCl); these cells have<br />enzymes called proton pumps, which secrete H+ ions<br />into the stomach cavity. The H+ ions unite with Cl–<br />ions that have diffused from the parietal cells to form<br />HCl in the lumen of the stomach. HCl converts<br />pepsinogen to pepsin, which then begins the digestion<br />of proteins to polypeptides, and also gives gastric juice<br />its pH of 1 to 2. This very acidic pH is necessary for<br />pepsin to function and also kills most microorganisms<br />that enter the stomach. The parietal cells also secrete<br />intrinsic factor, which is necessary for the absorption<br />of vitamin B12. Enteroendocrine cells called G cells<br />secrete the hormone gastrin.<br />Gastric juice is secreted in small amounts at the<br />sight or smell of food. This is a parasympathetic<br />response that ensures that some gastric juice will be<br />present in the stomach when food arrives. The pres-<br />The Digestive System 377<br />Esophagus Fundus of stomach<br />Longitudinal muscle<br />layer<br />Circular muscle layer<br />Oblique muscle layer<br />Body<br />Greater curvature<br />Rugae<br />Pylorus<br />Duodenum<br />Pyloric sphincter<br />Lesser curvature<br />Cardiac orifice<br />Mucous cell<br />Parietal cell<br />Chief cell<br />G cell<br />B<br />A<br />Figure 16–5. (A) The stomach in anterior view. The stomach wall has been sectioned to<br />show the muscle layers and the rugae of the mucosa. (B) Gastric pits (glands) showing the<br />types of cells present. See text for functions.<br />QUESTION: What is the function of the pyloric sphincter?<br />ence of food in the stomach causes the G cells to<br />secrete gastrin, a hormone that stimulates the secretion<br />of greater amounts of gastric juice.<br />The external muscle layer of the stomach consists<br />of three layers of smooth muscle: circular, longitudinal,<br />and oblique layers. These three layers are innervated<br />by the myenteric plexuses of the enteric nervous<br />system. Stimulatory impulses are carried from the<br />CNS by the vagus nerves (10th cranial) and provide<br />for very efficient mechanical digestion to change food<br />into a thick liquid called chyme. The pyloric sphincter<br />is usually contracted when the stomach is churning<br />food; it relaxes at intervals to permit small amounts of<br />chyme to pass into the duodenum. This sphincter then<br />contracts again to prevent the backup of intestinal<br />contents into the stomach (see Box 16–1: Disorders of<br />the Stomach).<br />SMALL INTESTINE<br />The small intestine is about 1 inch (2.5 cm) in diameter<br />and approximately 20 feet (6 m) long and extends<br />from the stomach to the cecum of the large intestine.<br />Within the abdominal cavity, the large intestine encircles<br />the coils of the small intestine (see Fig. 16–1).<br />The duodenum is the first 10 inches (25 cm) of the<br />small intestine. The common bile duct enters the duodenum<br />at the ampulla of Vater (or hepatopancreatic<br />ampulla). The jejunum is about 8 feet long, and the<br />ileum is about 11 feet in length. In a living person,<br />however, the small intestine is always contracted and is<br />therefore somewhat shorter.<br />Digestion is completed in the small intestine, and<br />the end products of digestion are absorbed into the<br />blood and lymph. The mucosa (see Fig. 16–4) has<br />simple columnar epithelium that includes cells with<br />microvilli and goblet cells that secrete mucus.<br />Enteroendocrine cells secrete the hormones of the<br />small intestine. Lymph nodules called Peyer’s patches<br />are especially abundant in the ileum to destroy<br />absorbed pathogens. The external muscle layer has the<br />typical circular and longitudinal smooth muscle layers<br />that mix the chyme with digestive secretions and propel<br />the chyme toward the colon. Stimulatory impulses<br />to the enteric nerves of these muscle layers are carried<br />by the vagus nerves. The waves of peristalsis, however,<br />can take place without stimulation by the central nerv-<br />378 The Digestive System<br />BOX 16–1 DISORDERS OF THE STOMACH<br />pyloric stenosis. Correcting this condition requires<br />surgery to widen the opening in the sphincter.<br />A gastric ulcer is an erosion of the mucosa of<br />the stomach. Because the normal stomach lining is<br />adapted to resist the corrosive action of gastric<br />juice, ulcer formation is the result of oversecretion<br />of HCl or undersecretion of mucus.<br />As erosion reaches the submucosa, small blood<br />vessels are ruptured and bleed. If vomiting occurs,<br />the vomitus has a “coffee-ground” appearance due<br />to the presence of blood acted on by gastric juice.<br />A more serious complication is perforation of the<br />stomach wall, with leakage of gastric contents into<br />the abdominal cavity, and peritonitis.<br />The bacterium called Helicobacter pylori is the<br />cause of most gastric ulcers. For many patients, a<br />few weeks of antibiotic therapy to eradicate this<br />bacterium has produced rapid healing of their<br />ulcers. This bacterium also seems to be responsible<br />for virtually all cases of stomach cancer.<br />The medications that decrease the secretion of<br />HCl are useful for ulcer patients not helped by<br />antibiotics.<br />Vomiting is the expulsion of stomach and intestinal<br />contents through the esophagus and mouth.<br />Stimuli include irritation of the stomach, motion<br />sickness, food poisoning, or diseases such as meningitis.<br />The vomiting center is in the medulla, which<br />coordinates the simultaneous contraction of the<br />diaphragm and the abdominal muscles. This<br />squeezes the stomach and upper intestine, expelling<br />their contents. As part of the reflex, the lower<br />esophageal sphincter relaxes, and the glottis closes.<br />If the glottis fails to close, as may happen in alcohol<br />or drug intoxication, aspiration of vomitus may<br />occur and result in fatal obstruction of the respiratory<br />passages.<br />Pyloric stenosis means that the opening of the<br />pyloric sphincter is narrowed, and emptying of the<br />stomach is impaired. This is most often a congenital<br />disorder caused by hypertrophy of the pyloric<br />sphincter. For reasons unknown, this condition is<br />more common in male infants than in female<br />infants. When the stomach does not empty efficiently,<br />its internal pressure increases. Vomiting<br />relieves the pressure; this is a classic symptom of<br />ous system; the enteric nervous system can function<br />independently and promote normal peristalsis.<br />There are three sources of digestive secretions that<br />function within the small intestine: the liver, the pancreas,<br />and the small intestine itself. We will return to<br />the small intestine after considering these other<br />organs.<br />LIVER<br />The liver (Fig. 16–6) consists of two large lobes, right<br />and left, and fills the upper right and center of the<br />abdominal cavity, just below the diaphragm. The<br />structural unit of the liver is the liver lobule, a<br />roughly hexagonal column of liver cells (hepatocytes).<br />Between adjacent lobules are branches of the hepatic<br />artery and portal vein. The capillaries of a lobule are<br />sinusoids, large and very permeable vessels between<br />the rows of liver cells. The sinusoids receive blood<br />from both the hepatic artery and portal vein, and it is<br />with this mixture of blood that the liver cells carry out<br />their functions. The hepatic artery brings oxygenated<br />blood, and the portal vein brings blood from the<br />digestive organs and spleen (see Fig. 13–7). Each lobule<br />has a central vein. The central veins of all the lobules<br />unite to form the hepatic veins, which take blood<br />out of the liver to the inferior vena cava.<br />The cells of the liver have many functions (which<br />are discussed in a later section), but their only digestive<br />function is the production of bile. Bile enters the<br />small bile ducts, called bile canaliculi, on the liver<br />cells, which unite to form larger ducts and finally<br />merge to form the hepatic duct, which takes bile out<br />of the liver (see Fig. 16–6). The hepatic duct unites<br />with the cystic duct of the gallbladder to form the<br />common bile duct, which takes bile to the duodenum.<br />Bile is mostly water and has an excretory function<br />in that it carries bilirubin and excess cholesterol to the<br />intestines for elimination in feces. The digestive function<br />of bile is accomplished by bile salts, which emulsify<br />fats in the small intestine. Emulsification means<br />that large fat globules are broken into smaller globules.<br />This is mechanical, not chemical, digestion; the<br />fat is still fat but now has more surface area to facilitate<br />chemical digestion.<br />Production of bile is stimulated by the hormone<br />secretin, which is produced by the duodenum when<br />food enters the small intestine. Table 16–2 summa-<br />The Digestive System 379<br />rizes the regulation of secretion of all digestive secretions.<br />GALLBLADDER<br />The gallbladder is a sac about 3 to 4 inches (7.5 to 10<br />cm) long located on the undersurface of the right lobe<br />of the liver. Bile in the hepatic duct of the liver flows<br />through the cystic duct into the gallbladder (see Fig.<br />16–6), which stores bile until it is needed in the small<br />intestine. The gallbladder also concentrates bile by<br />absorbing water (see Box 16–2: Gallstones).<br />When fatty foods enter the duodenum, the<br />enteroendocrine cells of the duodenal mucosa secrete<br />the hormone cholecystokinin. This hormone stimulates<br />contraction of the smooth muscle in the wall of<br />the gallbladder, which forces bile into the cystic duct,<br />then into the common bile duct, and on into the duodenum.<br />PANCREAS<br />The pancreas is located in the upper left abdominal<br />quadrant between the curve of the duodenum and the<br />spleen and is about 6 inches (15 cm) in length. The<br />endocrine functions of the pancreas were discussed in<br />Chapter 10, so only the exocrine functions will be<br />considered here. The exocrine glands of the pancreas<br />are called acini (singular: acinus) (Fig. 16–7). They<br />produce enzymes that are involved in the digestion of<br />all three types of complex food molecules.<br />The pancreatic enzyme amylase digests starch to<br />maltose. You may recall that this is the “backup”<br />enzyme for salivary amylase, though pancreatic amylase<br />is responsible for most digestion of starch. Lipase<br />converts emulsified fats to fatty acids and glycerol.<br />The emulsifying or fat-separating action of bile salts<br />increases the surface area of fats so that lipase works<br />effectively. Trypsinogen is an inactive enzyme that is<br />changed to active trypsin in the duodenum. Trypsin<br />digests polypeptides to shorter chains of amino acids.<br />The pancreatic enzyme juice is carried by small<br />ducts that unite to form larger ducts, then finally the<br />main pancreatic duct. An accessory duct may also be<br />present. The main pancreatic duct emerges from the<br />medial side of the pancreas and joins the common bile<br />duct to the duodenum (see Fig. 16–7).<br />The pancreas also produces a bicarbonate juice<br />(containing sodium bicarbonate), which is alkaline.<br />380<br />Right hepatic vein<br />Liver<br />Cystic<br />duct<br />Inferior vena cava<br />Left hepatic vein<br />Hepatic duct<br />Common bile duct<br />Hepatic artery<br />Portal vein<br />Central vein<br />(branch of<br />hepatic vein)<br />A<br />Gallbladder<br />Sinusoids<br />Bile duct<br />Liver lobule<br />Branch of portal vein<br />Branch of hepatic artery<br />Bile duct<br />Hepatocytes<br />B<br />Figure 16–6. (A) The liver and gallbladder with blood vessels and bile ducts.<br />(B) Magnified view of one liver lobule. See text for description.<br />QUESTION: In part B, trace the pathway of blood flow through a liver lobule.<br />Because the gastric juice that enters the duodenum<br />is very acidic, it must be neutralized to prevent damage<br />to the duodenal mucosa. This neutralizing is<br />accomplished by the sodium bicarbonate in pancreatic<br />juice, and the pH of the duodenal chyme is raised to<br />about 7.5.<br />Secretion of pancreatic juice is stimulated by the<br />hormones secretin and cholecystokinin, which are<br />produced by the duodenal mucosa when chyme enters<br />the small intestine. Secretin stimulates the production<br />of bicarbonate juice by the pancreas, and cholecystokinin<br />stimulates the secretion of the pancreatic<br />enzymes.<br />COMPLETION OF DIGESTION<br />AND ABSORPTION<br />SMALL INTESTINE<br />The secretion of the epithelium of the intestinal<br />glands (or crypts of Lieberkühn) is stimulated by the<br />The Digestive System 381<br />Table 16–2 REGULATION OF DIGESTIVE SECRETIONS<br />Secretion Nervous Regulation Chemical Regulation<br />Saliva<br />Gastric juice<br />Bile<br />Secretion by<br />the liver<br />Contraction of<br />the gallbladder<br />Enzyme pancreatic juice<br />Bicarbonate pancreatic juice<br />Intestinal juice<br />Presence of food in mouth or sight of food;<br />parasympathetic impulses along 7th and<br />9th cranial nerves<br />Sight or smell of food; parasympathetic<br />impulses along 10th cranial nerves<br />None<br />None<br />None<br />None<br />Presence of chyme in the duodenum; parasympathetic<br />impulses along 10th cranial nerves<br />None<br />Gastrin—produced by the G cells of<br />the gastric mucosa when food is<br />present in the stomach<br />Secretin—produced by the enteroendocrine<br />cells of the duodenum<br />when chyme enters<br />Cholecystokinin—produced by the<br />enteroendocrine cells of the duodenum<br />when chyme enters<br />Cholecystokinin—from the duodenum<br />Secretin—from the duodenum<br />None<br />BOX 16–2 GALLSTONES<br />Several treatments are available for gallstones.<br />Medications that dissolve gallstones work slowly,<br />over the course of several months, and are useful if<br />biliary obstruction is not severe. An instrument that<br />generates shock waves (called a lithotripter) may be<br />used to pulverize the stones into smaller pieces that<br />may easily pass into the duodenum; this procedure<br />is called lithotripsy. Surgery to remove the gallbladder<br />(cholecystectomy) is required in some<br />cases. The hepatic duct is then connected directly<br />to the common bile duct, and dilute bile flows into<br />the duodenum. Following such surgery, the patient<br />should avoid meals high in fats.<br />One of the functions of the gallbladder is to concentrate<br />bile by absorbing water. If the bile contains<br />a high concentration of cholesterol, absorption of<br />water may lead to precipitation and the formation of<br />cholesterol crystals. These crystals are gallstones.<br />If the gallstones are small, they will pass through<br />the cystic duct and common bile duct to the duodenum<br />without causing symptoms. If large, however,<br />the gallstones cannot pass out of the<br />gallbladder, and may cause mild to severe pain that<br />often radiates to the right shoulder. Obstructive<br />jaundice may occur if bile backs up into the liver<br />and bilirubin is reabsorbed into the blood.<br />presence of food in the duodenum. The intestinal<br />enzymes are the peptidases and sucrase, maltase, and<br />lactase. Peptidases complete the digestion of protein<br />by breaking down short polypeptide chains to amino<br />acids. Sucrase, maltase, and lactase, respectively,<br />digest the disaccharides sucrose, maltose, and lactose<br />to monosaccharides.<br />The enteroendocrine cells of the intestinal glands<br />secrete the hormones of the small intestine. Secretion<br />is stimulated by food entering the duodenum.<br />382 The Digestive System<br />Pyloric sphincter<br />Hepatic duct<br />Cystic duct<br />Duodenum<br />Main<br />pancreatic<br />duct<br />Pancreas<br />Superior mesenteric<br />artery and vein<br />Islets of<br />Langerhans<br />Ducts<br />Acini<br />Capillaries<br />Delta cell<br />Beta cell<br />Alpha cell<br />A<br />B<br />Common<br />bile duct<br />Ampulla of Vater<br />Figure 16–7. (A) The pancreas, sectioned to show the pancreatic ducts. The main pancreatic<br />duct joins the common bile duct. (B) Microscopic section showing acini with their<br />ducts and several islets of Langerhans.<br />QUESTION: In part B, what do the acini secrete?<br />A summary of the digestive secretions and their<br />functions is found in Table 16–1. Regulation of these<br />secretions is shown in Table 16–2.<br />ABSORPTION<br />Most absorption of the end products of digestion takes<br />place in the small intestine (although the stomach does<br />absorb water and alcohol). The process of absorption<br />requires a large surface area, which is provided by several<br />structural modifications of the small intestine;<br />these are shown in Fig. 16–8. Plica circulares, or circular<br />folds, are macroscopic folds of the mucosa and<br />submucosa, somewhat like accordion pleats. The<br />mucosa is further folded into projections called villi,<br />which give the inner surface of the intestine a velvetlike<br />appearance. Each columnar cell (except the<br />mucus-secreting goblet cells) of the villi also has<br />microvilli on its free surface. Microvilli are microscopic<br />folds of the cell membrane, and are collectively<br />The Digestive System 383<br />Small intestine<br />Plica<br />circulares<br />Intestinal<br />gland<br />Microvilli<br />Absorptive cell<br />Goblet cell<br />Lacteal<br />Capillary<br />network<br />Enteroendocrine cell<br />A<br />B<br />Figure 16–8. The small intestine. (A) Section through the small intestine showing plica<br />circulares. (B) Microscopic view of a villus showing the internal structure. The enteroendocrine<br />cells secrete the intestinal hormones.<br />QUESTION: What is the purpose of the villi? What other structures have the same purpose?<br />called the brush border. All of these folds greatly<br />increase the surface area of the intestinal lining. It is<br />estimated that if the intestinal mucosa could be flattened<br />out, it would cover more than 2000 square feet<br />(half a basketball court).<br />The absorption of nutrients takes place from the<br />lumen of the intestine into the vessels within the<br />villi. Refer to Fig. 16–8 and notice that within each villus<br />is a capillary network and a lacteal, which is a<br />dead-end lymph capillary. Water-soluble nutrients<br />are absorbed into the blood in the capillary networks.<br />Monosaccharides, amino acids, positive ions, and the<br />water-soluble vitamins (vitamin C and the B vitamins)<br />are absorbed by active transport. Negative ions may<br />be absorbed by either passive or active transport<br />mechanisms. Water is absorbed by osmosis following<br />the absorption of minerals, especially sodium. Certain<br />nutrients have additional special requirements for<br />their absorption: For example, vitamin B12 requires<br />the intrinsic factor produced by the parietal cells<br />of the gastric mucosa, and the efficient absorption of<br />calcium ions requires parathyroid hormone and vitamin<br />D.<br />Fat-soluble nutrients are absorbed into the lymph<br />in the lacteals of the villi. Bile salts are necessary for<br />the efficient absorption of fatty acids and the fat-soluble<br />vitamins (A, D, E, and K). Once absorbed, fatty<br />acids are recombined with glycerol to form triglycerides.<br />These triglycerides then form globules that<br />include cholesterol and protein; these lipid–protein<br />complexes are called chylomicrons. In the form of<br />chylomicrons, most absorbed fat is transported by the<br />lymph and eventually enters the blood in the left subclavian<br />vein.<br />Blood from the capillary networks in the villi does<br />not return directly to the heart but first travels<br />through the portal vein to the liver. You may recall the<br />importance of portal circulation, discussed in Chapter<br />13. This pathway enables the liver to regulate the<br />blood levels of glucose and amino acids, store certain<br />vitamins, and remove potential poisons from the<br />blood (see Box 16–3: Disorders of the Intestines).<br />384 The Digestive System<br />BOX 16–3 DISORDERS OF THE INTESTINES<br />Salmonella food poisoning is caused by bacteria<br />in the genus Salmonella. These are part of the<br />intestinal flora of animals, and animal foods such as<br />meat and eggs may be sources of infection. These<br />bacteria are not normal for people, and they cause<br />the intestines to secrete large amounts of fluid.<br />Symptoms include diarrhea, abdominal cramps,<br />and vomiting and usually last only a few days. For<br />elderly or debilitated people, however, salmonella<br />food poisoning may be very serious or even fatal.<br />Diverticula are small outpouchings through<br />weakened areas of the intestinal wall. They are<br />more likely to occur in the colon than in the small<br />intestine and may exist for years without causing<br />any symptoms. The presence of diverticula is called<br />diverticulosis. Inflammation of diverticula is<br />called diverticulitis, which is usually the result of<br />entrapment of feces and bacteria. Symptoms<br />include abdominal pain and tenderness and fever. If<br />uncomplicated, diverticulitis may be treated with<br />antibiotics and modifications in diet. The most serious<br />complication is perforation of diverticula, allowing<br />fecal material into the abdominal cavity, causing<br />peritonitis. A diet high in fiber is believed to be an<br />important aspect of prevention, to provide bulk in<br />the colon and prevent weakening of its wall.<br />Duodenal ulcers are erosions of the duodenal<br />wall caused by the gastric juice that enters from the<br />stomach. The most serious consequences are bleeding<br />and perforation.<br />Paralytic ileus is the cessation of contraction<br />of the smooth muscle layer of the intestine. This<br />is a possible complication of abdominal surgery,<br />but it may also be the result of peritonitis or<br />inflammation elsewhere in the abdominal cavity.<br />In the absence of peristalsis, intestinal obstruction<br />may occur. Bowel movements cease, and vomiting<br />occurs to relieve the pressure within the<br />alimentary tube. Treatment involves suctioning<br />the intestinal contents to eliminate any obstruction<br />and to allow the intestine to regain its normal<br />motility.<br />Lactose intolerance is the inability to digest<br />lactose because of deficiency of the enzyme lactase.<br />Lactase deficiency may be congenital, a consequence<br />of prematurity, or acquired later in life. The<br />delayed form is quite common among people of<br />African or Asian ancestry, and in part is genetic.<br />When lactose, or milk sugar, is not digested, it<br />undergoes fermentation in the intestine. Symptoms<br />include diarrhea, abdominal pain, bloating, and<br />flatulence (gas formation).<br />LARGE INTESTINE<br />The large intestine, also called the colon, is approximately<br />2.5 inches (6.3 cm) in diameter and 5 feet (1.5<br />m) in length. It extends from the ileum of the small<br />intestine to the anus, the terminal opening. The parts<br />of the colon are shown in Fig. 16–9. The cecum is the<br />first portion, and at its junction with the ileum is<br />the ileocecal valve, which is not a sphincter but serves<br />the same purpose. After undigested food (which is<br />now mostly cellulose) and water pass from the ileum<br />into the cecum, closure of the ileocecal valve prevents<br />the backflow of fecal material.<br />Attached to the cecum is the appendix, a small,<br />dead-end tube with abundant lymphatic tissue. The<br />appendix seems to be a vestigial organ, that is, one<br />whose size and function seem to be reduced. Although<br />there is abundant lymphatic tissue in the wall of the<br />appendix, the possibility that the appendix is concerned<br />with immunity is not known with certainty.<br />Appendicitis refers to inflammation of the appendix,<br />which may occur if fecal material becomes impacted<br />within it. This usually necessitates an appendectomy,<br />the surgical removal of the appendix.<br />The remainder of the colon consists of the ascending,<br />transverse, and descending colon, which encircle<br />the small intestine; the sigmoid colon, which turns<br />medially and downward; the rectum; and the anal<br />canal. The rectum is about 6 inches long, and the anal<br />canal is the last inch of the colon that surrounds the<br />anus. Clinically, however, the terminal end of the<br />colon is usually referred to as the rectum.<br />No digestion takes place in the colon. The only<br />secretion of the colonic mucosa is mucus, which lubricates<br />the passage of fecal material. The longitudinal<br />smooth muscle layer of the colon is in three bands<br />called taeniae coli. The rest of the colon is “gathered”<br />to fit these bands. This gives the colon a puckered<br />appearance; the puckers or pockets are called haustra,<br />which provide for more surface area within the colon.<br />The functions of the colon are the absorption of<br />water, minerals, and vitamins and the elimination of<br />undigestible material. About 80% of the water that<br />The Digestive System 385<br />Hepatic flexure<br />Haustra<br />Taeniae coli<br />Splenic flexure<br />Transverse colon<br />Ascending<br />colon<br />lleum<br />Descending<br />colon<br />lleocecal valve<br />Appendix<br />Cecum<br />Sigmoid colon<br />Rectum<br />Anus<br />Anal canal<br />Figure 16–9. The large intestine shown in<br />anterior view. The term flexure means a turn or<br />bend.<br />QUESTION: What is the function of the ileocecal<br />valve?<br />enters the colon is absorbed (400 to 800 mL per day).<br />Positive and negative ions are also absorbed. The vitamins<br />absorbed are those produced by the normal<br />flora, the trillions of bacteria that live in the colon.<br />Vitamin K is produced and absorbed in amounts usually<br />sufficient to meet a person’s daily need. Other<br />vitamins produced in smaller amounts include<br />riboflavin, thiamin, biotin, and folic acid. Everything<br />absorbed by the colon circulates first to the liver by<br />way of portal circulation. Yet another function of the<br />normal colon flora is to inhibit the growth of<br />pathogens (see Box 16–4: Infant Botulism).<br />ELIMINATION OF FECES<br />Feces consist of cellulose and other undigestible material,<br />dead and living bacteria, and water. Elimination<br />of feces is accomplished by the defecation reflex, a<br />spinal cord reflex that may be controlled voluntarily.<br />The rectum is usually empty until peristalsis of the<br />colon pushes feces into it. These waves of peristalsis<br />tend to occur after eating, especially when food enters<br />the duodenum. The wall of the rectum is stretched by<br />the entry of feces, and this is the stimulus for the defecation<br />reflex.<br />Stretch receptors in the smooth muscle layer of the<br />rectum generate sensory impulses that travel to the<br />sacral spinal cord. The returning motor impulses<br />cause the smooth muscle of the rectum to contract.<br />Surrounding the anus is the internal anal sphincter,<br />which is made of smooth muscle. As part of the reflex,<br />this sphincter relaxes, permitting defecation to take<br />place.<br />The external anal sphincter is made of skeletal<br />muscle and surrounds the internal anal sphincter (Fig.<br />16–10). If defecation must be delayed, the external<br />sphincter may be voluntarily contracted to close the<br />anus. The awareness of the need to defecate passes as<br />the stretch receptors of the rectum adapt. These<br />receptors will be stimulated again when the next wave<br />of peristalsis reaches the rectum (see Box 16–5: Fiber).<br />OTHER FUNCTIONS OF THE LIVER<br />The liver is a remarkable organ, and only the brain is<br />capable of a greater variety of functions. The liver<br />cells (hepatocytes) produce many enzymes that catalyze<br />many different chemical reactions. These reactions<br />are the functions of the liver. As blood flows<br />through the sinusoids (capillaries) of the liver (see Fig.<br />16–6), materials are removed by the liver cells, and the<br />products of the liver cells are secreted into the blood.<br />Some of the liver functions will already be familiar<br />to you. Others are mentioned again and discussed in<br />more detail in the next chapter. Because the liver<br />has such varied effects on so many body systems, we<br />will use the categories below to summarize the liver<br />functions.<br />1. Carbohydrate metabolism—As you know, the<br />liver regulates the blood glucose level. Excess glucose<br />is converted to glycogen (glycogenesis) when<br />blood glucose is high; the hormones insulin and<br />cortisol facilitate this process. During hypoglycemia<br />or stress situations, glycogen is converted<br />back to glucose (glycogenolysis) to raise the blood<br />glucose level. Epinephrine and glucagon are the<br />hormones that facilitate this process.<br />386 The Digestive System<br />BOX 16–4 INFANT BOTULISM<br />Botulism is most often acquired from food.<br />When the spores of the botulism bacteria are in<br />an anaerobic (without oxygen) environment<br />such as a can of food, they germinate into active<br />bacteria that produce a neurotoxin. If people<br />ingest food containing this toxin, they will<br />develop the paralysis that is characteristic of<br />botulism.<br />For infants less than 1 year of age, however,<br />ingestion of just the bacterial spores may be<br />harmful. The infant’s stomach does not produce<br />much HCl, so ingested botulism spores may not<br />be destroyed. Of equal importance, the infant’s<br />normal colon flora is not yet established. Without<br />the normal population of colon bacteria to provide<br />competition, spores of the botulism bacteria<br />may germinate and produce their toxin.<br />An affected infant becomes lethargic and<br />weak; paralysis may progress slowly or rapidly.<br />Treatment (antitoxin) is available, but may be<br />delayed if botulism is not suspected. Many cases<br />of infant botulism have been traced to honey<br />that was found to contain botulism spores. Such<br />spores are not harmful to older children and<br />adults, who have a normal colon flora that prevents<br />the botulism bacteria from becoming<br />established.<br />387<br />BOX 16–5 FIBER<br />no protective effect of fiber against colon cancer.<br />What we can say for sure is that fiber may not be<br />the only dietary or environmental factor involved.<br />Claims that high-fiber diets directly lower blood<br />levels of cholesterol and fats are not supported by<br />definitive clinical or experimental studies. One possible<br />explanation may be that a person whose diet<br />consists largely of high-fiber foods simply eats less<br />of the foods high in cholesterol and fats, and this is<br />the reason for that person’s lower blood levels of<br />fats and cholesterol.<br />Should people try to make great changes in their<br />diets? Probably not, not if they are careful to limit<br />fat intake and to include significant quantities of<br />vegetables and fruits. Besides the possible benefits<br />of fiber, unprocessed plant foods provide important<br />amounts of vitamins and minerals.<br />Fiber is a term we use to refer to the organic materials<br />in the cell walls of plants. These are mainly cellulose<br />and pectins. The role of dietary fiber and<br />possible benefits that a high-fiber diet may provide<br />are currently the focus of much research. It is<br />important to differentiate what is known from what<br />is, at present, merely speculation.<br />Many studies have shown that populations<br />(large groups of people, especially those of different<br />cultures) who consume high-fiber diets tend to<br />have a lower frequency of certain diseases. These<br />include diverticulitis, colon cancer, coronary artery<br />disease, diabetes, and hypertension. Such diseases<br />are much more common among populations<br />whose diets are low in vegetables, fruits, and whole<br />grains, and high in meat, dairy products, and<br />processed foods. In contrast, a 2005 study showed<br />Rectum<br />Anal canal<br />Anal<br />columns<br />Longitudinal muscle<br />Circular muscle<br />Rectal fold<br />Levator ani<br />muscle<br />Internal anal<br />sphincter<br />External anal<br />sphincter<br />Anus<br />A<br />B<br />Figure 16–10. (A) Internal and external anal sphincters shown in a frontal section<br />through the lower rectum and anal canal. (B) Position of rectum and anal canal relative to<br />pelvic bone.<br />QUESTION: The internal anal sphincter is a continuation of which part of the rectum?<br />The liver also changes other monosaccharides to<br />glucose. Fructose and galactose, for example, are<br />end products of the digestion of sucrose and lactose.<br />Because most cells, however, cannot readily<br />use fructose and galactose as energy sources, they<br />are converted by the liver to glucose, which is easily<br />used by cells.<br />2. Amino acid metabolism—The liver regulates<br />blood levels of amino acids based on tissue needs<br />for protein synthesis. Of the 20 different amino<br />acids needed for the production of human proteins,<br />the liver is able to synthesize 12, called the nonessential<br />amino acids. The chemical process by<br />which this is done is called transamination, the<br />transfer of an amino group (NH2) from an amino<br />acid present in excess to a free carbon chain that<br />forms a complete, new amino acid molecule. The<br />other eight amino acids, which the liver cannot<br />synthesize, are called the essential amino acids. In<br />this case, “essential” means that the amino acids<br />must be supplied by our food, because the liver<br />cannot manufacture them. Similarly, “non-essential”<br />means that the amino acids do not have to be<br />supplied in our food because the liver can make<br />them. All 20 amino acids are required in order to<br />make our body proteins.<br />Excess amino acids, those not needed right away<br />for protein synthesis, cannot be stored. However,<br />they do serve another useful purpose. By the<br />process of deamination, which also occurs in the<br />liver, the NH2 group is removed from an amino<br />acid, and the remaining carbon chain may be converted<br />to a simple carbohydrate molecule or to fat.<br />Thus, excess amino acids are utilized for energy<br />production: either for immediate energy or for the<br />potential energy stored as fat in adipose tissue. The<br />NH2 groups that were detached from the original<br />amino acids are combined to form urea, a waste<br />product that will be removed from the blood by the<br />kidneys and excreted in urine.<br />3. Lipid metabolism—The liver forms lipoproteins,<br />which as their name tells us, are molecules of lipids<br />and proteins, for the transport of fats in the blood<br />to other tissues. The liver also synthesizes cholesterol<br />and excretes excess cholesterol into bile to be<br />eliminated in feces.<br />Fatty acids are a potential source of energy, but<br />in order to be used in cell respiration they must be<br />broken down to smaller molecules. In the process<br />of beta-oxidation, the long carbon chains of fatty<br />acids are split into two-carbon molecules called<br />acetyl groups, which are simple carbohydrates.<br />These acetyl groups may be used by the liver cells<br />to produce ATP or may be combined to form<br />ketones to be transported in the blood to other<br />cells. These other cells then use the ketones to produce<br />ATP in cell respiration.<br />4. Synthesis of plasma proteins—This is a liver<br />function that you will probably remember from<br />Chapter 11. The liver synthesizes many of the proteins<br />that circulate in the blood. Albumin, the most<br />abundant plasma protein, helps maintain blood volume<br />by pulling tissue fluid into capillaries.<br />The clotting factors are also produced by the<br />liver. These, as you recall, include prothrombin,<br />fibrinogen, and Factor 8, which circulate in the<br />blood until needed in the chemical clotting mechanism.<br />The liver also synthesizes alpha and beta<br />globulins, which are proteins that serve as carriers<br />for other molecules, such as fats, in the blood.<br />5. Formation of bilirubin—This is another familiar<br />function: The liver contains fixed macrophages<br />that phagocytize old red blood cells (RBCs).<br />Bilirubin is then formed from the heme portion of<br />the hemoglobin. The liver also removes from the<br />blood the bilirubin formed in the spleen and red<br />bone marrow and excretes it into bile to be eliminated<br />in feces.<br />6. Phagocytosis by Kupffer cells—The fixed<br />macrophages of the liver are called Kupffer cells<br />(or stellate reticuloendothelial cells). Besides<br />destroying old RBCs, Kupffer cells phagocytize<br />pathogens or other foreign material that circulate<br />through the liver. Many of the bacteria that get to<br />the liver come from the colon. These bacteria are<br />part of the normal flora of the colon but would be<br />very harmful elsewhere in the body. The bacteria<br />that enter the blood with the water absorbed by the<br />colon are carried to the liver by way of portal circulation.<br />The Kupffer cells in the liver phagocytize<br />and destroy these bacteria, removing them from<br />the blood before the blood returns to the heart.<br />7. Storage—The liver stores the fat-soluble vitamins<br />A, D, E, and K, and the water-soluble vitamin B12.<br />Up to a 6- to 12-month supply of vitamins A and D<br />may be stored, and beef or chicken liver is an excellent<br />dietary source of these vitamins.<br />Also stored by the liver are the minerals iron and<br />copper. You already know that iron is needed for<br />hemoglobin and myoglobin and enables these pro-<br />388 The Digestive System<br />teins to bond to oxygen. Copper (as well as iron) is<br />part of some of the proteins needed for cell respiration,<br />and is part of some of the enzymes necessary<br />for hemoglobin synthesis.<br />8. Detoxification—The liver is capable of synthesizing<br />enzymes that will detoxify harmful substances,<br />that is, change them to less harmful ones. Alcohol,<br />for example, is changed to acetate, which is a twocarbon<br />molecule (an acetyl group) that can be used<br />in cell respiration.<br />Medications are all potentially toxic, but the<br />liver produces enzymes that break them down or<br />change them. When given in a proper dosage, a<br />medication exerts its therapeutic effect but is then<br />changed to less active substances that are usually<br />excreted by the kidneys. An overdose of a drug<br />means that there is too much of it for the liver to<br />detoxify in a given time, and the drug will remain<br />in the body with possibly harmful effects. This is<br />why alcohol should never be consumed when taking<br />medication. Such a combination may cause the<br />liver’s detoxification ability to be overworked and<br />ineffective, with the result that both the alcohol<br />and the medication will remain toxic for a longer<br />time. Barbiturates taken as sleeping pills after consumption<br />of alcohol have too often proved fatal for<br />just this reason.<br />Ammonia is a toxic substance produced by the<br />bacteria in the colon. Because it is soluble in water,<br />some ammonia is absorbed into the blood, but it is<br />carried first to the liver by portal circulation. The<br />liver converts ammonia to urea, a less toxic substance,<br />before the ammonia can circulate and damage<br />other organs, especially the brain. The urea<br />formed is excreted by the kidneys (see Box 16–6:<br />Hepatitis).<br />AGING AND THE<br />DIGESTIVE SYSTEM<br />Many changes can be expected in the aging digestive<br />system. The sense of taste becomes less acute, less<br />saliva is produced, and there is greater likelihood of<br />The Digestive System 389<br />BOX 16–6 HEPATITIS<br />include blood and semen. Hepatitis B may be<br />severe or even fatal, and approximately 10% of<br />those who recover become carriers of the virus.<br />Possible consequences of the carrier state are<br />chronic hepatitis progressing to cirrhosis or primary<br />liver cancer. Of equal importance, carriers are<br />sources of the virus for others, especially their sexual<br />partners.<br />A vaccine is available for hepatitis B, and healthcare<br />workers who have contact with blood, even<br />just occasional contact, should receive it. Other<br />potential recipients of the vaccine are the sexual<br />partners of carriers. Pediatricians now consider this<br />vaccine one of the standard ones for infants.<br />The hepatitis C virus is also present in body fluids<br />and is spread by blood or mucous membrane<br />contact. Most people develop chronic disease, but<br />many may remain asymptomatic for years after<br />being infected. With active disease the virus may<br />cause liver failure. The only therapy then is a liver<br />transplant.<br />It is important for healthcare personnel, and<br />their patients, to know that these types of hepatitis<br />are not spread by blood transfusions. Donated<br />blood is tested for all three viruses.<br />Hepatitis is inflammation of the liver caused<br />by any of several viruses. The most common of<br />these hepatitis viruses have been designated A, B,<br />and C, although there are others. Symptoms of hepatitis<br />include anorexia, nausea, fatigue, and possibly<br />jaundice. Severity of disease ranges from very mild<br />(even asymptomatic) to fatal. Hundreds of thousands<br />of cases of hepatitis occur in the United States<br />every year, and although liver inflammation is common<br />to all of them, the three hepatitis viruses have<br />different modes of transmission and different consequences<br />for affected people.<br />Hepatitis A is an intestinal virus that is spread<br />by the fecal–oral route. Food contaminated by the<br />hands of people with mild cases is the usual vehicle<br />of transmission, although shellfish harvested from<br />water contaminated with human sewage are<br />another possible source of this virus. Hepatitis A is<br />most often mild, recovery provides lifelong immunity,<br />and the carrier state is not known to occur. A<br />vaccine is available, but people who have been<br />exposed to hepatitis A may receive gamma globulin<br />by injection to prevent the disease.<br />Hepatitis B is contracted by exposure to the<br />body fluids of an infected person; these fluids<br />Function of the Digestive System—to break<br />down food into simple chemicals that can<br />be absorbed into the blood and lymph and<br />utilized by cells<br />Divisions of the Digestive System<br />1. Alimentary tube—oral cavity, pharynx, esophagus,<br />stomach, small intestine, large intestine. Digestion<br />takes place in the oral cavity, stomach, and small<br />intestine.<br />2. Accessory organs—salivary glands, teeth, tongue,<br />liver, gallbladder, and pancreas. Each contributes to<br />digestion.<br />Types of Digestion<br />1. Mechanical—breaks food into smaller pieces to<br />increase the surface area for the action of enzymes.<br />2. Chemical—enzymes break down complex organics<br />into simpler organics and inorganics; each enzyme<br />is specific for the food it will digest.<br />End Products of Digestion<br />1. Carbohydrates are digested to monosaccharides.<br />2. Fats are digested to fatty acids and glycerol.<br />3. Proteins are digested to amino acids.<br />4. Other end products are vitamins, minerals, and<br />water.<br />Oral Cavity—food enters by way of the<br />mouth<br />1. Teeth and tongue break up food and mix it with<br />saliva.<br />2. Tooth structure (see Fig. 16–2)—enamel covers the<br />crown and provides a hard chewing surface; dentin<br />is within the enamel and forms the roots; the pulp<br />cavity contains blood vessels and endings of the<br />trigeminal nerve; the periodontal membrane<br />produces cement to anchor the tooth in the jawbone.<br />3. The tongue is skeletal muscle innervated by the<br />hypoglossal nerves. Papillae on the upper surface<br />contain taste buds (facial and glossopharyngeal<br />nerves). Functions: taste, keeps food between the<br />teeth when chewing, elevates to push food backward<br />for swallowing.<br />4. Salivary glands—parotid, submandibular, and sublingual<br />(see Fig. 16–3); ducts take saliva to the oral<br />cavity.<br />390 The Digestive System<br />STUDY OUTLINE<br />periodontal disease and loss of teeth. Secretions are<br />reduced throughout the digestive system, and the<br />effectiveness of peristalsis diminishes. Indigestion may<br />become more frequent, especially if the LES loses its<br />tone, and there is a greater chance of esophageal damage.<br />In the colon, diverticula may form; these are bubble-<br />like outpouchings of the weakened wall of the<br />colon that may be asymptomatic or become infected.<br />Intestinal obstruction, of the large or small bowel,<br />occurs with greater frequency among the elderly.<br />Sluggish peristalsis contributes to constipation, which<br />in turn may contribute to the formation of hemorrhoids.<br />The risk of oral cancer or colon cancer also<br />increases with age.<br />The liver usually continues to function adequately<br />even well into old age, unless damaged by pathogens<br />such as the hepatitis viruses or by toxins such as alcohol.<br />There is a greater tendency for gallstones to form,<br />perhaps necessitating removal of the gallbladder.<br />Inflammation of the gallbladder (cholecystitis) is also<br />more frequent in older adults. In the absence of specific<br />diseases, the pancreas usually functions well,<br />although acute pancreatitis of unknown cause is somewhat<br />more likely in elderly people.<br />SUMMARY<br />The processes of the digestion of food and the absorption<br />of nutrients enable the body to use complex food<br />molecules for many purposes. Much of the food we eat<br />literally becomes part of us. The body synthesizes proteins<br />and lipids for the growth and repair of tissues and<br />produces enzymes to catalyze all of the reactions that<br />contribute to homeostasis. Some of our food provides<br />the energy required for growth, repair, movement,<br />sensation, and thinking. In the next chapter we will<br />discuss the chemical basis of energy production from<br />food and consider the relationship of energy production<br />to the maintenance of body temperature.<br />5. Saliva—amylase digests starch to maltose; water<br />dissolves food for tasting and moistens food for<br />swallowing; lysozyme inhibits the growth of bacteria<br />(see Tables 16–1 and 16–2).<br />Pharynx—food passageway from the oral<br />cavity to the esophagus<br />1. No digestion takes place.<br />2. Contraction of pharyngeal muscles is part of swallowing<br />reflex, regulated by the medulla.<br />Esophagus—food passageway from pharynx<br />to stomach<br />1. No digestion takes place.<br />2. Lower esophageal sphincter (LES) at junction with<br />stomach prevents backup of stomach contents.<br />Structural Layers of the Alimentary Tube<br />(see Fig. 16–4)<br />1. Mucosa (lining)—made of epithelial tissue that<br />produces the digestive secretions; lymph nodules<br />contain macrophages to phagocytize pathogens<br />that penetrate the mucosa; thin layer of smooth<br />muscle to ripple the epithelium.<br />2. Submucosa—areolar connective tissue with blood<br />vessels and lymphatic vessels; Meissner’s plexus is a<br />nerve network that innervates the mucosa, part of<br />the enteric nervous system that extends the entire<br />length of the alimentary tube.<br />3. External muscle layer—typically an inner circular<br />layer and an outer longitudinal layer of smooth<br />muscle; function is mechanical digestion and peristalsis;<br />innervated by Auerbach’s plexus, part of the<br />enteric nervous system; sympathetic impulses<br />decrease motility; parasympathetic impulses<br />increase motility.<br />4. Serosa—outermost layer; above the diaphragm is<br />fibrous connective tissue; below the diaphragm is<br />the mesentery (serous). The peritoneum (serous)<br />lines the abdominal cavity; serous fluid prevents<br />friction between the serous layers.<br />Stomach—in upper left abdominal quadrant;<br />a muscular sac that extends from the esophagus<br />to the small intestine (see Fig. 16–5)<br />1. Reservoir for food; begins the digestion of protein.<br />2. Gastric juice is secreted by gastric pits (see Tables<br />16–1 and 16–2).<br />3. The pyloric sphincter at the junction with the duodenum<br />prevents backup of intestinal contents.<br />Liver—consists of two lobes in the upper<br />right and center of the abdominal cavity<br />(see Figs. 16–1 and 16–6)<br />1. Functional unit is the hexagonal liver lobule: liver<br />cells, sinusoids, branches of the hepatic artery and<br />portal vein, and bile ducts.<br />2. The only digestive secretion is bile; the hepatic<br />duct takes bile out of the liver and unites with the<br />cystic duct of the gallbladder to form the common<br />bile duct to the duodenum.<br />3. Bile salts emulsify fats, a type of mechanical digestion<br />(see Table 16–1).<br />4. Excess cholesterol and bilirubin are excreted by the<br />liver into bile.<br />Gallbladder—on undersurface of right lobe<br />of liver (see Fig. 16–6)<br />1. Stores and concentrates bile until needed in the<br />duodenum (see Table 16–2).<br />2. The cystic duct joins the hepatic duct to form the<br />common bile duct.<br />Pancreas—in upper left abdominal quadrant<br />between the duodenum and the spleen (see<br />Fig. 16–1)<br />1. Pancreatic juice is secreted by acini, carried by pancreatic<br />duct to the common bile duct to the duodenum<br />(see Fig. 16–7).<br />2. Enzyme pancreatic juice contains enzymes for the<br />digestion of all three food types (see Tables 16–1<br />and 16–2).<br />3. Bicarbonate pancreatic juice neutralizes HCl from<br />the stomach in the duodenum.<br />Small Intestine—coiled within the center of<br />the abdominal cavity (see Fig. 16–1); extends<br />from stomach to colon<br />1. Duodenum—first 10 inches; the common bile duct<br />brings in bile and pancreatic juice. Jejunum (8 feet)<br />and ileum (11 feet).<br />2. Enzymes secreted by the intestinal glands complete<br />digestion (see Tables 16–1 and 16–2). Surface area<br />for absorption is increased by plica circulares, villi,<br />and microvilli (see Fig. 16–8); microvilli are the<br />brush border.<br />3. The villi contain capillary networks for the absorption<br />of water-soluble nutrients: monosaccharides,<br />The Digestive System 391<br />amino acids, vitamin C and the B vitamins, minerals,<br />and water. Blood from the small intestine goes<br />to the liver first by way of portal circulation.<br />4. The villi contain lacteals (lymph capillaries) for the<br />absorption of fat-soluble nutrients: vitamins A, D,<br />E, and K, fatty acids, and glycerol, which are combined<br />to form chylomicrons. Lymph from the small<br />intestine is carried back to the blood in the left subclavian<br />vein.<br />Large Intestine (colon)—extends from the<br />small intestine to the anus<br />1. Colon—parts (see Fig. 16–9): cecum, ascending<br />colon, transverse colon, descending colon, sigmoid<br />colon, rectum, anal canal.<br />2. Ileocecal valve—at the junction of the cecum and<br />ileum; prevents backup of fecal material into the<br />small intestine.<br />3. Colon—functions: absorption of water, minerals,<br />vitamins; elimination of undigestible material.<br />4. Normal flora—the bacteria of the colon; produce<br />vitamins, especially vitamin K, and inhibit the<br />growth of pathogens.<br />5. Defecation reflex—stimulus: stretching of the rectum<br />when peristalsis propels feces into it. Sensory<br />impulses go to the sacral spinal cord, and motor<br />impulses return to the smooth muscle of the rectum,<br />which contracts. The internal anal sphincter<br />relaxes to permit defecation. Voluntary control is<br />provided by the external anal sphincter, made of<br />skeletal muscle (see Fig. 16–10).<br />Liver—other functions<br />1. Carbohydrate metabolism—excess glucose is<br />stored in the form of glycogen and converted back<br />to glucose during hypoglycemia; fructose and<br />galactose are changed to glucose.<br />2. Amino acid metabolism—the non-essential amino<br />acids are synthesized by transamination; excess<br />amino acids are changed to carbohydrates or fats by<br />deamination; the amino groups are converted to<br />urea and excreted by the kidneys.<br />3. Lipid metabolism—formation of lipoproteins for<br />transport of fats in the blood; synthesis of cholesterol;<br />excretion of excess cholesterol into bile; betaoxidation<br />of fatty acids to form two-carbon acetyl<br />groups for energy use.<br />4. Synthesis of plasma proteins—albumin to help<br />maintain blood volume; clotting factors for blood<br />clotting; alpha and beta globulins as carrier molecules.<br />5. Formation of bilirubin—old RBCs are phagocytized,<br />and bilirubin is formed from the heme and<br />put into bile to be eliminated in feces.<br />6. Phagocytosis by Kupffer cells—fixed macrophages;<br />phagocytize old RBCs and bacteria, especially bacteria<br />absorbed by the colon.<br />7. Storage—vitamins: B12, A, D, E, and K, and the<br />minerals iron and copper.<br />8. Detoxification—liver enzymes change potential<br />poisons to less harmful substances; examples of<br />toxic substances are alcohol, medications, and<br />ammonia absorbed by the colon.<br />392 The Digestive System<br />REVIEW QUESTIONS<br />1. Name the organs of the alimentary tube, and<br />describe the location of each. Name the accessory<br />digestive organs, and describe the location of each.<br />(pp. 370, 372, 373, 376, 378, 379, 385)<br />2. Explain the purpose of mechanical digestion, and<br />give two examples. Explain the purpose of chemical<br />digestion, and give two examples. (pp. 370, 374)<br />3. Name the end products of digestion, and explain<br />how each is absorbed in the small intestine.<br />(pp. 370, 384)<br />4. Explain the function of teeth and tongue, salivary<br />amylase, enamel of teeth, lysozyme, and water of<br />saliva. (pp. 370–372)<br />5. Describe the function of the pharynx, esophagus,<br />and lower esophageal sphincter. (p. 373)<br />6. Name and describe the four layers of the alimentary<br />tube. (pp. 373, 376)<br />7. State the two general functions of the stomach and<br />the function of the pyloric sphincter. Explain the<br />function of pepsin, HCl, and mucus. (pp. 376–378)<br />8. Describe the general functions of the small intestine,<br />and name the three parts. Describe the structures<br />that increase the surface area of the small<br />intestine. (pp. 378, 383–384)<br />9. Explain how the liver, gallbladder, and pancreas<br />contribute to digestion. (pp. 379, 381)<br />1. Many people with GERD take proton-pump<br />inhibitors, medications that reduce stomach acid.<br />Why should these people be especially careful<br />about what they eat or drink?<br />2. The colon does not have villi as part of its mucosa.<br />Explain why villi are not necessary.<br />3. Food remains in the stomach for several hours.<br />Passage of food through the small intestine also<br />requires several hours. These two organs have very<br />different shapes. Explain why they are able to<br />retain food for so long, for efficient digestion and<br />absorption.<br />4. Diarrhea can be unpleasant, but does have a<br />purpose. Explain, and state the disadvantages as<br />well.<br />5. Explain how a spinal cord transection at the level of<br />T10 will affect the defecation reflex.<br />6. You have seen the word enteric (or entero) several<br />times in this chapter. What does it mean? Define<br />each of these: enteric bacilli, enterovirus, Enterococcus.<br />7. The word symbiosis indicates two different kinds of<br />living things, and literally means “together-life.”<br />Our own alimentary tube is a perfect example.<br />Explain, and state the advantages to each living<br />thing.<br />The Digestive System 393<br />FOR FURTHER THOUGHT<br />10. Describe the internal structure of a villus, and<br />explain how its structure is related to absorption.<br />(p. 384)<br />11. Name the parts of the large intestine, and<br />describe the function of the ileocecal valve.<br />(p. 385)<br />12. Describe the functions of the colon and of the<br />normal flora of the colon. (pp. 385–386)<br />13. With respect to the defecation reflex, explain the<br />stimulus, the part of the CNS directly involved,<br />the effector muscle, the function of the internal<br />anal sphincter, and the voluntary control possible.<br />(p. 386)<br />14. Name the vitamins and minerals stored in the<br />liver. Name the fixed macrophages of the liver,<br />and explain their function. (p. 388)<br />15. Describe how the liver regulates blood glucose<br />level. Explain the purpose of the processes of<br />deamination and transamination. (pp. 386, 388)<br />16. Name the plasma proteins produced by the liver,<br />and state the function of each. (p. 388)<br />17. Name the substances excreted by the liver into<br />bile. (p. 388)<br />394<br />CHAPTER 17<br />Chapter Outline<br />Body Temperature<br />Heat Production<br />Heat Loss<br />Heat loss through the skin<br />Heat loss through the respiratory tract<br />Heat loss through the urinary and digestive tracts<br />Regulation of Body Temperature<br />Mechanisms to increase heat loss<br />Mechanisms to conserve heat<br />Fever<br />Metabolism<br />Cell Respiration<br />Glycolysis<br />Krebs citric acid cycle<br />Cytochrome transport system<br />Proteins and fats as energy sources<br />Energy available from the three nutrient types<br />Synthesis Uses of Foods<br />Glucose<br />Amino acids<br />Fatty acids and glycerol<br />Vitamins and Minerals<br />Metabolic Rate<br />Aging and Metabolism<br />BOX 17–1 HEAT-RELATED DISORDERS<br />BOX 17–2 COLD-RELATED DISORDERS<br />BOX 17–3 KETOSIS<br />BOX 17–4 METABOLIC RATE<br />BOX 17–5 WEIGHT LOSS<br />BOX 17–6 LEPTIN AND BODY-MASS INDEX<br />Student Objectives<br />• State the normal range of human body temperature.<br />• Explain how cell respiration produces heat and the<br />factors that affect heat production.<br />• Describe the pathways of heat loss through the<br />skin and respiratory tract.<br />• Explain why the hypothalamus is called the “thermostat”<br />of the body.<br />• Describe the mechanisms to increase heat loss.<br />• Describe the mechanisms to conserve heat.<br />• Explain how a fever is caused and its advantages<br />and disadvantages.<br />• Define metabolism, anabolism, and catabolism.<br />• Describe what happens to a glucose molecule during<br />the three stages of cell respiration.<br />• State what happens to each of the products of cell<br />respiration.<br />• Explain how amino acids and fats may be used for<br />energy production.<br />• Describe the synthesis uses for glucose, amino<br />acids, and fats.<br />• Explain what is meant by metabolic rate and kilocalories.<br />• Describe the factors that affect a person’s metabolic<br />rate.<br />Body Temperature<br />and Metabolism<br />395<br />New Terminology<br />Anabolism (an-AB-uh-lizm)<br />Catabolism (kuh-TAB-uh-lizm)<br />Coenzyme (ko-EN-zime)<br />Conduction (kon-DUK-shun)<br />Convection (kon-VEK-shun)<br />Cytochromes (SIGH-toh-krohms)<br />Endogenous pyrogen (en-DOJ-en-us PYE-roh-jen)<br />Fever (FEE-ver)<br />Glycolysis (gly-KAHL-ah-sis)<br />Kilocalorie (KILL-oh-KAL-oh-ree)<br />Krebs cycle (KREBS SIGH-kuhl)<br />Pyrogen (PYE-roh-jen)<br />Radiation (RAY-dee-AY-shun)<br />Vitamins (VY-tah-mins)<br />Related Clinical Terminology<br />Antipyretic (AN-tigh-pye-RET-ik)<br />Basal metabolic rate (BAY-zuhl met-ah-BAHL-ik<br />RAYT)<br />Frostbite (FRAWST-bite)<br />Heat exhaustion (HEET eks-ZAWS-chun)<br />Heat stroke (HEET STROHK)<br />Hypothermia (HIGH-poh-THER-mee-ah)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />During every moment of our lives, our cells are<br />breaking down food molecules to obtain ATP (adenosine<br />triphosphate) for energy-requiring cellular<br />processes. Naturally, we are not aware of the process<br />of cell respiration, but we may be aware of one of the<br />products—energy in the form of heat. The human<br />body is indeed warm, and its temperature is regulated<br />very precisely. Though we cannot stand barefoot on<br />the ice of Antarctica for months in winter, as penguins<br />do, we can adapt to and survive a wide range of environmental<br />temperatures.<br />This chapter discusses the regulation of body temperature<br />and also discusses metabolism, which is the<br />total of all reactions that take place within the body.<br />These reactions include the energy-releasing ones of<br />cell respiration and energy-requiring ones such as<br />protein synthesis, or DNA synthesis for mitosis. As<br />you will see, body temperature and metabolism are<br />inseparable.<br />BODY TEMPERATURE<br />The normal range of human body temperature is<br />96.5° to 99.5°F (36° to 38°C), with an average oral<br />temperature of 98.6°F (37°C). (A 1992 study suggested<br />a slightly lower average oral temperature: 98.2°<br />or 36.8°. But everyone seems to prefer the “traditional”<br />average temperature.) Within a 24-hour<br />period, an individual’s temperature fluctuates 1° to 2°,<br />with the lowest temperatures occurring during sleep.<br />At either end of the age spectrum, however, temperature<br />regulation may not be as precise as it is in<br />older children or younger adults. Infants have more<br />surface area (skin) relative to volume and are likely to<br />lose heat more rapidly. In the elderly, the mechanisms<br />that maintain body temperature may not function as<br />efficiently as they once did, and changes in environmental<br />temperature may not be compensated for as<br />quickly or effectively. This is especially important to<br />remember when caring for patients who are very<br />young or very old.<br />HEAT PRODUCTION<br />Cell respiration, the process that releases energy from<br />food to produce ATP, also produces heat as one of its<br />energy products. Although cell respiration takes place<br />constantly, many factors influence the rate of this<br />process:<br />1. The hormone thyroxine (and T3), produced by the<br />thyroid gland, increases the rate of cell respiration<br />and heat production. The secretion of thyroxine is<br />regulated by the body’s rate of energy production,<br />the metabolic rate itself. (See Chapter 10 for a discussion<br />of the feedback mechanism involving the<br />hypothalamus and anterior pituitary gland and<br />Chapter 1 for an illustration.) When the metabolic<br />rate decreases, the thyroid gland is stimulated to<br />secrete more thyroxine. As thyroxine increases the<br />rate of cell respiration, a negative feedback mechanism<br />inhibits further secretion until metabolic rate<br />decreases again. Thus, thyroxine is secreted whenever<br />there is a need for increased cell respiration<br />and is probably the most important regulator of<br />day-to-day energy production.<br />2. In stress situations, epinephrine and norepinephrine<br />are secreted by the adrenal medulla, and the<br />sympathetic nervous system becomes more active.<br />Epinephrine increases the rate of cell respiration,<br />especially in organs such as the heart, skeletal muscles,<br />and liver. Sympathetic stimulation also increases<br />the activity of these organs. The increased<br />production of ATP to meet the demands of the<br />stress situation also means that more heat will be<br />produced.<br />3. Organs that are normally active (producing<br />ATP) are significant sources of heat when the body<br />is at rest. The skeletal muscles, for example, are<br />usually in a state of slight contraction called muscle<br />tone. Because even slight contraction requires ATP,<br />the muscles are also producing heat. This amounts<br />to about 25% of the total body heat at rest and<br />much more during exercise, when more ATP is<br />produced.<br />The liver is another organ that is continually<br />active, producing ATP to supply energy for its<br />many functions. As a result, the liver produces as<br />much as 20% of the total body heat at rest. The<br />heat produced by these active organs is dispersed<br />throughout the body by the blood. As the relatively<br />cooler blood flows through organs such as the muscles<br />and liver, the heat they produce is transferred<br />to the blood, warming it. The warmed blood circulates<br />to other areas of the body, distributing this<br />heat.<br />4. The intake of food also increases heat production,<br />because the metabolic activity of the digestive tract<br />is increased. Heat is generated as the digestive<br />396 Body Temperature and Metabolism<br />organs produce ATP for peristalsis and for the synthesis<br />of digestive enzymes.<br />5. Changes in body temperature also have an effect<br />on metabolic rate and heat production. This<br />becomes clinically important when a person has<br />a fever, an abnormally high body temperature.<br />The higher temperature increases the metabolic<br />rate, which increases heat production and<br />elevates body temperature further. Thus, a high<br />fever may trigger a vicious cycle of ever-increasing<br />heat production. Fever is discussed later in this<br />chapter.<br />The factors that affect heat production are summarized<br />in Table 17–1.<br />HEAT LOSS<br />The pathways of heat loss from the body are the skin,<br />the respiratory tract, and, to a lesser extent, the urinary<br />and digestive tracts.<br />Heat Loss through the Skin<br />Because the skin covers the body, most body heat is<br />lost from the skin to the environment. When the environment<br />is cooler than body temperature (as it usually<br />is), heat loss is unavoidable. The amount of heat that<br />is lost is determined by blood flow through the skin<br />and by the activity of sweat glands.<br />Blood flow through the skin influences the amount<br />of heat lost by the processes of radiation, conduction,<br />and convection. Radiation means that heat from the<br />body is transferred to cooler objects not touching the<br />skin, much as a radiator warms the contents of a room<br />(radiation starts to become less effective when the<br />environmental temperature rises above 88°F). Conduction<br />is the loss of heat to cooler air or objects, such<br />as clothing, that touch the skin. Convection means<br />that air currents move the warmer air away from the<br />skin surface and facilitate the loss of heat; this is why a<br />fan makes us feel cooler on hot days. Loss of heat by<br />convection also gives us the “wind chill factor” we<br />hear about in winter. A cold day that is windy will feel<br />colder than a cold day when the air is still, because the<br />wind blows the slightly warmer air surrounding the<br />body away, replacing it with colder air.<br />As you may recall from Chapter 5, the temperature<br />of the skin and the subsequent loss of heat are<br />determined by blood flow through the skin. The arterioles<br />in the dermis may constrict or dilate to decrease<br />or increase blood flow. In a cold environment, vasoconstriction<br />decreases blood flow through the dermis<br />and thereby decreases heat loss. In a warm environment,<br />vasodilation in the dermis increases blood flow<br />to the body surface and loss of heat to the environment.<br />The other mechanism by which heat is lost from<br />the skin is sweating. The eccrine sweat glands<br />secrete sweat (water) onto the skin surface, and excess<br />body heat evaporates the sweat. Think of running<br />water into a hot frying pan; the pan is rapidly cooled<br />as its heat vaporizes the water. Although sweating is<br />not quite as dramatic (no visible formation of steam),<br />the principle is just the same.<br />Sweating is most efficient when the humidity of the<br />surrounding air is low. Humidity is the percentage of<br />the maximum amount of water vapor the atmosphere<br />can contain. A humidity reading of 90% means that<br />the air is already 90% saturated with water vapor and<br />can hold little more. In such a situation, sweat does<br />not readily evaporate, but instead remains on the skin<br />even as more sweat is secreted. If the humidity is 40%,<br />Body Temperature and Metabolism 397<br />Table 17–1 FACTORS THAT AFFECT<br />HEAT PRODUCTION<br />Factor Effect<br />Thyroxine<br />Epinephrine and<br />sympathetic<br />stimulation<br />Skeletal muscles<br />Liver<br />Food intake<br />Higher body<br />temperature<br />• The most important regulator<br />of day-to-day metabolism;<br />increases use of foods for ATP<br />production, thereby increasing<br />heat production<br />• Important in stress situations;<br />increases the metabolic activity<br />of many organs; increases ATP<br />and heat production<br />• Normal muscle tone requires<br />ATP; the heat produced is<br />about 25% of the total body<br />heat at rest<br />• Always metabolically active;<br />produces as much as 20% of<br />total body heat at rest<br />• Increases activity of the GI<br />tract; increases ATP and heat<br />production<br />• Increases metabolic rate, which<br />increases heat production,<br />which further increases metabolic<br />rate and heat production;<br />may become detrimental during<br />high fevers<br />however, the air can hold a great deal more water<br />vapor, and sweat evaporates quickly from the skin surface,<br />removing excess body heat. In air that is completely<br />dry, a person may tolerate a temperature of<br />200°F for nearly 1 hour.<br />Although sweating is a very effective mechanism of<br />heat loss, it does have a disadvantage in that it requires<br />the loss of water in order to also lose heat. Water loss<br />during sweating may rapidly lead to dehydration, and<br />the water lost must be replaced by drinking fluids (see<br />Box 17–1: Heat-Related Disorders).<br />Small amounts of heat are also lost in what is called<br />“insensible water loss.” Because the skin is not like a<br />plastic bag, but is somewhat permeable to water, a<br />small amount of water diffuses through the skin and is<br />evaporated by body heat. Compared to sweating, however,<br />insensible water loss is a minor source of heat<br />loss.<br />Heat Loss through<br />the Respiratory Tract<br />Heat is lost from the respiratory tract as the warmth of<br />the respiratory mucosa evaporates some water from<br />the living epithelial surface. The water vapor formed<br />is exhaled, and a small amount of heat is lost.<br />Animals such as dogs that do not have numerous<br />sweat glands often pant in warm weather. Panting is<br />the rapid movement of air into and out of the upper<br />respiratory passages, where the warm surfaces evaporate<br />large amounts of water. In this way the animal<br />may lose large amounts of heat.<br />Heat Loss through the Urinary<br />and Digestive Tracts<br />When excreted, urine and feces are at body temperature,<br />and their elimination results in a very small<br />amount of heat loss.<br />The pathways of heat loss are summarized in Table<br />17–2.<br />REGULATION OF BODY TEMPERATURE<br />The hypothalamus is responsible for the regulation<br />of body temperature and is considered the “thermostat”<br />of the body. As the thermostat, the hypothalamus<br />maintains the “setting” of body temperature by balancing<br />heat production and heat loss to keep the body<br />at the set temperature.<br />To do this, the hypothalamus must receive information<br />about the temperature within the body and<br />about the environmental temperature. Specialized<br />neurons of the hypothalamus detect changes in the<br />temperature of the blood that flows through the brain.<br />The temperature receptors in the skin provide information<br />about the external temperature changes to<br />which the body is exposed. The hypothalamus then<br />integrates this sensory information and promotes the<br />necessary responses to maintain body temperature<br />within the normal range.<br />Mechanisms to Increase Heat Loss<br />In a warm environment or during exercise, the body<br />temperature tends to rise, and greater heat loss is<br />398 Body Temperature and Metabolism<br />BOX 17–1 HEAT-RELATED DISORDERS<br />of heat loss, but in high heat the sweating process<br />continues. As fluid loss increases, sweating stops to<br />preserve body fluid, and body temperature rises<br />rapidly (over 105°F , possibly as high as 110°F ).<br />The classic symptom of heat stroke is hot, dry<br />skin. The affected person often loses consciousness,<br />reflecting the destructive effect of such a high body<br />temperature on the brain. Treatment should involve<br />hospitalization so that IV fluids may be administered<br />and body temperature lowered under medical<br />supervision. A first-aid measure would be the<br />application of cool (not ice cold) water to as much<br />of the skin as possible. Fluids should never be forced<br />on an unconscious person, because the fluid may<br />be aspirated into the respiratory tract.<br />Heat exhaustion is caused by excessive sweating<br />with loss of water and salts, especially NaCl. The<br />affected person feels very weak, and the skin is usually<br />cool and clammy (moist). Body temperature is<br />normal or slightly below normal, the pulse is often<br />rapid and weak, and blood pressure may be low<br />because of fluid loss. Other symptoms may include<br />dizziness, vomiting, and muscle cramps. Treatment<br />involves rest and consumption of salty fluids or fruit<br />juices (in small amounts at frequent intervals).<br />Heat stroke is a life-threatening condition that<br />may affect elderly or chronically ill people on hot,<br />humid days, or otherwise healthy people who exercise<br />too strenuously during such weather. High<br />humidity makes sweating an ineffective mechanism<br />needed. This is accomplished by vasodilation in the<br />dermis and an increase in sweating. Vasodilation<br />brings more warm blood close to the body surface,<br />and heat is lost to the environment. However, if the<br />environmental temperature is close to or higher than<br />body temperature, this mechanism becomes ineffective.<br />The second mechanism is increased sweating, in<br />which excess body heat evaporates the sweat on the<br />skin surface. As mentioned previously, sweating<br />becomes inefficient when the atmospheric humidity is<br />high.<br />On hot days, heat production may also be<br />decreased by a decrease in muscle tone. This is why we<br />may feel very sluggish on hot days; our muscles are<br />even slightly less contracted than usual and are slower<br />to respond.<br />Mechanisms to Conserve Heat<br />In a cold environment, heat loss from the body is<br />unavoidable but may be reduced to some extent.<br />Vasoconstriction in the dermis shunts blood away<br />from the body surface, so that more heat is kept in the<br />core of the body. Sweating decreases, and will stop<br />completely if the temperature of the hypothalamus<br />falls below about 98.6°F. (Remember that the internal<br />temperature of the brain is higher than an oral temperature,<br />and is less subject to any changes in environmental<br />temperature.)<br />If these mechanisms are not sufficient to prevent<br />the body temperature from dropping, more heat may<br />be produced by increasing muscle tone. When this<br />greater muscle tone becomes noticeable and rhythmic,<br />it is called shivering and may increase heat production<br />by as much as five times the normal.<br />People also have behavioral responses to cold, and<br />these too are important to prevent heat loss. Such<br />things as putting on a sweater or going indoors reflect<br />our awareness of the discomfort of being cold. For<br />people (we do not have thick fur as do some other<br />mammals), these voluntary activities are of critical<br />importance to the prevention of excessive heat loss<br />when it is very cold (see Box 17–2: Cold-Related<br />Disorders).<br />FEVER<br />A fever is an abnormally high body temperature<br />and may accompany infectious diseases, extensive<br />physical trauma, cancer, or damage to the CNS. The<br />substances that may cause a fever are called pyrogens.<br />Pyrogens include bacteria, foreign proteins, and<br />chemicals released during inflammation. These<br />inflammatory chemicals are called endogenous pyrogens.<br />Endogenous means “generated from within.” It<br />is believed that pyrogens chemically affect the hypothalamus<br />and “raise the setting” of the hypothalamic<br />thermostat. The hypothalamus will then stimulate<br />responses by the body to raise body temperature to<br />this higher setting.<br />Let us use as a specific example a child who has a<br />strep throat. The bacterial and endogenous pyrogens<br />reset the hypothalamic thermostat upward, to 102°F.<br />At first, the body is “colder” than the setting of the<br />hypothalamus, and the heat conservation and production<br />mechanisms are activated. The child feels cold<br />and begins to shiver (chills). Eventually, sufficient heat<br />is produced to raise the body temperature to the hypothalamic<br />setting of 102°F. At this time, the child will<br />feel neither too warm nor too cold, because the body<br />temperature is what the hypothalamus wants.<br />As the effects of the pyrogens diminish, the hypothalamic<br />setting decreases, perhaps close to normal<br />again, 99°F. Now the child will feel warm, and the heat<br />loss mechanisms will be activated. Vasodilation in the<br />skin and sweating will occur until the body temperature<br />drops to the new hypothalamic setting. This<br />is sometimes referred to as the “crisis,” but actually<br />the crisis has passed, because sweating indicates that<br />Body Temperature and Metabolism 399<br />Table 17–2 PATHWAYS OF HEAT LOSS<br />Pathway Mechanism<br />Skin (major<br />pathway)<br />Respiratory tract<br />(secondary<br />pathway)<br />Urinary tract<br />(minor pathway)<br />Digestive tract<br />(minor pathway)<br />• Radiation and conduction—<br />heat is lost from the body to<br />cooler air or objects.<br />• Convection—air currents move<br />warm air away from the skin.<br />• Sweating—excess body heat<br />evaporates sweat on the skin<br />surface.<br />• Evaporation—body heat evaporates<br />water from the respiratory<br />mucosa, and water vapor<br />is exhaled.<br />• Urination—urine is at body<br />temperature when eliminated.<br />• Defecation—feces are at body<br />temperature when eliminated.<br />the body temperature is returning to normal. The<br />sequence of temperature changes during a fever is<br />shown in Fig. 17–1.<br />You may be wondering if a fever serves a useful purpose.<br />For low fevers that are the result of infection, the<br />answer is yes. White blood cells increase their activity<br />at moderately elevated temperatures, and the metabolism<br />of some pathogens is inhibited. Thus, a fever may<br />be beneficial in that it may shorten the duration of an<br />infection by accelerating the destruction of the<br />pathogen.<br />High fevers, however, may have serious consequences.<br />A fever increases the metabolic rate, which<br />increases heat production, which in turn raises body<br />temperature even more. This is a positive feedback<br />mechanism that will continue until an external event<br />(such as aspirin or death of the pathogens) acts as a<br />brake (see Fig. 1–3). When the body temperature rises<br />above 106°F, the hypothalamus begins to lose its ability<br />to regulate temperature. The proteins of cells,<br />especially the enzymes, are also damaged by such high<br />temperatures. Enzymes become denatured, that is,<br />lose their shape and do not catalyze the reactions necessary<br />within cells (see Fig. 2–9). As a result, cells<br />begin to die. This is most serious in the brain, because<br />neurons cannot be replaced, and cellular death is the<br />cause of brain damage that may follow a prolonged<br />high fever. The effects of changes in body temperature<br />on the hypothalamus are shown in Fig. 17–2.<br />A medication such as aspirin is called an antipyretic<br />because it lowers a fever, probably by affecting<br />the hypothalamic thermostat. To help lower a high<br />fever, the body may be cooled by sponging it with cool<br />water. The excessive body heat will cause this external<br />water to evaporate, thus reducing temperature. A very<br />high fever requires medical attention.<br />METABOLISM<br />The term metabolism encompasses all of the reactions<br />that take place in the body. Everything that happens<br />within us is part of our metabolism. The reactions<br />of metabolism may be divided into two major categories:<br />anabolism and catabolism.<br />Anabolism means synthesis or “formation” reactions,<br />the bonding together of smaller molecules to<br />form larger ones. The synthesis of hemoglobin by<br />cells of the red bone marrow, synthesis of glycogen by<br />liver cells, and synthesis of fat to be stored in adipose<br />tissue are all examples of anabolism. Such reactions<br />require energy, usually in the form of ATP.<br />400 Body Temperature and Metabolism<br />BOX 17–2 COLD-RELATED DISORDERS<br />slurred speech, drowsiness, and lack of coordination.<br />At this stage, people often do not realize the<br />seriousness of their condition, and if outdoors (ice<br />skating or skiing) may not seek a warmer environment.<br />In progressive hypothermia, breathing and<br />heart rate slow, and coma and death follow.<br />Other people at greater risk for hypothermia<br />include the elderly, whose temperature-regulating<br />mechanisms are no longer effective, and quadriplegics,<br />who have no sensation of cold in the body. For<br />both of these groups, heat production is or may be<br />low because of inactivity of skeletal muscles.<br />Artificial hypothermia may be induced during<br />some types of cardiovascular or neurologic surgery.<br />This carefully controlled lowering of body temperature<br />decreases the metabolic rate and need for<br />oxygen and makes possible prolonged surgery<br />without causing extensive tissue death in the<br />patient.<br />Frostbite is the freezing of part of the body.<br />Fingers, toes, the nose, and ears are most often<br />affected by prolonged exposure to cold, because<br />these areas have little volume in proportion to their<br />surface.<br />At first the skin tingles, then becomes numb. If<br />body fluids freeze, ice crystals may destroy capillaries<br />and tissues (because water expands when it<br />freezes), and blisters form. In the most severe cases<br />gangrene develops; that is, tissue dies because of<br />lack of oxygen.<br />Treatment of frostbite includes rewarming the<br />affected area. If skin damage is apparent, it should<br />be treated as if it were a burn injury.<br />Hypothermia is an abnormally low body temperature<br />(below 95°F) that is most often the result<br />of prolonged exposure to cold. Although the<br />affected person certainly feels cold at first, this sensation<br />may pass and be replaced by confusion,<br />Catabolism means decomposition, the breaking of<br />bonds of larger molecules to form smaller molecules.<br />Cell respiration is a series of catabolic reactions that<br />break down food molecules to carbon dioxide and<br />water. During catabolism, energy is often released and<br />used to synthesize ATP (the heat energy released was<br />discussed in the previous section). The ATP formed<br />during catabolism is then used for energy-requiring<br />anabolic reactions.<br />Most of our anabolic and catabolic reactions are<br />catalyzed by enzymes. Enzymes are proteins that<br />enable reactions to take place rapidly at body temperature<br />(see Chapter 2 to review the active site theory of<br />enzyme functioning). The body has thousands of<br />enzymes, and each is specific, that is, will catalyze only<br />one type of reaction. As you read the discussions that<br />follow, keep in mind the essential role of enzymes.<br />CELL RESPIRATION<br />You are already familiar with the summary reaction of<br />cell respiration,<br />C6H12O6 O2 → CO2 H2O ATP Heat,<br />(glucose)<br />the purpose of which is to produce ATP. Glucose contains<br />potential energy, and when it is broken down to<br />CO2 and H2O, this energy is released in the forms of<br />ATP and heat. The oxygen that is required comes<br />from breathing, and the CO2 formed is circulated to<br />the lungs to be exhaled. The water formed is called<br />metabolic water, and helps to meet our daily need for<br />water. Energy in the form of heat gives us a body temperature,<br />and the ATP formed is used for energyrequiring<br />reactions. Synthesis of ATP means that<br />Body Temperature and Metabolism 401<br />105˚<br />104˚<br />103˚<br />102˚<br />101˚<br />100˚<br />99˚<br />98˚<br />2 hr 4 hr 6 hr 8 hr 10 hr 12 hr 14 hr 16 hr 18 hr 20 hr 22 hr 24 hr<br />Body temperature<br />Time<br />Hypothalamic<br />thermostat<br />Actual body<br />temperature<br />Pyrogen affects hypothalamus<br />Vasoconstriction, shivering<br />Effect of pyrogen diminishes<br />Sweating, vasodilation<br />Figure 17–1. Changes in body temperature during an episode of fever. The body temperature<br />changes (purple line) lag behind the changes in the hypothalamic thermostat<br />(blue line) but eventually reach whatever the thermostat has called for.<br />QUESTION: In this cycle of fever, why do sweating and vasodilation occur when they do?<br />energy is used to bond a free phosphate molecule to<br />ADP (adenosine diphosphate). ADP and free phosphates<br />are present in cells after ATP has been broken<br />down for energy-requiring processes.<br />The breakdown of glucose summarized here is not<br />quite that simple, however, and involves a complex<br />series of reactions. Glucose is taken apart “piece by<br />piece,” with the removal of hydrogens and the splitting<br />of carbon–carbon bonds. This releases the energy<br />of glucose gradually, so that a significant portion<br />(about 40%) is available to synthesize ATP.<br />Cell respiration of glucose involves three major<br />stages: glycolysis, the Krebs citric acid cycle, and the<br />cytochrome (or electron) transport system. Although<br />402 Body Temperature and Metabolism<br />114˚<br />112˚<br />110˚<br />108˚<br />106˚<br />104˚<br />102˚<br />100˚<br />98˚<br />96˚<br />94˚<br />92˚<br />90˚<br />88˚<br />86˚<br />84˚<br />82˚<br />80˚<br />78˚<br />76˚<br />74˚<br />72˚<br />70˚<br />68˚<br />66˚<br />44˚<br />42˚<br />40˚<br />38˚<br />37˚<br />36˚<br />34˚<br />32˚<br />30˚<br />28˚<br />26˚<br />24˚<br />Upper limit of survival?<br />Body temperature Temperature regulation by<br />the hypothalamus<br />Heat stroke or high fever<br />Strenuous exercise or fever<br />Usual range of normal<br />Hypothermia<br />(cold weather or<br />immersion in<br />cold water)<br />Lower limit of survival?<br />˚F ˚C<br />Temperature regulation is<br />lost<br />Temperature regulation is<br />impaired<br />Temperature regulation is<br />efficient<br />Temperature regulation is<br />seriously impaired<br />Figure 17–2. Effects of changes in body temperature on the temperature-regulating<br />ability of the hypothalamus. Body temperature is shown in degrees Fahrenheit and degrees<br />Celsius.<br />QUESTION: Give a range of temperature that an average person would probably survive.<br />the details of each stage are beyond the scope of this<br />book, we will summarize the most important aspects<br />of each, and then relate to them the use of amino acids<br />and fats for energy. This simple summary is depicted<br />in Fig. 17–3.<br />Glycolysis<br />The enzymes for the reactions of glycolysis are found<br />in the cytoplasm of cells, and oxygen is not required<br />(glycolysis is an anaerobic process). Refer now to Fig.<br />17–3 as you read the following. In glycolysis, a sixcarbon<br />glucose molecule is broken down to two threecarbon<br />molecules of pyruvic acid. Two molecules of<br />ATP are necessary to start the process. The energy<br />they supply is called energy of activation and is necessary<br />to make glucose unstable enough to begin to break<br />down. As a result of these reactions, enough energy is<br />released to synthesize four molecules of ATP, for a net<br />gain of two ATP molecules per glucose molecule. Also<br />during glycolysis, two pairs of hydrogens are removed<br />by NAD, a carrier molecule that contains the vitamin<br />niacin. Two NAD molecules thus become 2NADH2,<br />and these attached hydrogen pairs will be transported<br />to the cytochrome transport system (stage 3).<br />If no oxygen is present in the cell, as may happen in<br />muscle cells during exercise, pyruvic acid is converted<br />to lactic acid, which causes muscle fatigue. If oxygen is<br />present, however, pyruvic acid continues into the next<br />stage, the Krebs citric acid cycle (or, more simply, the<br />Krebs cycle).<br />Krebs Citric Acid Cycle<br />The enzymes for the Krebs cycle (or citric acid<br />cycle) are located in the mitochondria of cells. This<br />second stage of cell respiration is aerobic, meaning<br />that oxygen is required. In a series of reactions, a pyruvic<br />acid molecule is “taken apart,” and its carbons are<br />converted to CO2. The first CO2 molecule is removed<br />by an enzyme that contains the vitamin thiamine.<br />This leaves a two-carbon molecule called an acetyl<br />group, which combines with a molecule called coenzyme<br />A to form acetyl coenzyme A (acetyl CoA). As<br />acetyl CoA continues in the Krebs cycle, two more<br />carbons are removed as CO2, and more pairs of hydrogens<br />are picked up by NAD and FAD (another carrier<br />molecule that contains the vitamin riboflavin).<br />NADH2 and FADH2 will carry their hydrogens to the<br />cytochrome transport system.<br />During the Krebs cycle, a small amount of energy<br />is released, enough to synthesize one molecule of ATP<br />(two per glucose). Notice also that a four-carbon molecule<br />(oxaloacetic acid) is regenerated after the formation<br />of CO2. This molecule will react with the next<br />acetyl CoA, which is what makes the Krebs cycle truly<br />a self-perpetuating cycle. The results of the stages of<br />cell respiration are listed in Table 17–3. Before you<br />continue, you may wish to look at that table to see just<br />where the process has gotten thus far.<br />Cytochrome Transport System<br />Cytochromes are proteins that contain either iron or<br />copper and are found in the mitochondria of cells.<br />The pairs of hydrogens that were once part of glucose<br />are brought to the cytochromes by the carrier molecules<br />NAD and FAD. Each hydrogen atom is then<br />split into its proton (H+ ion) and its electron. The<br />electrons of the hydrogens are passed from one<br />cytochrome to the next, and finally to oxygen. The<br />reactions of the electrons with the cytochromes<br />release most of the energy that was contained in the<br />glucose molecule, enough to synthesize 34 molecules<br />of ATP. As you can see, most of the ATP produced in<br />cell respiration comes from this third stage.<br />Finally, and very importantly, each oxygen atom<br />that has gained two electrons (from the cytochromes)<br />reacts with two of the H+ ions (protons) to form water.<br />The formation of metabolic water contributes to the<br />necessary intracellular fluid, and also prevents acidosis.<br />If H+ ions accumulated, they would rapidly lower<br />the pH of the cell. This does not happen, however,<br />because the H+ ions react with oxygen to form water,<br />and a decrease in pH is prevented.<br />The summary of the three stages of cell respiration<br />in Table 17–3 also includes the vitamins and minerals<br />that are essential for this process. An important overall<br />concept is the relationship between eating and<br />breathing. Eating provides us with a potential energy<br />source (often glucose) and with necessary vitamins and<br />minerals. However, to release the energy from food,<br />we must breathe. This is why we breathe. The oxygen<br />we inhale is essential for the completion of cell respiration,<br />and the CO2 produced is exhaled.<br />Proteins and Fats as Energy Sources<br />Although glucose is the preferred energy source for<br />cells, proteins and fats also contain potential energy<br />and are alternative energy sources in certain situations.<br />As you know, proteins are made of the smaller molecules<br />called amino acids, and the primary use for the<br />amino acids we obtain from food is the synthesis of<br />Body Temperature and Metabolism 403<br />404 Body Temperature and Metabolism<br />Carbohydrates Fats<br />Fatty acid<br />Pyruvic acid<br />Acetyl CoA<br />Citric acid<br />(Beta<br />oxidation<br />in the liver)<br />Glucose Glycerol<br />ATP<br />ATP<br />ATP<br />H+<br />H e<br />H+ H+ Cytochromes 34 ATP<br />Metabolic water<br />H2O<br />CO2<br />H2<br />H2<br />FAD<br />NH2<br />NH2<br />NH2<br />NH2<br />NH2<br />NH2<br />H2<br />NAD<br />NAD<br />Urea 3<br />4<br />C<br />C<br />C<br />C<br />C<br />C<br />C<br />C<br />C C<br />C<br />C C C C C C C C<br />C<br />C<br />C<br />C<br />C<br />C<br />C<br />C<br />C C C C C C C C C<br />C C C C C C C C<br />C C C C C C<br />C C<br />C<br />C<br />C C<br />C C<br />C C C<br />C C<br />C<br />C<br />C<br />C<br />C<br />CO2<br />CO2<br />=<br />Oxygen<br />Proteins<br />Amino<br />acids<br />(Digestion) (Digestion) (Digestion)<br />2<br />Coenzyme A<br />Acetyl<br />groups<br />Oxaloacetic<br />acid<br />2<br />Figure 17–3. Schematic representation of cell respiration. The breakdown of glucose is<br />shown in the center, amino acids on the left, and fatty acids and glycerol on the right. See<br />text for description.<br />QUESTION: To which two molecules can all three food types be converted to enter the citric<br />acid cycle?<br />new proteins. Excess amino acids, however, those not<br />needed immediately for protein synthesis, may be<br />used for energy production. In the liver, excess amino<br />acids are deaminated, that is, the amino group (NH2)<br />is removed. The remaining portion is converted to a<br />molecule that will fit into the Krebs cycle. For example,<br />a deaminated amino acid may be changed to a<br />three-carbon pyruvic acid or to a two-carbon acetyl<br />group. When these molecules enter the Krebs cycle,<br />the results are just the same as if they had come from<br />glucose. This is diagrammed in Fig. 17–3.<br />Fats are made of glycerol and fatty acids, which are<br />the end products of fat digestion. These molecules may<br />also be changed to ones that will take part in the Krebs<br />cycle, and the reactions that change them usually take<br />place in the liver. Glycerol is a three-carbon molecule<br />that can be converted to the three-carbon pyruvic acid,<br />which enters the Krebs cycle. In the process of betaoxidation,<br />the long carbon chains of fatty acids are split<br />into two-carbon acetyl groups, which enter a later step<br />in the Krebs cycle (see Fig. 17–3).<br />Both amino acids and fatty acids may be converted<br />by the liver to ketones, which are two- or four-carbon<br />molecules such as acetone and acetoacetic acid.<br />Although body cells can use ketones in cell respiration,<br />they do so slowly. In situations in which fats or<br />amino acids have become the primary energy sources,<br />a state called ketosis may develop; this is described in<br />Box 17–3: Ketosis. Excess amino acids may also be<br />converted to glucose; this is important to supply the<br />brain when dietary intake of carbohydrates is low. The<br />effects of hormones on the metabolism of food are<br />summarized in Table 17–4.<br />Energy Available from<br />the Three Nutrient Types<br />The potential energy in food is measured in units<br />called Calories or kilocalories. A calorie (lowercase<br />“c”) is the amount of energy needed to raise the temperature<br />of 1 gram of water 1°C. A kilocalorie or<br />Calorie (capital “C”) is 1000 times that amount of<br />energy.<br />One gram of carbohydrate yields about 4 kilocalories.<br />A gram of protein also yields about 4 kilocalories.<br />A gram of fat, however, yields 9 kilocalories, and<br />a gram of alcohol yields 7 kilocalories. This is why a<br />diet high in fat is more likely to result in weight gain<br />if the calories are not expended in energy-requiring<br />activities.<br />You may have noticed that calorie content is part of<br />the nutritional information on food labels. On such<br />labels the term calorie actually means Calorie or kilocalories<br />but is used for the sake of simplicity.<br />SYNTHESIS USES OF FOODS<br />Besides being available for energy production, each of<br />the three food types is used in anabolic reactions to<br />Body Temperature and Metabolism 405<br />Table 17–3 SUMMARY OF CELL RESPIRATION<br />Molecules That Vitamins or<br />Stage Enter the Process Results Minerals Needed<br />Glycolysis<br />(cytoplasm)<br />Krebs citric acid cycle<br />(mitochondria)<br />Cytochrome<br />transport system<br />(mitochondria)<br />• 2 ATP (net)<br />• 2 NADH2 (to cytochrome<br />transport system)<br />• 2 pyruvic acid (aerobic:<br />to Krebs cycle; anaerobic:<br />lactic acid formation)<br />• CO2 (exhaled)<br />• ATP (2 per glucose)<br />• 3 NADH2 and 1 FADH2 (to<br />cytochrome transport system)<br />• A 4-carbon molecule is regenerated<br />for the next cycle<br />• 34 ATP<br />• Metabolic water<br />• Niacin (part of NAD)<br />• Thiamine (for removal<br />of CO2)<br />• Niacin (part of NAD)<br />• Riboflavin (part of FAD)<br />• Pantothenic acid (part<br />of coenzyme A)<br />• Iron and copper (part<br />of some cytochromes)<br />Glucose—ATP needed as<br />energy of activation<br />Pyruvic acid—from glucose<br />or glycerol or excess<br />amino acids<br />or<br />Acetyl CoA—from fatty acids<br />or excess amino acids<br />NADH2 and FADH2—from<br />glycolysis or the Krebs<br />cycle<br />synthesize necessary materials for cells and tissues. A<br />simple summary of these reactions is shown in Fig.<br />17–4. The three food types and their end products of<br />digestion are at the bottom of the picture, and the<br />arrows going upward indicate synthesis and lead to the<br />products formed. You may wish to refer to Fig. 17–4<br />as you read the next sections.<br />Glucose<br />Glucose is the raw material for the synthesis of<br />another important monosaccharide, the pentose sugars<br />that are part of nucleic acids. Deoxyribose is the<br />five-carbon sugar found in DNA, and ribose is found<br />in RNA. This function of glucose is very important,<br />for without the pentose sugars our cells could neither<br />produce new chromosomes for cell division nor carry<br />out the process of protein synthesis.<br />Any glucose in excess of immediate energy needs or<br />the need for pentose sugars is converted to glycogen<br />in the liver and skeletal muscles. Glycogen is then an<br />energy source during states of hypoglycemia or during<br />exercise. If still more glucose is present, it will be<br />changed to fat and stored in adipose tissue.<br />Amino Acids<br />As mentioned previously, the primary uses for amino<br />acids are the synthesis of the non-essential amino<br />acids by the liver and the synthesis of new proteins in<br />all tissues. By way of review, we can mention some<br />proteins with which you are already familiar: keratin<br />and melanin in the epidermis; collagen in the dermis,<br />tendons, and ligaments; myosin, actin, and myoglobin<br />in muscle cells; hemoglobin in RBCs; antibodies produced<br />by WBCs; prothrombin and fibrinogen for<br />406 Body Temperature and Metabolism<br />Table 17–4 HORMONES THAT<br />REGULATE METABOLISM<br />Hormone (Gland) Effects<br />Thyroxine (thyroid<br />gland)<br />Growth hormone<br />(anterior pituitary)<br />Insulin (pancreas)<br />Glucagon (pancreas)<br />Cortisol (adrenal<br />cortex)<br />Epinephrine<br />(adrenal medulla)<br />• Increases use of all three<br />food types for energy (glucose,<br />fats, amino acids)<br />• Increases protein synthesis<br />• Increases amino acid transport<br />into cells<br />• Increases protein synthesis<br />• Increases use of fats for<br />energy<br />• Increases glucose transport<br />into cells and use for energy<br />• Increases conversion of glucose<br />to glycogen in liver<br />and muscles<br />• Increases transport of amino<br />acids and fatty acids into<br />cells to be used for synthesis<br />(not energy production)<br />• Increases conversion of<br />glycogen to glucose<br />• Increases use of amino acids<br />and fats for energy<br />• Increases conversion of glucose<br />to glycogen in liver<br />• Increases use of amino acids<br />and fats for energy<br />• Decreases protein synthesis<br />except in liver and GI tract<br />• Increases conversion of<br />glycogen to glucose<br />• Increases use of fats for<br />energy<br />BOX 17–3 KETOSIS<br />When fats and amino acids are to be used for<br />energy, they are often converted by the liver to<br />ketones. Ketones are organic molecules such as<br />acetone that may be changed to acetyl CoA and<br />enter the Krebs cycle. Other cells are able to use<br />ketones as an energy source, but they do so<br />slowly. When ketones are produced in small<br />amounts, as they usually are between meals, the<br />blood level does not rise sharply.<br />A state of ketosis exists when fats and proteins<br />become the primary energy sources, and<br />ketones accumulate in the blood faster than cells<br />can utilize them. Because ketones are organic<br />acids, they lower the pH of the blood. As the<br />blood ketone level rises, the kidneys excrete<br />ketones, but they must also excrete more water<br />as a solvent, which leads to dehydration.<br />Ketosis is clinically important in diabetes mellitus,<br />starvation, and eating disorders such as<br />anorexia nervosa. Diabetics whose disease is<br />poorly controlled may progress to ketoacidosis,<br />a form of metabolic acidosis that may lead to<br />confusion, coma, and death. Reversal of this<br />state requires a carbohydrate energy source and<br />the insulin necessary to utilize it.<br />clotting; albumin to maintain blood volume; pepsin<br />and amylase for digestion; growth hormone and<br />insulin; and the thousands of enzymes needed to catalyze<br />reactions within the body.<br />The amino acids we obtain from the proteins in<br />our food are used by our cells to synthesize all of these<br />proteins in the amounts needed by the body. Only<br />when the body’s needs for new proteins have been<br />met are amino acids used for energy production. But<br />notice in Fig. 17–4 what happens to excess amino<br />acids; they will be deaminated and converted to simple<br />carbohydrates and contribute to glycogen storage<br />or they may be changed to fat and stored in adipose<br />tissue.<br />Body Temperature and Metabolism 407<br />Proteins<br />(enzymes, structural)<br />Non-essential<br />amino acids<br />Transamination<br />Proteins<br />(Digestion)<br />Amino<br />acids<br />NH2<br />C<br />C<br />NH2<br />NH2<br />C<br />C<br />C<br />C<br />C<br />C<br />C C C C C C<br />C C C<br />C C C C C C C C<br />C C C C C C C C<br />(Digestion) (Digestion)<br />Glucose<br />Glycerol<br />Fatty acid<br />Carbohydrates Fats<br />Phospholipids<br />(cell membranes)<br />Pentose sugars<br />Excess<br />(deamination)<br />Excess<br />Glycogen<br />True fats<br />(adipose tissue)<br />Cholesterol<br />and other<br />steroids<br />Figure 17–4. Synthesis uses of foods. See text for description.<br />QUESTION: Excess amino acids can be used to synthesize carbohydrates or fats. Can any<br />other food be used to synthesize proteins?<br />Fatty Acids and Glycerol<br />The end products of fat digestion that are not needed<br />immediately for energy production may be stored as<br />fat (triglycerides) in adipose tissue. Most adipose tissue<br />is found subcutaneously and is potential energy for<br />times when food intake decreases. Notice in Table<br />17–4 that insulin promotes fat synthesis and storage.<br />One theory of weight gain proposes that a diet high in<br />sugars and starches stimulates the secretion of so<br />much insulin that fat can only be stored, not taken out<br />of storage and used for energy.<br />Fatty acids and glycerol are also used for the synthesis<br />of phospholipids, which are essential components<br />of all cell membranes. Myelin, for example, is a<br />phospholipid of the membranes of Schwann cells,<br />which form the myelin sheath of peripheral neurons.<br />The liver can synthesize most of the fatty acids<br />needed by the body. Two exceptions are linoleic acid<br />and linolenic acid, which are essential fatty acids and<br />must be obtained from the diet. Linoleic acid is part of<br />lecithin, which in turn is part of all cell membranes.<br />Vegetable oils are good sources of these essential fatty<br />acids.<br />When fatty acids are broken down in the process of<br />beta-oxidation, the resulting acetyl groups may also be<br />used for the synthesis of cholesterol, a steroid. This<br />takes place primarily in the liver, although all cells are<br />capable of synthesizing cholesterol for their cell membranes.<br />The liver uses cholesterol to synthesize bile<br />salts for the emulsification of fats in digestion. The<br />steroid hormones are also synthesized from cholesterol.<br />Cortisol and aldosterone are produced by the<br />adrenal cortex, estrogen and progesterone by the<br />ovaries, and testosterone by the testes.<br />VITAMINS AND MINERALS<br />Vitamins are organic molecules needed in very small<br />amounts for normal body functioning. Some vitamins<br />are coenzymes; that is, they are necessary for the<br />functioning of certain enzymes. Others are antioxidant<br />vitamins, including vitamins C, E, and betacarotene<br />(a precursor for vitamin A). Antioxidants<br />prevent damage from free radicals, which are molecules<br />that contain an unpaired electron and are highly<br />reactive. The reactions of free radicals can damage<br />DNA, cell membranes, and the cell organelles. Free<br />radicals are formed during some normal body reactions,<br />but smoking and exposure to pollution will<br />increase their formation. Antioxidant vitamins combine<br />with free radicals before they can react with cellular<br />components. Plant foods are good sources of<br />these vitamins. Table 17–5 summarizes some important<br />metabolic and nutritional aspects of the vitamins<br />we need.<br />Deficiencies of vitamins often result in disease:<br />vitamin C deficiency and scurvy, for example (see Box<br />4–2). Other deficiency diseases that have been known<br />for decades include pellagra (lack of niacin), beri-beri<br />(riboflavin), pernicious anemia (B12), and rickets (D).<br />More recently the importance of folic acid (folacin)<br />for the development of the fetal central nervous system<br />has been recognized. Adequate folic acid during<br />pregnancy can significantly decrease the chance of<br />spina bifida (open spinal column) and anencephaly<br />(absence of the cerebrum, always fatal) in a fetus. All<br />women should be aware of the need for extra (400<br />micrograms) folic acid during pregnancy.<br />Minerals are simple inorganic chemicals and have<br />a variety of functions, many of which you are already<br />familiar with. Table 17–6 lists some important aspects<br />of minerals. We will return to the minerals as part of<br />our study of fluid–electrolyte balance in Chapter 19.<br />METABOLIC RATE<br />Although the term metabolism is used to describe all<br />of the chemical reactions that take place within the<br />body, metabolic rate is usually expressed as an<br />amount of heat production. This is because many<br />body processes that utilize ATP also produce heat.<br />These processes include the contraction of skeletal<br />muscle, the pumping of the heart, and the normal<br />breakdown of cellular components. Therefore, it is<br />possible to quantify heat production as a measure of<br />metabolic activity.<br />As mentioned previously, the energy available from<br />food is measured in kilocalories (kcal). Kilocalories are<br />also the units used to measure the energy expended by<br />the body. During sleep, for example, energy expended<br />by a 150-pound person is about 60 to 70 kcal per hour.<br />Getting up and preparing breakfast increases energy<br />expenditure to 80 to 90 kcal per hour. For mothers<br />with several small children, this value may be significantly<br />higher. Clearly, greater activity results in<br />greater energy expenditure.<br />The energy required for merely living (lying quietly<br />in bed) is the basal metabolic rate (BMR). See<br />Box 17–4: Metabolic Rate for a formula to estimate<br />408 Body Temperature and Metabolism<br />409<br />Table 17–5 VITAMINS<br />Vitamin Functions Food Sources Comment<br />Water Soluble<br />Thiamine (B1)<br />Riboflavin (B2)<br />Niacin (nicotinamide)<br />Pyridoxine (B6)<br />B12 (cyanocobalamin)<br />Biotin<br />Folic acid (folacin)<br />Pantothenic acid<br />Vitamin C (ascorbic<br />acid)<br />Fat Soluble<br />Vitamin A<br />Vitamin D<br />Vitamin E<br />Vitamin K<br />• Conversion of pyruvic acid to acetyl<br />CoA in cell respiration<br />• Synthesis of pentose sugars<br />• Synthesis of acetylcholine<br />• Part of FAD in cell respiration<br />• Part of NAD in cell respiration<br />• Metabolism of fat for energy<br />• Part of enzymes needed for amino<br />acid metabolism and protein synthesis,<br />nucleic acid synthesis, synthesis of<br />antibodies<br />• Synthesis of DNA, especially in RBC<br />production<br />• Metabolism of amino acids for energy<br />• Synthesis of nucleic acids<br />• Metabolism of fatty acids and amino<br />acids<br />• Synthesis of DNA, especially in blood<br />cell production<br />• Contributes to development of fetal<br />CNS<br />• Part of coenzyme A in cell respiration,<br />use of amino acids and fats for energy<br />• Synthesis of collagen, especially for<br />wound healing<br />• Metabolism of amino acids<br />• Absorption of iron<br />• An antioxidant—prevents cellular<br />damage from free radicals<br />• Synthesis of rhodopsin<br />• Calcification of growing bones<br />• Maintenance of epithelial tissues<br />• Absorption of calcium and phosphorus<br />in the small intestine<br />• Contributes to immune responses,<br />action of insulin, and preservation of<br />muscle mass and strength<br />• An antioxidant—prevents destruction<br />of cell membranes<br />• Contributes to wound healing and<br />detoxifying ability of the liver<br />• Synthesis of prothrombin and<br />other clotting factors<br />• Meat, eggs,<br />legumes, green<br />leafy vegetables,<br />grains<br />• Meat, milk, cheese,<br />grains<br />• Meat, fish, grains,<br />legumes<br />• Meat, fish, grains,<br />yeast, yogurt<br />• Liver, meat, fish,<br />eggs, milk, cheese<br />• Yeast, liver, eggs<br />• Liver, grains,<br />legumes, leafy<br />green vegetables<br />• Meat, fish, grains,<br />legumes, vegetables<br />• Citrus fruits, tomatoes,<br />potatoes<br />• Yellow and green<br />vegetables, liver,<br />milk, eggs<br />• Fortified milk, egg<br />yolks, fish liver oils<br />• Nuts, wheat germ,<br />seed oils<br />• Liver, spinach,<br />cabbage<br />Rapidly destroyed by heat<br />Small amounts produced<br />by GI bacteria<br />Small amounts produced<br />by GI bacteria<br />Contains cobalt; intrinsic<br />factor required for<br />absorption<br />Small amounts produced<br />by GI bacteria<br />Small amounts produced<br />by GI bacteria<br />Small amounts produced<br />by GI bacteria<br />Rapidly destroyed by<br />heat<br />Stored in liver; bile salts<br />required for absorption<br />Produced in skin exposed<br />to UV rays; stored<br />in liver; bile salts<br />required for absorption<br />Stored in liver and adipose<br />tissue; bile salts<br />required for absorption<br />Large amounts produced<br />by GI bacteria; bile salts<br />required for absorption;<br />stored in liver<br />your own metabolic rate. A number of factors affect<br />the metabolic rate of an active person:<br />1. Exercise—Contraction of skeletal muscle increases<br />energy expenditure and raises metabolic rate (see<br />Box 17–5: Weight Loss).<br />2. Age—Metabolic rate is highest in young children<br />and decreases with age. The energy requirements<br />for growth and the greater heat loss by a<br />smaller body contribute to the higher rate in children.<br />After growth has stopped, metabolic rate<br />decreases about 2% per decade. If a person<br />410 Body Temperature and Metabolism<br />Table 17–6 MINERALS<br />Mineral Functions Food Sources Comment<br />Calcium<br />Phosphorus<br />Sodium<br />Potassium<br />Chlorine<br />Iron<br />Iodine<br />Sulfur<br />Magnesium<br />Manganese<br />Copper<br />Cobalt<br />Zinc<br />• Formation of bones and teeth<br />• Neuron and muscle functioning<br />• Blood clotting<br />• Formation of bones and teeth<br />• Part of DNA, RNA, and ATP<br />• Part of phosphate buffer system<br />• Contributes to osmotic pressure<br />of body fluids<br />• Nerve impulse transmission and<br />muscle contraction<br />• Part of bicarbonate buffer system<br />• Contributes to osmotic pressure of<br />body fluids<br />• Nerve impulse transmission and<br />muscle contraction<br />• Contributes to osmotic pressure of<br />body fluids<br />• Part of HCI in gastric juice<br />• Part of hemoglobin and myoglobin<br />• Part of some cytochromes in cell<br />respiration<br />• Part of thyroxine and T3<br />• Part of some amino acids<br />• Part of thiamine and biotin<br />• Formation of bone<br />• Metabolism of ATP–ADP<br />• Formation of urea<br />• Synthesis of fatty acids and cholesterol<br />• Synthesis of hemoglobin<br />• Part of some cytochromes in cell<br />respiration<br />• Synthesis of melanin<br />• Part of vitamin B12<br />• Part of carbonic anhydrase needed<br />for CO2 transport<br />• Part of peptidases needed for protein<br />digestion<br />• Necessary for normal taste sensation<br />• Involved in wound healing<br />• Milk, cheese, yogurt,<br />shellfish, leafy green<br />vegetables<br />• Milk, cheese, fish, meat<br />• Table salt, almost all<br />foods<br />• Virtually all foods<br />• Table salt<br />• Meat, shellfish, dried<br />apricots, legumes, eggs<br />• Iodized salt, seafood<br />• Meat, eggs<br />• Green vegetables,<br />legumes, seafood, milk<br />• Legumes, grains, nuts,<br />leafy green vegetables<br />• Liver, seafood, grains,<br />nuts, legumes<br />• Liver, meat, fish<br />• Meat, seafood, grains,<br />legumes<br />Vitamin D required for<br />absorption; stored in<br />bones<br />Vitamin D required for<br />absorption; stored in<br />bones<br />Most abundant cation ( )<br />in extracellular fluid<br />Most abundant cation ( )<br />in intracellular fluid<br />Most abundant anion ( )<br />in extracellular fluid<br />Stored in liver<br />Insulin and keratin require<br />sulfur<br />Part of chlorophyll in green<br />plants<br />Some stored in liver<br />Stored in liver<br />Vitamin B12 stored in liver<br />Body Temperature and Metabolism 411<br />BOX 17–4 METABOLIC RATE<br />Example: A 160-pound man:<br />1. 160 lb at 2.2 lb/kg = 73 kg<br />2. 73 kg x 1.0 kcal/kg/hr = 73 kcal/hr<br />3. 73 kcal/hr x 24 = 1752 kcal/day<br />To approximate the amount of energy actually<br />expended during an average day (24 hours), the<br />following percentages may be used:<br />Sedentary activity: add 40% to 50% of the BMR to<br />the BMR<br />Light activity: add 50% to 65% of the BMR to the<br />BMR<br />Moderate activity: add 65% to 75% of the BMR to<br />the BMR<br />Strenuous activity: add 75% to 100% of the BMR to<br />the BMR<br />Using our example of the 120-pound woman<br />with a BMR of 1200 kcal/day:<br />Sedentary: 1680 to 1800 kcal/day<br />Light: 1800 to 1980 kcal/day<br />Moderate: 1980 to 2100 kcal/day<br />Strenuous: 2100 to 2400 kcal/day<br />To estimate your own basal metabolic rate<br />(BMR), calculate kilocalories (kcal) used per hour as<br />follows:<br />For women: use the factor of 0.9 kcal per kilogram<br />(kg) of body weight<br />For men: use the factor of 1.0 kcal per kg of body<br />weight<br />Then multiply kcal/hour by 24 hours to determine<br />kcal per day.<br />Example: A 120-pound woman:<br />1. Change pounds to kilograms:<br />120 lb at 2.2 lb/kg = 55 kg<br />2. Multiply kg weight by the BMR factor:<br />55 kg x 0.9 kcal/kg/hr = 49.5 kcal/hr<br />3. Multiply kcal/hr by 24:<br />49.5 kcal/hr x 24 = 1188 kcal/day<br />(An approximate BMR, about 1200<br />kcal/day)<br />BOX 17–5 WEIGHT LOSS<br />food. Keeping track of daily caloric intake is an<br />important part of a decision to try to lose weight. It<br />is also important to remember that sustained loss of<br />fat usually does not exceed 1 to 2 pounds per week.<br />In part this is so because as calorie intake decreases,<br />the metabolic rate decreases. There will also be loss<br />of some body protein so that amino acids can be<br />converted to carbohydrates to supply the brain.<br />A sensible weight-loss diet will include carbohydrate<br />to supply energy needs, will have sufficient<br />protein (40 to 45 grams per day), and will be low in<br />animal fat. Including vegetables and fruits will supply<br />vitamins, minerals, and fiber.<br />Although diet books are often found on the bestseller<br />lists, there is no magic method that will result<br />in weight loss. Losing weight depends on one simple<br />fact: calorie expenditure in activity must exceed<br />calorie intake in food (the term calorie here will be<br />used to mean kilocalorie).<br />To lose 1 pound of body fat, which consists of fat,<br />water, and protein, 3500 calories of energy must be<br />expended. Although any form of exercise requires<br />calories, the more strenuous the exercise, the more<br />calories expended. Some examples are shown in the<br />accompanying table.<br />Most food packaging contains nutritional information,<br />including the calories per serving of the<br />Calories per 10 Calories per 10<br />minutes (average minutes (average<br />Activity for a 150-lb person) Activity for a 150-lb person)<br />Walking slowly<br />Walking briskly<br />Walking up stairs<br />Dancing (slow)<br />Dancing (fast)<br />30<br />45<br />170<br />40<br />65<br />Running (8 mph)<br />Cycling (10 mph)<br />Cycling (15 mph)<br />Swimming<br />120<br />70<br />115<br />100<br />becomes less active, the total decrease is almost 5%<br />per decade.<br />3. Body configuration of adults—Tall, thin people<br />usually have higher metabolic rates than do short,<br />stocky people of the same weight. This is so because<br />the tall, thin person has a larger surface area (proportional<br />to weight) through which heat is continuously<br />lost. The metabolic rate, therefore, is<br />slightly higher to compensate for the greater heat<br />loss. The variance of surface-to-weight ratios for<br />different body configurations is illustrated in<br />Fig. 17–5.<br />4. Sex hormones—Testosterone increases metabolic<br />activity to a greater degree than does estrogen, giving<br />men a slightly higher metabolic rate than<br />women. Also, men tend to have more muscle, an<br />active tissue, whereas women tend to have more<br />fat, a relatively inactive tissue.<br />5. Sympathetic stimulation—In stress situations, the<br />metabolism of many body cells is increased. Also<br />contributing to this are the hormones epinephrine<br />and norepinephrine. As a result, metabolic rate<br />increases.<br />6. Decreased food intake—If the intake of food<br />decreases for a prolonged period of time, metabolic<br />rate also begins to decrease. It is as if the body’s<br />metabolism is “slowing down” to conserve whatever<br />energy sources may still be available. (See also<br />Box 17–6: Leptin and Body-Mass Index.)<br />7. Climate—People who live in cold climates may<br />have metabolic rates 10% to 20% higher than people<br />who live in tropical regions. This is believed to<br />be due to the variations in the secretion of thyroxine,<br />the hormone most responsible for regulation<br />of metabolic rate. In a cold climate, the necessity<br />for greater heat production brings about an<br />412<br />Figure 17–5. Surface-to-weight ratios. Imagine that<br />the three shapes are people who all weigh the same<br />amount. The “tall, thin person” on the right has about<br />50% more surface area than does the “short, stocky person”<br />on the left. The more surface area (where heat is<br />lost), the higher the metabolic rate.<br />QUESTION: Which of these ratios best represents an<br />infant? (Rather than weight, think of inside-outside<br />proportion.)<br />Box 17–6 LEPTIN AND BODY-MASS INDEX<br />directly decreases fat storage in cells, and improves<br />the efficiency of the pancreatic cells that produce<br />insulin. What was first believed to be a simple chemical<br />signal has proved to be much more complex.<br />A good measure of leanness or fatness is the<br />body-mass index.<br />To calculate: Multiple weight in pounds by 703.<br />Divide by height in inches.<br />Divide again by height in inches = body-mass<br />index<br />Example: A person five foot six weighing 130<br />pounds.<br />130 x 703 = 91,390<br />91,390 66 = 1385<br />1385 66 = 20.98<br />The optimal body-mass index is considered to be<br />21. Any index over 25 is considered overweight.<br />The 1994 discovery of the hormone leptin was<br />reported to the general public in 1995, along with<br />speculation that leptin could become an anti-obesity<br />medication, which it has not. Leptin is a protein<br />produced by fat cells, and signals the hypothalamus<br />to release a chemical that acts as an appetite suppressant.<br />It seems to inform the brain of how much<br />stored fat the body has, and is therefore involved in<br />the regulation of body weight (along with many<br />other chemicals, some still unknown).<br />Another likely role for leptin is as a contributor to<br />the onset of puberty, especially in females. Girls<br />who are very thin, with little body fat, tend to have<br />a later first menstrual period than girls with average<br />body fat, and a certain level of body fat is necessary<br />for continued ovulation. Leptin may be the chemical<br />mediator of this information.<br />The most recent research indicates that leptin<br />Body Temperature<br />1. Normal range is 96.5° to 99.5°F (36° to 38°C),<br />with an average of 98.6°F (37°C).<br />2. Normal fluctuation in 24 hours is 1° to 2°F.<br />3. Temperature regulation in infants and the elderly is<br />not as precise as it is at other ages.<br />Heat Production<br />Heat is one of the energy products of cell respiration.<br />Many factors affect the total heat actually produced<br />(see Table 17–1).<br />1. Thyroxine from the thyroid gland—the most important<br />regulator of daily heat production. As metabolic<br />rate decreases, more thyroxine is secreted to<br />increase the rate of cell respiration.<br />2. Stress—sympathetic impulses and epinephrine and<br />norepinephrine increase the metabolic activity of<br />many organs, increasing the production of ATP<br />and heat.<br />3. Active organs continuously produce heat. Skeletal<br />muscle tone produces 25% of the total body heat at<br />rest. The liver provides up to 20% of the resting<br />body heat.<br />4. Food intake increases the activity of the digestive<br />organs and increases heat production.<br />5. Changes in body temperature affect metabolic rate.<br />A fever increases the metabolic rate, and more heat<br />is produced; this may become detrimental during<br />very high fevers.<br />Heat Loss (see Table 17–2)<br />1. Most heat is lost through the skin.<br />2. Blood flow through the dermis determines the<br />amount of heat that is lost by radiation, conduction,<br />and convection.<br />3. Vasodilation in the dermis increases blood flow and<br />heat loss; radiation and conduction are effective<br />only if the environment is cooler than the body.<br />4. Vasoconstriction in the dermis decreases blood<br />flow and conserves heat in the core of the body.<br />5. Sweating is a very effective heat loss mechanism;<br />excess body heat evaporates sweat on the skin surface;<br />sweating is most effective when the atmospheric<br />humidity is low.<br />6. Sweating also has a disadvantage in that water is<br />lost and must be replaced to prevent serious dehydration.<br />7. Heat is lost from the respiratory tract by the evaporation<br />of water from the warm respiratory<br />mucosa; water vapor is part of exhaled air.<br />8. A very small amount of heat is lost as urine and<br />feces are excreted at body temperature.<br />Body Temperature and Metabolism 413<br />STUDY OUTLINE<br />increased secretion of thyroxine and a higher metabolic<br />rate.<br />AGING AND METABOLISM<br />As mentioned in the previous section, metabolic rate<br />decreases with age. Elderly people who remain active,<br />however, can easily maintain a metabolic rate (energy<br />production) adequate for their needs as long as their<br />general health is good. Some elderly people subject to<br />physical or emotional disability, however, may be at<br />risk for malnutrition. Caregivers may assess such a risk<br />by asking how often the person eats every day; if<br />appetite is good, fair, or poor; and how the food tastes.<br />These simple questions may help ensure adequate<br />nutrition.<br />Sensitivity to external temperature changes may<br />decrease with age, and the regulation of body temperature<br />is no longer as precise. Sweat glands are not as<br />active, and prolonged high environmental temperatures<br />are a real danger for elderly people. In August<br />2003, in Europe, an unusually long and severe heat<br />wave was the cause of at least 25,000 deaths. Most of<br />those who died were elderly.<br />SUMMARY<br />Food is needed for the synthesis of new cells and tissues,<br />or is utilized to produce the energy required for<br />such synthesis reactions. As a consequence of metabolism,<br />heat energy is released to provide a constant<br />body temperature and permit the continuation of<br />metabolic activity. The metabolic pathways described<br />in this chapter are only a small portion of the body’s<br />total metabolism. Even this simple presentation, however,<br />suggests the great chemical complexity of the<br />functioning human being.<br />Regulation of Heat Loss<br />1. The hypothalamus is the thermostat of the body<br />and regulates body temperature by balancing heat<br />production and heat loss.<br />2. The hypothalamus receives information from its<br />own neurons (blood temperature) and from the<br />temperature receptors in the dermis.<br />3. Mechanisms to increase heat loss are vasodilation<br />in the dermis and increased sweating. Decreased<br />muscle tone will decrease heat production.<br />4. Mechanisms to conserve heat are vasoconstriction<br />in the dermis and decreased sweating. Increased<br />muscle tone (shivering) will increase heat production.<br />Fever—an abnormally elevated body temperature<br />1. Pyrogens are substances that cause a fever: bacteria,<br />foreign proteins, or chemicals released during<br />inflammation (endogenous pyrogens).<br />2. Pyrogens raise the setting of the hypothalamic<br />thermostat; the person feels cold and begins to<br />shiver to produce heat.<br />3. When the pyrogen has been eliminated, the hypothalamic<br />setting returns to normal; the person feels<br />warm, and sweating begins to lose heat to lower the<br />body temperature.<br />4. A low fever may be beneficial because it increases<br />the activity of WBCs and inhibits the activity of<br />some pathogens.<br />5. A high fever may be detrimental because enzymes<br />are denatured at high temperatures. This is most<br />critical in the brain, where cells that die cannot be<br />replaced.<br />Metabolism—all the reactions within the<br />body<br />1. Anabolism—synthesis reactions that usually<br />require energy in the form of ATP.<br />2. Catabolism—decomposition reactions that often<br />release energy in the form of ATP.<br />3. Enzymes catalyze most anabolic and catabolic reactions.<br />Cell Respiration—the breakdown of food<br />molecules to release their potential energy<br />and synthesize ATP (Fig. 17–3)<br />1. Glucose oxygen yields CO2 H2O ATP <br />heat.<br />2. The breakdown of glucose involves three stages:<br />glycolysis, the Krebs cycle, and the cytochrome<br />(electron) transport system (see also Table 17–3).<br />3. The oxygen necessary comes from breathing.<br />4. The water formed becomes part of intracellular<br />fluid; CO2 is exhaled; ATP is used for energyrequiring<br />reactions; heat provides a body temperature.<br />Proteins and Fats—as energy sources (see<br />Table 17–4 for hormonal regulation)<br />1. Excess amino acids are deaminated in the liver and<br />converted to pyruvic acid or acetyl groups to enter<br />the Krebs cycle. Amino acids may also be converted<br />to glucose to supply the brain (Fig. 17–3).<br />2. Glycerol is converted to pyruvic acid to enter the<br />Krebs cycle.<br />3. Fatty acids, in the process of beta-oxidation in the<br />liver, are split into acetyl groups to enter the Krebs<br />cycle; ketones are formed for transport to other<br />cells (see Fig. 17–3).<br />Energy Available from Food<br />1. Energy is measured in kilocalories (Calories):<br />kcal.<br />2. There are 4 kcal per gram of carbohydrate, 4 kcal<br />per gram of protein, 9 kcal per gram of fat. With<br />reference to food, kilocalories may be called calories.<br />Synthesis Uses of Foods (Fig. 17–4)<br />1. Glucose—used to synthesize the pentose sugars for<br />DNA and RNA; used to synthesize glycogen to<br />store energy in liver and muscles.<br />2. Amino acids—used to synthesize new proteins and<br />the non-essential amino acids; essential amino<br />acids must be obtained in the diet.<br />3. Fatty acids and glycerol—used to synthesize phospholipids<br />for cell membranes, triglycerides for fat<br />storage in adipose tissue, and cholesterol and other<br />steroids; essential fatty acids must be obtained in<br />the diet.<br />4. Any food eaten in excess will be changed to fat and<br />stored.<br />5. Vitamins and minerals—see Tables 17–5 and 17–6.<br />Metabolic Rate—heat production by the<br />body; measured in kcal<br />1. Basal metabolic rate (BMR) is the energy required<br />to maintain life (see Box 17–4); several factors<br />influence the metabolic rate of an active person.<br />414 Body Temperature and Metabolism<br />2. Age—metabolic rate is highest in young children<br />and decreases with age.<br />3. Body configuration—more surface area proportional<br />to weight (tall and thin) means a higher<br />metabolic rate.<br />4. Sex hormones—men usually have a higher metabolic<br />rate than do women; men have more muscle<br />proportional to fat than do women.<br />5. Sympathetic stimulation—metabolic activity increases<br />in stress situations.<br />6. Decreased food intake—metabolic rate decreases<br />to conserve available energy sources.<br />7. Climate—people who live in cold climates usually<br />have higher metabolic rates because of a greater<br />need for heat production.<br />Body Temperature and Metabolism 415<br />REVIEW QUESTIONS<br />1. State the normal range of human body temperature<br />in °F and °C. (p. 396)<br />2. State the summary equation of cell respiration, and<br />state what happens to (or the purpose of) each of<br />the products. (p. 401)<br />3. Describe the role of each in heat production: thyroxine,<br />skeletal muscles, stress situations, and the<br />liver. (p. 396)<br />4. Describe the two mechanisms of heat loss through<br />the skin, and explain the role of blood flow.<br />Describe how heat is lost through the respiratory<br />tract. (pp. 397–398)<br />5. Explain the circumstances that exist when sweating<br />and vasodilation in the dermis are not effective<br />mechanisms of heat loss. (p. 397)<br />6. Name the part of the brain that regulates body<br />temperature, and explain what is meant by a thermostat.<br />(p. 398)<br />7. Describe the responses by the body to a warm environment<br />and to a cold environment. (pp. 399)<br />8. Explain how pyrogens are believed to cause a fever,<br />and give two examples of pyrogens. (p. 399)<br />9. Define metabolism, anabolism, catabolism, kilocalorie,<br />and metabolic rate. (pp. 400, 401, 405,<br />408)<br />10. Name the three stages of the cell respiration<br />of glucose and state where in the cell each takes<br />place and whether or not oxygen is required.<br />(pp. 403)<br />11. For each, state the molecules that enter the<br />process and the results of the process: glycolysis,<br />Krebs cycle, and cytochrome transport system.<br />(pp. 403–405)<br />12. Explain how fatty acids, glycerol, and excess<br />amino acids are used for energy production in cell<br />respiration. (pp. 403, 405)<br />13. Describe the synthesis uses for glucose, amino<br />acids, and fatty acids. (pp. 406–408)<br />14. Describe four factors that affect the metabolic<br />rate of an active person. (pp. 410, 412)<br />15. If lunch consists of 60 grams of carbohydrate,<br />15 grams of protein, and 10 grams of fat, how<br />many kilocalories are provided by this meal?<br />(p. 405)<br />FOR FURTHER THOUGHT<br />1. For many people, iceberg lettuce is the vegetable<br />eaten most often. What does lettuce provide? What<br />does lettuce lack, compared to vegetables such as<br />broccoli?<br />2. Fourteen-year-old Donna has just decided that eating<br />meat is “gross,” and that she will be a vegetarian.<br />What difficulties are there with such a diet;<br />that is, what nutrients may be lacking? How may<br />they be obtained?<br />3. Studies with animals have shown that caloric<br />restriction may prolong life by protecting the brain<br />from some effects of aging. The animals’ diet was<br />about half the usual calories they would consume.<br />For people, 1250 to 1500 calories per day would be<br />restrictive (compared to the 2000 calories or more<br />that many of us in North America consume).<br />Would it be worth it for a life span of 110 years?<br />Describe the problems with such a diet.<br />4. Every summer small children are left alone in cars<br />“just for a few minutes,” while a parent does an<br />errand. The result may be tragic—severe brain<br />damage or death of the child from heat stroke.<br />Explain why small children are so susceptible to<br />heat.<br />5. Remember the Titanic, which sank in April of<br />1912? There were not enough lifeboats for everyone,<br />and many people were in the water of the<br />North Atlantic. They did have life jackets, and did<br />not drown, but many were dead within half an<br />hour. Explain why.<br />6. An elderly person and a quadriplegic person may<br />each have difficulties during cold weather. Explain<br />how the problem is a little different for each.<br />416 Body Temperature and Metabolism<br />CHAPTER 18<br />The Urinary System<br />417<br />418<br />CHAPTER 18<br />Chapter Outline<br />Kidneys<br />Internal Structure of the Kidney<br />The Nephron<br />Renal corpuscle<br />Renal tubule<br />Blood Vessels of the Kidney<br />Formation of Urine<br />Glomerular Filtration<br />Tubular Reabsorption<br />Mechanisms of reabsorption<br />Tubular Secretion<br />Hormones That Influence Reabsorption of Water<br />Summary of Urine Formation<br />The Kidneys and Acid–Base Balance<br />Other Functions of the Kidneys<br />Elimination of Urine<br />Ureters<br />Urinary Bladder<br />Urethra<br />The Urination Reflex<br />Characteristics of Urine<br />Aging and the Urinary System<br />BOX 18–1 FLOATING KIDNEY<br />BOX 18–2 RENAL FAILURE AND HEMODIALYSIS<br />BOX 18–3 ERYTHROPOIETIN<br />BOX 18–4 KIDNEY STONES<br />BOX 18–5 BLOOD TESTS AND KIDNEY FUNCTION<br />BOX 18–6 URINARY TRACT INFECTIONS<br />Student Objectives<br />• Describe the location and general function of each<br />organ of the urinary system.<br />• Name the parts of a nephron and the important<br />blood vessels associated with them.<br />• Explain how the following are involved in urine<br />formation: glomerular filtration, tubular reabsorption,<br />tubular secretion, and blood flow through the<br />kidney.<br />• Describe the mechanisms of tubular reabsorption,<br />and explain the importance of tubular secretion.<br />• Describe how the kidneys help maintain normal<br />blood volume and blood pressure.<br />• Name and state the functions of the hormones<br />that affect the kidneys.<br />• Describe how the kidneys help maintain normal<br />pH of blood and tissue fluid.<br />• Describe the urination reflex, and explain how voluntary<br />control is possible.<br />• Describe the characteristics of normal urine.<br />The Urinary System<br />419<br />New Terminology<br />Bowman’s capsule (BOW-manz KAP-suhl)<br />Detrusor muscle (de-TROO-ser)<br />External urethral sphincter (yoo-REE-thruhl<br />SFINK-ter)<br />Glomerular filtration rate (gloh-MER-yoo-ler fill-<br />TRAY-shun RAYT)<br />Glomerulus (gloh-MER-yoo-lus)<br />Internal urethral sphincter (yoo-REE-thruhl<br />SFINK-ter)<br />Juxtaglomerular cells ( JUKS-tah-gloh-MER-yoo-ler<br />SELLS)<br />Micturition (MIK-tyoo-RISH-un)<br />Nephron (NEFF-ron)<br />Nitrogenous wastes (nigh-TRAH-jen-us)<br />Peritubular capillaries (PER-ee-TOO-byoo-ler)<br />Renal corpuscle (REE-nuhl KOR-pus’l)<br />Renal filtrate (REE-nuhl FILL-trayt)<br />Renal tubule (REE-nuhl TOO-byoo’l)<br />Retroperitoneal (RE-troh-PER-i-toh-NEE-uhl)<br />Specific gravity (spe-SIF-ik GRA-vi-tee)<br />Threshold level (THRESH-hold LE-vuhl)<br />Trigone (TRY-gohn)<br />Ureter (YOOR-uh-ter)<br />Urethra (yoo-REE-thrah)<br />Urinary bladder (YOOR-i-NAR-ee BLA-der)<br />Related Clinical Terminology<br />Cystitis (sis-TIGH-tis)<br />Dysuria (dis-YOO-ree-ah)<br />Hemodialysis (HEE-moh-dye-AL-i-sis)<br />Nephritis (ne-FRY-tis)<br />Oliguria (AH-li-GYOO-ree-ah)<br />Polyuria (PAH-li-YOO-ree-ah)<br />Renal calculi (REE-nuhl KAL-kew-lye)<br />Renal failure (REE-nuhl FAYL-yer)<br />Uremia (yoo-REE-me-ah)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />The first successful human organ transplant was a<br />kidney transplant performed in 1953. Because the<br />donor and recipient were identical twins, rejection was<br />not a problem. Thousands of kidney transplants have<br />been performed since then, and the development of<br />immunosuppressive medications has permitted many<br />people to live normal lives with donated kidneys.<br />Although a person usually has two kidneys, one kidney<br />is sufficient to carry out the complex work required to<br />maintain homeostasis of the body fluids.<br />The urinary system consists of two kidneys, two<br />ureters, the urinary bladder, and the urethra (Fig.<br />18–1). The formation of urine is the function of the<br />kidneys, and the rest of the system is responsible for<br />eliminating the urine.<br />Body cells produce waste products such as urea,<br />420 The Urinary System<br />Diaphragm<br />Right kidney<br />Psoas major muscle<br />lliacus muscle<br />Right ureter<br />Urinary bladder<br />Urethra<br />Symphysis pubis<br />Trigone of bladder<br />Opening of ureter<br />Sacrum<br />Pelvis<br />Lumbar vertebra<br />Left common iliac<br />artery and vein<br />Left ureter<br />Left kidney<br />Left renal artery and vein<br />Superior mesenteric artery<br />Left adrenal gland<br />Inferior vena cava<br />Aorta<br />Ribs<br />Figure 18–1. The urinary system shown in anterior view.<br />QUESTION: Why is blood pressure relatively high in the kidneys? What do you see that<br />would suggest this?<br />creatinine, and ammonia, which must be removed<br />from the blood before they accumulate to toxic levels.<br />As the kidneys form urine to excrete these waste products,<br />they also accomplish several other important<br />functions:<br />1. Regulation of the volume of blood by excretion or<br />conservation of water<br />2. Regulation of the electrolyte content of the blood<br />by the excretion or conservation of minerals<br />3. Regulation of the acid–base balance of the blood by<br />excretion or conservation of ions such as H ions<br />or HCO3<br /> ions<br />4. Regulation of all of the above in tissue fluid<br />The process of urine formation, therefore, helps<br />maintain the normal composition, volume, and pH of<br />both blood and tissue fluid by removing those substances<br />that would upset the normal constancy and<br />balance of these extracellular fluids.<br />KIDNEYS<br />The two kidneys are located in the upper abdominal<br />cavity on either side of the vertebral column, behind<br />the peritoneum (retroperitoneal). The upper portions<br />of the kidneys rest on the lower surface of the<br />diaphragm and are enclosed and protected by the<br />lower rib cage (see Fig. 18–1). The kidneys are<br />embedded in adipose tissue that acts as a cushion and<br />is in turn covered by a fibrous connective tissue membrane<br />called the renal fascia, which helps hold the<br />kidneys in place (see Box 18–1: Floating Kidney).<br />Each kidney has an indentation called the hilus<br />on its medial side. At the hilus, the renal artery enters<br />the kidney, and the renal vein and ureter emerge. The<br />renal artery is a branch of the abdominal aorta, and the<br />renal vein returns blood to the inferior vena cava (see<br />Fig. 18–1). The ureter carries urine from the kidney to<br />the urinary bladder.<br />INTERNAL STRUCTURE<br />OF THE KIDNEY<br />In a coronal or frontal section of the kidney, three<br />areas can be distinguished (Fig. 18–2). The lateral and<br />middle areas are tissue layers, and the medial area at<br />the hilus is a cavity. The outer tissue layer is called the<br />renal cortex; it is made of renal corpuscles and convoluted<br />tubules. These are parts of the nephron and are<br />described in the next section. The inner tissue layer is<br />the renal medulla, which is made of loops of Henle<br />and collecting tubules (also parts of the nephron). The<br />renal medulla consists of wedge-shaped pieces called<br />renal pyramids. The tip of each pyramid is its apex or<br />papilla.<br />The third area is the renal pelvis; this is not a layer<br />of tissues, but rather a cavity formed by the expansion<br />of the ureter within the kidney at the hilus. Funnelshaped<br />extensions of the renal pelvis, called calyces<br />(singular: calyx), enclose the papillae of the renal pyramids.<br />Urine flows from the renal pyramids into the<br />calyces, then to the renal pelvis and out into the ureter.<br />THE NEPHRON<br />The nephron is the structural and functional unit of<br />the kidney. Each kidney contains approximately 1 million<br />nephrons. It is in the nephrons, with their associated<br />blood vessels, that urine is formed. Each nephron<br />has two major portions: a renal corpuscle and a renal<br />tubule. Each of these major parts has further subdivisions,<br />which are shown with their blood vessels in Fig.<br />18–3.<br />Renal Corpuscle<br />A renal corpuscle consists of a glomerulus surrounded<br />by a Bowman’s capsule. The glomerulus is a capillary<br />network that arises from an afferent arteriole<br />and empties into an efferent arteriole. The diameter<br />The Urinary System 421<br />BOX 18–1 FLOATING KIDNEY<br />A floating kidney is one that has moved out of its<br />normal position. This may happen in very thin<br />people whose renal cushion of adipose tissue is<br />thin, or it may be the result of a sharp blow to<br />the back that dislodges a kidney.<br />A kidney can function in any position; the<br />problem with a floating kidney is that the ureter<br />may become twisted or kinked. If urine cannot<br />flow through the ureter, the urine backs up and<br />collects in the renal pelvis. Incoming urine<br />from the renal tubules then backs up as well. If<br />the renal filtrate cannot flow out of Bowman’s<br />capsules, the pressure within Bowman’s capsules<br />increases, opposing the blood pressure in the<br />glomeruli. Glomerular filtration then cannot take<br />place efficiently. If uncorrected, this may lead to<br />permanent kidney damage.<br />422 The Urinary System<br />Renal corpuscle<br />Renal cortex<br />Renal tubule<br />Renal medulla<br />Papillary duct<br />B<br />D<br />Papilla of pyramid<br />Calyx<br />Renal pelvis<br />Renal artery<br />Renal vein<br />Interlobar<br />artery<br />Arcuate artery<br />Ureter<br />A<br />C<br />Nephron<br />Renal cortex<br />Renal medulla<br />(pyramids)<br />Figure 18–2. (A) Frontal section of the right kidney showing internal structure and<br />blood vessels. (B) The magnified section of the kidney shows several nephrons. (C) Vascular<br />cast of a kidney in lateral view. Red plastic fills the blood vessels. (D) Vascular cast in medial<br />view. Blood vessels have been removed; yellow plastic fills the renal pelvis. (Photographs C<br />and D by Dan Kaufman.)<br />QUESTION: Which main parts of a nephron are found in the renal cortex? Which areas of<br />a kidney have many blood vessels?<br />423<br />Bowman's capsule<br />(inner layer)<br />Bowman's capsule<br />(outer layer)<br />Efferent arteriole<br />Juxtaglomerular<br />cells<br />Afferent arteriole<br />Collecting tubule<br />Distal<br />convoluted tubule<br />Podocyte<br />Glomerulus<br />Proximal convoluted<br />tubule<br />Loop of Henle<br />Peritubular capillaries<br />Renal<br />cortex<br />Renal<br />medulla<br />Figure 18–3. A nephron with its associated blood vessels. Portions of the nephron have<br />been magnified. The arrows indicate the direction of blood flow and flow of renal filtrate.<br />See text for description.<br />QUESTION: How does the shape of a podocyte contribute to its function? How is the lining<br />of the proximal convoluted tubule specialized?<br />of the efferent arteriole is smaller than that of the<br />afferent arteriole, which helps maintain a fairly high<br />blood pressure in the glomerulus.<br />Bowman’s capsule (or glomerular capsule) is the<br />expanded end of a renal tubule; it encloses the<br />glomerulus. The inner layer of Bowman’s capsule is<br />made of podocytes; the name means “foot cells,” and<br />the “feet” of the podocytes are on the surface of the<br />glomerular capillaries. The arrangement of podocytes<br />creates pores, spaces between adjacent “feet,” which<br />make this layer very permeable. The outer layer of<br />Bowman’s capsule has no pores and is not permeable.<br />The space between the inner and outer layers of<br />Bowman’s capsule contains renal filtrate, the fluid that<br />is formed from the blood in the glomerulus and will<br />eventually become urine.<br />Renal Tubule<br />The renal tubule continues from Bowman’s capsule<br />and consists of the following parts: proximal convoluted<br />tubule (in the renal cortex), loop of Henle (or<br />loop of the nephron, in the renal medulla), and distal<br />convoluted tubule (in the renal cortex). The distal<br />convoluted tubules from several nephrons empty into<br />a collecting tubule. Several collecting tubules then<br />unite to form a papillary duct that empties urine into<br />a calyx of the renal pelvis.<br />Cross-sections of the parts of the renal tubule are<br />shown in Fig. 18–3. Notice how thin the walls of the<br />tubule are, and also the microvilli in the proximal convoluted<br />tubule. These anatomic characteristics provide<br />for efficient exchanges of materials, as you will see.<br />All parts of the renal tubule are surrounded by<br />peritubular capillaries, which arise from the efferent<br />arteriole. The peritubular capillaries will receive the<br />materials reabsorbed by the renal tubules; this is<br />described in the section on urine formation.<br />BLOOD VESSELS OF THE KIDNEY<br />The pathway of blood flow through the kidney is an<br />essential part of the process of urine formation. Blood<br />from the abdominal aorta enters the renal artery,<br />which branches extensively within the kidney into<br />smaller arteries (see Fig. 18–2). The smallest arteries<br />give rise to afferent arterioles in the renal cortex (see<br />Fig. 18–3). From the afferent arterioles, blood flows<br />into the glomeruli (capillaries), to efferent arterioles,<br />to peritubular capillaries, to veins within the kidney, to<br />the renal vein, and finally to the inferior vena cava.<br />Notice that in this pathway there are two sets of capillaries,<br />and recall that it is in capillaries that exchanges<br />take place between the blood and surrounding tissues.<br />Therefore, in the kidneys there are two sites of exchange.<br />The exchanges that take place between the<br />nephrons and the capillaries of the kidneys will form<br />urine from blood plasma.<br />Figure 18–2 shows two views of a vascular cast of a<br />kidney; the shape of the blood vessels has been preserved<br />in red plastic. You can see how dense the vasculature<br />of a kidney is, and most of these vessels are<br />capillaries.<br />FORMATION OF URINE<br />The formation of urine involves three major processes.<br />The first is glomerular filtration, which takes<br />place in the renal corpuscles. The second and third are<br />tubular reabsorption and tubular secretion, which take<br />place in the renal tubules.<br />GLOMERULAR FILTRATION<br />You may recall that filtration is the process in which<br />blood pressure forces plasma and dissolved material<br />out of capillaries. In glomerular filtration, blood<br />pressure forces plasma, dissolved substances, and small<br />proteins out of the glomeruli and into Bowman’s capsules.<br />This fluid is no longer plasma but is called renal<br />filtrate.<br />The blood pressure in the glomeruli, compared<br />with that in other capillaries, is relatively high, about<br />60 mmHg. The pressure in Bowman’s capsule is very<br />low, and its inner, podocyte layer is very permeable, so<br />that approximately 20% to 25% of the blood that<br />enters glomeruli becomes renal filtrate in Bowman’s<br />capsules. The blood cells and larger proteins are too<br />large to be forced out of the glomeruli, so they remain<br />in the blood. Waste products are dissolved in blood<br />plasma, so they pass into the renal filtrate. Useful<br />materials such as nutrients and minerals are also dissolved<br />in plasma and are also present in renal filtrate.<br />Filtration is not selective with respect to usefulness; it<br />is selective only with respect to size. Therefore, renal<br />filtrate is very much like blood plasma, except that<br />there is far less protein and no blood cells are present.<br />The glomerular filtration rate (GFR) is the<br />amount of renal filtrate formed by the kidneys in 1<br />minute, and averages 100 to 125 mL per minute. GFR<br />424 The Urinary System<br />may be altered if the rate of blood flow through the<br />kidney changes. If blood flow increases, the GFR<br />increases, and more filtrate is formed. If blood flow<br />decreases (as may happen following a severe hemorrhage),<br />the GFR decreases, less filtrate is formed, and<br />urinary output decreases (see Box 18–2: Renal Failure<br />and Hemodialysis).<br />TUBULAR REABSORPTION<br />Tubular reabsorption takes place from the renal<br />tubules into the peritubular capillaries. In a 24-hour<br />period, the kidneys form 150 to 180 liters of filtrate,<br />and normal urinary output in that time is 1 to 2 liters.<br />Therefore, it becomes apparent that most of the renal<br />filtrate does not become urine. Approximately 99% of<br />the filtrate is reabsorbed back into the blood in the<br />peritubular capillaries. Only about 1% of the filtrate<br />will enter the renal pelvis as urine.<br />Most reabsorption and secretion (about 65%) take<br />place in the proximal convoluted tubules, whose cells<br />have microvilli that greatly increase their surface area.<br />The distal convoluted tubules and collecting tubules<br />are also important sites for the reabsorption of water<br />(Fig. 18–4).<br />Mechanisms of Reabsorption<br />1. Active transport—the cells of the renal tubule use<br />ATP to transport most of the useful materials from<br />the filtrate to the blood. These useful materials<br />include glucose, amino acids, vitamins, and positive<br />ions.<br />For many of these substances, the renal tubules<br />have a threshold level of reabsorption. This means<br />that there is a limit to how much the tubules can<br />remove from the filtrate. For example, if the filtrate<br />level of glucose is normal (reflecting a normal<br />The Urinary System 425<br />BOX 18–2 RENAL FAILURE AND HEMODIALYSIS<br />artificial kidney machine to do what the patient’s<br />nephrons can no longer do. The patient’s blood is<br />passed through minute tubes surrounded by fluid<br />(dialysate) with the same chemical composition as<br />plasma. Waste products and excess minerals diffuse<br />out of the patient’s blood into the fluid of the<br />machine.<br />Although hemodialysis does prolong life for those<br />with chronic renal failure, it does not fully take the<br />place of functioning kidneys. The increasing success<br />rate of kidney transplants, however, does indeed<br />provide the possibility of a normal life for people<br />with chronic renal failure.<br />Renal failure, the inability of the kidneys to function<br />properly, may be the result of three general<br />causes, which may be called prerenal, intrinsic<br />renal, and postrenal.<br />“Prerenal” means that the problem is “before”<br />the kidneys, that is, in the blood flow to the kidneys.<br />Any condition that decreases blood flow to<br />the kidneys may result in renal damage and failure.<br />Examples are severe hemorrhage or very low blood<br />pressure following a heart attack (MI).<br />“Intrinsic renal” means that the problem is in the<br />kidneys themselves. Diabetes and hypertension<br />damage the blood vessels of the kidneys, and are<br />the causes of 70% of all cases of end-stage renal<br />failure. Bacterial infections of the kidneys or exposure<br />to chemicals (certain antibiotics) may cause<br />damage to the nephrons. Polycystic kidney disease<br />is a genetic disorder in which the kidney tubules<br />dilate and become nonfunctional. Severe damage<br />may not be apparent until age 40 to 60 years but<br />may then progress to renal failure.<br />“Postrenal” means that the problem is “after” the<br />kidneys, somewhere in the rest of the urinary tract.<br />Obstruction of urine flow may be caused by kidney<br />stones, a twisted ureter, or prostatic hypertrophy.<br />Treatment of renal failure involves correcting the<br />specific cause, if possible. If not possible, and kidney<br />damage is permanent, the person is said to have<br />chronic renal failure. Hemodialysis is the use of an<br />A<br />C<br />B<br />Box Figure 18–A Causes of renal failure. (A) Prerenal.<br />(B) Intrinsic renal. (C) Postrenal.<br />426 The Urinary System<br />Proximal convoluted tubule<br />H2O<br />Glucose<br />Amino acids<br />Small proteins<br />Minerals<br />Wastes<br />Bowman's capsule<br />Glomerulus<br />Efferent arteriole<br />Afferent arteriole<br />Distal<br />convoluted<br />tubule<br />Collecting<br />tubule<br />H2O<br />H+<br />Ammonia<br />H2O<br />Glucose<br />Amino acids<br />Small proteins<br />Minerals<br />H2O<br />H2O<br />Wastes<br />H+<br />Na+<br />K+<br />Urine<br />Artery<br />Vein<br />Creatinine<br />Medications<br />Peritubular<br />capillaries<br />Loop of Henle<br />Glomerular<br />filtration<br />Tubular<br />reabsorption<br />Tubular secretion<br />Figure 18–4. Schematic representation of glomerular filtration, tubular reabsorption,<br />and tubular secretion. The renal tubule has been uncoiled, and the peritubular capillaries<br />are shown adjacent to the tubule.<br />QUESTION: Describe tubular secretion; that is, it goes from where to where? What substances<br />may be secreted?<br />blood glucose level), the tubules will reabsorb all of<br />the glucose, and none will be found in the urine.<br />What happens is this: The number of glucose<br />transporter molecules in the membranes of the<br />tubule cells is sufficient to take in the number of<br />glucose molecules passing by in the filtrate. If,<br />however, the blood glucose level is above normal,<br />the amount of glucose in the filtrate will also be<br />above normal and will exceed the threshold level of<br />reabsorption. The number of glucose molecules to<br />be reabsorbed is more than the number of the<br />transporter molecules available to do so. In this situation,<br />therefore, some glucose will remain in the<br />filtrate and be present in urine.<br />The reabsorption of Ca 2 ions is increased by<br />parathyroid hormone (PTH). The parathyroid<br />glands secrete PTH when the blood calcium level<br />decreases. The reabsorption of Ca 2 ions by the<br />kidneys is one of the mechanisms by which the<br />blood calcium level is raised back to normal.<br />The hormone aldosterone, secreted by the adrenal<br />cortex, increases the reabsorption of Na ions<br />and the excretion of K ions. Besides regulating the<br />blood levels of sodium and potassium, aldosterone<br />also affects the volume of blood.<br />2. Passive transport—many of the negative ions that<br />are returned to the blood are reabsorbed following<br />the reabsorption of positive ions, because unlike<br />charges attract.<br />3. Osmosis—the reabsorption of water follows the<br />reabsorption of minerals, especially sodium ions.<br />The hormones that affect reabsorption of water are<br />discussed in the next section.<br />4. Pinocytosis—small proteins are too large to be<br />reabsorbed by active transport. They become<br />adsorbed to the membranes of the cells of the proximal<br />convoluted tubules. The cell membrane then<br />sinks inward and folds around the protein to take it<br />in (see Fig. 3–3 for depictions of this and the other<br />transport mechanisms). Normally all proteins in<br />the filtrate are reabsorbed; none is found in urine.<br />TUBULAR SECRETION<br />This mechanism also changes the composition of<br />urine. In tubular secretion, substances are actively<br />secreted from the blood in the peritubular capillaries<br />into the filtrate in the renal tubules. Waste products,<br />such as ammonia and some creatinine, and the metabolic<br />products of medications may be secreted into the<br />filtrate to be eliminated in urine. Hydrogen ions (H )<br />may be secreted by the tubule cells to help maintain<br />the normal pH of blood.<br />HORMONES THAT INFLUENCE<br />REABSORPTION OF WATER<br />Aldosterone is secreted by the adrenal cortex in response<br />to a high blood potassium level, to a low blood<br />sodium level, or to a decrease in blood pressure. When<br />aldosterone stimulates the reabsorption of Na ions,<br />water follows from the filtrate back to the blood. This<br />helps maintain normal blood volume and blood pressure.<br />You may recall that the antagonist to aldosterone is<br />atrial natriuretic peptide (ANP), which is secreted<br />by the atria of the heart when the atrial walls are<br />stretched by high blood pressure or greater blood volume.<br />ANP decreases the reabsorption of Na ions by<br />the kidneys; these remain in the filtrate, as does water,<br />and are excreted. By increasing the elimination of<br />sodium and water, ANP lowers blood volume and<br />blood pressure.<br />Antidiuretic hormone (ADH) is released by the<br />posterior pituitary gland when the amount of water in<br />the body decreases. Under the influence of ADH, the<br />distal convoluted tubules and collecting tubules are<br />able to reabsorb more water from the renal filtrate.<br />This helps maintain normal blood volume and blood<br />pressure, and also permits the kidneys to produce<br />urine that is more concentrated than body fluids.<br />Producing a concentrated urine is essential to prevent<br />excessive water loss while still excreting all the substances<br />that must be eliminated.<br />If the amount of water in the body increases, however,<br />the secretion of ADH diminishes, and the kidneys<br />will reabsorb less water. Urine then becomes<br />dilute, and water is eliminated until its concentration<br />in the body returns to normal. This may occur following<br />ingestion of excessive quantities of fluids.<br />SUMMARY OF URINE FORMATION<br />1. The kidneys form urine from blood plasma. Blood<br />flow through the kidneys is a major factor in determining<br />urinary output.<br />2. Glomerular filtration is the first step in urine formation.<br />Filtration is not selective in terms of usefulness<br />of materials; it is selective only in terms of<br />size. High blood pressure in the glomeruli forces<br />The Urinary System 427<br />plasma, dissolved materials, and small proteins into<br />Bowman’s capsules; the fluid is now called renal<br />filtrate.<br />3. Tubular reabsorption is selective in terms of usefulness.<br />Nutrients such as glucose, amino acids, and<br />vitamins are reabsorbed by active transport and<br />may have renal threshold levels. Positive ions are<br />reabsorbed by active transport and negative<br />ions are reabsorbed most often by passive transport.<br />Water is reabsorbed by osmosis, and small<br />proteins are reabsorbed by pinocytosis.<br />Reabsorption takes place from the filtrate in the<br />renal tubules to the blood in the peritubular capillaries.<br />4. Tubular secretion takes place from the blood in the<br />peritubular capillaries to the filtrate in the renal<br />tubules and can ensure that wastes such as creatinine<br />or excess H ions are actively put into the filtrate<br />to be excreted.<br />5. Hormones such as aldosterone, ANP, and ADH<br />influence the reabsorption of water and help maintain<br />normal blood volume and blood pressure. The<br />secretion of ADH determines whether a concentrated<br />or dilute urine will be formed.<br />6. Waste products remain in the renal filtrate and are<br />excreted in urine. The effects of hormones on the<br />kidneys are summarized in Table 18–1 and illustrated<br />in Fig. 18–5.<br />THE KIDNEYS AND<br />ACID–BASE BALANCE<br />The kidneys are the organs most responsible for maintaining<br />the pH of blood and tissue fluid within normal<br />ranges. They have the greatest ability to compensate<br />for the pH changes that are a normal part of body<br />metabolism or the result of disease, and to make the<br />necessary corrections.<br />This regulatory function of the kidneys is complex,<br />but at its simplest it may be described as follows. If<br />body fluids are becoming too acidic, the kidneys will<br />secrete more H ions into the renal filtrate and will<br />return more HCO3<br /> ions to the blood. This will help<br />raise the pH of the blood back to normal. The reactions<br />involved in such a mechanism are shown in Fig.<br />18–6, to which we will return later. First, however, let<br />us briefly consider how the kidneys will compensate<br />for body fluids that are becoming too alkaline. You<br />might expect the kidneys to do just the opposite of<br />what was just described, and that is just what happens.<br />The kidneys will return H ions to the blood and<br />excrete HCO3<br /> ions in urine. This will help lower the<br />pH of the blood back to normal.<br />Because the natural tendency is for body fluids to<br />become more acidic, let us look at the pH-raising<br />mechanism in more detail (see Fig. 18–6). The cells<br />of the renal tubules can secrete H ions or ammonia<br />in exchange for Na ions and, by doing so, influence<br />the reabsorption of other ions. Hydrogen ions are<br />obtained from the reaction of CO2 and water (or other<br />processes). An amine group from an amino acid is<br />combined with an H ion to form ammonia.<br />The tubule cell secretes the H ion and the ammonia<br />into the renal filtrate, and two Na ions are reabsorbed<br />in exchange. In the filtrate, the H ion and<br />ammonia form NH4<br /> (an ammonium radical), which<br />reacts with a chloride ion (Cl ) to form NH4Cl<br />(ammonium chloride) that is excreted in urine.<br />As the Na ions are returned to the blood in the<br />428 The Urinary System<br />Table 18–1 EFFECTS OF HORMONES ON THE KIDNEYS<br />Hormone (gland) Function<br />Antidiuretic hormone (ADH)<br />(posterior pituitary)<br />Parathyroid hormone (PTH)<br />(parathyroid glands)<br />Aldosterone<br />(adrenal cortex)<br />Atrial natriuretic peptide<br />(ANP) (atria of heart)<br />• Increases reabsorption of water from the filtrate to the blood.<br />• Increases reabsorption of Ca 2 ions from filtrate to the blood and excretion of<br />phosphate ions into the filtrate.<br />• Increases reabsorption of Na ions from the filtrate to the blood and excretion of<br />K ions into the filtrate. Water is reabsorbed following the reabsorption of sodium.<br />• Decreases reabsorption of Na ions, which remain in the filtrate. More sodium<br />and water are eliminated in urine.<br />peritubular capillaries, HCO3<br /> ions follow. Notice<br />what has happened: Two H ions have been excreted<br />in urine, and two Na ions and two HCO3<br /> ions have<br />been returned to the blood. As reactions like these<br />take place, the body fluids are prevented from becoming<br />too acidic.<br />Another mechanism used by the cells of the kidney<br />tubules to regulate pH is the phosphate buffer system,<br />which is described in Chapter 19.<br />OTHER FUNCTIONS<br />OF THE KIDNEYS<br />In addition to the functions described thus far, the kidneys<br />have other functions, some of which are not<br />directly related to the formation of urine. These functions<br />are secretion of renin (which does influence<br />urine formation), production of erythropoietin, and<br />activation of vitamin D.<br />1. Secretion of renin—When blood pressure decreases,<br />the juxtaglomerular ( juxta means “next<br />to”) cells in the walls of the afferent arterioles<br />secrete the enzyme renin. Renin then initiates the<br />renin-angiotensin mechanism to raise blood pressure.<br />This was first described in Chapter 13, and<br />the sequence of events is presented in Table 18–2.<br />The end product of this mechanism is angiotensin<br />II, which causes vasoconstriction and increases the<br />secretion of aldosterone, both of which help raise<br />blood pressure.<br />The Urinary System 429<br />ADH Increases reabsorption of H2O<br />ANP<br />PTH<br />Aldosterone<br />Blood<br />Urine<br />Increases reabsorption of Ca+2<br />Increases reabsorption of Na+<br />and excretion of K+<br />Decreases reabsorption of Na+<br />Figure 18–5. Effects of hormones on the kidneys.<br />QUESTION: Do any of these hormones affect both reabsorption and secretion? If so, how?<br />A normal blood pressure is essential to normal<br />body functioning. Perhaps the most serious change<br />is a sudden, drastic decrease in blood pressure, such<br />as would follow a severe hemorrhage. In response to<br />such a decrease, the kidneys will decrease filtration<br />and urinary output and will initiate the formation of<br />angiotensin II. In these ways the kidneys help<br />ensure that the heart has enough blood to pump to<br />maintain cardiac output and blood pressure.<br />2. Secretion of erythropoietin—This hormone is<br />secreted whenever the blood oxygen level decreases<br />(a state of hypoxia). Erythropoietin stimulates<br />the red bone marrow to increase the rate of RBC<br />production. With more RBCs in circulation, the<br />oxygen-carrying capacity of the blood is greater,<br />and the hypoxic state may be corrected (see also<br />Box 18–3: Erythropoietin).<br />3. Activation of vitamin D—This vitamin exists in<br />several structural forms that are converted to calcitriol<br />(D2) by the kidneys. Calcitriol is the most<br />active form of vitamin D, which increases the<br />absorption of calcium and phosphate in the small<br />intestine.<br />430 The Urinary System<br />Peritubular<br />capillary Filtrate<br />Renal tubule<br />Na+ Na+ Na+<br />HCO<br />3<br /> <br />Na+<br />HCO<br />3<br /> <br />HCO<br />3<br /> <br />HCO<br />3<br /> <br />CI CI <br />Na+<br />Na+<br />H CI +<br />H+<br />Na+<br />Na+<br />CO<br />2<br />H<br />2<br />O H<br />2<br />CO<br />3<br /> <br />CO<br />2<br />H<br />2<br />O H<br />2<br />CO<br />3<br /> <br />H+<br />H+<br /> <br /> <br /> <br />+<br />NH<br />3<br />NH<br />2<br />NH<br />NH 4<br />3<br />NH<br />4<br />Cl<br />Blood Urine<br />Figure 18–6. Renal regulation of acid–base<br />balance. The cells of the renal tubule secrete H <br />ions and ammonia into the filtrate and return<br />Na ions and HCO3<br /> ions to the blood in the<br />peritubular capillaries. See text for further description.<br />QUESTION: The cells of the renal tubule make<br />good use of CO2. What do the cells use CO2 for?<br />Table 18–2 THE RENIN-ANGIOTENSIN<br />MECHANISM<br />Sequence<br />1. Decreased blood pressure stimulates the kidneys to<br />secrete renin.<br />2. Renin splits the plasma protein angiotensinogen<br />(synthesized by the liver) to angiotensin I.<br />3. Angiotensin I is converted to angiotensin II by an<br />enzyme found in lung tissue and vascular endothelium.<br />4. Angiotensin II causes vasoconstriction and stimulates<br />the adrenal cortex to secrete aldosterone.<br />ELIMINATION OF URINE<br />The ureters, urinary bladder, and urethra do not<br />change the composition or amount of urine, but are<br />responsible for the periodic elimination of urine.<br />URETERS<br />Each ureter extends from the hilus of a kidney to the<br />lower, posterior side of the urinary bladder (see Fig.<br />18–1). Like the kidneys, the ureters are retroperitoneal,<br />that is, behind the peritoneum of the dorsal<br />abdominal cavity.<br />The smooth muscle in the wall of the ureter contracts<br />in peristaltic waves to propel urine toward the<br />urinary bladder. As the bladder fills, it expands and<br />compresses the lower ends of the ureters to prevent<br />backflow of urine.<br />URINARY BLADDER<br />The urinary bladder is a muscular sac below the peritoneum<br />and behind the pubic bones. In women, the<br />bladder is inferior to the uterus; in men, the bladder is<br />superior to the prostate gland. The bladder is a reservoir<br />for accumulating urine, and it contracts to eliminate<br />urine.<br />The mucosa of the bladder is transitional epithelium,<br />which permits expansion without tearing the<br />lining. When the bladder is empty, the mucosa<br />appears wrinkled; these folds are rugae, which also<br />permit expansion. On the floor of the bladder is a triangular<br />area called the trigone, which has no rugae<br />and does not expand. The points of the triangle are<br />the openings of the two ureters and that of the urethra<br />(Fig. 18–7).<br />The smooth muscle layer in the wall of the bladder<br />is called the detrusor muscle. It is a muscle in the<br />form of a sphere; when it contracts it becomes a<br />smaller sphere, and its volume diminishes. Around the<br />opening of the urethra the muscle fibers of the detrusor<br />form the internal urethral sphincter (or sphincter<br />of the bladder), which is involuntary.<br />URETHRA<br />The urethra (see Fig. 18–7) carries urine from the<br />bladder to the exterior. The external urethral<br />sphincter is made of the surrounding skeletal muscle<br />of the pelvic floor, and is under voluntary control.<br />In women, the urethra is 1 to 1.5 inches (2.5 to 4<br />cm) long and is anterior to the vagina. In men, the<br />urethra is 7 to 8 inches (17 to 20 cm) long. The first<br />part just outside the bladder is called the prostatic urethra<br />because it is surrounded by the prostate gland.<br />The next inch is the membranous urethra, around<br />which is the external urethral sphincter. The longest<br />portion is the cavernous urethra (or spongy or penile<br />urethra), which passes through the cavernous (or erectile)<br />tissue of the penis. The male urethra carries<br />semen as well as urine.<br />THE URINATION REFLEX<br />Urination may also be called micturition or voiding.<br />This reflex is a spinal cord reflex over which voluntary<br />control may be exerted. The stimulus for the reflex is<br />stretching of the detrusor muscle of the bladder. The<br />bladder can hold as much as 800 mL of urine, or even<br />more, but the reflex is activated long before the maximum<br />is reached.<br />When urine volume reaches 200 to 400 mL, the<br />stretching is sufficient to generate sensory impulses<br />that travel to the sacral spinal cord. Motor impulses<br />return along parasympathetic nerves to the detrusor<br />muscle, causing contraction. At the same time, the<br />internal urethral sphincter relaxes. If the external urethral<br />sphincter is voluntarily relaxed, urine flows into<br />the urethra, and the bladder is emptied.<br />Urination can be prevented by voluntary contraction<br />of the external urethral sphincter. However, if the<br />bladder continues to fill and be stretched, voluntary<br />control is eventually no longer possible.<br />The Urinary System 431<br />BOX 18–3 ERYTHROPOIETIN<br />Anemia is one of the most debilitating consequences<br />of renal failure, one that hemodialysis<br />cannot reverse. Diseased kidneys stop producing<br />erythropoietin, a natural stimulus for RBC<br />production. Erythropoietin can be produced by<br />genetic engineering and is available for hemodialysis<br />patients. In the past, their anemia could<br />only be treated with transfusions, which exposed<br />these patients to possible immunologic complications<br />of repeated exposure to donated blood<br />or to viral diseases. The synthetic erythropoietin<br />eliminates such risks. Others who benefit from<br />this medication are cancer patients and AIDS<br />patients with severe anemia.<br />CHARACTERISTICS OF URINE<br />The characteristics of urine include the physical and<br />chemical aspects that are often evaluated as part of a<br />urinalysis. Some of these are described in this section,<br />and others are included in Appendix D: Normal<br />Values for Some Commonly Used Urine Tests.<br />Amount—normal urinary output per 24 hours is 1<br />to 2 liters. Many factors can significantly change<br />output. Excessive sweating or loss of fluid through<br />432 The Urinary System<br />Internal<br />urethral sphincter<br />External<br />urethral sphincter<br />Urethra<br />Urethral orifice<br />Trigone<br />Ureter<br />Trigone<br />Rugae<br />Openings of<br />ureters<br />Detrusor muscle<br />Parietal peritoneum Ureter<br />Prostate gland<br />Prostatic urethra<br />Membranous<br />urethra<br />Cavernous (spongy)<br />urethra<br />Cavernous (erectile)<br />tissue of penis<br />A<br />B<br />Figure 18–7. (A) Frontal section of female urinary bladder and urethra. (B) Frontal section<br />of male urinary bladder and urethra.<br />QUESTION: Name the sphincters of the urinary system and state whether each is voluntary<br />or involuntary.<br />diarrhea will decrease urinary output (oliguria) to<br />conserve body water. Excessive fluid intake will<br />increase urinary output (polyuria). Consumption of<br />alcohol will also increase output because alcohol<br />inhibits the secretion of ADH, and the kidneys will<br />reabsorb less water.<br />Color—the typical yellow color of urine (from<br />urochrome, a breakdown product of bile) is often<br />referred to as “straw” or “amber.” Concentrated<br />urine is a deeper yellow (amber) than is dilute urine.<br />Freshly voided urine is also clear rather than cloudy.<br />Specific gravity—the normal range is 1.010 to 1.025;<br />this is a measure of the dissolved materials in urine.<br />The specific gravity of distilled water is 1.000, meaning<br />that there are no solutes present. Therefore, the<br />higher the specific gravity number, the more dissolved<br />material is present. Someone who has been<br />exercising strenuously and has lost body water in<br />sweat will usually produce less urine, which will<br />be more concentrated and have a higher specific<br />gravity.<br />The specific gravity of the urine is an indicator of<br />the concentrating ability of the kidneys: The kidneys<br />must excrete the waste products that are constantly<br />formed in as little water as possible.<br />pH—the pH range of urine is between 4.6 and 8.0,<br />with an average value of 6.0. Diet has the greatest<br />influence on urine pH. A vegetarian diet will result<br />in a more alkaline urine, whereas a high-protein<br />diet will result in a more acidic urine.<br />Constituents—urine is approximately 95% water,<br />which is the solvent for waste products and salts.<br />Salts are not considered true waste products because<br />they may well be utilized by the body when needed,<br />but excess amounts will be excreted in urine (see<br />Box 18–4: Kidney Stones).<br />Nitrogenous wastes—as their name indicates, all of<br />these wastes contain nitrogen. Urea is formed by<br />liver cells when excess amino acids are deaminated<br />to be used for energy production. Creatinine comes<br />from the metabolism of creatine phosphate, an<br />energy source in muscles. Uric acid comes from the<br />metabolism of nucleic acids, that is, the breakdown<br />of DNA and RNA. Although these are waste products,<br />there is always a certain amount of each in the<br />blood. Box 18–5: Blood Tests and Kidney Function<br />describes the relationship between blood levels of<br />these waste products and kidney function.<br />Other non-nitrogenous waste products include<br />small amounts of urobilin from the hemoglobin of<br />old RBCs (see Fig. 11–4), and may include the<br />metabolic products of medications. Table 18–3<br />summarizes the characteristics of urine.<br />When a substance not normally found in urine<br />does appear there, there is a reason for it. The reason<br />may be quite specific or more general. Table<br />18–4 lists some abnormal constituents of urine and<br />possible reasons for each (see also Box 18–6:<br />Urinary Tract Infections).<br />AGING AND THE URINARY SYSTEM<br />With age, the number of nephrons in the kidneys<br />decreases, often to half the original number by the age<br />of 70 to 80, and the kidneys lose some of their con-<br />The Urinary System 433<br />BOX 18–4 KIDNEY STONES<br />The entry of a kidney stone into a ureter may<br />cause intense pain (renal colic) and bleeding.<br />Obstruction of a ureter by a stone may cause<br />backup of urine and possible kidney damage.<br />Treatments include surgery to remove the stone, or<br />lithotripsy, the use of shock waves to crush the<br />stone into pieces small enough to be eliminated<br />without damage to the urinary tract. A recent study<br />links lithotripsy with an increased risk of diabetes or<br />hypertension later in life, though the mechanisms<br />that would bring about these conditions have not<br />yet been discovered.<br />Kidney stones, or renal calculi, are crystals of the<br />salts that are normally present in urine. A very high<br />concentration of salts in urine may trigger precipitation<br />of the salt and formation of crystals, which<br />can range in size from microscopic to 10 to 20 mm<br />in diameter. The most common type of kidney<br />stone is made of calcium salts; a less common type<br />is made of uric acid.<br />Kidney stones are most likely to form in the renal<br />pelvis. Predisposing factors include decreased fluid<br />intake or overingestion of minerals (as in mineral<br />supplements), both of which lead to the formation<br />of a very concentrated urine.<br />434 The Urinary System<br />Table 18–3 CHARACTERISTICS OF NORMAL URINE<br />Characteristic Description<br />Amount<br />Color<br />Specific gravity<br />pH<br />Composition<br />Nitrogenous wastes<br />1–2 liters per 24 hours; highly variable depending on fluid intake and water loss through the<br />skin and GI tract<br />Straw or amber; darker means more concentrated; should be clear, not cloudy<br />1.010–1.025; a measure of the dissolved material in urine; the lower the value, the more dilute<br />the urine<br />Average 6; range 4.6–8.0; diet has the greatest effect on urine pH<br />95% water; 5% salts and waste products<br />Urea—from amino acid metabolism<br />Creatinine—from muscle metabolism<br />Uric acid—from nucleic acid metabolism<br />Table 18–4 ABNORMAL CONSTITUENTS IN URINE<br />Characteristic Reason(s)<br />Glycosuria<br />(presence of<br />glucose)<br />Proteinuria<br />(presence of<br />protein)<br />Hematuria<br />(presence of<br />blood—RBCs)<br />Bacteriuria<br />(presence<br />of bacteria)<br />Ketonuria<br />(presence of<br />ketones)<br />As long as blood glucose levels are within normal limits, filtrate levels will also be normal and will<br />not exceed the threshold level for reabsorption. In an untreated diabetic, for example, blood<br />glucose is too high; therefore the filtrate glucose level is too high. The kidneys reabsorb glucose<br />up to their threshold level, but the excess remains in the filtrate and is excreted in urine.<br />Most plasma proteins are too large to be forced out of the glomeruli, and the small proteins that<br />enter the filtrate are reabsorbed by pinocytosis. The presence of protein in the urine indicates<br />that the glomeruli have become too permeable, as occurs in some types of kidney disease.<br />The presence of RBCs in urine may also indicate that the glomeruli have become too permeable.<br />Another possible cause might be bleeding somewhere in the urinary tract. Pinpointing the site<br />of bleeding would require specific diagnostic tests.<br />Bacteria give urine a cloudy rather than clear appearance; WBCs may be present also. The presence<br />of bacteria means that there is an infection somewhere in the urinary tract. Further diagnostic<br />tests would be needed to determine the precise location.<br />Ketones are formed from fats and proteins that are used for energy production. A trace of ketones<br />in urine is normal. Higher levels of ketones indicate an increased use of fats and proteins for<br />energy. This may be the result of malfunctioning carbohydrate metabolism (as in diabetes mellitus)<br />or simply the result of a high-protein diet.<br />BOX 18–5 BLOOD TESTS AND KIDNEY FUNCTION<br />impaired. Of the three, the creatinine level is probably<br />the most reliable indicator of kidney functioning.<br />Blood urea nitrogen (BUN) may vary considerably in<br />certain situations not directly related to the kidneys.<br />For example, BUN may be elevated as a consequence<br />of a high-protein diet or of starvation when<br />body protein is being broken down at a faster rate<br />than normal. Uric acid levels may also vary according<br />to diet. However, elevated blood levels of all<br />three nitrogenous wastes usually indicate impaired<br />glomerular filtration.<br />Waste products are normally present in the blood,<br />and the concentration of each varies within a normal<br />range. As part of the standard lab work called<br />blood chemistry, the levels of the three nitrogenous<br />waste products are determined (urea, creatinine,<br />and uric acid).<br />If blood levels of these three substances are<br />within normal ranges, it may be concluded that the<br />kidneys are excreting these wastes at normal rates.<br />If, however, these blood levels are elevated, one<br />possible cause is that kidney function has been<br />centrating ability. The glomerular filtration rate also<br />decreases, partly as a consequence of arteriosclerosis<br />and diminished renal blood flow. Despite these changes,<br />excretion of nitrogenous wastes usually remains<br />adequate.<br />The urinary bladder decreases in size, and the tone<br />of the detrusor muscle decreases. These changes may<br />lead to a need to urinate more frequently. Urinary<br />incontinence (the inability to control voiding) is not an<br />inevitable consequence of aging, and can be prevented<br />or minimized. Elderly people are, however, more at<br />risk for infections of the urinary tract, especially if<br />voiding leaves residual urine in the bladder.<br />SUMMARY<br />The kidneys are the principal regulators of the internal<br />environment of the body. The composition of all<br />body fluids is either directly or indirectly regulated by<br />the kidneys as they form urine from blood plasma.<br />The kidneys are also of great importance in the regulation<br />of the pH of the body fluids. These topics are<br />the subject of the next chapter.<br />The Urinary System 435<br />BOX 18–6 URINARY TRACT INFECTIONS<br />Symptoms include frequency of urination, painful<br />voiding, and low back pain. Nephritis (or pyelonephritis)<br />is inflammation of the kidneys. Although<br />this may be the result of a systemic bacterial infection,<br />nephritis is a common complication of<br />untreated lower urinary tract infections such as cystitis.<br />Possible symptoms are fever and flank pain (in<br />the area of the kidneys). Untreated nephritis may<br />result in severe damage to nephrons and progress<br />to renal failure.<br />Infections may occur anywhere in the urinary tract<br />and are most often caused by the microbial agents<br />of sexually transmitted diseases (see Chapter 20) or<br />by the bacteria that are part of the normal flora of<br />the colon. In women especially, the urinary and<br />anal openings are in close proximity, and colon bacteria<br />on the skin of the perineum may invade the<br />urinary tract. The use of urinary catheters in hospitalized<br />or bedridden patients may also be a factor if<br />sterile technique is not carefully followed.<br />Cystitis is inflammation of the urinary bladder.<br />STUDY OUTLINE<br />The urinary system consists of two kidneys,<br />two ureters, the urinary bladder, and the<br />urethra.<br />1. The kidneys form urine to excrete waste products<br />and to regulate the volume, electrolytes, and pH of<br />blood and tissue fluid.<br />2. The other organs of the system are concerned with<br />elimination of urine.<br />Kidneys (see Fig. 18–1)<br />1. Retroperitoneal on either side of the backbone in<br />the upper abdominal cavity; partially protected by<br />the lower rib cage.<br />2. Adipose tissue and the renal fascia cushion the kidneys<br />and help hold them in place.<br />3. Hilus—an indentation on the medial side; renal<br />artery enters, renal vein and ureter emerge.<br />Kidney—internal structure (see Fig. 18–2)<br />1. Renal cortex—outer tissue layer, made of renal corpuscles<br />and convoluted tubules.<br />2. Renal medulla (pyramids)—inner tissue layer,<br />made of loops of Henle and collecting tubules.<br />3. Renal pelvis—a cavity formed by the expanded end<br />of the ureter within the kidney at the hilus; extensions<br />around the papillae of the pyramids are called<br />calyces, which collect urine.<br />The Nephron—the functional unit of the kidney<br />(see Fig. 18–3); 1 million per kidney<br />1. Renal corpuscle—consists of a glomerulus surrounded<br />by a Bowman’s capsule.<br />• Glomerulus—a capillary network between an<br />afferent arteriole and an efferent arteriole.<br />• Bowman’s capsule—the expanded end of a renal<br />tubule that encloses the glomerulus; inner layer<br />is made of podocytes, has pores, and is very permeable;<br />contains renal filtrate (potential urine).<br />2. Renal tubule—consists of the proximal convoluted<br />tubule, loop of Henle, distal convoluted tubule,<br />and collecting tubule. Collecting tubules unite to<br />form papillary ducts that empty urine into the<br />calyces of the renal pelvis.<br />• Peritubular capillaries—arise from the efferent<br />arteriole and surround all parts of the renal<br />tubule.<br />Blood Vessels of the Kidney (see Figs. 18–1,<br />18–2, and 18–3)<br />1. Pathway: abdominal aorta → renal artery → small<br />arteries in the kidney → afferent arterioles →<br />glomeruli → efferent arterioles → peritubular capillaries<br />→ small veins in the kidney → renal vein →<br />inferior vena cava.<br />2. Two sets of capillaries provide for two sites of<br />exchanges between the blood and tissues in the<br />process of urine formation.<br />Formation of Urine (see Fig. 18–4)<br />1. Glomerular filtration—takes place from the<br />glomerulus to Bowman’s capsule. High blood pressure<br />(60 mmHg) in the glomerulus forces plasma,<br />dissolved materials, and small proteins out of the<br />blood and into Bowman’s capsule. The fluid is now<br />called filtrate. Filtration is selective only in terms of<br />size; blood cells and large proteins remain in the<br />blood.<br />2. GFR is 100 to 125 mL per minute. Increased blood<br />flow to the kidney increases GFR; decreased blood<br />flow decreases GFR.<br />3. Tubular reabsorption—takes place from the filtrate<br />in the renal tubule to the blood in the peritubular<br />capillaries; 99% of the filtrate is reabsorbed; only<br />1% becomes urine.<br />• Active transport—reabsorption of glucose,<br />amino acids, vitamins, and positive ions; threshold<br />level is a limit to the quantity that can be<br />reabsorbed.<br />• Passive transport—most negative ions follow the<br />reabsorption of positive ions.<br />• Osmosis—water follows the reabsorption of<br />minerals, especially sodium.<br />• Pinocytosis—small proteins are engulfed by<br />proximal tubule cells.<br />4. Tubular secretion—takes place from the blood in<br />the peritubular capillaries to the filtrate in the renal<br />tubule; creatinine and other waste products may be<br />secreted into the filtrate to be excreted in urine;<br />secretion of H ions helps maintain pH of blood.<br />5. Hormones that affect reabsorption—aldosterone,<br />atrial natriuretic peptide, antidiuretic hormone,<br />and parathyroid hormone—see Table 18–1 and<br />Fig. 18–5.<br />The Kidneys and Acid–Base Balance<br />1. The kidneys have the greatest capacity to compensate<br />for normal and abnormal pH changes.<br />2. If the body fluids are becoming too acidic, the kidneys<br />excrete H ions and return HCO3<br /> ions to<br />the blood (see Fig. 18–6).<br />3. If the body fluids are becoming too alkaline, the<br />kidneys return H ions to the blood and excrete<br />HCO3<br /> ions.<br />Other Functions of the Kidneys<br />1. Secretion of renin by juxtaglomerular cells when<br />blood pressure decreases (see Table 18–2). Angiotensin<br />II causes vasoconstriction and increases<br />secretion of aldosterone.<br />2. Secretion of erythropoietin in response to hypoxia;<br />stimulates red bone marrow to increase rate of<br />RBC production.<br />3. Activation of vitamin D—conversion of inactive<br />forms to the active form.<br />Elimination of Urine—the function of the<br />ureters, urinary bladder, and urethra<br />Ureters (see Figs. 18–1 and 18–7)<br />1. Each extends from the hilus of a kidney to the<br />lower posterior side of the urinary bladder.<br />2. Peristalsis of smooth muscle layer propels urine<br />toward bladder.<br />Urinary Bladder (see Figs. 18–1 and 18–7)<br />1. A muscular sac below the peritoneum and behind<br />the pubic bones; in women, below the uterus; in<br />men, above the prostate gland.<br />2. Mucosa—transitional epithelial tissue folded into<br />rugae; permit expansion without tearing.<br />3. Trigone—triangular area on bladder floor; no<br />rugae, does not expand; bounded by openings of<br />ureters and urethra.<br />436 The Urinary System<br />4. Detrusor muscle—the smooth muscle layer, a<br />spherical muscle; contracts to expel urine (reflex).<br />5. Internal urethral sphincter—involuntary; formed<br />by detrusor muscle fibers around the opening of<br />the urethra.<br />Urethra—takes urine from the bladder to<br />the exterior (see Fig. 18–7)<br />1. In women—1 to 1.5 inches long; anterior to vagina.<br />2. In men—7 to 8 inches long; passes through the<br />prostate gland and penis.<br />3. Has the external urethral sphincter: skeletal muscle<br />of pelvic floor (voluntary).<br />The Urination Reflex—also called micturition<br />or voiding<br />1. Stimulus: stretching of the detrusor muscle by<br />accumulating urine.<br />2. Sensory impulses to spinal cord, motor impulses<br />(parasympathetic) return to detrusor muscle, which<br />contracts; internal urethral sphincter relaxes.<br />3. The external urethral sphincter provides voluntary<br />control.<br />Characteristics of Urine (see Table 18–3)<br />Abnormal Constituents of Urine (see<br />Table 18–4)<br />The Urinary System 437<br />REVIEW QUESTIONS<br />1. Describe the location of the kidneys, ureters, urinary<br />bladder, and urethra. (pp. 421, 431)<br />2. Name the three areas of the kidney, and state what<br />each consists of. (p. 421)<br />3. Name the two major parts of a nephron. State the<br />general function of nephrons. (p. 421)<br />4. Name the parts of a renal corpuscle. What process<br />takes place here? Name the parts of a renal tubule.<br />What processes take place here? (pp. 421, 424)<br />5. State the mechanism of tubular reabsorption of<br />each of the following: (pp. 425, 427)<br />a. Water<br />b. Glucose<br />c. Small proteins<br />d. Positive ions<br />e. Negative ions<br />f. Amino acids<br />g. Vitamins<br />Also explain what is meant by a threshold level of<br />reabsorption.<br />6. Explain the importance of tubular secretion. (p.<br />427)<br />7. Describe the pathway of blood flow through the<br />kidney from the abdominal aorta to the inferior<br />vena cava. (p. 424)<br />8. Name the two sets of capillaries in the kidney, and<br />state the processes that take place in each. (pp. 424,<br />425)<br />9. Name the hormone that has each of these effects<br />on the kidneys: (pp. 428–429)<br />a. Promotes reabsorption of Na ions<br />b. Promotes direct reabsorption of water<br />c. Promotes reabsorption of Ca 2 ions<br />d. Promotes excretion of K ions<br />e. Decreases reabsorption of Na ions<br />10. In what circumstances will the kidneys excrete H <br />ions? What ions will be returned to the blood?<br />How will this affect the pH of blood? (p. 428)<br />11. In what circumstances do the kidneys secrete<br />renin, and what is its purpose? (p. 429)<br />12. In what circumstances do the kidneys secrete erythropoietin,<br />and what is its purpose? (p. 430)<br />13. Describe the function of the ureters and that of<br />the urethra. (p. 431)<br />14. With respect to the urinary bladder, describe<br />the function of rugae and the detrusor muscle.<br />(p. 431)<br />15. Describe the urination reflex in terms of stimulus,<br />part of the CNS involved, effector muscle, internal<br />urethral sphincter, and voluntary control.<br />(pp. 431)<br />16. Describe the characteristics of normal urine in<br />terms of appearance, amount, pH, specific gravity,<br />and composition. (pp. 432–433)<br />17. State the source of each of the nitrogenous waste<br />products: creatinine, uric acid, and urea. (p. 433)<br />1. The functioning of the kidneys may be likened to<br />cleaning your room by throwing everything out the<br />window, then going outside to retrieve what you<br />wish to keep, such as jammies and slippers. Imagine<br />the contents of a room, liken them to the materials<br />in the blood (you yourself are a kidney), and<br />describe what happens to each, and why.<br />2. Explain why fatty acids are not found in urine.<br />Under what circumstances are water-soluble vitamins<br />(such as vitamin C) found in urine?<br />3. Explain how a spinal cord transection at the level of<br />T11 will affect the urination reflex.<br />4. As part of his yearly physical for the college football<br />team, 20-year-old Patrick has a urinalysis,<br />which shows a high level of ketones. He is not<br />diabetic, and is not ill. What might cause the<br />high urine level of ketones? What blood chemistry<br />test (for nitrogenous wastes) would help confirm<br />this?<br />5. A patient being evaluated for food poisoning has a<br />blood pH of 7.33, a urine pH of 4.5, and a respiratory<br />rate of 28 per minute. What kind of pH imbalance<br />is this? Explain your reasoning step by step.<br />6. Erythropoietin, called EPO, has become a drug<br />used illegally by some athletes. Which athletes use<br />EPO, that is, in what kind of sports? What benefits<br />are they hoping for? What part of a CBC would<br />indicate that an athlete is taking EPO? Explain.<br />7. After a 4-hour workout on a hot June day, the high<br />school track coach tells her group to keep drinking<br />plenty of water. The girls assure their coach that<br />they will know just how to determine if they are sufficiently<br />hydrated that evening, that they have their<br />color scheme memorized. What do they mean?<br />438 The Urinary System<br />FOR FURTHER THOUGHTinternet fast worldhttp://www.blogger.com/profile/13869077830569899582noreply@blogger.com0tag:blogger.com,1999:blog-135611804747902727.post-84366868056021349472010-06-27T07:28:00.001-07:002010-06-27T07:57:00.739-07:00endocrine and moreQuestion Figure 9–A<br />CHAPTER 10<br />The Endocrine System<br />221<br />222<br />CHAPTER 10<br />Chapter Outline<br />Chemistry of Hormones<br />Regulation of Hormone Secretion<br />The Pituitary Gland<br />Posterior Pituitary Gland<br />Antidiuretic hormone<br />Oxytocin<br />Anterior Pituitary Gland<br />Growth hormone<br />Thyroid-stimulating hormone<br />Adrenocorticotropic hormone<br />Prolactin<br />Follicle-stimulating hormone<br />Luteinizing hormone<br />Thyroid Gland<br />Thyroxine and T3<br />Calcitonin<br />Parathyroid Glands<br />Parathyroid Hormone<br />Pancreas<br />Glucagon<br />Insulin<br />Adrenal Glands<br />Adrenal Medulla<br />Epinephrine and norepinephrine<br />Adrenal Cortex<br />Aldosterone<br />Cortisol<br />Ovaries<br />Estrogen<br />Progesterone<br />Inhibin<br />Testes<br />Testosterone<br />Inhibin<br />Other Hormones<br />Prostaglandins<br />Mechanisms of Hormone Action<br />The Two-Messenger Mechanism—Protein<br />Hormones<br />Action of Steroid Hormones<br />Aging and the Endocrine System<br />BOX 10–1 DISORDERS OF GROWTH HORMONE<br />BOX 10–2 DISORDERS OF THYROXINE<br />BOX 10–3 DIABETES MELLITUS<br />BOX 10–4 DISORDERS OF THE ADRENAL CORTEX<br />Student Objectives<br />• Name the endocrine glands and the hormones<br />secreted by each.<br />• Explain how a negative feedback mechanism<br />works.<br />• Explain how the hypothalamus is involved<br />in the secretion of hormones from the posterior<br />pituitary gland and anterior pituitary<br />gland.<br />• State the functions of oxytocin and antidiuretic<br />hormone, and explain the stimulus for secretion<br />of each.<br />• State the functions of the hormones of the anterior<br />pituitary gland, and state the stimulus for<br />secretion of each.<br />The Endocrine System<br />223<br />Student Objectives (Continued)<br />• State the functions of thyroxine and T3, and<br />describe the stimulus for their secretion.<br />• Explain how parathyroid hormone and calcitonin<br />work as antagonists.<br />• Explain how insulin and glucagon work as antagonists.<br />• State the functions of epinephrine and norepinephrine,<br />and explain their relationship to the sympathetic<br />division of the autonomic nervous system.<br />• State the functions of aldosterone and cortisol, and<br />describe the stimulus for secretion of each.<br />• State the functions of estrogen, progesterone,<br />testosterone, and inhibin and state the stimulus for<br />secretion of each.<br />• Explain what prostaglandins are made of, and state<br />some of their functions.<br />• Explain how the protein hormones are believed to<br />exert their effects.<br />• Explain how the steroid hormones are believed to<br />exert their effects.<br />New Terminology<br />Alpha cells (AL-fah SELLS)<br />Beta cells (BAY-tah SELLS)<br />Catecholamines (KAT-e-kohl-ah-MEENZ)<br />Corpus luteum (KOR-pus LOO-tee-um)<br />Gluconeogenesis (GLOO-koh-nee-oh-JEN-i-sis)<br />Glycogenesis (GLIGH-koh-JEN-i-sis)<br />Glycogenolysis (GLIGH-ko-jen-OL-i-sis)<br />Hypercalcemia (HIGH-per-kal-SEE-mee-ah)<br />Hyperglycemia (HIGH-per-gligh-SEE-mee-ah)<br />Hypocalcemia (HIGH-poh-kal-SEE-mee-ah)<br />Hypoglycemia (HIGH-poh-gligh-SEE-mee-ah)<br />Hypophysis (high-POFF-e-sis)<br />Islets of Langerhans (EYE-lets of LAHNG-er-hanz)<br />Prostaglandins (PRAHS-tah-GLAND-ins)<br />Renin-angiotensin mechanism (REE-nin AN-jeeoh-<br />TEN-sin)<br />Sympathomimetic (SIM-pah-tho-mi-MET-ik)<br />Target organ (TAR-get OR-gan)<br />Related Clinical Terminology<br />Acromegaly (AK-roh-MEG-ah-lee)<br />Addison’s disease (ADD-i-sonz)<br />Cretinism (KREE-tin-izm)<br />Cushing’s syndrome (KOOSH-ingz SIN-drohm)<br />Diabetes mellitus (DYE-ah-BEE-tis mel-LYE-tus)<br />Giantism (JIGH-an-tizm)<br />Goiter (GOY-ter)<br />Graves’ disease (GRAYVES)<br />Ketoacidosis (KEY-toh-ass-i-DOH-sis)<br />Myxedema (MIK-suh-DEE-mah)<br />Pituitary dwarfism (pi-TOO-i-TER-ee DWORFizm)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />We have already seen how the nervous system<br />regulates body functions by means of nerve impulses<br />and integration of information by the spinal cord and<br />brain. The other regulating system of the body is<br />the endocrine system, which consists of endocrine<br />glands that secrete chemicals called hormones. These<br />glands, and the names of the hormones they secrete,<br />are shown in Fig. 10–1.<br />Endocrine glands are ductless; that is, they do not<br />have ducts to take their secretions to specific sites.<br />224 The Endocrine System<br />Anterior: GH, TSH, ACTH<br />FSH, LH, Prolactin<br />Posterior: ADH, Oxytocin<br />Melatonin<br />PTH<br />Releasing hormones<br />for anterior pituitary<br />Thyroxine and T3<br />Calcitonin<br />THYMUS GLAND<br />Immune hormones<br />Insulin<br />Glucagon<br />Cortex: Aldosterone<br />Cortisol<br />Sex hormones<br />Medulla: Epinephrine<br />Norepinephrine<br />Testosterone<br />Inhibin<br />THYROID GLAND<br />HYPOTHALAMUS<br />PITUITARY (HYPOPHYSIS) GLAND<br />PINEAL GLAND<br />PARATHYROID GLANDS<br />ADRENAL (SUPRARENAL) GLANDS<br />PANCREAS<br />OVARIES<br />Estrogen<br />Progesterone<br />Inhibin<br />TESTES<br />Figure 10–1. The endocrine system. Locations of many endocrine glands. Both male<br />and female gonads (testes and ovaries) are shown.<br />QUESTION: Why is the location of the thyroid gland not really important for its function?<br />Instead, hormones are secreted directly into capillaries<br />and circulate in the blood throughout the body. Each<br />hormone then exerts very specific effects on certain<br />organs, called target organs or target tissues. Some<br />hormones, such as insulin and thyroxine, have many<br />target organs. Other hormones, such as calcitonin and<br />some pituitary gland hormones, have only one or a<br />few target organs.<br />In general, the endocrine system and its hormones<br />help regulate growth, the use of foods to produce<br />energy, resistance to stress, the pH of body fluids and<br />fluid balance, and reproduction. In this chapter we will<br />discuss the specific functions of the hormones and<br />how each contributes to homeostasis.<br />CHEMISTRY OF HORMONES<br />With respect to their chemical structure, hormones<br />may be classified into three groups: amines, proteins,<br />and steroids.<br />1. Amines—these simple hormones are structural<br />variations of the amino acid tyrosine. This group<br />includes thyroxine from the thyroid gland and epinephrine<br />and norepinephrine from the adrenal<br />medulla.<br />2. Proteins—these hormones are chains of amino<br />acids. Insulin from the pancreas, growth hormone<br />from the anterior pituitary gland, and calcitonin<br />from the thyroid gland are all proteins. Short<br />chains of amino acids may be called peptides.<br />Antidiuretic hormone and oxytocin, synthesized by<br />the hypothalamus, are peptide hormones.<br />3. Steroids—cholesterol is the precursor for the<br />steroid hormones, which include cortisol and aldosterone<br />from the adrenal cortex, estrogen and progesterone<br />from the ovaries, and testosterone from<br />the testes.<br />REGULATION OF<br />HORMONE SECRETION<br />Hormones are secreted by endocrine glands when<br />there is a need for them, that is, for their effects on<br />their target organs. The cells of endocrine glands<br />respond to changes in the blood or perhaps to other<br />hormones in the blood. These stimuli are the information<br />they use to increase or decrease secretion of<br />their own hormones. When a hormone brings about<br />its effects, the stimulus is reversed, and secretion of<br />the hormone decreases until the stimulus reoccurs.<br />You may recall from Chapter 1 that this is a negative<br />feedback mechanism, and the mechanism for thyroxine<br />was depicted in Fig. 1–3. Let us use insulin as a different<br />example here.<br />Insulin is secreted by the pancreas when the blood<br />glucose level is high; that is, hyperglycemia is the<br />stimulus for secretion of insulin. Once circulating<br />in the blood, insulin enables cells to remove glucose<br />from the blood so that it can be used for energy<br />production and enables the liver to store glucose<br />as glycogen. As a result of these actions of insulin,<br />the blood glucose level decreases, reversing the stimulus<br />for secretion of insulin. Insulin secretion then<br />decreases until the blood glucose level increases<br />again.<br />In any hormonal negative feedback mechanism,<br />information about the effects of the hormone is “fed<br />back” to the gland, which then decreases its secretion<br />of the hormone. This is why the mechanism is called<br />“negative”: The effects of the hormone reverse the<br />stimulus and decrease the secretion of the hormone.<br />The secretion of many other hormones is regulated in<br />a similar way.<br />The hormones of the anterior pituitary gland are<br />secreted in response to releasing hormones (also<br />called releasing factors) secreted by the hypothalamus.<br />You may recall this from Chapter 8. Growth hormone,<br />for example, is secreted in response to growth hormone–<br />releasing hormone (GHRH) from the hypothalamus.<br />As growth hormone exerts its effects, the<br />secretion of GHRH decreases, which in turn decreases<br />the secretion of growth hormone. This is another type<br />of negative feedback mechanism.<br />For each of the hormones to be discussed in this<br />chapter, the stimulus for its secretion will also be mentioned.<br />Some hormones function as an antagonistic<br />pair to regulate a particular aspect of blood chemistry;<br />these mechanisms will also be covered.<br />THE PITUITARY GLAND<br />The pituitary gland (or hypophysis) hangs by a short<br />stalk (infundibulum) from the hypothalamus and is<br />enclosed by the sella turcica of the sphenoid bone.<br />The Endocrine System 225<br />226 The Endocrine System<br />Figure 10–2. Hormones of the pituitary gland and their target organs.<br />QUESTION: Which pituitary hormones have other endocrine glands as their target organs?<br />Despite its small size, the pituitary gland regulates<br />many body functions. Its two major portions are the<br />posterior pituitary gland (neurohypophysis), which is<br />an extension of the nerve tissue of the hypothalamus,<br />and the anterior pituitary gland (adenohypophysis),<br />which is separate glandular tissue. All of the hormones<br />of the pituitary gland and their target organs are<br />shown in Fig. 10–2. It may be helpful for you to look<br />at this summary picture before you begin reading the<br />following sections.<br />POSTERIOR PITUITARY GLAND<br />The two hormones of the posterior pituitary gland<br />are actually produced by the hypothalamus and simply<br />stored in the posterior pituitary until needed. Their<br />release is stimulated by nerve impulses from the hypothalamus<br />(Fig. 10–3).<br />Antidiuretic Hormone<br />Antidiuretic hormone (ADH, also called vasopressin)<br />increases the reabsorption of water by kidney<br />tubules, which decreases the amount of urine formed.<br />The water is reabsorbed into the blood, so as urinary<br />output is decreased, blood volume is increased, which<br />helps maintain normal blood pressure. ADH also<br />decreases sweating, but the amount of water conserved<br />is much less than that conserved by the kidneys.<br />The stimulus for secretion of ADH is decreased<br />water content of the body. If too much water is lost<br />through sweating or diarrhea, for example, osmoreceptors<br />in the hypothalamus detect the increased<br />“saltiness” of body fluids. The hypothalamus then<br />transmits impulses to the posterior pituitary to<br />increase the secretion of ADH and decrease the loss of<br />more water in urine.<br />Any type of dehydration stimulates the secretion of<br />ADH to conserve body water. In the case of severe<br />hemorrhage, ADH is released in large amounts and<br />will also cause vasoconstriction, especially in arterioles,<br />which will help to raise or at least maintain blood<br />pressure. This function gives ADH its other name,<br />vasopressin.<br />Ingestion of alcohol inhibits the secretion of ADH<br />and increases urinary output. If alcohol intake is excessive<br />and fluid is not replaced, a person will feel thirsty<br />and dizzy the next morning. The thirst is due to the<br />loss of body water, and the dizziness is the result of<br />low blood pressure.<br />The Endocrine System 227<br />Hypothalamus<br />Releasing hormones<br />Capillaries in hypothalamus<br />Hypophyseal portal veins<br />Capillaries in<br />anterior pituitary<br />Hormones of<br />anterior pituitary<br />Lateral hypophyseal vein<br />Superior hypophyseal<br />arteries<br />Optic chiasma<br />B<br />Hypothalamus<br />Hypothalamic-hypophyseal tract<br />Posterior pituitary<br />Inferior hypophyseal<br />artery<br />Hormones of<br />posterior<br />pituitary<br />Posterior lobe vein<br />Optic chiasma<br />A<br />Figure 10–3. Structural relationships of hypothalamus and pituitary gland. (A) Posterior<br />pituitary stores hormones produced in the hypothalamus. (B) Releasing hormones of the<br />hypothalamus circulate directly to the anterior pituitary and influence its secretions. Notice<br />the two networks of capillaries.<br />QUESTION: In part A, name the hormones of the posterior pituitary. In part B, what stimulates<br />secretion of anterior pituitary hormones?<br />Oxytocin<br />Oxytocin stimulates contraction of the uterus at the<br />end of pregnancy and stimulates release of milk from<br />the mammary glands.<br />As labor begins, the cervix of the uterus is<br />stretched, which generates sensory impulses to the<br />hypothalamus, which in turn stimulates the posterior<br />pituitary to release oxytocin. Oxytocin then causes<br />strong contractions of the smooth muscle (myometrium)<br />of the uterus to bring about delivery of the<br />baby and the placenta. The secretion of oxytocin is<br />one of the few positive feedback mechanisms within<br />the body, and the external brake or shutoff of the<br />feedback cycle is delivery of the baby and the placenta.<br />It has been discovered that the placenta itself<br />secretes oxytocin at the end of gestation and in an<br />amount far higher than that from the posterior pituitary<br />gland. Research is continuing to determine the<br />exact mechanism and precise role of the placenta in<br />labor.<br />When a baby is breast-fed, the sucking of the baby<br />stimulates sensory impulses from the mother’s nipple<br />to the hypothalamus. Nerve impulses from the hypothalamus<br />to the posterior pituitary cause the release of<br />oxytocin, which stimulates contraction of the smooth<br />muscle cells around the mammary ducts. This release<br />of milk is sometimes called the “milk let-down” reflex.<br />The hormones of the posterior pituitary are summarized<br />in Table 10–1.<br />Both ADH and oxytocin are peptide hormones<br />with similar structure, having nine amino acids each.<br />And both have been found to influence aspects of<br />behavior such as nurturing and trustfulness. Certain<br />brain cells have receptors for vasopressin, and they<br />seem to be involved in creating the bonds that sustain<br />family life. Trust is part of many social encounters<br />such as friendship, school, sports and games, and buying<br />and selling, as well as family life. These two small<br />hormones seem to have some influence on us mentally<br />as well as physically.<br />ANTERIOR PITUITARY GLAND<br />The hormones of the anterior pituitary gland regulate<br />many body functions. They are in turn regulated<br />by releasing hormones from the hypothalamus.<br />These releasing hormones are secreted into capillaries<br />in the hypothalamus and pass through the hypophyseal<br />portal veins to another capillary network in the<br />anterior pituitary gland. Here, the releasing hormones<br />are absorbed and stimulate secretion of the anterior<br />pituitary hormones. This small but specialized pathway<br />of circulation is shown in Fig. 10–3. This pathway<br />permits the releasing hormones to rapidly stimulate<br />the anterior pituitary, without having to pass through<br />general circulation.<br />Growth Hormone<br />Growth hormone (GH) is also called somatotropin,<br />and it does indeed promote growth (see Fig. 10–4).<br />GH stimulates cells to produce insulin-like growth factors<br />(IGFs), intermediary molecules that bring about<br />the functions of GH. Growth hormone increases the<br />transport of amino acids into cells, and increases the<br />rate of protein synthesis. Amino acids cannot be stored<br />in the body, so when they are available, they must be<br />228 The Endocrine System<br />Table 10–1 HORMONES OF THE POSTERIOR PITUITARY GLAND<br />Hormone Function(s) Regulation of Secretion<br />Antidiuretic hormone<br />(ADH or vasopressin)<br />Oxytocin<br />• Increases water reabsorption<br />by the kidney tubules (water<br />returns to the blood)<br />• Decreases sweating<br />• Causes vasoconstriction<br />(in large amounts)<br />• Promotes contraction of<br />myometrium of uterus (labor)<br />• Promotes release of milk from<br />mammary glands<br />Decreased water content in the body (alcohol<br />inhibits secretion)<br />Nerve impulses from hypothalamus, the result of<br />stretching of cervix or stimulation of nipple<br />Secretion from placenta at end of gestation—<br />stimulus unknown<br />used in protein synthesis. Excess amino acids are<br />changed to carbohydrates or fat, for energy storage.<br />Growth hormone ensures that amino acids will be used<br />for whatever protein synthesis is necessary, before the<br />amino acids can be changed to carbohydrates. Growth<br />hormone also stimulates cell division in those tissues<br />capable of mitosis. These functions contribute to the<br />growth of the body during childhood, especially<br />growth of bones and muscles.<br />You may now be wondering if GH is secreted in<br />adults, and the answer is yes. The use of amino acids<br />for the synthesis of proteins is still necessary. Even if<br />the body is not growing in height, some tissues will<br />require new proteins for repair or replacement. GH<br />also stimulates the release of fat from adipose tissue<br />and the use of fats for energy production. This is<br />important any time we go for extended periods without<br />eating, no matter what our ages.<br />The secretion of GH is regulated by two releasing<br />hormones from the hypothalamus. Growth hormone–<br />releasing hormone (GHRH), which increases the<br />secretion of GH, is produced during hypoglycemia<br />and during exercise. Another stimulus for GHRH is a<br />high blood level of amino acids; the GH then secreted<br />will ensure the conversion of these amino acids into<br />protein. Somatostatin may also be called growth hormone<br />inhibiting hormone (GHIH), and, as its name<br />tells us, it decreases the secretion of GH. Somatostatin<br />is produced during hyperglycemia. Disorders of GH<br />secretion are discussed in Box 10–1.<br />The Endocrine System 229<br />Increases use of<br />fats for energy<br />Increases protein<br />synthesis<br />Increases<br />mitosis<br />Bone and<br />muscle<br />Liver and<br />other viscera<br />ATP<br />GH<br />Anterior<br />pituitary<br />Figure 10–4. Functions of growth hormone.<br />QUESTION: Which functions of growth hormone directly help bones and muscles to<br />grow?<br />Thyroid-Stimulating Hormone<br />Thyroid-stimulating hormone (TSH) is also called<br />thyrotropin, and its target organ is the thyroid gland.<br />TSH stimulates the normal growth of the thyroid and<br />the secretion of thyroxine (T4) and triiodothyronine<br />(T3). The functions of these thyroid hormones will be<br />covered later in this chapter.<br />The secretion of TSH is stimulated by thyrotropinreleasing<br />hormone (TRH) from the hypothalamus.<br />When metabolic rate (energy production) decreases,<br />TRH is produced.<br />Adrenocorticotropic Hormone<br />Adrenocorticotropic hormone (ACTH) stimulates<br />the secretion of cortisol and other hormones by the<br />adrenal cortex. Secretion of ACTH is increased by<br />corticotropin-releasing hormone (CRH) from the<br />hypothalamus. CRH is produced in any type of physiological<br />stress situation such as injury, disease, exercise,<br />or hypoglycemia (being hungry is stressful).<br />Prolactin<br />Prolactin, as its name suggests, is responsible for lactation.<br />More precisely, prolactin initiates and maintains<br />milk production by the mammary glands. The<br />regulation of secretion of prolactin is complex, involving<br />both prolactin-releasing hormone (PRH) and<br />prolactin-inhibiting hormone (PIH) from the hypothalamus.<br />The mammary glands must first be acted<br />upon by other hormones such as estrogen and progesterone,<br />which are secreted in large amounts by the<br />placenta during pregnancy. Then, after delivery of the<br />baby, prolactin secretion increases and milk is produced.<br />If the mother continues to breast-feed, prolactin<br />levels remain high.<br />Follicle-Stimulating Hormone<br />Follicle-stimulating hormone (FSH) is one of the<br />gonadotropic hormones; that is, it has its effects on<br />the gonads: the ovaries or testes. FSH is named for<br />one of its functions in women. Within the ovaries are<br />ovarian follicles that contain potential ova (egg cells).<br />FSH stimulates the growth of ovarian follicles; that is,<br />it initiates egg development in cycles of approximately<br />28 days. FSH also stimulates secretion of estrogen by<br />the follicle cells. In men, FSH initiates sperm production<br />within the testes.<br />The secretion of FSH is stimulated by the hypothalamus,<br />which produces gonadotropin-releasing<br />hormone (GnRH). FSH secretion is decreased by<br />inhibin, a hormone produced by the ovaries or testes.<br />Luteinizing Hormone<br />Luteinizing hormone (LH) is another gonadotropic<br />hormone. In women, LH is responsible for ovulation,<br />the release of a mature ovum from an ovarian follicle.<br />LH then stimulates that follicle to develop into the<br />corpus luteum, which secretes progesterone, also<br />under the influence of LH. In men, LH stimulates the<br />interstitial cells of the testes to secrete testosterone.<br />230 The Endocrine System<br />BOX 10–1 DISORDERS OF GROWTH HORMONE<br />not have this condition; they are tall as a result of<br />their genetic makeup and good nutrition.<br />In an adult, hypersecretion of GH is caused by a<br />pituitary tumor, and results in acromegaly. The<br />long bones cannot grow because the epiphyseal<br />discs are closed, but the growth of other bones is<br />stimulated. The jaw and other facial bones become<br />disproportionately large, as do the bones of the<br />hands and feet. The skin becomes thicker, and the<br />tongue also grows and may protrude. Other consequences<br />include compression of nerves by abnormally<br />growing bones and growth of the articular<br />cartilages, which then erode and bring on arthritis.<br />Treatment of acromegaly requires surgical removal<br />of the tumor or its destruction by radiation.<br />A deficiency or excess of growth hormone (GH)<br />during childhood will have marked effects on the<br />growth of a child. Hyposecretion of GH results in<br />pituitary dwarfism, in which the person may<br />attain a final height of only 3 to 4 feet but will have<br />normal body proportions. GH can now be produced<br />using genetic engineering and may be used<br />to stimulate growth in children with this disorder.<br />GH will not increase growth of children with the<br />genetic potential for short stature. Reports that GH<br />will reverse the effects of aging are simply not true.<br />Hypersecretion of GH results in giantism (or<br />gigantism), in which the long bones grow excessively<br />and the person may attain a height of 8 feet.<br />Most very tall people, such as basketball players, do<br />(LH is also called ICSH, interstitial cell stimulating<br />hormone.)<br />Secretion of LH is also regulated by GnRH from<br />the hypothalamus. We will return to FSH and LH, as<br />well as a discussion of the sex hormones, in Chapter<br />20.<br />The hormones of the anterior pituitary are summarized<br />in Table 10–2.<br />THYROID GLAND<br />The thyroid gland is located on the front and sides of<br />the trachea just below the larynx. Its two lobes are<br />connected by a middle piece called the isthmus. The<br />structural units of the thyroid gland are thyroid follicles,<br />which produce thyroxine (T4) and triiodothyronine<br />(T3). Iodine is necessary for the synthesis of<br />these hormones; thyroxine contains four atoms of<br />iodine, and T3 contains three atoms of iodine.<br />The third hormone produced by the thyroid<br />gland is calcitonin, which is secreted by parafollicular<br />cells. Its function is very different from those<br />of thyroxine and T3, which you may recall from<br />Chapter 6.<br />THYROXINE AND T3<br />Thyroxine (T4) and T3 have the same functions: regulation<br />of energy production and protein synthesis,<br />which contribute to growth of the body and to normal<br />body functioning throughout life (Fig. 10–5). Thyroxine<br />and T3 increase cell respiration of all food types<br />(carbohydrates, fats, and excess amino acids) and<br />thereby increase energy and heat production. They<br />also increase the rate of protein synthesis within cells.<br />Normal production of thyroxine and T3 is essential for<br />physical growth, normal mental development, and<br />maturation of the reproductive system. These hormones<br />are the most important day-to-day regulators<br />of metabolic rate; their activity is reflected in the func-<br />The Endocrine System 231<br />Table 10–2 HORMONES OF THE ANTERIOR PITUITARY GLAND<br />Hormone Function(s) Regulation of Secretion<br />Growth hormone (GH)<br />Thyroid-stimulating<br />hormone (TSH)<br />Adrenocorticotropic<br />hormone (ACTH)<br />Prolactin<br />Follicle-stimulating<br />hormone (FSH)<br />Luteinizing hormone<br />(LH) (ICSH)<br />• Increases rate of mitosis<br />• Increases amino acid transport into cells<br />• Increases rate of protein synthesis<br />• Increases use of fats for energy<br />• Increases secretion of thyroxine and T3<br />by thyroid gland<br />• Increases secretion of cortisol by the<br />adrenal cortex<br />• Stimulates milk production by the mammary<br />glands<br />In women:<br />• Initiates growth of ova in ovarian follicles<br />• Increases secretion of estrogen by follicle<br />cells<br />In men:<br />• Initiates sperm production in the testes<br />In women:<br />• Causes ovulation<br />• Causes the ruptured ovarian follicle to<br />become the corpus luteum<br />• Increases secretion of progesterone by<br />the corpus luteum<br />In men:<br />• Increases secretion of testosterone by<br />the interstitial cells of the testes<br />GHRH (hypothalamus) stimulates secretion<br />GHIH—somatostatin (hypothalamus)<br />inhibits secretion<br />TRH (hypothalamus)<br />CRH (hypothalamus)<br />PRH (hypothalamus) stimulates secretion<br />PIH (hypothalamus) inhibits secretion<br />GnRH (hypothalamus) stimulates secretion<br />Inhibin (ovaries or testes) inhibits secretion<br />GnRH (hypothalamus)<br />tioning of the brain, muscles, heart, and virtually all<br />other organs. Although thyroxine and T3 are not vital<br />hormones, in that they are not crucial to survival, their<br />absence greatly diminishes physical and mental<br />growth and abilities (see Box 10–2: Disorders of<br />Thyroxine).<br />Secretion of thyroxine and T3 is stimulated by<br />thyroid-stimulating hormone (TSH) from the anterior<br />pituitary gland (see also Fig. 1–3). When the<br />metabolic rate (energy production) decreases, this<br />change is detected by the hypothalamus, which<br />secretes thyrotropin releasing hormone (TRH). TRH<br />stimulates the anterior pituitary to secrete TSH,<br />which stimulates the thyroid to release thyroxine and<br />T3, which raise the metabolic rate by increasing<br />energy production. This negative feedback mechanism<br />then shuts off TRH from the hypothalamus until<br />the metabolic rate decreases again.<br />232 The Endocrine System<br />Thyroid gland<br />T4 and T3<br />Increase<br />cell respiration of<br />all foods<br />Increase<br />protein synthesis<br />Glucose<br />Fats<br />Excess<br />amino acids<br />ATP<br />Bone and muscle<br />Liver and viscera<br />Brain<br />Reproductive<br />organs<br />Figure 10–5. Functions of thyroxine and T3.<br />QUESTION: Which functions of thyroxine help bones and muscles to grow and maintain<br />their own functions?<br />CALCITONIN<br />Calcitonin decreases the reabsorption of calcium<br />and phosphate from the bones to the blood, thereby<br />lowering blood levels of these minerals. This function<br />of calcitonin helps maintain normal blood levels<br />of calcium and phosphate and also helps maintain<br />a stable, strong bone matrix. It is believed that<br />calcitonin exerts its most important effects during<br />childhood, when bones are growing. A form of calcitonin<br />obtained from salmon is used to help treat<br />osteoporosis.<br />The stimulus for secretion of calcitonin is hypercalcemia,<br />that is, a high blood calcium level. When<br />blood calcium is high, calcitonin ensures that no more<br />calcium will be removed from bones until there is a<br />real need for more calcium in the blood (Fig. 10–6).<br />The hormones of the thyroid gland are summarized in<br />Table 10–3.<br />PARATHYROID GLANDS<br />There are four parathyroid glands: two on the back<br />of each lobe of the thyroid gland (Fig. 10–7). The hormone<br />they produce is called parathyroid hormone.<br />PARATHYROID HORMONE<br />Parathyroid hormone (PTH) is an antagonist to calcitonin<br />and is important for the maintenance of normal<br />blood levels of calcium and phosphate. The target<br />organs of PTH are the bones, small intestine, and kidneys.<br />PTH increases the reabsorption of calcium and<br />phosphate from bones to the blood, thereby raising<br />their blood levels. Absorption of calcium and phosphate<br />from food in the small intestine, which also<br />requires vitamin D, is increased by PTH. This too<br />raises the blood levels of these minerals. In the kidneys,<br />PTH stimulates the activation of vitamin D and<br />increases the reabsorption of calcium and the excretion<br />of phosphate (more than is obtained from bones).<br />Therefore, the overall effect of PTH is to raise the<br />blood calcium level and lower the blood phosphate<br />level. The functions of PTH are summarized in Table<br />10–4.<br />Secretion of PTH is stimulated by hypocalcemia,<br />a low blood calcium level, and inhibited by hypercalcemia.<br />The antagonistic effects of PTH and calcitonin<br />are shown in Fig. 10–6. Together, these hormones<br />maintain blood calcium within a normal range.<br />Calcium in the blood is essential for the process of<br />blood clotting and for normal activity of neurons and<br />muscle cells.<br />As you might expect, a sustained hypersecretion of<br />PTH, such as is caused by a parathyroid tumor, would<br />remove calcium from bones and weaken them. It has<br />been found, however, that an intermittent, brief excess<br />of PTH, such as can occur by injection, will stimulate<br />the formation of more bone matrix, rather than matrix<br />reabsorption. This may seem very strange—the opposite<br />of what we would expect—but it shows how much<br />we have yet to learn about the body. PTH is being<br />investigated as a possible way to help prevent osteoporosis.<br />PANCREAS<br />The pancreas is located in the upper left quadrant of<br />the abdominal cavity, extending from the curve of the<br />duodenum to the spleen. Although the pancreas is<br />both an exocrine (digestive) gland as well as an<br />endocrine gland, only its endocrine function will<br />be discussed here. The hormone-producing cells of<br />the pancreas are called islets of Langerhans (pancre-<br />The Endocrine System 233<br />Table 10–3 HORMONES OF THE THYROID GLAND<br />Regulation<br />Hormone Function(s) of Secretion<br />Thyroxine (T4) and<br />triiodothyronine (T3)<br />Calcitonin<br />• Increase energy production from all food types<br />• Increase rate of protein synthesis<br />• Decreases the reabsorption of calcium and phosphate<br />from bones to blood<br />TSH (anterior pituitary)<br />Hypercalcemia<br />234 The Endocrine System<br />Bones<br />Ca+2 is retained in bone matrix<br />Accelerates calcium<br />absorption by bones<br />Hypercalcemia (high blood calcium)<br />Thyroid<br />Calcitonin Inhibits<br />Bones<br />Small intestine<br />Reabsorb Ca+2 to the blood<br />Hypocalcemia (low blood calcium)<br />Parathyroids<br />PTH<br />Kidneys<br />(vitamin D activated)<br />Figure 10–6. Calcitonin and parathyroid hormone (PTH) and their functions related to<br />the maintenance of the blood calcium level.<br />QUESTION: Which hormone helps keep calcium in bones? What vitamin does PTH help<br />activate, and where?<br />Table 10–4 HORMONE OF THE PARATHYROID GLANDS<br />Regulation<br />Hormone Functions of Secretion<br />Parathyroid hormone<br />(PTH)<br />• Increases the reabsorption of calcium and phosphate<br />from bone to blood<br />• Increases absorption of calcium and phosphate by<br />the small intestine<br />• Increases the reabsorption of calcium and the excretion<br />of phosphate by the kidneys; activates vitamin D<br />Hypocalcemia stimulates secretion.<br />Hypercalcemia inhibits<br />secretion.<br />atic islets; see Fig. 16–7); they contain alpha cells<br />that produce glucagon and beta cells that produce<br />insulin.<br />GLUCAGON<br />Glucagon stimulates the liver to change glycogen to<br />glucose (this process is called glycogenolysis, which<br />literally means “glycogen breakdown”) and to increase<br />the use of fats and excess amino acids for energy production.<br />The process of gluconeogenesis (literally,<br />“making new glucose”) is the conversion of excess<br />amino acids into simple carbohydrates that may enter<br />the reactions of cell respiration. The overall effect of<br />glucagon, therefore, is to raise the blood glucose level<br />and to make all types of food available for energy<br />production.<br />The secretion of glucagon is stimulated by hypoglycemia,<br />a low blood glucose level. Such a state may<br />occur between meals or during physiological stress situations<br />such as exercise (Fig. 10–8).<br />INSULIN<br />Insulin increases the transport of glucose from the<br />blood into cells by increasing the permeability of<br />cell membranes to glucose. (Brain, liver, and kidney<br />cells, however, are not dependent on insulin for<br />glucose intake.) Once inside cells, glucose is used in<br />The Endocrine System 235<br />Larynx<br />Thyroid<br />Parathyroid<br />glands<br />Trachea<br />Figure 10–7. Parathyroid glands in posterior view, on<br />lobes of the thyroid gland.<br />QUESTION: Which of the target organs of PTH may be<br />called a reservoir, and what do they store?<br />BOX 10–2 DISORDERS OF THYROXINE<br />(energy production) decreases, resulting in lethargy,<br />muscular weakness, slow heart rate, a feeling of<br />cold, weight gain, and a characteristic puffiness of<br />the face. The administration of thyroid hormones<br />will return the metabolic rate to normal.<br />Graves’ disease is an autoimmune disorder<br />that causes hypersecretion of thyroxine. The<br />autoantibodies seem to bind to TSH receptors on<br />the thyroid cells and stimulate secretion of excess<br />thyroxine. The symptoms are those that would be<br />expected when the metabolic rate is abnormally<br />elevated: weight loss accompanied by increased<br />appetite, increased sweating, fast heart rate, feeling<br />of warmth, and fatigue. Also present may be goiter<br />and exophthalmos, which is protrusion of the eyes.<br />Treatment is aimed at decreasing the secretion of<br />thyroxine by the thyroid, and medications or<br />radioactive iodine may be used to accomplish this.<br />Iodine is an essential component of thyroxine (and<br />T3), and a dietary deficiency of iodine causes goiter.<br />In an attempt to produce more thyroxine, the<br />thyroid cells become enlarged, and hence the thyroid<br />gland enlarges and becomes visible on the<br />front of the neck. The use of iodized salt has made<br />goiter a rare condition in many parts of the world.<br />Hyposecretion of thyroxine in a newborn has<br />devastating effects on the growth of the child.<br />Without thyroxine, physical growth is diminished,<br />as is mental development. This condition is called<br />cretinism, characterized by severe physical and<br />mental retardation. If the thyroxine deficiency is<br />detected shortly after birth, the child may be<br />treated with thyroid hormones to promote normal<br />development.<br />Hyposecretion of thyroxine in an adult is called<br />myxedema. Without thyroxine, the metabolic rate<br />cell respiration to produce energy. The liver and skeletal<br />muscles also change glucose to glycogen (glycogenesis,<br />which means “glycogen production”) to be<br />stored for later use. Insulin is also important in the<br />metabolism of other food types; it enables cells to take<br />in fatty acids and amino acids to use in the synthesis of<br />lipids and proteins (not energy production). Without<br />insulin, blood levels of lipids tend to rise and cells accumulate<br />excess fatty acids. With respect to blood glucose,<br />insulin decreases its level by promoting the use of<br />glucose for energy production. The antagonistic functions<br />of insulin and glucagon are shown in Fig. 10–8.<br />Insulin is a vital hormone; we cannot survive for<br />very long without it. A deficiency of insulin or in its<br />functioning is called diabetes mellitus, which is discussed<br />in Box 10–3: Diabetes Mellitus.<br />Secretion of insulin is stimulated by hyperglycemia,<br />a high blood glucose level. This state<br />occurs after eating, especially of meals high in carbohydrates.<br />As glucose is absorbed from the small intestine<br />into the blood, insulin is secreted to enable cells<br />to use the glucose for immediate energy. At the same<br />time, any excess glucose will be stored in the liver and<br />muscles as glycogen.<br />You will also notice in Fig. 16–7 the cells called<br />delta cells. These produce the hormone somatostatin,<br />which is identical to growth hormone–inhibiting hormone<br />from the hypothalamus. Pancreatic somatostatin<br />acts locally to inhibit the secretion of insulin and<br />glucagon, and it seems to slow the absorption of the<br />end products of digestion in the small intestine. The<br />hormones of the pancreas are summarized in Table<br />10–5.<br />ADRENAL GLANDS<br />The two adrenal glands are located one on top of<br />each kidney, which gives them their other name of<br />suprarenal glands. Each adrenal gland consists of two<br />parts: an inner adrenal medulla and an outer adrenal<br />cortex. The hormones produced by each part have<br />very different functions.<br />ADRENAL MEDULLA<br />The cells of the adrenal medulla secrete epinephrine<br />and norepinephrine, which collectively are called catecholamines<br />and are sympathomimetic. The secre-<br />236 The Endocrine System<br />Hyperglycemia<br />(High blood glucose)<br />Glucagon Liver<br />Liver changes<br />glycogen to<br />glucose and<br />converts amino<br />acids to<br />carbohydrates<br />Pancreas<br />Hypoglycemia<br />(Low blood glucose)<br />Cells use glucose<br />for energy production<br />Insulin<br />Cells<br />Liver<br />Skeletal<br />muscles<br />Liver and skeletal<br />muscles change<br />glucose to glycogen<br />Figure 10–8. Insulin and<br />glucagon and their functions<br />related to the maintenance of<br />the blood glucose level.<br />QUESTION: Which hormone<br />enables cells to use glucose<br />for energy production? What<br />is the stimulus for secretion of<br />this hormone?<br />The Endocrine System 237<br />tion of both hormones is stimulated by sympathetic<br />impulses from the hypothalamus, and their functions<br />duplicate and prolong those of the sympathetic division<br />of the autonomic nervous system (mimetic means<br />“to mimic”).<br />Epinephrine and Norepinephrine<br />Epinephrine (Adrenalin) and norepinephrine (noradrenalin)<br />are both secreted in stress situations and<br />help prepare the body for “fight or flight.” Norepinephrine<br />is secreted in small amounts, and its most<br />significant function is to cause vasoconstriction in the<br />skin, viscera, and skeletal muscles (that is, throughout<br />the body), which raises blood pressure.<br />Epinephrine, secreted in larger amounts, increases<br />the heart rate and force of contraction and stimulates<br />vasoconstriction in skin and viscera and vasodilation in<br />skeletal muscles. It also dilates the bronchioles,<br />decreases peristalsis, stimulates the liver to change<br />glycogen to glucose, increases the use of fats for<br />energy, and increases the rate of cell respiration. Many<br />of these effects do indeed seem to be an echo of sympathetic<br />responses, don’t they? Responding to stress is<br />so important that the body acts redundantly (that is,<br />exceeds what is necessary, or repeats itself) and has<br />both a nervous mechanism and a hormonal mechanism.<br />Epinephrine is actually more effective than sympathetic<br />stimulation, however, because the hormone<br />increases energy production and cardiac output to a<br />greater extent. The hormones of the adrenal medulla<br />are summarized in Table 10–6, and their functions are<br />shown in Fig. 10–9.<br />ADRENAL CORTEX<br />The adrenal cortex secretes three types of steroid<br />hormones: mineralocorticoids, glucocorticoids, and<br />Table 10–5 HORMONES OF THE PANCREAS<br />Regulation<br />Hormone Functions of Secretion<br />Glucagon<br />(alpha cells)<br />Insulin (beta cells)<br />Somatostatin<br />(delta cells)<br />• Increases conversion of glycogen to glucose in the liver<br />• Increases the use of excess amino acids and of fats for energy<br />• Increases glucose transport into cells and the use of glucose for<br />energy production<br />• Increases the conversion of excess glucose to glycogen in the liver<br />and muscles<br />• Increases amino acid and fatty acid transport into cells, and their<br />use in synthesis reactions<br />• Decreases secretion of insulin and glucagon<br />• Slows absorption of nutrients<br />Hypoglycemia<br />Hyperglycemia<br />Rising levels of insulin<br />and glucagon<br />Table 10–6 HORMONES OF THE ADRENAL MEDULLA<br />Regulation<br />Hormone Function(s) of Secretion<br />Norepinephrine<br />Epinephrine<br />• Causes vasoconstriction in skin, viscera, and skeletal muscles<br />• Increases heart rate and force of contraction<br />• Dilates bronchioles<br />• Decreases peristalsis<br />• Increases conversion of glycogen to glucose in the liver<br />• Causes vasodilation in skeletal muscles<br />• Causes vasoconstriction in skin and viscera<br />• Increases use of fats for energy<br />• Increases the rate of cell respiration<br />Sympathetic impulses<br />from the hypothalamus<br />in stress<br />situations<br />238 The Endocrine System<br />BOX 10–3 DIABETES MELLITUS<br />more water is lost as well, symptoms include<br />greater urinary output (polyuria) and thirst (polydipsia).<br />The long-term effects of hyperglycemia produce<br />distinctive vascular changes. The capillary walls<br />thicken, and exchange of gases and nutrients<br />diminishes. The most damaging effects are seen in<br />the skin (especially of the feet), the retina (diabetic<br />retinopathy), and the kidneys. Poorly controlled<br />diabetes may lead to dry gangrene, blindness, and<br />severe kidney damage. Atherosclerosis is common,<br />because faulty triglyceride metabolism is linked to<br />faulty glucose metabolism. Neuropathy (damage to<br />nerves) leads to impaired cutaneous sensation and<br />difficulty with fine movements, such as buttoning a<br />shirt. It is now possible for diabetics to prevent<br />much of this tissue damage by precise monitoring<br />of the blood glucose level and more frequent<br />administration of insulin. Insulin pumps are able to<br />more closely mimic the natural secretion of insulin.<br />A very serious potential problem for the type 1<br />diabetic is ketoacidosis. When glucose cannot be<br />used for energy, the body turns to fats and proteins,<br />which are converted by the liver to ketones.<br />Ketones are organic acids (acetone, acetoacetic<br />acid) that can be used in cell respiration, but cells<br />are not able to utilize them rapidly so ketones<br />accumulate in the blood. Ketones are acids, and<br />lower the pH of the blood as they accumulate. The<br />kidneys excrete excess ketones, but in doing so<br />excrete more water as well, which leads to dehydration<br />and worsens the acidosis. Without administration<br />of insulin to permit the use of glucose, and<br />IV fluids to restore blood volume to normal, ketoacidosis<br />will progress to coma and death.<br />There are two types of diabetes mellitus: Type 1<br />is called insulin-dependent diabetes and its onset is<br />usually in childhood (juvenile onset). Type 2<br />is called non–insulin-dependent diabetes, and its<br />onset is usually later in life (maturity onset).<br />Type 1 diabetes is characterized by destruction<br />of the beta cells of the islets of Langerhans and a<br />complete lack of insulin (see Box Figure 10–A);<br />onset is usually abrupt. Destruction of the beta cells<br />is an autoimmune response, perhaps triggered by a<br />virus. There may be a genetic predisposition,<br />because certain HLA types are found more frequently<br />in type 1 diabetics than in other children<br />(see Box 11–5: HLA). Insulin by injection (inhaled<br />insulin is undergoing clinical trials) is essential to<br />control type 1 diabetes. Research is continuing on<br />the use of immunosuppressant medications to try<br />to preserve some beta cells (if diagnosis is early),<br />and also on the transplantation of stem cells to<br />replace lost beta cells.<br />In type 2 diabetes, insulin is produced but cannot<br />exert its effects on cells because of a loss of<br />insulin receptors on cell membranes (see Box Figure<br />10–A). Onset of type 2 diabetes is usually gradual,<br />and risk factors include a family history of diabetes<br />and being overweight. Control may not require<br />insulin, but rather medications that enable insulin<br />to react with the remaining membrane receptors.<br />For those with a family history of diabetes, a low-fat<br />diet and regular exercise reduce the risk of developing<br />the disease. The commitment to exercise<br />must be lifelong but is well worth the effort,<br />because diabetes is very destructive.<br />Without insulin (or its effects) blood glucose level<br />remains high, and glucose is lost in urine. Since<br />A Normal B Type 1 C Type 2<br />Glucose<br />Insulin<br />Insulin Receptor<br />Box Figure 10–A (A) Cell membrane in normal state, with insulin receptors and<br />insulin to regulate glucose intake. (B) Cell membrane in type 1 diabetes: insulin not<br />present, glucose remains outside cell. (C) Cell membrane in type 2 diabetes: without<br />insulin receptors, glucose remains outside cell.<br />The Endocrine System 239<br />Norepinephrine Epinephrine<br />Dilates bronchioles<br />Increases conversion<br />of<br />glycogen to glucose<br />Increases use of<br />fats for energy<br />Increases cell<br />respiration<br />Increases rate and<br />force of contraction<br />Vasoconstriction<br />in skin<br />Vasoconstriction<br />in viscera<br />Decreases<br />peristalsis<br />Vasodilation<br />in<br />skeletal muscle<br />Vasoconstriction in<br />skeletal muscle<br />Adrenal medulla<br />Figure 10–9. Functions of epinephrine and norepinephrine.<br />QUESTION: Do epinephrine and norepinephrine have the same effect on skeletal muscle?<br />Explain your answer.<br />sex hormones. The sex hormones, “female” estrogens<br />and “male” androgens (similar to testosterone), are<br />produced in very small amounts, and their importance<br />is not known with certainty. They may contribute to<br />rapid body growth during early puberty. They may<br />also be important in supplying estrogen to women<br />after menopause and to men throughout life (see the<br />“Estrogen” section later in this chapter).<br />The functions of the other adrenal cortical hormones<br />are well known, however, and these are considered<br />vital hormones.<br />Aldosterone<br />Aldosterone is the most abundant of the mineralocorticoids,<br />and we will use it as a representative of this<br />group of hormones. The target organs of aldosterone<br />are the kidneys, but there are important secondary<br />effects as well. Aldosterone increases the reabsorption<br />of sodium and the excretion of potassium by the kidney<br />tubules. Sodium ions (Na ) are returned to the<br />blood, and potassium ions (K ) are excreted in urine.<br />Look at Fig. 10–10 as you read the following.<br />As Na ions are reabsorbed, hydrogen ions (H )<br />may be excreted in exchange. This is one mechanism<br />to prevent the accumulation of excess H ions, which<br />would cause acidosis of body fluids. Also, as Na ions<br />are reabsorbed, negative ions such as chloride (Cl )<br />and bicarbonate (HCO3<br />–) follow the Na ions back to<br />the blood, and water follows by osmosis. This indirect<br />effect of aldosterone, the reabsorption of water by the<br />kidneys, is very important to maintain normal blood<br />volume and blood pressure. In summary, then, aldosterone<br />maintains normal blood levels of sodium and<br />potassium, and contributes to the maintenance of normal<br />blood pH, blood volume, and blood pressure.<br />A number of factors stimulate the secretion of<br />aldosterone. These are a deficiency of sodium, loss of<br />blood or dehydration that lowers blood pressure, or an<br />elevated blood level of potassium. Low blood pressure<br />or blood volume activates the renin-angiotensin<br />mechanism of the kidneys. This mechanism is discussed<br />in Chapters 13 and 18, so we will say for now<br />that the process culminates in the formation of a<br />chemical called angiotensin II. Angiotensin II causes<br />vasoconstriction and stimulates the secretion of aldosterone<br />by the adrenal cortex. Aldosterone then<br />increases sodium and water retention by the kidneys<br />to help restore blood volume and blood pressure to<br />normal.<br />Cortisol<br />We will use cortisol as a representative of the group<br />of hormones called glucocorticoids, because it is<br />responsible for most of the actions of this group<br />(Fig. 10–11). Cortisol increases the use of fats and<br />excess amino acids (gluconeogenesis) for energy and<br />decreases the use of glucose. This is called the glucosesparing<br />effect, and it is important because it conserves<br />240 The Endocrine System<br />Adrenal cortex<br />Aldosterone<br />K+ ions<br />excreted<br />Na+ ions<br />reabsorbed<br />HCO3<br />– ions reabsorbed<br />H2O reabsorbed<br />H+ ions excreted<br />Blood volume,<br />blood pressure,<br />and pH are<br />maintained<br />Figure 10–10. Functions of aldosterone. Direct and indirect functions are shown.<br />QUESTION: What ions does aldosterone have a direct effect on, and what is the effect?<br />glucose for use by the brain. Cortisol is secreted in any<br />type of physiological stress situation: disease, physical<br />injury, hemorrhage, fear or anger, exercise, and<br />hunger. Although most body cells easily use fatty acids<br />and excess amino acids in cell respiration, brain cells<br />do not, so they must have glucose. By enabling other<br />cells to use the alternative energy sources, cortisol<br />ensures that whatever glucose is present will be available<br />to the brain.<br />Cortisol also has an anti-inflammatory effect.<br />During inflammation, histamine from damaged tissues<br />makes capillaries more permeable, and the lysosomes<br />of damaged cells release their enzymes, which<br />help break down damaged tissue but may also cause<br />destruction of nearby healthy tissue. Cortisol blocks<br />the effects of histamine and stabilizes lysosomal membranes,<br />preventing excessive tissue destruction.<br />Inflammation is a beneficial process up to a point, and<br />is an essential first step if tissue repair is to take place.<br />It may, however, become a vicious cycle of damage,<br />inflammation, more damage, more inflammation,<br />and so on—a positive feedback mechanism. Normal<br />cortisol secretion seems to be the brake, to limit<br />the inflammation process to what is useful for tissue<br />repair, and to prevent excessive tissue destruction.<br />Too much cortisol, however, decreases the immune<br />response, leaving the body susceptible to infection<br />and significantly slowing the healing of damaged<br />tissue (see Box 10–4: Disorders of the Adrenal<br />Cortex).<br />The Endocrine System 241<br />Adrenal cortex<br />Cortisol<br />Conserves<br />glucose<br />Limits<br />inflammation<br />Increases use of<br />excess<br />amino acids<br />Increases use<br />of fats<br />ATP<br />ATP<br />Adipose tissue<br />Amino acids<br />Glucose<br />Brain<br />Glycogen<br />Liver<br />Most tissues<br />Figure 10–11. Functions of cortisol.<br />QUESTION: Which food types will be used for energy by most tissues? Which food type<br />may be stored?<br />The direct stimulus for cortisol secretion is ACTH<br />from the anterior pituitary gland, which in turn is<br />stimulated by corticotropin releasing hormone (CRH)<br />from the hypothalamus. CRH is produced in the<br />physiological stress situations mentioned earlier.<br />Although we often think of epinephrine as a hormone<br />important in stress, cortisol is also important. The<br />hormones of the adrenal cortex are summarized in<br />Table 10–7.<br />OVARIES<br />The ovaries are located in the pelvic cavity, one on<br />each side of the uterus. The hormones produced by<br />the ovaries are the steroids estrogen and progesterone,<br />and the protein inhibin. Although their functions are<br />an integral part of Chapters 20 and 21, we will briefly<br />discuss some of them here.<br />242 The Endocrine System<br />Table 10–7 HORMONES OF THE ADRENAL CORTEX<br />Hormone Functions Regulation of Secretion<br />Aldosterone<br />Cortisol<br />• Increases reabsorption of Na ions by<br />the kidneys to the blood<br />• Increases excretion of K ions by the<br />kidneys in urine<br />• Increases use of fats and excess amino<br />acids for energy<br />• Decreases use of glucose for energy<br />(except for the brain)<br />• Increases conversion of glucose to<br />glycogen in the liver<br />• Anti-inflammatory effect: stabilizes lysosomes<br />and blocks the effects of histamine<br />Low blood Na level<br />Low blood volume or blood pressure<br />High blood K level<br />ACTH (anterior pituitary) during<br />physiological stress<br />BOX 10–4 DISORDERS OF THE ADRENAL CORTEX<br />The cause may be a pituitary tumor that increases<br />ACTH secretion or a tumor of the adrenal cortex<br />itself.<br />Excessive cortisol promotes fat deposition in the<br />trunk of the body, while the extremities remain<br />thin. The skin becomes thin and fragile, and healing<br />after injury is slow. The bones also become fragile<br />because osteoporosis is accelerated. Also characteristic<br />of this syndrome is the rounded appearance of<br />the face. Treatment is aimed at removal of the cause<br />of the hypersecretion, whether it be a pituitary or<br />adrenal tumor.<br />Cushing’s syndrome may also be seen in people<br />who receive corticosteroids for medical reasons.<br />Transplant recipients or people with rheumatoid<br />arthritis or severe asthma who must take corticosteroids<br />may exhibit any of the above symptoms. In<br />such cases, the disadvantages of this medication<br />must be weighed against the benefits provided.<br />Addison’s disease is the result of hyposecretion of<br />the adrenol cortical hormones. Most cases are idiopathic,<br />that is, of unknown cause; atrophy of the<br />adrenal cortex decreases both cortisol and aldosterone<br />secretion.<br />Deficiency of cortisol is characterized by hypoglycemia,<br />decreased gluconeogenesis, and depletion<br />of glycogen in the liver. Consequences are<br />muscle weakness and the inability to resist physiological<br />stress. Aldosterone deficiency leads to retention<br />of potassium and excretion of sodium and<br />water in urine. The result is severe dehydration, low<br />blood volume, and low blood pressure. Without<br />treatment, circulatory shock and death will follow.<br />Treatment involves administration of hydrocortisone;<br />in high doses this will also compensate for the<br />aldosterone deficiency.<br />Cushing’s syndrome is the result of hypersecretion<br />of the adrenal cortex, primarily cortisol.<br />The Endocrine System 243<br />ESTROGEN<br />Estrogen is secreted by the follicle cells of the ovary;<br />secretion is stimulated by FSH from the anterior pituitary<br />gland. Estrogen promotes the maturation of the<br />ovum in the ovarian follicle and stimulates the growth<br />of blood vessels in the endometrium (lining) of the<br />uterus in preparation for a possible fertilized egg.<br />The secondary sex characteristics in women also<br />develop in response to estrogen. These include<br />growth of the duct system of the mammary glands,<br />growth of the uterus, and the deposition of fat subcutaneously<br />in the hips and thighs. The closure of the<br />epiphyseal discs in long bones is brought about by<br />estrogen, and growth in height stops. Estrogen is also<br />believed to lower blood levels of cholesterol and triglycerides.<br />For women before the age of menopause<br />this is beneficial in that it decreases the risk of atherosclerosis<br />and coronary artery disease.<br />Research suggests that estrogen no longer be considered<br />only a “female” hormone. Estrogen seems to<br />have effects on many organs, including the brain, the<br />heart, and blood vessels. In the brain, testosterone<br />from the testes or the adrenal cortex can be converted<br />to estrogen, which may be important for memory,<br />especially for older people. Estrogen seems to have<br />non-reproductive functions in both men and women,<br />although we cannot yet be as specific as we can be<br />with the reproductive functions in women, mentioned<br />previously.<br />PROGESTERONE<br />When a mature ovarian follicle releases an ovum, the<br />follicle becomes the corpus luteum and begins to<br />secrete progesterone in addition to estrogen. This is<br />stimulated by LH from the anterior pituitary gland.<br />Progesterone promotes the storage of glycogen<br />and the further growth of blood vessels in the endometrium,<br />which thus becomes a potential placenta.<br />The secretory cells of the mammary glands also<br />develop under the influence of progesterone.<br />Both progesterone and estrogen are secreted by the<br />placenta during pregnancy; these functions are covered<br />in Chapter 21.<br />INHIBIN<br />The corpus luteum secretes another hormone, called<br />inhibin. Inhibin helps decrease the secretion of FSH<br />by the anterior pituitary gland, and GnRH by the<br />hypothalamus.<br />TESTES<br />The testes are located in the scrotum, a sac of skin<br />between the upper thighs. Two hormones, testosterone<br />and inhibin, are secreted by the testes.<br />TESTOSTERONE<br />Testosterone is a steroid hormone secreted by the<br />interstitial cells of the testes; the stimulus for secretion<br />is LH from the anterior pituitary gland.<br />Testosterone promotes maturation of sperm in the<br />seminiferous tubules of the testes; this process begins<br />at puberty and continues throughout life. At puberty,<br />testosterone stimulates development of the male secondary<br />sex characteristics. These include growth of<br />all the reproductive organs, growth of facial and body<br />hair, growth of the larynx and deepening of the voice,<br />and growth (protein synthesis) of the skeletal muscles.<br />Testosterone also brings about closure of the epiphyses<br />of the long bones.<br />INHIBIN<br />The hormone inhibin is secreted by the sustentacular<br />cells of the testes; the stimulus for secretion is<br />increased testosterone. The function of inhibin is to<br />decrease the secretion of FSH by the anterior pituitary<br />gland. The interaction of inhibin, testosterone, and<br />the anterior pituitary hormones maintains spermatogenesis<br />at a constant rate.<br />OTHER HORMONES<br />Melatonin is a hormone produced by the pineal<br />gland, which is located at the back of the third ventricle<br />of the brain. The secretion of melatonin is greatest<br />during darkness and decreases when light enters the<br />eye and the retina signals the hypothalamus. A recent<br />discovery is that the retina also produces melatonin,<br />which seems to indicate that the eyes and pineal gland<br />work with the biological clock of the hypothalamus. In<br />other mammals, melatonin helps regulate seasonal<br />reproductive cycles. For people, melatonin definitely<br />stimulates the onset of sleep and increases its duration.<br />244 The Endocrine System<br />Other claims, such as that melatonin strengthens the<br />immune system or prevents cellular damage and<br />aging, are without evidence as yet.<br />There are other organs that produce hormones that<br />have only one or a few target organs. For example, the<br />stomach and duodenum produce hormones that regulate<br />aspects of digestion and appetite. Adipose tissue<br />produces the appetite-suppressing hormone leptin.<br />The thymus gland produces hormones necessary for<br />the normal functioning of the immune system, and the<br />kidneys produce a hormone that stimulates red blood<br />cell production. All of these will be discussed in later<br />chapters.<br />PROSTAGLANDINS<br />Prostaglandins (PGs) are made by virtually all cells<br />from the phospholipids of their cell membranes. They<br />differ from other hormones in that they do not circulate<br />in the blood to target organs, but rather exert<br />their effects locally, where they are produced.<br />There are many types of prostaglandins, designated<br />by the letters A through I, as in PGA, PGB, and so on.<br />Prostaglandins have many functions, and we will list<br />only a few of them here. Prostaglandins are known to<br />be involved in inflammation, pain mechanisms, blood<br />clotting, vasoconstriction and vasodilation, contraction<br />of the uterus, reproduction, secretion of digestive<br />glands, and nutrient metabolism. Current research<br />is directed at determining the normal functioning of<br />prostaglandins in the hope that many of them may<br />eventually be used clinically.<br />One familiar example may illustrate the widespread<br />activity of prostaglandins. For minor pain such as a<br />headache, many people take aspirin. Aspirin inhibits<br />the synthesis of prostaglandins involved in pain mechanisms<br />and usually relieves the pain. Some people,<br />however, such as those with rheumatoid arthritis, may<br />take large amounts of aspirin to diminish pain and<br />inflammation. These people may bruise easily because<br />blood clotting has been impaired. This too is an effect<br />of aspirin, which blocks the synthesis of prostaglandins<br />necessary for blood clotting.<br />MECHANISMS OF<br />HORMONE ACTION<br />Exactly how hormones exert their effects on their target<br />organs involves a number of complex processes,<br />which will be presented simply here.<br />A hormone must first bond to a receptor for it on<br />or in the target cell. Cells respond to certain hormones<br />and not to others because of the presence of<br />specific receptors, which are proteins. These receptor<br />proteins may be part of the cell membrane or within<br />the cytoplasm or nucleus of the target cells. A hormone<br />will affect only those cells that have its specific<br />receptors. Liver cells, for example, have cell membrane<br />receptors for insulin, glucagon, growth hormone,<br />and epinephrine; bone cells have receptors for<br />growth hormone, PTH, and calcitonin. Cells of the<br />ovaries and testes do not have receptors for PTH and<br />calcitonin, but do have receptors for FSH and LH,<br />which bone cells and liver cells do not have. Once a<br />hormone has bonded to a receptor on or in its target<br />cell, other reactions will take place.<br />THE TWO-MESSENGER MECHANISM—<br />PROTEIN HORMONES<br />The two-messenger mechanism of hormone action<br />involves “messengers” that make something happen,<br />that is, stimulate specific reactions. Protein hormones<br />usually bond to receptors of the cell membrane,<br />and the hormone is called the first messenger.<br />The hormone–receptor bonding activates the enzyme<br />adenyl cyclase on the inner surface of the cell membrane.<br />Adenyl cyclase synthesizes a substance called<br />cyclic adenosine monophosphate (cyclic AMP or<br />cAMP) from ATP, and cyclic AMP is the second messenger.<br />Cyclic AMP activates specific enzymes within the<br />cell, which bring about the cell’s characteristic<br />response to the hormone. These responses include a<br />change in the permeability of the cell membrane to a<br />specific substance, an increase in protein synthesis,<br />activation of other enzymes, or the secretion of a cellular<br />product.<br />In summary, a cell’s response to a hormone is determined<br />by the enzymes within the cell, that is, the reactions<br />of which the cell is capable. These reactions are<br />brought about by the first messenger, the hormone,<br />which stimulates the formation of the second messenger,<br />cyclic AMP. Cyclic AMP then activates the cell’s<br />enzymes to elicit a response to the hormone (Fig.<br />10–12).<br />ACTION OF STEROID HORMONES<br />Steroid hormones are soluble in the lipids of the cell<br />membrane and diffuse easily into a target cell. Once<br />The Endocrine System 245<br />inside the cell, the steroid hormone combines with a<br />protein receptor in the cytoplasm, and this steroidprotein<br />complex enters the nucleus of the cell. Within<br />the nucleus, the steroid-protein complex activates specific<br />genes, which begin the process of protein synthesis.<br />The enzymes produced bring about the cell’s<br />characteristic response to the hormone (see Fig.<br />10–12).<br />AGING AND THE<br />ENDOCRINE SYSTEM<br />Most of the endocrine glands decrease their secretions<br />with age, but normal aging usually does not lead to<br />serious hormone deficiencies. There are decreases in<br />adrenol cortical hormones, for example, but the levels<br />are usually sufficient to maintain homeostasis of water,<br />electrolytes, and nutrients. The decreased secretion of<br />growth hormone leads to a decrease in muscle mass<br />and an increase in fat storage. A lower basal metabolic<br />rate is common in elderly people as the thyroid slows<br />its secretion of thyroxine. Unless specific pathologies<br />develop, however, the endocrine system usually continues<br />to function adequately in old age.<br />SUMMARY<br />The hormones of endocrine glands are involved in virtually<br />all aspects of normal body functioning. The<br />growth and repair of tissues, the utilization of food to<br />produce energy, responses to stress, the maintenance<br />of the proper levels and pH of body fluids, and the<br />continuance of the human species all depend on hormones.<br />Some of these topics will be discussed in later<br />chapters. As you might expect, you will be reading<br />about the functions of many of these hormones again<br />and reviewing their important contributions to the<br />maintenance of homeostasis.<br />A<br />B<br />Figure 10–12. Mechanisms<br />of hormone action. (A) Twomessenger<br />mechanism of the<br />action of protein hormones.<br />(B) Action of steroid hormones.<br />See text for description.<br />QUESTION: What must a cell<br />have in order to be a target cell<br />for a particular hormone?<br />246 The Endocrine System<br />Endocrine glands are ductless glands that<br />secrete hormones into the blood. Hormones<br />exert their effects on target organs or tissues.<br />Chemistry of Hormones<br />1. Amines—structural variations of the amino acid<br />tyrosine; thyroxine, epinephrine.<br />2. Proteins—chains of amino acids; peptides are short<br />chains. Insulin, GH, glucagon are proteins; ADH<br />and oxytocin are peptides.<br />3. Steroids—made from cholesterol; cortisol, aldosterone,<br />estrogen, testosterone.<br />Regulation of Hormone Secretion<br />1. Hormones are secreted when there is a need for<br />their effects. Each hormone has a specific stimulus<br />for secretion.<br />2. The secretion of most hormones is regulated by<br />negative feedback mechanisms: As the hormone<br />exerts its effects, the stimulus for secretion is<br />reversed, and secretion of the hormone decreases.<br />Pituitary Gland (Hypophysis)—hangs from<br />hypothalamus by the infundibulum; enclosed<br />by sella turcica of sphenoid bone (see Figs.<br />10–1 and 10–2)<br />1. Posterior Pituitary (Neurohypophysis)—stores<br />hormones produced by the hypothalamus (Figs.<br />10–2 and 10–3 and Table 10–1).<br />• ADH—increases water reabsorption by the kidneys,<br />decreases sweating, in large amounts causes<br />vasoconstriction. Result: decreases urinary output<br />and increases blood volume; increases BP.<br />Stimulus: nerve impulses from hypothalamus<br />when body water decreases.<br />• Oxytocin—stimulates contraction of myometrium<br />of uterus during labor and release of milk<br />from mammary glands. Stimulus: nerve impulses<br />from hypothalamus as cervix is stretched or as<br />infant sucks on nipple.<br />2. Anterior Pituitary (Adenohypophysis)—secretions<br />are regulated by releasing hormones from the<br />hypothalamus (Fig. 10–3 and Table 10–2).<br />• GH—through intermediary molecules, IGFs,<br />GH increases amino acid transport into cells<br />and increases protein synthesis; increases rate<br />of mitosis; increases use of fats for energy<br />(Fig. 10–4). Stimulus: GHRH from the hypothalamus.<br />• TSH—increases secretion of thyroxine and T3<br />by the thyroid. Stimulus: TRH from the hypothalamus.<br />• ACTH—increases secretion of cortisol by the<br />adrenal cortex. Stimulus: CRH from the hypothalamus.<br />• Prolactin—initiates and maintains milk production<br />by the mammary glands. Stimulus: PRH<br />from the hypothalamus.<br />• FSH—In women: initiates development of ova in<br />ovarian follicles and secretion of estrogen by follicle<br />cells.<br />In men: initiates sperm development in the testes.<br />Stimulus: GnRH from the hypothalamus.<br />• LH—In women: stimulates ovulation, transforms<br />mature follicle into corpus luteum and stimulates<br />secretion of progesterone.<br />In men: stimulates secretion of testosterone by<br />the testes. Stimulus: GnRH from the hypothalamus.<br />Thyroid Gland—on front and sides of trachea<br />below the larynx (see Fig. 10–1 and<br />Table 10–3)<br />• Thyroxine (T4) and T3—(Fig. 10–5) produced by<br />thyroid follicles. Increase use of all food types for<br />energy and increase protein synthesis. Necessary<br />for normal physical, mental, and sexual development.<br />Stimulus: TSH from the anterior pituitary.<br />• Calcitonin—produced by parafollicular cells.<br />Decreases reabsorption of calcium from bones<br />and lowers blood calcium level. Stimulus: hypercalcemia.<br />Parathyroid Glands—four; two on posterior<br />of each lobe of thyroid (see Figs. 10–6 and<br />10–7 and Table 10–4)<br />• PTH—increases reabsorption of calcium and<br />phosphate from bones to the blood; increases<br />absorption of calcium and phosphate by the<br />small intestine; increases reabsorption of calcium<br />and excretion of phosphate by the kidneys, and<br />activates vitamin D. Result: raises blood calcium<br />and lowers blood phosphate levels. Stimulus:<br />hypocalcemia. Inhibitor: hypercalcemia.<br />STUDY OUTLINE<br />Pancreas—extends from curve of duodenum<br />to the spleen. Islets of Langerhans contain<br />alpha cells and beta cells (see Figs. 10–1 and<br />10–8 and Table 10–5)<br />• Glucagon—secreted by alpha cells. Stimulates<br />liver to change glycogen to glucose; increases use<br />of fats and amino acids for energy. Result: raises<br />blood glucose level. Stimulus: hypoglycemia.<br />• Insulin—secreted by beta cells. Increases use of<br />glucose by cells to produce energy; stimulates<br />liver and muscles to change glucose to glycogen;<br />increases cellular intake of fatty acids and amino<br />acids to use for synthesis of lipids and proteins.<br />Result: lowers blood glucose level. Stimulus: hyperglycemia.<br />• Somatostatin—inhibits secretion of insulin and<br />glucagon.<br />Adrenal Glands—one on top of each kidney;<br />each has an inner adrenal medulla and an<br />outer adrenal cortex (see Fig. 10–1)<br />1. Adrenal Medulla—produces catecholamines in<br />stress situations (Table 10–6 and Fig. 10–9).<br />• Norepinephrine—stimulates vasoconstriction<br />and raises blood pressure.<br />• Epinephrine—increases heart rate and force,<br />causes vasoconstriction in skin and viscera and<br />vasodilation in skeletal muscles; dilates bronchioles;<br />slows peristalsis; causes liver to change<br />glycogen to glucose; increases use of fats for<br />energy; increases rate of cell respiration. Stimulus:<br />sympathetic impulses from the hypothalamus.<br />2. Adrenal Cortex—produces mineralocorticoids,<br />glucocorticoids, and very small amounts of sex hormones<br />(function not known with certainty) (Table<br />10–7).<br />• Aldosterone—(Fig. 10–10) increases reabsorption<br />of sodium and excretion of potassium by<br />the kidneys. Results: hydrogen ions are excreted<br />in exchange for sodium; chloride and bicarbonate<br />ions and water follow sodium back to the<br />blood; maintains normal blood pH, blood volume,<br />and blood pressure. Stimulus: decreased<br />blood sodium or elevated blood potassium;<br />decreased blood volume or blood pressure (activates<br />the renin-angiotensin mechanism of the<br />kidneys).<br />• Cortisol—(Fig. 10–11) increases use of fats and<br />amino acids for energy; decreases use of glucose<br />to conserve glucose for the brain; anti-inflammatory<br />effect: blocks effects of histamine and stabilizes<br />lysosomes to prevent excessive tissue<br />damage. Stimulus: ACTH from hypothalamus<br />during physiological stress.<br />Ovaries—in pelvic cavity on either side of<br />uterus (see Fig. 10–1)<br />• Estrogen—produced by follicle cells. Promotes<br />maturation of ovum; stimulates growth of blood<br />vessels in endometrium; stimulates development<br />of secondary sex characteristics: growth of duct<br />system of mammary glands, growth of uterus, fat<br />deposition. Promotes closure of epiphyses of<br />long bones; lowers blood levels of cholesterol<br />and triglycerides. Stimulus: FSH from anterior<br />pituitary.<br />• Progesterone—produced by the corpus luteum.<br />Promotes storage of glycogen and further<br />growth of blood vessels in the endometrium;<br />promotes growth of secretory cells of mammary<br />glands. Stimulus: LH from anterior pituitary.<br />• Inhibin—inhibits secretion of FSH.<br />Testes—in scrotum between the upper<br />thighs (see Fig. 10–1)<br />• Testosterone—produced by interstitial cells.<br />Promotes maturation of sperm in testes; stimulates<br />development of secondary sex characteristics:<br />growth of reproductive organs, facial and<br />body hair, larynx, skeletal muscles; promotes closure<br />of epiphyses of long bones. Stimulus: LH<br />from anterior pituitary.<br />• Inhibin—produced by sustentacular cells.<br />Inhibits secretion of FSH to maintain a constant<br />rate of sperm production. Stimulus: increased<br />testosterone.<br />Other Hormones<br />• Melatonin—secreted by the pineal gland during<br />darkness; brings on sleep.<br />• Prostaglandins—synthesized by cells from the<br />phospholipids of their cell membranes; exert<br />their effects locally. Are involved in inflammation<br />and pain, reproduction, nutrient metabolism,<br />changes in blood vessels, blood clotting.<br />Mechanisms of Hormone Action (see Fig.<br />10–12)<br />1. A hormone affects cells that have receptors for it.<br />Receptors are proteins that may be part of the cell<br />The Endocrine System 247<br />248 The Endocrine System<br />membrane, or within the cytoplasm or nucleus of<br />the target cell.<br />• The two-messenger mechanism: a protein hormone<br />(1st messenger) bonds to a membrane<br />receptor; stimulates formation of cyclic AMP<br />(2nd messenger), which activates the cell’s<br />enzymes to bring about the cell’s characteristic<br />response to the hormone.<br />• Steroid hormones diffuse easily through cell<br />membranes and bond to cytoplasmic receptors.<br />Steroid-protein complex enters the nucleus and<br />activates certain genes, which initiate protein<br />synthesis.<br />REVIEW QUESTIONS<br />1. Use the following to describe a negative feedback<br />mechanism: TSH, TRH, decreased metabolic rate,<br />thyroxine and T3. (p. 232)<br />2. Name the two hormones stored in the posterior<br />pituitary gland. Where are these hormones produced?<br />State the functions of each of these hormones.<br />(pp. 227–228)<br />3. Name the two hormones of the anterior pituitary<br />gland that affect the ovaries or testes, and state<br />their functions. (p. 230)<br />4. Describe the antagonistic effects of PTH and calcitonin<br />on bones and on blood calcium level. State<br />the other functions of PTH. (p. 233)<br />5. Describe the antagonistic effects of insulin and<br />glucagon on the liver and on blood glucose level.<br />(pp. 235–236)<br />6. Describe how cortisol affects the use of foods for<br />energy. Explain the anti-inflammatory effects of<br />cortisol. (pp. 240–241)<br />7. State the effect of aldosterone on the kidneys.<br />Describe the results of this effect on the composition<br />of the blood. (p. 240)<br />8. When are epinephrine and norepinephrine<br />secreted? Describe the effects of these hormones.<br />(p. 237)<br />9. Name the hormones necessary for development<br />of egg cells in the ovaries. Name the hormones<br />necessary for development of sperm in the testes.<br />(p. 243)<br />10. State what prostaglandins are made from. State<br />three functions of prostaglandins. (p. 244)<br />11. Name the hormones that promote the growth of<br />the endometrium of the uterus in preparation for<br />a fertilized egg, and state precisely where each<br />hormone is produced. (p. 243)<br />12. State the functions of thyroxine and T3. For what<br />aspects of growth are these hormones necessary?<br />(pp. 231–232)<br />13. Explain the functions of GH as they are related to<br />normal growth. (pp. 228–229)<br />14. State the direct stimulus for secretion of each of<br />these hormones: (pp. 227, 229, 232, 233, 235, 240,<br />241, 243)<br />a. Thyroxine<br />b. Insulin<br />c. Cortisol<br />d. PTH<br />e. Aldosterone<br />f. Calcitonin<br />g. GH<br />h. Glucagon<br />i. Progesterone<br />j. ADH<br />FOR FURTHER THOUGHT<br />1. During a soccer game, 12-year-old Alicia got in a<br />tangle with another player, fell hard on her hand,<br />and fractured her radius. She is going to be fine,<br />though she will be wearing a cast for a few weeks.<br />What hormones were secreted immediately after<br />the injury? What functions do they have? What<br />hormones will contribute to the healing of the<br />fracture, and how?<br />The Endocrine System 249<br />2. Darren is 15 years old, tall for his age, but he wants<br />to build more muscle. He decides that he will eat<br />only protein foods, because, he says, “Muscle is<br />protein, so protein will make protein, and the more<br />protein, the more muscle.” In part he is correct,<br />and in part incorrect. Explain, and name the hormones<br />involved in protein metabolism; state how<br />each affects protein metabolism.<br />3. Many people love pasta, others love potatoes, and<br />still others love rice. Name the hormones involved<br />in carbohydrate metabolism, and, for each, explain<br />its specific function.<br />4. Unfortunately, many fast-food meals are well<br />over 50% fat. Name the hormones involved in<br />fat metabolism, and, for each, explain its specific<br />function.<br />5. You have read about the liver several times in this<br />chapter, and often seen its picture as a target organ.<br />Many functions of the liver are stimulated by hormones.<br />Name as many hormones as you can think<br />of with effects on the liver, and state the function of<br />each.<br />250<br />CHAPTER 11<br />Chapter Outline<br />Characteristics of Blood<br />Plasma<br />Blood Cells<br />Red Blood Cells<br />Function<br />Production and maturation<br />Life span<br />Blood types<br />White Blood Cells<br />Classification<br />Functions<br />Platelets<br />Function<br />Prevention of abnormal clotting<br />BOX 11–1 ANEMIA<br />BOX 11–2 JAUNDICE<br />BOX 11–3 RH DISEASE OF THE NEWBORN<br />BOX 11–4 LEUKEMIA<br />BOX 11–5 WHITE BLOOD CELL TYPES: HLA<br />BOX 11–6 HEMOPHILIA<br />BOX 11–7 DISSOLVING CLOTS<br />Student Objectives<br />• Describe the composition and explain the functions<br />of blood plasma.<br />• Name the primary hemopoietic tissue and the<br />kinds of blood cells produced.<br />• State the function of red blood cells, including the<br />protein and the mineral involved.<br />• Name the nutrients necessary for red blood cell<br />production, and state the function of each.<br />• Explain how hypoxia may change the rate of red<br />blood cell production.<br />• Describe what happens to red blood cells that have<br />reached the end of their life span; what happens to<br />the hemoglobin?<br />• Explain the ABO and Rh blood types.<br />• Name the five kinds of white blood cells and<br />describe the function of each.<br />• State what platelets are, and explain how they are<br />involved in hemostasis.<br />• Describe the three stages of chemical blood clotting.<br />• Explain how abnormal clotting is prevented in the<br />vascular system.<br />• State the normal values in a complete blood count.<br />Blood<br />251<br />New Terminology<br />ABO group (A-B-O GROOP)<br />Albumin (al-BYOO-min)<br />Bilirubin (BILL-ee-roo-bin)<br />Chemical clotting (KEM-i-kuhl KLAH-ting)<br />Embolism (EM-boh-lizm)<br />Erythrocyte (e-RITH-roh-sight)<br />Hemoglobin (HEE-moh-GLOW-bin)<br />Hemostasis (HEE-moh-STAY-sis)<br />Heparin (HEP-ar-in)<br />Immunity (im-YOO-ni-tee)<br />Leukocyte (LOO-koh-sight)<br />Macrophage (MAK-roh-fahj)<br />Normoblast (NOR-moh-blast)<br />Reticulocyte (re-TIK-yoo-loh-sight)<br />Rh factor (R-H FAK-ter)<br />Thrombocyte (THROM-boh-sight)<br />Thrombus (THROM-bus)<br />Related Clinical Terminology<br />Anemia (uh-NEE-mee-yah)<br />Differential count (DIFF-er-EN-shul KOWNT)<br />Erythroblastosis fetalis (e-RITH-roh-blass-TOH-sis<br />fee-TAL-is)<br />Hematocrit (hee-MAT-oh-krit)<br />Hemophilia (HEE-moh-FILL-ee-ah)<br />Jaundice (JAWN-diss)<br />Leukemia (loo-KEE-mee-ah)<br />Leukocytosis (LOO-koh-sigh-TOH-sis)<br />RhoGAM (ROH-gam)<br />Tissue typing (TISH-yoo-TIGH-ping)<br />Typing and cross-matching (TIGH-ping and<br />KROSS-match-ing)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />One of the simplest and most familiar life-saving<br />medical procedures is a blood transfusion. As you<br />know, however, the blood of one individual is not<br />always compatible with that of another person. The<br />ABO blood types were discovered in the early 1900s<br />by Karl Landsteiner, an Austrian-American. He<br />also contributed to the discovery of the Rh factor in<br />1940. In the early 1940s, Charles Drew, an African-<br />American, developed techniques for processing and<br />storing blood plasma, which could then be used in<br />transfusions for people with any blood type. When we<br />donate blood today, our blood may be given to a recipient<br />as whole blood, or it may be separated into its<br />component parts, and recipients will then receive only<br />those parts they need, such as red cells, plasma, factor<br />8, or platelets. Each of these parts has a specific function,<br />and all of the functions of blood are essential to<br />our survival.<br />The general functions of blood are transportation,<br />regulation, and protection. Materials transported by<br />the blood include nutrients, waste products, gases, and<br />hormones. The blood helps regulate fluid–electrolyte<br />balance, acid–base balance, and the body temperature.<br />Protection against pathogens is provided by white<br />blood cells, and the blood clotting mechanism prevents<br />excessive loss of blood after injuries. Each of these<br />functions is covered in more detail in this chapter.<br />CHARACTERISTICS OF BLOOD<br />Blood has distinctive physical characteristics:<br />Amount—a person has 4 to 6 liters of blood, depending<br />on his or her size. Of the total blood volume in<br />the human body, 38% to 48% is composed of the<br />various blood cells, also called formed elements. The<br />remaining 52% to 62% of the blood volume is<br />plasma, the liquid portion of blood (Fig. 11–1).<br />Color—you’re probably saying to yourself, “Of<br />course, it’s red!” Mention is made of this obvious<br />fact, however, because the color does vary. Arterial<br />blood is bright red because it contains high levels of<br />oxygen. Venous blood has given up much of its oxygen<br />in tissues, and has a darker, dull red color. This<br />may be important in the assessment of the source of<br />bleeding. If blood is bright red, it is probably from<br />a severed artery, and dark red blood is probably<br />venous blood.<br />pH—the normal pH range of blood is 7.35 to 7.45,<br />which is slightly alkaline. Venous blood normally<br />has a lower pH than does arterial blood because of<br />the presence of more carbon dioxide.<br />Viscosity—this means thickness or resistance to flow.<br />Blood is about three to five times thicker than<br />water. Viscosity is increased by the presence of<br />blood cells and the plasma proteins, and this thickness<br />contributes to normal blood pressure.<br />PLASMA<br />Plasma is the liquid part of blood and is approximately<br />91% water. The solvent ability of water<br />enables the plasma to transport many types of substances.<br />Nutrients absorbed in the digestive tract, such<br />as glucose, amino acids, and minerals, are circulated to<br />all body tissues. Waste products of the tissues, such as<br />urea and creatinine, circulate through the kidneys and<br />are excreted in urine. Hormones produced by<br />endocrine glands are carried in the plasma to their target<br />organs, and antibodies are also transported in<br />plasma. Most of the carbon dioxide produced by cells<br />is carried in the plasma in the form of bicarbonate ions<br />(HCO3<br />–). When the blood reaches the lungs, the CO2<br />is re-formed, diffuses into the alveoli, and is exhaled.<br />Also in the plasma are the plasma proteins. The<br />clotting factors prothrombin, fibrinogen, and others<br />are synthesized by the liver and circulate until activated<br />to form a clot in a ruptured or damaged blood<br />vessel. Albumin is the most abundant plasma protein.<br />It too is synthesized by the liver. Albumin contributes<br />to the colloid osmotic pressure of blood, which pulls<br />tissue fluid into capillaries. This is important to maintain<br />normal blood volume and blood pressure. Other<br />plasma proteins are called globulins. Alpha and beta<br />globulins are synthesized by the liver and act as carriers<br />for molecules such as fats. The gamma globulins<br />are antibodies produced by lymphocytes. Antibodies<br />initiate the destruction of pathogens and provide us<br />with immunity.<br />Plasma also carries body heat. Heat is one of the<br />by-products of cell respiration (the production of ATP<br />in cells). Blood is warmed by flowing through active<br />organs such as the liver and muscles. This heat is distributed<br />to cooler parts of the body as blood continues<br />to circulate.<br />252 Blood<br />253<br />Other body tissues and fluids 92% Blood<br />8%<br />Total body weight<br />Blood plasma 52–62% Blood cells 38–48% Blood volume<br />Water 91.5% Erythrocytes 4.5–6.0 million Blood cells<br />(per microliter)<br />Other substances<br />Other substances<br />1.5% 7%<br />Thrombocytes 150,000 – 300,000<br />Nutrients<br />Hormones<br />Nitrogenous<br />wastes<br />Respiratory<br />gases<br />Electrolytes<br />Fibrinogen 7%<br />Globulins<br />38%<br />Albumins<br />55%<br />Proteins<br />Basophils 0.5–1.0%<br />Eosinophils 1–3%<br />Monocytes 3–8%<br />Lymphocytes<br />20–35%<br />Neutrophils<br />55–70%<br />Proteins Leukocytes 5,000–10,000<br />Leukocytes<br />Figure 11–1. Components of blood and the relationship of blood to other body tissues.<br />QUESTION: Blood plasma is mostly what substance? Which blood cells are the most<br />numerous?<br />BLOOD CELLS<br />There are three kinds of blood cells: red blood cells,<br />white blood cells, and platelets. Blood cells are produced<br />from stem cells in hemopoietic tissue. After<br />birth this is primarily the red bone marrow, found<br />in flat and irregular bones such as the sternum,<br />hip bone, and vertebrae. Lymphocytes mature and<br />divide in lymphatic tissue, found in the spleen,<br />lymph nodes, and thymus gland. The thymus contains<br />stem cells that produce T lymphocytes, and the stem<br />cells in other lymphatic tissue also produce lymphocytes.<br />RED BLOOD CELLS<br />Also called erythrocytes, red blood cells (RBCs)<br />are biconcave discs, which means their centers are<br />thinner than their edges. You may recall from Chapter<br />3 that red blood cells are the only human cells without<br />nuclei. Their nuclei disintegrate as the red blood<br />cells mature and are not needed for normal functioning.<br />A normal RBC count ranges from 4.5 to 6.0 million<br />cells per microliter ( L) of blood (1 microliter 1<br />mm3 one millionth of a liter, a very small volume).<br />RBC counts for men are often toward the high end of<br />this range; those for women are often toward the low<br />end. Another way to measure the amount of RBCs is<br />the hematocrit. This test involves drawing blood into<br />a thin glass tube called a capillary tube, and centrifuging<br />the tube to force all the cells to one end. The percentages<br />of cells and plasma can then be determined.<br />Because RBCs are by far the most abundant of the<br />blood cells, a normal hematocrit range is just like that<br />of the total blood cells: 38% to 48%. Both RBC count<br />and hematocrit (Hct) are part of a complete blood<br />count (CBC).<br />Function<br />Red blood cells contain the protein hemoglobin<br />(Hb), which gives them the ability to carry oxygen.<br />Each red blood cell contains approximately 300 million<br />hemoglobin molecules, each of which can bond to<br />four oxygen molecules (see Box Fig. 3–B). In the pulmonary<br />capillaries, RBCs pick up oxygen and oxyhemoglobin<br />is formed. In the systemic capillaries,<br />hemoglobin gives up much of its oxygen and becomes<br />reduced hemoglobin. A determination of hemoglobin<br />level is also part of a CBC; the normal range is 12 to<br />18 grams per 100 mL of blood. Essential to the formation<br />of hemoglobin is the mineral iron; there are<br />four atoms of iron in each molecule of hemoglobin. It<br />is the iron that actually bonds to the oxygen and also<br />makes RBCs red.<br />Hemoglobin is also able to bond to carbon dioxide<br />(CO2), and does transport some CO2 from the tissues<br />to the lungs. But hemoglobin accounts for only about<br />10% of total CO2 transport (most is carried in the<br />plasma as bicarbonate ions).<br />Production and Maturation<br />Red blood cells are formed in red bone marrow (RBM)<br />in flat and irregular bones. Within red bone marrow<br />are precursor cells called stem cells. Recall from<br />Chapter 3 that stem cells are unspecialized cells that<br />may develop, or differentiate, in several ways. The<br />stem cells of the red bone marrow may also be called<br />hemocytoblasts (hemo “blood,” cyto “cell,” blast<br /> “producing”), and they constantly undergo mitosis<br />to produce all the kinds of blood cells, many of which<br />are RBCs (Figs. 11–2 and 11–3). The rate of production<br />is very rapid (estimated at several million new<br />RBCs per second), and a major regulating factor is<br />oxygen. If the body is in a state of hypoxia, or lack of<br />oxygen, the kidneys produce a hormone called erythropoietin,<br />which stimulates the red bone marrow to<br />increase the rate of RBC production (that is, the rate<br />of stem cell mitosis). This will occur following hemorrhage<br />or if a person stays for a time at a higher altitude.<br />As a result of the action of erythropoietin, more RBCs<br />will be available to carry oxygen and correct the<br />hypoxic state.<br />The stem cells that will become RBCs go through<br />a number of developmental stages, only the last two of<br />which we will mention (see Fig. 11–2). The normoblast<br />is the last stage with a nucleus, which then<br />disintegrates. The reticulocyte has fragments of the<br />endoplasmic reticulum, which are visible when blood<br />smears are stained for microscopic evaluation. These<br />immature cells are usually found in the red bone marrow,<br />although a small number of reticulocytes in the<br />peripheral circulation is considered normal (up to<br />1.5% of the total RBCs). Large numbers of reticulocytes<br />or normoblasts in the circulating blood mean<br />that the number of mature RBCs is not sufficient to<br />254 Blood<br />255<br />Macrophage<br />Plasma cell<br />T cell<br />Monocyte<br />Thrombocytes<br />(platelets)<br />Stem cell Lymphoblast<br />B cell<br />Natural<br />killer<br />cell<br />Eosinophil<br />Megakaryocyte<br />Normoblasts<br />Reticulocytes<br />Erythrocytes<br />Basophil<br />Neutrophil<br />Band cell<br />Figure 11–2. Production of blood cells. Stem cells are found primarily in red bone marrow<br />and are the precursor cells for all the types of blood cells.<br />QUESTION: Where are normoblasts and reticulocytes usually found, and why?<br />carry the oxygen needed by the body. Such situations<br />include hemorrhage, or when mature RBCs have<br />been destroyed, as in Rh disease of the newborn, and<br />malaria.<br />The maturation of red blood cells requires many<br />nutrients. Protein and iron are necessary for the synthesis<br />of hemoglobin and become part of hemoglobin<br />molecules. Copper is part of some enzymes involved<br />in hemoglobin synthesis. The vitamins folic acid and<br />B12 are required for DNA synthesis in the stem cells of<br />the red bone marrow. As these cells undergo mitosis,<br />they must continually produce new sets of chromosomes.<br />Vitamin B12 is also called the extrinsic factor<br />because its source is external, our food. Parietal cells<br />of the stomach lining produce the intrinsic factor, a<br />chemical that combines with the vitamin B12 in food to<br />prevent its digestion and promote its absorption in the<br />small intestine. A deficiency of either vitamin B12 or<br />the intrinsic factor results in pernicious anemia<br />(see Box 11–1: Anemia).<br />Life Span<br />Red blood cells live for approximately 120 days.<br />As they reach this age they become fragile and are<br />removed from circulation by cells of the tissue<br />macrophage system (formerly called the reticuloendothelial<br />or RE system). The organs that contain<br />macrophages (literally, “big eaters”) are the liver,<br />spleen, and red bone marrow. Look at Fig. 11–4 as you<br />read the following. The old RBCs are phagocytized<br />and digested by macrophages, and the iron they contained<br />is put into the blood to be returned to the red<br />bone marrow to be used for the synthesis of new<br />hemoglobin. If not needed immediately for this purpose,<br />excess iron is stored in the liver. The iron of<br />256 Blood<br />Figure 11–3. Blood cells.<br />(A) Red blood cells, platelets, and a<br />basophil. (B) Lymphocyte (left) and<br />neutrophil (right). (C) Eosinophil.<br />(D) Monocytes. (E) Megakaryocyte<br />with platelets. (A–E 600)<br />(F) Normal bone marrow ( 200).<br />(From Harmening, DM: Clinical<br />Hematology and Fundamentals<br />of Hemostasis, ed. 3. FA Davis,<br />Philadelphia, 1997, pp 14, 17, 19,<br />26, 48, with permission.)<br />QUESTION: Look at the RBCs in<br />picture B. Why do they have pale<br />centers?<br />A B<br />D<br />E F<br />C<br />Blood 257<br />BOX 11–1 ANEMIA<br />Aplastic anemia is suppression of the red bone<br />marrow, with decreased production of RBCs, WBCs,<br />and platelets. This is a very serious disorder that<br />may be caused by exposure to radiation, certain<br />chemicals such as benzene, or some medications.<br />There are several antibiotics that must be used with<br />caution since they may have this potentially fatal<br />side effect.<br />Hemolytic anemia is any disorder that causes<br />rupture of RBCs before the end of their normal life<br />span. Sickle-cell anemia and Rh disease of the newborn<br />are examples. Another example is malaria, in<br />which a protozoan parasite reproduces in RBCs and<br />destroys them. Hemolytic anemias are often characterized<br />by jaundice because of the increased production<br />of bilirubin.<br />Anemia is a deficiency of red blood cells, or insufficient<br />hemoglobin within the red blood cells. There<br />are many different types of anemia.<br />Iron-deficiency anemia is caused by a lack of<br />dietary iron, and there is not enough of this mineral<br />to form sufficient hemoglobin. A person with this<br />type of anemia may have a normal RBC count and<br />a normal hematocrit, but the hemoglobin level will<br />be below normal.<br />A deficiency of vitamin B12, which is found only<br />in animal foods, leads to pernicious anemia, in<br />which the RBCs are large, misshapen, and fragile.<br />Another cause of this form of anemia is lack of the<br />intrinsic factor due to autoimmune destruction of<br />the parietal cells of the stomach lining.<br />Sickle-cell anemia has already been discussed<br />in Chapter 3. It is a genetic disorder of hemoglobin,<br />which causes RBCs to sickle, clog capillaries, and<br />rupture.<br />A B<br />C D<br />Box Figure 11–A Anemia. (A) Iron-deficiency anemia; notice the pale, oval RBCs ( 400).<br />(B) Pernicious anemia, with large, misshapen RBCs ( 400). (C) Sickle-cell anemia ( 400).<br />(D) Aplastic anemia, bone marrow ( 200). (A, B, and C from Listen, Look, and Learn, Vol<br />3; Coagulation, Hematology. The American Society of Clinical Pathologists Press, Chicago,<br />1973, with permission. D from Harmening, DM: Clinical Hematology and Fundamentals of<br />Hemostasis, ed 3. FA Davis, Philadelphia, 1997, p 49, with permission.)<br />RBCs is actually recycled over and over again. The<br />globin or protein portion of the hemoglobin molecule<br />is also recycled. It is digested to its amino acids, which<br />may then be used for the synthesis of new proteins.<br />Another part of the hemoglobin molecule is the<br />heme portion, which cannot be recycled and is a waste<br />product. The heme is converted to bilirubin by<br />macrophages. The liver removes bilirubin from circulation<br />and excretes it into bile; bilirubin is a bile pigment.<br />Bile is secreted by the liver into the duodenum<br />and passes through the small intestine and colon, so<br />bilirubin is eliminated in feces, and gives feces their<br />characteristic brown color. In the colon some bilirubin<br />is changed to urobilinogen by the colon bacteria.<br />258 Blood<br />New RBCs<br />formed in<br />red bone marrow<br />Used to make<br />new RBCs<br />Iron Heme Globin<br />Stored in<br />liver<br />Bilirubin<br />Small<br />intestine<br />Large intestine<br />Bilirubin<br />Colon<br />bacteria<br />Urobilin<br />Urobilin<br />Urine<br />Amino acids<br />Protein synthesis<br />Macrophages in<br />liver, spleen, and<br />red bone marrow<br />phagocytize old RBCs<br />Kidney<br />RBCs Circulate<br />120 days<br />Figure 11–4. Life cycle of red blood cells. See text for description.<br />QUESTION: Which components of old RBCs are recycled? Which is excreted? (Go to the<br />macrophage and follow the arrows.)<br />Some urobilinogen may be absorbed into the blood,<br />but it is changed to urobilin and excreted by the kidneys<br />in urine. If bilirubin is not excreted properly, perhaps<br />because of liver disease such as hepatitis, it<br />remains in the blood. This may cause jaundice, a condition<br />in which the whites of the eyes appear yellow.<br />This yellow color may also be seen in the skin of lightskinned<br />people (see Box 11–2: Jaundice).<br />Blood Types<br />Our blood types are genetic; that is, we inherit genes<br />from our parents that determine our own types. There<br />are many red blood cell factors or types; we will discuss<br />the two most important ones: the ABO group<br />and the Rh factor. (The genetics of blood types is discussed<br />in Chapter 21.)<br />The ABO group contains four blood types: A, B,<br />AB, and O. The letters A and B represent antigens<br />(protein-oligosaccharides) on the red blood cell membrane.<br />A person with type A blood has the A antigen<br />on the RBCs, and someone with type B blood has the<br />B antigen. Type AB means that both A and B antigens<br />are present, and type O means that neither the A nor<br />the B antigen is present.<br />Circulating in the plasma of each person are natural<br />antibodies for those antigens not present on the<br />RBCs. Therefore, a type A person has anti-B antibodies<br />in the plasma; a type B person has anti-A antibodies;<br />a type AB person has neither anti-A nor anti-B<br />antibodies; and a type O person has both anti-A and<br />anti-B antibodies (see Table 11–1 and Fig. 11–5).<br />These natural antibodies are of great importance<br />for transfusions. If possible, a person should receive<br />Blood 259<br />BOX 11–2 JAUNDICE<br />born; these are hemolytic anemias. As excessive<br />numbers of RBCs are destroyed, bilirubin is formed<br />at a faster rate than the liver can excrete it. The<br />bilirubin that the liver cannot excrete remains in the<br />blood and causes jaundice. Another name for this<br />type is hemolytic jaundice.<br />Posthepatic jaundice means that the problem<br />is “after” the liver. The liver excretes bilirubin into<br />bile, which is stored in the gallbladder and then<br />moved to the small intestine. If the bile ducts are<br />obstructed, perhaps by gallstones or inflammation<br />of the gallbladder, bile cannot pass to the small<br />intestine and backs up in the liver. Bilirubin may<br />then be reabsorbed back into the blood and cause<br />jaundice. Another name for this type is obstructive<br />jaundice.<br />Jaundice is not a disease, but rather a sign caused<br />by excessive accumulation of bilirubin in the blood.<br />Because one of the liver’s many functions is the<br />excretion of bilirubin, jaundice may be a sign of<br />liver disease such as hepatitis or cirrhosis. This may<br />be called hepatic jaundice, because the problem<br />is with the liver.<br />Other types of jaundice are prehepatic jaundice<br />and posthepatic jaundice: The name of each tells us<br />where the problem is. Recall that bilirubin is the<br />waste product formed from the heme portion of<br />the hemoglobin of old RBCs. Prehepatic jaundice<br />means that the problem is “before” the liver;<br />that is, hemolysis of RBCs is taking place at a more<br />rapid rate. Rapid hemolysis is characteristic of sicklecell<br />anemia, malaria, and Rh disease of the new-<br />Table 11–1 ABO BLOOD TYPES<br />Antigens Present Antibodies Present Percentage in U.S. Population*<br />Type on RBCs in Plasma White Black Asian<br />A A anti-B 40 27 31<br />B B anti-A 11 20 26<br />AB both A and B neither anti-A nor anti-B 4 4 8<br />O neither A nor B both anti-A and anti-B 45 49 35<br />*Average.<br />260<br />Type A<br />Type<br />AB<br />Red blood cells Plasma Anti-A serum Anti-B serum<br />ABO blood types Typing and cross-matching<br />Type A<br />Type B Type B<br />Type AB Type AB<br />Type O Type O<br />A antigens B antibodies<br />B antigens A antibodies<br />A and B antigens Neither A nor B antibodies<br />Neither A<br />nor B antigens<br />A and B antibodies<br />Type<br />O<br />Type<br />A<br />Type<br />A<br />Type<br />O<br />Type<br />B<br />Type<br />B<br />Universal donor<br />Type<br />AB<br />Universal recipient<br />A B<br />C<br />Figure 11–5. (A) The ABO blood types. Schematic representation of antigens on the<br />RBCs and antibodies in the plasma. (B) Typing and cross-matching. The A or B antiserum<br />causes agglutination of RBCs with the matching antigen. (C) Acceptable transfusions are<br />diagrammed and presuppose compatible Rh factors.<br />QUESTION: In part C, find your blood type. To whom (that is, to which blood types) can<br />you donate blood?<br />blood of his or her own type; only if this type is not<br />available should another type be given. For example,<br />let us say that a type A person needs a transfusion to<br />replace blood lost in hemorrhage. If this person were<br />to receive type B blood, what would happen? The type<br />A recipient has anti-B antibodies that would bind to<br />the type B antigens of the RBCs of the donated blood.<br />The type B RBCs would first clump (agglutination)<br />then rupture (hemolysis), thus defeating the purpose<br />of the transfusion. An even more serious consequence<br />is that the hemoglobin of the ruptured RBCs, now<br />called free hemoglobin, may clog the capillaries of the<br />kidneys and lead to renal damage or renal failure. You<br />can see why typing and cross-matching of donor and<br />recipient blood in the hospital laboratory is so important<br />before any transfusion is given (see Fig. 11–5).<br />This procedure helps ensure that donated blood will<br />not bring about a hemolytic transfusion reaction in<br />the recipient.<br />You may have heard of the concept that a person<br />with type O blood is a “universal donor.” Usually, a<br />unit of type O negative blood may be given to people<br />with any other blood type. This is so because type O<br />RBCs have neither the A nor the B antigens and will<br />not react with whatever antibodies the recipient may<br />have. If only one unit (1 pint) of blood is given, the<br />anti-A and anti-B antibodies in the type O blood<br />plasma will be diluted in the recipient’s blood plasma<br />and will not have a harmful effect on the recipient’s<br />RBCs. The term negative, in O negative, the universal<br />donor, refers to the Rh factor, which we will now<br />consider.<br />The Rh factor is another antigen (often called D)<br />that may be present on RBCs. People whose RBCs<br />have the Rh antigen are Rh positive; those without<br />the antigen are Rh negative. Rh-negative people do<br />not have natural antibodies to the Rh antigen, and<br />for them this antigen is foreign. If an Rh-negative<br />person receives Rh-positive blood by mistake,<br />antibodies will be formed just as they would be<br />to bacteria or viruses. A first mistaken transfusion<br />often does not cause problems, because antibody production<br />is slow upon the first exposure to Rh-positive<br />RBCs. A second transfusion, however, when anti-Rh<br />antibodies are already present, will bring about a<br />transfusion reaction, with hemolysis and possible kidney<br />damage (see also Box 11–3: Rh Disease of the<br />Newborn).<br />WHITE BLOOD CELLS<br />White blood cells (WBCs) are also called leukocytes.<br />There are five kinds of WBCs; all are larger than<br />RBCs and have nuclei when mature. The nucleus may<br />be in one piece or appear as several lobes or segments.<br />Special staining for microscopic examination gives<br />each kind of WBC a distinctive appearance (see Figs.<br />11–2 and 11–3).<br />Blood 261<br />BOX 11–3 Rh DISEASE OF THE NEWBORN<br />baby will be born anemic and jaundiced from the<br />loss of RBCs. Such an infant may require a gradual<br />exchange transfusion to remove the blood with the<br />maternal antibodies and replace it with Rh-negative<br />blood. The baby will continue to produce its own<br />Rh-positive RBCs, which will not be destroyed once<br />the maternal antibodies have been removed.<br />Much better than treatment, however, is prevention.<br />If an Rh-negative woman delivers an Rhpositive<br />baby, she should be given RhoGAM within<br />72 hours after delivery. RhoGAM is an anti-Rh antibody<br />that will destroy any fetal RBCs that have<br />entered the mother’s circulation before her immune<br />system can respond and produce antibodies. The<br />RhoGAM antibodies themselves break down within<br />a few months. The woman’s next pregnancy will be<br />like the first, as if she had never been exposed to<br />Rh-positive RBCs.<br />Rh disease of the newborn may also be called<br />erythroblastosis fetalis and is the result of an Rh<br />incompatibility between mother and fetus. During<br />a normal pregnancy, maternal blood and fetal<br />blood do not mix in the placenta. However, during<br />delivery of the placenta (the “afterbirth” that follows<br />the birth of the baby), some fetal blood may<br />enter maternal circulation.<br />If the woman is Rh negative and her baby is Rh<br />positive, this exposes the woman to Rh-positive<br />RBCs. In response, her immune system will now<br />produce anti-Rh antibodies following this first delivery.<br />In a subsequent pregnancy, these maternal<br />antibodies will cross the placenta and enter fetal circulation.<br />If this next fetus is also Rh positive, the<br />maternal antibodies will cause destruction (hemolysis)<br />of the fetal RBCs. In severe cases this may result<br />in the death of the fetus. In less severe cases, the<br />A normal WBC count (part of a CBC) is 5,000 to<br />10,000 per L. Notice that this number is quite small<br />compared to a normal RBC count. Many of our<br />WBCs are not circulating within blood vessels but are<br />carrying out their functions in tissue fluid or in lymphatic<br />tissue.<br />Classification<br />The five kinds of white blood cells, all produced in the<br />red bone marrow (and some lymphocytes in lymphatic<br />tissue), may be classified in two groups: granular and<br />agranular. The granular leukocytes are the neutrophils,<br />eosinophils, and basophils, which usually<br />have nuclei in two or more lobes or segments, and<br />have distinctly colored granules when stained.<br />Neutrophils have light blue granules, eosinophils have<br />red granules, and basophils have dark blue granules.<br />The agranular leukocytes are lymphocytes and<br />monocytes, which have nuclei in one piece.<br />Monocytes are usually quite a bit larger than lymphocytes.<br />A differential WBC count (part of a CBC) is<br />the percentage of each kind of leukocyte. Normal<br />ranges are listed in Table 11–2, along with other normal<br />values of a CBC.<br />Functions<br />White blood cells all contribute to the same general<br />function, which is to protect the body from infectious<br />disease and to provide immunity to certain diseases.<br />Each kind of leukocyte makes a contribution to this<br />very important aspect of homeostasis.<br />Neutrophils and monocytes are capable of the<br />phagocytosis of pathogens. Neutrophils are the more<br />abundant phagocytes, but the monocytes are the more<br />efficient phagocytes, because they differentiate into<br />macrophages, which also phagocytize dead or damaged<br />tissue at the site of any injury, helping to make<br />tissue repair possible. During an infection, neutrophils<br />are produced more rapidly, and the immature forms,<br />called band cells (see Fig. 11–2), may appear in<br />greater numbers in peripheral circulation (band cells<br />are usually less than 10% of the total neutrophils).<br />The term “band” refers to the nucleus that has not yet<br />become segmented, and may look somewhat like a<br />dumbbell.<br />Eosinophils are believed to detoxify foreign<br />proteins and will phagocytize anything labeled with<br />antibodies. This is especially important in allergic<br />reactions and parasitic infections such as trichinosis (a<br />worm parasite). Basophils contain granules of heparin<br />and histamine. Heparin is an anticoagulant that helps<br />prevent abnormal clotting within blood vessels.<br />Histamine, you may recall, is released as part of the<br />inflammation process, and it makes capillaries more<br />permeable, allowing tissue fluid, proteins, and white<br />blood cells to accumulate in the damaged area.<br />There are two major kinds of lymphocytes, T cells<br />and B cells, and a less numerous third kind called natural<br />killer cells. For now we will say that T cells (or T<br />lymphocytes) help recognize foreign antigens and may<br />directly destroy some foreign antigens. B cells (or B<br />lymphocytes) become plasma cells that produce antibodies<br />to foreign antigens. Both T cells and B cells<br />provide memory for immunity. Natural killer cells<br />(NK cells) destroy foreign cells by chemically rupturing<br />their membranes. These functions of lymphocytes<br />are discussed in the context of the mechanisms of<br />immunity in Chapter 14.<br />As mentioned earlier, leukocytes function in tissue<br />fluid as well as in the blood. Many WBCs are capable<br />of self-locomotion (ameboid movement) and are able<br />to squeeze between the cells of capillary walls and out<br />into tissue spaces. Macrophages provide a good example<br />of the dual locations of leukocytes. Some<br />macrophages are “fixed,” that is, stationary in organs<br />such as the liver, spleen, and red bone marrow (part of<br />the tissue macrophage or RE system—the same<br />262 Blood<br />Table 11–2 COMPLETE BLOOD COUNT<br />Measurement Normal Range*<br />Red blood cells<br />Hemoglobin<br />Hematocrit<br />Reticulocytes<br />White blood cells (total)<br />Neutrophils<br />Eosinophils<br />Basophils<br />Lymphocytes<br />Monocytes<br />Platelets<br />*The values on hospital lab slips may vary somewhat but<br />will be very similar to the normal ranges given here.<br />• 4.5–6.0 million/ L<br />• 12–18 grams/100 mL<br />• 38%–48%<br />• 0%–1.5%<br />• 5000–10,000/ L<br />• 55%–70%<br />• 1%–3%<br />• 0.5%–1%<br />• 20%–35%<br />• 3%–8%<br />• 150,000–300,000/ L<br />macrophages that phagocytize old RBCs) and in the<br />lymph nodes. They phagocytize pathogens that circulate<br />in blood or lymph through these organs. Other<br />“wandering” macrophages move about in tissue fluid,<br />especially in the areolar connective tissue of mucous<br />membranes and below the skin. Pathogens that gain<br />entry into the body through natural openings or<br />through breaks in the skin are usually destroyed by the<br />macrophages and other leukocytes in connective tissue<br />before they can cause serious disease. The alveoli<br />of the lungs, for example, have macrophages that<br />very efficiently destroy pathogens that enter with<br />inhaled air.<br />A high WBC count, called leukocytosis, is often<br />an indication of infection. Leukopenia is a low WBC<br />count, which may be present in the early stages of diseases<br />such as tuberculosis. Exposure to radiation or to<br />chemicals such as benzene may destroy WBCs and<br />lower the total count. Such a person is then very susceptible<br />to infection. Leukemia, or malignancy of<br />leukocyte-forming tissues, is discussed in Box 11–4:<br />Leukemia.<br />The white blood cell types (analogous to RBC types<br />such as the ABO group) are called human leukocyte<br />antigens (HLA) and are discussed in Box 11–5: White<br />Blood Cell Types: HLA.<br />PLATELETS<br />The more formal name for platelets is thrombocytes,<br />which are not whole cells but rather fragments or<br />pieces of cells. Some of the stem cells in the red bone<br />marrow differentiate into large cells called megakaryocytes<br />(see Figs. 11–2 and 11–3), which break up into<br />small pieces that enter circulation. These small, oval,<br />circulating pieces are platelets, which may last for 5 to<br />9 days, if not utilized before that. Thrombopoietin is<br />a hormone produced by the liver that increases the<br />rate of platelet production.<br />A normal platelet count (part of a CBC) is 150,000<br />to 300,000/ L (the high end of the range may be<br />extended to 500,000). Thrombocytopenia is the term<br />for a low platelet count.<br />Function<br />Platelets are necessary for hemostasis, which means<br />prevention of blood loss. There are three mechanisms,<br />Blood 263<br />BOX 11–4 LEUKEMIA<br />Chemotherapy may bring about cure or remission<br />for some forms of leukemia, but other forms<br />remain resistant to treatment and may be fatal<br />within a few months of diagnosis. In such cases, the<br />cause of death is often pneumonia or some other<br />serious infection, because the abnormal white<br />blood cells cannot prevent the growth and spread<br />of pathogens within the body.<br />Leukemia is the term for malignancy of the bloodforming<br />tissue. There are many types of leukemia,<br />which are classified as acute or chronic, by the<br />types of abnormal cells produced, and by either<br />childhood or adult onset.<br />In general, leukemia is characterized by an overproduction<br />of immature white blood cells. These<br />immature cells cannot perform their normal functions,<br />and the person becomes very susceptible to<br />infection. As a greater proportion of the body’s<br />nutrients are used by malignant cells, the production<br />of other blood cells decreases. Severe anemia is<br />a consequence of decreased red blood cell production,<br />and the tendency to hemorrhage is the result<br />of decreased platelets.<br />Box Figure 11–B Leukemia. Notice the many<br />darkly staining WBCs ( 300). (From Sacher, RA, and<br />McPherson, RA: Widmann’s Clinical Interpretation<br />of Laboratory Tests, ed 11. FA Davis, Philadelphia,<br />2000, with permission.)<br />and platelets are involved in each. Two of these mechanisms<br />are shown in Fig. 11–6.<br />1. Vascular spasm—when a large vessel such as an<br />artery or vein is severed, the smooth muscle in its<br />wall contracts in response to the damage (called the<br />myogenic response). Platelets in the area of the<br />rupture release serotonin, which also brings about<br />vasoconstriction. The diameter of the vessel is<br />thereby made smaller, and the smaller opening may<br />then be blocked by a blood clot. If the vessel did<br />not constrict first, the clot that forms would quickly<br />be washed out by the force of the blood pressure.<br />2. Platelet plugs—when capillaries rupture, the<br />damage is too slight to initiate the formation of a<br />blood clot. The rough surface, however, causes<br />platelets to change shape (become spiky) and<br />become sticky. These activated platelets stick to the<br />edges of the break and to each other. The platelets<br />form a mechanical barrier or wall to close off the<br />break in the capillary. Capillary ruptures are quite<br />frequent, and platelet plugs, although small, are all<br />that is needed to seal them.<br />Would platelet plugs be effective for breaks in<br />larger vessels? No, they are too small, and though<br />they do form, they are washed away (until a clot<br />begins to form that can contain them). Would vascular<br />spasm be effective for capillaries? Again, the<br />answer is no, because capillaries have no smooth<br />muscle and cannot constrict at all.<br />264 Blood<br />BOX 11–5 WHITE BLOOD CELL TYPES: HLA<br />the HLA types of a donated organ to see if one or<br />several will match the HLA types of the potential<br />recipient. If even one HLA type matches, the chance<br />of rejection is lessened. Although all transplant<br />recipients (except corneal) must receive immunosuppressive<br />medications to prevent rejection, such<br />medications make them more susceptible to infection.<br />The closer the HLA match of the donated<br />organ, the lower the dosage of such medications,<br />and the less chance of serious infections. (The<br />chance of finding a perfect HLA match in the general<br />population is estimated at 1 in 20,000.)<br />There is yet another aspect of the importance of<br />HLA: People with certain HLA types seem to be<br />more likely to develop certain non-infectious diseases.<br />For example, type 1 (insulin-dependent) diabetes<br />mellitus is often found in people with HLA<br />DR3 or DR4, and a form of arthritis of the spine<br />called ankylosing spondylitis is often found in those<br />with HLA B27. These are not genes for these diseases,<br />but may be predisposing factors. What may<br />happen is this: A virus enters the body and stimulates<br />the immune system to produce antibodies.<br />The virus is destroyed, but one of the person’s own<br />antigens is so similar to the viral antigen that the<br />immune system continues its activity and begins to<br />destroy this similar part of the body. Another possibility<br />is that a virus damages a self-antigen to the<br />extent that it is now so different that it will be perceived<br />as foreign. These are two theories of how<br />autoimmune diseases are triggered, which is the<br />focus of much research in the field of immunology.<br />Human leukocyte antigens (HLA) are antigens<br />on WBCs that are representative of the antigens<br />present on all the cells of an individual. These are<br />our “self” antigens that identify cells that belong in<br />the body.<br />Recall that in the ABO blood group of RBCs,<br />there are only two antigens, A and B, and four<br />possible types: A, B, AB, and O. HLA antigens are<br />also given letter names. HLA A, B, and C are called<br />class I proteins, with from 100 to more than 400<br />possibilities for the specific protein each can be. The<br />several class II proteins are given various D designations<br />and, again, there are many possibilities for<br />each. Each person has two genes for each HLA type,<br />because these types are inherited, just as RBC types<br />are inherited. Members of the same family may<br />have some of the same HLA types, and identical<br />twins have exactly the same HLA types.<br />The purpose of the HLA types is to provide a<br />“self” comparison for the immune system to use<br />when pathogens enter the body. The T lymphocytes<br />compare the “self” antigens on macrophages<br />to the antigens on bacteria and viruses. Because<br />these antigens do not match ours, they are recognized<br />as foreign; this is the first step in the destruction<br />of a pathogen.<br />The surgical transplantation of organs has also<br />focused on the HLA. The most serious problem for<br />the recipient of a transplanted heart or kidney is<br />rejection of the organ and its destruction by the<br />immune system. You may be familiar with the term<br />tissue typing. This process involves determining<br />3. Chemical clotting—The stimulus for clotting is a<br />rough surface within a vessel, or a break in the vessel,<br />which also creates a rough surface. The more<br />damage there is, the faster clotting begins, usually<br />within 15 to 120 seconds.<br />The clotting mechanism is a series of reactions<br />involving chemicals that normally circulate in the<br />blood and others that are released when a vessel is<br />damaged.<br />The chemicals involved in clotting include platelet<br />factors, chemicals released by damaged tissues, calcium<br />ions, and the plasma proteins prothrombin, fibrinogen,<br />Factor 8, and others synthesized by the liver.<br />(These clotting factors are also designated by Roman<br />numerals; Factor 8 would be Factor VIII.) Vitamin K<br />is necessary for the liver to synthesize prothrombin<br />and several other clotting factors (Factors 7, 9, and 10).<br />Most of our vitamin K is produced by the bacteria that<br />live in the colon; the vitamin is absorbed as the colon<br />absorbs water and may be stored in the liver.<br />Chemical clotting is usually described in three<br />stages, which are listed in Table 11–3 and illustrated in<br />Fig. 11–7. Stage 1 begins when a vessel is cut or damaged<br />internally, and includes all of the factors shown.<br />As you follow the pathway, notice that the product of<br />stage 1 is prothrombin activator, which may also be<br />called prothrombinase. Each name tells us something.<br />The first name suggests that this chemical activates<br />prothrombin, and that is true. The second name ends<br />in “ase,” which indicates that this is an enzyme. The<br />traditional names for enzymes use the substrate of the<br />enzyme as the first part of the name, and add “ase.” So<br />this chemical must be an enzyme whose substrate is<br />Blood 265<br />Skin is cut and<br />blood escapes from a<br />capillary and an<br />arteriole.<br />Capillary<br />Arteriole<br />Platelets<br />Fibrin<br />In the capillary, platelets<br />stick to the ruptured wall<br />and form a platelet plug.<br />In the arteriole, chemical<br />clotting forms a fibrin clot.<br />Clot retraction pulls the<br />edges of the break together.<br />Figure 11–6. Hemostasis. Platelet<br />plug formation in a capillary and<br />chemical clotting and clot retraction<br />in an arteriole.<br />QUESTION: Look at the diameter of<br />the arteriole (compared to that of<br />the capillary) and explain why<br />platelet plugs would not be sufficient<br />to stop the bleeding.<br />prothrombin, and that is also true. The stages of clotting<br />may be called a cascade, where one leads to the<br />next, as inevitable as water flowing downhill. Prothrombin<br />activator, the product of stage 1, brings<br />about the stage 2 reaction: converting prothrombin to<br />thrombin. The product of stage 2, thrombin, brings<br />about the stage 3 reaction: converting fibrinogen to<br />fibrin (see Box 11–6: Hemophilia).<br />The clot itself is made of fibrin, the product of<br />stage 3. Fibrin is a thread-like protein. Many strands<br />of fibrin form a mesh that traps RBCs and platelets,<br />and creates a wall across the break in the vessel.<br />Once the clot has formed and bleeding has stopped,<br />clot retraction and fibrinolysis occur. Clot retraction<br />requires platelets, ATP, and Factor 13 and involves<br />folding of the fibrin threads to pull the edges of the<br />rupture in the vessel wall closer together. This will<br />make the area to be repaired smaller. The platelets<br />contribute in yet another way, because as they disintegrate<br />they release platelet-derived growth factor<br />(PDGF), which stimulates the repair of blood vessels<br />(growth of their tissues). As repair begins, the clot is<br />dissolved, a process called fibrinolysis. It is important<br />that the clot be dissolved, because it is a rough surface,<br />266 Blood<br />Table 11–3 CHEMICAL CLOTTING<br />Clotting Stage Factors Needed Reaction<br />Stage 1<br />Stage 2<br />Stage 3<br />• Platelet factors<br />• Chemicals from damaged tissue<br />(tissue thromboplastin)<br />• Factors 5,7,8,9,10,11,12<br />• Calcium ions<br />• Prothrombin activator from stage 1<br />• Prothrombin<br />• Calcium ions<br />• Thrombin from stage 2<br />• Fibrinogen<br />• Calcium ions<br />• Factor 13 (fibrin stabilizing factor)<br />Platelet factors tissue thromboplastin <br />other clotting factors calcium ions<br />form prothrombin activator<br />Prothrombin activator converts prothrombin<br />to thrombin<br />Thrombin converts fibrinogen to fibrin<br />BOX 11–6 HEMOPHILIA<br />cure) has become possible with factor 8 obtained<br />from blood donors. The Factor 8 is extracted from<br />the plasma of donated blood and administered in<br />concentrated form to hemophiliacs, enabling them<br />to live normal lives.<br />In what is perhaps the most tragic irony of medical<br />progress, many hemophiliacs were inadvertently<br />infected with HIV, the virus that causes AIDS.<br />Before 1985, there was no test to detect HIV in<br />donated blood, and the virus was passed to hemophiliacs<br />in the very blood product that was meant<br />to control their disease and prolong their lives.<br />Today, all donated blood and blood products are<br />tested for HIV, and the risk of AIDS transmission to<br />hemophiliacs, or anyone receiving donated blood,<br />is now very small.<br />There are several forms of hemophilia; all are<br />genetic and are characterized by the inability of<br />the blood to clot properly. Hemophilia A is the<br />most common form and involves a deficiency of<br />clotting Factor 8. The gene for hemophilia A is<br />located on the X chromosome, so this is a sexlinked<br />trait, with the same pattern of inheritance<br />as red-green color blindness and Duchenne’s muscular<br />dystrophy.<br />Without factor 8, the first stage of chemical<br />clotting cannot be completed, and prothrombin<br />activator is not formed. Without treatment, a<br />hemophiliac experiences prolonged bleeding after<br />even minor injuries and extensive internal bleeding,<br />especially in joints subjected to the stresses of<br />weight-bearing. In recent years, treatment (but not<br />Blood 267<br />Factors 5, 7, 8, 9,<br />10, 11, 12<br />Platelet factors<br />Chemicals from<br />damaged tissue Calcium<br />ions<br />Prothrombin<br />activator<br />Prothrombin<br />Calcium<br />ions<br />Thrombin<br />Fibrinogen<br />Calcium ions<br />Factor 13<br />Fibrin<br />Stage 1<br />Stage 2<br />Stage 3<br />Liver<br />Figure 11–7. Stages of chemical blood clotting.<br />QUESTION: Based only on this picture, explain why the liver is a vital organ.<br />and if it were inside a vessel it would stimulate more<br />and unnecessary clotting, which might eventually<br />obstruct blood flow.<br />Prevention of Abnormal Clotting<br />Clotting should take place to stop bleeding, but too<br />much clotting would obstruct vessels and interfere<br />with normal circulation of blood. Clots do not usually<br />form in intact vessels because the endothelium (simple<br />squamous epithelial lining) is very smooth and<br />repels the platelets and clotting factors. If the lining<br />becomes roughened, as happens with the lipid deposits<br />of atherosclerosis, a clot will form.<br />Heparin, produced by basophils, is a natural anticoagulant<br />that inhibits the clotting process (although<br />heparin is called a “blood thinner,” it does not “thin”<br />or dilute the blood in any way; rather it prevents a<br />chemical reaction from taking place). The liver produces<br />a globulin called antithrombin, which combines<br />with and inactivates excess thrombin. Excess thrombin<br />would exert a positive feedback effect on the clotting<br />cascade, and result in the splitting of more prothrombin<br />to thrombin, more clotting, more thrombin<br />formed, and so on. Antithrombin helps to prevent this,<br />as does the fibrin of the clot, which adsorbs excess<br />thrombin and renders it inactive. All of these factors<br />are the external brake for this positive feedback mechanism.<br />Together they usually limit the fibrin formed to<br />what is needed to create a useful clot but not an<br />obstructive one.<br />Thrombosis refers to clotting in an intact vessel;<br />the clot itself is called a thrombus. Coronary thrombosis,<br />for example, is abnormal clotting in a coronary<br />artery, which will decrease the blood (oxygen) supply<br />to part of the heart muscle. An embolism is a clot<br />or other tissue transported from elsewhere that lodges<br />in and obstructs a vessel (see Box 11–7: Dissolving<br />Clots).<br />SUMMARY<br />All of the functions of blood described in this chapter—<br />transport, regulation, and protection—contribute<br />to the homeostasis of the body as a whole.<br />However, these functions could not be carried out if<br />the blood did not circulate properly. The circulation<br />of blood throughout the blood vessels depends upon<br />the proper functioning of the heart, the pump of the<br />circulatory system, which is the subject of our next<br />chapter.<br />268 Blood<br />BOX 11–7 DISSOLVING CLOTS<br />Abnormal clots may cause serious problems in<br />coronary arteries, pulmonary arteries, cerebral<br />vessels, and even veins in the legs. However, if<br />clots can be dissolved before they cause death of<br />tissue, normal circulation and tissue functioning<br />may be restored.<br />One of the first substances used to dissolve<br />clots in coronary arteries was streptokinase,<br />which is actually a bacterial toxin produced<br />by some members of the genus Streptococcus.<br />Streptokinase did indeed dissolve clots, but its<br />use created the possibility of clot destruction<br />throughout the body, with serious hemorrhage a<br />potential consequence.<br />Safer chemicals called third-generation<br />thrombolytics are now used (thrombo “clot”<br />and lytic “to lyse” or “split”). In a case of coronary<br />thrombosis, if a thrombolytic can be administered<br />within a few hours, the clot may be<br />dissolved and permanent heart damage prevented.<br />The same procedure is also used to prevent<br />permanent brain damage after strokes<br />(CVAs) caused by blood clots.<br />STUDY OUTLINE<br />The general functions of blood are transportation,<br />regulation, and protection.<br />Characteristics of Blood<br />1. Amount—4 to 6 liters; 38% to 48% is cells; 52% to<br />62% is plasma (Fig. 11–1).<br />2. Color—arterial blood has a high oxygen content<br />and is bright red; venous blood has less oxygen and<br />is dark red.<br />3. pH—7.35 to 7.45; venous blood has more CO2 and<br />a lower pH than arterial blood.<br />4. Viscosity—thickness or resistance to flow; due to<br />the presence of cells and plasma proteins; contributes<br />to normal blood pressure.<br />Plasma—the liquid portion of blood<br />1. 91% water.<br />2. Plasma transports nutrients, wastes, hormones,<br />antibodies, and CO2 as HCO3<br />–.<br />3. Plasma proteins: clotting factors are synthesized by<br />the liver; albumin is synthesized by the liver and<br />provides colloid osmotic pressure that pulls tissue<br />fluid into capillaries to maintain normal blood volume<br />and blood pressure; alpha and beta globulins<br />are synthesized by the liver and are carriers for fats<br />and other substances in the blood; gamma globulins<br />are antibodies produced by lymphocytes.<br />Blood Cells<br />1. Formed elements are RBCs, WBCs, and platelets<br />(Figs. 11–2 and 11–3).<br />2. After birth the primary hemopoietic tissue is the<br />red bone marrow, which contains stem cells.<br />Lymphocytes mature and divide in the lymphatic<br />tissue of the spleen, lymph nodes, and thymus,<br />which also have stem cells for lymphocytes.<br />Red Blood Cells—erythrocytes (see Table<br />11–2 for normal values)<br />1. Biconcave discs; no nuclei when mature.<br />2. RBCs carry O2 bonded to the iron in hemoglobin.<br />3. RBCs are formed in the RBM from hemocytoblasts<br />(stem cells, the precursor cells).<br />4. Hypoxia stimulates the kidneys to produce the hormone<br />erythropoietin, which increases the rate of<br />RBC production in the RBM.<br />5. Immature RBCs: normoblasts (have nuclei) and<br />reticulocytes; large numbers in peripheral circulation<br />indicate a need for more RBCs to carry<br />oxygen.<br />6. Vitamin B12 is the extrinsic factor, needed for DNA<br />synthesis (mitosis) in stem cells in the RBM.<br />Intrinsic factor is produced by the parietal cells of<br />the stomach lining; it combines with B12 to prevent<br />its digestion and promote its absorption.<br />7. RBCs live for 120 days and are then phagocytized<br />by macrophages in the liver, spleen, and RBM. The<br />iron is returned to the RBM or stored in the liver.<br />The heme of the hemoglobin is converted to bilirubin,<br />which the liver excretes into bile to be eliminated<br />in feces. Colon bacteria change bilirubin to<br />urobilinogen. Any urobilinogen absorbed is converted<br />to urobilin and excreted by the kidneys in<br />urine (Fig. 11–4). Jaundice is the accumulation of<br />bilirubin in the blood, perhaps due to liver disease.<br />8. ABO blood types are hereditary. The type indicates<br />the antigen(s) on the RBCs (see Table 11–1 and<br />Fig. 11–5); antibodies in plasma are for those antigens<br />not present on the RBCs and are important<br />for transfusions.<br />9. The Rh type is also hereditary. Rh positive means<br />that the D antigen is present on the RBCs; Rh negative<br />means that the D antigen is not present on the<br />RBCs. Rh-negative people do not have natural<br />antibodies but will produce them if given Rhpositive<br />blood.<br />White Blood Cells—leukocytes (see Table<br />11–2 for normal values)<br />1. Larger than RBCs; have nuclei when mature; produced<br />in the red bone marrow, except some lymphocytes<br />produced in the thymus (Figs. 11–2 and<br />11–3).<br />2. Granular WBCs are the neutrophils, eosinophils,<br />and basophils.<br />3. Agranular WBCs are the lymphocytes and monocytes.<br />4. Neutrophils and monocytes phagocytize pathogens;<br />monocytes become macrophages, which also<br />phagocytize dead tissue.<br />5. Eosinophils detoxify foreign proteins during allergic<br />reactions and parasitic infections; they phagocytize<br />anything labeled with antibodies.<br />6. Basophils contain the anticoagulant heparin and<br />histamine, which contributes to inflammation.<br />7. Lymphocytes: T cells, B cells, and natural killer<br />cells. T cells recognize foreign antigens and destroy<br />them. B cells become plasma cells, which produce<br />antibodies to foreign antigens. NK cells<br />destroy foreign cell membranes.<br />8. WBCs carry out their functions in tissue fluid and<br />lymphatic tissue, as well as in the blood.<br />Platelets—thrombocytes (see Table 11–2 for<br />normal values)<br />1. Platelets are formed in the RBM and are fragments<br />of megakaryocytes; the hormone thrombopoietin<br />from the liver increases platelet production.<br />2. Platelets are involved in all mechanisms of hemostasis<br />(prevention of blood loss) (Fig. 11–6).<br />3. Vascular spasm—large vessels constrict when<br />damaged, the myogenic response. Platelets release<br />serotonin, which also causes vasoconstriction. The<br />break in the vessel is made smaller and may be<br />closed with a blood clot.<br />Blood 269<br />4. Platelet plugs—rupture of a capillary creates a<br />rough surface to which platelets stick and form a<br />barrier over the break.<br />5. Chemical clotting involves platelet factors, chemicals<br />from damaged tissue, prothrombin, fibrinogen<br />and other clotting factors synthesized by the liver,<br />and calcium ions. See Table 11–3 and Fig. 11–7 for<br />the three stages of chemical clotting. The clot is<br />formed of fibrin threads that form a mesh over the<br />break in the vessel.<br />6. Clot retraction is the folding of the fibrin threads<br />to pull the cut edges of the vessel closer together to<br />facilitate repair. Fibrinolysis is the dissolving of the<br />clot once it has served its purpose.<br />7. Abnormal clotting (thrombosis) is prevented by the<br />very smooth endothelium (simple squamous<br />epithelium) that lines blood vessels; heparin, which<br />inhibits the clotting process; and antithrombin<br />(synthesized by the liver), which inactivates excess<br />thrombin.<br />270 Blood<br />REVIEW QUESTIONS<br />1. Name four different kinds of substances transported<br />in blood plasma. (p. 252)<br />2. Name the precursor cell of all blood cells. Name<br />the primary hemopoietic tissue and state its locations.<br />(pp. 254)<br />3. State the normal values (CBC) for RBCs,<br />WBCs, platelets, hemoglobin, and hematocrit.<br />(p. 262)<br />4. State the function of RBCs; include the protein and<br />mineral needed. (p. 254)<br />5. Explain why iron, protein, folic acid, vitamin B12,<br />and the intrinsic factor are needed for RBC production.<br />(pp. 256)<br />6. Explain how bilirubin is formed and excreted.<br />(pp. 258–259)<br />7. Explain what will happen if a person with type O<br />positive blood receives a transfusion of type A negative<br />blood. (p. 261)<br />8. Name the WBC with each of the following functions:<br />(p. 262)<br />a. Become macrophages and phagocytize dead<br />tissue<br />b. Produce antibodies<br />c. Detoxify foreign proteins<br />d. Phagocytize pathogens<br />e. Contain the anticoagulant heparin<br />f. Recognize antigens as foreign<br />g. Secrete histamine during inflammation<br />9. Explain how and why platelet plugs form in ruptured<br />capillaries. (p. 264)<br />10. Explain how vascular spasm prevents excessive<br />blood loss when a large vessel is severed.<br />(p. 264)<br />11. With respect to chemical blood clotting:<br />(pp. 265–266)<br />a. Name the mineral necessary<br />b. Name the organ that produces many of the<br />clotting factors<br />c. Name the vitamin necessary for prothrombin<br />synthesis<br />d. State what the clot itself is made of<br />12. Explain what is meant by clot retraction and fibrinolysis.<br />(p. 266)<br />13. State two ways abnormal clotting is prevented in<br />the vascular system. (p. 268)<br />14. Explain what is meant by blood viscosity, the factors<br />that contribute, and why viscosity is important.<br />(p. 252)<br />15. State the normal pH range of blood. What gas has<br />an effect on blood pH? (p. 252)<br />16. Define anemia, leukocytosis, and thrombocytopenia.<br />(pp. 257, 263)<br />1. Explain why type AB blood may be called the<br />“universal recipient” for blood transfusions.<br />Explain why this would not be true if the transfusion<br />required 6 units (about 3 liters) of blood.<br />2. The liver has many functions that are directly<br />related to the composition and functions of blood.<br />Name as many as you can.<br />3. Constructing a brick wall requires bricks and bricklayers.<br />List all the nutrients that are needed for<br />RBC production, and indicate which are bricks and<br />which are bricklayers.<br />4. Anthony moved from New Jersey to a mountain<br />cabin in Colorado, 8000 feet above sea level. When<br />he first arrived, his hematocrit was 44%. After 6<br />months in his new home, what would you expect<br />his hematocrit to be? Explain your answer, and<br />what brought about the change.<br />5. The lab results for a particular patient show these<br />CBC values:<br />RBCs—4.2 million/ L<br />Hct—40%<br />Hb—13 g/100 mL<br />WBCs—8,500/ L<br />Platelets—30,000/ L<br />Is this patient healthy, or would you expect any<br />symptoms of a disorder? Explain your answer.<br />6. Using the model in Question 5, make a list of possible<br />CBC values for a patient with iron-deficiency<br />anemia. Then make a list of possible CBC values<br />for a person with aplastic anemia.<br />7. An artificial blood may someday be available; many<br />are being tested. What specific function of blood<br />will it definitely have? Are there any advantages to<br />an artificial blood compared with blood from a<br />human donor?<br />8. Disseminated intravascular coagulation (DIC) is a<br />serious condition that may follow certain kinds of<br />infections or traumas. First, explain what the name<br />means. This is best done one word at a time. In<br />DIC, clotting becomes a vicious cycle, and the<br />blood is depleted of clotting factors. What do<br />you think will be the consequence for the affected<br />person?<br />Blood 271<br />FOR FURTHER THOUGHT<br />272<br />CHAPTER 12<br />Chapter Outline<br />Location and Pericardial Membranes<br />Chambers—Vessels and Valves<br />Right Atrium<br />Left Atrium<br />Right Ventricle<br />Left Ventricle<br />Coronary Vessels<br />Cardiac Cycle and Heart Sounds<br />Cardiac Conduction Pathway<br />Heart Rate<br />Cardiac Output<br />Regulation of Heart Rate<br />Aging and the Heart<br />BOX 12–1 CORONARY ARTERY DISEASE<br />BOX 12–2 HEART MURMUR<br />BOX 12–3 ELECTROCARDIOGRAM<br />BOX 12–4 ARRHYTHMIAS<br />Student Objectives<br />• Describe the location of the heart, the pericardial<br />membranes, and the endocardium.<br />• Name the chambers of the heart and the vessels<br />that enter or leave each.<br />• Name the valves of the heart, and explain their<br />functions.<br />• Describe coronary circulation, and explain its purpose.<br />• Describe the cardiac cycle.<br />• Explain how heart sounds are created.<br />• Name the parts of the cardiac conduction pathway,<br />and explain why it is the sinoatrial node that initiates<br />each beat.<br />• Explain stroke volume, cardiac output, and<br />Starling’s law of the heart.<br />• Explain how the nervous system regulates heart<br />rate and force of contraction.<br />The Heart<br />273<br />New Terminology<br />Aorta (ay-OR-tah)<br />Atrium (AY-tree-um)<br />Cardiac cycle (KAR-dee-yak SIGH-kuhl)<br />Cardiac output (KAR-dee-yak OUT-put)<br />Coronary arteries (KOR-uh-na-ree AR-tuh-rees)<br />Diastole (dye-AS-tuh-lee)<br />Endocardium (EN-doh-KAR-dee-um)<br />Epicardium (EP-ee-KAR-dee-um)<br />Mediastinum (ME-dee-ah-STYE-num)<br />Mitral valve (MY-truhl VALV)<br />Myocardium (MY-oh-KAR-dee-um)<br />Sinoatrial (SA) node (SIGH-noh-AY-tree-al NOHD)<br />Stroke volume (STROHK VAHL-yoom)<br />Systole (SIS-tuh-lee)<br />Tricuspid valve (try-KUSS-pid VALV)<br />Venous return (VEE-nus ree-TURN)<br />Ventricle (VEN-tri-kuhl)<br />Related Clinical Terminology<br />Arrhythmia (uh-RITH-me-yah)<br />Ectopic focus (ek-TOP-ik FOH-kus)<br />Electrocardiogram (ECG) (ee-LEK-troh-KARdee-<br />oh-GRAM)<br />Fibrillation (fi-bri-LAY-shun)<br />Heart murmur (HART MUR-mur)<br />Ischemic (iss-KEY-mik)<br />Myocardial infarction (MY-oh-KAR-dee-yuhl<br />in-FARK-shun)<br />Pulse (PULS)<br />Stenosis (ste-NOH-sis)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />In the embryo, the heart begins to beat at 4 weeks of<br />age, even before its nerve supply has been established.<br />If a person lives to be 80 years old, his or her heart<br />continues to beat an average of 100,000 times a day,<br />every day for each of those 80 years. Imagine trying to<br />squeeze a tennis ball 70 times a minute. After a few<br />minutes, your arm muscles would begin to tire. Then<br />imagine increasing your squeezing rate to 120 times a<br />minute. Most of us could not keep that up very long,<br />but that is what the heart does during exercise. A<br />healthy heart can increase its rate and force of contraction<br />to meet the body’s need for more oxygen,<br />then return to its resting rate and keep on beating as if<br />nothing very extraordinary had happened. In fact, it<br />isn’t extraordinary at all; this is the job the heart is<br />meant to do.<br />The primary function of the heart is to pump blood<br />through the arteries, capillaries, and veins. As you<br />learned in the previous chapter, blood transports oxygen<br />and nutrients and has other important functions<br />as well. The heart is the pump that keeps blood circulating<br />properly.<br />LOCATION AND<br />PERICARDIAL MEMBRANES<br />The heart is located in the thoracic cavity between the<br />lungs. This area is called the mediastinum. The base<br />of the cone-shaped heart is uppermost, behind the<br />sternum, and the great vessels enter or leave here. The<br />apex (tip) of the heart points downward and is just<br />above the diaphragm to the left of the midline. This is<br />why we may think of the heart as being on the left<br />side, because the strongest beat can be heard or felt<br />here.<br />The heart is enclosed in the pericardial membranes,<br />of which there are three layers (Fig. 12–1).<br />The outermost is the fibrous pericardium, a loosefitting<br />sac of strong fibrous connective tissue that<br />extends inferiorly over the diaphragm and superiorly<br />over the bases of the large vessels that enter and leave<br />the heart. The serous pericardium is a folded membrane;<br />the fold gives it two layers, parietal and visceral.<br />Lining the fibrous pericardium is the parietal pericardium.<br />On the surface of the heart muscle is the<br />visceral pericardium, often called the epicardium.<br />Between the parietal and visceral pericardial membranes<br />is serous fluid, which prevents friction as the<br />heart beats.<br />CHAMBERS—VESSELS AND VALVES<br />The walls of the four chambers of the heart are made<br />of cardiac muscle called the myocardium. The chambers<br />are lined with endocardium, simple squamous<br />epithelium that also covers the valves of the heart and<br />continues into the vessels as their lining (endothelium).<br />The important physical characteristic of the<br />endocardium is not its thinness, but rather its smoothness.<br />This very smooth tissue prevents abnormal<br />blood clotting, because clotting would be initiated by<br />contact of blood with a rough surface.<br />274 The Heart<br />Endocardium<br />Parietal<br />Myocardium pericardium<br />(heart muscle)<br />Epicardium<br />(visceral pericardium)<br />Fibrous pericardium<br />(pericardial sac)<br />Pericardial cavity<br />Figure 12–1. Layers of the wall of the heart and the<br />pericardial membranes. The endocardium is the lining of<br />the chambers of the heart. The fibrous pericardium is the<br />outermost layer.<br />QUESTION: What is found between the parietal and visceral<br />pericardial layers, and what is its function?<br />The upper chambers of the heart are the right and<br />left atria (singular: atrium), which have relatively thin<br />walls and are separated by a common wall of myocardium<br />called the interatrial septum. The lower chambers<br />are the right and left ventricles, which have<br />thicker walls and are separated by the interventricular<br />septum (Fig. 12–2). As you will see, the atria<br />receive blood, either from the body or the lungs, and<br />the ventricles pump blood to either the lungs or the<br />body.<br />RIGHT ATRIUM<br />The two large caval veins return blood from the body<br />to the right atrium (see Fig. 12–2). The superior vena<br />cava carries blood from the upper body, and the inferior<br />vena cava carries blood from the lower body.<br />From the right atrium, blood will flow through the<br />right atrioventricular (AV) valve, or tricuspid valve,<br />into the right ventricle.<br />The tricuspid valve is made of three flaps (or cusps)<br />of endocardium reinforced with connective tissue. The<br />general purpose of all valves in the circulatory system<br />is to prevent backflow of blood. The specific purpose<br />of the tricuspid valve is to prevent backflow of blood<br />from the right ventricle to the right atrium when the<br />right ventricle contracts. As the ventricle contracts,<br />blood is forced behind the three valve flaps, forcing<br />them upward and together to close the valve.<br />LEFT ATRIUM<br />The left atrium receives blood from the lungs, by way<br />of four pulmonary veins. This blood will then flow<br />into the left ventricle through the left atrioventricular<br />(AV) valve, also called the mitral valve or bicuspid<br />(two flaps) valve. The mitral valve prevents backflow<br />of blood from the left ventricle to the left atrium when<br />the left ventricle contracts.<br />Another function of the atria is the production of a<br />hormone involved in blood pressure maintenance.<br />When the walls of the atria are stretched by increased<br />blood volume or blood pressure, the cells produce<br />atrial natriuretic peptide (ANP), also called atrial<br />natriuretic hormone (ANH). (The ventricles of the<br />heart produce a similar hormone called B-type natriuretic<br />peptide, or BNP, but we will use ANP as the<br />representative cardiac hormone.) ANP decreases the<br />reabsorption of sodium ions by the kidneys, so that<br />more sodium ions are excreted in urine, which in turn<br />increases the elimination of water. The loss of water<br />lowers blood volume and blood pressure. You may<br />have noticed that ANP is an antagonist to the hormone<br />aldosterone, which raises blood pressure.<br />RIGHT VENTRICLE<br />When the right ventricle contracts, the tricuspid valve<br />closes and the blood is pumped to the lungs through<br />the pulmonary artery (or trunk). At the junction of this<br />large artery and the right ventricle is the pulmonary<br />semilunar valve (or more simply, pulmonary valve).<br />Its three flaps are forced open when the right ventricle<br />contracts and pumps blood into the pulmonary<br />artery. When the right ventricle relaxes, blood tends<br />to come back, but this fills the valve flaps and closes<br />the pulmonary valve to prevent backflow of blood into<br />the right ventricle.<br />Projecting into the lower part of the right ventricle<br />are columns of myocardium called papillary muscles<br />(see Fig. 12–2). Strands of fibrous connective tissue,<br />the chordae tendineae, extend from the papillary<br />muscles to the flaps of the tricuspid valve. When the<br />right ventricle contracts, the papillary muscles also<br />contract and pull on the chordae tendineae to prevent<br />inversion of the tricuspid valve. If you have ever had<br />your umbrella blown inside out by a strong wind, you<br />can see what would happen if the flaps of the tricuspid<br />valve were not anchored by the chordae tendineae and<br />papillary muscles.<br />LEFT VENTRICLE<br />The walls of the left ventricle are thicker than those of<br />the right ventricle, which enables the left ventricle to<br />contract more forcefully. The left ventricle pumps<br />blood to the body through the aorta, the largest artery<br />of the body. At the junction of the aorta and the left<br />ventricle is the aortic semilunar valve (or aortic<br />valve) (see Fig. 12–2). This valve is opened by the<br />force of contraction of the left ventricle, which also<br />closes the mitral valve. The aortic valve closes when<br />the left ventricle relaxes, to prevent backflow of blood<br />from the aorta to the left ventricle. When the mitral<br />(left AV) valve closes, it prevents backflow of blood to<br />the left atrium; the flaps of the mitral valve are also<br />anchored by chordae tendineae and papillary muscles.<br />All of the valves are shown in Fig. 12–3, which also<br />depicts the fibrous skeleton of the heart. This is<br />fibrous connective tissue that anchors the outer edges<br />The Heart 275<br />276 The Heart<br />Brachiocephalic (trunk) artery<br />Superior vena cava<br />Right pulmonary artery<br />Right atrium<br />Right coronary artery<br />Inferior vena cava<br />A<br />Left subclavian artery<br />Left internal jugular vein<br />Left common carotid artery<br />Aortic arch Left pulmonary artery<br />(to lungs)<br />Left atrium<br />Left pulmonary veins<br />(from lungs)<br />Circumflex artery<br />Left coronary artery<br />Left coronary vein<br />Left anterior descending<br />artery<br />Left ventricle<br />Right ventricle Aorta<br />Brachiocephalic artery<br />Superior vena cava<br />Left common carotid artery<br />Left subclavian artery<br />Aortic arch<br />Right pulmonary artery<br />Right pulmonary veins<br />Right pulmonary veins<br />Right atrium<br />Inferior vena cava<br />Tricuspid<br />valve<br />Pulmonary<br />semilunar valve<br />Left pulmonary artery<br />Left atrium<br />Left pulmonary veins<br />Mitral valve<br />Left ventricle<br />Aortic semilunar valve<br />Interventricular septum<br />Chordae Apex<br />tendineae Right ventricle<br />Papillary<br />muscles<br />B<br />Figure 12–2. (A) Anterior view of the heart and major blood vessels. (B) Frontal section<br />of the heart in anterior view, showing internal structures.<br />QUESTION: In part B, in the right atrium, what do the blue arrows represent?<br />of the valve flaps and keeps the valve openings from<br />stretching. It also separates the myocardium of the<br />atria and ventricles and prevents the contraction of<br />the atria from reaching the ventricles except by way of<br />the normal conduction pathway.<br />As you can see from this description of the chambers<br />and their vessels, the heart is really a double, or<br />two-sided, pump. The right side of the heart receives<br />deoxygenated blood from the body and pumps it to<br />the lungs to pick up oxygen and release carbon dioxide.<br />The left side of the heart receives oxygenated<br />blood from the lungs and pumps it to the body. Both<br />pumps work simultaneously; that is, both atria contract<br />together, followed by the contraction of both<br />ventricles. Aspects of the anatomy of the heart are<br />summarized in Table 12–1.<br />CORONARY VESSELS<br />The right and left coronary arteries are the first<br />branches of the ascending aorta, just beyond the aortic<br />semilunar valve (Fig. 12–4). The two arteries branch<br />into smaller arteries and arterioles, then to capillaries.<br />The coronary capillaries merge to form coronary<br />The Heart 277<br />Pulmonary semilunar<br />valve<br />Aortic semilunar<br />valve Tricuspid<br />valve<br />Fibrous<br />skeleton<br />Bicuspid (mitral) valve<br />Posterior<br />Coronary artery<br />Figure 12–3. Heart valves in superior view. The atria<br />have been removed. The fibrous skeleton of the heart is<br />also shown.<br />QUESTION: When do the mitral and tricuspid valves<br />close, and why is this important?<br />Table 12–1 ANATOMY OF THE HEART<br />Structure Description<br />Epicardium<br />Myocardium<br />Endocardium<br />Right atrium (RA)<br />Tricuspid valve<br />Right ventricle (RV)<br />Pulmonary semilunar valve<br />Left atrium (LA)<br />Mitral valve<br />Left ventricle (LV)<br />Aortic semilunar valve<br />Papillary muscles and<br />chordae tendineae<br />Fibrous skeleton of the heart<br />Serous membrane on the surface of the myocardium<br />Heart muscle; forms the walls of the four chambers<br />Endothelium that lines the chambers and covers the valves; smooth to prevent abnormal<br />clotting<br />Receives deoxygenated blood from the body by way of the superior and inferior caval<br />veins<br />Right AV valve; prevents backflow of blood from the RV to the RA when the RV<br />contracts<br />Pumps blood to the lungs by way of the pulmonary artery<br />Prevents backflow of blood from the pulmonary artery to the RV when the RV relaxes<br />Receives oxygenated blood from the lungs by way of the four pulmonary veins<br />Left AV valve; prevents backflow of blood from the LV to the LA when the LV contracts<br />Pumps blood to the body by way of the aorta<br />Prevents backflow of blood from the aorta to the LV when the LV relaxes<br />In both the RV and LV; prevent inversion of the AV valves when the ventricles contract<br />Fibrous connective tissue that anchors the four heart valves, prevents enlargement of<br />the valve openings, and electrically insulates the ventricles from the atria<br />veins, which empty blood into a large coronary sinus<br />that returns blood to the right atrium.<br />The purpose of the coronary vessels is to supply<br />blood to the myocardium itself, because oxygen is<br />essential for normal myocardial contraction. If a coronary<br />artery becomes obstructed, by a blood clot for<br />example, part of the myocardium becomes ischemic,<br />that is, deprived of its blood supply. Prolonged<br />ischemia will create an infarct, an area of necrotic<br />(dead) tissue. This is a myocardial infarction, commonly<br />called a heart attack (see also Box 12–1: Coronary<br />Artery Disease).<br />CARDIAC CYCLE<br />AND HEART SOUNDS<br />The cardiac cycle is the sequence of events in one<br />heartbeat. In its simplest form, the cardiac cycle is the<br />simultaneous contraction of the two atria, followed a<br />fraction of a second later by the simultaneous contraction<br />of the two ventricles. Systole is another term for<br />contraction. The term for relaxation is diastole. You<br />are probably familiar with these terms as they apply to<br />blood pressure readings. If we apply them to the cardiac<br />cycle, we can say that atrial systole is followed by<br />ventricular systole. There is, however, a significant<br />difference between the movement of blood from the<br />atria to the ventricles and the movement of blood<br />from the ventricles to the arteries. The events of the<br />cardiac cycle are shown in Fig. 12–5. In this traditional<br />representation, the cardiac cycle is depicted in a circle,<br />because one heartbeat follows another, and the beginning<br />of atrial systole is at the top (12 o’clock). The size<br />of the segment or arc of the circle indicates how long<br />it takes. Find the segment for atrial systole and the one<br />for ventricular systole, and notice how much larger<br />(meaning “longer”) ventricular systole is. Do you<br />think this might mean that ventricular contraction is<br />more important than atrial contraction? It does, as you<br />will see. Refer to Fig. 12–5 as you read the following.<br />We will begin at the bottom (6 o’clock) where the atria<br />are in the midst of diastole and the ventricles have just<br />completed their systole. The entire heart is relaxed<br />and the atria are filling with blood.<br />Blood is constantly flowing from the veins into<br />both atria. As more blood accumulates, its pressure<br />forces open the right and left AV valves. Two-thirds<br />of the atrial blood flows passively into the ventricles<br />(which brings us to 12 o’clock); the atria then<br />contract to pump the remaining blood into the ventricles.<br />Following their contraction, the atria relax and the<br />ventricles begin to contract. Ventricular contraction<br />forces blood against the flaps of the right and left AV<br />valves and closes them; the force of blood also opens<br />the aortic and pulmonary semilunar valves. As the<br />ventricles continue to contract, they pump blood into<br />the arteries. Notice that blood that enters the arteries<br />must all be pumped. The ventricles then relax, and at<br />the same time blood continues to flow into the atria,<br />and the cycle begins again.<br />278 The Heart<br />Aorta<br />Left coronary artery<br />Anterior<br />interventricular branch<br />Great cardiac<br />vein<br />Coronary sinus<br />Posterior<br />artery and<br />vein<br />Small<br />Right coronary artery cardiac vein<br />A and vein B<br />Figure 12–4. (A) Coronary vessels<br />in anterior view. The pulmonary<br />artery has been cut to show the left<br />coronary artery emerging from the<br />ascending aorta. (B) Coronary vessels<br />in posterior view. The coronary sinus<br />empties blood into the right atrium.<br />QUESTION: What is the function of<br />the coronary vessels?<br />The Heart 279<br />Ventricular diastole 0.5 sec<br />Atrial systole<br />0.1 sec<br />Most atrial blood<br />flows passively<br />into ventricles<br />Remainder<br />of atrial blood<br />is pumped into<br />ventricles<br />AV valves open<br />AV valves close<br />Semilunar valves open<br />Semilunar valves close<br />Ventricular systole 0.3 sec<br />Atrial diastole 0.7 sec<br />Ventricular blood<br />is pumped into<br />arteries<br />Figure 12–5. The cardiac cycle<br />depicted in one heartbeat (pulse: 75). The<br />outer circle represents the ventricles, the<br />middle circle the atria, and the inner circle<br />the movement of blood and its effect on<br />the heart valves. See text for description.<br />QUESTION: What makes the AV valves<br />close and the semilunar valves open?<br />The important distinction here is that most blood<br />flows passively from atria to ventricles, but all blood<br />to the arteries is actively pumped by the ventricles.<br />For this reason, the proper functioning of the ventricles<br />is much more crucial to survival than is atrial<br />functioning.<br />You may be asking “All this in one heartbeat?” The<br />answer is yes. The cardiac cycle is this precise<br />sequence of events that keeps blood moving from the<br />veins, through the heart, and into the arteries.<br />The cardiac cycle also creates the heart sounds:<br />Each heartbeat produces two sounds, often called lubdup,<br />that can be heard with a stethoscope. The first<br />sound, the loudest and longest, is caused by ventricular<br />systole closing the AV valves. The second sound is<br />caused by the closure of the aortic and pulmonary<br />semilunar valves. If any of the valves do not close<br />properly, an extra sound called a heart murmur may<br />be heard (see Box 12–2: Heart Murmur).<br />CARDIAC CONDUCTION PATHWAY<br />The cardiac cycle is a sequence of mechanical events<br />that is regulated by the electrical activity of the<br />myocardium. Cardiac muscle cells have the ability<br />to contract spontaneously; that is, nerve impulses are<br />not required to cause contraction. The heart generates<br />its own beat, and the electrical impulses follow a<br />very specific route throughout the myocardium. You<br />may find it helpful to refer to Fig. 12–6 as you read the<br />following.<br />The natural pacemaker of the heart is the sinoatrial<br />(SA) node, a specialized group of cardiac muscle cells<br />located in the wall of the right atrium just below the<br />opening of the superior vena cava. The SA node is<br />considered specialized because it has the most rapid<br />rate of contraction, that is, it depolarizes more rapidly<br />than any other part of the myocardium (60 to 80 times<br />per minute). As you may recall, depolarization is the<br />rapid entry of Na ions and the reversal of charges on<br />either side of the cell membrane. The cells of the SA<br />node are more permeable to Na ions than are other<br />cardiac muscle cells. Therefore, they depolarize more<br />rapidly, then contract and initiate each heartbeat.<br />From the SA node, impulses for contraction travel<br />to the atrioventricular (AV) node, located in the<br />lower interatrial septum. The transmission of impulses<br />from the SA node to the AV node and to the rest of the<br />atrial myocardium brings about atrial systole.<br />280 The Heart<br />BOX 12–1 CORONARY ARTERY DISEASE<br />Other predisposing factors for atherosclerosis<br />include cigarette smoking, diabetes mellitus, and<br />high blood pressure. Any one of these may cause<br />damage to the lining of coronary arteries, which is<br />the first step in the abnormal deposition of cholesterol.<br />A diet high in cholesterol and saturated fats<br />and high blood levels of these lipids will increase<br />the rate of cholesterol deposition.<br />A possible chemical marker of risk is a high blood<br />level of homocysteine. Homocysteine is a metabolic<br />product of the essential amino acid methionine,<br />and may be converted back to methionine or further<br />changed and excreted by the kidneys. A high<br />blood level of homocysteine may indicate inflammation<br />of the walls of arteries. Yet another chemical<br />marker of inflammation is C-reactive protein (CRP).<br />There is still much to learn about the role of inflammation<br />in atherosclerosis, but simple blood tests for<br />chemical markers may someday provide a diagnosis<br />before heart damage occurs.<br />When coronary artery disease becomes lifethreatening,<br />coronary artery bypass surgery may be<br />performed. In this procedure, a synthetic vessel or a<br />vein (such as the saphenous vein of the leg) is<br />grafted around the obstructed coronary vessel to<br />restore blood flow to the myocardium. This is not a<br />cure, for atherosclerosis may occur in a grafted vein<br />or at other sites in the coronary arteries.<br />Coronary artery disease results in decreased blood<br />flow to the myocardium. If blood flow is diminished<br />but not completely obstructed, the person may<br />experience difficulty breathing and angina, which is<br />chest pain caused by lack of oxygen to part of the<br />heart muscle. If blood flow is completely blocked,<br />however, the result is a myocardial infarction<br />(necrosis of cardiac muscle).<br />The most common cause of coronary artery disease<br />is atherosclerosis. Plaques of cholesterol form<br />in the walls of a coronary artery; this narrows the<br />lumen (cavity) and creates a rough surface where a<br />clot (thrombus) may form (see Box Fig. 12–A). A<br />predisposing factor for such clot formation, one that<br />cannot be changed, is a family history of coronary<br />artery disease. There is no “gene for heart attacks,”<br />but we do have genes for the enzymes involved in<br />cholesterol metabolism. Many of these are liver<br />enzymes that regulate the transport of cholesterol in<br />the blood in the form of lipoproteins and regulate<br />the liver’s excretion of excess cholesterol in bile.<br />Some people, therefore, have a greater tendency<br />than others to have higher blood levels of cholesterol<br />and certain lipoproteins. In women before<br />menopause, estrogen is believed to exert a protective<br />effect by lowering blood lipid levels. This is why<br />heart attacks in the 30- to 50-year-old age range are<br />less frequent in women than in men.<br />Normal artery Atherosclerotic artery<br />A B<br />Box Figure 12–A (A) Cross-section of normal coronary artery. (B) Coronary artery with<br />atherosclerosis narrowing the lumen.<br />Recall that the fibrous skeleton of the heart separates<br />the atrial myocardium from the ventricular<br />myocardium; the fibrous connective tissue acts as electrical<br />insulation between the two sets of chambers.<br />The only pathway for impulses from the atria to the<br />ventricles, therefore, is the atrioventricular bundle<br />(AV bundle), also called the bundle of His. The AV<br />bundle is within the upper interventricular septum; it<br />receives impulses from the AV node and transmits<br />them to the right and left bundle branches. From the<br />The Heart 281<br />SA node<br />Left atrium<br />Right<br />atrium<br />AV node<br />Right<br />ventricle<br />AV bundle<br />(Bundle of His)<br />Purkinje<br />fibers<br />Left ventricle<br />Left bundle branch<br />Right bundle branch<br />R<br />P wave<br />Q S<br />T wave<br />P-R Interval QRS<br />Complex<br />ST Segment<br />S-T Interval<br />Figure 12–6. Conduction pathway of the heart. Anterior view of the interior of the<br />heart. The electrocardiogram tracing is of one normal heartbeat. See text and Box 12–3 for<br />description.<br />QUESTION: What structure is the pacemaker of the heart, and what is its usual rate of<br />depolarization?<br />BOX 12–2 HEART MURMUR<br />close and prevent backflow during ventricular systole.<br />Some valve defects involve a narrowing (stenosis)<br />and are congenital; that is, the child is born with<br />an abnormally narrow valve. In aortic stenosis, for<br />example, blood cannot easily pass from the left ventricle<br />to the aorta. The ventricle must then work<br />harder to pump blood through the narrow valve to<br />the arteries, and the turbulence created is also heard<br />as a systolic murmur.<br />Children sometimes have heart murmurs that<br />are called “functional” because no structural cause<br />can be found. These murmurs usually disappear<br />with no adverse effects on the child.<br />A heart murmur is an abnormal or extra heart sound<br />caused by a malfunctioning heart valve. The function<br />of heart valves is to prevent backflow of blood,<br />and when a valve does not close properly, blood will<br />regurgitate (go backward), creating turbulence that<br />may be heard with a stethoscope.<br />Rheumatic heart disease is a now uncommon<br />complication of a streptococcal infection. In rheumatic<br />fever, the heart valves are damaged by an<br />abnormal response by the immune system. Erosion<br />of the valves makes them “leaky” and inefficient,<br />and a murmur of backflowing blood will be heard.<br />Mitral valve regurgitation, for example, will be heard<br />as a systolic murmur, because this valve is meant to<br />bundle branches, impulses travel along Purkinje<br />fibers to the rest of the ventricular myocardium and<br />bring about ventricular systole. The electrical activity<br />of the atria and ventricles is depicted by an electrocardiogram<br />(ECG); this is discussed in Box 12–3:<br />Electrocardiogram.<br />If the SA node does not function properly, the AV<br />node will initiate the heartbeat, but at a slower rate (50<br />to 60 beats per minute). The AV bundle is also capable<br />of generating the beat of the ventricles, but at a<br />much slower rate (15 to 40 beats per minute). This<br />may occur in certain kinds of heart disease in which<br />transmission of impulses from the atria to the ventricles<br />is blocked.<br />Arrhythmias are irregular heartbeats; their effects<br />range from harmless to life-threatening. Nearly<br />everyone experiences heart palpitations (becoming<br />aware of an irregular beat) from time to time. These<br />are usually not serious and may be the result of<br />too much caffeine, nicotine, or alcohol. Much more<br />serious is ventricular fibrillation, a very rapid and<br />uncoordinated ventricular beat that is totally inef-<br />282 The Heart<br />BOX 12–3 ELECTROCARDIOGRAM<br />throughout the ventricular myocardium. The T<br />wave represents repolarization of the ventricles<br />(atrial repolarization does not appear as a separate<br />wave because it is masked by the QRS complex).<br />Detailed interpretation of abnormal ECGs is<br />beyond the scope of this book, but in general, the<br />length of each wave and the time intervals between<br />waves are noted. An ECG may be helpful in the<br />diagnosis of coronary atherosclerosis, which deprives<br />the myocardium of oxygen, or of rheumatic<br />fever or other valve disorders that result in enlargement<br />of a chamber of the heart and prolong a specific<br />wave of an ECG. For example, the enlargement<br />of the left ventricle that is often a consequence of<br />hypertension may be indicated by an abnormal<br />QRS complex.<br />A heartbeat is a series of electrical events, and the<br />electrical changes generated by the myocardium<br />can be recorded by placing electrodes on the body<br />surface. Such a recording is called an electrocardiogram<br />(ECG) (see Fig. 12–6).<br />A typical ECG consists of three distinguishable<br />waves or deflections: the P wave, the QRS complex,<br />and the T wave. Each represents a specific electrical<br />event; all are shown in Fig. 12–6 in a normal ECG<br />tracing.<br />The P wave represents depolarization of the<br />atria, that is, the transmission of electrical impulses<br />from the SA node throughout the atrial myocardium.<br />The QRS complex represents depolarization of<br />the ventricles as the electrical impulses spread<br />BOX 12–4 ARRHYTHMIAS<br />rillating ventricles are not pumping, and cardiac<br />output decreases sharply.<br />Ventricular fibrillation may follow a non-fatal<br />heart attack (myocardial infarction). Damaged cardiac<br />muscle cells may not be able to maintain a normal<br />state of polarization, and they depolarize<br />spontaneously and rapidly. From this ectopic focus,<br />impulses spread to other parts of the ventricular<br />myocardium in a rapid and haphazard pattern, and<br />the ventricles quiver rather than contract as a unit.<br />It is often possible to correct ventricular fibrillation<br />with the use of an electrical defibrillator. This<br />instrument delivers an electric shock to the heart,<br />which causes the entire myocardium to depolarize<br />and contract, then relax. If the first part of the heart<br />to recover is the SA node (which usually has the<br />most rapid rate of contraction), a normal heartbeat<br />may be restored.<br />Arrhythmias (also called dysrhythmias) are irregular<br />heartbeats caused by damage to part of the<br />conduction pathway, or by an ectopic focus,<br />which is a beat generated in part of the<br />myocardium other than the SA node.<br />Flutter is a very rapid but fairly regular heartbeat.<br />In atrial flutter, the atria may contract up to<br />300 times per minute. Because atrial pumping is<br />not crucial, however, blood flow to the ventricles<br />may be maintained for a time, and flutter may not<br />be immediately life-threatening. Ventricular flutter<br />is usually only a brief transition between ventricular<br />tachycardia and fibrillation.<br />Fibrillation is very rapid and uncoordinated<br />contractions. Ventricular fibrillation is a medical<br />emergency that must be quickly corrected to prevent<br />death. Normal contraction of the ventricles is<br />necessary to pump blood into the arteries, but fib-<br />fective for pumping blood (see Box 12–4: Arrhythmias).<br />HEART RATE<br />A healthy adult has a resting heart rate (pulse) of 60 to<br />80 beats per minute, which is the rate of depolarization<br />of the SA node. (The SA node actually has a<br />slightly faster rate, closer to 100 beats per minute, but<br />is slowed by parasympathetic nerve impulses to what<br />we consider a normal resting rate.) A rate less than 60<br />(except for athletes) is called bradycardia; a prolonged<br />or consistent rate greater than 100 beats per minute is<br />called tachycardia.<br />A child’s normal heart rate may be as high as 100<br />beats per minute, that of an infant as high as 120, and<br />that of a near-term fetus as high as 140 beats per<br />minute. These higher rates are not related to age, but<br />rather to size: the smaller the individual, the higher the<br />metabolic rate and the faster the heart rate. Parallels<br />may be found among animals of different sizes; the<br />heart rate of a mouse is about 200 beats per minute<br />and that of an elephant about 30 beats per minute.<br />Let us return to the adult heart rate and consider<br />the person who is in excellent physical condition. As<br />you may know, well-conditioned athletes have low<br />resting pulse rates. Those of basketball players are<br />often around 50 beats per minute, and the pulse of a<br />marathon runner often ranges from 35 to 40 beats per<br />minute. To understand why this is so, remember that<br />the heart is a muscle. When our skeletal muscles are<br />exercised, they become stronger and more efficient.<br />The same is true for the heart; consistent exercise<br />makes it a more efficient pump, as you will see in the<br />next section.<br />CARDIAC OUTPUT<br />Cardiac output is the amount of blood pumped by a<br />ventricle in 1 minute. A certain level of cardiac output<br />is needed at all times to transport oxygen to tissues and<br />to remove waste products. During exercise, cardiac<br />output must increase to meet the body’s need for more<br />oxygen. We will return to exercise after first considering<br />resting cardiac output.<br />To calculate cardiac output, we must know the<br />pulse rate and how much blood is pumped per beat.<br />Stroke volume is the term for the amount of blood<br />pumped by a ventricle per beat; an average resting<br />stroke volume is 60 to 80 mL per beat. A simple formula<br />then enables us to determine cardiac output:<br />Cardiac output stroke volume pulse (heart rate)<br />Let us put into this formula an average resting<br />stroke volume, 70 mL, and an average resting pulse,<br />70 beats per minute (bpm):<br />Cardiac output 70 mL 70 bpm<br />Cardiac output 4900 mL per minute<br />(approximately 5 liters)<br />Naturally, cardiac output varies with the size of the<br />person, but the average resting cardiac output is 5 to 6<br />liters per minute. Notice that this amount is just about<br />the same as a person’s average volume of blood. At<br />rest, the heart pumps all of the blood in the body<br />within about a minute. Changes are possible, depending<br />on circumstances and extent of physical activity.<br />If we now reconsider the athlete, you will be able to<br />see precisely why the athlete has a low resting pulse. In<br />our formula, we will use an average resting cardiac<br />output (5 liters) and an athlete’s pulse rate (50):<br />Cardiac output stroke volume pulse<br />5000 mL stroke volume 50 bpm<br />5000/50 stroke volume<br />100 mL stroke volume<br />Notice that the athlete’s resting stroke volume is<br />significantly higher than the average. The athlete’s<br />more efficient heart pumps more blood with each beat<br />and so can maintain a normal resting cardiac output<br />with fewer beats.<br />Now let us see how the heart responds to exercise.<br />Heart rate (pulse) increases during exercise, and so<br />does stroke volume. The increase in stroke volume is<br />the result of Starling’s law of the heart, which states<br />that the more the cardiac muscle fibers are stretched,<br />the more forcefully they contract. During exercise,<br />more blood returns to the heart; this is called venous<br />return. Increased venous return stretches the myocardium<br />of the ventricles, which contract more forcefully<br />and pump more blood, thereby increasing stroke volume.<br />Therefore, during exercise, our formula might<br />be the following:<br />Cardiac output stroke volume pulse<br />Cardiac output 100 mL 100 bpm<br />Cardiac output 10,000 mL (10 liters)<br />The Heart 283<br />This exercise cardiac output is twice the resting<br />cardiac output we first calculated, which should not be<br />considered unusual. The cardiac output of a healthy<br />young person may increase up to four times the resting<br />level during strenuous exercise. This difference is<br />the cardiac reserve, the extra volume the heart can<br />pump when necessary. If resting cardiac output is 5<br />liters and exercise cardiac output is 20 liters, the cardiac<br />reserve is 15 liters. The marathon runner’s cardiac<br />output may increase six times or more compared to<br />the resting level, and cardiac reserve is even greater<br />than for the average young person; this is the result of<br />the marathoner’s extremely efficient heart. Because of<br />Starling’s law, it is almost impossible to overwork a<br />healthy heart. No matter how much the volume of<br />venous return increases, the ventricles simply pump<br />more forcefully and increase the stroke volume and<br />cardiac output.<br />Also related to cardiac output, and another measure<br />of the health of the heart, is the ejection fraction.<br />This is the percent of the blood in a ventricle that is<br />pumped during systole. A ventricle does not empty<br />completely when it contracts, but should pump out<br />60% to 70% of the blood within it. A lower percentage<br />would indicate that the ventricle is weakening.<br />These aspects of physiology are summarized in Table<br />12–2.<br />REGULATION OF HEART RATE<br />Although the heart generates and maintains its own<br />beat, the rate of contraction can be changed to adapt<br />to different situations. The nervous system can and<br />does bring about necessary changes in heart rate as<br />well as in force of contraction.<br />The medulla of the brain contains the two cardiac<br />centers, the accelerator center and the inhibitory<br />center. These centers send impulses to the heart<br />along autonomic nerves. Recall from Chapter 8 that<br />the autonomic nervous system has two divisions: sympathetic<br />and parasympathetic. Sympathetic impulses<br />from the accelerator center along sympathetic nerves<br />increase heart rate and force of contraction during<br />exercise and stressful situations. Parasympathetic<br />impulses from the inhibitory center along the vagus<br />nerves decrease the heart rate. At rest these impulses<br />slow down the depolarization of the SA node to what<br />we consider a normal resting rate, and they also slow<br />the heart after exercise is over.<br />Our next question might be: What information is<br />received by the medulla to initiate changes? Because<br />the heart pumps blood, it is essential to maintain normal<br />blood pressure. Blood contains oxygen, which all<br />tissues must receive continuously. Therefore, changes<br />in blood pressure and oxygen level of the blood are<br />stimuli for changes in heart rate.<br />You may also recall from Chapter 9 that pressoreceptors<br />and chemoreceptors are located in the<br />carotid arteries and aortic arch. Pressoreceptors in<br />the carotid sinuses and aortic sinus detect changes in<br />blood pressure. Chemoreceptors in the carotid<br />bodies and aortic body detect changes in the oxygen<br />content of the blood. The sensory nerves for the<br />carotid receptors are the glossopharyngeal (9th cranial)<br />nerves; the sensory nerves for the aortic arch<br />284 The Heart<br />Table 12–2 PHYSIOLOGY OF THE HEART<br />Aspect and<br />Normal Range Description<br />Heart rate (pulse):<br />60–80 bpm<br />Stroke volume:<br />60–80 mL/beat<br />Cardiac output:<br />5–6 L/min<br />Ejection fraction:<br />60%–70%<br />Cardiac reserve:<br />15 liters or more<br />Generated by the SA node, propagated through the conduction pathway; parasympathetic<br />impulses (vagus nerves) decrease the rate; sympathetic impulses increase the rate<br />The amount of blood pumped by a ventricle in one beat<br />The volume of blood pumped by a ventricle in 1 minute; stroke volume x pulse<br />The percentage of blood within a ventricle that is pumped out per beat<br />The difference between resting cardiac output and maximum cardiac output during exercise<br />receptors are the vagus (10th cranial) nerves. If we<br />now put all of these facts together in a specific example,<br />you will see that the regulation of heart rate is a<br />reflex. Figure 12–7 depicts all of the structures just<br />mentioned.<br />A person who stands up suddenly from a lying position<br />may feel light-headed or dizzy for a few moments,<br />because blood pressure to the brain has decreased<br />abruptly. The drop in blood pressure is detected by<br />pressoreceptors in the carotid sinuses—notice that<br />they are “on the way” to the brain, a very strategic<br />location. The drop in blood pressure causes fewer<br />impulses to be generated by the pressoreceptors.<br />These impulses travel along the glossopharyngeal<br />nerves to the medulla, and the decrease in the frequency<br />of impulses stimulates the accelerator center.<br />The accelerator center generates impulses that are carried<br />by sympathetic nerves to the SA node, AV node,<br />and ventricular myocardium. As heart rate and force<br />increase, blood pressure to the brain is raised to normal,<br />and the sensation of light-headedness passes.<br />When blood pressure to the brain is restored to normal,<br />the heart receives more parasympathetic impulses<br />from the inhibitory center along the vagus nerves to<br />the SA node and AV node. These parasympathetic<br />impulses slow the heart rate to a normal resting pace.<br />The heart will also be the effector in a reflex stimulated<br />by a decrease in the oxygen content of the<br />blood. The aortic receptors are strategically located so<br />as to detect such an important change as soon as blood<br />leaves the heart. The reflex arc in this situation would<br />be (1) aortic chemoreceptors, (2) vagus nerves (sensory),<br />(3) accelerator center in the medulla, (4) sympathetic<br />nerves, and (5) the heart muscle, which will<br />increase its rate and force of contraction to circulate<br />more oxygen to correct the hypoxia.<br />Recall also from Chapter 10 that the hormone<br />epinephrine is secreted by the adrenal medulla in<br />stressful situations. One of the many functions of epinephrine<br />is to increase heart rate and force of contraction.<br />This will help supply more blood to tissues in<br />need of more oxygen.<br />The Heart 285<br />Accelerator center<br />Inhibitory center<br />Medulla<br />Thoracic spinal cord<br />Sympathetic nerves<br />Right ventricle<br />Vagus (motor) nerves<br />Vagus (sensory)<br />nerve<br />Glossopharyngeal<br />nerves<br />Carotid sinus and<br />carotid body<br />Common<br />carotid<br />arteries<br />Aortic arch<br />Aortic sinus<br />and aortic body<br />SA node<br />Bundle of His<br />AV node<br />Figure 12–7. Nervous regulation of the heart. The brain and spinal cord are shown on<br />the left. The heart and major blood vessels are shown on the right.<br />QUESTION: Sympathetic impulses to the ventricles will have what effect?<br />The heart pumps blood, which creates blood<br />pressure, and circulates oxygen, nutrients,<br />and other substances. The heart is located in<br />the mediastinum, the area between the<br />lungs in the thoracic cavity.<br />Pericardial Membranes—three layers that<br />enclose the heart (see Fig. 12–1)<br />1. The outer, fibrous pericardium, made of fibrous<br />connective tissue, is a loose-fitting sac that surrounds<br />the heart and extends over the diaphragm<br />and the bases of the great vessels.<br />2. The parietal pericardium is a serous membrane<br />that lines the fibrous pericardium.<br />3. The visceral pericardium, or epicardium, is a serous<br />membrane on the surface of the myocardium.<br />4. Serous fluid between the parietal and visceral pericardial<br />membranes prevents friction as the heart<br />beats.<br />Chambers of the Heart (see Fig. 12–2 and<br />Table 12–1)<br />1. Cardiac muscle tissue, the myocardium, forms the<br />walls of the four chambers of the heart.<br />2. Endocardium lines the chambers and covers the<br />valves of the heart; is simple squamous epithelium<br />that is very smooth and prevents abnormal clotting.<br />3. The right and left atria are the upper chambers,<br />separated by the interatrial septum. The atria<br />receive blood from veins.<br />4. The right and left ventricles are the lower chambers,<br />separated by the interventricular septum. The<br />ventricles pump blood into arteries.<br />Right Atrium<br />1. Receives blood from the upper body by way of the<br />superior vena cava and receives blood from the<br />lower body by way of the inferior vena cava.<br />2. The tricuspid (right AV) valve prevents backflow of<br />blood from the right ventricle to the right atrium<br />when the right ventricle contracts.<br />Left Atrium<br />1. Receives blood from the lungs by way of four pulmonary<br />veins.<br />2. The mitral (left AV or bicuspid) valve prevents<br />backflow of blood from the left ventricle to the left<br />atrium when the left ventricle contracts.<br />3. The walls of the atria produce atrial natriuretic<br />peptide when stretched by increased blood volume<br />or BP. ANP increases the loss of Na ions and<br />water in urine, which decreases blood volume and<br />BP to normal.<br />286 The Heart<br />STUDY OUTLINE<br />AGING AND THE HEART<br />The heart muscle becomes less efficient with age, and<br />there is a decrease in both maximum cardiac output<br />and heart rate, although resting levels may be more<br />than adequate. The health of the myocardium<br />depends on its blood supply, and with age there is<br />greater likelihood that atherosclerosis will narrow the<br />coronary arteries. Atherosclerosis is the deposition of<br />cholesterol on and in the walls of the arteries, which<br />decreases blood flow and forms rough surfaces that<br />may cause intravascular clot formation.<br />High blood pressure (hypertension) causes the left<br />ventricle to work harder; it may enlarge and outgrow<br />its blood supply, thus becoming weaker. A weak ventricle<br />is not an efficient pump, and such weakness may<br />progress to congestive heart failure; such a progression<br />may be slow, or may be rapid. The heart valves<br />may become thickened by fibrosis, leading to heart<br />murmurs and less efficient pumping. Arrhythmias are<br />also more common with age, as the cells of the conduction<br />pathway become less efficient.<br />SUMMARY<br />As you can see, the nervous system regulates the functioning<br />of the heart based on what the heart is supposed<br />to do. The pumping of the heart maintains<br />normal blood pressure and proper oxygenation of tissues,<br />and the nervous system ensures that the heart<br />will be able to meet these demands in different situations.<br />Blood pressure and the blood vessels are the<br />subjects of the next chapter.<br />Right Ventricle—has relatively thin walls<br />1. Pumps blood to the lungs through the pulmonary<br />artery.<br />2. The pulmonary semilunar valve prevents backflow<br />of blood from the pulmonary artery to the right<br />ventricle when the right ventricle relaxes.<br />3. Papillary muscles and chordae tendineae prevent<br />inversion of the right AV valve when the right ventricle<br />contracts.<br />Left Ventricle—has thicker walls than does<br />the right ventricle<br />1. Pumps blood to the body through the aorta.<br />2. The aortic semilunar valve prevents backflow of<br />blood from the aorta to the left ventricle when the<br />left ventricle relaxes.<br />3. Papillary muscles and chordae tendineae prevent<br />inversion of the left AV valve when the left ventricle<br />contracts.<br />4. The heart is a double pump: The right side of the<br />heart receives deoxygenated blood from the body<br />and pumps it to the lungs; the left side of the heart<br />receives oxygenated blood from the lungs and<br />pumps it to the body. Both sides of the heart work<br />simultaneously.<br />Coronary Vessels (see Fig. 12–4)<br />1. Pathway: ascending aorta to right and left coronary<br />arteries, to smaller arteries, to capillaries, to coronary<br />veins, to the coronary sinus, to the right<br />atrium.<br />2. Coronary circulation supplies oxygenated blood to<br />the myocardium.<br />3. Obstruction of a coronary artery causes a myocardial<br />infarction: death of an area of myocardium due<br />to lack of oxygen.<br />Cardiac Cycle—the sequence of events in one<br />heartbeat (see Fig. 12–5)<br />1. The atria continually receive blood from the veins;<br />as pressure within the atria increases, the AV valves<br />are opened.<br />2. Two-thirds of the atrial blood flows passively<br />into the ventricles; atrial contraction pumps the<br />remaining blood into the ventricles; the atria then<br />relax.<br />3. The ventricles contract, which closes the AV valves<br />and opens the aortic and pulmonary semilunar<br />valves.<br />4. Ventricular contraction pumps all blood into the<br />arteries. The ventricles then relax. Meanwhile,<br />blood is filling the atria, and the cycle begins again.<br />5. Systole means contraction; diastole means relaxation.<br />In the cardiac cycle, atrial systole is followed<br />by ventricular systole. When the ventricles are in<br />systole, the atria are in diastole.<br />6. The mechanical events of the cardiac cycle keep<br />blood moving from the veins through the heart and<br />into the arteries.<br />Heart Sounds—two sounds per heartbeat:<br />lub-dup<br />1. The first sound is created by closure of the AV<br />valves during ventricular systole.<br />2. The second sound is created by closure of the aortic<br />and pulmonary semilunar valves.<br />3. Improper closing of a valve results in a heart murmur.<br />Cardiac Conduction Pathway—the pathway<br />of impulses during the cardiac cycle (see Fig.<br />12–6)<br />1. The SA node in the wall of the right atrium initiates<br />each heartbeat; the cells of the SA node are<br />more permeable to Na ions and depolarize more<br />rapidly than any other part of the myocardium.<br />2. The AV node is in the lower interatrial septum.<br />Depolarization of the SA node spreads to the AV<br />node and to the atrial myocardium and brings<br />about atrial systole.<br />3. The AV bundle (bundle of His) is in the upper<br />interventricular septum; the first part of the ventricles<br />to depolarize.<br />4. The right and left bundle branches in the interventricular<br />septum transmit impulses to the Purkinje<br />fibers in the ventricular myocardium, which complete<br />ventricular systole.<br />5. An electrocardiogram (ECG) depicts the electrical<br />activity of the heart (see Fig. 12–6).<br />6. If part of the conduction pathway does not function<br />properly, the next part will initiate contraction, but<br />at a slower rate.<br />7. Arrhythmias are irregular heartbeats; their effects<br />range from harmless to life-threatening.<br />Heart Rate<br />1. Healthy adult: 60 to 80 beats per minute (heart rate<br />equals pulse); children and infants have faster<br />The Heart 287<br />pulses because of their smaller size and higher<br />metabolic rate.<br />2. A person in excellent physical condition has a slow<br />resting pulse because the heart is a more efficient<br />pump and pumps more blood per beat.<br />Cardiac Output (see Table 12–2)<br />1. Cardiac output is the amount of blood pumped by<br />a ventricle in 1 minute.<br />2. Stroke volume is the amount of blood pumped by a<br />ventricle in one beat; average is 60 to 80 mL.<br />3. Cardiac output equals stroke volume pulse; average<br />resting cardiac output is 5 to 6 liters.<br />4. Starling’s law of the heart—the more cardiac muscle<br />fibers are stretched, the more forcefully they<br />contract.<br />5. During exercise, stroke volume increases as venous<br />return increases and stretches the myocardium of<br />the ventricles (Starling’s law).<br />6. During exercise, the increase in stroke volume and<br />the increase in pulse result in an increase in cardiac<br />output: two to four times the resting level.<br />7. Cardiac reserve is the difference between resting<br />cardiac output and the maximum cardiac output;<br />may be 15 liters or more.<br />8. The ejection fraction is the percent of its total<br />blood that a ventricle pumps per beat; average is<br />60% to 70%.<br />Regulation of Heart Rate (see Fig. 12–7)<br />1. The heart generates its own beat, but the nervous<br />system brings about changes to adapt to different<br />situations.<br />2. The medulla contains the cardiac centers: the<br />accelerator center and the inhibitory center.<br />3. Sympathetic impulses to the heart increase rate and<br />force of contraction; parasympathetic impulses<br />(vagus nerves) to the heart decrease heart rate.<br />4. Pressoreceptors in the carotid and aortic sinuses<br />detect changes in blood pressure.<br />5. Chemoreceptors in the carotid and aortic bodies<br />detect changes in the oxygen content of the blood.<br />6. The glossopharyngeal nerves are sensory for the<br />carotid receptors. The vagus nerves are sensory for<br />the aortic receptors.<br />7. If blood pressure to the brain decreases, pressoreceptors<br />in the carotid sinuses detect this decrease<br />and send fewer sensory impulses along the glossopharyngeal<br />nerves to the medulla. The accelerator<br />center dominates and sends motor impulses<br />along sympathetic nerves to increase heart rate and<br />force of contraction to restore blood pressure to<br />normal.<br />8. A similar reflex is activated by hypoxia.<br />9. Epinephrine from the adrenal medulla increases<br />heart rate and force of contraction during stressful<br />situations.<br />288 The Heart<br />REVIEW QUESTIONS<br />1. Describe the location of the heart with respect to<br />the lungs and to the diaphragm. (p. 274)<br />2. Name the three pericardial membranes. Where<br />is serous fluid found and what is its function?<br />(p. 274)<br />3. Describe the location and explain the function of<br />endocardium. (p. 274)<br />4. Name the veins that enter the right atrium; name<br />those that enter the left atrium. For each, where<br />does the blood come from? (p. 275)<br />5. Name the artery that leaves the right ventricle;<br />name the artery that leaves the left ventricle. For<br />each, where is the blood going? (p. 275)<br />6. Explain the purpose of the right and left AV valves<br />and the purpose of the aortic and pulmonary semilunar<br />valves. (p. 275)<br />7. Describe the coronary system of vessels and<br />explain the purpose of coronary circulation. (p.<br />278)<br />8. Define systole, diastole, and cardiac cycle.<br />(p. 278)<br />9. Explain how movement of blood from atria to<br />ventricles differs from movement of blood from<br />ventricles to arteries. (p. 279)<br />10. Explain why the heart is considered a double<br />pump. Trace the path of blood from the right<br />atrium back to the right atrium, naming the<br />chambers of the heart and their vessels through<br />which the blood passes. (pp. 275, 277)<br />11. Name the parts, in order, of the cardiac conduction<br />pathway. Explain why it is the SA node that<br />generates each heartbeat. State a normal range of<br />heart rate for a healthy adult. (pp. 279–283)<br />12. Calculate cardiac output if stroke volume is 75<br />mL and pulse is 75 bpm. Using the cardiac output<br />you just calculated as a resting normal, what is the<br />stroke volume of a marathoner whose resting<br />pulse is 40 bpm? (p. 283)<br />13. Name the two cardiac centers and state their location.<br />Sympathetic impulses to the heart have what<br />effect? Parasympathetic impulses to the heart<br />have what effect? Name the parasympathetic<br />nerves to the heart. (p. 284)<br />14. State the locations of arterial pressoreceptors and<br />chemoreceptors, what they detect, and their sensory<br />nerves. (p. 284)<br />15. Describe the reflex arc to increase heart rate and<br />force when blood pressure to the brain decreases.<br />(p. 285)<br />The Heart 289<br />FOR FURTHER THOUGHT<br />1. Endocarditis may be caused by bacteria or fungi<br />that erode, or wear away, the heart valves, or in<br />some cases make the valves bumpy (these bumps<br />are called vegetations—think cauliflower). Explain<br />the possible consequences of this.<br />2. Bob, a college freshman, is telling his new friends<br />that he has been running seriously for 6 years,<br />and can run a marathon in a little over 3 hours. His<br />friends aren’t sure they should believe him, but<br />don’t want to spend 3 hours waiting while Bob<br />runs 26 miles. Bob says that he can prove he is<br />telling the truth in 1 minute. Can he? Explain why<br />or why not.<br />3. A neighbor, Mrs. G., age 62, tells you that she<br />“doesn’t feel right” and is suddenly tired for no<br />apparent reason. She denies having chest pain,<br />though she admits to a “full” feeling she calls indigestion.<br />You suspect that she may be having a heart<br />attack. What question can you ask to help you be<br />more sure? Explain the physiological basis for your<br />question.<br />4. Several types of artificial hearts are being developed<br />and tested. What are the three essential<br />characteristics a truly useful artificial heart must<br />have? One is obvious, the others, perhaps not as<br />much so.<br />290<br />CHAPTER 13<br />C<br />Chapter Outline<br />Arteries<br />Veins<br />Anastomoses<br />Capillaries<br />Exchanges in Capillaries<br />Pathways of Circulation<br />Pulmonary Circulation<br />Systemic Circulation<br />Hepatic Portal Circulation<br />Fetal Circulation<br />Velocity of Blood Flow<br />Blood Pressure<br />Maintenance of Systemic Blood Pressure<br />Distribution of Blood Flow<br />Regulation of Blood Pressure<br />Intrinsic Mechanisms<br />Nervous Mechanisms<br />Aging and the Vascular System<br />BOX 13–1 DISORDERS OF ARTERIES<br />BOX 13–2 DISORDERS OF VEINS<br />BOX 13–3 PULSE SITES<br />BOX 13–4 HYPERTENSION<br />BOX 13–5 CIRCULATORY SHOCK<br />Student Objectives<br />• Describe the structure of arteries and veins, and<br />relate their structure to function.<br />• Explain the purpose of arterial and venous anastomoses.<br />• Describe the structure of capillaries, and explain<br />the exchange processes that take place in capillaries.<br />• Describe the pathway and purpose of pulmonary<br />circulation.<br />• Name the branches of the aorta and their distributions.<br />• Name the major systemic veins, and the parts of<br />the body they drain of blood.<br />• Describe the pathway and purpose of hepatic portal<br />circulation.<br />• Describe the modifications of fetal circulation, and<br />explain the purpose of each.<br />• Explain the importance of slow blood flow in capillaries.<br />• Define blood pressure, and state the normal<br />ranges for systemic and pulmonary blood pressure.<br />• Explain the factors that maintain systemic blood<br />pressure.<br />• Explain how the heart and kidneys are involved in<br />the regulation of blood pressure.<br />• Explain how the medulla and the autonomic nervous<br />system regulate the diameter of blood vessels.<br />The Vascular System<br />291<br />New Terminology<br />Anastomosis (a-NAS-ti-MOH-sis)<br />Arteriole (ar-TIR-ee-ohl)<br />Circle of Willis (SIR-kuhl of WILL-iss)<br />Ductus arteriosus (DUK-tus ar-TIR-ee-OH-sis)<br />Foramen ovale (for-RAY-men oh-VAHL-ee)<br />Hepatic portal (hep-PAT-ik POOR-tuhl)<br />Peripheral resistance (puh-RIFF-uh-ruhl ree-ZIStense)<br />Placenta (pluh-SEN-tah)<br />Precapillary sphincter (pre-KAP-i-lar-ee SFINK-ter)<br />Sinusoid (SIGH-nuh-soyd)<br />Umbilical arteries (uhm-BILL-i-kull AR-tuh-rees)<br />Umbilical vein (uhm-BILL-i-kull VAIN)<br />Venule (VEN-yool)<br />Related Clinical Terminology<br />Anaphylactic (AN-uh-fi-LAK-tik)<br />Aneurysm (AN-yur-izm)<br />Arteriosclerosis (ar-TIR-ee-oh-skle-ROH-sis)<br />Hypertension (HIGH-per-TEN-shun)<br />Hypovolemic (HIGH-poh-voh-LEEM-ik)<br />Phlebitis (fle-BY-tis)<br />Pulse deficit (PULS DEF-i-sit)<br />Septic shock (SEP-tik SHAHK)<br />Varicose veins (VAR-i-kohs VAINS).<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />The role of blood vessels in the circulation of blood<br />has been known since 1628, when William Harvey, an<br />English anatomist, demonstrated that blood in veins<br />always flowed toward the heart. Before that time, it<br />was believed that blood was static or stationary, some<br />of it within the vessels but the rest sort of in puddles<br />throughout the body. Harvey showed that blood<br />indeed does move, and only in the blood vessels<br />(though he did not know of the existence of capillaries).<br />In the centuries that followed, the active (rather<br />than merely passive) roles of the vascular system were<br />discovered, and all contribute to homeostasis.<br />The vascular system consists of the arteries, capillaries,<br />and veins through which the heart pumps blood<br />throughout the body. As you will see, the major “business”<br />of the vascular system, which is the exchange of<br />materials between the blood and tissues, takes place in<br />the capillaries. The arteries and veins, however, are<br />just as important, transporting blood between the capillaries<br />and the heart.<br />Another important topic of this chapter will be<br />blood pressure (BP), which is the force the blood<br />exerts against the walls of the vessels. Normal blood<br />pressure is essential for circulation and for some of the<br />material exchanges that take place in capillaries.<br />ARTERIES<br />Arteries carry blood from the heart to capillaries;<br />smaller arteries are called arterioles. If we look at an<br />artery in cross-section, we find three layers (or tunics)<br />of tissues, each with different functions (Fig. 13–1).<br />The innermost layer, the tunica intima, is the only<br />part of a vessel that is in contact with blood. It is made<br />of simple squamous epithelium called endothelium.<br />This lining is the same type of tissue that forms the<br />endocardium, the lining of the chambers of the heart.<br />As you might guess, its function is also the same: Its<br />extreme smoothness prevents abnormal blood clotting.<br />The endothelium of vessels, however, also produces<br />nitric oxide (NO), which is a vasodilator. The<br />tunica media, or middle layer, is made of smooth<br />muscle and elastic connective tissue. Both of these tissues<br />are involved in the maintenance of normal blood<br />pressure, especially diastolic blood pressure when the<br />heart is relaxed. The smooth muscle is the tissue<br />affected by the vasodilator NO; relaxation of this muscle<br />tissue brings about dilation of the vessel. Smooth<br />muscle also has a nerve supply; sympathetic nerve<br />impulses bring about vasoconstriction. Fibrous connective<br />tissue forms the outer layer, the tunica<br />externa. This tissue is very strong, which is important<br />to prevent the rupture or bursting of the larger arteries<br />that carry blood under high pressure (see Box 13–1:<br />Disorders of Arteries).<br />The outer and middle layers of large arteries are<br />quite thick. In the smallest arterioles, only individual<br />smooth muscle cells encircle the tunica intima. As<br />mentioned, the smooth muscle layer enables arteries<br />to constrict or dilate. Such changes in diameter are<br />regulated by the medulla and autonomic nervous system,<br />and will be discussed in a later section on blood<br />pressure.<br />VEINS<br />Veins carry blood from capillaries back to the heart;<br />the smaller veins are called venules. The same three<br />tissue layers are present in veins as in the walls of<br />arteries, but there are some differences when compared<br />to the arterial layers. The inner layer of veins is<br />smooth endothelium, but at intervals this lining is<br />folded to form valves (see Fig. 13–1). Valves prevent<br />backflow of blood and are most numerous in veins of<br />the legs, where blood must often return to the heart<br />against the force of gravity.<br />The middle layer of veins is a thin layer of smooth<br />muscle. It is thin because veins do not regulate blood<br />pressure and blood flow into capillaries as arteries do.<br />Veins can constrict extensively, however, and this<br />function becomes very important in certain situations<br />such as severe hemorrhage. The outer layer of veins is<br />also thin; not as much fibrous connective tissue is necessary<br />because blood pressure in veins is very low.<br />ANASTOMOSES<br />An anastomosis is a connection, or joining, of vessels,<br />that is, artery to artery or vein to vein. The general<br />purpose of these connections is to provide alternate<br />pathways for the flow of blood if one vessel becomes<br />obstructed.<br />An arterial anastomosis helps ensure that blood<br />will get to the capillaries of an organ to deliver oxygen<br />292 The Vascular System<br />and nutrients and to remove waste products. There<br />are arterial anastomoses, for example, between some<br />of the coronary arteries that supply blood to the<br />myocardium.<br />A venous anastomosis helps ensure that blood will<br />be able to return to the heart in order to be pumped<br />again. Venous anastomoses are most numerous among<br />the veins of the legs, where the possibility of obstruction<br />increases as a person gets older (see Box 13–2:<br />Disorders of Veins).<br />CAPILLARIES<br />Capillaries carry blood from arterioles to venules.<br />Their walls are only one cell in thickness; capillaries<br />are actually the extension of the endothelium, the simple<br />squamous lining, of arteries and veins (see Fig.<br />13–1). Some tissues do not have capillaries; these are<br />the epidermis, cartilage, and the lens and cornea of the<br />eye.<br />Most tissues, however, have extensive capillary networks.<br />The quantity or volume of capillary networks<br />in an organ reflects the metabolic activity of the organ.<br />The functioning of the kidneys, for example, depends<br />upon a good blood supply. Fig. 18–2 shows a vascular<br />cast of a kidney; you can see how dense the vessels are,<br />most of which are capillaries. In contrast, a tendon<br />such as the Achilles tendon at the heel or the patellar<br />tendon at the knee would have far fewer vessels,<br />because fibrous connective tissue is far less metabolically<br />active.<br />The Vascular System 293<br />Tunica externa<br />External elastic<br />lamina<br />Tunica<br />media<br />Internal elastic<br />lamina<br />Endothelium (lining)<br />Artery<br />Arteriole<br />Endothelial<br />cells<br />Smooth muscle<br />Precapillary<br />sphincter<br />Capillary<br />Blood flow<br />Venule<br />Vein<br />Valve<br />Tunica<br />intima<br />Tunica<br />externa<br />Tunica<br />media<br />Figure 13–1. Structure of<br />an artery, arteriole, capillary<br />network, venule, and vein.<br />See text for description.<br />QUESTION: What tissue is the<br />tunica media made of, and<br />how is this layer different in an<br />artery and in a vein?<br />Blood flow into capillary networks is regulated by<br />smooth muscle cells called precapillary sphincters,<br />found at the beginning of each network (see Fig. 13–1).<br />Precapillary sphincters are not regulated by the nervous<br />system but rather constrict or dilate depending on<br />the needs of the tissues. Because there is not enough<br />blood in the body to fill all of the capillaries at once,<br />precapillary sphincters are usually slightly constricted.<br />In an active tissue that requires more oxygen, such as<br />exercising muscle, the precapillary sphincters dilate to<br />increase blood flow. These automatic responses ensure<br />that blood, the volume of which is constant, will circulate<br />where it is needed most.<br />Some organs have another type of capillary called<br />sinusoids, which are larger and more permeable than<br />are other capillaries. The permeability of sinusoids<br />permits large substances such as proteins and blood<br />cells to enter or leave the blood. Sinusoids are found<br />in the red bone marrow and spleen, where blood cells<br />enter or leave the blood, and in organs such as the<br />294 The Vascular System<br />BOX 13–1 DISORDERS OF ARTERIES<br />The most common sites for aneurysm formation<br />are the cerebral arteries and the aorta, especially<br />the abdominal aorta. Rupture of a cerebral<br />aneurysm is a possible cause of a cerebrovascular<br />accident (CVA). Rupture of an aortic aneurysm is<br />life-threatening and requires immediate corrective<br />surgery. The damaged portion of the artery is<br />removed and replaced with a graft. Such surgery<br />may also be performed when an aneurysm is found<br />before it ruptures.<br />Atherosclerosis—this condition has been mentioned<br />previously; see Chapters 2 and 12.<br />Arteriosclerosis—although commonly called<br />“hardening of the arteries,” arteriosclerosis really<br />means that the arteries lose their elasticity, and their<br />walls become weakened. Arteries carry blood under<br />high pressure, so deterioration of their walls is part<br />of the aging process.<br />Aneurysm—a weak portion of an arterial wall<br />may bulge out, forming a sac or bubble called an<br />aneurysm. Arteriosclerosis is a possible cause, but<br />some aneurysms are congenital. An aneurysm may<br />be present for many years without any symptoms<br />and may only be discovered during diagnostic procedures<br />for some other purpose.<br />BOX 13–2 DISORDERS OF VEINS<br />to pool in the leg veins, stretching their walls. If the<br />veins become overly stretched, the valves within<br />them no longer close properly. These incompetent<br />valves no longer prevent backflow of blood, leading<br />to further pooling and even further stretching of<br />the walls of the veins. Varicose veins may cause discomfort<br />and cramping in the legs, or become even<br />more painful. Severe varicosities may be removed<br />surgically.<br />This condition may also develop during pregnancy,<br />when the enlarged uterus presses against<br />the iliac veins and slows blood flow into the inferior<br />vena cava. Varicose veins of the anal canal are called<br />hemorrhoids, which may also be a result of pregnancy<br />or of chronic constipation and straining to<br />defecate. Hemorrhoids that cause discomfort or<br />pain may also be removed surgically. Developments<br />in laser surgery have made this a simpler procedure<br />than it was in the past.<br />Phlebitis—inflammation of a vein. This condition<br />is most common in the veins of the legs, because<br />they are subjected to great pressure as the blood is<br />returned to the heart against the force of gravity.<br />Often no specific cause can be determined, but<br />advancing age, obesity, and blood disorders may<br />be predisposing factors.<br />If a superficial vein is affected, the area may be<br />tender or painful, but blood flow is usually maintained<br />because there are so many anastomoses<br />among these veins. Deep vein phlebitis is potentially<br />more serious, with the possibility of clot formation<br />(thrombophlebitis) and subsequent dislodging of<br />the clot to form an embolism.<br />Varicose veins—swollen and distended veins that<br />occur most often in the superficial veins of the legs.<br />This condition may develop in people who must sit<br />or stand in one place for long periods of time.<br />Without contraction of the leg muscles, blood tends<br />liver and pituitary gland, which produce and secrete<br />proteins into the blood.<br />EXCHANGES IN CAPILLARIES<br />Capillaries are the sites of exchanges of materials<br />between the blood and the tissue fluid surrounding<br />cells. Some of these substances move from the blood<br />to tissue fluid, and others move from tissue fluid to the<br />blood. The processes by which these substances are<br />exchanged are illustrated in Fig. 13–2.<br />Gases move by diffusion, that is, from their area of<br />greater concentration to their area of lesser concentration.<br />Oxygen, therefore, diffuses from the blood in<br />systemic capillaries to the tissue fluid, and carbon<br />dioxide diffuses from tissue fluid to the blood to be<br />brought to the lungs and exhaled.<br />Let us now look at the blood pressure as blood<br />enters capillaries from the arterioles. Blood pressure<br />here is about 30 to 35 mmHg, and the pressure of the<br />surrounding tissue fluid is much lower, about 2<br />mmHg. Because the capillary blood pressure is higher,<br />the process of filtration occurs, which forces plasma<br />and dissolved nutrients out of the capillaries and into<br />tissue fluid. This is how nutrients such as glucose,<br />amino acids, and vitamins are brought to cells.<br />Blood pressure decreases as blood reaches the<br />venous end of capillaries, but notice that proteins<br />such as albumin have remained in the blood. Albumin<br />contributes to the colloid osmotic pressure (COP)<br />of blood; this is an “attracting” pressure, a “pulling”<br />rather than a “pushing” pressure. At the venous end<br />of capillaries, the presence of albumin in the blood<br />pulls tissue fluid into the capillaries, which also brings<br />into the blood the waste products produced by<br />cells. The tissue fluid that returns to the blood<br />also helps maintain normal blood volume and blood<br />pressure.<br />The Vascular System 295<br />Precapillary sphincter Tissue fluid Cells Capillary<br />Tissue fluid -<br />Hydrostatic pressure<br />2 mmHg<br />COP 4 mmHg<br />CO2<br />CO2<br />CO2<br />O2<br />O2<br />O2<br />Plasma, glucose,<br />amino acids, vitamins<br />B. P. 33 mmHg<br />Albumin - COP 25 mmHg<br />B. P. 15 mmHg<br />Albumin - COP 25 mmHg<br />Tissue fluid,<br />waste products<br />VENOUS END<br />Outward forces 19<br />Inward forces 27<br />ARTERIAL END<br />Outward forces 37<br />Inward forces 27<br />Figure 13–2. Exchanges between blood in a systemic capillary and the surrounding tissue<br />fluid. Arrows depict the direction of movement. Filtration takes place at the arterial end<br />of the capillary. Osmosis takes place at the venous end. Gases are exchanged by diffusion.<br />QUESTION: Of all the pressures shown here, which one is the highest, and what process<br />does it bring about?<br />The amount of tissue fluid formed is slightly<br />greater than the amount returned to the capillaries. If<br />this were to continue, blood volume would be gradually<br />depleted. The excess tissue fluid, however, enters<br />lymph capillaries. Now called lymph, it will be<br />returned to the blood to be recycled again as plasma,<br />thus maintaining blood volume. This is discussed further<br />in Chapter 14.<br />PATHWAYS OF CIRCULATION<br />The two major pathways of circulation are pulmonary<br />and systemic. Pulmonary circulation begins at the<br />right ventricle, and systemic circulation begins at the<br />left ventricle. Hepatic portal circulation is a special<br />segment of systemic circulation that will be covered<br />separately. Fetal circulation involves pathways that are<br />present only before birth and will also be discussed<br />separately.<br />PULMONARY CIRCULATION<br />The right ventricle pumps blood into the pulmonary<br />artery (or trunk), which divides into the right and left<br />pulmonary arteries, one going to each lung. Within<br />the lungs each artery branches extensively into smaller<br />arteries and arterioles, then to capillaries. The pulmonary<br />capillaries surround the alveoli of the lungs; it<br />is here that exchanges of oxygen and carbon dioxide<br />take place. The capillaries unite to form venules,<br />which merge into veins, and finally into the two pulmonary<br />veins from each lung that return blood to the<br />left atrium. This oxygenated blood will then travel<br />through the systemic circulation. (Notice that the pulmonary<br />veins contain oxygenated blood; these are the<br />only veins that carry blood with a high oxygen content.<br />The blood in systemic veins has a low oxygen<br />content; it is systemic arteries that carry oxygenated<br />blood.)<br />SYSTEMIC CIRCULATION<br />The left ventricle pumps blood into the aorta, the<br />largest artery of the body. We will return to the aorta<br />and its branches in a moment, but first we will summarize<br />the rest of systemic circulation. The branches<br />of the aorta take blood into arterioles and capillary networks<br />throughout the body. Capillaries merge to form<br />venules and veins. The veins from the lower body take<br />blood to the inferior vena cava; veins from the upper<br />body take blood to the superior vena cava. These two<br />caval veins return blood to the right atrium. The major<br />arteries and veins are shown in Figs. 13–3 to 13–5, and<br />their functions are listed in Tables 13–1 and 13–2.<br />The aorta is a continuous vessel, but for the sake of<br />precise description it is divided into sections that are<br />named anatomically: ascending aorta, aortic arch, thoracic<br />aorta, and abdominal aorta. The ascending aorta<br />is the first inch that emerges from the top of the left<br />ventricle. The arch of the aorta curves posteriorly over<br />the heart and turns downward. The thoracic aorta<br />continues down through the chest cavity and through<br />the diaphragm. Below the level of the diaphragm, the<br />abdominal aorta continues to the level of the 4th lumbar<br />vertebra, where it divides into the two common<br />iliac arteries. Along its course, the aorta has many<br />branches through which blood travels to specific<br />organs and parts of the body.<br />The ascending aorta has only two branches: the<br />right and left coronary arteries, which supply blood to<br />the myocardium. This pathway of circulation was<br />described previously in Chapter 12.<br />The aortic arch has three branches that supply<br />blood to the head and arms: the brachiocephalic<br />artery, left common carotid artery, and left subclavian<br />artery. The brachiocephalic (literally, “arm-head”)<br />artery is very short and divides into the right common<br />carotid artery and right subclavian artery. The right<br />and left common carotid arteries extend into the neck,<br />where each divides into an internal carotid artery and<br />external carotid artery, which supply the head. The<br />right and left subclavian arteries are in the shoulders<br />behind the clavicles and continue into the arms. As the<br />artery enters another body area (it may not “branch,”<br />simply continue), its name changes: The subclavian<br />artery becomes the axillary artery, which becomes the<br />brachial artery. The branches of the carotid and subclavian<br />arteries are diagrammed in Figs. 13–3 and<br />13–5. As you look at these diagrams, keep in mind that<br />the name of the vessel often tells us where it is. The<br />facial artery, for example, is found in the face.<br />Some of the arteries in the head contribute to an<br />important arterial anastomosis, the circle of Willis<br />(or cerebral arterial circle), which is a “circle” of arteries<br />around the pituitary gland (Fig. 13–6). The circle<br />of Willis is formed by the right and left internal<br />carotid arteries and the basilar artery, which is the<br />union of the right and left vertebral arteries (branches<br />of the subclavian arteries). The brain is always active,<br />296 The Vascular System<br />(text continued on page 299)<br />The Vascular System 297<br />Occipital<br />Internal carotid<br />Vertebral<br />Brachiocephalic<br />Aortic arch<br />Maxillary<br />Facial<br />External carotid<br />Common carotid<br />Subclavian<br />Axillary<br />Pulmonary<br />Celiac<br />Left gastric<br />Hepatic<br />Splenic<br />Superior mesenteric<br />Abdominal aorta<br />Right common iliac<br />Internal iliac<br />External iliac<br />Femoral<br />Popliteal<br />Anterior tibial<br />Posterior tibial<br />Intercostal<br />Brachial<br />Renal<br />Gonadal<br />Inferior mesenteric<br />Radial<br />Ulnar<br />Deep palmar arch<br />Superficial palmar arch<br />Deep femoral<br />Figure 13–3. Systemic arteries. The aorta and its major branches are shown in anterior<br />view.<br />QUESTION: Can you find three arteries named after bones? After organs?<br />298 The Vascular System<br />Superior sagittal sinus<br />Inferior sagittal sinus<br />Straight sinus<br />Transverse sinus<br />Vertebral<br />External jugular<br />Internal jugular<br />Subclavian<br />Brachiocephalic<br />Pulmonary<br />Hepatic<br />Hepatic portal<br />Left gastric<br />Renal<br />Splenic<br />Inferior mesenteric<br />Internal iliac<br />Femoral<br />External iliac<br />Great saphenous<br />Popliteal<br />Small saphenous<br />Anterior tibial<br />Anterior facial<br />Superior vena cava<br />Axillary<br />Cephalic<br />Hemiazygos<br />Intercostal<br />Inferior vena cava<br />Brachial<br />Basilic<br />Gonadal<br />Superior mesenteric<br />Dorsal arch<br />Volar digital<br />Dorsal arch<br />Common iliac<br />Figure 13–4. Systemic veins shown in anterior view.<br />QUESTION: Can you find three veins with the same name as the accompanying artery?<br />even during sleep, and must have a constant flow of<br />blood to supply oxygen and remove waste products.<br />For this reason there are four vessels that bring blood<br />to the circle of Willis. From this anastomosis, several<br />paired arteries (the cerebral arteries) extend into the<br />brain itself.<br />The thoracic aorta and its branches supply the<br />chest wall and the organs within the thoracic cavity.<br />These vessels are listed in Table 13–1.<br />The abdominal aorta gives rise to arteries that supply<br />the abdominal wall and organs and to the common<br />iliac arteries, which continue into the legs. Notice in<br />Fig. 13–3 that the common iliac artery becomes the<br />external iliac artery, which becomes the femoral artery,<br />which becomes the popliteal artery; the same vessel<br />has different names based on location. These vessels<br />are also listed in Table 13–1 (see Box 13–3: Pulse<br />Sites).<br />The systemic veins drain blood from organs or<br />parts of the body and often parallel their correspond-<br />The Vascular System 299<br />Figure 13–5. Arteries and veins of the head and neck shown in right lateral view. Veins<br />are labeled on the left. Arteries are labeled on the right.<br />QUESTION: Which vein is the counterpart of the common carotid artery?<br />300 The Vascular System<br />Table 13–1 MAJOR SYSTEMIC ARTERIES<br />Branches of the Ascending Aorta and Aortic Arch<br />Artery Branch of Region supplied<br />Coronary a.<br />Brachiocephalic a.<br />Right common carotid a.<br />Right subclavian a.<br />Left common carotid a.<br />Left subclavian a.<br />External carotid a.<br />Superficial temporal a.<br />Internal carotid a.<br />Ophthalmic a.<br />Vertebral a.<br />Axillary a.<br />Brachial a.<br />Radial a.<br />Ulnar a.<br />Volar arch<br />Branches of the Thoracic Aorta<br />Artery Region Supplied<br />Intercostal a. (9 pairs)<br />Superior phrenic a.<br />Pericardial a.<br />Esophageal a.<br />Bronchial a.<br />Branches of the Abdominal Aorta<br />Artery Region Supplied<br />Inferior phrenic a.<br />Lumbar a.<br />Middle sacral a.<br />Celiac a.<br />Hepatic a.<br />Left gastric a.<br />Splenic a.<br />Superior mesenteric a.<br />Suprarenal a.<br />Renal a.<br />Inferior mesenteric a.<br />Ascending aorta<br />Aortic arch<br />Brachiocephalic a.<br />Brachiocephalic a.<br />Aortic arch<br />Aortic arch<br />Common carotid a.<br />External carotid a.<br />Common carotid a.<br />Internal carotid a.<br />Subclavian a.<br />Subclavian a.<br />Axillary a.<br />Brachial a.<br />Brachial a.<br />Radial and ulnar a.<br />• Myocardium<br />• Right arm and head<br />• Right side of head<br />• Right shoulder and arm<br />• Left side of head<br />• Left shoulder and arm<br />• Superficial head<br />• Scalp<br />• Brain (circle of Willis)<br />• Eye<br />• Cervical vertebrae and circle of Willis<br />• Armpit<br />• Upper arm<br />• Forearm<br />• Forearm<br />• Hand<br />• Skin, muscles, bones of trunk<br />• Diaphragm<br />• Pericardium<br />• Esophagus<br />• Bronchioles and connective tissue of the lungs<br />• Diaphragm<br />• Lumbar area of back<br />• Sacrum, coccyx, buttocks<br />• (see branches)<br />• Liver<br />• Stomach<br />• Spleen, pancreas<br />• Small intestine, part of colon<br />• Adrenal glands<br />• Kidneys<br />• Most of colon and rectum<br />ing arteries. The most important veins are diagrammed<br />in Fig. 13–4 and listed in Table 13–2.<br />HEPATIC PORTAL CIRCULATION<br />Hepatic portal circulation is a subdivision of systemic<br />circulation in which blood from the abdominal<br />digestive organs and spleen circulates through the<br />liver before returning to the heart.<br />Blood from the capillaries of the stomach, small<br />intestine, colon, pancreas, and spleen flows into two<br />large veins, the superior mesenteric vein and the<br />splenic vein, which unite to form the portal vein (Fig.<br />13–7). The portal vein takes blood into the liver,<br />where it branches extensively and empties blood into<br />the sinusoids, the capillaries of the liver (see also Fig.<br />16–6). From the sinusoids, blood flows into hepatic<br />veins, to the inferior vena cava and back to the right<br />atrium. Notice that in this pathway there are two sets<br />of capillaries, and keep in mind that it is in capillaries<br />that exchanges take place. Let us use some specific<br />examples to show the purpose and importance of portal<br />circulation.<br />Glucose from carbohydrate digestion is absorbed<br />into the capillaries of the small intestine; after a big<br />meal this may greatly increase the blood glucose level.<br />If this blood were to go directly back to the heart and<br />then circulate through the kidneys, some of the glucose<br />might be lost in urine. However, blood from the<br />small intestine passes first through the liver sinusoids,<br />and the liver cells remove the excess glucose and store<br />it as glycogen. The blood that returns to the heart will<br />then have a blood glucose level in the normal range.<br />Another example: Alcohol is absorbed into the capillaries<br />of the stomach. If it were to circulate directly<br />throughout the body, the alcohol would rapidly impair<br />the functioning of the brain. Portal circulation, however,<br />takes blood from the stomach to the liver, the<br />organ that can detoxify the alcohol and prevent its<br />detrimental effects on the brain. Of course, if alcohol<br />consumption continues, the blood alcohol level rises<br />faster than the liver’s capacity to detoxify, and the wellknown<br />signs of alcohol intoxication appear.<br />As you can see, this portal circulation pathway<br />enables the liver to modify the blood from the digestive<br />organs and spleen. Some nutrients may be stored<br />or changed, bilirubin from the spleen is excreted into<br />bile, and potential poisons are detoxified before the<br />blood returns to the heart and the rest of the body.<br />FETAL CIRCULATION<br />The fetus depends upon the mother for oxygen and<br />nutrients and for the removal of carbon dioxide and<br />The Vascular System 301<br />Table 13–1 MAJOR SYSTEMIC ARTERIES (Continued)<br />Branches of the Abdominal Aorta<br />Artery Region Supplied<br />Testicular or ovarian a.<br />Common iliac a.<br />Internal iliac a.<br />External iliac a.<br />Femoral a.<br />Popliteal a.<br />Anterior tibial a.<br />Dorsalis pedis<br />Plantar arches<br />Posterior tibial a.<br />Peroneal a.<br />Plantar arches<br />• Testes or ovaries<br />• The two large vessels that receive blood from the<br />abdominal aorta; each branches as follows:<br />• Bladder, rectum, reproductive organs<br />• Lower pelvis to leg<br />• Thigh<br />• Back of knee<br />• Front of lower leg<br />• Top of ankle and foot<br />• Foot<br />• Back of lower leg<br />• Medial lower leg<br />• Foot<br />302 The Vascular System<br />Table 13–2 MAJOR SYSTEMIC VEINS<br />Vein Vein Joined Region Drained<br />Head and Neck<br />Cranial venous sinuses<br />Internal jugular v.<br />External jugular v.<br />Subclavian v.<br />Brachiocephalic v.<br />Superior vena cava<br />Arm and Shoulder<br />Radial v.<br />Ulnar v.<br />Cephalic v.<br />Basilic v.<br />Brachial v.<br />Axillary v.<br />Subclavian v.<br />Trunk<br />Brachiocephalic v.<br />Azygos v.<br />Hepatic v.<br />Renal v.<br />Testicular or ovarian v.<br />Internal iliac v.<br />External iliac v.<br />Common iliac v.<br />Leg and Hip<br />Anterior and posterior tibial v.<br />Popliteal v.<br />Small saphenous v.<br />Great saphenous v.<br />Femoral v.<br />External iliac v.<br />Common iliac v.<br />Inferior vena cava<br />Internal jugular v.<br />Brachiocephalic v.<br />Subclavian v.<br />Brachiocephalic v.<br />Superior vena cava<br />Right atrium<br />Brachial v.<br />Brachial v.<br />Axillary v.<br />Axillary v.<br />Axillary v.<br />Subclavian v.<br />Brachiocephalic v.<br />Superior vena cava<br />Superior vena cava<br />Inferior vena cava<br />Inferior vena cava<br />Inferior vena cava<br />and left renal v.<br />Common iliac v.<br />Common iliac v.<br />Inferior vena cava<br />Popliteal v.<br />Femoral v.<br />Popliteal v.<br />Femoral v.<br />External iliac v.<br />Common iliac v.<br />Inferior vena cava<br />Right atrium<br />• Brain, including reabsorbed CSF<br />• Face and neck<br />• Superficial face and neck<br />• Shoulder<br />• Upper body<br />• Upper body<br />• Forearm and hand<br />• Forearm and hand<br />• Superficial arm and forearm<br />• Superficial upper arm<br />• Upper arm<br />• Armpit<br />• Shoulder<br />• Upper body<br />• Deep structures of chest and abdomen; links<br />inferior vena cava to superior vena cava<br />• Liver<br />• Kidney<br />• Testes or ovaries<br />• Rectum, bladder, reproductive organs<br />• Leg and abdominal wall<br />• Leg and lower abdomen<br />• Lower leg and foot<br />• Knee<br />• Superficial leg and foot<br />• Superficial foot, leg, and thigh<br />• Thigh<br />• Leg and abdominal wall<br />• Leg and lower abdomen<br />• Lower body<br />The Vascular System 303<br />other waste products. The site of exchange between<br />fetus and mother is the placenta, which contains fetal<br />and maternal blood vessels that are very close to one<br />another (see Figs. 13–8 and 21–5). The blood of the<br />fetus does not mix with the blood of the mother; substances<br />are exchanged by diffusion and active transport<br />mechanisms.<br />The fetus is connected to the placenta by the<br />umbilical cord, which contains two umbilical arteries<br />and one umbilical vein (see Fig. 13–8). The umbilical<br />arteries are branches of the fetal internal iliac arteries;<br />they carry blood from the fetus to the placenta. In the<br />placenta, carbon dioxide and waste products in the<br />fetal blood enter maternal circulation, and oxygen<br />Posterior communicating<br />Anterior cerebral<br />Circle of Willis<br />Anterior communicating<br />Posterior<br />cerebral<br />Middle<br />cerebral<br />Basilar<br />Internal<br />carotid<br />External carotid<br />Vertebral<br />Left<br />common carotid<br />Right<br />common carotid<br />ARTERIES<br />Anterior<br />cerebral<br />Anterior<br />communicating<br />Anterior<br />cerebral<br />Middle<br />cerebral<br />Internal<br />carotid<br />Posterior<br />cerebral<br />Posterior<br />communicating<br />Basilar<br />Vertebral<br />Spinal cord<br />Cerebellum<br />Medulla<br />Pons<br />Cerebrum<br />(temporal lobe)<br />Cerebrum<br />(frontal lobe)<br />Figure 13–6. Circle of Willis. This anastomosis is formed by the following arteries: internal<br />carotid, anterior communicating, posterior communicating, and basilar. The cerebral<br />arteries extend from the circle of Willis into the brain. The box shows these vessels in an<br />inferior view of the brain.<br />QUESTION: Why do so many vessels contribute to the circle of Willis?<br />and nutrients from the mother’s blood enter fetal<br />circulation.<br />The umbilical vein carries this oxygenated blood<br />from the placenta to the fetus. Within the body of the<br />fetus, the umbilical vein branches: One branch takes<br />some blood to the fetal liver, but most of the blood<br />passes through the ductus venosus to the inferior<br />vena cava, to the right atrium. After birth, when the<br />umbilical cord is cut, the remnants of these fetal vessels<br />constrict and become nonfunctional.<br />The other modifications of fetal circulation concern<br />the fetal heart and large arteries (also shown in<br />304 The Vascular System<br />Inferior vena cava<br />Hepatic vein<br />Liver<br />Portal vein<br />Superior<br />mesenteric V<br />Right colic V.<br />Ascending colon<br />Left gastric V.<br />Right gastric V.<br />Splenic V.<br />Spleen<br />Stomach<br />Pancreas<br />Left gastroepiploic V.<br />Inferior mesenteric V.<br />Left colic V.<br />Descending colon<br />Figure 13–7. Hepatic portal<br />circulation. Portions of<br />some of the digestive organs<br />have been removed to show<br />the veins that unite to form<br />the portal vein. See text for<br />description.<br />QUESTION: The blood in the<br />portal vein is going to what<br />organ? Where is the blood<br />coming from?<br />BOX 13–3 PULSE SITES<br />Popliteal—the popliteal artery at the back of the<br />knee.<br />Dorsalis pedis—the dorsalis pedis artery on the<br />top of the foot (commonly called the pedal pulse).<br />Pulse rate is, of course, the heart rate. However,<br />if the heart is beating weakly, a radial pulse may be<br />lower than an apical pulse (listening to the heart<br />itself with a stethoscope). This is called a pulse<br />deficit and indicates heart disease of some kind.<br />When taking a pulse, the careful observer also<br />notes the rhythm and force of the pulse. Abnormal<br />rhythms may reflect cardiac arrhythmias, and the<br />force of the pulse (strong or weak) is helpful in<br />assessing the general condition of the heart and<br />arteries.<br />A pulse is the heartbeat that is felt at an arterial site.<br />What is felt is not actually the force exerted by the<br />blood, but the force of ventricular contraction<br />transmitted through the walls of the arteries. This is<br />why pulses are not felt in veins; they are too far<br />from the heart for the force to be detectable.<br />The most commonly used pulse sites are:<br />Radial—the radial artery on the thumb side of the<br />wrist.<br />Carotid—the carotid artery lateral to the larynx in<br />the neck.<br />Temporal—the temporal artery just in front of the<br />ear.<br />Femoral—the femoral artery at the top of the<br />thigh.<br />Fig. 13–8). Because the fetal lungs are deflated and do<br />not provide for gas exchange, blood is shunted away<br />from the lungs and to the body. The foramen ovale is<br />an opening in the interatrial septum that permits some<br />blood to flow from the right atrium to the left atrium,<br />not, as usual, to the right ventricle. The blood that<br />does enter the right ventricle is pumped into the pulmonary<br />artery. The ductus arteriosus is a short vessel<br />that diverts most of the blood in the pulmonary<br />artery to the aorta, to the body. Both the foramen<br />ovale and the ductus arteriosus permit blood to bypass<br />the fetal lungs.<br />Just after birth, the baby breathes and expands its<br />lungs, which pulls more blood into the pulmonary circulation.<br />More blood then returns to the left atrium,<br />and a flap on the left side of the foramen ovale is<br />closed. The ductus arteriosus constricts, probably in<br />response to the higher oxygen content of the blood,<br />The Vascular System 305<br />Aortic arch<br />Ductus arteriosus<br />Pulmonary artery<br />Left atrium<br />Foramen<br />ovale<br />Right<br />atrium<br />Inferior vena cava<br />Aorta<br />Ductus venosus<br />Umbilical cord<br />Navel of fetus<br />Umbilical arteries<br />Umbilical vein<br />Placenta<br />Internal<br />iliac arteries<br />Maternal<br />blood vessels<br />Figure 13–8. Fetal circulation. Fetal heart and blood vessels are shown on the left.<br />Arrows depict the direction of blood flow. The placenta and umbilical blood vessels are<br />shown on the right. See text for description.<br />QUESTION: Find the foramen ovale in the fetal heart. Which way does blood flow through<br />it, and why?<br />and pulmonary circulation becomes fully functional<br />within a few days.<br />VELOCITY OF BLOOD FLOW<br />The velocity, or speed, with which blood flows differs<br />in the various parts of the vascular system. Velocity is<br />inversely related (meaning as one value goes up, the<br />other goes down) to the cross-sectional area of the<br />particular segment of the vascular system. Refer to<br />Fig. 13–9 as you read the following. The aorta receives<br />all the blood from the left ventricle, its cross-sectional<br />area is small, about 3 cm2 (1 sq. inch), and the blood<br />moves very rapidly, at least 30 cm per second (about 12<br />inches). Each time the aorta or any artery branches,<br />the total cross-sectional area becomes larger, and the<br />speed of blood flow decreases. Think of a river that<br />306 The Vascular System<br />4,000<br />3,000<br />2,000<br />1,000<br />30<br />20<br />10<br />120<br />100<br />80<br />60<br />40<br />20<br />0<br />Blood pressure<br />(mm Hg)<br />Pulse<br />pressure<br />Blood velocity<br />(cm/sec)<br />Cross-sectional<br />area (cm2)<br />Diastolic pressure<br />Systolic pressure<br />Aorta<br />Arteries<br />Arterioles<br />Capillaries<br />Venules<br />Veins<br />Vena Cavae<br />A<br />B<br />C<br />D<br />Figure 13–9. Characteristics of the vascular<br />system. (A) Schematic of the branching<br />of vessels. (B) Cross-sectional area in<br />square centimeters. (C) Blood velocity in<br />centimeters per second. (D) Systemic<br />blood pressure changes. Notice that systolic<br />and diastolic pressures become one<br />pressure in the capillaries.<br />QUESTION: Look at the cross-sectional<br />area and blood velocity. As area increases,<br />what happens to velocity? Where is velocity<br />slowest?<br />begins in a narrow bed and is flowing rapidly. If the<br />river bed widens, the water spreads out to fill it and<br />flows more slowly. If the river were to narrow again,<br />the water would flow faster. This is just what happens<br />in the vascular system.<br />The capillaries in total have the greatest crosssectional<br />area, and blood velocity there is slowest, less<br />than 0.1 cm per second. When capillaries unite to<br />form venules, and then veins, the cross-sectional area<br />decreases and blood flow speeds up.<br />Recall that it is in capillary networks that exchanges<br />of nutrients, wastes, and gases take place between the<br />blood and tissue fluid. The slow rate of blood flow in<br />capillaries permits sufficient time for these essential<br />exchanges. Think of a train slowing down (not actually<br />stopping) at stations to allow people to jump on<br />and off, then speeding up again to get to the next station.<br />The capillaries are the “stations” of the vascular<br />system.<br />The more rapid blood velocity in other vessels<br />makes circulation time quite short. This is the time it<br />takes for blood to go from the right ventricle to the<br />lungs, back to the heart to be pumped by the left ventricle<br />to the body, and return to the heart again.<br />Circulation time is about 1 minute or less, and ensures<br />an adequate exchange of gases.<br />BLOOD PRESSURE<br />Blood pressure is the force the blood exerts against<br />the walls of the blood vessels. Filtration in capillaries<br />depends upon blood pressure; filtration brings nutrients<br />to tissues, and as you will see in Chapter 18, is the<br />first step in the formation of urine. Blood pressure is<br />one of the “vital signs” often measured, and indeed a<br />normal blood pressure is essential to life.<br />The pumping of the ventricles creates blood pressure,<br />which is measured in mmHg (millimeters of<br />mercury). When a systemic blood pressure reading is<br />taken, two numbers are obtained: systolic and diastolic,<br />as in 110/70 mmHg. Systolic pressure is always<br />the higher of the two and represents the blood pressure<br />when the left ventricle is contracting. The lower<br />number is the diastolic pressure, when the left ventricle<br />is relaxed and does not exert force. Diastolic pressure<br />is maintained by the arteries and arterioles and is<br />discussed in a later section.<br />Systemic blood pressure is highest in the aorta,<br />which receives all of the blood pumped by the left ventricle.<br />As blood travels farther away from the heart,<br />blood pressure decreases (see Fig. 13–9). The brachial<br />artery is most often used to take a blood pressure reading;<br />here a normal systolic range is 90 to 120 mmHg,<br />and a normal diastolic range is 60 to 80 mmHg. In the<br />arterioles, blood pressure decreases further, and systolic<br />and diastolic pressures merge into one pressure.<br />At the arterial end of capillary networks, blood pressure<br />is about 30 to 35 mmHg, decreasing to 12 to 15<br />mmHg at the venous end of capillaries. This is high<br />enough to permit filtration but low enough to prevent<br />rupture of the capillaries. As blood flows through<br />veins, the pressure decreases further, and in the caval<br />veins, blood pressure approaches zero as blood enters<br />the right atrium.<br />The upper limit of the normal blood pressure range<br />is now 120/80 mmHg. The levels of 125 to 139/85 to<br />89 mmHg, once considered high-normal, are now<br />called “prehypertension,” that is, with the potential to<br />become even higher. A systemic blood pressure consistently<br />higher than the normal range is called hypertension<br />(see also Box 13–4: Hypertension). A lower<br />than normal blood pressure is called hypotension.<br />The regulation of systemic blood pressure is discussed<br />in a later section.<br />Pulmonary blood pressure is created by the right<br />ventricle, which has relatively thin walls and thus exerts<br />about one-sixth the force of the left ventricle. The<br />result is that pulmonary arterial pressure is always low:<br />20 to 25/8 to 10 mmHg, and in pulmonary capillaries<br />is lower still. This is important to prevent filtration in<br />pulmonary capillaries, which in turn prevents tissue<br />fluid from accumulating in the alveoli of the lungs.<br />MAINTENANCE OF SYSTEMIC<br />BLOOD PRESSURE<br />Because blood pressure is so important, many physiological<br />factors and processes interact to keep blood<br />pressure within normal limits:<br />1. Venous return—the amount of blood that returns<br />to the heart by way of the veins. Venous return<br />is important because the heart can pump only<br />the blood it receives. If venous return decreases,<br />the cardiac muscle fibers will not be stretched, the<br />force of ventricular systole will decrease (Starling’s<br />law), and blood pressure will decrease. This is what<br />might happen following a severe hemorrhage.<br />When the body is horizontal, venous return can<br />be maintained fairly easily, but when the body is<br />The Vascular System 307<br />vertical, gravity must be overcome to return blood<br />from the lower body to the heart. Three mechanisms<br />help promote venous return: constriction of<br />veins, the skeletal muscle pump, and the respiratory<br />pump.<br />Veins contain smooth muscle, which enables<br />them to constrict and force blood toward the heart;<br />the valves prevent backflow of blood. The second<br />mechanism is the skeletal muscle pump, which is<br />especially effective for the deep veins of the legs.<br />These veins are surrounded by skeletal muscles<br />that contract and relax during normal activities<br />such as walking. Contractions of the leg muscles<br />squeeze the veins to force blood toward the heart.<br />The third mechanism is the respiratory pump,<br />which affects veins that pass through the chest cavity.<br />The pressure changes of inhalation and exhalation<br />alternately expand and compress the veins, and<br />blood is returned to the heart.<br />2. Heart rate and force—in general, if heart rate and<br />force increase, blood pressure increases; this is<br />what happens during exercise. However, if the<br />heart is beating extremely rapidly, the ventricles<br />may not fill completely between beats, and cardiac<br />output and blood pressure will decrease.<br />3. Peripheral resistance—this term refers to the<br />resistance the vessels offer to the flow of blood.<br />The arteries and veins are usually slightly constricted,<br />which maintains normal diastolic blood<br />pressure. It may be helpful to think of the vessels<br />as the “container” for the blood. If a person’s body<br />has 5 liters of blood, the “container” must be<br />smaller in order for the blood to exert a pressure<br />against its walls. This is what normal vasoconstriction<br />does: It makes the container (the vessels)<br />smaller than the volume of blood so that the blood<br />will exert pressure even when the left ventricle is<br />relaxed.<br />308 The Vascular System<br />BOX 13–4 HYPERTENSION<br />trophy. This abnormal growth of the myocardium,<br />however, is not accompanied by a corresponding<br />growth in coronary capillaries, and the blood supply<br />of the left ventricle may not be adequate for all situations.<br />Exercise, for example, puts further<br />demands on the heart, and the person may experience<br />angina due to a lack of oxygen or a myocardial<br />infarction if there is a severe oxygen deficiency.<br />Although several different kinds of medications<br />(diuretics, vasodilators) are used to treat hypertension,<br />people with moderate hypertension may limit<br />their dependence on medications by following certain<br />guidelines:<br />1. Don’t smoke, because nicotine stimulates vasoconstriction,<br />which raises BP. Smoking also damages<br />arteries, contributing to arteriosclerosis.<br />2. Lose weight if overweight. A weight loss of as little<br />as 10 pounds can lower BP. A diet high in<br />fruits and vegetables may, for some people, contribute<br />to lower BP.<br />3. Cut salt intake in half. Although salt consumption<br />may not be the cause of hypertension,<br />reducing salt intake may help lower blood pressure<br />by decreasing blood volume.<br />4. Exercise on a regular basis. A moderate amount<br />of aerobic exercise (such as a half hour walk<br />every day) is beneficial for the entire cardiovascular<br />system and may also contribute to weight<br />loss.<br />Hypertension is high blood pressure, that is, a resting<br />systemic pressure consistently above the normal<br />range (90 to 120/60 to 80 mmHg). Clinicians now<br />consider 125 to 139/85 to 89 mmHg to be prehypertension.<br />A systolic reading of 140 to 159 mmHg<br />or a diastolic reading of 90 to 99 mmHg may be<br />called stage 1 hypertension, and a systolic reading<br />above 160 mmHg or a diastolic reading above 100<br />mmHg may be called stage 2 hypertension.<br />The term “essential hypertension” means that no<br />specific cause can be determined; most cases are in<br />this category. For some people, however, an overproduction<br />of renin by the kidneys is the cause<br />of their hypertension. Excess renin increases the<br />production of angiotensin II, which raises blood<br />pressure. Although hypertension often produces no<br />symptoms, the long-term consequences may be<br />very serious. Chronic hypertension has its greatest<br />effects on the arteries and on the heart.<br />Although the walls of arteries are strong, hypertension<br />weakens them and contributes to arteriosclerosis.<br />Such weakened arteries may rupture or<br />develop aneurysms, which may in turn lead to a<br />CVA or kidney damage.<br />Hypertension affects the heart because the left<br />ventricle must now pump blood against the higher<br />arterial pressure. The left ventricle works harder<br />and, like any other muscle, enlarges as more work is<br />demanded; this is called left ventricular hyper-<br />If more vasoconstriction occurs, blood pressure<br />will increase (the container has become even<br />smaller). This is what happens in a stress situation,<br />when greater vasoconstriction is brought about<br />by sympathetic impulses. If vasodilation occurs,<br />blood pressure will decrease (the container is<br />larger). After eating a large meal, for example, there<br />is extensive vasodilation in the digestive tract to<br />supply more oxygenated blood for digestive activities.<br />To keep blood pressure within the normal<br />range, vasoconstriction must, and does, occur elsewhere<br />in the body. This is why strenuous exercise<br />should be avoided right after eating; there is not<br />enough blood to completely supply oxygen to exercising<br />muscles and an active digestive tract at the<br />same time.<br />4. Elasticity of the large arteries—when the left<br />ventricle contracts, the blood that enters the large<br />arteries stretches their walls. The arterial walls are<br />elastic and absorb some of the force. When the left<br />ventricle relaxes, the arterial walls recoil or snap<br />back, which helps keep diastolic pressure within<br />the normal range. Normal elasticity, therefore,<br />lowers systolic pressure, raises diastolic pressure,<br />and maintains a normal pulse pressure. (Pulse pressure<br />is the difference between systolic and diastolic<br />pressure. The usual ratio of systolic to diastolic to<br />pulse pressure is approximately 3:2:1. For example,<br />with a blood pressure of 120/80 mmHg, the pulse<br />pressure is 40, and the ratio is 120:80:40, or 3:2:1.)<br />5. Viscosity of the blood—normal blood viscositydepends upon the presence of red blood cells and<br />plasma proteins, especially albumin. Having too<br />many red blood cells is rare but does occur in the<br />disorder called polycythemia vera and in people<br />who are heavy smokers. This will increase blood<br />viscosity and blood pressure.<br />A decreased number of red blood cells, as is seen<br />with severe anemia, or decreased albumin, as may<br />occur in liver disease or kidney disease, will<br />decrease blood viscosity and blood pressure. In<br />these situations, other mechanisms such as vasoconstriction<br />will maintain blood pressure as close to<br />normal as is possible.<br />6. Loss of blood—a small loss of blood, as when<br />donating a pint of blood, will cause a temporary<br />drop in blood pressure followed by rapid compensation<br />in the form of a more rapid heart rate and<br />greater vasoconstriction. After a severe hemorrhage,<br />however, these compensating mechanisms<br />may not be sufficient to maintain normal blood<br />pressure and blood flow to the brain. Although a<br />person may survive loss of 50% of the body’s total<br />blood, the possibility of brain damage increases as<br />more blood is lost and not rapidly replaced.<br />7. Hormones—several hormones have effects on<br />blood pressure. You may recall them from Chapters<br />10 and 12, but we will summarize them here and in<br />Fig. 13–10. The adrenal medulla secretes norepinephrine<br />and epinephrine in stress situations.<br />Norepinephrine stimulates vasoconstriction, which<br />raises blood pressure. Epinephrine also causes vasoconstriction,<br />and increases heart rate and force of<br />contraction, both of which increase blood pressure.<br />Antidiuretic hormone (ADH) is secreted by the<br />posterior pituitary gland when the water content of<br />the body decreases. ADH increases the reabsorption<br />of water by the kidneys to prevent further loss<br />of water in urine and a further decrease in blood<br />pressure.<br />Aldosterone, a hormone from the adrenal cortex,<br />has a similar effect on blood volume. When<br />blood pressure decreases, secretion of aldosterone<br />stimulates the reabsorption of Na ions by the kidneys.<br />Water follows sodium back to the blood,<br />which maintains blood volume to prevent a further<br />drop in blood pressure.<br />Atrial natriuretic peptide (ANP), secreted by the<br />atria of the heart, functions in opposition to aldosterone.<br />ANP increases the excretion of Na ions<br />and water by the kidneys, which decreases blood<br />volume and lowers blood pressure.<br />DISTRIBUTION OF BLOOD FLOW<br />An individual’s blood volume remains relatively constant<br />within the normal range appropriate to the size<br />of the person. Active tissues, however, require more<br />blood, that is, more oxygen, than do less active tissues.<br />As active tissues and organs receive a greater proportion<br />of the total blood flow, less active organs must<br />receive less, or blood pressure will decrease markedly.<br />As mentioned previously, precapillary sphincters<br />dilate in active tissues and constrict in less active ones.<br />The arterioles also constrict to reduce blood flow to<br />less active organs. This ensures that metabolically<br />active organs will receive enough oxygen to function<br />properly and that blood pressure for the body as a<br />whole will be maintained within normal limits.<br />An example will be helpful here; let us use the<br />body at rest and the body during exercise. Consult<br />Fig. 13–11 as you read the following. Resting cardiac<br />The Vascular System 309<br />output is approximately 5000 mL per minute. Exercise<br />cardiac output is three times that, about 15,000 mL<br />per minute. Keep in mind that the volume of blood is<br />the same in both cases, but that during exercise the<br />blood is being circulated more rapidly.<br />Compare the amounts of blood flowing to various<br />organs and tissues during exercise and at rest. During<br />exercise, the heart receives about three times as much<br />blood as it does when the body is at rest. The very<br />active skeletal muscles receive about ten times as much<br />blood. The skin, as an organ of heat loss, receives<br />about four times as much blood. Other organs, however,<br />can function adequately with less blood. Blood<br />flow is reduced to the digestive tract, to the kidneys,<br />and to other parts of the body such as bones.<br />When the exercise ceases, cardiac output will<br />gradually return to the resting level, as will blood flow<br />to the various organs. These changes in the distribution<br />of blood ensure sufficient oxygen for active tissues<br />and an appropriate blood pressure for the body as a<br />whole.<br />REGULATION OF BLOOD PRESSURE<br />The mechanisms that regulate systemic blood pressure<br />may be divided into two types: intrinsic mechanisms<br />and nervous mechanisms. The nervous mechanisms<br />involve the nervous system, and the intrinsic mechanisms<br />do not require nerve impulses.<br />INTRINSIC MECHANISMS<br />The term intrinsic means “within.” Intrinsic mechanisms<br />work because of the internal characteristics of<br />certain organs. The first such organ is the heart. When<br />310 The Vascular System<br />Adrenal gland<br />Pituitary<br />Heart<br />Norepinephrine<br />Epinephrine<br />Aldosterone<br />ADH<br />ANP<br />Increases<br />excretion of<br />Na+,<br />water follows<br />Kidney<br />Increases<br />reabsorption<br />of H2O<br />Increases<br />reabsorption of<br />Na+,<br />water follows<br />Increases rate<br />and force of<br />contraction<br />Vasoconstriction<br />Raises<br />B.P.<br />Lowers<br />B.P.<br />Figure 13–10. Hormones that affect blood pressure. See text for further description.<br />QUESTION: Which two hormones have opposite functions, and what are these functions?<br />venous return increases, cardiac muscle fibers are<br />stretched, and the ventricles pump more forcefully<br />(Starling’s law). Thus, cardiac output and blood pressure<br />increase. This is what happens during exercise,<br />when a higher blood pressure is needed. When exercise<br />ends and venous return decreases, the heart pumps<br />less forcefully, which helps return blood pressure to a<br />normal resting level.<br />The second intrinsic mechanism involves the kidneys.<br />When blood flow through the kidneys decreases,<br />the process of filtration decreases and less urine is<br />formed. This decrease in urinary output preserves<br />blood volume so that it does not decrease further.<br />Following severe hemorrhage or any other type of<br />dehydration, this is very important to maintain blood<br />pressure.<br />The kidneys are also involved in the reninangiotensin<br />mechanism. When blood pressure<br />decreases, the kidneys secrete the enzyme renin,<br />which initiates a series of reactions that result in the<br />formation of angiotensin II. These reactions are<br />described in Table 13–3 and depicted in Fig. 13–12.<br />Angiotensin II causes vasoconstriction and stimulates<br />secretion of aldosterone by the adrenal cortex, both of<br />which will increase blood pressure.<br />NERVOUS MECHANISMS<br />The medulla and the autonomic nervous system are<br />directly involved in the regulation of blood pressure.<br />The first of these nervous mechanisms concerns the<br />heart; this was described previously, so we will not<br />review it here but refer you to Chapter 12 and Fig.<br />12–7, as well as Fig. 13–13.<br />The Vascular System 311<br />Skeletal muscle<br />Blood distribution (mL/min)<br />Resting cardiac output<br />5,000 mL/min<br />Exercise cardiac output<br />15,000 mL/min<br />Heart Brain Skin GI tract Kidneys Rest of body<br />Resting<br />215 mL<br />% of<br />total<br />% of<br />total<br />Exercise<br />1,035 mL 645 mL 430 mL 1,205 mL 950 mL 515 mL<br />645 mL<br />10,710 mL 645 mL<br />1,635 mL<br />510 mL<br />510 mL<br />330 mL<br />4% 21% 13% 9% 24% 19% 10%<br />4.5% 71% 4.5% 11% 3.5% 3.5% 2%<br />Figure 13–11. Blood flow through various organs when the body is at rest and during<br />exercise. For each organ, the percentage of the total blood flow is given.<br />QUESTION: During exercise, which organs have the greatest increase in blood flow?<br />Which organs have the greatest decrease?<br />The second nervous mechanism involves peripheral<br />resistance, that is, the degree of constriction of the<br />arteries and arterioles and, to a lesser extent, the veins<br />(see Fig. 13–13). The medulla contains the vasomotor<br />center, which consists of a vasoconstrictor area and a<br />vasodilator area. The vasodilator area may depress the<br />vasoconstrictor area to bring about vasodilation,<br />which will decrease blood pressure. The vasoconstrictor<br />area may bring about more vasoconstriction by<br />way of the sympathetic division of the autonomic<br />nervous system.<br />Sympathetic vasoconstrictor fibers innervate the<br />smooth muscle of all arteries and veins, and several<br />impulses per second along these fibers maintain normal<br />vasoconstriction. More impulses per second bring<br />about greater vasoconstriction, and fewer impulses per<br />second cause vasodilation. The medulla receives the<br />information to make such changes from the pressoreceptors<br />in the carotid sinuses and the aortic sinus.<br />The inability to maintain normal blood pressure is one<br />aspect of circulatory shock (see Box 13–5: Circulatory<br />Shock).<br />312 The Vascular System<br />ANGIOTENSINOGEN<br />Liver<br />Lung and<br />vascular endothelium<br />CONVERTING ENZYME<br />ANGIOTENSIN I ANGIOTENSIN II<br />Vasoconstriction<br />Systemic<br />arteries<br />Increased<br />B. P.<br />Increased Na+ and<br />H2O reabsorption<br />ALDOSTERONE<br />Adrenal cortex<br />RENIN<br />Decreased<br />B. P.<br />Kidney<br />Figure 13–12. The renin-angiotensin mechanism. Begin at “Decreased B. P.” and see<br />Table 13–3 for numbered steps.<br />QUESTION: Where is renin produced? What are the functions of angiotensin II?<br />Table 13–3 THE RENIN-ANGIOTENSIN<br />MECHANISM<br />1. Decreased blood pressure stimulates the kidneys to<br />secrete renin.<br />2. Renin splits the plasma protein angiotensinogen (synthesized<br />by the liver) to angiotensin I.<br />3. Angiotensin I is converted to angiotensin II by an<br />enzyme (called converting enzyme) secreted by lung<br />tissue and vascular endothelium.<br />4. Angiotensin II:<br />• causes vasoconstriction<br />• stimulates the adrenal cortex to secrete aldosterone<br />313<br />Inhibit<br />vasomotor<br />center<br />Vasodilation<br />decreases<br />peripheral<br />resistance<br />Heart rate<br />slows,<br />decreases<br />cardiac output<br />B. P.<br />decreases to<br />normal range<br />B. P.<br />decreases<br />further<br />Stimulate<br />cardioinhibitory<br />center<br />Pressoreceptors in<br />carotid and aortic<br />sinuses stimulated<br />B. P.<br />increases<br />further<br />B. P.<br />increases to<br />normal<br />range<br />Vasoconstriction<br />increases<br />peripheral<br />resistance<br />Heart rate<br />and cardiac<br />output<br />increase<br />Stimulate<br />vasomotor<br />center<br />Stimulate<br />cardioaccelerator<br />center<br />Pressoreceptors in<br />carotid and aortic<br />sinuses inhibited<br />Figure 13–13. Nervous mechanisms that regulate blood pressure. See text for<br />description.<br />QUESTION: What kind of sensory information is used to make changes in BP, and<br />where are the receptors located?<br />AGING AND THE<br />VASCULAR SYSTEM<br />It is believed that the aging of blood vessels, especially<br />arteries, begins in childhood, although the effects are<br />not apparent for decades. The cholesterol deposits<br />of atherosclerosis are to be expected with advancing<br />age, with the most serious consequences in the coronary<br />arteries. A certain degree of arteriosclerosis is to<br />be expected, and average resting blood pressure may<br />increase, which further damages arterial walls.<br />Consequences include stroke and left-sided heart<br />failure.<br />The veins also deteriorate with age; their thin walls<br />weaken and stretch, making their valves incompetent.<br />This is most likely to occur in the veins of the legs;<br />their walls are subject to great pressure as blood is<br />returned to the heart against the force of gravity.<br />Varicose veins and phlebitis are more likely to occur<br />among elderly people.<br />SUMMARY<br />Although the vascular system does form passageways<br />for the blood, you can readily see that the blood vessels<br />are not simply pipes through which the blood<br />flows. The vessels are not passive tubes, but rather<br />active contributors to homeostasis. The arteries and<br />veins help maintain blood pressure, and the capillaries<br />provide sites for the exchanges of materials between<br />the blood and the tissues. Some very important sites of<br />exchange are discussed in the following chapters: the<br />lungs, the digestive tract, and the kidneys.<br />314 The Vascular System<br />BOX 13–5 CIRCULATORY SHOCK<br />Stages of Shock<br />Compensated shock—the responses by the body<br />maintain cardiac output. Following a small hemorrhage,<br />for example, the heart rate increases,<br />the blood vessels constrict, and the kidneys<br />decrease urinary output to conserve water. These<br />responses help preserve blood volume and maintain<br />blood pressure, cardiac output, and blood flow<br />to tissues.<br />Progressive shock—the state of shock leads to<br />more shock. Following a severe hemorrhage, cardiac<br />output decreases and the myocardium itself is<br />deprived of blood. The heart weakens, which further<br />decreases cardiac output. Arteries that are<br />deprived of their blood supply cannot remain constricted.<br />As the arteries dilate, venous return<br />decreases, which in turn decreases cardiac output.<br />Progressive shock is a series of such vicious cycles,<br />and medical intervention is required to restore cardiac<br />output to normal.<br />Irreversible shock—no amount of medical assistance<br />can restore cardiac output to normal. The<br />usual cause of death is that the heart has been damaged<br />too much to recover. A severe myocardial<br />infarction, massive hemorrhage, or septicemia may<br />all be fatal despite medical treatment.<br />Circulatory shock is any condition in which cardiac<br />output decreases to the extent that tissues are<br />deprived of oxygen and waste products accumulate.<br />Causes of Shock<br />Cardiogenic shock occurs most often after a<br />severe myocardial infarction but may also be the<br />result of ventricular fibrillation. In either case, the<br />heart is no longer an efficient pump, and cardiac<br />output decreases.<br />Hypovolemic shock is the result of decreased<br />blood volume, often due to severe hemorrhage.<br />Other possible causes are extreme sweating (heat<br />stroke) or extreme loss of water through the kidneys<br />(diuresis) or intestines (diarrhea). In these<br />situations, the heart simply does not have enough<br />blood to pump, and cardiac output decreases.<br />Anaphylactic shock, also in this category, is a massive<br />allergic reaction in which great amounts of<br />histamine increase capillary permeability and<br />vasodilation throughout the body. Much plasma is<br />then lost to tissue spaces, which decreases blood<br />volume, blood pressure, and cardiac output.<br />Septic shock is the result of septicemia, the<br />presence of bacteria in the blood. The bacteria and<br />damaged tissues release inflammatory chemicals<br />that cause vasodilation and extensive loss of plasma<br />into tissue spaces.<br />The vascular system consists of the arteries,<br />capillaries, and veins through which blood<br />travels.<br />Arteries (and arterioles) (see Fig. 13–1)<br />1. Carry blood from the heart to capillaries; three layers<br />in their walls.<br />2. Inner layer (tunica intima): simple squamous<br />epithelial tissue (endothelium), very smooth to prevent<br />abnormal blood clotting; secretes nitric oxide<br />(NO), a vasodilator.<br />3. Middle layer (tunica media): smooth muscle and<br />elastic connective tissue; contributes to maintenance<br />of diastolic blood pressure (BP).<br />4. Outer layer (tunica externa): fibrous connective tissue<br />to prevent rupture.<br />5. Constriction or dilation is regulated by the autonomic<br />nervous system.<br />Veins (and venules) (see Fig. 13–1)<br />1. Carry blood from capillaries to the heart; three layers<br />in walls.<br />2. Inner layer: endothelium folded into valves to prevent<br />the backflow of blood.<br />3. Middle layer: thin smooth muscle, because veins<br />are not as important in the maintenance of BP.<br />4. Outer layer: thin fibrous connective tissue because<br />veins do not carry blood under high pressure.<br />Anastomoses—connections between vessels<br />of the same type<br />1. Provide alternate pathways for blood flow if one<br />vessel is blocked.<br />2. Arterial anastomoses provide for blood flow to the<br />capillaries of an organ (e.g., circle of Willis to the<br />brain).<br />3. Venous anastomoses provide for return of blood to<br />the heart and are most numerous in veins of the<br />legs.<br />Capillaries (see Figs. 13–1 and 13–2)<br />1. Carry blood from arterioles to venules.<br />2. Walls are one cell thick (simple squamous epithelial<br />tissue) to permit exchanges between blood and tissue<br />fluid.<br />3. Oxygen and carbon dioxide are exchanged by<br />diffusion.<br />4. BP in capillaries brings nutrients to tissues and<br />forms tissue fluid in the process of filtration.<br />5. Albumin in the blood provides colloid osmotic<br />pressure, which pulls waste products and tissue fluid<br />into capillaries. The return of tissue fluid maintains<br />blood volume and BP.<br />6. Precapillary sphincters regulate blood flow into<br />capillary networks based on tissue needs; in active<br />tissues they dilate; in less active tissues they constrict.<br />7. Sinusoids are very permeable capillaries found in<br />the liver, spleen, pituitary gland, and red bone marrow<br />to permit proteins and blood cells to enter or<br />leave the blood.<br />Pathways of Circulation<br />1. Pulmonary: Right ventricle →pulmonary artery →<br />pulmonary capillaries (exchange of gases) → pulmonary<br />veins → left atrium.<br />2. Systemic: left ventricle → aorta → capillaries in<br />body tissues → superior and inferior caval veins →<br />right atrium (see Table 13–1 and Fig. 13–3 for systemic<br />arteries and Table 13–2 and Fig. 13–4 for systemic<br />veins).<br />3. Hepatic portal circulation: blood from the digestive<br />organs and spleen flows through the portal<br />vein to the liver before returning to the heart.<br />Purpose: the liver stores some nutrients or regulates<br />their blood levels and detoxifies potential poisons<br />before blood enters the rest of peripheral<br />circulation (see Fig. 13–7).<br />Fetal Circulation—the fetus depends on the<br />mother for oxygen and nutrients and for the<br />removal of waste products (see Fig. 13–8)<br />1. The placenta is the site of exchange between fetal<br />blood and maternal blood.<br />2. Umbilical arteries (two) carry blood from the fetus<br />to the placenta, where CO2 and waste products<br />enter maternal circulation.<br />3. The umbilical vein carries blood with O2 and nutrients<br />from the placenta to the fetus.<br />4. The umbilical vein branches; some blood<br />flows through the fetal liver; most blood flows<br />through the ductus venosus to the fetal inferior<br />vena cava.<br />5. The foramen ovale permits blood to flow from the<br />The Vascular System 315<br />STUDY OUTLINE<br />right atrium to the left atrium to bypass the fetal<br />lungs.<br />6. The ductus arteriosus permits blood to flow from<br />the pulmonary artery to the aorta to bypass the<br />fetal lungs.<br />7. These fetal structures become nonfunctional after<br />birth, when the umbilical cord is cut and breathing<br />takes place.<br />Velocity of Blood Flow (see Fig. 13–9)<br />1. Velocity is inversely related to the cross-sectional<br />area of a segment of the vascular system.<br />2. The total capillaries have the greatest crosssectional<br />area and slowest blood flow.<br />3. Slow flow in the capillaries is important to permit<br />sufficient time for exchange of gases, nutrients, and<br />wastes.<br />Blood Pressure (BP)—the force exerted by<br />the blood against the walls of the blood vessels<br />(Fig. 13–9)<br />1. BP is measured in mmHg: systolic/diastolic.<br />Systolic pressure occurs during ventricular contraction;<br />diastolic pressure occurs during ventricular<br />relaxation.<br />2. Normal range of systemic arterial BP: 90 to 120/60<br />to 80 mmHg.<br />3. BP in capillaries is 30 to 35 mmHg at the arterial<br />end and 12 to 15 mmHg at the venous end—high<br />enough to permit filtration but low enough to prevent<br />rupture of the capillaries.<br />4. BP decreases in the veins and approaches zero in<br />the caval veins.<br />5. Pulmonary BP is always low (the right ventricle<br />pumps with less force): 20 to 25/8 to 10 mmHg.<br />This low BP prevents filtration and accumulation<br />of tissue fluid in the alveoli.<br />Maintenance of Systemic BP<br />1. Venous return—the amount of blood that returns<br />to the heart. If venous return decreases, the heart<br />contracts less forcefully (Starling’s law) and BP<br />decreases. The mechanisms that maintain venous<br />return when the body is vertical are:<br />• Constriction of veins with the valves preventing<br />backflow of blood<br />• Skeletal muscle pump—contraction of skeletal<br />muscles, especially in the legs, squeezes the deep<br />veins<br />• Respiratory pump—the pressure changes of<br />inhalation and exhalation expand and compress<br />the veins in the chest cavity<br />2. Heart rate and force—if heart rate and force<br />increase, BP increases.<br />3. Peripheral resistance—the resistance of the arteries<br />and arterioles to the flow of blood. These vessels<br />are usually slightly constricted to maintain normal<br />diastolic BP. Greater vasoconstriction will increase<br />BP; vasodilation will decrease BP. In the body,<br />vasodilation in one area requires vasoconstriction<br />in another area to maintain normal BP.<br />4. Elasticity of the large arteries—ventricular systole<br />stretches the walls of large arteries, which recoil<br />during ventricular diastole. Normal elasticity lowers<br />systolic BP, raises diastolic BP, and maintains<br />normal pulse pressure.<br />5. Viscosity of blood—depends on RBCs and plasma<br />proteins, especially albumin. Severe anemia tends<br />to decrease BP. Deficiency of albumin as in liver or<br />kidney disease tends to decrease BP. In these cases,<br />compensation such as greater vasoconstriction will<br />keep BP close to normal.<br />6. Loss of blood—a small loss will be rapidly compensated<br />for by faster heart rate and greater vasoconstriction.<br />After severe hemorrhage, these<br />mechanisms may not be sufficient to maintain normal<br />BP.<br />7. Hormones—(see Fig. 13–10) (a) Norepinephrine<br />stimulates vasoconstriction, which raises BP; (b)<br />epinephrine increases cardiac output and raises BP;<br />(c) ADH increases water reabsorption by the kidneys,<br />which increases blood volume and BP; (d)<br />aldosterone increases reabsorption of Na ions by<br />the kidneys; water follows Na and increases blood<br />volume and BP; (e) ANP increases excretion of<br />Na ions and water by the kidneys, which decreases<br />blood volume and BP.<br />Distribution of Blood Flow<br />1. Metabolically active tissues require more oxygen,<br />and receive a greater proportion of the blood volume<br />as it circulates (see Fig. 13–11).<br />2. Blood flow is increased by the dilation of arterioles<br />and precapillary sphincters.<br />3. In less active tissues, arterioles and precapillary<br />sphincters constrict.<br />4. Organs receive sufficient oxygen, and BP for the<br />body is maintained within the normal range.<br />316 The Vascular System<br />Regulation of Blood Pressure—intrinsic<br />mechanisms and nervous mechanisms<br />Intrinsic Mechanisms<br />1. The heart—responds to increased venous return by<br />pumping more forcefully (Starling’s law), which<br />increases cardiac output and BP.<br />2. The kidneys—decreased blood flow decreases<br />filtration, which decreases urinary output to preserve<br />blood volume. Decreased BP stimulates the<br />kidneys to secrete renin, which initiates the reninangiotensin<br />mechanism (Table 13–3 and Fig.<br />13–12) that results in the formation of angiotensin<br />II, which causes vasoconstriction and stimulates<br />secretion of aldosterone.<br />Nervous Mechanisms (see Fig. 13–13)<br />1. Heart rate and force—see also Chapter 12.<br />2. Peripheral resistance—the medulla contains the<br />vasomotor center, which consists of a vasoconstrictor<br />area and a vasodilator area. The vasodilator area<br />brings about vasodilation by suppressing the vasoconstrictor<br />area. The vasoconstrictor area maintains<br />normal vasoconstriction by generating several<br />impulses per second along sympathetic vasoconstrictor<br />fibers to all arteries and veins. More<br />impulses per second increase vasoconstriction and<br />raise BP; fewer impulses per second bring about<br />vasodilation and a drop in BP.<br />The Vascular System 317<br />REVIEW QUESTIONS<br />1. Describe the structure of the three layers of the<br />walls of arteries, and state the function of each<br />layer. Describe the structural differences in these<br />layers in veins, and explain the reason for each difference.<br />(p. 292)<br />2. Describe the structure and purpose of anastomoses,<br />and give a specific example. (pp. 292–293)<br />3. Describe the structure of capillaries. State the<br />process by which each of the following is<br />exchanged between capillaries and tissue fluid:<br />nutrients, oxygen, waste products, and carbon<br />dioxide. (pp. 293–295)<br />4. State the part of the body supplied by each of the<br />following arteries: (pp. 297, 300)<br />a. Bronchial<br />b. Femoral<br />c. Hepatic<br />d. Brachial<br />e. Inferior mesenteric<br />f. Internal carotid<br />g. Subclavian<br />h. Intercostal<br />5. Describe the pathway of blood flow in hepatic portal<br />circulation. Use a specific example to explain the<br />purpose of portal circulation. (pp. 301, 304)<br />6. Begin at the right ventricle and describe the pathway<br />of pulmonary circulation. Explain the purpose<br />of this pathway. (p. 296)<br />7. Name the fetal structure with each of the following<br />functions: (pp. 303–305)<br />a. Permits blood to flow from the right atrium to<br />the left atrium<br />b. Carries blood from the placenta to the fetus<br />c. Permits blood to flow from the pulmonary<br />artery to the aorta<br />d. Carry blood from the fetus to the placenta<br />e. Carries blood from the umbilical vein to the<br />inferior vena cava<br />8. Describe the three mechanisms that promote<br />venous return when the body is vertical.<br />(pp. 307–308)<br />9. Explain how the normal elasticity of the large<br />arteries affects both systolic and diastolic blood<br />pressure. (p. 309)<br />10. Explain how Starling’s law of the heart is involved<br />in the maintenance of blood pressure. (p. 307)<br />11. Name two hormones involved in the maintenance<br />of blood pressure, and state the function of each.<br />(p. 309)<br />12. Describe two different ways the kidneys respond<br />to decreased blood flow and blood pressure.<br />(p. 311)<br />13. State two compensations that will maintain blood<br />pressure after a small loss of blood. (p. 309)<br />14. State the location of the vasomotor center and<br />name its two parts. Name the division of the<br />autonomic nervous system that carries impulses<br />to blood vessels. Which blood vessels? Which<br />tissue in these vessels? Explain why normal<br />vasoconstriction is important. Explain how<br />greater vasoconstriction is brought about. Explain<br />how vasodilation is brought about. How will<br />each of these changes affect blood pressure?<br />(pp. 312–313)<br />318 The Vascular System<br />FOR FURTHER THOUGHT<br />1. Some old textbooks used the term descending aorta.<br />Explain what is meant by that, and why it is not a<br />very good term. Explain why an aneurysm of the<br />aorta is quite likely to rupture sooner or later.<br />2. Renee, a nurse, is first on the scene of a car accident.<br />The driver has been thrown from the car, and<br />even from 15 feet away, Renee knows that a large<br />artery in the man’s leg has been severed. How does<br />she know this? What two things does she see?<br />Renee stops the bleeding, but the ambulance has<br />not arrived. She wants to assess the man’s condition<br />after he has lost so much blood. She cannot take a<br />blood pressure, but what other vital sign can be<br />helpful? Explain. If Renee could take a blood pressure<br />reading, what might it be? Might it be within<br />the normal range? Explain.<br />3. A friend tells you that her grandmother has a tendency<br />to develop blood clots in the veins of her<br />legs. Your friend fears that her grandmother will<br />have a stroke as a result. How would you explain<br />that a stroke from a clot there is not likely? Because<br />you are a good friend, you want to explain the serious<br />result that may occur. How would you do that?<br />4. Some people with hypertension take prescribed<br />diuretics. Some call these “water pills.” Is this an<br />accurate name? How can a diuretic help lower<br />blood pressure? What disadvantage does the use of<br />diuretics have?<br />5. Sinusoids are found in the liver and pituitary gland.<br />For each of these organs, name four specific large<br />molecules that enter the blood by way of sinusoids.<br />CHAPTER 14<br />The Lymphatic System<br />and Immunity<br />319<br />320internet fast worldhttp://www.blogger.com/profile/13869077830569899582noreply@blogger.com0tag:blogger.com,1999:blog-135611804747902727.post-5076407649569735012010-06-27T06:54:00.002-07:002010-06-27T07:40:06.703-07:00nevrvous system & musculrCHAPTER 7<br />The Muscular System<br />135<br />136<br />CHAPTER 7<br />Chapter Outline<br />Muscle Structure<br />Muscle Arrangements<br />Antagonistic muscles<br />Synergistic muscles<br />The Role of the Brain<br />Muscle Tone<br />Exercise<br />Muscle Sense<br />Energy Sources for Muscle Contraction<br />Muscle Fiber—Microscopic Structure<br />Sarcolemma—Polarization<br />Sarcolemma—Depolarization<br />Contraction—The Sliding Filament Mechanism<br />Responses to Exercise—Maintaining<br />Homeostasis<br />Aging and the Muscular System<br />Major Muscles of the Body<br />Muscles of the Head and Neck<br />Muscles of the Trunk<br />Muscles of the Shoulder and Arm<br />Muscles of the Hip and Leg<br />BOX 7–1 ANABOLIC STEROIDS<br />BOX 7–2 TETANUS AND BOTULISM<br />BOX 7–3 MUSCULAR DYSTROPHY<br />BOX 7–4 MYASTHENIA GRAVIS<br />BOX 7–5 COMMON INJECTION SITES<br />Student Objectives<br />• Name the organ systems directly involved in<br />movement, and state how they are involved.<br />• Describe muscle structure in terms of muscle cells,<br />tendons, and bones.<br />• Describe the difference between antagonistic and<br />synergistic muscles, and explain why such arrangements<br />are necessary.<br />• Explain the role of the brain with respect to skeletal<br />muscle.<br />• Define muscle tone and explain its importance.<br />• Explain the difference between isotonic and isometric<br />exercise.<br />• Define muscle sense and explain its importance.<br />• Name the energy sources for muscle contraction,<br />and state the simple equation for cell respiration.<br />• Explain the importance of hemoglobin and myoglobin,<br />oxygen debt, and lactic acid.<br />• Describe the neuromuscular junction and state the<br />function of each part.<br />• Describe the structure of a sarcomere.<br />• Explain the following in terms of ions and charges:<br />polarization, depolarization, and repolarization.<br />• Describe the sliding filament mechanism of muscle<br />contraction.<br />• Describe some of the body’s responses to exercise<br />and explain how each maintains homeostasis.<br />• Learn the major muscles of the body and their<br />functions.<br />The Muscular System<br />137<br />New Terminology<br />Actin (AK-tin)<br />Antagonistic muscles (an-TAG-on-ISS-tik MUSSuhls)<br />Creatine phosphate (KREE-ah-tin FOSS-fate)<br />Depolarization (DE-poh-lahr-i-ZAY-shun)<br />Fascia (FASH-ee-ah)<br />Insertion (in-SIR-shun)<br />Isometric (EYE-so-MEH-trik)<br />Isotonic (EYE-so-TAHN-ik)<br />Lactic acid (LAK-tik ASS-id)<br />Muscle fatigue (MUSS-uhl fah-TEEG)<br />Muscle sense (MUSS-uhl SENSE)<br />Muscle tone (MUSS-uhl TONE)<br />Myoglobin (MYE-oh-GLOW-bin)<br />Myosin (MYE-oh-sin)<br />Neuromuscular junction (NYOOR-oh-MUSS-kyooler<br />JUNK-shun)<br />Origin (AHR-i-jin)<br />Oxygen debt (AHKS-ah-jen DET)<br />Polarization (POH-lahr-i-ZAY-shun)<br />Prime mover (PRIME MOO-ver)<br />Sarcolemma (SAR-koh-LEM-ah)<br />Sarcomeres (SAR-koh-meers)<br />Synergistic muscles (SIN-er-JIS-tik MUSS-uhls)<br />Tendon (TEN-dun)<br />Related Clinical Terminology<br />Anabolic steroids (an-a-BOLL-ik STEER-oyds)<br />Atrophy (AT-ruh-fee)<br />Botulism (BOTT-yoo-lizm)<br />Hypertrophy (high-PER-truh-fee)<br />Intramuscular injection (IN-trah-MUSS-kyoo-ler in-<br />JEK-shun)<br />Muscular dystrophy (MUSS-kyoo-ler DIS-truh-fee)<br />Myalgia (my-AL-jee-ah)<br />Myasthenia gravis (MY-ass-THEE-nee-yuh GRAHviss)<br />Myopathy (my-AH-puh-thee)<br />Paralysis (pah-RAL-i-sis)<br />Range-of-motion exercises (RANJE-of-MOH-shun<br />EKS-err-sigh-zez)<br />Sex-linked trait (SEX LINKED TRAYT)<br />Tetanus (TET-uh-nus)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />Do you like to dance? Most of us do, or we may<br />simply enjoy watching good dancers. The grace and<br />coordination involved in dancing result from the<br />interaction of many of the organ systems, but the one<br />you think of first is probably the muscular system.<br />There are more than 600 muscles in the human<br />body. Most of these muscles are attached to the bones<br />of the skeleton by tendons, although a few muscles<br />are attached to the undersurface of the skin. The primary<br />function of the muscular system is to move<br />the skeleton. The muscle contractions required for<br />movement also produce heat, which contributes to the<br />maintenance of a constant body temperature. The<br />other body systems directly involved in movement<br />are the nervous, respiratory, and circulatory systems.<br />The nervous system transmits the electrochemical<br />impulses that cause muscle cells to contract. The respiratory<br />system exchanges oxygen and carbon dioxide<br />between the air and blood. The circulatory system<br />brings oxygen to the muscles and takes carbon<br />dioxide away.<br />These interactions of body systems are covered in<br />this chapter, which focuses on the skeletal muscles.<br />You may recall from Chapter 4 that there are two<br />other types of muscle tissue: smooth muscle and cardiac<br />muscle. These types of muscle tissue will be discussed<br />in other chapters in relation to the organs of<br />which they are part. Before you continue, you may<br />find it helpful to go back to Chapter 4 and review the<br />structure and characteristics of skeletal muscle tissue.<br />In this chapter we will begin with the gross (large)<br />anatomy and physiology of muscles, then discuss the<br />microscopic structure of muscle cells and the biochemistry<br />of muscle contraction.<br />MUSCLE STRUCTURE<br />All muscle cells are specialized for contraction. When<br />these cells contract, they shorten and pull a bone to<br />produce movement. Each skeletal muscle is made of<br />thousands of individual muscle cells, which also may<br />be called muscle fibers (see Fig. 7–3 later in this<br />chapter). Depending on the work a muscle is required<br />to do, variable numbers of muscle fibers contract.<br />When picking up a pencil, for example, only a small<br />portion of the muscle fibers in each finger muscle will<br />contract. If the muscle has more work to do, such as<br />picking up a book, more muscle fibers will contract to<br />accomplish the task.<br />Muscles are anchored firmly to bones by tendons.<br />Most tendons are rope-like, but some are flat; a flat<br />tendon is called an aponeurosis. (See Fig. 7–9 later in<br />this chapter for the epicranial aponeurosis, but before<br />you look, decide what epicranial means.) Tendons are<br />made of fibrous connective tissue, which, you may<br />remember, is very strong and merges with the fascia<br />that covers the muscle and with the periosteum, the<br />fibrous connective tissue membrane that covers bones.<br />A muscle usually has at least two tendons, each<br />attached to a different bone. The more immobile or<br />stationary attachment of the muscle is its origin; the<br />more movable attachment is called the insertion. The<br />muscle itself crosses the joint of the two bones to<br />which it is attached, and when the muscle contracts it<br />pulls on its insertion and moves the bone in a specific<br />direction.<br />MUSCLE ARRANGEMENTS<br />Muscles are arranged around the skeleton so as to<br />bring about a variety of movements. The two general<br />types of arrangements are the opposing antagonists<br />and the cooperative synergists.<br />Antagonistic Muscles<br />Antagonists are opponents, so we use the term antagonistic<br />muscles for muscles that have opposing or<br />opposite functions. An example will be helpful here—<br />refer to Fig. 7–1 as you read the following. The biceps<br />brachii is the muscle on the front of the upper arm.<br />The origin of the biceps is on the scapula (there are<br />actually two tendons, hence the name biceps), and the<br />insertion is on the radius. When the biceps contracts,<br />it flexes the forearm, that is, bends the elbow (see<br />Table 7–2 later in this chapter). Recall that when a<br />muscle contracts, it gets shorter and pulls. Muscles<br />cannot push, for when they relax they exert no force.<br />Therefore, the biceps can bend the elbow but cannot<br />straighten it; another muscle is needed. The triceps<br />brachii is located on the back of the upper arm. Its origins<br />(the prefix tri tells you that there are three of<br />them) are on the scapula and humerus, and its insertion<br />is on the ulna. When the triceps contracts and<br />pulls, it extends the forearm, that is, straightens the<br />elbow.<br />138 The Muscular System<br />Joints that are capable of a variety of movements<br />have several sets of antagonists. Notice how many<br />ways you can move your upper arm at the shoulder, for<br />instance. Abducting (laterally raising) the arm is the<br />function of the deltoid. Adducting the arm is brought<br />about by the pectoralis major and latissimus dorsi.<br />Flexion of the arm (across the chest) is also a function<br />of the pectoralis major, and extension of the arm<br />(behind the back) is also a function of the latissimus<br />dorsi. All of these muscles are described and depicted<br />in the tables and figures later in the chapter. Without<br />antagonistic muscles, this variety of movements would<br />not be possible.<br />You may be familiar with range-of-motion (or<br />ROM) exercises that are often recommended for<br />patients confined to bed. Such exercises are designed<br />to stretch and contract the antagonistic muscles of a<br />joint to preserve as much muscle function and joint<br />mobility as possible.<br />Synergistic Muscles<br />Synergistic muscles are those with the same function,<br />or those that work together to perform a particular<br />function. Recall that the biceps brachii flexes the<br />forearm. The brachioradialis, with its origin on the<br />humerus and insertion on the radius, also flexes<br />the forearm. There is even a third flexor of the forearm,<br />the brachialis. You may wonder why we need<br />three muscles to perform the same function, and the<br />explanation lies in the great mobility of the hand. If<br />the hand is palm up, the biceps does most of the work<br />of flexing and may be called the prime mover. When<br />the hand is thumb up, the brachioradialis is in position<br />to be the prime mover, and when the hand is palm<br />down, the brachialis becomes the prime mover. If you<br />have ever tried to do chin-ups, you know that it is<br />much easier with your palms toward you than with<br />palms away from you. This is because the biceps is a<br />larger, and usually much stronger, muscle than is the<br />brachialis.<br />Muscles may also be called synergists if they help to<br />stabilize or steady a joint to make a more precise<br />movement possible. If you drink a glass of water, the<br />biceps brachii may be the prime mover to flex the<br />forearm. At the same time, the muscles of the shoulder<br />keep that joint stable, so that the water gets to<br />your mouth, not over your shoulder or down your<br />chin. The shoulder muscles are considered synergists<br />for this movement because their contribution makes<br />the movement effective.<br />THE ROLE OF THE BRAIN<br />Even our simplest movements require the interaction<br />of many muscles, and the contraction of skeletal muscles<br />depends on the brain. The nerve impulses for<br />movement come from the frontal lobes of the cerebrum.<br />The cerebrum is the largest part of the brain;<br />the frontal lobes are beneath the frontal bone. The<br />The Muscular System 139<br />Radius<br />Ulna Humerus<br />Triceps (contracted)<br />Biceps (relaxed)<br />Scapula<br />Triceps (relaxed)<br />Biceps (contracted)<br />A Extension B Flexion<br />Figure 7–1. Antagonistic muscles. (A) Extension of the forearm. (B) Flexion of the<br />forearm.<br />QUESTION: When the biceps contracts, what happens to its length, and what kind of force<br />does it exert?<br />motor areas of the frontal lobes generate electrochemical<br />impulses that travel along motor nerves to<br />muscle fibers, causing the muscle fibers to contract.<br />For a movement to be effective, some muscles must<br />contract while others relax. When walking, for example,<br />antagonistic muscles on the front and back of the<br />thigh or the lower leg will alternate their contractions<br />and relaxations, and our steps will be smooth and efficient.<br />This is what we call coordination, and we do not<br />have to think about making it happen. Coordination<br />takes place below the level of conscious thought and is<br />regulated by the cerebellum, which is located below<br />the occipital lobes of the cerebrum.<br />MUSCLE TONE<br />Except during certain stages of sleep, most of our<br />muscles are in a state of slight contraction; this is what<br />is known as muscle tone. When sitting upright, for<br />example, the tone of your neck muscles keeps your<br />head up, and the tone of your back muscles keeps your<br />back straight. This is an important function of muscle<br />tone for human beings, because it helps us to maintain<br />an upright posture. For a muscle to remain slightly<br />contracted, only a few of the muscle fibers in that<br />muscle must contract. Alternate fibers contract so that<br />the muscle as a whole does not become fatigued. This<br />is similar to a pianist continuously rippling her fingers<br />over the keys of the piano—some notes are always<br />sounding at any given moment, but the notes that are<br />sounding are always changing. This contraction of<br />alternate fibers, muscle tone, is also regulated by the<br />cerebellum of the brain.<br />Muscle fibers need the energy of ATP (adenosine<br />triphosphate) in order to contract. When they produce<br />ATP in the process of cell respiration, muscle<br />fibers also produce heat. The heat generated by normal<br />muscle tone is approximately 25% of the total<br />body heat at rest. During exercise, of course, heat production<br />increases significantly.<br />EXERCISE<br />Good muscle tone improves coordination. When<br />muscles are slightly contracted, they can react more<br />rapidly if and when greater exertion is necessary.<br />Muscles with poor tone are usually soft and flabby, but<br />exercise will improve muscle tone.<br />There are two general types of exercise: isotonic<br />and isometric. In isotonic exercise, muscles contract<br />and bring about movement. Jogging, swimming, and<br />weight lifting are examples. Isotonic exercise improves<br />muscle tone, muscle strength, and, if done repetitively<br />against great resistance (as in weight lifting), muscle<br />size. This type of exercise also improves cardiovascular<br />and respiratory efficiency, because movement<br />exerts demands on the heart and respiratory muscles.<br />If done for 30 minutes or longer, such exercise may be<br />called aerobic, because it strengthens the heart and respiratory<br />muscles as well as the muscles attached to the<br />skeleton.<br />Isotonic contractions are of two kinds, concentric<br />or eccentric. A concentric contraction is the shortening<br />of a muscle as it exerts force. An eccentric<br />contraction is the lengthening of a muscle as it still<br />exerts force. Imagine lifting a book straight up (or try<br />it); the triceps brachii contracts and shortens to<br />straighten the elbow and raise the book, a concentric<br />contraction. Now imagine slowly lowering the book.<br />The triceps brachii is still contracting even as it is<br />lengthening, exerting force to oppose gravity (which<br />would make the book drop quickly). This is an eccentric<br />contraction.<br />Isometric exercise involves contraction without<br />movement. If you put your palms together and push<br />one hand against the other, you can feel your arm<br />muscles contracting. If both hands push equally, there<br />will be no movement; this is isometric contraction.<br />Such exercises will increase muscle tone and muscle<br />strength but are not considered aerobic. When the<br />body is moving, the brain receives sensory information<br />about this movement from the joints involved,<br />and responds with reflexes that increase heart rate and<br />respiration. Without movement, the brain does not<br />get this sensory information, and heart rate and<br />breathing do not increase nearly as much as they<br />would during an equally strenuous isotonic exercise.<br />Many of our actions involve both isotonic and isometric<br />contractions. Pulling open a door requires isotonic<br />contractions of arm muscles, but if the door is<br />then held open for someone else, those contractions<br />become isometric. Picking up a pencil is isotonic;<br />holding it in your hand is isometric. Walking uphill<br />involves concentric isotonic contractions, and may be<br />quite strenuous. Walking downhill seems easier, but is<br />no less complex. The eccentric isotonic contractions<br />involved make each step a precisely aimed and con-<br />140 The Muscular System<br />trolled fall against gravity. Without such control<br />(which we do not have to think about) a downhill walk<br />would quickly become a roll. These various kinds of<br />contractions are needed for even the simplest activities.<br />(With respect to increasing muscle strength, see<br />Box 7–1: Anabolic Steroids.)<br />MUSCLE SENSE<br />When you walk up a flight of stairs, do you have to<br />look at your feet to be sure each will get to the next<br />step? Most of us don’t (an occasional stumble doesn’t<br />count), and for this freedom we can thank our<br />muscle sense. Muscle sense (proprioception) is the<br />brain’s ability to know where our muscles are and what<br />they are doing, without our having to consciously<br />look at them.<br />Within muscles are receptors called stretch receptors<br />(proprioceptors or muscle spindles). The general<br />function of all sensory receptors is to detect changes.<br />The function of stretch receptors is to detect changes<br />in the length of a muscle as it is stretched. The sensory<br />impulses generated by these receptors are interpreted<br />by the brain as a mental “picture” of where the<br />muscle is.<br />We can be aware of muscle sense if we choose to be,<br />but usually we can safely take it for granted. In fact,<br />that is what we are meant to do. Imagine what life<br />would be like if we had to watch every move to be sure<br />that a hand or foot performed its intended action.<br />Even simple activities such as walking or eating would<br />require our constant attention.<br />At times, we may become aware of our muscle<br />sense. Learning a skill such as typing or playing the<br />guitar involves very precise movements of the fingers,<br />and beginners will often watch their fingers to be sure<br />they are moving properly. With practice, however,<br />the movements simply “feel” right, which means that<br />the brain has formed a very good mental picture of the<br />task. Muscle sense again becomes unconscious, and<br />the experienced typist or guitarist need not watch<br />every movement.<br />All sensation is a function of brain activity, and<br />muscle sense is no exception. The impulses for muscle<br />sense are integrated in the parietal lobes of the cerebrum<br />(conscious muscle sense) and in the cerebellum<br />(unconscious muscle sense) to be used to promote<br />coordination.<br />ENERGY SOURCES FOR<br />MUSCLE CONTRACTION<br />Before discussing the contraction process itself, let us<br />look first at how muscle fibers obtain the energy they<br />need to contract. The direct source of energy for muscle<br />contraction is ATP. ATP, however, is not stored in<br />large amounts in muscle fibers and is depleted in a few<br />seconds.<br />The secondary energy sources are creatine phosphate<br />and glycogen. Creatine phosphate is, like<br />ATP, an energy-transferring molecule. When it is broken<br />down (by an enzyme) to creatine, phosphate, and<br />energy, the energy is used to synthesize more ATP.<br />Most of the creatine formed is used to resynthesize<br />creatine phosphate, but some is converted to creatinine,<br />a nitrogenous waste product that is excreted by<br />the kidneys.<br />The most abundant energy source in muscle fibers<br />The Muscular System 141<br />BOX 7–1 ANABOLIC STEROIDS<br />will increase muscle size, but there are hazards,<br />some of them very serious. Side effects of such<br />self-medication include liver damage, kidney damage,<br />disruption of reproductive cycles, and<br />mental changes such as irritability and aggressiveness.<br />Female athletes may develop increased growth<br />of facial and body hair and may become sterile as a<br />result of the effects of a male hormone on their own<br />hormonal cycles.<br />Anabolic steroids are synthetic drugs very similar<br />in structure and action to the male hormone testosterone.<br />Normal secretion of testosterone, beginning<br />in males at puberty, increases muscle size and<br />is the reason men usually have larger muscles than<br />do women.<br />Some athletes, both male and female, both<br />amateur and professional, take anabolic steroids to<br />build muscle mass and to increase muscle strength.<br />There is no doubt that the use of anabolic steroids<br />is glycogen. When glycogen is needed to provide<br />energy for sustained contractions (more than a few<br />seconds), it is first broken down into the glucose molecules<br />of which it is made. Glucose is then further broken<br />down in the process of cell respiration to produce<br />ATP, and muscle fibers may continue to contract.<br />Recall from Chapter 2 our simple reaction for cell<br />respiration:<br />Glucose O2 → CO2 H2O ATP heat<br />Look first at the products of this reaction. ATP will<br />be used by the muscle fibers for contraction. The heat<br />produced will contribute to body temperature, and if<br />exercise is strenuous, will increase body temperature.<br />The water becomes part of intracellular water, and the<br />carbon dioxide is a waste product that will be exhaled.<br />Now look at what is needed to release energy from<br />glucose: oxygen. Muscles have two sources of oxygen.<br />The blood delivers a continuous supply of oxygen<br />from the lungs, which is carried by the hemoglobin in<br />red blood cells. Within muscle fibers themselves there<br />is another protein called myoglobin, which stores<br />some oxygen within the muscle cells. Both hemoglobin<br />and myoglobin contain the mineral iron, which<br />enables them to bond to oxygen. (Iron also makes<br />both molecules red, and it is myoglobin that gives<br />muscle tissue a red or dark color.)<br />During strenuous exercise, the oxygen stored in<br />myoglobin is quickly used up, and normal circulation<br />may not deliver oxygen fast enough to permit the<br />completion of cell respiration. Even though the respiratory<br />rate increases, the muscle fibers may literally<br />run out of oxygen. This state is called oxygen debt,<br />and in this case, glucose cannot be completely broken<br />down into carbon dioxide and water. If oxygen is not<br />present (or not present in sufficient amounts), glucose<br />is converted to an intermediate molecule called lactic<br />acid, which causes muscle fatigue.<br />In a state of fatigue, muscle fibers cannot contract<br />efficiently, and contraction may become painful. To be<br />in oxygen debt means that we owe the body some oxygen.<br />Lactic acid from muscles enters the blood and<br />circulates to the liver, where it is converted to pyruvic<br />acid, a simple carbohydrate (three carbons, about<br />half a glucose molecule). This conversion requires<br />ATP, and oxygen is needed to produce the necessary<br />ATP in the liver. This is why, after strenuous exercise,<br />the respiratory rate and heart rate remain high for a<br />time and only gradually return to normal. Another<br />name proposed for this state is recovery oxygen<br />uptake, which is a little longer but also makes sense.<br />Oxygen uptake means a faster and deeper respiratory<br />rate. What is this uptake for? For recovery from strenuous<br />exercise.<br />MUSCLE FIBER—<br />MICROSCOPIC STRUCTURE<br />We will now look more closely at a muscle fiber, keeping<br />in mind that there are thousands of these cylindrical<br />cells in one muscle. Each muscle fiber has its own<br />motor nerve ending; the neuromuscular junction is<br />where the motor neuron terminates on the muscle<br />fiber (Fig. 7–2). The axon terminal is the enlarged tip<br />of the motor neuron; it contains sacs of the neurotransmitter<br />acetylcholine (ACh). The membrane of<br />the muscle fiber is the sarcolemma, which contains<br />receptor sites for acetylcholine, and an inactivator<br />called cholinesterase. The synapse (or synaptic cleft)<br />is the small space between the axon terminal and the<br />sarcolemma.<br />Within the muscle fiber are thousands of individual<br />contracting units called sarcomeres, which are<br />arranged end to end in cylinders called myofibrils.<br />The structure of a sarcomere is shown in Fig. 7–3:<br />The Z lines are the end boundaries of a sarcomere.<br />Filaments of the protein myosin are in the center of<br />the sarcomere, and filaments of the protein actin are<br />at the ends, attached to the Z lines. Myosin filaments<br />are anchored to the Z lines by the protein titin.<br />Myosin and actin are the contractile proteins of a<br />muscle fiber. Their interactions produce muscle contraction.<br />Also present are two inhibitory proteins, troponin<br />and tropomyosin, which are part of the actin<br />filaments and prevent the sliding of actin and myosin<br />when the muscle fiber is relaxed.<br />Surrounding the sarcomeres is the sarcoplasmic<br />reticulum, the endoplasmic reticulum of muscle cells.<br />The sarcoplasmic reticulum is a reservoir for calcium<br />ions (Ca 2), which are essential for the contraction<br />process.<br />All of these parts of a muscle fiber are involved in<br />the contraction process. Contraction begins when a<br />nerve impulse arrives at the axon terminal and stimulates<br />the release of acetylcholine. Acetylcholine generates<br />electrical changes (the movement of ions) at the<br />sarcolemma of the muscle fiber. These electrical<br />changes initiate a sequence of events within the muscle<br />fiber that is called the sliding filament mechanism<br />of muscle contraction. We will begin our<br />discussion with the sarcolemma.<br />142 The Muscular System<br />SARCOLEMMA—POLARIZATION<br />When a muscle fiber is relaxed, the sarcolemma is<br />polarized (has a resting potential), which refers to a<br />difference in electrical charges between the outside<br />and the inside. During polarization, the outside of<br />the sarcolemma has a positive charge relative to the<br />inside, which is said to have a negative charge. Sodium<br />ions (Na ) are more abundant outside the cell, and<br />potassium ions (K ) and negative ions are more abundant<br />inside (Fig. 7–4).<br />The Na ions outside tend to diffuse into the cell,<br />and the sodium pump transfers them back out. The<br />K ions inside tend to diffuse outside, and the potas-<br />The Muscular System 143<br />ACh<br />Muscle<br />fiber<br />Synaptic<br />cleft<br />Vesicles of<br />acetylcholine<br />Axon terminal<br />Mitochondria<br />Motor neuron<br />Sarcolemma<br />1<br />2<br />3<br />ACh receptor<br />Cholinesterase<br />Na+<br />Na+<br />Na+<br />Sarcomere<br />T tubule<br />Figure 7–2. Structure of the neuromuscular junction, showing an axon terminal adjacent<br />to the sarcolemma of a muscle fiber. Schematic of events: (1) Acetylcholine is about<br />to bond to the ACh receptor in the sarcolemma. (2) Channel opens to allow Na ions into<br />the muscle cell. (3) Cholinesterase inactivates acetylcholine.<br />QUESTION: What event opens a sodium channel in the sarcolemma?<br />B Bundles of<br />muscle cells<br />Muscle cells (fibers)<br />Myofibril<br />Fascia and<br />connective tissue<br />Myofibrils<br />C Muscle fiber<br />Sarcomere<br />Z line<br />Myosin filaments<br />Myosin cross bridges<br />Troponin<br />Tropomyosin<br />Myosin-binding<br />site<br />E Muscle filaments<br />D Sarcomere<br />Titin<br />filament<br />Actin<br />filament<br />Z line<br />Myosin filament<br />Transverse<br />tubule<br />Sarcoplasmic<br />reticulum<br />Sarcolemma<br />A Entire muscle<br />Actin<br />Figure 7–3. Structure of skeletal muscle. (A) Entire muscle. (B) Bundles of muscle cells<br />within a muscle. (C) Single muscle fiber, microscopic structure. (D) A sarcomere.<br />(E) Structure of muscle filaments.<br />QUESTION: What is the unit of contraction of a muscle fiber?<br />144<br />sium pump returns them inside. Both of these pumps<br />are active transport mechanisms, which, you may<br />recall, require ATP. Muscle fibers use ATP to maintain<br />a high concentration of Na ions outside the cell and<br />a high concentration of K inside. The pumps, therefore,<br />maintain polarization and relaxation until a nerve<br />impulse stimulates a change.<br />SARCOLEMMA—DEPOLARIZATION<br />When a nerve impulse arrives at the axon terminal, it<br />causes the release of acetylcholine, which diffuses<br />across the synapse and bonds to ACh receptors on<br />the sarcolemma. By doing so, acetylcholine makes the<br />sarcolemma very permeable to Na ions, which rush<br />into the cell. This makes the inside of the sarcolemma<br />positive relative to the outside, which is now considered<br />negative. This reversal of charges is called depolarization.<br />The electrical impulse thus generated<br />(called an action potential) then spreads along the<br />entire sarcolemma of a muscle fiber. The sarcolemma<br />has inward folds called T tubules (transverse tubules,<br />shown in Fig. 7–2), which carry the action potential to<br />the interior of the muscle cell. Depolarization initiates<br />changes within the cell that bring about contraction.<br />The electrical changes that take place at the sarcolemma<br />are summarized in Table 7–1 and shown in<br />Fig. 7–4.<br />The Muscular System 145<br />Na+ Na+ Na+ Na+ Na+ Na+<br />Na+ Na+<br />Na+ Na+ Na+ Na+<br />Na+ Na+ Na+ Na+ Na+ Na+<br />K+ K+<br />K+ K+ K+ K+<br />K+ K+ K+ K+<br />ACh<br />A<br />B<br />C<br />Polarization<br />Depolarization<br />Repolarization<br />K+ K+ K+ K+<br />Figure 7–4. Electrical charges and ion concentrations<br />at the sarcolemma. (A) Polarization, when the muscle<br />fiber is relaxed. (B) Wave of depolarization in response to<br />stimulus of acetylcholine. (C) Wave of repolarization.<br />QUESTION: Which ion enters the cell during depolarization?<br />Which ion leaves during repolarization?<br />Table 7–1 SARCOLEMMA—<br />ELECTRICAL CHANGES<br />State or Event Description<br />Resting Potential<br />Polarization<br />Action Potential<br />Depolarization<br />Repolarization<br />• Sarcolemma has a ( ) charge<br />outside and a ( ) charge inside.<br />• Na ions are more abundant<br />outside the cell; as they diffuse<br />inward, the sodium pump<br />returns them outside.<br />• K ions are more abundant<br />Inside the cell; as they diffuse<br />out, the potassium pump<br />returns them inside.<br />• ACh makes the sarcolemma very<br />permeable to Na ions, which<br />rush into the cell.<br />• Reversal of charges on the sarcolemma:<br />now ( ) outside and<br />( ) inside.<br />• The reversal of charges spreads<br />along the entire sarcolemma<br />• Cholinesterase at the sarcolemma<br />inactivates ACh.<br />• Sarcolemma becomes very permeable<br />to K ions, which rush<br />out of the cell.<br />• Restoration of charges on the<br />sarcolemma: ( ) outside and<br />( ) inside.<br />• The sodium and potassium<br />pumps return Na ions outside<br />and K ions inside.<br />• The muscle fiber is now able to<br />respond to ACh released by<br />another nerve impulse arriving<br />at the axon terminal.<br />CONTRACTION—THE SLIDING<br />FILAMENT MECHANISM<br />All of the parts of a muscle fiber and the electrical<br />changes described earlier are involved in the contraction<br />process, which is a precise sequence of events<br />called the sliding filament mechanism.<br />In summary, a nerve impulse causes depolarization<br />of a muscle fiber, and this electrical change enables the<br />myosin filaments to pull the actin filaments toward the<br />center of the sarcomere, making the sarcomere<br />shorter. All of the sarcomeres shorten and the muscle<br />fiber contracts. A more detailed description of this<br />process is the following:<br />1. A nerve impulse arrives at the axon terminal;<br />acetylcholine is released and diffuses across the<br />synapse.<br />2. Acetylcholine makes the sarcolemma more permeable<br />to Na ions, which rush into the cell.<br />3. The sarcolemma depolarizes, becoming negative<br />outside and positive inside. The T tubules bring<br />the reversal of charges to the interior of the muscle<br />cell.<br />4. Depolarization stimulates the release of Ca 2 ions<br />from the sarcoplasmic reticulum. Ca 2 ions bond<br />to the troponin–tropomyosin complex, which<br />shifts it away from the actin filaments.<br />5. Myosin splits ATP to release its energy; bridges<br />on the myosin attach to the actin filaments and<br />pull them toward the center of the sarcomere,<br />thus making the sarcomere shorter (Fig. 7–5).<br />6. All of the sarcomeres in a muscle fiber shorten—<br />the entire muscle fiber contracts.<br />7. The sarcolemma repolarizes: K ions leave the<br />cell, restoring a positive charge outside and a negative<br />charge inside. The pumps then return Na <br />ions outside and K ions inside.<br />8. Cholinesterase in the sarcolemma inactivates<br />acetylcholine.<br />9. Subsequent nerve impulses will prolong contraction<br />(more acetylcholine is released).<br />10. When there are no further impulses, the muscle<br />fiber will relax and return to its original length.<br />Steps 1 through 8 of this sequence describe a single<br />muscle fiber contraction (called a twitch) in response to<br />a single nerve impulse. Because all of this takes place<br />in less than a second, useful movements would not be<br />possible if muscle fibers relaxed immediately after<br />contracting. Normally, however, nerve impulses arrive<br />in a continuous stream and produce a sustained contraction<br />called tetanus, which is a normal state not to<br />be confused with the disease tetanus (see Box 7–2:<br />Tetanus and Botulism). When in tetanus, muscle<br />fibers remain contracted and are capable of effective<br />movements. In a muscle such as the biceps brachii that<br />flexes the forearm, an effective movement means that<br />many of its thousands of muscle fibers are in tetanus,<br />a sustained contraction.<br />As you might expect with such a complex process,<br />muscle contraction may be impaired in many different<br />ways. Perhaps the most obvious is the loss of nerve<br />impulses to muscle fibers, which can occur when<br />nerves or the spinal cord are severed, or when a stroke<br />146 The Muscular System<br />BOX 7–2 TETANUS AND BOTULISM<br />tetanus the cause of death is spasm of the respiratory<br />muscles.<br />Botulism is usually a type of food poisoning, but<br />it is not characterized by typical food poisoning<br />symptoms such as diarrhea or vomiting. The neurotoxin<br />produced by the botulism bacteria (Clostridium<br />botulinum) prevents the release of acetylcholine at<br />neuromuscular junctions. Without acetylcholine,<br />muscle fibers cannot contract, and muscles become<br />paralyzed. Early symptoms of botulism include<br />blurred or double vision and difficulty speaking or<br />swallowing. Weakness and paralysis spread to other<br />muscle groups, eventually affecting all voluntary<br />muscles. Without rapid treatment with the antitoxin<br />(the specific antibody to this toxin), botulism is fatal<br />because of paralysis of the respiratory muscles.<br />Some bacteria cause disease by producing toxins. A<br />neurotoxin is a chemical that in some way disrupts<br />the normal functioning of the nervous system.<br />Because skeletal muscle contraction depends on<br />nerve impulses, the serious consequences for the<br />individual may be seen in the muscular system.<br />Tetanus is characterized by the inability of muscles<br />to relax. The toxin produced by the tetanus<br />bacteria (Clostridium tetani) affects the nervous system<br />in such a way that muscle fibers receive too<br />many impulses, and muscles go into spasms. Lockjaw,<br />the common name for tetanus, indicates one<br />of the first symptoms, which is difficulty opening<br />the mouth because of spasms of the masseter muscles.<br />Treatment requires the antitoxin (an antibody<br />to the toxin) to neutralize the toxin. In untreated<br />(cerebrovascular accident) occurs in the frontal<br />lobes of the cerebrum. Without nerve impulses, skeletal<br />muscles become paralyzed, unable to contract.<br />Paralyzed muscles eventually atrophy, that is, become<br />smaller from lack of use. Other disorders that affect<br />muscle functioning are discussed in Box 7–3: Muscular<br />Dystrophy and Box 7–4: Myasthenia Gravis.<br />RESPONSES TO EXERCISE—<br />MAINTAINING HOMEOSTASIS<br />Although entire textbooks are devoted to exercise<br />physiology, we will discuss it only briefly here as an<br />example of the body’s ability to maintain homeostasis.<br />Engaging in moderate or strenuous exercise is a physiological<br />stress situation, a change that the body must<br />cope with and still maintain a normal internal environment,<br />that is, homeostasis.<br />Some of the body’s responses to exercise are diagrammed<br />in Fig. 7–6; notice how they are related to<br />cell respiration. As you can see, the respiratory and<br />cardiovascular systems make essential contributions to<br />exercise. The integumentary system also has a role,<br />since it eliminates excess body heat. Although not<br />shown, the nervous system is also directly involved, as<br />we have seen. The brain generates the impulses for<br />muscle contraction, coordinates those contractions,<br />and regulates heart rate, breathing rate, and the diameter<br />of blood vessels. The next time you run up a flight<br />The Muscular System 147<br />A Relaxed muscle<br />B Contracted muscle<br />Sarcolemma<br />T tubule<br />Sarcoplasmic<br />reticulum<br />Calcium<br />ions<br />Calcium ions released from<br />sarcoplasmic reticulum<br />Calcium ions<br />bonded to<br />troponin<br />Myosin cross bridges<br />attach to actin<br />Actin<br />Myosin-binding site<br />Tropomyosin<br />Troponin<br />Figure 7–5. Sliding filament mechanism. (A) Sarcomere in relaxed muscle fiber.<br />(B) Sarcomere in contracted muscle fiber. See text for description.<br />QUESTION: During contraction, which filaments do the pulling?<br />BOX 7–3 MUSCULAR DYSTROPHY<br />Muscular dystrophy is really a group of genetic<br />diseases in which muscle tissue is replaced by<br />fibrous connective tissue or by fat. Neither of these<br />tissues is capable of contraction, and the result is<br />progressive loss of muscle function. The most common<br />form is Duchenne’s muscular dystrophy, in<br />which the loss of muscle function affects not only<br />skeletal muscle but also cardiac muscle. Death usually<br />occurs before the age of 20 due to heart failure,<br />and at present there is no cure.<br />Duchenne’s muscular dystrophy is a sexlinked<br />(or X-linked) trait, which means that the<br />gene for it is on the X chromosome and is recessive.<br />The female sex chromosomes are XX. If one X chromosome<br />has a gene for muscular dystrophy, and<br />the other X chromosome has a dominant gene for<br />normal muscle function, the woman will not have<br />muscular dystrophy but will be a carrier who may<br />pass the muscular dystrophy gene to her children.<br />The male sex chromosomes are XY, and the Y has<br />no gene at all for muscle function, that is, no gene<br />to prevent the expression of the gene on the X<br />chromosome. If the X chromosome has a gene for<br />muscular dystrophy, the male will have the disease.<br />This is why Duchenne’s muscular dystrophy is more<br />common in males; the presence of only one gene<br />means the disease will be present.<br />The muscular dystrophy gene on the X chromosome<br />has been located, and the protein the gene<br />codes for has been named dystrophin. Dystrophin is<br />necessary for the stability of the sarcolemma and<br />the proper movement of ions. Treatments for muscular<br />dystrophy that are being investigated include<br />the injection of normal muscle cells or stem cells<br />into affected muscles, and the insertion (using<br />viruses) of normal genes for dystrophin into<br />affected muscle cells.<br />Increased muscle<br />contraction<br />Increased cell<br />respiration<br />Increased ATP<br />production<br />Increased need<br />for O2<br />Increased CO2<br />production<br />Increased heat<br />production<br />Increased<br />sweating<br />Increased respiration<br />Increased heart rate<br />Vasodilation in muscles<br />Figure 7–6. Responses of the<br />body during exercise.<br />QUESTION: Name all the organ<br />systems depicted here.<br />BOX 7–4 MYASTHENIA GRAVIS<br />which acetylcholine bonds and stimulates the entry<br />of Na ions. Without these receptors, the acetylcholine<br />released by the axon terminal cannot cause<br />depolarization of a muscle fiber.<br />Treatment of myasthenia gravis may involve<br />anticholinesterase medications. Recall that cholinesterase<br />is present in the sarcolemma to inactivate<br />acetylcholine and prevent continuous, unwanted<br />impulses. If this action of cholinesterase is inhibited,<br />acetylcholine remains on the sarcolemma for a<br />longer time and may bond to any remaining receptors<br />to stimulate depolarization and contraction.<br />Myasthenia gravis is an autoimmune disorder<br />characterized by extreme muscle fatigue even after<br />minimal exertion. Women are affected more often<br />than are men, and symptoms usually begin in middle<br />age. Weakness may first be noticed in the facial or<br />swallowing muscles and may progress to other muscles.<br />Without treatment, the respiratory muscles will<br />eventually be affected, and respiratory failure is the<br />cause of death.<br />In myasthenia gravis, the autoantibodies (selfantibodies)<br />destroy the acetylcholine receptors<br />on the sarcolemma. These receptors are the sites to<br />148<br />of stairs, hurry to catch a bus, or just go dancing, you<br />might reflect a moment on all of the things that are<br />actually happening to your body . . . after you catch<br />your breath.<br />AGING AND THE<br />MUSCULAR SYSTEM<br />With age, muscle cells die and are replaced by fibrous<br />connective tissue or by fat. Regular exercise, however,<br />delays atrophy of muscles. Although muscles become<br />slower to contract and their maximal strength decreases,<br />exercise can maintain muscle functioning at a<br />level that meets whatever a person needs for daily<br />activities. The lifting of small weights is recommended<br />as exercise for elderly people, women as well<br />as men. Such exercise also benefits the cardiovascular,<br />respiratory, and skeletal systems.<br />The loss of muscle fibers also contributes to a loss<br />of proprioception, because the brain is getting less<br />information about where and how the body is positioned.<br />The loss of muscle sense contributes to<br />unsteadiness in elderly people and to an impaired<br />sense of balance, which in turn may lead to a fall.<br />Simple awareness of this may help an elderly person<br />prevent such accidents.<br />MAJOR MUSCLES OF THE BODY<br />The actions that muscles perform are listed in Table<br />7–2 and some are shown in Fig. 7–7. Most are in pairs<br />as antagonistic functions.<br />After the brief summaries of the muscles of each<br />body area that follow, the major muscles are shown in<br />Fig. 7–8. They are listed, according to body area, in<br />Tables 7–3 through 7–7, with associated Figs. 7–9<br />through 7–13, respectively. When you study the diagrams<br />of these muscles, and the tables that accompany<br />them, keep in mind the types of joints formed by the<br />bones of their origins and insertions. Muscles pull<br />bones to produce movement, and if you can remember<br />the joints involved, you can easily learn the locations<br />and actions of the muscles.<br />The name of the muscle may also be helpful, and<br />again, many of the terms are ones you have already<br />learned. Some examples: “abdominis” refers to an<br />abdominal muscle, “femoris” to a thigh muscle,<br />“brachii” to a muscle of the upper arm, “oculi” to an<br />The Muscular System 149<br />eye muscle, and so on. Other parts of muscle names<br />may be words such as “longus” or “maximus” that<br />tell you about size, or “flexor” that tells you about<br />function.<br />Muscles that are sites for intramuscular injections<br />are shown in Box 7–5.<br />BOX 7–5 COMMON INJECTION SITES<br />Intramuscular injections are used when rapid<br />absorption is needed, because muscle has a<br />good blood supply. Common sites are the buttock<br />(gluteus medius), the lateral thigh (vastus lateralis),<br />and the shoulder (deltoid). These sites are<br />shown; also shown are the large nerves to be<br />avoided when giving such injections.<br />Box Figure 7–A Sites for intramuscular injections.<br />Posterior view of right side of body.<br />150 The Muscular System<br />MUSCLES OF THE HEAD AND NECK<br />Three general groups of muscles are found in the head<br />and neck: those that move the head or neck, the muscles<br />of facial expression, and the muscles for chewing.<br />The muscles that turn or bend the head, such as the<br />sternocleidomastoids (flexion) and the pair of splenius<br />capitis muscles (extension), are anchored to the skull<br />and to the clavicle and sternum anteriorly or the vertebrae<br />posteriorly. The muscles for smiling or frowning<br />or raising our eyebrows in disbelief are anchored<br />to the bones of the head or to the undersurface of the<br />skin of the face. The masseter is an important chewing<br />muscle in that it raises the mandible (closes the jaw).<br />Flexion<br />Flexion<br />Extension<br />Extension<br />Abduction<br />Adduction<br />Abduction<br />Adduction<br />Figure 7–7. Actions of muscles.<br />QUESTION: Crossing the arm in front of the chest would<br />be which of these actions?<br />Table 7–2 ACTIONS OF MUSCLES<br />Action Definition<br />Flexion<br />Extension<br />Adduction<br />Abduction<br />Pronation<br />Supination<br />Dorsiflexion<br />Plantar flexion<br />Rotation<br />Most are grouped in pairs of antagonistic functions.<br />• To decrease the angle of a joint<br />• To increase the angle of a joint<br />• To move closer to the midline<br />• To move away from the midline<br />• To turn the palm down<br />• To turn the palm up<br />• To elevate the foot<br />• To lower the foot (point the toes)<br />• To move a bone around its<br />longitudinal axis<br />The Muscular System 151<br />MUSCLES OF THE TRUNK<br />The muscles of the trunk cannot be described with<br />one or two general functions. Some form the wall of<br />the trunk and bend the trunk, such as the rectus abdominis<br />(f lexion) and the sacrospinalis group (extension).<br />The trapezius (both together form the shape of<br />a trapezoid) is a large muscle that can raise (shrug) the<br />shoulder or pull it back, and can help extend the head.<br />Other muscles found on the trunk help move the arm<br />at the shoulder. The pectoralis major is a large muscle<br />of the chest that pulls the arm across the chest (flexion<br />and adduction). On the posterior side of the trunk, the<br />latissimus dorsi pulls the arm downward and behind<br />the back (extension and adduction). These muscles<br />have their origins on the bones of the trunk, the sternum,<br />the or vertebrae, which are strong, stable<br />anchors. Another set of muscles forms the pelvic floor,<br />where the muscles support the pelvic organs and assist<br />with urination and defecation. Yet another category is<br />the muscles that are concerned with breathing. These<br />are the intercostal muscles between the ribs and the<br />diaphragm that separates the thoracic and abdominal<br />cavities (see Fig. 15–6).<br />MUSCLES OF THE<br />SHOULDER AND ARM<br />The triangular deltoid muscle covers the point of the<br />shoulder like a cap, and can pull the humerus to the<br />side (abduction), forward (flexion), or backward<br />(extension). You already know the functions of the<br />biceps brachii and triceps brachii, the muscles that<br />form the bulk of the upper arm. Other muscles partially<br />in the upper arm help bend the elbow (flexion).<br />The muscles that form the bulk of the forearm are the<br />flexors and extensors of the hand and fingers. You can<br />demonstrate this yourself by clasping the middle of<br />your right forearm with your left hand, then moving<br />your right hand at the wrist and closing and opening a<br />fist; you can both feel and see the hand and finger<br />muscles at work.<br />MUSCLES OF THE HIP AND LEG<br />The hip muscles that move the thigh are anchored to<br />the pelvic bone and cross the hip joint to the femur.<br />Among these are the gluteus maximus (extension),<br />gluteus medius (abduction), and iliopsoas (flexion).<br />The muscles that form the thigh include the quadriceps<br />group anteriorly and the hamstring group<br />posteriorly. For most people, the quadriceps is<br />stronger than the hamstrings, which is why athletes<br />more often have a “pulled hamstring” rather than a<br />“pulled quadriceps.” Movement of the knee joint<br />depends on thigh muscles and lower leg muscles.<br />Movement of the foot depends on lower leg muscles<br />such as the gastrocnemius (dorsiflexion or flexion) and<br />the tibialis anterior (plantar flexion or extension).<br />152<br />Brachioradialis<br />Biceps<br />brachii Brachialis<br />Triceps<br />brachii<br />Latissimus dorsi<br />External oblique<br />Gluteus medius<br />Gluteus maximus<br />Vastus lateralis<br />Biceps femoris<br />Semitendinosus<br />Soleus<br />A<br />Achilles tendon<br />Trapezius<br />Deltoid<br />Infraspinatus<br />Teres major<br />Triceps brachii<br />Brachioradialis<br />Adductor magnus<br />Gracilis<br />Semimembranosus<br />Gastrocnemius<br />Figure 7–8. Major muscles of the body. (A) Posterior view.<br />B<br />Pectineus<br />Masseter<br />Sternocleidomastoid<br />Deltoid<br />Pectoralis major<br />Brachialis<br />Biceps<br />brachii<br />Brachioradialis<br />Gastrocnemius<br />Tibialis anterior<br />Soleus<br />Vastus medialis<br />Vastus lateralis<br />Gracilis<br />Rectus femoris<br />Adductor longus<br />Sartorius<br />Iliopsoas<br />Rectus abdominis<br />External oblique<br />Triceps<br />brachii<br />Figure 7–8. Major muscles of the body. (B) Anterior view.<br />QUESTION: Find a muscle named for: shape, size, location, a bone it is near, and function.<br />153<br />154<br />Levator labii<br />superioris<br />Zygomaticus<br />Orbicularis oris<br />Mentalis<br />Platysma<br />Anterior—left lateral view<br />Orbicularis oculi<br />Temporalis<br />Buccinator<br />Masseter<br />Sternohyoid<br />Sternocleidomastoid<br />Trapezius<br />Frontalis<br />Epicranial<br />aponeurosis<br />Figure 7–9. Muscles of the<br />head and neck in anterior, left-lateral<br />view.<br />QUESTION: In what way are both<br />orbicularis muscles similar?<br />Table 7–3 MUSCLES OF THE HEAD AND NECK<br />Muscle Function Origin Insertion<br />Frontalis<br />Orbicularis oculi<br />Orbicularis oris<br />Masseter<br />Buccinator<br />Sternocleidomastoid<br />Semispinalis capitis<br />(a deep muscle)<br />Splenius capitis<br />Raises eyebrows, wrinkles skin<br />of forehead<br />Closes eye<br />Puckers lips<br />Closes jaw<br />Pulls corners of mouth laterally<br />Turns head to opposite side<br />(both—flex head and neck)<br />Turns head to same side (both—<br />extend head and neck)<br />Turns head to same side (both—<br />extend head)<br />• epicranial aponeurosis<br />• medial side of orbit<br />• encircles mouth<br />• maxilla and zygomatic<br />• maxilla and mandible<br />• sternum and clavicle<br />• 7th cervical and first 6<br />thoracic vertebrae<br />• 7th cervical and first 4<br />thoracic vertebrae<br />• skin above supraorbital<br />margin<br />• encircles eye<br />• skin at corners of mouth<br />• mandible<br />• orbicularis oris<br />• temporal bone (mastoid<br />process)<br />• occipital bone<br />• occipital bone<br />155<br />Table 7–4 MUSCLES OF THE TRUNK<br />Muscle Function Origin Insertion<br />Trapezius<br />External intercostals<br />Internal intercostals<br />Diaphragm<br />Rectus abdominis<br />External oblique<br />Sacrospinalis group<br />(deep muscles)<br />Raises, lowers, and adducts<br />shoulders<br />Pull ribs up and out (inhalation)<br />Pull ribs down and in (forced<br />exhalation)<br />Flattens (down) to enlarge<br />chest cavity for inhalation<br />Flexes vertebral column, compresses<br />abdomen<br />Rotates and flexes vertebral column,<br />compresses abdomen<br />Extends vertebral column<br />• occipital bone and all<br />thoracic vertebrae<br />• superior rib<br />• inferior rib<br />• last 6 costal cartilages<br />and lumbar vertebrae<br />• pubic bones<br />• lower 8 ribs<br />• ilium, lumbar, and<br />some thoracic vertebrae<br />• spine of scapula and<br />clavicle<br />• inferior rib<br />• superior rib<br />• central tendon<br />• 5th–7th costal cartilages<br />and xiphoid process<br />• iliac crest and linea alba<br />• ribs, cervical, and<br />thoracic vertebrae<br />External oblique<br />Internal oblique<br />Transversus<br />abdominis<br />Rectus abdominis<br />Sternocleidomastoid<br />Trapezius<br />Pectoralis major<br />Serratus anterior<br />Trapezius<br />Splenius capitis<br />Deltoid<br />Teres major<br />Infraspinatus<br />Rhomboideus<br />major<br />Gluteus maximus<br />Latissimus<br />dorsi<br />External<br />oblique<br />A<br />B<br />Figure 7–10. Muscles of the trunk. (A) Anterior view. (B) Posterior view.<br />QUESTION: Which muscles of the trunk move the arm? Why are they on the trunk?<br />156 The Muscular System<br />Deltoid<br />Biceps<br />Brachialis<br />Extensor carpi<br />radialis longus<br />Extensor carpi<br />radialis longus<br />Brachioradialis<br />Flexor<br />carpi radialis<br />Extensor carpi<br />radialis brevis<br />Abductor<br />pollicis<br />brevis<br />Abductor<br />pollicis<br />Triceps<br />Palmaris longus<br />Flexor pollicis longus<br />Deltoid<br />Brachialis<br />Brachioradialis<br />Extensor carpi<br />radialis brevis<br />Extensor<br />digitorum<br />A B<br />Flexor digitorum<br />superficialis<br />Abductor<br />pollicis longus<br />Extensor<br />pollicis brevis<br />Anconeus<br />Flexor carpi ulnaris<br />Extensor carpi ulnaris<br />Extensor digiti minimi<br />Figure 7–11. Muscles of the arm. (A) Anterior view. (B) Posterior view.<br />QUESTION: Where are the muscles that flex the fingers located? How did you know?<br />The Muscular System 157<br />Table 7–5 MUSCLES OF THE SHOULDER AND ARM<br />Muscle Function Origin Insertion<br />Deltoid<br />Pectoralis major<br />Latissimus dorsi<br />Teres major<br />Triceps brachii<br />Biceps brachii<br />Brachioradialis<br />Abducts the humerus<br />Flexes and adducts the<br />humerus<br />Extends and adducts<br />the humerus<br />Extends and adducts<br />the humerus<br />Extends the forearm<br />Flexes the forearm<br />Flexes the forearm<br />• scapula and clavicle<br />• clavicle, sternum, 2nd–6th costal<br />cartilages<br />• last 6 thoracic vertebrae, all lumbar<br />vertebrae, sacrum, iliac crest<br />• scapula<br />• humerus and scapula<br />• scapula<br />• humerus<br />• humerus<br />• humerus<br />• humerus<br />• humerus<br />• ulna<br />• radius<br />• radius<br />Table 7–6 MUSCLES OF THE HIP AND LEG<br />Muscle Function Origin Insertion<br />Iliopsoas<br />Gluteus maximus<br />Gluteus medius<br />Quadriceps femoris group:<br />Rectus femoris<br />Vastus lateralis<br />Vastus medialis<br />Vastus intermedius<br />Hamstring group<br />Biceps femoris<br />Semimembranosus<br />Semitendinosus<br />Adductor group<br />Sartorius<br />Gastrocnemius<br />Soleus<br />Tibialis anterior<br />Flexes femur<br />Extends femur<br />Abducts femur<br />Flexes femur and extends<br />lower leg<br />Extends femur and flexes<br />lower leg<br />Adducts femur<br />Flexes femur and lower leg<br />Plantar flexes foot<br />Plantar flexes foot<br />Dorsiflexes foot<br />• ilium, lumbar vertebrae<br />• iliac crest, sacrum, coccyx<br />• ilium<br />• ilium and femur<br />• ischium<br />• ischium and pubis<br />• ilium<br />• femur<br />• tibia and fibula<br />• tibia<br />• femur<br />• femur<br />• femur<br />• tibia<br />• tibia and fibula<br />• femur<br />• tibia<br />• calcaneus (Achilles tendon)<br />• calcaneus (Achilles tendon)<br />• metatarsals<br />158<br />Sartorius<br />Rectus femoris<br />Vastus lateralis<br />Peroneus longus<br />Tibialis anterior<br />Extensor digitorum brevis<br />Extensor digitorum longus<br />Extensor<br />hallucis brevis<br />Iliopsoas<br />Pectineus<br />Adductor<br />longus<br />Adductor<br />magnus<br />Gracilis<br />Semitendinosus<br />Vastus medialis<br />Semimembranosus<br />Gastrocnemius<br />Soleus<br />A<br />B<br />Flexor<br />digitorum longus<br />Peroneus brevis<br />Extensor hallucis longus<br />Gluteus maximus<br />Vastus lateralis<br />Plantaris<br />Peroneus longus<br />Biceps femoris<br />Figure 7–12. Muscles of the leg. (A) Anterior view. (B) Posterior view.<br />QUESTION: How does the gastrocnemius compare in size to the tibialis anterior? What is<br />the reason for this difference?<br />The Muscular System 159<br />Clitoris<br />Urethra<br />Vagina<br />Ischium<br />Central<br />tendon<br />Anus<br />Gluteus<br />maximus<br />Anococcygeal<br />ligament<br />Coccyx<br />Ischiocavernosus<br />Bulbospongiosus<br />Transverse perineus<br />Levator ani<br />External<br />anal<br />sphincter<br />Coccygeus<br />Figure 7–13. Muscles of the female pelvic floor.<br />QUESTION: In women, what organs are directly supported by this “floor” of muscles?<br />Table 7–7 MUSCLES OF THE PELVIC FLOOR<br />Muscle Function Origin Insertion<br />Levator ani<br />Coccygeus<br />Ischiocavernosus<br />Bulbospongiosus<br />Transverse perineus<br />(superficial and deep)<br />External anal sphincter<br />Supports pelvic organs, especially during<br />defecation, urination, coughing,<br />and forced exhalation; constricts<br />anus, urethra, and vagina<br />Supports pelvic organs, especially during<br />defecation, urination, coughing,<br />and forced exhalation<br />Erection of clitoris in female, penis in<br />male<br />Assists urination; erection in female;<br />erection and ejaculation in male<br />Assists urination in female; urination and<br />ejaculation in male<br />Closes anus<br />• pubis and ischium<br />• ischium<br />• ischium and pubis<br />• central tendon of<br />perineum<br />• ischium<br />• anococcygeal<br />ligament<br />• coccyx, anal canal,<br />urethra<br />• coccyx and sacrum<br />• clitoris or penis<br />• fasciae, pubic arch,<br />clitoris, or penis<br />• central tendon of<br />perineum<br />• central tendon of<br />perineum<br />STUDY OUTLINE<br />Organ Systems Involved in Movement<br />1. Muscular—moves the bones.<br />2. Skeletal—bones are moved, at their joints, by muscles.<br />3. Nervous—transmits impulses to muscles to cause<br />contraction.<br />4. Respiratory—exchanges O2 and CO2 between the<br />air and blood.<br />5. Circulatory—transports O2 to muscles and removes<br />CO2.<br />Muscle Structure<br />1. Muscle fibers (cells) are specialized to contract,<br />shorten, and produce movement.<br />2. A skeletal muscle is made of thousands of<br />muscle fibers. Varying movements require contrac-<br />tion of variable numbers of muscle fibers in a<br />muscle.<br />3. Tendons attach muscles to bone; the origin is the<br />more stationary bone, the insertion is the more<br />movable bone. A tendon merges with the fascia of<br />a muscle and the periosteum of a bone; all are made<br />of fibrous connective tissue.<br />Muscle Arrangements<br />1. Antagonistic muscles have opposite functions. A<br />muscle pulls when it contracts, but exerts no force<br />when it relaxes and it cannot push. When one muscle<br />pulls a bone in one direction, another muscle is<br />needed to pull the bone in the other direction (see<br />also Table 7–2 and Fig. 7–1).<br />2. Synergistic muscles have the same function and<br />alternate as the prime mover depending on the<br />position of the bone to be moved. Synergists also<br />stabilize a joint to make a more precise movement<br />possible.<br />3. The frontal lobes of the cerebrum generate the<br />impulses necessary for contraction of skeletal muscles.<br />The cerebellum regulates coordination.<br />Muscle Tone—the state of slight contraction<br />present in muscles<br />1. Alternate fibers contract to prevent muscle fatigue;<br />regulated by the cerebellum.<br />2. Good tone helps maintain posture, produces 25%<br />of body heat (at rest), and improves coordination.<br />3. Isotonic exercise involves contraction with movement;<br />improves tone and strength and improves<br />cardiovascular and respiratory efficiency (aerobic<br />exercise).<br />• Concentric contraction—muscle exerts force<br />while shortening.<br />• Eccentric contraction—muscle exerts force<br />while lengthening.<br />4. Isometric exercise involves contraction without<br />movement; improves tone and strength but is not<br />aerobic.<br />Muscle Sense—proprioception: knowing<br />where our muscles are without looking<br />at them<br />1. Permits us to perform everyday activities without<br />having to concentrate on muscle position.<br />2. Stretch receptors (proprioceptors) in muscles<br />respond to stretching and generate impulses that<br />the brain interprets as a mental “picture” of where<br />the muscles are. Parietal lobes: conscious muscle<br />sense; cerebellum: unconscious muscle sense used<br />to promote coordination.<br />Energy Sources for Muscle Contraction<br />1. ATP is the direct source; the ATP stored in muscles<br />lasts only a few seconds.<br />2. Creatine phosphate is a secondary energy source; is<br />broken down to creatine phosphate energy.<br />The energy is used to synthesize more ATP. Some<br />creatine is converted to creatinine, which must be<br />excreted by the kidneys. Most creatine is used for<br />the resynthesis of creatine phosphate.<br />3. Glycogen is the most abundant energy source and<br />is first broken down to glucose. Glucose is broken<br />down in cell respiration:<br />Glucose O2 → CO2 H2O ATP heat<br />ATP is used for contraction; heat contributes to<br />body temperature; H2O becomes part of intracellular<br />fluid; CO2 is eventually exhaled.<br />4. Oxygen is essential for the completion of cell respiration.<br />Hemoglobin in red blood cells carries<br />oxygen to muscles; myoglobin stores oxygen in<br />muscles; both of these proteins contain iron, which<br />enables them to bond to oxygen.<br />5. Oxygen debt (recovery oxygen uptake): Muscle<br />fibers run out of oxygen during strenuous exercise,<br />and glucose is converted to lactic acid, which causes<br />fatigue. Breathing rate remains high after exercise<br />to deliver more oxygen to the liver, which converts<br />lactic acid to pyruvic acid, a simple carbohydrate<br />(ATP required).<br />Muscle Fiber—microscopic structure<br />1. Neuromuscular junction: axon terminal and sarcolemma;<br />the synapse is the space between. The<br />axon terminal contains acetylcholine (a neurotransmitter),<br />and the sarcolemma contains cholinesterase<br />(an inactivator) (see Fig. 7–2).<br />2. Sarcomeres are the contracting units of a muscle<br />fiber. Myosin and actin filaments are the contracting<br />proteins of sarcomeres. Troponin and tropomyosin<br />are proteins that inhibit the sliding of<br />myosin and actin when the muscle fiber is relaxed<br />(see Figs. 7–3 and 7–5).<br />3. The sarcoplasmic reticulum surrounds the sarcomeres<br />and is a reservoir for calcium ions.<br />4. Polarization (resting potential): When the muscle<br />fiber is relaxed, the sarcolemma has a ( ) charge<br />160 The Muscular System<br />1. Name the organ systems directly involved in<br />movement, and for each state how they are<br />involved. (p. 138)<br />2. State the function of tendons. Name the part of a<br />muscle and a bone to which a tendon is attached.<br />(p. 138)<br />3. State the term for: (pp. 138–139)<br />a. Muscles with the same function<br />b. Muscles with opposite functions<br />c. The muscle that does most of the work in a<br />movement<br />4. Explain why antagonistic muscle arrangements<br />are necessary. Give two examples. (p. 138)<br />5. State three reasons why good muscle tone is<br />important. (p. 140)<br />6. Explain why muscle sense is important. Name the<br />receptors involved and state what they detect.<br />(p. 141)<br />7. With respect to muscle contraction, state the<br />functions of the cerebellum and the frontal lobes<br />of the cerebrum. (p. 140)<br />8. Name the direct energy source for muscle contraction.<br />Name the two secondary energy sources.<br />Which of these is more abundant? (p. 141)<br />9. State the simple equation of cell respiration and<br />what happens to each of the products of this reaction.<br />(p. 142)<br />10. Name the two sources of oxygen for muscle<br />fibers. State what the two proteins have in common.<br />(p. 142)<br />11. Explain what is meant by oxygen debt. What is<br />needed to correct oxygen debt, and where does it<br />come from? (p. 142)<br />12. Name these parts of the neuromuscular junction:<br />(p. 142)<br />a. The membrane of the muscle fiber<br />b. The end of the motor neuron<br />c. The space between neuron and muscle cell<br />State the locations of acetylcholine and cholinesterase.<br />13. Name the contracting proteins of sarcomeres, and<br />describe their locations in a sarcomere. Where is<br />the sarcoplasmic reticulum and what does it contain?<br />(p. 142)<br />The Muscular System 161<br />REVIEW QUESTIONS<br />outside and a ( ) charge inside. Na ions are more<br />abundant outside the cell and K ions are more<br />abundant inside the cell. The Na and K pumps<br />maintain these relative concentrations on either<br />side of the sarcolemma (see Table 7–1 and Fig.<br />7–4).<br />5. Depolarization: This process is started by a nerve<br />impulse. Acetylcholine released by the axon terminal<br />makes the sarcolemma very permeable to Na <br />ions, which enter the cell and cause a reversal of<br />charges to ( ) outside and ( ) inside. The depolarization<br />spreads along the entire sarcolemma and<br />initiates the contraction process. Folds of the sarcolemma<br />called T tubules carry the depolarization<br />into the interior of the muscle cell.<br />Contraction—the sliding filament mechanism<br />(see Fig. 7–5)<br />1. Depolarization stimulates a sequence of events<br />that enables myosin filaments to pull the actin filaments<br />to the center of the sarcomere, which shortens.<br />2. All of the sarcomeres in a muscle fiber contract in<br />response to a nerve impulse; the entire cell contracts.<br />3. Tetanus is a sustained contraction brought about by<br />continuous nerve impulses; all our movements<br />involve tetanus.<br />4. Paralysis: Muscles that do not receive nerve<br />impulses are unable to contract and will atrophy.<br />Paralysis may be the result of nerve damage, spinal<br />cord damage, or brain damage.<br />Responses to Exercise—maintaining homeostasis<br />See section in chapter and Fig. 7–6.<br />Major Muscles<br />See Tables 7–2 through 7–7 and Figs. 7–7 through<br />7–13.<br />1. In an accident with farm machinery, Mr. R. had his<br />left arm severed just below the elbow. Mrs. R.<br />stopped the bleeding, called for an ambulance,<br />and packed the severed arm in ice for the EMTs<br />to take to the hospital. Will Mr. R. ever be able<br />to move the fingers of his left hand again? What<br />structures must be reattached, and what has to<br />happen?<br />2. Name all of the muscles you can think of that move<br />the thigh at the hip. Group them as synergists, if<br />possible. Then pair those groups or individual<br />muscles as antagonists.<br />3. Muscle contraction is important for posture.<br />Muscles oppose each other, contracting equally to<br />keep us upright. Picture the body in anatomic position,<br />and describe what would happen if each of<br />these muscles relaxed completely:<br />Semispinalis capitis<br />Masseter<br />Rectus abdominis<br />Sacrospinalis<br />Quadriceps femoris<br />Gluteus maximus<br />4. An exercise for skiers involves sitting against a wall<br />as if you were sitting in a chair, but without a chair.<br />Thighs should be parallel to the floor and the knees<br />should make a 90o angle. Try it. What kind of<br />exercise is this? Which muscles are doing most of<br />the work (which ones begin to hurt)? Which do<br />you think would be easier: 3 minutes of this exercise<br />or 3 minutes of jogging? Can you think of an<br />explanation?<br />5. Can you juggle? Don’t just say “no”—have you<br />ever tried? Find some old tennis balls and try juggling<br />two balls with one hand, or three balls with<br />two hands. Explain how muscle sense is involved in<br />juggling.<br />Now try to imagine what it would be like to be<br />without muscle sense. Some people do not have<br />muscle sense in certain parts of their bodies. Who<br />are these people, and what has happened that cost<br />them their muscle sense (and muscle contraction)?<br />162 The Muscular System<br />FOR FURTHER THOUGHT<br />14. In terms of ions and charges, describe: (p. 145)<br />a. Polarization<br />b. Depolarization<br />c. Repolarization<br />15. With respect to the sliding filament mechanism,<br />explain the function of: (p. 146)<br />a. Acetylcholine<br />b. Calcium ions<br />c. Myosin and actin<br />d. Troponin and tropomyosin<br />e. Cholinesterase<br />16. State three of the body’s physiological responses<br />to exercise, and explain how each helps maintain<br />homeostasis. (pp. 147–148)<br />17. Find the major muscles on yourself, and state a<br />function of each muscle<br />CHAPTER 8<br />The Nervous System<br />163<br />164<br />CHAPTER 8<br />Chapter Outline<br />Nervous System Divisions<br />Nerve Tissue<br />Synapses<br />Types of Neurons<br />Nerves and Nerve Tracts<br />The Nerve Impulse<br />The Spinal Cord<br />Spinal Nerves<br />Spinal Cord Reflexes<br />Reflex arc<br />The Brain<br />Ventricles<br />Medulla<br />Pons<br />Midbrain<br />Cerebellum<br />Hypothalamus<br />Thalamus<br />Cerebrum<br />Frontal lobes<br />Parietal lobes<br />Temporal lobes<br />Occipital lobes<br />Association areas<br />Basal ganglia<br />Corpus callosum<br />Meninges and Cerebrospinal Fluid<br />Cranial Nerves<br />The Autonomic Nervous System<br />Autonomic Pathways<br />Sympathetic Division<br />Parasympathetic Division<br />Neurotransmitters<br />Aging and the Nervous System<br />BOX 8–1 MULTIPLE MCLEROSIS<br />BOX 8–2 SHINGLES<br />BOX 8–3 SPINAL CORD INJURIES<br />BOX 8–4 CEREBROVASCULAR ACCIDENTS<br />BOX 8–5 APHASIA<br />BOX 8–6 ALZHEIMER’S DISEASE<br />BOX 8–7 PARKINSON’S DISEASE<br />BOX 8–8 LUMBAR PUNCTURE<br />Student Objectives<br />• Name the divisions of the nervous system and the<br />parts of each, and state the general functions of<br />the nervous system.<br />• Name the parts of a neuron and state the function<br />of each.<br />• Explain the importance of Schwann cells in the<br />peripheral nervous system and neuroglia in the<br />central nervous system.<br />• Describe the electrical nerve impulse, and describe<br />impulse transmission at synapses.<br />• Describe the types of neurons, nerves, and nerve<br />tracts.<br />• State the names and numbers of the spinal nerves,<br />and their destinations.<br />• Explain the importance of stretch reflexes and<br />flexor reflexes.<br />• State the functions of the parts of the brain; be<br />able to locate each part on a diagram.<br />• Name the meninges and describe their locations.<br />The Nervous System<br />165<br />Student Objectives (Continued)<br />• State the locations and functions of cerebrospinal<br />fluid.<br />• Name the cranial nerves, and state their functions.<br />• Explain how the sympathetic division of the autonomic<br />nervous system enables the body to adapt<br />to a stress situation.<br />• Explain how the parasympathetic division of the<br />autonomic nervous system promotes normal body<br />functioning in relaxed situations.<br />New Terminology<br />Afferent (AFF-uh-rent)<br />Autonomic nervous system (AW-toh-NOM-ik)<br />Cauda equina (KAW-dah ee-KWHY-nah)<br />Cerebral cortex (se-REE-bruhl KOR-teks)<br />Cerebrospinal fluid (se-REE-broh-SPY-nuhl)<br />Choroid plexus (KOR-oid PLEK-sus)<br />Corpus callosum (KOR-pus kuh-LOH-sum)<br />Cranial nerves (KRAY-nee-uhl NERVS)<br />Efferent (EFF-uh-rent)<br />Gray matter (GRAY MA-TUR)<br />Neuroglia (new-ROG-lee-ah)<br />Neurolemma (NYOO-ro-LEM-ah)<br />Parasympathetic (PAR-uh-SIM-puh-THET-ik)<br />Reflex (REE-fleks)<br />Somatic (soh-MA-tik)<br />Spinal nerves (SPY-nuhl NERVS)<br />Sympathetic (SIM-puh-THET-ik)<br />Ventricles of brain (VEN-trick’ls)<br />Visceral (VISS-er-uhl)<br />White matter (WIGHT MA-TUR)<br />Related Clinical Terminology<br />Alzheimer’s disease (ALZ-high-mer’s)<br />Aphasia (ah-FAY-zee-ah)<br />Blood–brain barrier (BLUHD BRAYNE)<br />Cerebrovascular accident (CVA) (se-REE-broh-<br />VAS-kyoo-lur)<br />Lumbar puncture (LUM-bar PUNK-chur)<br />Meningitis (MEN-in-JIGH-tis)<br />Multiple sclerosis (MS) (MULL-ti-puhl skle-<br />ROH-sis)<br />Neuralgia (new-RAL-jee-ah)<br />Neuritis (new-RYE-tis)<br />Neuropathy (new-RAH-puh-thee)<br />Parkinson’s disease (PAR-kin-son’s)<br />Remission (ree-MISH-uhn)<br />Spinal shock (SPY-nuhl SHAHK)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />Most of us can probably remember being told,<br />when we were children, not to touch the stove or some<br />other source of potential harm. Because children are<br />curious, such warnings often go unheeded. The<br />result? Touching a hot stove brings about an immediate<br />response of pulling away and a vivid memory of<br />painful fingers. This simple and familiar experience<br />illustrates the functions of the nervous system:<br />1. To detect changes and feel sensations<br />2. To initiate appropriate responses to changes<br />3. To organize information for immediate use and<br />store it for future use<br />The nervous system is one of the regulating systems<br />(the endocrine system is the other and is discussed<br />in Chapter 10). Electrochemical impulses of<br />the nervous system make it possible to obtain information<br />about the external or internal environment<br />and do whatever is necessary to maintain homeostasis.<br />Some of this activity is conscious, but much of it happens<br />without our awareness.<br />NERVOUS SYSTEM DIVISIONS<br />The nervous system has two divisions. The central<br />nervous system (CNS) consists of the brain and<br />spinal cord. The peripheral nervous system (PNS)<br />consists of cranial nerves and spinal nerves. The PNS<br />includes the autonomic nervous system (ANS).<br />The peripheral nervous system relays information<br />to and from the central nervous system, and the brain<br />is the center of activity that integrates this information,<br />initiates responses, and makes us the individuals<br />we are.<br />NERVE TISSUE<br />Nerve tissue was briefly described in Chapter 4, so we<br />will begin by reviewing what you already know and<br />then add to it.<br />Nerve cells are called neurons, or nerve fibers.<br />Whatever their specific functions, all neurons have the<br />same physical parts. The cell body contains the<br />nucleus (Fig. 8–1) and is essential for the continued<br />life of the neuron. As you will see, neuron cell bodies<br />are found in the central nervous system or close to it<br />in the trunk of the body. In these locations, cell bodies<br />are protected by bone. There are no cell bodies in the<br />arms and legs, which are much more subject to injury.<br />Dendrites are processes (extensions) that transmit<br />impulses toward the cell body. The one axon of a neuron<br />transmits impulses away from the cell body. It is<br />the cell membrane of the dendrites, cell body, and<br />axon that carries the electrical nerve impulse.<br />In the peripheral nervous system, axons and dendrites<br />are “wrapped” in specialized cells called<br />Schwann cells (see Fig. 8–1). During embryonic<br />development, Schwann cells grow to surround the<br />neuron processes, enclosing them in several layers of<br />Schwann cell membrane. These layers are the myelin<br />sheath; myelin is a phospholipid that electrically insulates<br />neurons from one another. Without the myelin<br />sheath, neurons would short-circuit, just as electrical<br />wires would if they were not insulated (see Box 8–1:<br />Multiple Sclerosis).<br />The spaces between adjacent Schwann cells, or segments<br />of the myelin sheath, are called nodes of<br />Ranvier (neurofibril nodes). These nodes are the parts<br />of the neuron cell membrane that depolarize when an<br />electrical impulse is transmitted (see “The Nerve<br />Impulse” section, on pages 171–172).<br />The nuclei and cytoplasm of the Schwann cells are<br />wrapped around the outside of the myelin sheath and<br />are called the neurolemma, which becomes very<br />important if nerves are damaged. If a peripheral nerve<br />is severed and reattached precisely by microsurgery,<br />the axons and dendrites may regenerate through the<br />tunnels formed by the neurolemmas. The Schwann<br />cells are also believed to produce a chemical growth<br />factor that stimulates regeneration. Although this regeneration<br />may take months, the nerves may eventually<br />reestablish their proper connections, and the<br />person may regain some sensation and movement in<br />the once-severed limb.<br />In the central nervous system, the myelin sheaths<br />are formed by oligodendrocytes, one of the neuroglia<br />(glial cells), the specialized cells found only in<br />the brain and spinal cord. Because no Schwann cells<br />are present, however, there is no neurolemma, and<br />regeneration of neurons does not occur. This is why<br />severing of the spinal cord, for example, results in permanent<br />loss of function. Another kind of neuroglia are<br />microglia,which are constantly moving, phagocytizing<br />cellular debris, damaged cells, and pathogens.<br />166 The Nervous System<br />Yet another type of glial cell is the astrocyte (literally,<br />“star cell”). In the embryo, these cells provide a<br />framework for the migrating neurons that will form<br />the brain. Thereafter, the extensions of astrocytes are<br />wrapped around brain capillaries and contribute to the<br />blood–brain barrier, which prevents potentially<br />harmful waste products in the blood from diffusing<br />out into brain tissue. These waste products are normal<br />in the blood and tissue fluid, but brain tissue is much<br />more sensitive to even low levels of them than are<br />other tissues such as muscle tissue or connective tissue.<br />The capillaries of the brain also contribute to this<br />barrier, because they are less permeable than are other<br />capillaries. A disadvantage of the blood–brain barrier<br />is that some useful medications cannot cross it, and<br />the antibodies produced by lymphocytes cross only<br />with difficulty. This becomes an important consideration<br />when treating brain infections or other diseases<br />The Nervous System 167<br />Afferent (sensory) neuron<br />Axon terminal<br />Axon<br />Nucleus<br />Cell body<br />Functional dendrite<br />Myelin sheath<br />Receptors<br />Dendrites<br />Nucleus<br />Axon terminal<br />Efferent (motor) neuron<br />Cell body<br />Axon<br />Schwann cell nucleus<br />Myelin sheath<br />Node of<br />Ranvier<br />A B<br />Schwann cell<br />Axon<br />Neurolemma<br />Layers of myelin sheath<br />C<br />Figure 8–1. Neuron structure.<br />(A) A typical sensory neuron.<br />(B) A typical motor neuron.<br />The arrows indicate the direction<br />of impulse transmission.<br />(C) Details of the myelin sheath<br />and neurolemma formed by<br />Schwann cells.<br />QUESTION: The axon terminal<br />of the motor neuron would be<br />found at what kinds of effectors?<br />or disorders (Table 8–1 summarizes the functions of<br />the neuroglia).<br />SYNAPSES<br />Neurons that transmit impulses to other neurons do<br />not actually touch one another. The small gap or space<br />between the axon of one neuron and the dendrites or<br />cell body of the next neuron is called the synapse.<br />Within the synaptic knob (terminal end) of the presynaptic<br />axon is a chemical neurotransmitter that is<br />released into the synapse by the arrival of an electrical<br />nerve impulse (Fig. 8–2). The neurotransmitter diffuses<br />across the synapse, combines with specific receptor<br />sites on the cell membrane of the postsynaptic<br />neuron, and there generates an electrical impulse that<br />is, in turn, carried by this neuron’s axon to the next<br />synapse, and so forth. A chemical inactivator at the<br />cell body or dendrite of the postsynaptic neuron<br />quickly inactivates the neurotransmitter. This prevents<br />unwanted, continuous impulses, unless a new<br />impulse from the first neuron releases more neurotransmitter.<br />Many synapses are termed excitatory, because the<br />neurotransmitter causes the postsynaptic neuron to<br />depolarize (become more negative outside as Na ions<br />enter the cell) and transmit an electrical impulse to<br />another neuron, muscle cell, or gland. Some synapses,<br />however, are inhibitory, meaning that the neurotransmitter<br />causes the postsynaptic neuron to hyperpolarize<br />(become even more positive outside as K ions<br />leave the cell or Cl ions enter the cell) and therefore<br />not transmit an electrical impulse. Such inhibitory<br />synapses are important, for example, for slowing the<br />heart rate, and for balancing the excitatory impulses<br />transmitted to skeletal muscles. With respect to the<br />skeletal muscles, this inhibition prevents excessive<br />contraction and is important for coordination.<br />168 The Nervous System<br />BOX 8–1 MULTIPLE SCLEROSIS<br />protect the axon. Because loss of myelin may<br />occur in many parts of the central nervous system,<br />the symptoms vary, but they usually include muscle<br />weakness or paralysis, numbness or partial loss<br />of sensation, double vision, and loss of spinal<br />cord reflexes, including those for urination and<br />defecation.<br />The first symptoms usually appear between the<br />ages of 20 and 40 years, and the disease may<br />progress either slowly or rapidly. Some MS patients<br />have remissions, periods of time when their symptoms<br />diminish, but remissions and progression of<br />the disease are not predictable. There is still no cure<br />for MS, but therapies include suppression of the<br />immune response, and interferon, which seems to<br />prolong remissions in some patients. The possibility<br />of stimulating remyelination of neurons is also<br />being investigated.<br />Multiple sclerosis (MS) is a demyelinating disease;<br />that is, it involves deterioration of the myelin<br />sheath of neurons in the central nervous system.<br />Without the myelin sheath, the impulses of these<br />neurons are short-circuited and do not reach their<br />proper destinations, and the neuron axons are<br />damaged and gradually die.<br />Multiple sclerosis is an autoimmune disorder<br />that may be triggered by a virus or bacterial infection.<br />Research has also uncovered a genetic component<br />to some clusters of MS cases in families.<br />Exactly how such genes would increase a person’s<br />susceptibility to an autoimmune disease is not<br />yet known. In MS, the autoantibodies destroy<br />the oligodendrocytes, the myelin-producing neuroglia<br />of the central nervous system, which results<br />in the formation of scleroses, or plaques of scar<br />tissue, that do not provide electrical insulation or<br />Table 8–1 NEUROGLIA<br />Name Function<br />Oligodendrocytes<br />Microglia<br />Astrocytes<br />Ependyma<br />• Produce the myelin sheath to<br />electrically insulate neurons of<br />the CNS.<br />• Capable of movement and<br />phagocytosis of pathogens<br />and damaged tissue.<br />• Support neurons, help maintain<br />K level, contribute to the<br />blood–brain barrier.<br />• Line the ventricles of the<br />brain; many of the cells have<br />cilia; involved in circulation of<br />cerebrospinal fluid.<br />One important consequence of the presence of<br />synapses is that they ensure one-way transmission of<br />impulses in a living person. A nerve impulse cannot go<br />backward across a synapse because there is no neurotransmitter<br />released by the dendrites or cell body.<br />Neurotransmitters can be released only by a neuron’s<br />axon, which does not have receptor sites for it, as does<br />the postsynaptic membrane. Keep this in mind when<br />we discuss the types of neurons later in the chapter.<br />An example of a neurotransmitter is acetylcholine,<br />which is found at neuromuscular junctions, in the<br />CNS, and in much of the peripheral nervous system.<br />Acetylcholine usually makes a postsynaptic membrane<br />more permeable to Na ions, which brings about<br />depolarization of the postsynaptic neuron. Cholinesterase<br />is the inactivator of acetylcholine. There are<br />many other neurotransmitters, especially in the central<br />nervous system. These include dopamine, GABA,<br />norepinephrine, glutamate, and serotonin. Each of<br />these neurotransmitters has its own chemical inactivator.<br />Some neurotransmitters are reabsorbed into the<br />neurons that secreted them; this process is called<br />reuptake and also terminates the effect of the transmitter.<br />The complexity and variety of synapses make them<br />frequent targets of medications. For example, drugs<br />that alter mood or behavior often act on specific neurotransmitters<br />in the brain, and antihypertensive<br />drugs affect synapse transmission at the smooth muscle<br />of blood vessels.<br />The Nervous System 169<br />Na+<br />Na+<br />Na+<br />Axon of presynaptic<br />neuron<br />Vesicles of neurotransmitter Receptor site<br />Inactivator<br />(cholinesterase)<br />Dendrite of<br />postsynaptic<br />neuron<br />Inactivated<br />neurotransmitter<br />Neurotransmitter<br />(acetylcholine)<br />Mitochondrion<br />Figure 8–2. Impulse transmission at a synapse. The arrow indicates the direction of the<br />electrical impulse.<br />QUESTION: Is this an excitatory synapse or an inhibitory synapse? Explain your answer.<br />TYPES OF NEURONS<br />Neurons may be classified into three groups: sensory<br />neurons, motor neurons, and interneurons (Fig. 8–3).<br />Sensory neurons (or afferent neurons) carry impulses<br />from receptors to the central nervous system.<br />Receptors detect external or internal changes and<br />send the information to the CNS in the form of<br />impulses by way of the afferent neurons. The central<br />nervous system interprets these impulses as a sensation.<br />Sensory neurons from receptors in skin, skeletal<br />muscles, and joints are called somatic; those from<br />receptors in internal organs are called visceral sensory<br />neurons.<br />Motor neurons (or efferent neurons) carry<br />impulses from the central nervous system to effectors.<br />The two types of effectors are muscles and glands. In<br />response to impulses, muscles contract or relax and<br />glands secrete. Motor neurons linked to skeletal muscle<br />are called somatic; those to smooth muscle, cardiac<br />muscle, and glands are called visceral.<br />170 The Nervous System<br />Dorsal root<br />Dorsal root ganglion<br />Cell body of<br />sensory neuron<br />Dendrite of<br />sensory neuron<br />Receptor Ventral root<br />Axon of motor neuron<br />Synaptic knobs<br />Effector muscle<br />Cell body of motor neuron<br />Gray matter<br />White matter<br />Spinothalamic tract<br />Rubrospinal tract<br />Corticospinal tract<br />Dorsal column<br />Central canal<br />Interneuron<br />Synapse<br />Figure 8–3. Cross-section of the spinal cord and the three types of neurons. Spinal nerve<br />roots and their neurons are shown on the left side. Spinal nerve tracts are shown in the<br />white matter on the right side. All tracts and nerves are bilateral (both sides).<br />QUESTION: The dorsal column is an ascending tract, and the corticospinal tract is<br />descending. Explain what this means.<br />Sensory and motor neurons make up the peripheral<br />nervous system. Visceral motor neurons form the<br />autonomic nervous system, a specialized subdivision<br />of the PNS that will be discussed later in this chapter.<br />Interneurons are found entirely within the central<br />nervous system. They are arranged so as to carry only<br />sensory or motor impulses, or to integrate these functions.<br />Some interneurons in the brain are concerned<br />with thinking, learning, and memory.<br />A neuron carries impulses in only one direction.<br />This is the result of the neuron’s structure and location,<br />as well as its physical arrangement with other<br />neurons and the resulting pattern of synapses. The<br />functioning nervous system, therefore, is an enormous<br />network of “one-way streets,” and there is no danger<br />of impulses running into and canceling one another<br />out.<br />NERVES AND NERVE TRACTS<br />A nerve is a group of axons and/or dendrites of many<br />neurons, with blood vessels and connective tissue.<br />Sensory nerves are made only of sensory neurons.<br />The optic nerves for vision and olfactory nerves for<br />smell are examples of nerves with a purely sensory<br />function. Motor nerves are made only of motor neurons;<br />autonomic nerves are motor nerves. A mixed<br />nerve contains both sensory and motor neurons. Most<br />of our peripheral nerves, such as the sciatic nerves in<br />the legs, are mixed nerves.<br />The term nerve tract refers to groups of neurons<br />within the central nervous system. All the neurons in<br />a nerve tract are concerned with either sensory or<br />motor activity. These tracts are often referred to as<br />white matter; the myelin sheaths of the neurons give<br />them a white color.<br />THE NERVE IMPULSE<br />The events of an electrical nerve impulse are the same<br />as those of the electrical impulse generated in muscle<br />fibers, which is discussed in Chapter 7. Stated simply,<br />a neuron not carrying an impulse is in a state of polarization,<br />with Na ions more abundant outside the<br />cell, and K ions and negative ions more abundant<br />inside the cell. The neuron has a positive charge on<br />the outside of the cell membrane and a relative negative<br />charge inside. A stimulus (such as a neurotransmitter)<br />makes the membrane very permeable to Na <br />ions, which rush into the cell. This brings about<br />depolarization, a reversal of charges on the membrane.<br />The outside now has a negative charge, and the<br />inside has a positive charge.<br />As soon as depolarization takes place, the neuron<br />membrane becomes very permeable to K ions, which<br />rush out of the cell. This restores the positive charge<br />outside and the negative charge inside, and is called<br />repolarization. (The term action potential refers to<br />depolarization followed by repolarization.) Then the<br />sodium and potassium pumps return Na ions outside<br />and K ions inside, and the neuron is ready to respond<br />to another stimulus and transmit another impulse. An<br />action potential in response to a stimulus takes place<br />very rapidly and is measured in milliseconds. An individual<br />neuron is capable of transmitting hundreds of<br />action potentials (impulses) each second. A summary<br />of the events of nerve impulse transmission is given in<br />Table 8–2.<br />Transmission of electrical impulses is very rapid.<br />The presence of an insulating myelin sheath increases<br />the velocity of impulses, since only the nodes of<br />Ranvier depolarize. This is called saltatory conduction.<br />Many of our neurons are capable of transmitting<br />impulses at a speed of many meters per second.<br />Imagine a person 6 feet (about 2 meters) tall who stubs<br />his toe; sensory impulses travel from the toe to the<br />brain in less than a second (crossing a few synapses<br />along the way). You can see how the nervous system<br />can communicate so rapidly with all parts of the body,<br />and why it is such an important regulatory system.<br />At synapses, nerve impulse transmission changes<br />from electrical to chemical and depends on the release<br />of neurotransmitters. Although diffusion across<br />synapses is slow, the synapses are so small that this<br />does not significantly affect the velocity of impulses in<br />a living person.<br />THE SPINAL CORD<br />The spinal cord transmits impulses to and from the<br />brain and is the integrating center for the spinal cord<br />reflexes. Although this statement of functions is very<br />brief and sounds very simple, the spinal cord is of<br />great importance to the nervous system and to the<br />body as a whole.<br />Enclosed within the vertebral canal and the meninges,<br />the spinal cord is well protected from mechanical<br />The Nervous System 171<br />injury. In length, the spinal cord extends from the<br />foramen magnum of the occipital bone to the disc<br />between the first and second lumbar vertebrae.<br />A cross-section of the spinal cord is shown in Fig.<br />8–3; refer to it as you read the following. The internal<br />gray matter is shaped like the letter H; gray matter<br />consists of the cell bodies of motor neurons and<br />interneurons. The external white matter is made of<br />myelinated axons and dendrites of interneurons.<br />These nerve fibers are grouped into nerve tracts based<br />on their functions. Ascending tracts (such as the dorsal<br />columns and spinothalamic tracts) carry sensory<br />impulses to the brain. Descending tracts (such as the<br />corticospinal and rubrospinal tracts) carry motor<br />impulses away from the brain. Lastly, find the central<br />canal; this contains cerebrospinal fluid and is continuous<br />with cavities in the brain called ventricles.<br />SPINAL NERVES<br />There are 31 pairs of spinal nerves, those that emerge<br />from the spinal cord. The nerves are named according<br />to their respective vertebrae: 8 cervical pairs, 12 thoracic<br />pairs, 5 lumbar pairs, 5 sacral pairs, and 1 very<br />small coccygeal pair. These are shown in Fig. 8–4;<br />notice that each nerve is designated by a letter and a<br />number. The 8th cervical nerve is C8, the 1st thoracic<br />nerve is T1, and so on.<br />In general, the cervical nerves supply the back of<br />the head, neck, shoulders, arms, and diaphragm (the<br />phrenic nerves). The first thoracic nerve also contributes<br />to nerves in the arms. The remaining thoracic<br />nerves supply the trunk of the body. The lumbar and<br />sacral nerves supply the hips, pelvic cavity, and legs.<br />Notice that the lumbar and sacral nerves hang below<br />the end of the spinal cord (in order to reach their<br />proper openings to exit from the vertebral canal); this<br />is called the cauda equina, literally, the “horse’s tail.”<br />Some of the important peripheral nerves and their<br />destinations are listed in Table 8–3.<br />Each spinal nerve has two roots, which are neurons<br />entering or leaving the spinal cord (see Fig. 8–3). The<br />dorsal root is made of sensory neurons that carry<br />impulses into the spinal cord. The dorsal root ganglion<br />is an enlarged part of the dorsal root that contains<br />the cell bodies of the sensory neurons. The term<br />ganglion means a group of cell bodies outside the<br />CNS. These cell bodies are within the vertebral canal<br />and are thereby protected from injury (see Box 8–2:<br />Shingles).<br />The ventral root is the motor root; it is made of<br />the axons of motor neurons carrying impulses from<br />the spinal cord to muscles or glands. The cell bodies<br />of these motor neurons, as mentioned previously, are<br />in the gray matter of the spinal cord. When the two<br />nerve roots merge, the spinal nerve thus formed is a<br />mixed nerve.<br />SPINAL CORD REFLEXES<br />When you hear the term reflex, you may think of an<br />action that “just happens,” and in part this is so. A<br />reflex is an involuntary response to a stimulus, that is,<br />172 The Nervous System<br />Table 8–2 THE NERVE IMPULSE<br />State or Event Description<br />Polarization<br />(the neuron<br />is not carrying<br />an electrical<br />impulse)<br />Depolarization<br />(generated<br />by a stimulus)<br />Propagation of<br />the impulse<br />from point<br />of stimulus<br />Repolarization<br />(immediately<br />follows<br />depolarization)<br />• Neuron membrane has a ( )<br />charge outside and a ( ) charge<br />inside.<br />• Na ions are more abundant outside<br />the cell.<br />• K ions and negative ions are<br />more abundant inside the cell.<br />Sodium and potassium pumps<br />maintain these ion concentrations.<br />• Neuron membrane becomes very<br />permeable to Na ions, which<br />rush into the cell.<br />• The neuron membrane then has a<br />( ) charge outside and a ( )<br />charge inside.<br />• Depolarization of part of the<br />membrane makes adjacent membrane<br />very permeable to Na ions,<br />and subsequent depolarization,<br />which similarly affects the next<br />part of the membrane, and so on.<br />• The depolarization continues<br />along the membrane of the neuron<br />to the end of the axon.<br />• Neuron membrane becomes very<br />permeable to K ions, which rush<br />out of the cell. This restores the<br />( ) charge outside and ( )<br />charge inside the membrane.<br />• The Na ions are returned outside<br />and the K ions are returned<br />inside by the sodium and potassium<br />pumps.<br />• The neuron is now able to<br />respond to another stimulus and<br />generate another impulse.<br />173<br />Spinal cord<br />Phrenic nerve<br />Intercostal nerves<br />Radial nerve<br />Median nerve<br />Ulnar nerve<br />Cauda equina<br />Femoral nerve<br />Sciatic nerve<br />Cervical plexus<br />Brachial plexus<br />Lumbar plexus<br />Sacral plexus<br />C1<br />C2<br />C3<br />C4<br />C5<br />C6<br />C7<br />C8<br />T1<br />T2<br />T3<br />T4<br />T5<br />T6<br />T7<br />T8<br />T9<br />T10<br />T11<br />T12<br />L1<br />L2<br />L3<br />L4<br />L5<br />S1<br />S2<br />S3<br />S4<br />S5<br />CO1<br />Figure 8–4. The spinal cord<br />and spinal nerves. The distribution<br />of spinal nerves is shown<br />only on the left side. The nerve<br />plexuses are labeled on the<br />right side. A nerve plexus is a<br />network of neurons from several<br />segments of the spinal cord<br />that combine to form nerves to<br />specific parts of the body. For<br />example, the radial and ulnar<br />nerves to the arm emerge from<br />the brachial plexus (see also<br />Table 8–3).<br />QUESTION: Where does the<br />spinal cord end? Why is this<br />important clinically?<br />an automatic action stimulated by a specific change of<br />some kind. Spinal cord reflexes are those that do not<br />depend directly on the brain, although the brain may<br />inhibit or enhance them. We do not have to think<br />about these reflexes, which is very important, as you<br />will see.<br />Reflex Arc<br />A reflex arc is the pathway that nerve impulses travel<br />when a reflex is elicited, and there are five essential<br />parts:<br />1. Receptors—detect a change (the stimulus) and<br />generate impulses.<br />2. Sensory neurons—transmit impulses from receptors<br />to the CNS.<br />3. Central nervous system—contains one or more<br />synapses (interneurons may be part of the pathway).<br />4. Motor neurons—transmit impulses from the<br />CNS to the effector.<br />5. Effector—performs its characteristic action.<br />Let us now look at the reflex arc of a specific reflex,<br />the patellar (or knee-jerk) reflex, with which you are<br />174 The Nervous System<br />Table 8–3 MAJOR PERIPHERAL NERVES<br />Spinal Nerves<br />Nerve That Contribute Distribution<br />Phrenic<br />Radial<br />Median<br />Ulnar<br />Intercostal<br />Femoral<br />Sciatic<br />C3–C5<br />C5–C8, T1<br />C5–C8, T1<br />C8, T1<br />T2–T12<br />L2–L4<br />L4–S3<br />• Diaphragm<br />• Skin and muscles of posterior arm, forearm, and hand; thumb and first 2 fingers<br />• Skin and muscles of anterior arm, forearm, and hand<br />• Skin and muscles of medial arm, forearm, and hand; little finger and ring finger<br />• Intercostal muscles, abdominal muscles; skin of trunk<br />• Skin and muscles of anterior thigh, medial leg, and foot<br />• Skin and muscles of posterior thigh, leg and foot<br />BOX 8–2 SHINGLES<br />Shingles is caused by the same virus that causes<br />chickenpox: the herpes varicella-zoster virus.<br />Varicella is chickenpox, which many of us probably<br />had as children (there is now a vaccine). When a<br />person recovers from chickenpox, the virus may<br />survive in a dormant (inactive) state in the dorsal<br />root ganglia of some spinal nerves. For most people,<br />the immune system is able to prevent reactivation<br />of the virus. With increasing age, however, the<br />immune system is not as effective, and the virus<br />may become active and cause zoster, or shingles.<br />The virus is present in sensory neurons, often<br />those of the trunk, but the damage caused by the<br />virus is seen in the skin over the affected nerve. The<br />raised, red lesions of shingles are often very painful<br />and follow the course of the nerve on the skin external<br />to it. Pain may continue even after the rash<br />heals; this is postherpetic neuralgia. Occasionally<br />the virus may affect a cranial nerve and cause facial<br />paralysis called Bell’s palsy (7th cranial) or extensive<br />facial lesions, or, rarely, blindness. Although not a<br />cure, some antiviral medications lessen the duration<br />of the illness. A vaccine is being developed for<br />adults. Though it may not completely prevent shingles,<br />it is expected to lessen the chance of postherpetic<br />neuralgia.<br />Box Figure 8–A Lesions of shingles on skin of trunk. (From<br />Goldsmith, LA, Lazarus, GS, and Tharp, MD: Adult and Pediatric<br />Dermatology: A Color Guide to Diagnosis and Treatment. FA<br />Davis, Philadelphia, 1997, p 307, with permission.)<br />probably familiar. In this reflex, a tap on the patellar<br />tendon just below the kneecap causes extension of the<br />lower leg. This is a stretch reflex, which means that a<br />muscle that is stretched will automatically contract.<br />Refer now to Fig. 8–5 as you read the following:<br />In the quadriceps femoris muscle are (1) stretch<br />receptors that detect the stretching produced by striking<br />the patellar tendon. These receptors generate<br />impulses that are carried along (2) sensory neurons in<br />the femoral nerve to (3) the spinal cord. In the spinal<br />cord, the sensory neurons synapse with (4) motor neurons<br />(this is a two-neuron reflex). The motor neurons<br />in the femoral nerve carry impulses back to (5) the<br />quadriceps femoris, the effector, which contracts and<br />extends the lower leg.<br />The patellar reflex is one of many used clinically to<br />determine whether the nervous system is functioning<br />properly. If the patellar reflex were absent in a patient,<br />the problem could be in the thigh muscle, the femoral<br />nerve, or the spinal cord. Further testing would be<br />needed to determine the precise break in the reflex<br />arc. If the reflex is normal, however, that means that<br />all parts of the reflex arc are intact. So the testing of<br />reflexes may be a first step in the clinical assessment of<br />neurologic damage.<br />You may be wondering why we have such reflexes,<br />these stretch reflexes. What is their importance in our<br />everyday lives? Imagine a person standing upright—is<br />the body perfectly still? No, it isn’t, because gravity<br />exerts a downward pull. However, if the body tilts to<br />the left, the right sides of the leg and trunk are<br />stretched, and these stretched muscles automatically<br />contract and pull the body upright again. This is the<br />purpose of stretch reflexes; they help keep us upright<br />without our having to think about doing so. If the<br />brain had to make a decision every time we swayed a<br />bit, all our concentration would be needed just to<br />remain standing. Since these are spinal cord reflexes,<br />the brain is not directly involved. The brain may<br />become aware that a reflex has taken place, but that<br />involves another set of neurons carrying impulses to<br />the brain.<br />Flexor reflexes (or withdrawal reflexes) are<br />another type of spinal cord reflex. The stimulus is<br />something painful and potentially harmful, and the<br />response is to pull away from it. If you inadvertently<br />touch a hot stove, you automatically pull your hand<br />away. Flexor reflexes are three-neuron reflexes,<br />because sensory neurons synapse with interneurons in<br />the spinal cord, which in turn synapse with motor<br />neurons. Again, however, the brain does not have to<br />make a decision to protect the body; the flexor reflex<br />does that automatically (see Box 8–3: Spinal Cord<br />Injuries). The brain may know that the reflex has<br />taken place, and may even learn from the experience,<br />but that requires different neurons, not the reflex arc.<br />The Nervous System 175<br />Gray matter<br />Biceps<br />femoris<br />muscle<br />(relaxes)<br />(4) Motor neuron<br />(3) Synapse in<br />spinal cord<br />Ventral root<br />Stimulus<br />(1) Stretch receptor<br />(5) Quadriceps femoris muscle<br />(contracts)<br />Femoral nerve Dorsal root<br />(2) Sensory neuron Dorsal root ganglion<br />Figure 8–5. Patellar reflex. The<br />reflex arc is shown. See text for<br />description.<br />QUESTION: Why is this reflex<br />called a stretch reflex?<br />THE BRAIN<br />The brain consists of many parts that function as an<br />integrated whole. The major parts are the medulla,<br />pons, and midbrain (collectively called the brain<br />stem), the cerebellum, the hypothalamus, the thalamus,<br />and the cerebrum. These parts are shown in Fig.<br />8–6. We will discuss each part separately, but keep in<br />mind that they are all interconnected and work<br />together.<br />VENTRICLES<br />The ventricles are four cavities within the brain: two<br />lateral ventricles, the third ventricle, and the fourth<br />ventricle (Fig. 8–7). Each ventricle contains a capillary<br />network called a choroid plexus, which forms cerebrospinal<br />fluid (CSF) from blood plasma. Cerebrospinal<br />fluid is the tissue fluid of the central nervous<br />system; its circulation and functions will be discussed<br />in the section on meninges.<br />MEDULLA<br />The medulla extends from the spinal cord to the pons<br />and is anterior to the cerebellum. Its functions are<br />those we think of as vital (as in “vital signs”). The<br />medulla contains cardiac centers that regulate heart<br />rate, vasomotor centers that regulate the diameter of<br />blood vessels and, thereby, blood pressure, and respiratory<br />centers that regulate breathing. You can see<br />why a crushing injury to the occipital bone may be<br />rapidly fatal—we cannot survive without the medulla.<br />Also in the medulla are reflex centers for coughing,<br />sneezing, swallowing, and vomiting.<br />PONS<br />The pons bulges anteriorly from the upper part of the<br />medulla. Within the pons are two respiratory centers<br />that work with those in the medulla to produce a normal<br />breathing rhythm. (The function of all the respiratory<br />centers is discussed in Chapter 15.) The many<br />other neurons in the pons ( pons is from the Latin for<br />“bridge”) connect the medulla with other parts of the<br />brain.<br />MIDBRAIN<br />The midbrain extends from the pons to the hypothalamus<br />and encloses the cerebral aqueduct, a tunnel<br />that connects the third and fourth ventricles. Several<br />176 The Nervous System<br />BOX 8–3 SPINAL CORD INJURIES<br />need to urinate or defecate. Nor will voluntary control<br />of these reflexes be possible, because inhibiting<br />impulses from the brain can no longer reach the<br />lower segments of the spinal cord.<br />Potentially less serious injuries are those in which<br />the spinal cord is crushed rather than severed, and<br />treatment is aimed at preserving whatever function<br />remains. Minimizing inflammation and stimulating<br />the production of nerve growth factors are aspects<br />of such treatment.<br />Perhaps the most challenging research is the<br />attempt to stimulate severed spinal cords to regenerate.<br />Partial success has been achieved in rats<br />and mice, with Schwann cells transplanted from<br />their peripheral nerves and nerve growth factors<br />produced by genetically engineered cells. The use<br />of stem cells has also been successful in rats. The<br />researchers caution, however, that it will take some<br />time before their procedures will be tested on<br />people.<br />Injuries to the spinal cord are most often caused by<br />auto accidents, falls, and gunshot wounds. The<br />most serious injury is transection, or severing, of the<br />spinal cord. If, for example, the spinal cord is severed<br />at the level of the 8th thoracic segment, there<br />will be paralysis and loss of sensation below that<br />level. Another consequence is spinal shock, the atleast-<br />temporary loss of spinal cord reflexes. In this<br />example, the spinal cord reflexes of the lower trunk<br />and legs will not occur. The stretch reflexes and<br />flexor reflexes of the legs will be at least temporarily<br />abolished, as will the urination and defecation<br />reflexes. Although these reflexes do not depend<br />directly on the brain, spinal cord neurons depend<br />on impulses from the brain to enhance their own<br />ability to generate impulses.<br />As spinal cord neurons below the injury recover<br />their ability to generate impulses, these reflexes,<br />such as the patellar reflex, often return. Urination<br />and defecation reflexes may also be reestablished,<br />but the person will not have an awareness of the<br />177<br />Frontal lobe<br />Corpus callosum<br />Parietal lobe<br />Occipital lobe<br />Midbrain<br />Cerebellum<br />Choroid plexus in<br />fourth ventricle<br />Spinal cord<br />Medulla<br />Pons<br />Temporal lobe<br />Pituitary gland<br />Hypothalamus<br />Optic nerve<br />Thalamus<br />Choroid plexus in<br />third ventricle<br />Basal<br />ganglia<br />Temporal<br />lobe<br />Longitudinal fissure<br />Cerebral cortex<br />Corpus callosum<br />Lateral ventricle<br />Thalamus<br />Third ventricle<br />Hypothalamus<br />Optic tracts<br />A<br />B<br />Hippocampus<br />Figure 8–6. (A) Midsagittal section of the brain as seen from the left side. This medial<br />plane shows internal anatomy as well as the lobes of the cerebrum. (B) Frontal section of<br />the brain in anterior view.<br />QUESTION: Find the corpus callosum in parts A and B, and describe its shape. What is its<br />function?<br />different kinds of reflexes are integrated in the midbrain,<br />including visual and auditory reflexes. If you see<br />a wasp flying toward you, you automatically duck or<br />twist away; this is a visual reflex, as is the coordinated<br />movement of the eyeballs. Turning your head (ear) to<br />a sound is an example of an auditory reflex. The midbrain<br />is also concerned with what are called righting<br />reflexes, those that keep the head upright and maintain<br />balance or equilibrium.<br />CEREBELLUM<br />The cerebellum is separated from the medulla and<br />pons by the fourth ventricle and is inferior to the<br />occipital lobes of the cerebrum. As you already know,<br />many of the functions of the cerebellum are concerned<br />with movement. These include coordination, regulation<br />of muscle tone, the appropriate trajectory and<br />endpoint of movements, and the maintenance of posture<br />and equilibrium. Notice that these are all involuntary;<br />that is, the cerebellum functions below the<br />level of conscious thought. This is important to permit<br />the conscious brain to work without being overburdened.<br />If you decide to pick up a pencil, for example,<br />the impulses for arm movement come from the cerebrum.<br />The cerebellum then modifies these impulses so<br />that your arm and finger movements are coordinated,<br />and you don’t reach past the pencil.<br />The cerebellum seems also to be involved in certain<br />sensory functions. For example, if you close your eyes<br />and someone places a tennis ball in one hand and a<br />baseball in the other, could you tell which was which?<br />Certainly you could, by the “feel” of each: the texture<br />and the weight or heft. If you pick up a plastic container<br />of coffee (with a lid on it) could you tell if the<br />cup is full, half-full, or empty? Again, you certainly<br />could. Do you have to think about it? No. The cerebellum<br />is, in part, responsible for this ability.<br />To regulate equilibrium, the cerebellum (and midbrain)<br />uses information about gravity and movement<br />provided by receptors in the inner ears. These receptors<br />are discussed further in Chapter 9.<br />HYPOTHALAMUS<br />Located superior to the pituitary gland and inferior to<br />the thalamus, the hypothalamus is a small area of the<br />brain with many diverse functions:<br />178 The Nervous System<br />Lateral ventricles<br />Parietal lobe<br />Occipital lobe<br />Cerebral aqueduct<br />Fourth ventricle<br />Cerebellum<br />Central canal of spinal cord<br />Medulla<br />Pons<br />Temporal lobe<br />Third ventricle<br />Figure 8–7. Ventricles of the brain as projected into the interior of the brain, which is<br />seen from the left side.<br />QUESTION: Describe the extent of each lateral ventricle.<br />1. Production of antidiuretic hormone (ADH) and<br />oxytocin; these hormones are then stored in the<br />posterior pituitary gland. ADH enables the kidneys<br />to reabsorb water back into the blood and thus<br />helps maintain blood volume. Oxytocin causes contractions<br />of the uterus to bring about labor and<br />delivery.<br />2. Production of releasing hormones (also called<br />releasing factors) that stimulate the secretion of<br />hormones by the anterior pituitary gland. Because<br />these hormones are covered in Chapter 10, a single<br />example will be given here: The hypothalamus produces<br />growth hormone releasing hormone<br />(GHRH), which stimulates the anterior pituitary<br />gland to secrete growth hormone (GH).<br />3. Regulation of body temperature by promoting<br />responses such as sweating in a warm environment<br />or shivering in a cold environment (see Chapter<br />17).<br />4. Regulation of food intake; the hypothalamus is<br />believed to respond to changes in blood nutrient<br />levels, to chemicals secreted by fat cells, and to hormones<br />secreted by the gastrointestinal tract. For<br />example, during a meal, after a certain duration of<br />digestion, the small intestine produces a hormone<br />that circulates to the hypothalamus and brings<br />about a sensation of satiety, or fullness, and we tend<br />to stop eating.<br />5. Integration of the functioning of the autonomic<br />nervous system, which in turn regulates the activity<br />of organs such as the heart, blood vessels, and<br />intestines. This will be discussed in more detail<br />later in this chapter.<br />6. Stimulation of visceral responses during emotional<br />situations. When we are angry, heart rate usually<br />increases. Most of us, when embarrassed, will blush,<br />which is vasodilation in the skin of the face. These<br />responses are brought about by the autonomic<br />nervous system when the hypothalamus perceives a<br />change in emotional state. The neurologic basis of<br />our emotions is not well understood, and the visceral<br />responses to emotions are not something most<br />of us can control.<br />7. Regulation of body rhythms such as secretion of<br />hormones, sleep cycles, changes in mood, or mental<br />alertness. This is often referred to as our biological<br />clock, the rhythms as circadian rhythms,<br />meaning “about a day.” If you have ever had to stay<br />awake for 24 hours, you know how disorienting it<br />can be, until the hypothalamic biological clock has<br />been reset.<br />THALAMUS<br />The thalamus is superior to the hypothalamus and<br />inferior to the cerebrum. The third ventricle is a narrow<br />cavity that passes through both the thalamus and<br />hypothalamus. Many of the functions of the thalamus<br />are concerned with sensation. Sensory impulses to the<br />brain (except those for the sense of smell) follow neuron<br />pathways that first enter the thalamus, which<br />groups the impulses before relaying them to the cerebrum,<br />where sensations are felt. For example, holding<br />a cup of hot coffee generates impulses for heat, touch<br />and texture, and the shape of the cup (muscle sense),<br />but we do not experience these as separate sensations.<br />The thalamus integrates the impulses from the cutaneous<br />receptors and from the cerebellum, that is, puts<br />them together in a sort of electrochemical package, so<br />that the cerebrum feels the whole and is able to interpret<br />the sensation quickly.<br />Some sensations, especially unpleasant ones such as<br />pain, are believed to be felt by the thalamus. However,<br />the thalamus cannot localize the sensation; that is, it<br />does not know where the painful sensation is. The<br />sensory areas of the cerebrum are required for localization<br />and precise awareness.<br />The thalamus may also suppress unimportant<br />sensations. If you are reading an enjoyable book, you<br />may not notice someone coming into the room. By<br />temporarily blocking minor sensations, the thalamus<br />permits the cerebrum to concentrate on important<br />tasks.<br />Parts of the thalamus are also involved in alertness<br />and awareness (being awake and knowing we are), and<br />others contribute to memory. For these functions, as<br />for others, the thalamus works very closely with the<br />cerebrum.<br />CEREBRUM<br />The largest part of the human brain is the cerebrum,<br />which consists of two hemispheres separated by the<br />longitudinal fissure. At the base of this deep groove is<br />the corpus callosum, a band of 200 million neurons<br />that connects the right and left hemispheres. Within<br />each hemisphere is a lateral ventricle.<br />The surface of the cerebrum is gray matter called<br />the cerebral cortex. Gray matter consists of cell bodies<br />of neurons, which carry out the many functions of<br />the cerebrum. Internal to the gray matter is white<br />matter, made of myelinated axons and dendrites that<br />connect the lobes of the cerebrum to one another and<br />to all other parts of the brain.<br />The Nervous System 179<br />In the human brain the cerebral cortex is folded<br />extensively. The folds are called convolutions or<br />gyri and the grooves between them are fissures or<br />sulci (you can see the folding of the cortex in the<br />frontal section of the brain in Fig. 8–6). This folding<br />permits the presence of millions more neurons in<br />the cerebral cortex. The cerebral cortex of an animal<br />such as a dog or cat does not have this extensive<br />folding. This difference enables us to read, speak,<br />do long division, write poetry and songs, and do<br />so many other “human” things that dogs and cats cannot<br />do.<br />The cerebral cortex is divided into lobes that have<br />the same names as the cranial bones external to them.<br />Therefore, each hemisphere has a frontal lobe, parietal<br />lobe, temporal lobe, and occipital lobe (Fig. 8–8).<br />These lobes have been mapped; that is, certain areas<br />are known to be associated with specific functions. We<br />will discuss the functions of the cerebrum according to<br />these mapped areas.<br />Frontal Lobes<br />Within the frontal lobes are the motor areas that<br />generate the impulses for voluntary movement. The<br />largest portions are for movement of the hands and<br />face, those areas with many muscles capable of very<br />fine or precise movements. It is the large size of the<br />motor area devoted to them that gives these muscles<br />their precision. The left motor area controls movement<br />on the right side of the body, and the right<br />motor area controls the left side of the body. This is<br />why a patient who has had a cerebrovascular accident,<br />or stroke, in the right frontal lobe will have paralysis<br />of muscles on the left side (see Box 8–4: Cerebrovascular<br />Accidents).<br />Anterior to the motor areas are the premotor<br />areas, which are concerned with learned motor skills<br />that require a sequence of movements. Tying shoelaces,<br />for example, seems almost automatic to us; we<br />forget having learned it. It is not a reflex, however;<br />180 The Nervous System<br />Frontal lobe<br />Premotor area<br />Motor area<br />General sensory area<br />Sensory association<br />area<br />Parietal lobe<br />Occipital lobe<br />Visual area<br />Auditory area<br />Temporal lobe<br />Auditory<br />association<br />area<br />Visual association<br />area<br />Motor speech area<br />Orbitofrontal<br />cortex<br />Figure 8–8. Left cerebral hemisphere showing some of the functional areas that have<br />been mapped.<br />QUESTION: What sensations are felt in the general sensory area?<br />rather the premotor cortex has learned the sequence<br />so well that we are able to repeat it without consciously<br />thinking about it.<br />The parts of the frontal lobes just behind the eyes<br />are the prefrontal or orbitofrontal cortex. This area<br />is concerned with things such as keeping emotional<br />responses appropriate to the situation, realizing that<br />there are standards of behavior (laws or rules of a<br />game or simple courtesy) and following them, and<br />anticipating and planning for the future. An example<br />may be helpful to put all this together: Someone with<br />damage to the prefrontal area might become enraged<br />if his pen ran out of ink during class, might throw the<br />pen at someone, and might not think that a pen will be<br />needed tomorrow and that it is time to go buy one. As<br />you can see, the prefrontal cortex is very important for<br />social behavior, and greatly contributes to what makes<br />us human.<br />Also in the frontal lobe, usually only the left lobe<br />for most right-handed people, is Broca’s motor<br />speech area, which controls the movements of the<br />mouth involved in speaking.<br />Parietal Lobes<br />The general sensory areas in the parietal lobes<br />receive impulses from receptors in the skin and feel<br />and interpret the cutaneous sensations. The left area is<br />for the right side of the body and vice versa. These<br />areas also receive impulses from stretch receptors in<br />muscles for conscious muscle sense. The largest portions<br />of these areas are for sensation in the hands and<br />face, those parts of the body with the most cutaneous<br />receptors and the most muscle receptors. The taste<br />areas, which overlap the parietal and temporal lobes,<br />receive impulses from taste buds on the tongue and<br />elsewhere in the oral cavity.<br />Temporal Lobes<br />The olfactory areas in the temporal lobes receive<br />impulses from receptors in the nasal cavities for the<br />sense of smell. The olfactory association area learns<br />the meaning of odors such as the smell of sour milk, or<br />fire, or brownies baking in the oven, and enables the<br />thinking cerebrum to use that information effectively.<br />The Nervous System 181<br />BOX 8–4 CEREBROVASCULAR ACCIDENTS<br />they cause is very widespread or affects vital centers<br />in the medulla or pons.<br />For CVAs of the thrombus type, a clot-dissolving<br />drug may help reestablish blood flow. To be effective,<br />however, the drug must be administered<br />within 3 hours of symptom onset (see also Box<br />11–7).<br />Recovery from a CVA depends on its location<br />and the extent of damage, as well as other factors.<br />One of these is the redundancy of the brain.<br />Redundancy means repetition or exceeding what<br />is necessary; the cerebral cortex has many more<br />neurons than we actually use in daily activities.<br />The characteristic of plasticity means that these<br />neurons are available for use, especially in younger<br />people (less than 50 years of age). When a patient<br />recovers from a disabling stroke, what has often<br />happened is that the brain has established new<br />pathways, with previously little-used neurons now<br />carrying impulses “full time.” Such recovery is<br />highly individual and may take months. Yet another<br />important factor is that CVA patients be started on<br />rehabilitation therapy as soon as their condition<br />permits.<br />A cerebrovascular accident (CVA), or stroke, is<br />damage to a blood vessel in the brain, resulting in<br />lack of oxygen to that part of the brain. Possible<br />types of vessel damage are thrombosis or hemorrhage.<br />A thrombus is a blood clot, which most often is<br />a consequence of atherosclerosis, abnormal lipid<br />deposits in cerebral arteries. The rough surface<br />stimulates clot formation, which obstructs the<br />blood flow to the part of the brain supplied by<br />the artery. The symptoms depend on the part of the<br />brain affected and may be gradual in onset if clot<br />formation is slow. Approximately 80% of CVAs are<br />of this type.<br />A hemorrhage, the result of arteriosclerosis or<br />aneurysm of a cerebral artery, allows blood out<br />into brain tissue, which destroys brain neurons by<br />putting excessive pressure on them as well as<br />depriving them of oxygen. Onset of symptoms in<br />this type of CVA is usually rapid.<br />If, for example, the CVA is in the left frontal lobe,<br />paralysis of the right side of the body will occur.<br />Speech may also be affected if the speech areas are<br />involved. Some CVAs are fatal because the damage<br />The auditory areas, as their name suggests, receive<br />impulses from receptors in the inner ear for hearing.<br />The auditory association area is quite large. Part of it<br />is concerned with the meanings of words we hear, that<br />is, with speech. Other parts are for the interpretation<br />of sounds such as thunder during a storm, an ambulance<br />siren, or a baby crying. Without proper interpretation,<br />we would hear the sound but would not know<br />what it meant, and could not respond appropriately.<br />Also in the temporal and parietal lobes in the left<br />hemisphere (for most of us) are other speech areas<br />concerned with the thought that precedes speech.<br />Each of us can probably recall (and regret) times when<br />we have “spoken without thinking,” but in actuality<br />that is not possible. The thinking takes place very rapidly<br />and is essential in order to be able to speak (see<br />Box 8–5: Aphasia).<br />Occipital Lobes<br />Impulses from the retinas of the eyes travel along the<br />optic nerves to the visual areas in the occipital lobes.<br />These areas “see.” The visual association areas interpret<br />what is seen, and enable the thinking cerebrum to<br />use the information. Imagine looking at a clock.<br />Seeing the clock is far different from being able to<br />interpret it. At one time we learned to interpret the<br />clock face and hands, and now we do not have to consciously<br />decide what time the clock is reading. We can<br />simply use that information, such as hurrying a bit so<br />as not to be late to class. Other parts of the occipital<br />lobes are concerned with spatial relationships; things<br />such as judging distance and seeing in three dimensions,<br />or the ability to read a map and relate it to the<br />physical world.<br />The cerebral cortex has the characteristic of neural<br />plasticity, the ability to adapt to changing needs, to<br />recruit different neurons for certain functions, as may<br />occur during childhood or recovery from a stroke.<br />Another example is the visual cortex of a person who<br />is born blind. The neurons in the occipital lobes that<br />would have been used for vision will often be used for<br />another function; some may become part of an auditory<br />area that is used to localize sounds and estimate<br />their distance. Those of us who can see may not rely<br />on hearing for localization; we simply look at where<br />we think the sound came from. A blind person cannot<br />do this, and may have an extensive mental catalogue of<br />sounds, meanings of sounds, distances of sounds, and<br />so on, some of these in the part of the cortex that normally<br />is for vision.<br />The younger the person, the more plastic the brain.<br />The brains of children are extraordinarily adaptable.<br />As we get older, this ability diminishes, but is still<br />present.<br />Association Areas<br />As you can see in Fig. 8–8, many parts of the cerebral<br />cortex are not concerned with movement or a particu-<br />182 The Nervous System<br />BOX 8–5 APHASIA<br />Auditory aphasia is “word deafness,” caused<br />by damage to an interpretation area. The person<br />can still hear but cannot comprehend what the<br />words mean. Visual aphasia is “word blindness”;<br />the person can still see perfectly well, but cannot<br />make sense of written words (the person retains<br />the ability to understand spoken words). Imagine<br />how you would feel if wms qsbbcljw jmqr rfc<br />yzgjgrw rm pcyb. Frustrating isn’t it? You know<br />that those symbols are letters, but you cannot<br />“decode” them right away. Those “words”<br />were formed by shifting the alphabet two letters<br />(A C, B D, C E, etc.), and would normally<br />be read as: “you suddenly lost the ability to read.”<br />That may give you a small idea of what word blindness<br />is like.<br />Our use of language sets us apart from other<br />animals and involves speech, reading, and writing.<br />Language is the use of symbols (words) to designate<br />objects and to express ideas. Damage to<br />the speech areas or interpretation areas of the<br />cerebrum may impair one or more aspects of a person’s<br />ability to use language; this is called aphasia.<br />Aphasia may be a consequence of a cerebrovascular<br />accident, or of physical trauma to the skull<br />and brain such as a head injury sustained in an<br />automobile accident. If the motor speech (Broca’s)<br />area is damaged, the person is still able to understand<br />written and spoken words and knows what<br />he wants to say, but he cannot say it. Without coordination<br />and impulses from the motor speech area,<br />the muscles used for speech cannot contract to<br />form words properly.<br />lar sensation. These may be called association areas<br />and perhaps are what truly make us individuals. It is<br />probably these areas that give each of us a personality,<br />a sense of humor, and the ability to reason and use<br />logic. Learning and memory are also functions of<br />these areas.<br />Although much has been learned about the formation<br />of memories, the processes are still incompletely<br />understood and mostly beyond the scope of this book.<br />Briefly, however, we can say that memories of things<br />such as people or books or what you did last summer<br />involve the hippocampus (from the Greek for<br />“seahorse,” because of its shape), part of the temporal<br />lobe on the floor of the lateral ventricle. The two hippocampi<br />seem to collect information from many areas<br />of the cerebral cortex. When you meet a friend, for<br />example, the memory emerges as a whole: “Here’s<br />Fred,” not in pieces. People whose hippocampi are<br />damaged cannot form new memories that last more<br />than a few seconds.<br />The right hippocampus is also believed to be<br />involved in spatial cognition (literally: “space thinking”).<br />For example, if you are in school and a friend<br />asks you the shortest way to your home, you will probably<br />quickly form a mental map. You can see how<br />much memory that involves (streets, landmarks, and<br />so on), but the hippocampus can take it a step further<br />and make your memories three-dimensional and mentally<br />visible. You can see your way home. That is spatial<br />cognition.<br />It is believed that most, if not all, of what we have<br />experienced or learned is stored somewhere in the<br />brain. Sometimes a trigger may bring back memories;<br />a certain scent or a song could act as possible triggers.<br />Then we find ourselves recalling something from the<br />past and wondering where it came from.<br />The loss of personality due to destruction of<br />brain neurons is perhaps most dramatically seen in<br />Alzheimer’s disease (see Box 8–6: Alzheimer’s Disease).<br />Basal Ganglia<br />The basal ganglia are paired masses of gray matter<br />within the white matter of the cerebral hemispheres<br />(see Fig. 8–6). Their functions are certain subconscious<br />aspects of voluntary movement, and they work<br />with the cerebellum. The basal ganglia help regulate<br />muscle tone, and they coordinate accessory movements<br />such as swinging the arms when walking or gesturing<br />while speaking. The most common disorder of<br />the basal ganglia is Parkinson’s disease (see Box 8–7:<br />Parkinson’s Disease).<br />Corpus Callosum<br />As mentioned previously, the corpus callosum is a<br />band of nerve fibers that connects the left and right<br />cerebral hemispheres. This enables each hemisphere<br />to know of the activity of the other. This is especially<br />important for people because for most of us, the left<br />hemisphere contains speech areas and the right hemisphere<br />does not. The corpus callosum, therefore, lets<br />The Nervous System 183<br />BOX 8–6 ALZHEIMER’S DISEASE<br />of another protein called beta-amyloid that are<br />damaging to neurons.<br />A defective gene has been found in some<br />patients who have late-onset Alzheimer’s disease,<br />the most common type. Yet another gene seems to<br />trigger increased synthesis of beta-amyloid. Some<br />research is focused on the interaction of these<br />genes and on inflammation as a contributing factor<br />to this type of brain damage.<br />It is likely that the treatment of Alzheimer’s disease<br />will one day mean delaying its onset with a<br />variety of medications, each targeted at a different<br />aspect of this complex disease. Early diagnosis will<br />be very important, and this is yet another avenue of<br />research.<br />In the United States, Alzheimer’s disease, a progressive,<br />incurable form of mental deterioration,<br />affects approximately 5 million people and is the<br />cause of 100,000 deaths each year. The first symptoms,<br />which usually begin after age 65, are memory<br />lapses and slight personality changes. As the<br />disease progresses, there is total loss of memory,<br />reasoning ability, and personality, and those with<br />advanced disease are unable to perform even the<br />simplest tasks or self-care.<br />Structural changes in the brains of Alzheimer’s<br />patients may be seen at autopsy. Neurofibrillary<br />tangles are abnormal fibrous proteins found in cells<br />of the cerebral cortex in areas important for memory<br />and reasoning. Also present are plaques made<br />the left hemisphere know what the right hemisphere is<br />thinking about, and the right hemisphere know what<br />the left hemisphere is thinking and talking about. A<br />brief example may be helpful. If you put your left hand<br />behind your back and someone places a pencil in your<br />hand (you are not looking at it) and asks you what it is,<br />would you be able to say? Yes, you would. You would<br />feel the shape and weight of the pencil, find the point<br />and the eraser. The sensory impulses from your left<br />hand are interpreted as “pencil” by the general sensory<br />area in your right parietal lobe. Your right hemisphere<br />probably cannot speak, but its thoughts can be conveyed<br />by way of the corpus callosum to the left hemisphere,<br />which does have speech areas. Your left<br />hemisphere can say that you are holding a pencil.<br />Other aspects of the “division of labor” of our cerebral<br />hemispheres are beyond the scope of this book, but it<br />is a fascinating subject that you may wish to explore<br />further.<br />MENINGES AND<br />CEREBROSPINAL FLUID<br />The connective tissue membranes that cover the brain<br />and spinal cord are called meninges; the three layers<br />are illustrated in Fig. 8–9. The thick outermost layer,<br />made of fibrous connective tissue, is the dura mater<br />(Latin for “tough mother”), which lines the skull and<br />vertebral canal. The middle arachnoid membrane<br />(arachnids are spiders) is made of web-like strands of<br />connective tissue. The innermost pia mater (Latin for<br />“gentle mother”) is a very thin membrane on the surface<br />of the spinal cord and brain. Between the arachnoid<br />and the pia mater is the subarachnoid space,<br />which contains cerebrospinal fluid (CSF), the tissue<br />fluid of the central nervous system.<br />Recall the ventricles (cavities) of the brain: two lateral<br />ventricles, the third ventricle, and the fourth ventricle.<br />Each contains a choroid plexus, a capillary<br />network that forms cerebrospinal fluid from blood<br />plasma. This is a continuous process, and the cerebrospinal<br />fluid then circulates in and around the central<br />nervous system (Fig. 8–10).<br />From the lateral and third ventricles, cerebrospinal<br />fluid flows through the fourth ventricle, then to the<br />central canal of the spinal cord, and to the cranial and<br />spinal subarachnoid spaces. As more cerebrospinal<br />fluid is formed, you might expect that some must be<br />reabsorbed, and that is just what happens. From the<br />cranial subarachnoid space, cerebrospinal fluid is reabsorbed<br />through arachnoid villi into the blood in<br />cranial venous sinuses (large veins within the<br />double-layered cranial dura mater). The cerebrospinal<br />fluid becomes blood plasma again, and the rate of<br />reabsorption normally equals the rate of production.<br />Since cerebrospinal fluid is tissue fluid, one of its<br />functions is to bring nutrients to CNS neurons and to<br />remove waste products to the blood as the fluid is<br />reabsorbed. The other function of cerebrospinal fluid<br />is to act as a cushion for the central nervous system.<br />The brain and spinal cord are enclosed in fluid-filled<br />membranes that absorb shock. You can, for example,<br />shake your head vigorously without harming your<br />brain. Naturally, this protection has limits; very sharp<br />or heavy blows to the skull will indeed cause damage<br />to the brain.<br />Examination of cerebrospinal fluid may be used in<br />the diagnosis of certain diseases (see Box 8–8: Lumbar<br />Puncture).<br />184 The Nervous System<br />BOX 8–7 PARKINSON’S DISEASE<br />Parkinson’s disease is a disorder of the basal<br />ganglia whose cause is unknown, and though<br />there is a genetic component in some families, it<br />is probably not the only factor. The disease usually<br />begins after the age of 60. Neurons in the<br />basal ganglia that produce the neurotransmitter<br />dopamine begin to degenerate and die, and the<br />deficiency of dopamine causes specific kinds<br />of muscular symptoms. Tremor, or involuntary<br />shaking, of the hands is probably the most common<br />symptom. The accessory movements regulated<br />by the basal ganglia gradually diminish,<br />and the affected person walks slowly without<br />swinging the arms. A mask-like face is characteristic<br />of this disease, as the facial muscles become<br />rigid. Eventually all voluntary movements become<br />slower and much more difficult, and balance<br />is seriously impaired.<br />Dopamine itself cannot be used to treat<br />Parkinson’s disease because it does not cross the<br />blood–brain barrier. A substance called L-dopa<br />does cross and can be converted to dopamine by<br />brain neurons. Unfortunately, L-dopa begins to<br />lose its therapeutic effectiveness within a few<br />years.<br />Other medications in use do not provide a<br />cure. Some researchers suggest that implants of<br />stem cells may offer the best hope of meaningful<br />therapy.<br />185<br />Central canal<br />Pia mater<br />Arachnoid<br />membrane<br />Dura<br />mater<br />Gray matter<br />White matter<br />Spinal nerve roots<br />Dorsal root ganglion<br />Subarachnoid<br />space<br />Spinal nerve<br />Skin<br />Dura mater<br />Superior sagittal sinus<br />Cerebral cortex<br />Cerebrum<br />(white matter)<br />Arachnoid membrane<br />Subarachnoid space<br />Pia mater<br />A<br />B<br />Arachnoid villi<br />Skull<br />Figure 8–9. Structure of the meninges. (A) Meninges of the spinal cord. (B) Frontal section<br />through the top of the skull showing the double-layered cranial dura mater and one<br />of the cranial venous sinuses.<br />QUESTION: Describe the structural difference between the spinal dura mater and the cranial<br />dura mater.<br />CRANIAL NERVES<br />The 12 pairs of cranial nerves emerge from the brain<br />stem or other parts of the brain—they are shown in<br />Fig. 8–11. The name cranial indicates their origin, and<br />many of them do carry impulses for functions involving<br />the head. Some, however, have more far-reaching<br />destinations.<br />The impulses for the senses of smell, taste, sight,<br />hearing, and equilibrium are all carried by cranial<br />nerves to their respective sensory areas in the brain.<br />Some cranial nerves carry motor impulses to muscles<br />of the face and eyes or to the salivary glands. The<br />186 The Nervous System<br />Subarachnoid space<br />Cranial<br />meninges<br />Dura mater<br />Arachnoid<br />Fourth ventricle<br />Arachnoid villus<br />Choroid plexus of<br />fourth ventricle<br />Corpus<br />callosum<br />Cerebellum<br />Cerebral aqueduct<br />Pia mater<br />Cranial venous sinus<br />Cerebrum<br />Subarachnoid space<br />Central canal<br />Pons<br />Medulla<br />Spinal cord<br />Spinal meninges<br />Pia mater<br />Arachnoid<br />Dura mater<br />Subarachnoid space<br />Hypothalamus<br />Third ventricle<br />Choroid plexus of<br />third ventricle<br />Choroid plexus of<br />lateral ventricle<br />Lateral<br />ventricle<br />Figure 8–10. Formation, circulation, and reabsorption of cerebrospinal fluid. See text for<br />description.<br />QUESTION: In this pathway, where is the CSF reabsorbed, and into what?<br />vagus nerves (vagus means “wanderer”) branch extensively<br />to the larynx, heart, stomach and intestines, and<br />bronchial tubes.<br />The functions of the cranial nerves are summarized<br />in Table 8–4.<br />THE AUTONOMIC<br />NERVOUS SYSTEM<br />The autonomic nervous system (ANS) is actually<br />part of the peripheral nervous system in that it consists<br />of motor portions of some cranial and spinal nerves.<br />Because its functioning is so specialized, however, the<br />autonomic nervous system is usually discussed as a<br />separate entity, as we will do here.<br />Making up the autonomic nervous system are visceral<br />motor neurons to smooth muscle, cardiac muscle,<br />and glands. These are the visceral effectors;<br />muscle will either contract or relax, and glands will<br />either increase or decrease their secretions.<br />The ANS has two divisions: sympathetic and<br />parasympathetic. Often, they function in opposition<br />to each other, as you will see. The activity of both divisions<br />is integrated by the hypothalamus, which<br />ensures that the visceral effectors will respond appropriately<br />to the situation.<br />AUTONOMIC PATHWAYS<br />An autonomic nerve pathway from the central nervous<br />system to a visceral effector consists of two motor<br />neurons that synapse in a ganglion outside the CNS<br />(Fig. 8–12). The first neuron is called the preganglionic<br />neuron, from the CNS to the ganglion. The<br />second neuron is called the postganglionic neuron,<br />from the ganglion to the visceral effector. The ganglia<br />are actually the cell bodies of the postganglionic<br />neurons.<br />SYMPATHETIC DIVISION<br />Another name for the sympathetic division is thoracolumbar<br />division, which tells us where the sympathetic<br />preganglionic neurons originate. Their cell<br />The Nervous System 187<br />BOX 8–8 LUMBAR PUNCTURE<br />Box Figure 8–B Cerebrospinal fluid from a patient with<br />meningitis. The bacteria are streptococci, found in pairs. The<br />large cells are WBCs. ( 500) (From Sacher, RA, and<br />McPherson, RA: Widmann’s Clinical Interpretation of<br />Laboratory Tests, ed. 11. FA Davis, Philadelphia, 2000, Plate<br />52, with permission.)<br />A lumbar puncture (spinal tap) is a diagnostic<br />procedure that involves the removal of cerebrospinal<br />fluid to determine its pressure and constituents.<br />As the name tells us, the removal, using a<br />syringe, is made in the lumbar area. Because the<br />spinal cord ends between the 1st and 2nd lumbar<br />vertebrae, the needle is usually inserted between<br />the 4th and 5th lumbar vertebrae. The meningeal<br />sac containing cerebrospinal fluid extends to the<br />end of the lumbar vertebrae, permitting access to<br />the cerebrospinal fluid with little chance of damaging<br />the spinal cord.<br />Cerebrospinal fluid is a circulating fluid and has<br />a normal pressure of 70 to 200 mmH2O. An abnormal<br />pressure usually indicates an obstruction in circulation,<br />which may be caused by infection, a<br />tumor, or mechanical injury. Other diagnostic<br />tests would be needed to determine the precise<br />cause.<br />Perhaps the most common reason for a lumbar<br />puncture is suspected meningitis, which may be<br />caused by several kinds of bacteria. If the patient<br />does have meningitis, the cerebrospinal fluid will be<br />cloudy rather than clear and will be examined for<br />the presence of bacteria and many white blood<br />cells. A few WBCs in CSF is normal, because WBCs<br />are found in all tissue fluid.<br />Another abnormal constituent of cerebrospinal<br />fluid is red blood cells. Their presence indicates<br />bleeding somewhere in the central nervous system.<br />There may be many causes, and again, further testing<br />would be necessary.<br />188 The Nervous System<br />Optic<br />chiasma<br />Figure 8–11. Cranial<br />nerves and their distributions.<br />The brain is shown in an inferior<br />view. See Table 8–4 for<br />descriptions.<br />QUESTION: Which cranial<br />nerves bring about secretion<br />of saliva? Which nerve brings<br />about gastric and intestinal<br />secretion?<br />bodies are in the thoracic segments and some of the<br />lumbar segments of the spinal cord. Their axons<br />extend to the sympathetic ganglia, most of which are<br />located in two chains just outside the spinal column<br />(see Fig. 8–12). Within the ganglia are the synapses<br />between preganglionic and postganglionic neurons;<br />the postganglionic axons then go to the visceral effectors.<br />One preganglionic neuron often synapses with<br />many postganglionic neurons to many effectors. This<br />anatomic arrangement has physiological importance:<br />The sympathetic division brings about widespread<br />responses in many organs.<br />The sympathetic division is dominant in stressful<br />situations, which include anger, fear, or anxiety, as<br />well as exercise. For our prehistoric ancestors, stressful<br />situations often involved the need for intense physical<br />activity—the “fight or flight response.” Our<br />nervous systems haven’t changed very much in 50,000<br />years, and if you look at Table 8–5, you will see the<br />kinds of responses the sympathetic division stimulates.<br />The heart rate increases, vasodilation in skeletal muscles<br />supplies them with more oxygen, the bronchioles<br />dilate to take in more air, and the liver changes glycogen<br />to glucose to supply energy. At the same time,<br />digestive secretions decrease and peristalsis slows;<br />these are not important in a stress situation.<br />Vasoconstriction in the skin and viscera shunts blood<br />to more vital organs such as the heart, muscles, and<br />brain. All of these responses enabled our ancestors to<br />stay and fight or to get away from potential danger.<br />Even though we may not always be in life-threatening<br />situations during stress (such as figuring out our<br />income taxes), our bodies are prepared for just that.<br />PARASYMPATHETIC DIVISION<br />The other name for the parasympathetic division is<br />the craniosacral division. The cell bodies of parasympathetic<br />preganglionic neurons are in the brain stem<br />and the sacral segments of the spinal cord. Their axons<br />are in cranial nerve pairs 3, 7, 9, and 10 and in some<br />sacral nerves and extend to the parasympathetic ganglia.<br />These ganglia are very close to or actually in the<br />visceral effector (see Fig. 8–12), and contain the postganglionic<br />cell bodies, with very short axons to the<br />cells of the effector.<br />In the parasympathetic division, one preganglionic<br />neuron synapses with just a few postganglionic neurons<br />to only one effector. With this anatomic arrangement,<br />very localized (one organ) responses are possible.<br />The parasympathetic division dominates in relaxed<br />(non-stress) situations to promote normal functioning<br />of several organ systems. Digestion will be efficient,<br />with increased secretions and peristalsis; defecation<br />and urination may occur; and the heart will beat at a<br />normal resting rate. Other functions of this division<br />are listed in Table 8–5.<br />Notice that when an organ receives both sympathetic<br />and parasympathetic impulses, the responses are<br />opposites. Such an arrangement makes maintaining an<br />appropriate level of activity quite simple, as in changing<br />the heart rate to meet the needs of a situation.<br />Notice also that some visceral effectors receive only<br />sympathetic impulses. In such cases, the opposite<br />response is brought about by a decrease in sympathetic<br />impulses. Secretion by the sweat glands is an example.<br />NEUROTRANSMITTERS<br />Recall that neurotransmitters enable nerve impulses to<br />cross synapses. In autonomic pathways there are two<br />synapses: one between preganglionic and postganglionic<br />neurons, and the second between postganglionic<br />neurons and visceral effectors.<br />Acetylcholine is the transmitter released by all<br />preganglionic neurons, both sympathetic and para-<br />The Nervous System 189<br />Table 8–4 CRANIAL NERVES<br />Number and Name Function(s)<br />I Olfactory<br />II Optic<br />III Oculomotor<br />IV Trochlear<br />V Trigeminal<br />VI Abducens<br />VII Facial<br />VIII Acoustic (vestibulocochlear)<br />IX Glossopharyngeal<br />X Vagus<br />XI Accessory<br />XII Hypoglossal<br />• Sense of smell<br />• Sense of sight<br />• Movement of the eyeball; constriction of pupil in bright light or for near vision<br />• Movement of eyeball<br />• Sensation in face, scalp, and teeth; contraction of chewing muscles<br />• Movement of the eyeball<br />• Sense of taste; contraction of facial muscles; secretion of saliva<br />• Sense of hearing; sense of equilibrium<br />• Sense of taste; sensory for cardiac, respiratory, and blood pressure reflexes;<br />contraction of pharynx; secretion of saliva<br />• Sensory in cardiac, respiratory, and blood pressure reflexes; sensory and motor<br />to larynx (speaking); decreases heart rate; contraction of alimentary tube<br />(peristalsis); increases digestive secretions<br />• Contraction of neck and shoulder muscles; motor to larynx (speaking)<br />• Movement of the tongue<br />Sympathetic<br />Eye Ciliary ganglion<br />Parasympathetic<br />Salivary<br />glands<br />Pons<br />Otic<br />ganglion<br />Vagus nerve<br />Pterygopalatine<br />ganglion<br />Submandibular<br />ganglion<br />Midbrain<br />III<br />Medulla<br />VII<br />IX<br />Trachea<br />Preganglionic<br />neuron<br />Preganglionic neurons<br />Postganglionic<br />neuron<br />Postganglionic<br />neurons<br />Celiac ganglion<br />Adrenal gland<br />Chain of<br />sympathetic<br />ganglia<br />Inferior<br />mesenteric<br />ganglion<br />Kidney<br />Pancreas<br />Superior<br />mesenteric<br />ganglion<br />Large<br />intestine<br />Bronchioles<br />Heart<br />Stomach<br />Small<br />intestine<br />Colon<br />Rectum<br />Reproductive<br />organs<br />T1<br />T2<br />T3<br />T4<br />T5<br />T6<br />T7<br />T8<br />T9<br />T10<br />T11<br />T12<br />L1<br />L2<br />S2<br />S3<br />S4<br />X<br />Bladder<br />Figure 8–12. The autonomic nervous system. The sympathetic division is shown on the<br />left, and the parasympathetic division is shown on the right (both divisions are bilateral).<br />QUESTION: Do both or just one division of the ANS supply the heart? What is the purpose<br />of this arrangement?<br />190<br />sympathetic; it is inactivated by cholinesterase in<br />postganglionic neurons. Parasympathetic postganglionic<br />neurons all release acetylcholine at the<br />synapses with their visceral effectors. Most sympathetic<br />postganglionic neurons release the transmitter<br />norepinephrine at the synapses with the effector cells.<br />Norepinephrine is inactivated by either catechol-Omethyl<br />transferase (COMT) or monoamine oxidase<br />(MAO), or it may be removed from the synapse by<br />reuptake.<br />AGING AND THE<br />NERVOUS SYSTEM<br />The aging brain does lose neurons, but this is only a<br />small percentage of the total and not the usual cause of<br />mental impairment in elderly people. (Far more common<br />causes are depression, malnutrition, hypotension,<br />and the side effects of medications.) Some forgetfulness<br />is to be expected, however, as is a decreased ability<br />for rapid problem solving, but most memory<br />should remain intact. Voluntary movements become<br />slower, as do reflexes and reaction time. Think of driving<br />a car, an ability most of us take for granted. For<br />elderly people, with their slower perceptions and reaction<br />times, greater consciousness of driving is necessary.<br />As the autonomic nervous system ages, dry eyes and<br />constipation may become problems. Transient hypotension<br />may be the result of decreased sympathetic<br />stimulation of vasoconstriction. In most cases, however,<br />elderly people who are aware of these aspects of<br />aging will be able to work with their physicians or<br />nurses to adapt to them.<br />SUMMARY<br />The nervous system regulates many of our simplest<br />and our most complex activities. The impulses generated<br />and carried by the nervous system are an example<br />of the chemical level of organization of the body.<br />These nerve impulses then regulate the functioning of<br />tissues, organs, and organ systems, which permits us to<br />perceive and respond to the world around us and the<br />changes within us. The detection of such changes is<br />the function of the sense organs, and they are the subject<br />of our next chapter.<br />The Nervous System 191<br />Table 8–5 FUNCTIONS OF THE AUTONOMIC NERVOUS SYSTEM<br />Organ Sympathetic Response Parasympathetic Response<br />Heart (cardiac muscle)<br />Bronchioles (smooth muscle)<br />Iris (smooth muscle)<br />Salivary glands<br />Stomach and intestines (smooth muscle)<br />Stomach and intestines (glands)<br />Internal anal sphincter<br />Urinary bladder (smooth muscle)<br />Internal urethral sphincter<br />Liver<br />Pancreas<br />Sweat glands<br />Blood vessels in skin and viscera<br />(smooth muscle)<br />Blood vessels in skeletal muscle<br />(smooth muscle)<br />Adrenal glands<br />• Increase rate<br />• Dilate<br />• Pupil dilates<br />• Decrease secretion<br />• Decrease peristalsis<br />• Decrease secretion<br />• Contracts to prevent defecation<br />• Relaxes to prevent urination<br />• Contracts to prevent urination<br />• Changes glycogen to glucose<br />• Secretes glucagon<br />• Increase secretion<br />• Constrict<br />• Dilate<br />• Increase secretion of epinephrine<br />and norepinephrine<br />• Decrease rate (to normal)<br />• Constrict (to normal)<br />• Pupil constricts (to normal)<br />• Increase secretion (to normal)<br />• Increase peristalsis for normal digestion<br />• Increase secretion for normal digestion<br />• Relaxes to permit defecation<br />• Contracts for normal urination<br />• Relaxes to permit urination<br />• None<br />• Secretes insulin and digestive enzymes<br />• None<br />• None<br />• None<br />• None<br />192 The Nervous System<br />STUDY OUTLINE<br />Functions of the Nervous System<br />1. Detect changes and feel sensations.<br />2. Initiate responses to changes.<br />3. Organize and store information.<br />Nervous System Divisions<br />1. Central nervous system (CNS)—brain and spinal<br />cord.<br />2. Peripheral nervous system (PNS)—cranial nerves<br />and spinal nerves.<br />Nerve Tissue—neurons (nerve fibers) and<br />specialized cells (Schwann, neuroglia)<br />1. Neuron cell body contains the nucleus; cell bodies<br />are in the CNS or in the trunk and are protected by<br />bone.<br />2. Axon carries impulses away from the cell body;<br />dendrites carry impulses toward the cell body.<br />3. Schwann cells in PNS: Layers of cell membrane<br />form the myelin sheath to electrically insulate neurons;<br />nodes of Ranvier are spaces between adjacent<br />Schwann cells. Nuclei and cytoplasm of Schwann<br />cells form the neurolemma, which is essential for<br />regeneration of damaged axons or dendrites.<br />4. Oligodendrocytes in CNS form the myelin<br />sheaths; microglia phagocytize pathogens and<br />damaged cells; astrocytes contribute to the<br />blood–brain barrier (see Table 8–1).<br />5. Synapse—the space between the axon of one neuron<br />and the dendrites or cell body of the next neuron.<br />A neurotransmitter carries the impulse across<br />a synapse and is then destroyed by a chemical inactivator.<br />Synapses make impulse transmission one<br />way in the living person.<br />Types of Neurons—nerve fibers<br />1. Sensory—carry impulses from receptors to the<br />CNS; may be somatic (from skin, skeletal muscles,<br />joints) or visceral (from internal organs).<br />2. Motor—carry impulses from the CNS to effectors;<br />may be somatic (to skeletal muscle) or visceral (to<br />smooth muscle, cardiac muscle, or glands). Visceral<br />motor neurons make up the autonomic nervous<br />system.<br />3. Interneurons—entirely within the CNS.<br />Nerves and Nerve Tracts<br />1. Sensory nerve—made only of sensory neurons.<br />2. Motor nerve—made only of motor neurons.<br />3. Mixed nerve—made of both sensory and motor<br />neurons.<br />4. Nerve tract—a nerve within the CNS; also called<br />white matter.<br />The Nerve Impulse—see Table 8–2<br />1. Polarization—neuron membrane has a ( ) charge<br />outside and a ( ) charge inside.<br />2. Depolarization—entry of Na ions and reversal of<br />charges on either side of the membrane.<br />3. Impulse transmission is rapid, often several meters<br />per second.<br />• Saltatory conduction—in a myelinated neuron<br />only the nodes of Ranvier depolarize; increases<br />speed of impulses.<br />The Spinal Cord<br />1. Functions: transmits impulses to and from the<br />brain, and integrates the spinal cord reflexes.<br />2. Location: within the vertebral canal; extends from<br />the foramen magnum to the disc between the 1st<br />and 2nd lumbar vertebrae.<br />3. Cross-section: internal H-shaped gray matter contains<br />cell bodies of motor neurons and interneurons;<br />external white matter is the myelinated axons<br />and dendrites of interneurons.<br />4. Ascending tracts carry sensory impulses to the<br />brain; descending tracts carry motor impulses away<br />from the brain.<br />5. Central canal contains cerebrospinal fluid and is<br />continuous with the ventricles of the brain.<br />Spinal Nerves—see Table 8–3 for major<br />peripheral nerves<br />1. Eight cervical pairs to head, neck, shoulder, arm,<br />and diaphragm; 12 thoracic pairs to trunk; 5 lumbar<br />pairs and 5 sacral pairs to hip, pelvic cavity, and<br />leg; 1 very small coccygeal pair.<br />2. Cauda equina—the lumbar and sacral nerves that<br />extend below the end of the spinal cord.<br />3. Each spinal nerve has two roots: dorsal or sensory<br />root; dorsal root ganglion contains cell bodies of<br />sensory neurons; ventral or motor root; the two<br />roots unite to form a mixed spinal nerve.<br />Spinal Cord Reflexes—do not depend directly<br />on the brain<br />1. A reflex is an involuntary response to a stimulus.<br />2. Reflex arc—the pathway of nerve impulses during a<br />reflex: (1) receptors, (2) sensory neurons, (3) CNS<br />with one or more synapses, (4) motor neurons,<br />(5) effector that responds.<br />3. Stretch reflex—a muscle that is stretched will contract;<br />these reflexes help keep us upright against<br />gravity. The patellar reflex is also used clinically to<br />assess neurologic functioning, as are many other<br />reflexes (Fig. 8–5).<br />4. Flexor reflex—a painful stimulus will cause withdrawal<br />of the body part; these reflexes are protective.<br />The Brain—many parts that function as an<br />integrated whole; see Figs. 8–6 and 8–8 for<br />locations<br />1. Ventricles—four cavities: two lateral, 3rd, 4th; each<br />contains a choroid plexus that forms cerebrospinal<br />fluid (Figs. 8–6 and 8–7).<br />2. Medulla—regulates the vital functions of heart<br />rate, breathing, and blood pressure; regulates<br />reflexes of coughing, sneezing, swallowing, and<br />vomiting.<br />3. Pons—contains respiratory centers that work with<br />those in the medulla.<br />4. Midbrain—contains centers for visual reflexes,<br />auditory reflexes, and righting (equilibrium)<br />reflexes.<br />5. Cerebellum—regulates coordination of voluntary<br />movement, muscle tone, stopping movements, and<br />equilibrium; contributes to sensations involving<br />texture and weight.<br />6. Hypothalamus—produces antidiuretic hormone<br />(ADH), which increases water reabsorption by the<br />kidneys; produces oxytocin, which promotes uterine<br />contractions for labor and delivery; produces<br />releasing hormones that regulate the secretions of<br />the anterior pituitary gland; regulates body temperature;<br />regulates food intake; integrates the functioning<br />of the autonomic nervous system (ANS);<br />promotes visceral responses to emotional situations;<br />acts as a biological clock that regulates body<br />rhythms.<br />7. Thalamus—groups sensory impulses as to body<br />part before relaying them to the cerebrum; awareness<br />of pain but inability to localize; suppresses<br />unimportant sensations to permit concentration;<br />contributes to alertness and awareness, and to<br />memory.<br />8. Cerebrum—two hemispheres connected by the<br />corpus callosum, which permits communication<br />between the hemispheres. The cerebral cortex is<br />the surface gray matter, which consists of cell bodies<br />of neurons and is folded extensively into convolutions.<br />The internal white matter consists of nerve<br />tracts that connect the lobes of the cerebrum to one<br />another and to other parts of the brain.<br />• Frontal lobes—motor areas initiate voluntary<br />movement; premotor area regulates sequences of<br />movements for learned skills; prefrontal area for<br />aspects of social behavior; Broca’s motor speech<br />area (left hemisphere) regulates the movements<br />involved in speech.<br />• Parietal lobes—general sensory area feels and<br />interprets the cutaneous senses and conscious<br />muscle sense; taste area extends into temporal<br />lobe, for sense of taste; speech areas (left hemisphere)<br />for thought before speech.<br />• Temporal lobes—auditory areas for hearing and<br />interpretation; olfactory areas for sense of smell<br />and interpretation; speech areas for thought<br />before speech.<br />• Occipital lobes—visual areas for vision; interpretation<br />areas for spatial relationships.<br />• Association areas—in all lobes, for abstract<br />thinking, reasoning, learning, memory, and<br />personality. The hippocampi are essential for<br />the formation of memories. Neural plasticity is<br />the ability of the brain to adapt to changing<br />needs.<br />• Basal ganglia—gray matter within the cerebral<br />hemispheres; regulate accessory movements and<br />muscle tone.<br />Meninges and Cerebrospinal Fluid (CSF) (see<br />Figs. 8–9 and 8–10)<br />1. Three meningeal layers made of connective tissue:<br />outer—dura mater; middle—arachnoid membrane;<br />inner—pia mater; all three enclose the brain and<br />spinal cord.<br />2. Subarachnoid space contains CSF, the tissue fluid<br />of the CNS.<br />3. CSF is formed continuously in the ventricles<br />of the brain by choroid plexuses, from blood<br />plasma.<br />4. CSF circulates from the ventricles to the central<br />canal of the spinal cord and to the cranial and<br />spinal subarachnoid spaces.<br />5. CSF is reabsorbed from the cranial subarachnoid<br />space through arachnoid villi into the blood in the<br />cranial venous sinuses. The rate of reabsorption<br />equals the rate of production.<br />The Nervous System 193<br />6. As tissue fluid, CSF brings nutrients to CNS neurons<br />and removes waste products. CSF also acts as<br />a shock absorber to cushion the CNS.<br />Cranial Nerves—12 pairs of nerves that<br />emerge from the brain (see Fig. 8–11)<br />1. Concerned with vision, hearing and equilibrium,<br />taste and smell, and many other functions.<br />2. See Table 8–4 for the functions of each pair.<br />The Autonomic Nervous System (ANS) (see<br />Fig. 8–12 and Table 8–5)<br />1. Has two divisions: sympathetic and parasympathetic;<br />their functioning is integrated by the hypothalamus.<br />2. Consists of motor neurons to visceral effectors:<br />smooth muscle, cardiac muscle, and glands.<br />3. An ANS pathway consists of two neurons that<br />synapse in a ganglion:<br />• Preganglionic neurons—from the CNS to the<br />ganglia<br />• Postganglionic neurons—from the ganglia to the<br />effectors<br />• Most sympathetic ganglia are in two chains<br />just outside the vertebral column; parasympathetic<br />ganglia are very near or in the visceral<br />effectors.<br />4. Neurotransmitters: acetylcholine is released by<br />all preganglionic neurons and by parasympathetic<br />postganglionic neurons; the inactivator is<br />cholinesterase. Norepinephrine is released by most<br />sympathetic postganglionic neurons; the inactivator<br />is COMT or MAO.<br />5. Sympathetic division—dominates during stress situations;<br />responses prepare the body to meet physical<br />demands.<br />6. Parasympathetic division—dominates in relaxed<br />situations to permit normal functioning.<br />194 The Nervous System<br />REVIEW QUESTIONS<br />1. Name the divisions of the nervous system and state<br />the parts of each. (p. 166)<br />2. State the function of the following parts of nerve<br />tissue: (pp. 166–167)<br />a. Axon<br />b. Dendrites<br />c. Myelin sheath<br />d. Neurolemma<br />e. Microglia<br />f. Astrocytes<br />3. Explain the difference between: (pp. 170–171)<br />a. Sensory neurons and motor neurons<br />b. Interneurons and nerve tracts<br />4. Describe an electrical nerve impulse in terms of<br />charges on either side of the neuron membrane.<br />Describe how a nerve impulse crosses a synapse.<br />(pp. 168–169, 171)<br />5. With respect to the spinal cord: (p. 172)<br />a. Describe its location<br />b. State what gray matter and white matter are<br />made of<br />c. State the function of the dorsal root, ventral<br />root, and dorsal root ganglion<br />6. State the names and number of pairs of spinal<br />nerves. State the part of the body supplied by<br />the phrenic nerves, radial nerves, and sciatic nerves.<br />(pp. 172, 174)<br />7. Define reflex, and name the five parts of a reflex<br />arc. (pp. 172, 174)<br />8. Define stretch reflexes, and explain their practical<br />importance. Define flexor reflexes, and explain<br />their practical importance. (p. 175)<br />9. Name the part of the brain concerned with each of<br />the following: (pp. 176–179)<br />a. Regulates body temperature<br />b. Regulates heart rate<br />c. Suppresses unimportant sensations<br />d. Regulates respiration (two parts)<br />e. Regulates food intake<br />f. Regulates coordination of voluntary movement<br />g. Regulates secretions of the anterior pituitary<br />gland<br />h. Regulates coughing and sneezing<br />i. Regulates muscle tone<br />j. Regulates visual and auditory reflexes<br />k. Regulates blood pressure<br />10. Name the part of the cerebrum concerned with<br />each of the following: (pp. 179–183)<br />a. Feels the cutaneous sensations<br />b. Contains the auditory areas<br />c. Contains the visual areas<br />d. Connects the cerebral hemispheres<br />e. Regulates accessory movements<br />f. Contains the olfactory areas<br />g. Initiates voluntary movement<br />h. Contains the speech areas (for most people)<br />11. Name the three layers of the meninges, beginning<br />with the outermost. (p. 184)<br />12. State all the locations of cerebrospinal fluid. What<br />is CSF made from? Into what is CSF reabsorbed?<br />State the functions of CSF. (p. 184)<br />13. State a function of each of the following cranial<br />nerves: (p. 189)<br />a. Glossopharyngeal<br />b. Olfactory<br />c. Trigeminal<br />d. Facial<br />e. Vagus (three functions)<br />14. Explain how the sympathetic division of the ANS<br />helps the body adapt to a stress situation; give<br />three specific examples. (pp. 188–189)<br />15. Explain how the parasympathetic division of the<br />ANS promotes normal body functioning; give<br />three specific examples. (pp. 189, 191)<br />The Nervous System 195<br />FOR FURTHER THOUGHT<br />1. Your friend Fred was telling a story, with eloquent<br />gestures, while making a salad. He missed the<br />tomato with the knife, cut his hand badly, and<br />needed quite a few stitches. A local anesthetic was<br />used. How might a local anesthetic stop nerve<br />impulses? (Remember that a nerve impulse is very<br />simple.) What part of Fred’s brain got him into<br />trouble?<br />2. Some pesticides kill insects by interfering with<br />cholinesterase. We have cholinesterase too, and<br />may be adversely affected. What would be the<br />symptoms of such pesticide poisoning?<br />3. We cannot live without a central nervous system.<br />Describe all the ways in which the central nervous<br />system is protected.<br />4. Older drivers are sometimes said to have “lost their<br />reflexes.” Is this really true? Explain.<br />5. Look at Question Figure 8–A. Starting at the top<br />of column A, read the words down as fast as you<br />can. For column B, start at the top and name the<br />colors as fast as you can. Did you have any trouble?<br />Now column C: Start at the top and name the colors—<br />do not read the words—as fast as you can.<br />Was there any difference? Explain why.<br />Question Figure 8–A<br />A B C<br />196<br />CHAPTER 9<br />Chapter Outline<br />Sensory Pathway<br />Characteristics of Sensations<br />Cutaneous Senses<br />Referred Pain<br />Muscle Sense<br />Sense of Taste<br />Sense of Smell<br />Hunger and Thirst<br />The Eye<br />Eyelids and the Lacrimal Apparatus<br />Eyeball<br />Layers of the eyeball<br />Cavities of the eyeball<br />Physiology of Vision<br />The Ear<br />Outer Ear<br />Middle Ear<br />Inner Ear<br />Cochlea<br />Utricle and saccule<br />Semicircular canals<br />Arterial Receptors<br />Aging and the Senses<br />BOX 9–1 CATARACTS<br />BOX 9–2 GLAUCOMA<br />BOX 9–3 ERRORS OF REFRACTION<br />BOX 9–4 NIGHT BLINDNESS AND COLOR BLINDNESS<br />BOX 9–5 DEAFNESS<br />BOX 9–6 MOTION SICKNESS<br />Student Objectives<br />• Explain the general purpose of sensations.<br />• Name the parts of a sensory pathway, and state the<br />function of each.<br />• Describe the characteristics of sensations.<br />• Name the cutaneous senses, and explain their<br />purpose.<br />• Explain referred pain and its importance.<br />• Explain the importance of muscle sense.<br />• Describe the pathways for the senses of taste and<br />smell, and explain how these senses are interrelated.<br />• Name the parts of the eye and their functions.<br />• Describe the physiology of vision.<br />• Name the parts of the ear and their functions.<br />• Describe the physiology of hearing.<br />• Describe the physiology of equilibrium.<br />• Explain the importance of the arterial pressoreceptors<br />and chemoreceptors.<br />The Senses<br />197<br />New Terminology<br />Adaptation (A-dap-TAY-shun)<br />After-image (AFF-ter-im-ije)<br />Aqueous humor (AY-kwee-us HYOO-mer)<br />Cochlea (KOK-lee-ah)<br />Cones (KOHNES)<br />Conjunctiva (KON-junk-TIGH-vah)<br />Contrast (KON-trast)<br />Cornea (KOR-nee-ah)<br />Eustachian tube (yoo-STAY-shee-un TOOB)<br />Iris (EYE-ris)<br />Lacrimal glands (LAK-ri-muhl)<br />Olfactory receptors (ohl-FAK-toh-ree)<br />Organ of Corti (KOR-tee)<br />Projection (proh-JEK-shun)<br />Referred pain (ree-FURRD PAYNE)<br />Retina (RET-i-nah)<br />Rhodopsin (roh-DOP-sin)<br />Rods (RAHDS)<br />Sclera (SKLER-ah)<br />Semicircular canals (SEM-ee-SIR-kyoo-lur)<br />Tympanic membrane (tim-PAN-ik)<br />Vitreous humor (VIT-ree-us HYOO-mer)<br />Related Clinical Terminology<br />Age-related macular degeneration (MAK-yoo-lar<br />de-gen-er-AY-shun)<br />Amblyopia (am-blee-OH-pee-uh)<br />Astigmatism (uh-STIG-mah-TIZM)<br />Cataract (KAT-uh-rakt)<br />Color blindness (KUHL-or BLIND-ness)<br />Conjunctivitis (kon-JUNK-ti-VIGH-tis)<br />Deafness (DEFF-ness)<br />Detached retina (dee-TACHD)<br />Glaucoma (glaw-KOH-mah)<br />Hyperopia (HIGH-per-OH-pee-ah)<br />Motion sickness (MOH-shun)<br />Myopia (my-OH-pee-ah)<br />Night blindness (NITE BLIND-ness)<br />Otitis media (oh-TIGH-tis MEE-dee-ah)<br />Phantom pain (FAN-tum)<br />Presbyopia (PREZ-bee-OH-pee-ah)<br />Strabismus (strah-BIZ-miss)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />Our senses constantly provide us with information<br />about our surroundings: We see, hear, and touch. The<br />senses of taste and smell enable us to enjoy the flavor<br />of our food or warn us that food has spoiled and may<br />be dangerous to eat. Our sense of equilibrium keeps us<br />upright. We also get information from our senses<br />about what is happening inside the body. The pain of<br />a headache, for example, prompts us to do something<br />about it, such as take aspirin. In general, this is the<br />purpose of sensations: to enable the body to respond<br />appropriately to ever-changing situations and maintain<br />homeostasis.<br />SENSORY PATHWAY<br />The impulses involved in sensations follow very precise<br />pathways, which all have the following parts:<br />1. Receptors—detect changes (stimuli) and generate<br />impulses. Receptors are usually very specific with<br />respect to the kinds of changes they respond to.<br />Those in the retina detect light rays, those in the<br />nasal cavities detect vapors, and so on. Once a specific<br />stimulus has affected receptors, however, they<br />all respond in the same way by generating electrical<br />nerve impulses.<br />2. Sensory neurons—transmit impulses from receptors<br />to the central nervous system. These sensory<br />neurons are found in both spinal nerves and cranial<br />nerves, but each carries impulses from only one<br />type of receptor.<br />3. Sensory tracts—white matter in the spinal cord or<br />brain that transmits the impulses to a specific part<br />of the brain.<br />4. Sensory areas—most are in the cerebral cortex.<br />These areas feel and interpret the sensations.<br />Learning to interpret sensations begins in infancy,<br />without our awareness of it, and continues throughout<br />life.<br />CHARACTERISTICS OF SENSATIONS<br />Certain characteristics of sensations will help you<br />understand how the sensory areas work with information<br />from the receptors.<br />1. Projection—the sensation seems to come from the<br />area where the receptors were stimulated. If you<br />touch this book, the sensation of touch seems to be<br />in your hand but is actually being felt by your cerebral<br />cortex. That it is indeed the brain that feels<br />sensations is demonstrated by patients who feel<br />phantom pain after amputation of a limb. After<br />loss of a hand, for example, the person may still feel<br />that the hand is really there. Why does this happen?<br />The receptors in the hand are no longer present,<br />but the severed nerve endings continue to<br />generate impulses. These impulses arrive in the<br />parietal lobe area for the hand, and the brain does<br />what it has always done and creates the projection,<br />the feeling that the hand is still there. For most<br />amputees, phantom pain diminishes as the severed<br />nerves heal, but the person often experiences a<br />phantom “presence” of the missing part. This may<br />be helpful when learning to use an artificial limb.<br />2. Intensity—some sensations are felt more distinctly<br />and to a greater degree than are others. A weak<br />stimulus such as dim light will affect a small number<br />of receptors, but a stronger stimulus, such as<br />bright sunlight, will stimulate many more receptors.<br />When more receptors are stimulated, more<br />impulses will arrive in the sensory area of the brain.<br />The brain “counts” the impulses and projects a<br />more intense sensation.<br />3. Contrast—the effect of a previous or simultaneous<br />sensation on a current sensation, which may then<br />be exaggerated or diminished. Again, this is a function<br />of the brain, which constantly compares sensations.<br />If, on a very hot day, you jump into a<br />swimming pool, the water may feel quite cold at<br />first. The brain compares the new sensation to the<br />previous one, and since there is a significant difference<br />between the two, the water will seem colder<br />than it actually is.<br />4. Adaptation—becoming unaware of a continuing<br />stimulus. Receptors detect changes, but if the stimulus<br />continues it may not be much of a change, and<br />the receptors will generate fewer impulses. The<br />water in the swimming pool that seemed cold at<br />first seems to “warm up” after a few minutes. The<br />water has not changed temperature, and the receptors<br />for cold have no changes to detect, therefore<br />they generate fewer impulses. The sensation of<br />cold lessens, and we interpret or feel that as<br />increasing warmth. For another example, look at<br />your left wrist (or perhaps the right one). Many of<br />us wear a watch and are probably unaware of its<br />198 The Senses<br />presence on the arm most of the time. The cutaneous<br />receptors for touch or pressure adapt very<br />quickly to a continuing stimulus, and if there is no<br />change, there is nothing for the receptors to detect.<br />5. After-image—the sensation remains in the consciousness<br />even after the stimulus has stopped. A<br />familiar example is the bright after-image seen after<br />watching a flashbulb go off. The very bright light<br />strongly stimulates receptors in the retina, which<br />generate many impulses that are perceived as an<br />intense sensation that lasts longer than the actual<br />stimulus.<br />CUTANEOUS SENSES<br />The dermis of the skin and the subcutaneous tissue<br />contain receptors for the sensations of touch, pressure,<br />heat, cold, and pain. The receptors for pain, heat, and<br />cold are free nerve endings, which also respond to<br />any intense stimulus. Intense pressure, for example,<br />may be felt as pain. The receptors for touch and pressure<br />are encapsulated nerve endings, meaning that<br />there is a cellular structure around the nerve ending<br />(Fig. 9–1).<br />The cutaneous senses provide us with information<br />about the external environment and also about<br />the skin itself. Much of the information about the<br />environment is not of great importance and is<br />processed at a subconscious level (suppressed by the<br />thalamus), though we can choose to be aware of it. For<br />example, could you distinguish a cotton T-shirt from<br />denim jeans by touch alone? Probably, but you might<br />not realize that you can do that until you try it by, say,<br />sorting laundry in the dark. If you were walking barefoot,<br />could you tell if you were walking on a carpet, a<br />The Senses 199<br />Free nerve<br />endings<br />(temperature<br />receptor)<br />Merkel disc<br />(touch receptor)<br />Ruffini corpuscle<br />(pressure receptor) Meissner corpuscle<br />(touch receptor)<br />Pacinian corpuscle<br />(pressure receptor)<br />Subcutaneous<br />tissue<br />Dermis<br />Epidermis<br />Free nerve endings<br />(pain receptor)<br />Figure 9–1. Cutaneous receptors in a section of the skin. Free nerve endings and encapsulated<br />nerve endings are shown.<br />QUESTION: In which layers are most of the cutaneous receptors located?<br />wood floor, concrete, or beach sand? Yes, you could.<br />But are we usually aware of the sensation from the<br />soles of our feet? If all is going well, probably not.<br />Some people with diabetes develop diabetic neuropathy,<br />damage to nerves that impairs sensation, and they<br />may say that a wood floor feels like walking on cotton<br />balls or that the buttons of a shirt feel too large or too<br />small. They are aware of such odd sensations simply<br />because the feelings are odd. For most of us, the touch<br />of the wood floor is not brought to awareness because<br />it is what the brain expects from past experience,<br />but if the floor has splinters or if the beach sand is hot,<br />we are certainly aware. This is information we can<br />bring to our conscious minds if necessary, but usually<br />do not.<br />As for the skin itself, if you have ever had poison ivy<br />or chickenpox, you may remember the itching sensation<br />of the rash. An itch is actually a mild pain sensation,<br />which may become real pain if not scratched.<br />Why does scratching help relieve some itches, besides<br />by removing an external irritant? One proposed<br />mechanism is that scratching is a bit more painful than<br />the itch, and the impulses it generates can distract the<br />brain from the impulses from the itch. Scratching will<br />not help relieve the itch of poison ivy, chickenpox, or<br />a mosquito bite, however, because the irritating chemicals<br />are in the skin, not on it. In such cases, scratching<br />may do more damage and worsen inflammation at<br />the site.<br />The sensory areas for the skin are in the parietal<br />lobes. You may recall from Chapter 5 that the sensitivity<br />of an area of skin is determined by the number<br />of receptors present. The number of receptors corresponds<br />to the size of the sensory area in the cerebral<br />cortex. The largest parts of this sensory cortex are for<br />the parts of the skin with the most receptors, that is,<br />the hands and face.<br />As mentioned previously, sensory areas are not<br />merely passive recipients of impulses. Consider the<br />sensation of wetness. It is a distinct sensation, but<br />there are no receptors for “wet” in the skin. Where<br />does the sensation come from? Where all sensation<br />comes from: the brain. The parietal lobes have learned<br />to associate the simultaneous reception of temperature<br />and pressure impulses with “wet.” You can demonstrate<br />this for yourself by putting on a plastic glove<br />and dunking your fingers in a cup of water. Your fingers<br />will feel wet, though they are perfectly dry inside<br />the glove. Wetness is a learned sensation, created by<br />the brain.<br />REFERRED PAIN<br />Free nerve endings are also found in internal organs.<br />The smooth muscle of the small intestine, for example,<br />has free nerve endings that are stimulated by<br />excessive stretching or contraction; the resulting pain<br />is called visceral pain. Sometimes pain that originates<br />in an internal organ may be felt in a cutaneous area;<br />this is called referred pain. The pain of a heart attack<br />(myocardial infarction) may be felt in the left arm and<br />shoulder, or the pain of gallstones may be felt in the<br />right shoulder.<br />This referred pain is actually a creation of the<br />brain. Within the spinal cord are sensory tracts<br />that are shared by cutaneous impulses and visceral<br />impulses. Cutaneous impulses are much more frequent,<br />and the brain correctly projects the sensation to<br />the skin. When the impulses come from an organ such<br />as the heart, however, the brain may still project the<br />sensation to the “usual” cutaneous area. The brain<br />projects sensation based on past experience, and cutaneous<br />pain is far more common than visceral pain.<br />Knowledge of referred pain, as in the examples mentioned<br />earlier, may often be helpful in diagnosis.<br />MUSCLE SENSE<br />Muscle sense (also called proprioception or kinesthetic<br />sense) was discussed in Chapter 7 and will be<br />reviewed only briefly here. Stretch receptors (also<br />called proprioceptors or muscle spindles) detect<br />stretching of muscles and generate impulses, which<br />enable the brain to create a mental picture to know<br />where the muscles are and how they are positioned.<br />Conscious muscle sense is felt by the parietal lobes.<br />Unconscious muscle sense is used by the cerebellum<br />to coordinate voluntary movements. We do not have<br />to see our muscles to be sure that they are performing<br />their intended actions. Muscle sense also contributes<br />to our ability to distinguish the shape of objects.<br />SENSE OF TASTE<br />The receptors for taste are found in taste buds, most<br />of which are in papillae on the tongue (Fig. 9–2).<br />These chemoreceptors detect chemicals in solution<br />in the mouth. The chemicals are foods and the solvent<br />is saliva (if the mouth is very dry, taste is very<br />200 The Senses<br />201<br />Figure 9–2. Structures concerned with the senses of smell and taste, shown in a midsagittal<br />section of the head.<br />QUESTION: If we sniff something pungent, why can we often taste it as well? (Follow the<br />inhaled air.)<br />indistinct). There are five (perhaps more) general<br />types of taste receptors: sweet, sour, salty, bitter, and<br />savory. Savory (also called umami or glutamate) is a<br />taste like grilled meat. We experience many more different<br />tastes, however, because foods are often complex<br />chemicals that stimulate different combinations<br />of receptors, and the sense of smell also contributes to<br />our perception of food.<br />Some taste preferences have been found to be<br />genetic. People with more than the average number of<br />taste buds find broccoli very bitter, whereas people<br />with fewer taste buds may like the taste.<br />The impulses from taste buds are transmitted by<br />the facial and glossopharyngeal (7th and 9th cranial)<br />nerves to the taste areas in the parietal-temporal cortex.<br />The sense of taste is important because it makes<br />eating enjoyable. Some medications may interfere<br />with the sense of taste, and this sense becomes less<br />acute as we get older. These may be contributing factors<br />to poor nutrition in certain patients and in the<br />elderly.<br />SENSE OF SMELL<br />The receptors for smell (olfaction) are chemoreceptors<br />that detect vaporized chemicals that have been<br />sniffed into the upper nasal cavities (see Fig. 9–2). Just<br />as there are specific taste receptors, there are also<br />specific scent receptors, and research indicates that<br />humans have several hundred different receptors.<br />When stimulated by vapor molecules, olfactory receptors<br />generate impulses carried by the olfactory<br />nerves (1st cranial) through the ethmoid bone to the<br />olfactory bulbs. The pathway for these impulses ends<br />in the olfactory areas of the temporal lobes. Vapors<br />may stimulate many combinations of receptors, and it<br />has been estimated that the human brain is capable of<br />distinguishing among 10,000 different scents.<br />That may seem impressive, but the human sense of<br />smell is very poorly developed compared to those<br />of other animals. Dogs, for example, have a sense of<br />smell about 2000 times more acute than that of people.<br />(It has been said that most people live in a world<br />of sights, whereas dogs live in a world of smells.) As<br />mentioned earlier, however, much of what we call taste<br />is actually the smell of food. If you have a cold and<br />your nasal cavities are stuffed up, food just doesn’t<br />taste as good as it usually does. Adaptation occurs relatively<br />quickly with odors. Pleasant scents may be<br />sharply distinct at first but rapidly seem to dissipate or<br />fade, and even unpleasant scents may fade with long<br />exposure.<br />HUNGER AND THIRST<br />Hunger and thirst may be called visceral sensations,<br />in that they are triggered by internal changes. Hunger<br />is a sensation that seems to be far more complex than<br />was first thought, but thirst seems to be somewhat<br />simpler. The receptors for both senses are specialized<br />cells in the hypothalamus. Receptors for hunger are<br />believed to detect changes in blood nutrient levels, the<br />blood levels of hormones from the stomach and small<br />intestine, and a hormone released by adipose tissue; all<br />of these chemical signals are collected by the hypothalamus.<br />The receptors for thirst detect changes in<br />the body water content, which is actually the water-tosalt<br />proportion.<br />Naturally we do not feel these sensations in the<br />hypothalamus: They are projected. Hunger is projected<br />to the stomach, which contracts. Thirst is projected<br />to the mouth and pharynx, and less saliva is<br />produced.<br />If not satisfied by eating, the sensation of hunger<br />gradually diminishes, that is, adaptation occurs. The<br />reason is that after blood nutrient levels decrease, they<br />become stable as fat in adipose tissue is used for<br />energy. With little or no digestive activity in the gastrointestinal<br />tract, secretion of hormones diminishes.<br />With no sharp fluctuations of the chemical signals, the<br />receptors in the hypothalamus have few changes to<br />detect, and hunger becomes much less intense.<br />In contrast, the sensation of thirst, if not satisfied by<br />drinking, continues to worsen. There is no adaptation.<br />As body water is lost, the amount keeps decreasing<br />and does not stabilize. Therefore, there are constant<br />changes for the receptors to detect, and prolonged<br />thirst may be painful.<br />THE EYE<br />The eye contains the receptors for vision and a<br />refracting system that focuses light rays on the receptors<br />in the retina. We will begin our discussion, however,<br />with the accessory structures of the eye, then<br />later return to the eye itself and the physiology of<br />vision.<br />202 The Senses<br />EYELIDS AND THE<br />LACRIMAL APPARATUS<br />The eyelids contain skeletal muscle that enables the<br />eyelids to close and cover the front of the eyeball.<br />Eyelashes along the border of each eyelid help keep<br />dust out of the eyes. The eyelids are lined with a thin<br />membrane called the conjunctiva, which is also<br />folded over the white of the eye and merges with the<br />corneal epithelium. Inflammation of this membrane,<br />called conjunctivitis, may be caused by allergies or by<br />certain bacteria or viruses, and makes the eyes red,<br />itchy, and watery.<br />Tears are produced by the lacrimal glands,<br />located at the upper, outer corner of the eyeball,<br />within the orbit (Fig. 9–3). Secretion of tears occurs<br />constantly, but is increased by the presence of irritating<br />chemicals (onion vapors, for example) or dust,<br />and in certain emotional situations (sad or happy).<br />Small ducts take tears to the anterior of the eyeball,<br />and blinking spreads the tears and washes the surface<br />of the eye. Tears are mostly water, with about 1%<br />sodium chloride, similar to other body fluids. Tears<br />also contain lysozyme, an enzyme that inhibits the<br />growth of most bacteria on the wet, warm surface<br />of the eye. At the medial corner of the eyelids are<br />two small openings into the superior and inferior<br />lacrimal canals. These ducts take tears to the lacrimal<br />sac (in the lacrimal bone), which leads to the<br />nasolacrimal duct, which empties tears into the<br />nasal cavity. This is why crying often makes the nose<br />run.<br />EYEBALL<br />Most of the eyeball is within and protected by the<br />orbit, formed by the lacrimal, maxilla, zygomatic,<br />frontal, sphenoid, and ethmoid bones. The six extrinsic<br />muscles of the eye (Fig. 9–4) are attached to this<br />bony socket and to the surface of the eyeball. There<br />are four rectus (straight) muscles that move the eyeball<br />up and down or side to side; the name tells you which<br />direction. The medial rectus muscle, for example,<br />pulls the eyeball medially, as if to look at the nose. The<br />two oblique (slanted) muscles rotate the eye. The cranial<br />nerves that innervate these muscles are the oculomotor,<br />trochlear, and abducens (3rd, 4th, and 6th<br />cranial nerves, respectively). The very rapid and complex<br />coordination of these muscles in both eyes is, fortunately,<br />not something we have to think about. The<br />convergence of both eyes on an object is very important<br />to ensure a single image (that is, to prevent double<br />vision) and to give us depth perception and a<br />three-dimensional world.<br />The Senses 203<br />Lacrimal ducts<br />Lacrimal gland<br />Conjunctiva<br />Lacrimal canals<br />Lacrimal sac<br />Nasolacrimal duct<br />Nasal cavity<br />Figure 9–3. Lacrimal apparatus<br />shown in an anterior<br />view of the right eye.<br />QUESTION: Where do tears<br />usually end up?<br />Layers of the Eyeball<br />In its wall, the eyeball has three layers: the outer<br />sclera, middle choroid layer, and inner retina (Fig.<br />9–5). The sclera is the thickest layer and is made of<br />fibrous connective tissue that is visible as the white of<br />the eye. The most anterior portion is the cornea,<br />which differs from the rest of the sclera in that it is<br />transparent. The cornea has no capillaries, covers the<br />iris and pupil inside the eye, and is the first part of the<br />eye that refracts, or bends, light rays.<br />The choroid layer contains blood vessels and a<br />dark blue pigment (derived from melanin) that<br />absorbs light within the eyeball and thereby prevents<br />glare (just as does the black interior of a camera). The<br />anterior portion of the choroid is modified into more<br />specialized structures: the ciliary body and the iris.<br />The ciliary body (muscle) is a circular muscle that<br />surrounds the edge of the lens and is connected to the<br />lens by suspensory ligaments. The lens is made of a<br />transparent, elastic protein, and, like the cornea, has<br />no capillaries (see Box 9–1: Cataracts). The shape of<br />the lens is changed by the ciliary muscle, which<br />enables the eye to focus light from objects at varying<br />distances from the eye.<br />Just in front of the lens is the circular iris, the colored<br />part of the eye; its pigment is a form of melanin.<br />What we call “eye color” is the color of the iris and is<br />a genetic characteristic, just as skin color is. Two sets<br />of smooth muscle fibers in the iris change the diameter<br />of the pupil, the central opening. Contraction of<br />the radial fibers dilates the pupil; this is a sympathetic<br />response. Contraction of the circular fibers constricts<br />the pupil; this is a parasympathetic response (oculomotor<br />nerves). Pupillary constriction is a reflex that<br />protects the retina from intense light or that permits<br />more acute near vision, as when reading.<br />The retina lines the posterior two-thirds of the<br />eyeball and contains the visual receptors, the rods and<br />cones (Fig. 9–6). Rods detect only the presence of<br />light, whereas cones detect colors, which, as you may<br />know from physics, are the different wavelengths of<br />visible light. Rods are proportionally more abundant<br />toward the periphery, or edge, of the retina. Our best<br />vision in dim light or at night, for which we depend<br />on the rods, is at the sides of our visual fields. Cones<br />are most abundant in the center of the retina, especially<br />an area called the macula lutea directly behind<br />the center of the lens on what is called the visual axis.<br />The fovea, which contains only cones, is a small<br />depression in the macula and is the area for best color<br />vision.<br />An important cause of vision loss for people over<br />65 years of age is age-related macular degeneration<br />204 The Senses<br />Eyelid<br />Eyelashes<br />Cornea<br />Eyeball<br />Inferior rectus muscle<br />Superior rectus muscle<br />Optic nerve<br />Lateral rectus muscle<br />Inferior oblique muscle<br />Levator palpebrae<br />superioris muscle<br />Figure 9–4. Extrinsic muscles<br />of the eye. Lateral view of<br />left eye (the medial rectus<br />and superior oblique are not<br />shown).<br />QUESTION: Contraction of<br />the inferior rectus muscle will<br />have what effect on the eyeball?<br />(AMD), that is, loss of central vision, and some cases<br />seem to have a genetic component. In the dry form of<br />AMD, small fatty deposits impair circulation to the<br />macula, and cells die from lack of oxygen. In the wet<br />form of AMD, abnormal blood vessels begin leaking<br />into the retina, and cells in the macula die from the<br />damaging effects of blood outside its vessels. The<br />macula, the center of the visual field, is the part of<br />The Senses 205<br />Lens<br />Cornea<br />Conjunctiva<br />Pupil<br />Inferior rectus muscle<br />Canal of Schlemm<br />Suspensory ligament<br />Iris<br />Anterior cavity<br />Sclera<br />Vitreous humor in<br />posterior cavity<br />Choroid<br />Retina<br />Optic nerve<br />Optic disc<br />Retinal artery and vein<br />Fovea in macula lutea<br />Ciliary body (muscle)<br />Corneal<br />epithelium<br />Figure 9–5. Internal anatomy of the eyeball.<br />QUESTION: What is the function of the iris?<br />BOX 9–1 CATARACTS<br />and blurry vision throughout the visual field is<br />the result. Small cataracts may be destroyed<br />by laser surgery. Artificial lenses are available, and<br />may be surgically implanted to replace an extensively<br />cloudy lens. The artificial lens is not<br />adjustable, however, and the person may require<br />glasses or contact lenses for vision at certain<br />distances.<br />The lens of the eye is normally transparent but may<br />become opaque; this cloudiness or opacity is called<br />a cataract. Cataract formation is most common<br />among elderly people. With age, the proteins of the<br />lens break down and lose their transparency. Longterm<br />exposure to ultraviolet light (sunlight) seems to<br />be a contributing factor, as is smoking.<br />The cloudy lens does not refract light properly,<br />the retina we use most: for reading, for driving, for<br />recognizing people, and for any kind of close work.<br />People of all ages should be aware of this condition<br />and that smoking and exposure to ultraviolet rays are<br />risk factors.<br />When light strikes the retina, the rods and cones<br />generate impulses. These impulses are carried by ganglion<br />neurons, which all converge at the optic disc<br />(see Figs. 9–5 and 9–6) and pass through the wall of<br />the eyeball as the optic nerve. There are no rods or<br />cones in the optic disc, so this part of the retina is<br />sometimes called the “blind spot.” We are not aware<br />of a blind spot in our field of vision, however, in part<br />because the eyes are constantly moving, and in part<br />because the brain “fills in” the blank spot to create a<br />“complete” picture.<br />206 The Senses<br />Sclera<br />Pigment cells Choroid<br />Bipolar<br />neurons<br />Ganglion<br />neurons<br />Light waves<br />Cone<br />Rod<br />Optic nerve<br />Optic nerve<br />fibers<br />Figure 9–6. Microscopic structure of the retina in the area of the optic disc. See text for<br />description.<br />QUESTION: Which type of neuron forms the optic nerve? Which cells are the photoreceptors?<br />Cavities of the Eyeball<br />There are two cavities within the eye: the posterior<br />cavity and the anterior cavity. The larger, posterior<br />cavity is found between the lens and retina and contains<br />vitreous humor (or vitreous body). This semisolid<br />substance keeps the retina in place. If the eyeball<br />is punctured and vitreous humor is lost, the retina may<br />fall away from the choroid; this is one possible cause<br />of a detached retina.<br />The anterior cavity is found between the back of<br />the cornea and the front of the lens, and contains<br />aqueous humor, the tissue fluid of the eyeball.<br />Aqueous humor is formed by capillaries in the ciliary<br />body, flows anteriorly through the pupil, and is reabsorbed<br />by the canal of Schlemm (small veins also<br />called the scleral venous sinus) at the junction of the<br />iris and cornea. Because aqueous humor is tissue fluid,<br />you would expect it to have a nourishing function, and<br />it does. Recall that the lens and cornea have no capillaries;<br />they are nourished by the continuous flow of<br />aqueous humor (see Box 9–2: Glaucoma).<br />PHYSIOLOGY OF VISION<br />For us to see, light rays must be focused on the retina,<br />and the resulting nerve impulses must be transmitted<br />to the visual areas of the cerebral cortex in the<br />brain.<br />Refraction of light rays is the deflection or bending<br />of a ray of light as it passes through one object and<br />into another object of greater or lesser density. The<br />refraction of light within the eye takes place in the following<br />pathway of structures: the cornea, aqueous<br />humor, lens, and vitreous humor. The lens is the only<br />adjustable part of the refraction system. When looking<br />at distant objects, the ciliary muscle is relaxed and the<br />lens is elongated and thin. When looking at near<br />objects, the ciliary muscle contracts to form a smaller<br />circle, the elastic lens recoils and bulges in the middle,<br />and has greater refractive power (see Box 9–3: Errors<br />of Refraction).<br />When light rays strike the retina, they stimulate<br />chemical reactions in the rods and cones. In rods, the<br />chemical rhodopsin breaks down to form scotopsin<br />and retinal (a derivative of vitamin A). This chemical<br />reaction generates an electrical impulse, and<br />rhodopsin is then resynthesized in a slower reaction.<br />Adaptation to darkness, such as going outside at night,<br />takes a little while because being in a well-lit area has<br />broken down most of the rhodopsin in the rods, and<br />resynthesis of rhodopsin is slow. The opposite situation,<br />perhaps being suddenly awakened by a bright<br />light, can seem almost painful. What happens is this:<br />In darkness the rods have resynthesized a full supply of<br />rhodopsin, and the sudden bright light breaks down<br />all the rhodopsin at the same time. The barrage of<br />The Senses 207<br />BOX 9–2 GLAUCOMA<br />cannot easily integrate with the normal image of<br />the other eye. When both eyes are affected, the<br />person may not become aware of the gradual loss<br />of peripheral vision, because close work such as<br />reading does not require the edges of the visual<br />fields.<br />Glaucoma may often be controlled with medications<br />that constrict the pupil and flatten the iris,<br />thus opening up access to the canal of Schlemm. If<br />these or other medications are not effective, laser<br />surgery may be used to create a larger drainage<br />canal.<br />Anyone over the age of 40 should have a test for<br />glaucoma; anyone with a family history of glaucoma<br />should have this test annually, as should those<br />with diabetes or high blood pressure. If diagnosed<br />early, glaucoma is treatable, and blindness can usually<br />be prevented.<br />The presence of aqueous humor in the anterior cavity<br />of the eye creates a pressure called intraocular<br />pressure. An increase in this pressure is an important<br />risk factor for glaucoma, which is now defined<br />as a group of disorders that damage the optic nerve<br />and cause loss of vision. Other risk factors include<br />high blood pressure and diabetes. In the most<br />common form of glaucoma, aqueous humor is not<br />reabsorbed properly into the canal of Schlemm.<br />Increased pressure in the anterior cavity is transmitted<br />to the lens, the vitreous humor, and the retina<br />and optic nerve. As pressure on the retina increases,<br />halos may be seen around bright lights, and peripheral<br />vision is lost. Frequently, however, there are no<br />symptoms. A person with glaucoma may not notice<br />the shrinking visual field in one eye before vision<br />loss is far advanced. This happens because the brain<br />will suppress a faulty image from one eye that it<br />208<br />BOX 9–3 ERRORS OF REFRACTION<br />Normal visual acuity is referred to as 20/20; that is,<br />the eye should and does clearly see an object 20<br />feet away. Nearsightedness (myopia) means<br />that the eye sees near objects well but not distant<br />ones. If an eye has 20/80 vision, this means that<br />what the normal eye can see at 80 feet, the nearsighted<br />eye can see only if the object is brought to<br />20 feet away. The nearsighted eye focuses images<br />from distant objects in front of the retina, because<br />the eyeball is too long or the lens too thick. These<br />structural characteristics of the eye are hereditary.<br />Correction requires a concave lens to spread out<br />light rays before they strike the eye.<br />Farsightedness (hyperopia) means that the<br />eye sees distant objects well. Such an eye may have<br />an acuity of 20/10, that is, it sees at 20 feet what the<br />normal eye can see only at 10 feet. The farsighted<br />eye focuses light from near objects “behind” the<br />retina, because the eyeball is too short or the lens<br />too thin. Correction requires a convex lens to converge<br />light rays before they strike the eye.<br />As we get older, most of us will become more farsighted<br />(presbyopia). As the aging lens loses its<br />elasticity, it is not as able to recoil and thicken for near<br />vision, and glasses for reading are often necessary.<br />Astigmatism is another error of refraction,<br />caused by an irregular curvature of the cornea or<br />lens that scatters light rays and blurs the image on<br />the retina. Correction requires a lens ground specifically<br />for the curvature of the individual eye.<br />Normal eye<br />Nearsighted<br />Farsighted<br />Astigmatic<br />Corrected<br />Corrected<br />Box Figure 9–A Errors of refraction compared to normal eye. Corrective lenses are shown for nearsightedness<br />and farsightedness.<br />impulses generated is very intense, and the brain may<br />interpret any intense sensation as pain. A few minutes<br />later the bright light seems fine because the rods are<br />recycling their rhodopsin slowly, and it is not breaking<br />down all at once.<br />Chemical reactions in the cones, also involving<br />retinal, are brought about by different wavelengths of<br />light. It is believed that there are three types of cones:<br />red-absorbing, blue-absorbing, and green-absorbing<br />cones. Each type absorbs wavelengths over about a<br />third of the visible light spectrum, so red cones,<br />for example, absorb light of the red, orange, and<br />yellow wavelengths. The chemical reactions in cones<br />also generate electrical impulses (see Box 9–4: Night<br />Blindness and Color Blindness).<br />The impulses from the rods and cones are transmitted<br />to ganglion neurons (see Fig. 9–6); these converge<br />at the optic disc and become the optic nerve,<br />which passes posteriorly through the wall of the eyeball.<br />Ganglion neurons also seem to have a photoreceptor<br />chemical (called melanopsin) that may<br />contribute to the daily resetting of our biological<br />clocks.<br />The optic nerves from both eyes come together at<br />the optic chiasma (or chiasm), just in front of the<br />pituitary gland (see Fig. 8–11). Here, the medial fibers<br />of each optic nerve cross to the other side. This crossing<br />permits each visual area to receive impulses from<br />both eyes, which is important for binocular vision.<br />The visual areas are in the occipital lobes of the<br />cerebral cortex. Although each eye transmits a slightly<br />different picture (look straight ahead and close one<br />eye at a time to see the difference between the two pictures),<br />the visual areas put them together, or integrate<br />them, to make a single image that has depth and three<br />dimensions. This is called binocular vision. The<br />visual areas also right the image, because the image on<br />the retina is upside down. The image on film in a camera<br />is also upside down, but we don’t even realize that<br />because we look at the pictures right side up. The<br />brain just as automatically ensures that we see our<br />world right side up.<br />Also for near vision, the pupils constrict to block<br />out peripheral light rays that would otherwise blur the<br />image, and the eyes converge even further to keep the<br />images on the corresponding parts of both retinas.<br />The Senses 209<br />BOX 9–4 NIGHT BLINDNESS AND COLOR BLINDNESS<br />color blindness on his X chromosome has no gene<br />at all for color vision on his Y chromosome and will<br />be color blind.<br />Night blindness, the inability to see well in dim<br />light or at night, is usually caused by a deficiency of<br />vitamin A, although some night blindness may<br />occur with aging. Vitamin A is necessary for the synthesis<br />of rhodopsin in the rods. Without sufficient<br />vitamin A, there is not enough rhodopsin present to<br />respond to low levels of light.<br />Color blindness is a genetic disorder in which<br />one of the three sets of cones is lacking or nonfunctional.<br />Total color blindness, the inability to see<br />any colors at all, is very rare. The most common<br />form is red-green color blindness, which is the<br />inability to distinguish between these colors. If<br />either the red cones or green cones are nonfunctional,<br />the person will still see most colors, but will<br />not have the contrast that the non-working set of<br />cones would provide. So red and green shades will<br />look somewhat similar, without the definite difference<br />most of us see. This is a sex-linked trait; the<br />recessive gene is on the X chromosome. A woman<br />with one gene for color blindness and a gene for<br />normal color vision on her other X chromosome will<br />not be color blind but may pass the gene for color<br />blindness to her children. A man with a gene for<br />Box Figure 9–B Example of color patterns used to<br />detect color blindness.<br />The importance of pupil constriction can be demonstrated<br />by looking at this page through a pinhole in a<br />piece of paper. You will be able to read with the page<br />much closer to your eye because the paper blocks out<br />light from the sides.<br />The importance of convergence can be demonstrated<br />by looking at your finger placed on the tip of<br />your nose. You can feel your eyes move medially<br />(“cross”) in maximum convergence. If the eyes don’t<br />converge, the result is double vision; the brain cannot<br />make the very different images into one, and settles<br />for two. This is temporary, however, because the brain<br />does not like seeing double and will eventually suppress<br />one image.<br />You have probably heard of the condition called<br />“lazy eye” (the formal name is strabismus), in which<br />a person’s eyes (the visual axis of each) cannot be<br />directed at precisely the same point. True convergence<br />is not possible and, if untreated, the brain simply will<br />not use the lazy eye image. That eye may stop focusing<br />and become functionally blind because the brain is<br />ignoring the nerve impulses from it. Such loss of<br />vision is called amblyopia. Correction of a lazy eye<br />may involve eye exercises (to make the lazy eye<br />straighten out), a patch over the good eye (to make the<br />lazy eye straighten out and take over), or surgery to<br />correct an imbalance of the extrinsic muscles. You can<br />show yourself the benefits of converging eyes the next<br />time you are a passenger in a car (not the driver). As<br />the car is moving, close one eye. Does the oncoming<br />landscape seem to flatten out, lose dimension? This is<br />loss of depth perception and some of the three dimensionality<br />that our brains provide.<br />THE EAR<br />The ear consists of three areas: the outer ear, the middle<br />ear, and the inner ear (Fig. 9–7). The ear contains<br />the receptors for two senses: hearing and equilibrium.<br />These receptors are all found in the inner ear.<br />OUTER EAR<br />The outer ear consists of the auricle and the ear<br />canal. The auricle, or pinna, is made of cartilage covered<br />with skin. For animals such as dogs, whose ears<br />are movable, the auricle may act as a funnel for sound<br />waves. For people, however, the flat and stationary<br />auricle is not important. Hearing would not be negatively<br />affected without it, although those of us who<br />wear glasses would have our vision impaired without<br />our auricles. The ear canal is lined with skin that contains<br />ceruminous glands. It may also be called the<br />external auditory meatus, and is a tunnel into the<br />temporal bone, curving slightly forward and down.<br />MIDDLE EAR<br />The middle ear is an air-filled cavity in the temporal<br />bone. The eardrum, or tympanic membrane, is<br />stretched across the end of the ear canal and vibrates<br />when sound waves strike it. These vibrations are<br />transmitted to the three auditory bones: the malleus,<br />incus, and stapes (see Fig. 9–7). The stapes then<br />transmits vibrations to the fluid-filled inner ear at the<br />oval window.<br />The eustachian tube (auditory tube) extends from<br />the middle ear to the nasopharynx and permits air to<br />enter or leave the middle ear cavity. The air pressure<br />in the middle ear must be the same as the external<br />atmospheric pressure in order for the eardrum to<br />vibrate properly. You may have noticed your ears<br />“popping” when in an airplane or when driving to a<br />higher or lower altitude. Swallowing or yawning creates<br />the “pop” by opening the eustachian tubes and<br />equalizing the air pressures.<br />The eustachian tubes of children are short and<br />nearly horizontal and may permit bacteria to spread<br />from the pharynx to the middle ear. This is why otitis<br />media may be a complication of a strep throat.<br />INNER EAR<br />Within the temporal bone, the inner ear is a cavity<br />called the bony labyrinth (a labyrinth is a series of<br />interconnecting paths or tunnels, somewhat like a<br />maze but without dead ends; see Fig. 9–7), which is<br />lined with membrane called the membranous<br />labyrinth. Perilymph is the fluid found between bone<br />and membrane, and endolymph is the fluid within the<br />membranous structures of the inner ear. These structures<br />are the cochlea, concerned with hearing, and the<br />utricle, saccule, and semicircular canals, all concerned<br />with equilibrium (Fig. 9–8).<br />Cochlea<br />The cochlea is shaped like a snail shell with two-anda-<br />half structural turns. Internally, the cochlea is partitioned<br />into three fluid-filled canals. The medial canal<br />210 The Senses<br />211<br />Auricle<br />Malleus<br />Incus<br />Stapes Vestibular branch<br />Temporal bone Semicircular canals<br />External auditory meatus<br />Cochlear branch<br />Cochlea<br />Eustachian tube<br />Tympanic<br />membrane<br />Vestibule<br />Acoustic or<br />8th<br />cranial nerve<br />B<br />A<br />Semicircular<br />canals<br />Malleus Cochlea<br />Stapes<br />Incus<br />Eardrum<br />Mastoid<br />sinus<br />C<br />Figure 9–7. (A) Outer, middle, and inner ear structures as shown in a frontal section<br />through the right temporal bone. (B) Section of temporal bone with the auditory bones.<br />(C) Section of temporal bone showing bony labyrinth of inner ear. The colors on the bone<br />are artificial. (Photographs by Dan Kaufman.)<br />QUESTION: What structure is first to vibrate when sound waves enter the ear canal? What<br />is second?<br />is the cochlear duct, the floor of which is the basilar<br />membrane that supports the receptors for hearing in<br />the organ of Corti (spiral organ). The receptors are<br />called hair cells (their projections are not “hair,” of<br />course, but rather are specialized microvilli called<br />stereocilia), which contain endings of the cochlear<br />branch of the 8th cranial nerve. Overhanging the hair<br />cells is the tectorial membrane (Fig. 9–9).<br />Very simply, the process of hearing involves the<br />transmission of vibrations and the generation of nerve<br />impulses. When sound waves enter the ear canal,<br />vibrations are transmitted by the following sequence<br />of structures: eardrum, malleus, incus, stapes, oval<br />window of the inner ear, and perilymph and<br />endolymph within the cochlea. Imagine the vibrations<br />in the fluids as ripples or waves. The basilar membrane<br />ripples and pushes the hair cells of the organ of<br />Corti against the tectorial membrane. When the hair<br />cells bend, they generate impulses that are carried by<br />the 8th cranial nerve to the brain. As you may recall,<br />the auditory areas are in the temporal lobes of the<br />cerebral cortex. It is here that sounds are heard and<br />interpreted (see Box 9–5: Deafness).<br />The auditory areas also enable us to determine<br />from which direction a sound is coming. Simply<br />stated, the auditory areas count and compare the number<br />of impulses coming from each inner ear. For<br />example, if more impulses arrive from the left cochlea<br />than from the right one, the sound will be projected to<br />the left. If the source of a sound is directly above your<br />head, the sound may seem to come from all directions,<br />because each auditory area is receiving approximately<br />the same number of impulses and cannot project the<br />sensation to one side or the other.<br />The final structure in the hearing pathway is the<br />round window (see Fig. 9–8). The membrane-covered<br />212 The Senses<br />Semicircular canals<br />Endolymph<br />Crista<br />Saccule<br />Vestibular nerve<br />Cochlear nerve<br />Vestibulocochlear<br />nerve<br />Scala tympani<br />Cochlear duct<br />Scala vestibuli<br />Cochlea<br />Round window<br />Oval window<br />Utricle<br />Ampulla<br />Figure 9–8. Inner ear structures. The arrows show the transmission of vibrations during<br />hearing.<br />QUESTION: What is the function of the round window?<br />The Senses 213<br />Cochlear duct filled with endolymph<br />Vestibular membrane<br />Vestibular canal (from the oval window)<br />Supporting cells<br />Hair cells<br />Basilar membrane<br />Tympanic canal<br />(to the round window)<br />Semicircular canals<br />Oval window<br />Round window<br />Cochlea<br />Nerve fibers of<br />8th cranial nerve<br />Tectorial membrane<br />A<br />B<br />Figure 9–9. Organ of Corti. (A) Inner ear structures. (B) Magnification of organ of Corti<br />within the cochlea.<br />QUESTION: What do the canals and cochlear duct contain (air or fluid)? What are the<br />receptors for hearing?<br />round window, just below the oval window, is important<br />to relieve pressure. When the stapes pushes in the<br />fluid at the oval window, the round window bulges<br />out, which prevents damage to the hair cells.<br />Utricle and Saccule<br />The utricle and saccule are membranous sacs in an<br />area called the vestibule, between the cochlea and<br />semicircular canals. Within the utricle and saccule are<br />hair cells embedded in a gelatinous membrane with<br />tiny crystals of calcium carbonate called otoliths.<br />Gravity pulls on the otoliths and bends the hair cells<br />as the position of the head changes (Fig. 9–10). The<br />impulses generated by these hair cells are carried by<br />the vestibular portion of the 8th cranial nerve to the<br />cerebellum, the midbrain, and the temporal lobes of<br />the cerebrum.<br />The cerebellum and midbrain use this information<br />to maintain equilibrium at a subconscious level. We<br />can, of course, be aware of the position of the head,<br />and it is the cerebrum that provides awareness.<br />214 The Senses<br />BOX 9–5 DEAFNESS<br />measles). Deterioration of the hair cells in the<br />cochlea is a natural consequence of aging, and the<br />acuity of hearing diminishes as we get older. For<br />example, it may be more difficult for an elderly person<br />to distinguish conversation from background<br />noise. Chronic exposure to loud noise accelerates<br />degeneration of the hair cells and onset of this type<br />of deafness. Listening to music by way of earphones<br />is also believed to increase the risk of this type of<br />damage.<br />Central deafness—damage to the auditory areas<br />in the temporal lobes. This type of deafness is rare<br />but may be caused by a brain tumor, meningitis, or<br />a cerebrovascular accident in the temporal lobe.<br />Deafness is the inability to hear properly; the types<br />are classified according to the part of the hearing<br />process that is not functioning normally:<br />Conduction deafness—impairment of one of the<br />structures that transmits vibrations. Examples of this<br />type are a punctured eardrum, arthritis of the auditory<br />bones, or a middle ear infection in which fluid<br />fills the middle ear cavity.<br />Nerve deafness—impairment of the 8th cranial<br />nerve or the receptors for hearing in the cochlea.<br />The 8th cranial nerve may be damaged by some<br />antibiotics used to treat bacterial infections. Nerve<br />deafness is a rare complication of some viral infections<br />such as mumps or congenital rubella (German<br />Central deafness<br />Nerve deafness<br />Conduction deafness<br />Box Figure 9–C Types of deafness.<br />Semicircular Canals<br />The three semicircular canals are fluid-filled membranous<br />ovals oriented in three different planes. At the<br />base of each is an enlarged portion called the ampulla<br />(see Fig. 9–8), which contains hair cells (the crista) that<br />are affected by movement. As the body moves forward,<br />for example, the hair cells are bent backward<br />at first and then straighten (see Fig. 9–10). The bending<br />of the hair cells generates impulses carried by the<br />vestibular branch of the 8th cranial nerve to the<br />cerebellum, midbrain, and temporal lobes of the cerebrum.<br />These impulses are interpreted as starting or<br />stopping, and accelerating or decelerating, or changing<br />direction, and this information is used to maintain<br />equilibrium while we are moving (see Box 9–6: Motion<br />Sickness).<br />In summary then, the utricle and saccule provide<br />information about the position of the body at rest,<br />while the semicircular canals provide information<br />about the body in motion. Of course, there is some<br />overlap, and the brain puts all the information together<br />to create a single sense of body position.<br />The Senses 215<br />Utricle<br />Saccule<br />Otoliths<br />Hair cells<br />Gravity<br />Head tilted<br />8th cranial<br />nerve<br />Head upright<br />Ampulla<br />Crista<br />Ampulla<br />Hair<br />cells<br />8th cranial<br />nerve<br />1.<br />At rest<br />2.<br />Starting<br />3.<br />Moving<br />4.<br />Stopping<br />5.<br />At rest<br />A<br />B<br />Figure 9–10. Physiology of<br />equilibrium. (A) Utricle and<br />saccule. (B) Semicircular canals.<br />See text for description.<br />QUESTION: In part A, what<br />causes the hair cells to bend? In<br />part B, what causes the hair<br />cells to sway?<br />ARTERIAL RECEPTORS<br />The aorta and carotid arteries contain receptors that<br />detect changes in the blood. The aortic arch, which<br />receives blood pumped by the left ventricle of the<br />heart, curves over the top of the heart. The left and<br />right carotid arteries are branches of the aortic arch<br />that take blood through the neck on the way to the<br />brain. In each of these vessels are pressoreceptors and<br />chemoreceptors (see Fig. 12–7).<br />Pressoreceptors in the carotid sinuses and aortic<br />sinus detect changes in blood pressure. Chemoreceptors<br />in the carotid bodies and the aortic body detect<br />changes in the oxygen and carbon dioxide content and<br />the pH of blood. The impulses generated by these<br />receptors do not give rise to sensations that we feel but<br />rather are information used to make any necessary<br />changes in respiration or circulation. We will return<br />to this in later chapters, so one example will suffice<br />for now.<br />If the blood level of oxygen decreases significantly,<br />this change (hypoxia) is detected by carotid and aortic<br />chemoreceptors. The sensory impulses are carried by<br />the glossopharyngeal (9th cranial) and vagus (10th<br />cranial) nerves to the medulla. Centers in the medulla<br />may then increase the respiratory rate and the heart<br />rate to obtain and circulate more oxygen. These are<br />the respiratory and cardiac reflexes that were mentioned<br />in Chapter 8 as functions of the glossopharyngeal<br />and vagus nerves. The importance of these<br />reflexes is readily apparent: to maintain normal blood<br />levels of oxygen and carbon dioxide and to maintain<br />normal blood pressure.<br />AGING AND THE SENSES<br />All of the senses may be diminished in old age. In the<br />eye, cataracts may make the lens opaque. The lens<br />also loses its elasticity and the eye becomes more<br />farsighted, a condition called presbyopia. The risk<br />of glaucoma increases, and elderly people should be<br />tested for it because treatment is available that can<br />prevent blindness. Macular degeneration, in which<br />central vision becomes impaired first, is a major cause<br />of vision loss for people over 65. Reading and close<br />work of any kind become difficult.<br />In the ear, cumulative damage to the hair cells in<br />the organ of Corti usually becomes apparent some<br />time after the age of 60. Hair cells that have been<br />damaged in a lifetime of noise cannot be replaced<br />(regrowth of cochlear hair cells has been stimulated in<br />guinea pigs, but not yet in people). The deafness of<br />old age ranges from slight to profound; very often<br />high-pitched sounds are lost first, while hearing<br />may still be adequate for low-pitched sounds. The<br />sense of equilibrium may be diminished; the body is<br />slower to react to tilting, and falls may become more<br />frequent.<br />Both taste and smell become less acute with age,<br />which may contribute to poor nutrition in elderly<br />people.<br />SUMMARY<br />Changes take place all around us as well as within<br />us. If the body could not respond appropriately to<br />environmental and internal changes, homeostasis<br />would soon be disrupted, resulting in injury, illness, or<br />even death. To respond appropriately to changes, the<br />brain must know what they are. Conveying this information<br />to our brains is the function of our senses.<br />Although we may sometimes take our senses for<br />granted, we could not survive for very long without<br />them.<br />You have just read about the great variety of internal<br />and external changes that are detected by the sense<br />organs. You are also familiar with the role of the nervous<br />system in the regulation of the body’s responses.<br />In the next chapter we will discuss the other regulatory<br />system, the endocrine system. The hormones<br />of the endocrine glands are produced in response to<br />changes, and their regulatory effects all contribute to<br />homeostasis.<br />216 The Senses<br />BOX 9–6 MOTION SICKNESS<br />Motion sickness is characterized by cold sweats,<br />hyperventilation, nausea, and vomiting when the<br />person is exposed to repetitive motion that is<br />unexpected or unfamiliar, or that cannot be controlled.<br />Seasickness is a type of motion sickness,<br />as is carsickness (why children are carsick more<br />often than adults is not known).<br />Some people are simply not affected by the<br />rolling of a ship or train; for others, the constant<br />stimulation of the receptors for position first<br />becomes uncomfortable, then nauseating. For<br />those who know they are susceptible to motion<br />sickness, medications are available for use before<br />traveling by plane, train, boat, or car.<br />Purpose of Sensations—to detect changes in<br />the external or internal environment to<br />enable the body to respond appropriately<br />to maintain homeostasis<br />Sensory Pathway—pathway of impulses for a<br />sensation<br />1. Receptors—detect a change (usually very specific)<br />and generate impulses.<br />2. Sensory neurons—transmit impulses from receptors<br />to the CNS.<br />3. Sensory tracts—white matter in the CNS.<br />4. Sensory area—most are in the cerebral cortex; feels<br />and interprets the sensation.<br />Characteristics of Sensations<br />1. Projection—the sensation seems to come from the<br />area where the receptors were stimulated, even<br />though it is the brain that truly feels the sensation.<br />2. Intensity—the degree to which a sensation is felt; a<br />strong stimulus affects more receptors, more<br />impulses are sent to the brain and are interpreted as<br />a more intense sensation.<br />3. Contrast—the effect of a previous or simultaneous<br />sensation on a current sensation as the brain compares<br />them.<br />4. Adaptation—becoming unaware of a continuing<br />stimulus; if the stimulus remains constant, there is<br />no change for receptors to detect.<br />5. After-image—the sensation remains in the consciousness<br />after the stimulus has stopped.<br />Cutaneous Senses—provide information<br />about the external environment and the<br />skin itself<br />1. The dermis has free nerve endings that are receptors<br />for pain, heat, and cold, and encapsulated nerve<br />endings that are receptors for touch and pressure<br />(see Fig. 9–1).<br />2. Sensory areas are in parietal lobes.<br />3. Referred pain is visceral pain that is felt as cutaneous<br />pain. Common pathways in the CNS carry<br />both cutaneous and visceral impulses; the brain<br />usually projects sensation to the cutaneous area.<br />Muscle Sense—knowing where our muscles<br />are without looking at them<br />1. Stretch receptors in muscles detect stretching.<br />2. Sensory areas for conscious muscle sense are in<br />parietal lobes.<br />3. Cerebellum uses unconscious muscle sense to<br />coordinate voluntary movement.<br />Sense of Taste (see Fig. 9–2)<br />1. Chemoreceptors are in taste buds on the tongue;<br />detect chemicals (foods) in solution (saliva) in the<br />mouth.<br />2. Five basic tastes: sweet, sour, salty, bitter, and<br />savory; foods stimulate combinations of receptors.<br />3. Pathway: facial and glossopharyngeal nerves to<br />taste areas in parietal-temporal lobes.<br />Sense of Smell (see Fig. 9–2)<br />1. Chemoreceptors are in upper nasal cavities; several<br />hundred different ones; detect vaporized chemicals<br />(many combinations possible).<br />2. Pathway: olfactory nerves to olfactory bulbs to<br />olfactory areas in the temporal lobes.<br />3. Smell contributes greatly to what we call taste.<br />Hunger and Thirst—visceral (internal)<br />sensations<br />1. Receptors for hunger: in hypothalamus, detect<br />changes in GI hormones and nutrient levels in the<br />blood; hunger is projected to the stomach; adaptation<br />does occur.<br />2. Receptors for thirst: in hypothalamus, osmoreceptors<br />detect changes in body water (water–salt proportions);<br />thirst is projected to the mouth and<br />pharynx; adaptation does not occur.<br />The Eye (see Figs. 9–3 through 9–6)<br />1. Eyelids and eyelashes keep dust out of eyes; conjunctivae<br />line the eyelids and cover white of eye.<br />2. Lacrimal glands produce tears, which flow across<br />the eyeball to two lacrimal ducts, to lacrimal sac to<br />nasolacrimal duct to nasal cavity. Tears wash the<br />anterior eyeball and contain lysozyme to inhibit<br />bacterial growth.<br />3. The eyeball is protected by the bony orbit (socket).<br />4. The six extrinsic muscles move the eyeball; innervated<br />by the 3rd, 4th, and 6th cranial nerves.<br />5. Sclera—outermost layer of the eyeball, made of<br />fibrous connective tissue; anterior portion is the<br />transparent cornea, the first light-refracting structure.<br />6. Choroid layer—middle layer of eyeball; dark blue<br />pigment absorbs light to prevent glare within the<br />eyeball.<br />The Senses 217<br />STUDY OUTLINE<br />7. Ciliary body (muscle) and suspensory ligaments—<br />change shape of lens, which is made of a transparent,<br />elastic protein and which refracts light.<br />8. Iris—two sets of smooth muscle fibers regulate<br />diameter of pupil, that is, how much light strikes<br />the retina.<br />9. Retina—innermost layer of eyeball; contains rods<br />and cones.<br />• Rods—detect light; abundant toward periphery<br />of retina.<br />• Cones—detect color; abundant in center of<br />retina.<br />• Fovea—in the center of the macula lutea; contains<br />only cones; area of best color vision.<br />• Optic disc—no rods or cones; optic nerve<br />passes through eyeball.<br />10. Posterior cavity contains vitreous humor (semisolid)<br />that keeps the retina in place.<br />11. Anterior cavity contains aqueous humor that<br />nourishes the lens and cornea; made by capillaries<br />of the ciliary body, flows through pupil, is reabsorbed<br />to blood at the canal of Schlemm.<br />Physiology of Vision<br />1. Refraction (bending and focusing) pathway of<br />light: cornea, aqueous humor, lens, vitreous<br />humor.<br />2. Lens is adjustable; ciliary muscle relaxes for distant<br />vision, and lens is thin. Ciliary muscle contracts for<br />near vision, and elastic lens thickens and has<br />greater refractive power.<br />3. Light strikes retina and stimulates chemical reactions<br />in the rods and cones.<br />4. In rods: rhodopsin breaks down to scotopsin and<br />retinal (from vitamin A), and an electrical impulse<br />is generated. In cones: specific wavelengths of light<br />are absorbed (red, blue, green); chemical reactions<br />generate nerve impulses.<br />5. Ganglion neurons from the rods and cones form<br />the optic nerve, which passes through the eyeball at<br />the optic disc.<br />6. Optic chiasma—site of the crossover of medial<br />fibers of both optic nerves, permitting binocular<br />vision.<br />7. Visual areas in occipital lobes—each area receives<br />impulses from both eyes; both areas create one<br />image from the two slightly different images of<br />each eye; both areas right the upside-down retinal<br />image.<br />The Ear (see Figs. 9–7 through 9–10)<br />1. Outer ear—auricle or pinna has no real function<br />for people; ear canal curves forward and down into<br />temporal bone.<br />2. Middle ear—eardrum at end of ear canal vibrates<br />when sound waves strike it. Auditory bones:<br />malleus, incus, stapes; transmit vibrations to inner<br />ear at oval window.<br />• Eustachian tube—extends from middle ear to<br />nasopharynx; allows air in and out of middle<br />ear to permit eardrum to vibrate; air pressure<br />in middle ear should equal atmospheric pressure.<br />3. Inner ear—bony labyrinth in temporal bone, lined<br />with membranous labyrinth. Perilymph is fluid<br />between bone and membrane; endolymph is fluid<br />within membrane. Membranous structures are the<br />cochlea, utricle and saccule, and semicircular<br />canals.<br />4. Cochlea—snail-shell shaped; three internal canals;<br />cochlear duct contains receptors for hearing: hair<br />cells in the organ of Corti; these cells contain<br />endings of the cochlear branch of the 8th cranial<br />nerve.<br />5. Physiology of hearing—sound waves stimulate<br />vibration of eardrum, malleus, incus, stapes, oval<br />window of inner ear, perilymph and endolymph of<br />cochlea, and hair cells of organ of Corti. When hair<br />cells bend, impulses are generated and carried by<br />the 8th cranial nerve to the auditory areas in the<br />temporal lobes. Round window prevents pressure<br />damage to the hair cells.<br />6. Utricle and saccule—membranous sacs in the<br />vestibule; each contains hair cells that are affected<br />by gravity. When position of the head changes,<br />otoliths bend the hair cells, which generate<br />impulses along the vestibular branch of the 8th cranial<br />nerve to the cerebellum, midbrain, and cerebrum.<br />Impulses are interpreted as position of the<br />head at rest.<br />7. Semicircular canals—three membranous ovals in<br />three planes; enlarged base is the ampulla, which<br />contains hair cells (crista) that are affected by<br />movement. As body moves, hair cells bend in opposite<br />direction, generate impulses along vestibular<br />branch of 8th cranial nerve to cerebellum, midbrain,<br />and cerebrum. Impulses are interpreted as<br />movement of the body, changing speed, stopping<br />or starting.<br />218 The Senses<br />Arterial Receptors—in large arteries; detect<br />changes in blood<br />1. Aortic arch—curves over top of heart. Aortic sinus<br />contains pressoreceptors; aortic body contains<br />chemoreceptors; sensory nerve is vagus (10th<br />cranial).<br />2. Right and left carotid arteries in the neck; carotid<br />sinus contains pressoreceptors; carotid body contains<br />chemoreceptors; sensory nerve is the glossopharyngeal<br />(9th cranial).<br />3. Pressoreceptors detect changes in blood pressure;<br />chemoreceptors detect changes in pH or oxygen<br />and CO2 levels in the blood. This information is<br />used by the vital centers in the medulla to change<br />respiration or circulation to maintain normal blood<br />oxygen and CO2 and normal blood pressure.<br />The Senses 219<br />REVIEW QUESTIONS<br />1. State the two general functions of receptors.<br />Explain the purpose of sensory neurons and sensory<br />tracts. (p. 198)<br />2. Name the receptors for the cutaneous senses, and<br />explain the importance of this information.<br />(p. 199)<br />3. Name the receptors for muscle sense and the parts<br />of the brain concerned with muscle sense.<br />(p. 200)<br />4. State what the chemoreceptors for taste and smell<br />detect. Name the cranial nerve(s) for each of these<br />senses and the lobe of the cerebrum where each is<br />felt. (pp. 200, 202)<br />5. Name the part of the eye with each of the following<br />functions: (pp. 203–206)<br />a. Change the shape of the lens<br />b. Contains the rods and cones<br />c. Forms the white of the eye<br />d. Form the optic nerve<br />e. Keep dust out of eye<br />f. Changes the size of the pupil<br />g. Produce tears<br />h. Absorbs light within the eyeball to prevent glare<br />6. With respect to vision: (pp. 207, 209)<br />a. Name the structures and substances that refract<br />light rays (in order)<br />b. State what cones detect and what rods detect.<br />What happens within these receptors when light<br />strikes them?<br />c. Name the cranial nerve for vision and the lobe<br />of the cerebrum that contains the visual area<br />7. With respect to the ear: (pp. 210–215)<br />a. Name the parts of the ear that transmit the<br />vibrations of sound waves (in order)<br />b. State the location of the receptors for hearing<br />c. State the location of the receptors that respond<br />to gravity<br />d. State the location of the receptors that respond<br />to motion<br />e. State the two functions of the 8th cranial nerve<br />f. Name the lobe of the cerebrum concerned with<br />hearing<br />g. Name the two parts of the brain concerned with<br />maintaining balance and equilibrium<br />8. Name the following: (p. 216)<br />a. The locations of arterial chemoreceptors, and<br />state what they detect<br />b. The locations of arterial pressoreceptors, and<br />state what they detect<br />c. The cranial nerves involved in respiratory and<br />cardiac reflexes, and state the part of the brain<br />that regulates these vital functions<br />9. Explain each of the following: adaptation, afterimage,<br />projection, contrast. (pp. 198–199)<br />FOR FURTHER THOUGHT<br />1. Why are the inner ear labyrinths filled with fluid<br />rather than air? One reason is directly concerned<br />with hearing, the other with survival.<br />2. Michael’s summer job in his town was collecting<br />garbage. At first he thought the garbage and the<br />truck smelled awful, but by the time he went home<br />for lunch he decided that he didn’t mind it at all.<br />Explain what happened, and why his mother did<br />mind.<br />3. When we are out in very cold weather, why don’t<br />our eyes freeze shut? Try to think of two reasons.<br />4. You probably have at one time hit your “funny<br />bone,” which is really the ulnar nerve where it<br />crosses the elbow. Such a whack is very painful, and<br />not just in the elbow but all the way down the forearm<br />to the ring and little fingers of the hand. This<br />is referred pain. Explain why it happens, and name<br />the characteristic of sensations that it illustrates.<br />5. Albinism is a genetic characteristic in which<br />melanin is not produced; it may occur in just<br />about any type of animal. As you probably know,<br />an albino person will have white skin and hair.<br />Describe the consequences for the person’s<br />eyes.<br />6. We sometimes hear that blind people have a better<br />sense of hearing than do sighted people. Do you<br />think this is really true? Explain. Name two other<br />senses a blind person may especially depend upon.<br />Explain.<br />7. Look at Question Figure 9–A. In part A, which rectangle<br />seems wider, the upper one or the lower<br />one? Measure them, and explain your answer. Part<br />B shows a Necker cube. Look at the cube and let<br />your eyes relax. What seems to happen? Why do<br />you think this happens? Part C has some lines and<br />some shaded blocks. But what do we see? Explain.<br />8. Look at Question Figure 9–B. Part A shows a normal<br />visual field. Parts B, C, and D are the visual<br />fields of eye disorders you have read about in this<br />chapter. Try to name each, with a reason for your<br />answer.<br />220 The Senses<br />A Normal<br />B<br />C<br />D<br />Question Figure 9–B<br />A B<br />Cinternet fast worldhttp://www.blogger.com/profile/13869077830569899582noreply@blogger.com0tag:blogger.com,1999:blog-135611804747902727.post-27534417022230847102010-06-27T06:54:00.000-07:002010-06-27T07:02:41.626-07:00muscles67<br />CHAPTER 4<br />Chapter Outline<br />Epithelial Tissue<br />Simple Squamous Epithelium<br />Stratified Squamous Epithelium<br />Transitional Epithelium<br />Simple Cuboidal Epithelium<br />Simple Columnar Epithelium<br />Ciliated Epithelium<br />Glands<br />Unicellular glands<br />Multicellular glands<br />Connective Tissue<br />Blood<br />Areolar Connective Tissue<br />Adipose Tissue<br />Fibrous Connective Tissue<br />Elastic Connective Tissue<br />Bone<br />Cartilage<br />Muscle Tissue<br />Skeletal Muscle<br />Smooth Muscle<br />Cardiac Muscle<br />Nerve Tissue<br />Membranes<br />Epithelial Membranes<br />Serous membranes<br />Mucous membranes<br />Connective Tissue Membranes<br />Aging and Tissues<br />68<br />BOX 4–1 CYSTIC FIBROSIS<br />BOX 4–2 VITAMIN C AND COLLAGEN<br />BOX 4–3 COSMETIC COLLAGEN<br />Student Objectives<br />• Describe the general characteristics of each of the<br />four major categories of tissues.<br />• Describe the functions of the types of epithelial<br />tissues with respect to the organs in which they<br />are found.<br />• Describe the functions of the connective tissues,<br />and relate them to the functioning of the body or<br />a specific organ system.<br />• Explain the differences, in terms of location and<br />function, among skeletal muscle, smooth muscle,<br />and cardiac muscle.<br />• Name the three parts of a neuron and state the<br />function of each. Name the organs made of nerve<br />tissue.<br />• Describe the locations of the pleural membranes,<br />the pericardial membranes, and the peritoneummesentery.<br />State the function of serous fluid in<br />each of these locations.<br />• State the locations of mucous membranes and the<br />functions of mucus.<br />• Name some membranes made of connective<br />tissue.<br />• Explain the difference between exocrine and<br />endocrine glands, and give an example of each.<br />Tissues and Membranes<br />69<br />New Terminology<br />Bone (BOWNE)<br />Cartilage (KAR-ti-lidj)<br />Chondrocyte (KON-droh-sight)<br />Collagen (KAH-lah-jen)<br />Connective tissue (kah-NEK-tiv TISH-yoo)<br />Elastin (eh-LAS-tin)<br />Endocrine gland (EN-doh-krin GLAND)<br />Epithelial tissue (EP-i-THEE-lee-uhl TISH-yoo)<br />Exocrine gland (EK-so-krin GLAND)<br />Hemopoietic (HEE-moh-poy-ET-ik)<br />Matrix (MAY-triks)<br />Mucous membrane (MEW-kuss MEM-brayn)<br />Muscle tissue (MUSS-uhl TISH-yoo)<br />Myocardium (MY-oh-KAR-dee-um)<br />Nerve tissue (NERV TISH-yoo)<br />Neuron (NYOOR-on)<br />Neurotransmitter (NYOOR-oh-TRANS-mih-ter)<br />Osteocyte (AHS-tee-oh-sight)<br />Plasma (PLAZ-mah)<br />Secretion (see-KREE-shun)<br />Serous membrane (SEER-us MEM-brayn)<br />Synapse (SIN-aps)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />Atissue is a group of cells with similar structure<br />and function. The tissue contributes to the functioning<br />of the organs in which it is found. You may recall<br />that in Chapter 1 the four major groups of tissues were<br />named and very briefly described. These four groups<br />are epithelial, connective, muscle, and nerve tissue.<br />This chapter presents more detailed descriptions of<br />the tissues in these four categories. For each tissue, its<br />functions are related to the organs of which it is a part.<br />Also in this chapter is a discussion of membranes,<br />which are sheets of tissues. As you might expect, each<br />type of membrane has its specific locations and functions.<br />EPITHELIAL TISSUE<br />Epithelial tissues are found on surfaces as either coverings<br />(outer surfaces) or linings (inner surfaces).<br />Because they have no capillaries of their own, epithelial<br />tissues receive oxygen and nutrients from the<br />blood supply of the connective tissue beneath them.<br />Many epithelial tissues are capable of secretion and<br />may be called glandular epithelium, or more simply,<br />glands.<br />Classification of the epithelial tissues is based on<br />the type of cell of which the tissue is made, its characteristic<br />shape, and the number of layers of cells. There<br />are three distinctive shapes: squamous cells are flat,<br />cuboidal cells are cube shaped, and columnar cells<br />are tall and narrow. “Simple” is the term for a single<br />layer of cells, and “stratified” means that many layers<br />of cells are present (Fig. 4–1).<br />SIMPLE SQUAMOUS EPITHELIUM<br />Simple squamous epithelium is a single layer of flat<br />cells (Fig. 4–2). These cells are very thin and very<br />smooth—these are important physical characteristics.<br />The alveoli (air sacs) of the lungs are simple squamous<br />epithelium. The thinness of the cells permits the diffusion<br />of gases between the air and blood.<br />Another location of this tissue is capillaries, the<br />smallest blood vessels. Capillary walls are only one cell<br />thick, which permits the exchange of gases, nutrients,<br />and waste products between the blood and tissue fluid.<br />The interior surface of capillaries is also very smooth<br />(and these cells continue as the lining of the arteries,<br />veins, and heart); this is important because it prevents<br />abnormal blood clotting within blood vessels.<br />70 Tissues and Membranes<br />Columnar<br />Cuboidal<br />Squamous<br />Simple columnar<br />Simple cuboidal<br />Simple squamous<br />Stratified squamous<br />Shapes Simple Stratified<br />Figure 4–1. Classification of epithelial tissues<br />based on the shape of the cells and the number of<br />layers of cells.<br />QUESTION: Which of these might be best for efficient<br />diffusion, and why?<br />STRATIFIED SQUAMOUS EPITHELIUM<br />Stratified squamous epithelium consists of many<br />layers of mostly flat cells, although lower cells are<br />rounded. Mitosis takes place in the lowest layer to<br />continually produce new cells to replace those worn<br />off the surface (see Fig. 4–2). This type of epithelium<br />makes up the epidermis of the skin, where it is called<br />“keratinizing” because the protein keratin is produced,<br />and the surface cells are dead. Stratified squamous<br />epithelium of the non-keratinizing type lines the oral<br />cavity, the esophagus, and, in women, the vagina. In<br />these locations the surface cells are living and make up<br />the mucous membranes of these organs. In all of its<br />body locations, this tissue is a barrier to microorganisms<br />because the cells of which it is made are very<br />close together. The more specialized functions of the<br />epidermis will be covered in the next chapter.<br />TRANSITIONAL EPITHELIUM<br />Transitional epithelium is a type of stratified epithelium<br />in which the surface cells change shape from<br />round to squamous. The urinary bladder is lined with<br />transitional epithelium. When the bladder is empty,<br />the surface cells are rounded (see Fig. 4–2). As the<br />Tissues and Membranes 71<br />Free surface<br />Stratified squamous<br />Connective tissues<br />Transitional<br />Simple squamous<br />Example: Lung (approximately 430X)<br />Alveolar sacs<br />Example: Esophagus<br />(approximately 430X)<br />B<br />Free surface<br />Example: Urinary bladder (approximately 430X)<br />A C<br />Figure 4–2. Epithelial tissues. (A) Simple squamous. (B) Stratified squamous.<br />(C) Transitional.<br />QUESTION: Which two of these tissues seem to be most related in structure?<br />bladder fills, these cells become flattened. Transitional<br />epithelium enables the bladder to fill and stretch without<br />tearing the lining.<br />SIMPLE CUBOIDAL EPITHELIUM<br />Simple cuboidal epithelium is a single layer of cubeshaped<br />cells (Fig. 4–3). This type of tissue makes up<br />the functional units of the thyroid gland and salivary<br />glands. These are examples of glandular epithelium;<br />their function is secretion. In these glands the<br />cuboidal cells are arranged in small spheres and secrete<br />into the cavity formed by the sphere. In the thyroid<br />gland, the cuboidal epithelium secretes the thyroid<br />hormones; thyroxine is an example. In the salivary<br />glands the cuboidal cells secrete saliva. Cuboidal<br />epithelium also makes up portions of the kidney<br />tubules. Here the cells have microvilli (see Fig. 1–1),<br />and their function is the reabsorption of useful materials<br />back to the blood.<br />SIMPLE COLUMNAR EPITHELIUM<br />Columnar cells are taller than they are wide and are<br />specialized for secretion and absorption. The stomach<br />lining is made of columnar epithelium that<br />secretes gastric juice for digestion. The lining of the<br />small intestine (see Fig. 4–3) secretes digestive<br />enzymes, but these cells also absorb the end products<br />of digestion from the cavity of the intestine into the<br />blood and lymph. To absorb efficiently, the columnar<br />cells of the small intestine have microvilli, which you<br />72 Tissues and Membranes<br />Thyroid secretions (hormones)<br />Simple cuboidal<br />Simple columnar<br />Goblet cells<br />Cilia<br />Connective tissue<br />Ciliated<br />Example: Small intestine<br />(approximately 430X)<br />B<br />Example: Trachea (approximately 430X)<br />A C<br />Example: Thyroid gland (approximately 430X)<br />Figure 4–3. Epithelial tissues. (A) Simple cuboidal. (B) Simple columnar. (C) Ciliated.<br />QUESTION: What is the function of the cilia that line the trachea?<br />may recall are folds of the cell membrane on their free<br />surfaces (see Fig. 3–2). These microscopic folds<br />greatly increase the surface area for absorption.<br />Yet another type of columnar cell is the goblet cell,<br />which is a unicellular gland. Goblet cells secrete<br />mucus and are found in the lining of the intestines<br />and the lining of parts of the respiratory tract such as<br />the trachea. Mucous membranes will be described in a<br />later section.<br />CILIATED EPITHELIUM<br />Ciliated epithelium consists of columnar cells that<br />have cilia on their free surfaces (see Fig. 4–3). Recall<br />from Chapter 3 that the function of cilia is to sweep<br />materials across the cell surface. Ciliated epithelium<br />lines the nasal cavities, larynx, trachea, and large<br />bronchial tubes. The cilia sweep mucus, with trapped<br />dust and bacteria from the inhaled air, toward the<br />pharynx to be swallowed. Bacteria are then destroyed<br />by the hydrochloric acid in the stomach. The air that<br />reaches the lungs is almost entirely free of pathogens<br />and particulate pollution.<br />Another location of ciliated epithelium in women is<br />the lining of the fallopian tubes. The cilia here sweep<br />the ovum, which has no means of self-locomotion,<br />toward the uterus.<br />The epithelial tissues are summarized in Table 4–1.<br />GLANDS<br />Glands are cells or organs that secrete something;<br />that is, they produce a substance that has a function<br />either at that site or at a more distant site.<br />Unicellular Glands<br />Unicellular means “one cell.” Goblet cells are an<br />example of unicellular glands. As mentioned earlier,<br />goblet cells are found in the lining of the respiratory<br />and digestive tracts. Their secretion is mucus (see also<br />Box 4–1: Cystic Fibrosis).<br />Multicellular Glands<br />Most glands are made of many similar cells, or of a<br />variety of cells with their secretions mingled into a<br />collective secretion. Multicellular glands may be<br />divided into two major groups: exocrine glands and<br />endocrine glands.<br />Exocrine glands have ducts (tubes) to take the<br />secretion away from the gland to the site of its function.<br />Salivary glands, for example, secrete saliva that is<br />carried by ducts to the oral cavity. Sweat glands secrete<br />sweat that is transported by ducts to the skin surface,<br />where it can be evaporated by excess body heat. The<br />gastric glands of the stomach lining contain different<br />kinds of cells (see Fig. 16–5), which produce<br />Tissues and Membranes 73<br />Table 4–1 TYPES OF EPITHELIAL TISSUE<br />Type Structure Location and Function<br />Simple squamous<br />Stratified squamous<br />Transitional<br />Cuboidal<br />Columnar<br />Ciliated<br />One layer of flat cells<br />Many layers of cells; surface cells<br />flat; lower cells rounded;<br />lower layer undergoes mitosis<br />Many layers of cells; surface cells<br />change from rounded to flat<br />One layer of cube-shaped cells<br />One layer of column-shaped cells<br />One layer of columnar cells with<br />cilia on their free surfaces<br />• Alveoli of the lungs—thin to permit diffusion of gases<br />• Capillaries—thin to permit exchanges of materials;<br />smooth to prevent abnormal blood clotting<br />• Epidermis—surface cells are dead; a barrier to pathogens<br />• Lining of esophagus, vagina—surface cells are living; a<br />barrier to pathogens<br />• Lining of urinary bladder—permits expansion without<br />tearing the lining<br />• Thyroid gland—secretes thyroxine<br />• Salivary glands—secrete saliva<br />• Kidney tubules—permit reabsorption of useful materials<br />back to the blood<br />• Lining of stomach—secretes gastric juice<br />• Lining of small intestine—secretes enzymes and absorbs<br />end products of digestion (microvilli present)<br />• Lining of trachea—sweeps mucus and dust to the pharynx<br />• Lining of fallopian tube—sweeps ovum toward uterus<br />hydrochloric acid and the enzyme pepsin. Both of<br />these secretions are part of gastric juice.<br />Endocrine glands are ductless glands. The secretions<br />of endocrine glands are a group of chemicals<br />called hormones, which enter capillaries and are circulated<br />throughout the body. Hormones then bring<br />about specific effects in their target organs. These<br />effects include aspects of growth, use of minerals and<br />other nutrients, and regulation of blood pressure, and<br />will be covered in more detail in Chapter 10.<br />Examples of endocrine glands are the thyroid gland,<br />adrenal glands, and pituitary gland.<br />The pancreas is an organ that is both an exocrine<br />and an endocrine gland. The exocrine portions secrete<br />digestive enzymes that are carried by ducts to the<br />duodenum of the small intestine, their site of action.<br />The endocrine portions of the pancreas, called pancreatic<br />islets or islets of Langerhans, secrete the hormones<br />insulin and glucagon directly into the blood.<br />CONNECTIVE TISSUE<br />There are several kinds of connective tissue, some of<br />which may at first seem more different than alike. The<br />types of connective tissue include areolar, adipose,<br />fibrous, and elastic tissue as well as blood, bone, and<br />cartilage; these are summarized in Table 4–2. A characteristic<br />that all connective tissues have in common is<br />the presence of a matrix in addition to cells. The<br />matrix is a structural network or solution of nonliving<br />intercellular material. Each connective tissue<br />has its own specific kind of matrix. The matrix of<br />blood, for example, is blood plasma, which is mostly<br />water. The matrix of bone is made primarily of calcium<br />salts, which are hard and strong. As each type of<br />connective tissue is described in the following sections,<br />mention will be made of the types of cells present<br />as well as the kind of matrix.<br />BLOOD<br />Although blood is the subject of Chapter 11, a brief<br />description will be given here. Blood consists of cells<br />and plasma; cells are the living portion. The matrix of<br />blood is plasma, which is about 52% to 62% of the<br />total blood volume in the body. The water of plasma<br />contains dissolved salts, nutrients, gases, and waste<br />products. As you might expect, one of the primary<br />functions of plasma is transport of these materials<br />within the body.<br />Blood cells are produced from stem cells in the red<br />bone marrow, the body’s primary hemopoietic tissue<br />(blood-forming tissue), which is found in flat and<br />irregular bones such as the hip bone and vertebrae.<br />The blood cells are red blood cells, platelets, and the<br />five kinds of white blood cells: neutrophils,<br />eosinophils, basophils, monocytes, and lymphocytes<br />(see Figs. 4–4 and 11–2). Lymphocytes mature and<br />divide in lymphatic tissue, which makes up the spleen,<br />74 Tissues and Membranes<br />BOX 4–1 CYSTIC FIBROSIS<br />defensin, a bacterium called Pseudomonas aeruginosa<br />stimulates the lung cells to produce copious<br />thick mucus, an ideal growth environment for bacteria.<br />Defensive white blood cells cannot get<br />through the thick mucus, and their activity mistakenly<br />destroys lung tissue. A person with CF has<br />clogged bronchial tubes, frequent episodes of<br />pneumonia, and, ultimately, lungs that cannot carry<br />out gas exchange. CF is a chronic, progressive disease<br />that is eventually fatal unless a lung transplant<br />is performed.<br />CF is one of several disorders believed to be correctable<br />by gene therapy, but because it involves<br />human subjects, this kind of work proceeds very<br />slowly.<br />Cystic fibrosis (CF) is a genetic disorder (there are<br />many forms) of certain exocrine glands including<br />the salivary glands, the sweat glands, the pancreas,<br />and the mucous glands of the respiratory tract.<br />In the pancreas, thick mucus clogs the ducts and<br />prevents pancreatic enzymes from reaching the<br />small intestine, thus impairing digestion, especially<br />of fats. But the most serious effects of CF are in the<br />lungs. The genetic mistake in CF often involves a<br />gene called CFTR, which codes for chloride ion<br />channels (proteins) in the membranes of epithelial<br />cells. In the lungs, the defective channels are<br />destroyed (by proteasomes), which causes a change<br />in the composition of the tissue fluid around the<br />cells. This change inactivates defensin, a natural<br />antibiotic produced by lung tissue. In the absence of<br />the lymph nodes, and the thymus gland. The thymus<br />also contains stem cells, but they produce only a<br />subset of lymphocytes. Stem cells are present in the<br />spleen and lymph nodes as well, though the number of<br />lymphocytes they produce is a small fraction of the<br />total.<br />The blood cells make up 38% to 48% of the total<br />blood, and each type of cell has its specific function.<br />Red blood cells (RBCs) carry oxygen bonded to the<br />iron in their hemoglobin. White blood cells (WBCs)<br />destroy pathogens by phagocytosis, the production of<br />antibodies, or other chemical methods, and provide us<br />with immunity to some diseases. Platelets prevent<br />blood loss; the process of blood clotting involves<br />platelets.<br />AREOLAR CONNECTIVE TISSUE<br />The cells of areolar (or loose) connective tissue are<br />called fibroblasts. A blast cell is a “producing” cell,<br />and fibroblasts produce protein fibers. Collagen fibers<br />are very strong; elastin fibers are elastic, that is, able<br />to return to their original length, or recoil, after being<br />stretched. These protein fibers and tissue fluid make<br />up the matrix, or non-living portion, of areolar connective<br />tissue (see Fig. 4–4). Also within the matrix are<br />mast cells that release inflammatory chemicals when<br />tissue is damaged, and many white blood cells, which<br />are capable of self-locomotion. Their importance here<br />is related to the locations of areolar connective tissue.<br />Areolar tissue is found beneath the dermis of the<br />Tissues and Membranes 75<br />Blood<br />Areolar<br />Adipose<br />White blood cell<br />Fibroblast<br />Collagen fibers<br />Elastin fiber<br />Adipocytes<br />(Approximately 430X)<br />Red blood cells<br />White blood cell<br />(Approximately 300X)<br />(Approximately 150X)<br />A<br />B<br />C<br />Platelets<br />Figure 4–4. Connective tissues. (A) Blood. (B) Areolar. (C) Adipose.<br />QUESTION: What is the matrix of blood, and what is found in adipocytes?<br />skin and beneath the epithelial tissue of all the body<br />systems that have openings to the environment. Recall<br />that one function of white blood cells is to destroy<br />pathogens. How do pathogens enter the body? Many<br />do so through breaks in the skin. Bacteria and viruses<br />also enter with the air we breathe and the food we eat,<br />and some may get through the epithelial linings of the<br />respiratory and digestive tracts and cause tissue damage.<br />Areolar connective tissue with its mast cells and<br />many white blood cells is strategically placed to intercept<br />pathogens before they get to the blood and circulate<br />throughout the body.<br />ADIPOSE TISSUE<br />The cells of adipose tissue are called adipocytes and<br />are specialized to store fat in microscopic droplets.<br />True fats are the chemical form of long-term energy<br />storage. Excess nutrients have calories that are not<br />wasted but are converted to fat to be stored for use<br />when food intake decreases. Any form of excess calories,<br />whether in the form of fats, carbohydrates, or<br />amino acids from protein, may be changed to triglycerides<br />and stored. The amount of matrix in adipose<br />tissue is small and consists of tissue fluid and a few collagen<br />fibers (see Fig. 4–4).<br />Most fat is stored subcutaneously in the areolar<br />connective tissue between the dermis and the muscles.<br />This layer varies in thickness among individuals; the<br />more excess calories consumed, the thicker the layer.<br />As mentioned in Chapter 2, adipose tissue also cushions<br />organs such as the eyes and kidneys.<br />Recent research has discovered that adipose tissue<br />does much more than provide a cushion or store<br />energy. Adipose tissue is now considered an endocrine<br />tissue, because it produces at least one hormone.<br />Leptin is an appetite-suppressing hormone secreted<br />by adipocytes to signal the hypothalamus in the brain<br />that fat storage is sufficient (see also Chapter 17).<br />When leptin secretion diminishes, appetite increases.<br />Adipocytes secrete at least two chemicals that help<br />regulate the use of insulin in glucose and fat metabolism.<br />Adipose tissue is also involved in inflammation,<br />the body’s first response to injury, in that it produces<br />cytokines, chemicals that activate white blood cells.<br />Our adipose tissue is not simply an inert depository of<br />76 Tissues and Membranes<br />Fibrous tissue<br />Collagen<br />fibers<br />Fibroblasts<br />Example: Trachea<br />(approximately 430X)<br />B<br />A<br />C<br />(Approximately 430X)<br />Haversian canal<br />Canaliculi<br />Matrix<br />Osteocytes<br />Bone<br />Chondrocytes<br />Matrix<br />Cartilage<br />Example: Tendons<br />(approximately 430X)<br />Figure 4–5. Connective tissues. (A) Fibrous. (B) Cartilage. (C) Bone.<br />QUESTION: What is the matrix of fibrous tissue, and of bone?<br />fat, rather it is part of the complex systems that ensure<br />we are nourished properly or that protect us from<br />pathogens that get through the skin.<br />FIBROUS CONNECTIVE TISSUE<br />Fibrous connective tissue consists mainly of parallel<br />(regular) collagen fibers with a few fibroblasts scattered<br />among them (Fig. 4–5). This parallel arrangement<br />of collagen provides great strength, yet is<br />flexible. The locations of this tissue are related to<br />the need for flexible strength. The outer walls of arteries<br />are reinforced with fibrous connective tissue,<br />because the blood in these vessels is under high pressure.<br />The strong outer wall prevents rupture of the<br />artery (see also Box 4–2: Vitamin C and Collagen).<br />Tendons and ligaments are made of fibrous connective<br />tissue. Tendons connect muscle to bone; ligaments<br />connect bone to bone. When the skeleton is moved,<br />these structures must be able to withstand the great<br />mechanical forces exerted upon them.<br />Fibrous connective tissue has a relatively poor<br />blood supply, which makes repair a slow process. If<br />you have ever had a severely sprained ankle (which<br />Tissues and Membranes 77<br />Table 4–2 TYPES OF CONNECTIVE TISSUE<br />Type Structure Location and Function<br />Blood<br />Areolar (loose)<br />Adipose<br />Fibrous<br />Elastic<br />Bone<br />Cartilage<br />Within blood vessels<br />• Plasma—transports materials<br />• RBCs—carry oxygen<br />• WBCs—destroy pathogens<br />• Platelets—prevent blood loss<br />Subcutaneous<br />• Connects skin to muscles; WBCs destroy pathogens<br />Mucous membranes (digestive, respiratory, urinary,<br />reproductive tracts)<br />• WBCs destroy pathogens<br />Subcutaneous<br />• Stores excess energy<br />• Produces chemicals that influence appetite, use<br />of nutrients, and inflammation<br />Around eyes and Kidneys<br />• Cushions<br />Tendons and ligaments (regular)<br />• Strong to withstand forces of movement of joints<br />Dermis (irregular)<br />• The strong inner layer of the skin<br />Walls of large arteries<br />• Helps maintain blood pressure Around alveoli in lungs<br />• Promotes normal exhalation<br />Bones<br />• Support the body<br />• Protect internal organs from mechanical injury<br />• Store excess calcium<br />• Contain and protect red bone marrow<br />Wall of trachea<br />• Keeps airway open<br />On joint surfaces of bones<br />• Smooth to prevent friction<br />Tip of nose and outer ear<br />• Support<br />Between vertebrae<br />• Absorb shock<br />Plasma (matrix) and red blood cells,<br />white blood cells, and platelets<br />Fibroblasts and a matrix of tissue<br />fluid, collagen, and elastin fibers<br />Adipocytes that store fat (little matrix)<br />Mostly collagen fibers (matrix) with<br />few fibroblasts<br />Mostly elastin fibers (matrix) with few<br />fibroblasts<br />Osteocytes in a matrix of calcium<br />salts and collagen<br />Chondrocytes in a flexible protein<br />matrix<br />means the ligaments have been overly stretched), you<br />know that complete healing may take several months.<br />An irregular type of fibrous connective tissue forms<br />the dermis of the skin and the fasciae (membranes)<br />around muscles. Although the collagen fibers here are<br />not parallel to one another, the tissue is still strong.<br />The dermis is different from other fibrous connective<br />tissue in that it has a good blood supply (see also Box<br />4–3: Cosmetic Collagen).<br />ELASTIC CONNECTIVE TISSUE<br />As its name tells us, elastic connective tissue is primarily<br />elastin fibers. One of its locations is in the walls<br />of large arteries. These vessels are stretched when the<br />heart contracts and pumps blood, then they recoil, or<br />snap back, when the heart relaxes. This recoil helps<br />keep the blood moving away from the heart, and is<br />important to maintain normal blood pressure.<br />Elastic connective tissue is also found surrounding<br />the alveoli of the lungs. The elastic fibers are stretched<br />during inhalation, then recoil during exhalation to<br />squeeze air out of the lungs. If you pay attention to<br />your breathing for a few moments, you will notice that<br />normal exhalation does not require “work” or energy.<br />This is because of the normal elasticity of the lungs.<br />BONE<br />The prefix that designates bone is “osteo,” so bone<br />cells are called osteocytes. The matrix of bone is<br />made of calcium salts and collagen and is strong, hard,<br />78 Tissues and Membranes<br />BOX 4–2 VITAMIN C AND COLLAGEN<br />Many people take extra vitamin C, for various reasons.<br />Vitamin C has several functions, and an important<br />one is the synthesis of collagen.<br />Imagine the protein collagen as a ladder with<br />three uprights and rungs that connect adjacent<br />uprights. Vitamin C is essential for forming the<br />“rungs,” without which the uprights will not stay<br />together as a strong unit. Collagen formed in the<br />absence of vitamin C is weak, and the effects of<br />weak collagen are dramatically seen in the disease<br />called scurvy.<br />In 1753 James Lind, a Scottish surgeon, recommended<br />to the British Navy that lime juice be taken<br />on long voyages to prevent scurvy among the<br />sailors. Scurvy is characterized by bleeding gums<br />and loss of teeth, poor healing of wounds, fractures,<br />and bleeding in the skin, joints, and elsewhere in<br />the body. The lime juice did prevent this potentially<br />fatal disease, as did consumption of fresh fruits and<br />vegetables, although at the time no one knew why.<br />Vitamin C was finally isolated in the laboratory in<br />1928.<br />BOX 4–3 COSMETIC COLLAGEN<br />system as foreign tissue. More seriously, an autoimmune<br />response may be triggered in some individuals,<br />and the immune system may begin to destroy<br />the person’s own connective tissue.<br />In an effort to avoid these problems, some cosmetic<br />surgeons now use the person’s own collagen<br />and fat, which may be extracted from the thigh,<br />hip, or abdomen. The long-term consequences and<br />outcomes of such procedures have yet to be evaluated.<br />We might remember that for many years the<br />use of silicone injections had been considered safe.<br />Silicone injections are now banned by the FDA,<br />since we now know that they carry significant risk<br />of serious tissue damage.<br />Collagen is the protein that makes tendons, ligaments,<br />and other connective tissues strong. In 1981,<br />the Food and Drug Administration (FDA) approved<br />the use of cattle collagen by injection for cosmetic<br />purposes, to minimize wrinkles and scars. Indeed,<br />collagen injected below the skin will flatten out<br />deep facial wrinkles and make them less prominent,<br />and many people have had this seemingly simple<br />cosmetic surgery.<br />There are, however, drawbacks. Injected collagen<br />lasts only a few months; the injections must be<br />repeated several times a year, and they are expensive.<br />Some people have allergic reactions to the cattle<br />collagen, which is perceived by the immune<br />and not flexible. In the shafts of long bones such as the<br />femur, the osteocytes, matrix, and blood vessels are<br />in very precise arrangements called haversian systems<br />or osteons (see Fig. 4–5). Bone has a good blood<br />supply, which enables it to serve as a storage site for<br />calcium and to repair itself relatively rapidly after a<br />simple fracture. Some bones, such as the sternum<br />(breastbone) and pelvic bone, contain red bone marrow,<br />the primary hemopoietic tissue that produces<br />blood cells.<br />Other functions of bone tissue are related to the<br />strength of bone matrix. The skeleton supports the<br />body, and some bones protect internal organs from<br />mechanical injury. A more complete discussion of<br />bone is found in Chapter 6.<br />CARTILAGE<br />The protein–carbohydrate matrix of cartilage does<br />not contain calcium salts, and also differs from that of<br />bone in that it contains more water, which makes it<br />resilient. It is firm, yet smooth and flexible. Cartilage<br />is found on the joint surfaces of bones, where its<br />smooth surface helps prevent friction. The tip of the<br />nose and external ear are supported by flexible cartilage.<br />The wall of the trachea, the airway to the lungs,<br />contains firm rings of cartilage to maintain an open air<br />passageway. Discs of cartilage are found between the<br />vertebrae of the spine. Here the cartilage is a firm<br />cushion; it absorbs shock and permits movement.<br />Within the cartilage matrix are the chondrocytes,<br />or cartilage cells (see Fig. 4–5). There are no capillaries<br />within the cartilage matrix, so these cells are nourished<br />by diffusion through the matrix, a slow process.<br />This becomes clinically important when cartilage is<br />damaged, for repair will take place very slowly or not<br />at all. Athletes sometimes damage cartilage within the<br />knee joint. Such damaged cartilage is usually surgically<br />removed in order to preserve as much joint mobility as<br />possible.<br />MUSCLE TISSUE<br />Muscle tissue is specialized for contraction. When<br />muscle cells contract, they shorten and bring about<br />some type of movement. There are three types of<br />muscle tissue: skeletal, smooth, and cardiac (Table<br />4–3). The movements each can produce have very different<br />purposes.<br />SKELETAL MUSCLE<br />Skeletal muscle may also be called striated muscle or<br />voluntary muscle. Each name describes a particular<br />aspect of this tissue, as you will see. The skeletal muscle<br />cells are cylindrical, have several nuclei each, and<br />appear striated, or striped (Fig. 4–6). The striations<br />are the result of the precise arrangement of the contracting<br />proteins within the cells.<br />Skeletal muscle tissue makes up the muscles that<br />are attached to bones. These muscles are supplied<br />with motor nerves, and thus move the skeleton. They<br />also produce a significant amount of heat, which is<br />important to help maintain the body’s constant temperature.<br />Each muscle cell has its own motor nerve<br />ending. The nerve impulses that can then travel to the<br />muscles are essential to cause contraction. Although<br />we do not have to consciously plan all our movements,<br />the nerve impulses for them originate in the cerebrum,<br />the “thinking” part of the brain.<br />Let us return to the three names for this tissue:<br />“skeletal” describes its location, “striated” describes its<br />appearance, and “voluntary” describes how it functions.<br />The skeletal muscles and their functioning are<br />the subject of Chapter 7.<br />SMOOTH MUSCLE<br />Smooth muscle may also be called involuntary muscle<br />or visceral muscle. The cells of smooth muscle<br />have tapered ends, a single nucleus, and no striations<br />(see Fig. 4–6). Although nerve impulses do bring<br />about contractions, this is not something most of us<br />can control, hence the name involuntary. The term visceral<br />refers to internal organs, many of which contain<br />smooth muscle. The functions of smooth muscle are<br />actually functions of the organs in which the muscle is<br />found.<br />In the stomach and intestines, smooth muscle contracts<br />in waves called peristalsis to propel food<br />through the digestive tract. In the walls of arteries and<br />veins, smooth muscle constricts or dilates the vessels<br />to maintain normal blood pressure. The iris of the eye<br />has two sets of smooth muscle fibers to constrict or<br />dilate the pupil, which regulates the amount of light<br />that strikes the retina.<br />Other functions of smooth muscle are mentioned<br />in later chapters. This is an important tissue that you<br />will come across again and again in our study of the<br />human body.<br />Tissues and Membranes 79<br />80<br />Skeletal muscle<br />(Approximately 430X)<br />A<br />Smooth muscle<br />(Approximately 430X)<br />B<br />Cardiac muscle<br />(Approximately 430X)<br />C<br />Intercalated<br />discs<br />Figure 4–6. Muscle tissues. (A) Skeletal. (B) Smooth. (C) Cardiac.<br />QUESTION: Which kinds of muscle cells have striations? What forms these striations?<br />Table 4–3 TYPES OF MUSCLE TISSUE<br />Type Structure Location and Function Effect of Nerve Impulses<br />Skeletal<br />Smooth<br />Cardiac<br />Large cylindrical cells with<br />striations and several<br />nuclei each<br />Small tapered cells with no<br />striations and one nucleus<br />each<br />Branched cells with faint striations<br />and one nucleus<br />each<br />Attached to bones<br />• Moves the skeleton and<br />produces heat<br />Walls of arteries<br />• Maintains blood pressure<br />Walls of stomach and intestines<br />• Peristalsis<br />Iris of eye<br />• Regulates size of pupil<br />Walls of the chambers of the heart<br />• Pumps blood<br />Essential to cause contraction<br />(voluntary)<br />Bring about contraction or<br />regulate the rate of contraction<br />(involuntary)<br />Regulate only the rate of contraction<br />CARDIAC MUSCLE<br />The cells of the heart, cardiac muscle, are shown in<br />Fig. 4–6. They are branched, have one nucleus each,<br />and have faint striations. The cell membranes at the<br />ends of these cells are somewhat folded and fit into<br />matching folds of the membranes of the next cells.<br />(Interlock the fingers of both hands to get an idea of<br />what these adjacent membranes look like.) These<br />interlocking folds are called intercalated discs, and<br />permit the electrical impulses of muscle contraction to<br />pass swiftly from cell to cell. This enables the heart to<br />beat in a very precise wave of contraction from the<br />upper chambers to the lower chambers. Cardiac muscle<br />as a whole is called the myocardium, and forms<br />the walls of the four chambers of the heart. Its function,<br />therefore, is the function of the heart, to pump<br />blood. The contractions of the myocardium create<br />blood pressure and keep blood circulating throughout<br />the body, so that the blood can carry out its many<br />functions.<br />Cardiac muscle cells have the ability to contract by<br />themselves. Thus the heart maintains its own beat.<br />The role of nerve impulses is to increase or decrease<br />the heart rate, depending upon whatever is needed<br />by the body in a particular situation. We will return to<br />the heart in Chapter 12.<br />NERVE TISSUE<br />Nerve tissue consists of nerve cells called neurons<br />and some specialized cells found only in the nervous<br />system. The nervous system has two divisions: the<br />central nervous system (CNS) and the peripheral<br />nervous system (PNS). The brain and spinal cord are<br />the organs of the CNS. They are made of neurons and<br />specialized cells called neuroglia. The CNS and the<br />neuroglia are discussed in detail in Chapter 8. The<br />PNS consists of all of the nerves that emerge from<br />the CNS and supply the rest of the body. These nerves<br />are made of neurons and specialized cells called<br />Schwann cells. The Schwann cells form the myelin<br />sheath to electrically insulate neurons.<br />Neurons are capable of generating and transmitting<br />electrochemical impulses. There are many different<br />kinds of neurons, but they all have the same basic<br />structure (Fig. 4–7). The cell body contains the<br />nucleus and is essential for the continuing life of the<br />neuron. An axon is a process (the term “process” here<br />means “something that sticks out,” a cellular extension)<br />that carries impulses away from the cell body; a<br />neuron has only one axon. Dendrites are processes<br />that carry impulses toward the cell body; a neuron<br />may have several dendrites. A nerve impulse travels<br />along the cell membrane of a neuron, and is electrical,<br />but where neurons meet there is a small space called a<br />synapse, which an electrical impulse cannot cross. At<br />a synapse, between the axon of one neuron and the<br />dendrite or cell body of the next neuron, impulse<br />transmission depends upon chemicals called neurotransmitters.<br />A summary of nerve tissue is found in<br />Table 4–4, and each of these aspects of nerve tissue is<br />covered in more detail in Chapter 8.<br />Nerve tissue makes up the brain, spinal cord, and<br />Tissues and Membranes 81<br />Table 4–4 NERVE TISSUE<br />Part Structure Function<br />Neuron (nerve cell)<br />Cell body<br />Axon<br />Dendrites<br />Synapse<br />Neurotransmitters<br />Neuroglia<br />Schwann cells<br />Contains the nucleus<br />Cellular process (extension)<br />Cellular process (extension)<br />Space between axon of one neuron and the<br />dendrite or cell body of the next neuron<br />Chemicals released by axons<br />Specialized cells in the central nervous<br />system<br />Specialized cells in the peripheral nervous<br />system<br />• Regulates the functioning of the neuron<br />• Carries impulses away from the cell body<br />• Carry impulses toward the cell body<br />• Transmits impulses from one neuron<br />to others<br />• Transmit impulses across synapses<br />• Form myelin sheaths and other functions<br />• Form the myelin sheaths around neurons<br />peripheral nerves. As you can imagine, each of these<br />organs has very specific functions. For now, we will<br />just mention the categories of the functions of nerve<br />tissue. These include feeling and interpreting sensation,<br />initiation of movement, the rapid regulation of<br />body functions such as heart rate and breathing, and<br />the organization of information for learning and<br />memory.<br />MEMBRANES<br />Membranes are sheets of tissue that cover or line surfaces<br />or that separate organs or parts (lobes) of organs<br />from one another. Many membranes produce secretions<br />that have specific functions. The two major categories<br />of membranes are epithelial membranes and<br />connective tissue membranes.<br />EPITHELIAL MEMBRANES<br />There are two types of epithelial membranes, serous<br />and mucous. Each type is found in specific locations<br />within the body and secretes a fluid. These fluids are<br />called serous fluid and mucus.<br />Serous Membranes<br />Serous membranes are sheets of simple squamous<br />epithelium that line some closed body cavities and<br />cover the organs in these cavities (Fig. 4–8). The<br />pleural membranes are the serous membranes of the<br />thoracic cavity. The parietal pleura lines the chest wall<br />and the visceral pleura covers the lungs. (Notice that<br />line means “on the inside” and cover means “on the<br />outside.” These terms cannot be used interchangeably,<br />because each indicates a different location.) The pleural<br />membranes secrete serous fluid, which prevents<br />friction between them as the lungs expand and recoil<br />during breathing.<br />The heart, in the thoracic cavity between the lungs,<br />has its own set of serous membranes. The parietal<br />pericardium lines the fibrous pericardium (a connective<br />tissue membrane), and the visceral pericardium,<br />or epicardium, is on the surface of the heart muscle<br />(see also Fig. 12–1). Serous fluid is produced to prevent<br />friction as the heart beats.<br />In the abdominal cavity, the peritoneum is the<br />serous membrane that lines the cavity. The mesentery,<br />or visceral peritoneum, is folded over and covers<br />the abdominal organs. Here, the serous fluid prevents<br />friction as the stomach and intestines contract and<br />slide against other organs (see also Fig. 16–4).<br />Mucous Membranes<br />Mucous membranes line the body tracts (systems)<br />that have openings to the environment. These are the<br />respiratory, digestive, urinary, and reproductive tracts.<br />The epithelium of a mucous membrane (mucosa)<br />varies with the different organs involved. The mucosa<br />of the esophagus and of the vagina is stratified squamous<br />epithelium; the mucosa of the trachea is ciliated<br />epithelium; the mucosa of the stomach is columnar<br />epithelium.<br />82 Tissues and Membranes<br />Figure 4–7. Nerve tissue of the central nervous system<br />(CNS).<br />QUESTION: How many processes does the central neuron<br />have, and what are they called?<br />Cell body<br />Neuron<br />Neurons Nucleus<br />(Approximately 250X)<br />Neuroglia<br />83<br />Mucous Membranes<br />Parietal Pleura<br />Visceral Pleura<br />Mesentery<br />Peritoneum<br />Figure 4–8. Epithelial membranes. Mucous membranes line body tracts that open to the<br />environment. Serous membranes are found within closed body cavities such as the thoracic<br />and abdominal cavities. See text for further description.<br />QUESTION: Name another organ covered by mesentery.<br />The mucus secreted by these membranes keeps the<br />lining epithelial cells wet. Remember that these are<br />living cells, and if they dry out, they will die. In the<br />digestive tract, mucus also lubricates the surface to<br />permit the smooth passage of food. In the respiratory<br />tract the mucus traps dust and bacteria, which are then<br />swept to the pharynx by ciliated epithelium.<br />CONNECTIVE TISSUE MEMBRANES<br />Many membranes are made of connective tissue.<br />Because these will be covered with the organ systems<br />of which they are a part, their locations and functions<br />are summarized in Table 4–5.<br />AGING AND TISSUES<br />As mentioned in the previous chapter, aging takes<br />place at the cellular level, but of course is apparent in<br />the groups of cells we call tissues. In muscle tissue, for<br />example, the proteins that bring about contraction<br />deteriorate and are not repaired or replaced. The same<br />is true of collagen and elastin, the proteins of connective<br />tissue such as the dermis of the skin. Other aspects<br />of the aging of tissues will be more meaningful to you<br />in the context of the functions of organs and systems,<br />so we will save those for the following chapters.<br />SUMMARY<br />The tissues and membranes described in this chapter<br />are more complex than the individual cells of which<br />they are made. However, we have only reached an<br />intermediate level with respect to the structural and<br />functional complexity of the body as a whole. The following<br />chapters are concerned with the organ systems,<br />the most complex level. In the descriptions of the<br />organs of these systems, you will find mention of the<br />tissues and their contributions to each organ and<br />organ system.<br />84 Tissues and Membranes<br />Table 4–5 CONNECTIVE TISSUE<br />MEMBRANES<br />Membrane Location and Function<br />Superficial<br />fascia<br />Periosteum<br />Perichondrium<br />Synovial<br />Deep fascia<br />Meninges<br />Fibrous<br />pericardium<br />• Between the skin and muscles;<br />adipose tissue stores fat<br />• Covers each bone; contains<br />blood vessels that enter the<br />bone<br />• Anchors tendons and ligaments<br />• Covers cartilage; contains capillaries,<br />the only blood supply for<br />cartilage<br />• Lines joint cavities; secretes synovial<br />fluid to prevent friction<br />when joints move<br />• Covers each skeletal muscle;<br />anchors tendons<br />• Cover the brain and spinal cord;<br />contain cerebrospinal fluid<br />• Forms a sac around the heart;<br />lined by the serous parietal pericardium<br />STUDY OUTLINE<br />A tissue is a group of cells with similar structure<br />and function. The four main groups of<br />tissues are epithelial, connective, muscle, and<br />nerve.<br />Epithelial Tissue—found on surfaces; have no<br />capillaries; some are capable of secretion;<br />classified as to shape of cells and number of<br />layers of cells (see Table 4–1 and Figs. 4–1,<br />4–2, and 4–3)<br />1. Simple squamous—one layer of flat cells; thin and<br />smooth. Sites: alveoli (to permit diffusion of gases);<br />capillaries (to permit exchanges between blood and<br />tissues).<br />2. Stratified squamous—many layers of mostly flat<br />cells; mitosis takes place in lowest layer. Sites: epidermis,<br />where surface cells are dead (a barrier to<br />pathogens); lining of mouth; esophagus; and vagina<br />(a barrier to pathogens).<br />3. Transitional—stratified, yet surface cells are<br />rounded and flatten when stretched. Site: urinary<br />bladder (to permit expansion without tearing the<br />lining).<br />4. Simple cuboidal—one layer of cube-shaped cells.<br />Sites: thyroid gland (to secrete thyroid hormones);<br />salivary glands (to secrete saliva); kidney tubules (to<br />reabsorb useful materials back to the blood).<br />5. Simple columnar—one layer of column-shaped<br />cells. Sites: stomach lining (to secrete gastric juice);<br />small intestinal lining (to secrete digestive enzymes<br />and absorb nutrients—microvilli increase surface<br />area for absorption).<br />6. Ciliated—columnar cells with cilia on free surfaces.<br />Sites: trachea (to sweep mucus and bacteria to the<br />pharynx); fallopian tubes (to sweep ovum to<br />uterus).<br />7. Glands—epithelial tissues that produce secretions.<br />• Unicellular—one-celled glands. Goblet cells<br />secrete mucus in the respiratory and digestive<br />tracts.<br />• Multicellular—many-celled glands.<br />Exocrine glands have ducts; salivary glands<br />secrete saliva into ducts that carry it to the oral<br />cavity.<br />Endocrine glands secrete hormones directly<br />into capillaries (no ducts); thyroid gland secretes<br />thyroxine.<br />Connective Tissue—all have a non-living<br />intercellular matrix and specialized cells (see<br />Table 4–2 and Figs. 4–4 and 4–5)<br />1. Blood—the matrix is plasma, mostly water; transports<br />materials in the blood. Red blood cells carry<br />oxygen; white blood cells destroy pathogens and<br />provide immunity; platelets prevent blood loss, as<br />in clotting. Blood cells are made in red bone marrow.<br />2. Areolar (loose)—cells are fibroblasts, which produce<br />protein fibers: collagen is strong, elastin is<br />elastic; the matrix is collagen, elastin, and tissue<br />fluid. White blood cells and mast cells are also<br />present. Sites: below the dermis and below the<br />epithelium of tracts that open to the environment<br />(to destroy pathogens that enter the body).<br />3. Adipose—cells are adipocytes that store fat; little<br />matrix. Sites: between the skin and muscles (to<br />store energy); around the eyes and kidneys (to<br />cushion). Also involved in appetite, use of insulin,<br />and inflammation.<br />4. Fibrous—mostly matrix, strong collagen fibers;<br />cells are fibroblasts. Regular fibrous sites: tendons<br />(to connect muscle to bone); ligaments (to connect<br />bone to bone); poor blood supply, slow healing.<br />Irregular fibrous sites: dermis of the skin and the<br />fascia around muscles.<br />5. Elastic—mostly matrix, elastin fibers. Sites: walls of<br />large arteries (to maintain blood pressure); around<br />alveoli (to promote normal exhalation).<br />6. Bone—cells are osteocytes; matrix is calcium salts<br />and collagen, strong and not flexible; good blood<br />supply, rapid healing. Sites: bones of the skeleton<br />(to support the body and protect internal organs<br />from mechanical injury).<br />7. Cartilage—cells are chondrocytes; protein matrix is<br />firm yet flexible; no capillaries in matrix, very slow<br />healing. Sites: joint surfaces of bones (to prevent<br />friction); tip of nose and external ear (to support);<br />wall of trachea (to keep air passage open); discs<br />between vertebrae (to absorb shock).<br />Muscle Tissue—specialized to contract and<br />bring about movement (see Table 4–3 and<br />Fig. 4–6)<br />1. Skeletal—also called striated or voluntary muscle.<br />Cells are cylindrical, have several nuclei, and have<br />striations. Each cell has a motor nerve ending;<br />nerve impulses are essential to cause contraction.<br />Site: skeletal muscles attached to bones (to move<br />the skeleton and produce heat).<br />2. Smooth—also called visceral or involuntary muscle.<br />Cells have tapered ends, one nucleus each, and<br />no striations. Contraction is not under voluntary<br />control. Sites: stomach and intestines (peristalsis);<br />walls of arteries and veins (to maintain blood pressure);<br />iris (to constrict or dilate pupil).<br />3. Cardiac—cells are branched, have one nucleus<br />each, and faint striations. Site: walls of the four<br />chambers of the heart (to pump blood; nerve<br />impulses regulate the rate of contraction).<br />Nerve Tissue—neurons are specialized to<br />generate and transmit impulses (see Table<br />4–4 and Fig. 4–7)<br />1. Cell body contains the nucleus; axon carries<br />impulses away from the cell body; dendrites carry<br />impulses toward the cell body.<br />2. A synapse is the space between two neurons; a neurotransmitter<br />carries the impulse across a synapse.<br />Tissues and Membranes 85<br />3. Specialized cells in nerve tissue are neuroglia in the<br />CNS and Schwann cells in the PNS.<br />4. Sites: brain; spinal cord; and peripheral nerves (to<br />provide sensation, movement, regulation of body<br />functions, learning, and memory).<br />Membranes—sheets of tissue on surfaces, or<br />separating organs or lobes<br />1. Epithelial membranes (see Fig. 4–8)<br />• Serous membranes—in closed body cavities; the<br />serous fluid prevents friction between the two<br />layers of the serous membrane.<br />Thoracic cavity—partial pleura lines chest<br />wall; visceral pleura covers the lungs.<br />Pericardial sac—parietal pericardium lines<br />fibrous pericardium; visceral pericardium (epicardium)<br />covers the heart muscle.<br />Abdominal cavity—peritoneum lines the<br />abdominal cavity; mesentery covers the<br />abdominal organs.<br />• Mucous membranes—line body tracts that open<br />to the environment: respiratory, digestive, urinary,<br />and reproductive. Mucus keeps the living<br />epithelium wet; provides lubrication in the digestive<br />tract; traps dust and bacteria in the respiratory<br />tract.<br />2. Connective tissue membranes—see Table 4–5.<br />86 Tissues and Membranes<br />REVIEW QUESTIONS<br />1. Explain the importance of each tissue in its location:<br />(pp. 70, 73, 79)<br />a. Simple squamous epithelium in the alveoli of<br />the lungs<br />b. Ciliated epithelium in the trachea<br />c. Cartilage in the trachea<br />2. Explain the importance of each tissue in its location:<br />(pp. 77, 79)<br />a. Bone tissue in bones<br />b. Cartilage on the joint surfaces of bones<br />c. Fibrous connective tissue in ligaments<br />3. State the functions of red blood cells, white blood<br />cells, and platelets. (p. 75)<br />4. Name two organs made primarily of nerve tissue,<br />and state the general functions of nerve tissue.<br />(p. 81)<br />5. State the location and function of cardiac muscle.<br />(p. 81)<br />6. Explain the importance of each of these tissues in<br />the small intestine: smooth muscle and columnar<br />epithelium. (pp. 72, 79)<br />7. State the precise location of each of the following<br />membranes: (p. 82)<br />a. Peritoneum<br />b. Visceral pericardium<br />c. Parietal pleura<br />8. State the function of: (pp. 74, 82, 84)<br />a. Serous fluid<br />b. Mucus<br />c. Blood plasma<br />9. State two functions of skeletal muscles. (p. 79)<br />10. Name three body tracts lined with mucous membranes.<br />(p. 82)<br />11. Explain how endocrine glands differ from exocrine<br />glands. (pp. 73–74)<br />12. State the function of adipose tissue: (p. 76)<br />a. Around the eyes<br />b. Between the skin and muscles<br />13. State the location of: (p. 84)<br />a. Meninges<br />b. Synovial membranes<br />14. State the important physical characteristics of collagen<br />and elastin, and name the cells that produce<br />these protein fibers (p. 75)<br />1. A friend suffers a knee injury involving damage to<br />bone, cartilage, and ligaments. What can you tell<br />your friend about the healing of these tissues?<br />2. Stratified squamous keratinizing epithelium is an<br />excellent barrier to pathogens in the epidermis of<br />the skin. Despite the fact that it is such a good barrier,<br />this tissue would not be suitable for the lining<br />of the trachea or small intestine. Explain why.<br />3. Many tissues have protective functions, but it is<br />important to be specific about the kind of protection<br />provided. Name at least three tissues with a<br />protective function, and state what each protects<br />the body (or parts of the body) against.<br />4. Why is blood classified as a connective tissue?<br />What does it connect? What kinds of connections<br />does it make?<br />Tissues and Membranes 87<br />FOR FURTHER THOUGHT<br />88<br />CHAPTER 5<br />Chapter Outline<br />The Skin<br />Epidermis<br />Stratum germinativum<br />Stratum corneum<br />Langerhans cells<br />Melanocytes<br />Dermis<br />Hair follicles<br />Nail follicles<br />Receptors<br />Glands<br />Blood vessels<br />Subcutaneous Tissue<br />Aging and the Integumentary System<br />BOX 5–1 BURNS<br />BOX 5–2 PREVENTING SKIN CANCER: COMMON<br />SENSE AND SUNSCREENS<br />BOX 5–3 COMMON SKIN DISORDERS<br />BOX 5–4 ADMINISTERING MEDICATIONS<br />Student Objectives<br />• Name the two major layers of the skin and the<br />tissue of which each is made.<br />• State the locations and describe the functions of<br />the stratum germinativum and stratum corneum.<br />• Describe the function of Langerhans cells.<br />• Describe the function of melanocytes and melanin.<br />• Describe the functions of hair and nails.<br />• Name the cutaneous senses and explain their<br />importance.<br />• Describe the functions of the secretions of sebaceous<br />glands, ceruminous glands, and eccrine<br />sweat glands.<br />• Describe how the arterioles in the dermis respond<br />to heat, cold, and stress.<br />• Name the tissues that make up the subcutaneous<br />tissue, and describe their functions.<br />The Integumentary System<br />89<br />New Terminology<br />Arterioles (ar-TEER-ee-ohls)<br />Ceruminous gland/Cerumen (suh-ROO-mi-nus<br />GLAND/suh-ROO-men)<br />Dermis (DER-miss)<br />Eccrine sweat gland (EK-rin SWET GLAND)<br />Epidermis (EP-i-DER-miss)<br />Hair follicle (HAIR FAH-li-kull)<br />Keratin (KER-uh-tin)<br />Melanin (MEL-uh-nin)<br />Melanocyte (muh-LAN-oh-sight)<br />Nail follicle (NAIL FAH-li-kull)<br />Papillary layer (PAP-i-LAR-ee LAY-er)<br />Receptors (ree-SEP-turs)<br />Sebaceous gland/Sebum (suh-BAY-shus<br />GLAND/SEE-bum)<br />Stratum corneum (STRA-tum KOR-nee-um)<br />Stratum germinativum (STRA-tum JER-min-ah-<br />TEE-vum)<br />Subcutaneous tissue (SUB-kew-TAY-nee-us TISHyoo)<br />Vasoconstriction (VAY-zoh-kon-STRIK-shun)<br />Vasodilation (VAY-zoh-dye-LAY-shun)<br />Related Clinical Terminology<br />Acne (AK-nee)<br />Alopecia (AL-oh-PEE-she-ah)<br />Biopsy (BYE-op-see)<br />Carcinoma (KAR-sin-OH-mah)<br />Circulatory shock (SIR-kew-lah-TOR-ee SHAHK)<br />Contusion (kon-TOO-zhun)<br />Decubitus ulcer (dee-KEW-bi-tuss UL-ser)<br />Dehydration (DEE-high-DRAY-shun)<br />Dermatology (DER-muh-TAH-luh-gee)<br />Eczema (EK-zuh-mah)<br />Erythema (ER-i-THEE-mah)<br />Histamine (HISS-tah-meen)<br />Hives (HIGH-VZ)<br />Inflammation (IN-fluh-MAY-shun)<br />Melanoma (MEL-ah-NOH-mah)<br />Nevus (NEE-vus)<br />Pruritus (proo-RYE-tus)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />The integumentary system consists of the skin, its<br />accessory structures such as hair and sweat glands, and<br />the subcutaneous tissue below the skin. The skin is<br />made of several different tissue types and is considered<br />an organ. Because the skin covers the surface of the<br />body, one of its functions is readily apparent: It separates<br />the internal environment of the body from the<br />external environment and prevents the entry of many<br />harmful substances. The subcutaneous tissue directly<br />underneath the skin connects it to the muscles and has<br />other functions as well.<br />THE SKIN<br />The two major layers of the skin are the outer<br />epidermis and the inner dermis. Each of these layers<br />is made of different tissues and has very different functions.<br />EPIDERMIS<br />The epidermis is made of stratified squamous keratinizing<br />epithelial tissue and is thickest on the palms<br />and soles. The cells that are most abundant are called<br />keratinocytes, and there are no capillaries present<br />between them. Although the epidermis may be further<br />subdivided into four or five sublayers, two of these are<br />of greatest importance: the innermost layer, the stratum<br />germinativum, and the outermost layer, the stratum<br />corneum (Fig. 5–1).<br />Stratum Germinativum<br />The stratum germinativum may also be called the<br />stratum basale. Each name tells us something about<br />this layer. To germinate means “to sprout” or “to<br />grow.” Basal means the “base” or “lowest part.” The<br />stratum germinativum is the base of the epidermis, the<br />innermost layer in which mitosis takes place. New<br />cells are continually being produced, pushing the<br />older cells toward the skin surface. These cells produce<br />the protein keratin, and as they get farther away<br />from the capillaries in the dermis, they die. As dead<br />cells are worn off the skin’s surface, they are replaced<br />by cells from the lower layers. Scattered among the<br />keratinocytes of the stratum germinativum are very<br />different cells called Merkel cells (or Merkel discs);<br />these are receptors for the sense of touch (Fig. 5–2).<br />The living keratinocytes are able to synthesize<br />antimicrobial peptides called defensins; these and<br />other chemicals are produced following any injury to<br />the skin, as part of the process of inflammation.<br />Defensins rupture the membranes of pathogens such<br />as bacteria that may enter by way of breaks in the skin.<br />The living portion of the epidermis also produces a<br />vitamin; the cells have a form of cholesterol that, on<br />exposure to ultraviolet light, is changed to vitamin D<br />(then modified by the liver and kidneys to the most<br />active form, called 1,25-D, or calcitriol, which is considered<br />a hormone). This is why vitamin D is sometimes<br />referred to as the “sunshine vitamin.” People<br />who do not get much sunlight depend more on nutritional<br />sources of vitamin D, such as fortified milk. But<br />sunlight is probably the best way to get vitamin D, and<br />15 minutes a day a few times a week is often enough.<br />Vitamin D is important for the absorption of calcium<br />and phosphorus from food in the small intestine; this<br />function has been known for years. Recent research,<br />however, suggests that vitamin D is also involved in<br />maintaining muscle strength, especially in elderly people,<br />in the functioning of insulin, and in some immune<br />responses, where it may be protective for some types<br />of cancer.<br />Stratum Corneum<br />The stratum corneum, the outermost epidermal<br />layer, consists of many layers of dead cells; all that is<br />left is their keratin. The protein keratin is relatively<br />waterproof, and though the stratum corneum should<br />not be thought of as a plastic bag encasing the body, it<br />does prevent most evaporation of body water. Also of<br />importance, keratin prevents the entry of water. Without<br />a waterproof stratum corneum, it would be impossible<br />to swim in a pool or even take a shower without<br />damaging our cells.<br />The stratum corneum is also a barrier to pathogens<br />and chemicals. Most bacteria and other microorganisms<br />cannot penetrate unbroken skin. The flaking of<br />dead cells from the skin surface helps remove microorganisms,<br />and the fatty acids in sebum help inhibit<br />their growth. Most chemicals, unless they are corrosive,<br />will not get through unbroken skin to the living<br />tissue within. One painful exception is the sap of poison<br />ivy. This resin does penetrate the skin and initiates<br />an allergic reaction in susceptible people. The inflammatory<br />response that characterizes allergies causes<br />90 The Integumentary System<br />blisters and severe itching. The importance of the<br />stratum corneum becomes especially apparent when it<br />is lost (see Box 5–1: Burns).<br />Certain minor changes in the epidermis are<br />undoubtedly familiar to you. When first wearing new<br />shoes, for example, the skin of the foot may be subjected<br />to friction. This will separate layers of the epidermis,<br />or separate the epidermis from the dermis,<br />and tissue fluid may collect, causing a blister. If the<br />skin is subjected to pressure, the rate of mitosis in the<br />stratum germinativum will increase and create a<br />thicker epidermis; we call this a callus. Although calluses<br />are more common on the palms and soles, they<br />may occur on any part of the skin.<br />The Integumentary System 91<br />Sebaceous gland<br />Receptor for touch<br />(encapsulated)<br />Hair follicle<br />Receptor for pressure(encapsulated)<br />Pore<br />Stratum germinativum<br />Stratum corneum<br />Epidermis<br />Dermis<br />Papillary layer<br />with capillaries<br />Pilomotor<br />muscle<br />Subcutaneous<br />tissue<br />Fascia of<br />muscle<br />Adipose tissue<br />Eccrine sweat gland<br />Free nerve ending<br />Nerve<br />Arteriole<br />Venule<br />Figure 5–1. Skin. Structure of the skin and subcutaneous tissue.<br />QUESTION: Which layers of the integumentary system have blood vessels?<br />92<br />Stratum corneum<br />Langerhans cell<br />Mitosis<br />Capillary<br />Stratum germinativum<br />Sensory neuron<br />Dermis<br />Melanocyte<br />Merkel cell<br />Figure 5–2. The epidermis, showing<br />the different kinds of cells present<br />and the blood supply in the<br />upper dermis.<br />QUESTION: Which type of cell<br />shown is capable of self-locomotion,<br />and what does it carry?<br />BOX 5–1 BURNS<br />Burns of the skin may be caused by flames, hot<br />water or steam, sunlight, electricity, or corrosive<br />chemicals. The severity of burns ranges from minor<br />to fatal, and the classification of burns is based on<br />the extent of damage.<br />First-Degree Burn—only the superficial epidermis<br />is burned, and is painful but not blistered. Lightcolored<br />skin will appear red due to localized<br />vasodilation in the damaged area. Vasodilation is<br />part of the inflammatory response that brings more<br />blood to the injured site.<br />Second-Degree Burn—deeper layers of the epidermis<br />are affected. Another aspect of inflammation<br />is that damaged cells release histamine,<br />which makes capillaries more permeable. More<br />plasma leaves these capillaries and becomes tissue<br />fluid, which collects at the burn site, creating blisters.<br />The burned skin is often very painful.<br />Third-Degree Burn—the entire epidermis is<br />charred or burned away, and the burn may extend<br />into the dermis or subcutaneous tissue. Often such<br />a burn is not painful at first, if the receptors in the<br />dermis have been destroyed.<br />Extensive third-degree burns are potentially lifethreatening<br />because of the loss of the stratum<br />corneum. Without this natural barrier, living tissue is<br />exposed to the environment and is susceptible to<br />infection and dehydration.<br />Bacterial infection is a serious problem for burn<br />patients; the pathogens may get into the blood<br />(septicemia) and quickly spread throughout<br />the body. Dehydration may also be fatal if medical<br />intervention does not interrupt and correct the<br />following sequence: Tissue fluid evaporates from<br />the burned surface, and more plasma is pulled<br />out of capillaries into the tissue spaces. As more<br />plasma is lost, blood volume and blood pressure<br />decrease. This is called circulatory shock; eventually<br />the heart simply does not have enough blood<br />to pump, and heart failure is the cause of death. To<br />prevent these serious consequences, third-degree<br />burns are covered with donor skin or artificial skin<br />until skin grafts of the patient’s own skin can be put<br />in place.<br />(Continued on following page)<br />The Integumentary System 93<br />BOX 5–1 BURNS (Continued)<br />Normal skin<br />First-degree burn<br />Second-degree burn<br />Third-degree burn<br />Box Figure 5–A Normal skin section and representative sections showing first-degree, seconddegree,<br />and third-degree burns.<br />Langerhans Cells<br />Within the epidermis are Langerhans cells, which<br />are also called dendritic cells because of their<br />branched appearance when they move (see Fig. 5–2).<br />These cells originate in the red bone marrow, and are<br />quite mobile. They are able to phagocytize foreign<br />material, such as bacteria that enter the body through<br />breaks in the skin. With such ingested pathogens, the<br />Langerhans cells migrate to lymph nodes and present<br />the pathogen to lymphocytes, a type of white blood<br />cell. This triggers an immune response such as the<br />production of antibodies (antibodies are proteins that<br />label foreign material for destruction). Because of its<br />position adjacent to the external environment, the skin<br />is an important component of the body’s protective<br />responses, though many of the exact aspects of this<br />have yet to be determined. Immunity is covered in<br />Chapter 14.<br />Melanocytes<br />Another type of cell found in the lower epidermis is<br />the melanocyte, which is also shown in Fig. 5–2.<br />Melanocytes produce another protein, a pigment<br />called melanin. People of the same size have approximately<br />the same number of melanocytes, though these<br />cells may differ in their level of activity. In people with<br />dark skin, the melanocytes continuously produce large<br />amounts of melanin. The melanocytes of lightskinned<br />people produce less melanin. The activity of<br />melanocytes is genetically regulated; skin color is one<br />of our hereditary characteristics.<br />In all people, melanin production is increased by<br />exposure of the skin to ultraviolet rays, which are part<br />of sunlight and are damaging to living cells. As more<br />melanin is produced, it is taken in by the epidermal<br />cells as they are pushed toward the surface. This gives<br />the skin a darker color, which prevents further exposure<br />of the living stratum germinativum to ultraviolet<br />rays. People with dark skin already have good protection<br />against the damaging effects of ultraviolet rays;<br />people with light skin do not (see Box 5–2: Preventing<br />Skin Cancer: Common Sense and Sunscreens).<br />Melanin also gives color to hair, though its protective<br />function is confined to the hair of the head. Two parts<br />of the eye obtain their color from melanin: the iris and<br />the interior choroid layer of the eyeball (the eye is discussed<br />in Chapter 9).The functions of the epidermis<br />and its cells are summarized in Table 5–1.<br />94 The Integumentary System<br />BOX 5–2 PREVENTING SKIN CANCER: COMMON SENSE AND SUNSCREENS<br />A B<br />Anyone can get skin cancer, and the most important<br />factor is exposure to sunlight. Light-skinned<br />people are, of course, more susceptible to the<br />effects of ultraviolet (UV) rays, which may trigger<br />mutations in living epidermal cells.<br />Squamous cell carcinoma and basal cell carcinoma<br />(see A in Box Fig. 5–B) are the most common<br />forms of skin cancer. The lesions are visible as<br />changes in the normal appearance of the skin, and<br />a biopsy (microscopic examination of a tissue specimen)<br />is used to confirm the diagnosis. These lesions<br />usually do not metastasize rapidly, and can be<br />completely removed using simple procedures<br />Malignant melanoma (see B in Box Fig. 5–B) is<br />a more serious form of skin cancer, which begins in<br />melanocytes. Any change in a pigmented spot or<br />mole (nevus) should prompt a person to see a doctor.<br />Melanoma is serious not because of its growth<br />in the skin, but because it may metastasize very<br />rapidly to the lungs, liver, or other vital organ.<br />Researchers are testing individualized vaccines for<br />people who have had melanoma. The purpose of<br />the vaccine is to stimulate the immune system<br />strongly enough to prevent a second case, for such<br />recurrences are often fatal.<br />Although the most common forms of skin cancer<br />are readily curable, prevention is a better strategy.<br />We cannot, and we would not want to, stay out of<br />the sun altogether (because sunlight may be the<br />best way to get sufficient vitamin D), but we may<br />be able to do so when sunlight is most damaging.<br />During the summer months, UV rays are especially<br />intense between 10 A.M. and 2 P.M. If we are or<br />must be outdoors during this time, dermatologists<br />recommend use of a sunscreen.<br />Sunscreens contain chemicals such as PABA<br />(para-amino benzoic acid) that block UV rays and<br />prevent them from damaging the epidermis. An<br />SPF (sun protection factor) of 15 or higher is considered<br />good protection. Use of a sunscreen on<br />exposed skin not only helps prevent skin cancer but<br />also prevents sunburn and its painful effects. It is<br />especially important to prevent children from getting<br />severely sunburned, because such burns have<br />been linked to the development of skin cancer years<br />later.<br />Box Figure 5–B (A) Classic basal cell carcinoma on face. (B) Melanoma in finger web. (From<br />Goldsmith, LA, Lazarus, GS, and Tharp, MD: Adult and Pediatric Dermatology: A Color Guide to<br />Diagnosis and Treatment. FA Davis, 1997, pp 137 and 144, with permission.)<br />DERMIS<br />The dermis is made of an irregular type of fibrous<br />connective tissue, irregular meaning that the fibers are<br />not parallel, but run in all directions. Fibroblasts produce<br />both collagen and elastin fibers. Recall that collagen<br />fibers are strong, and elastin fibers are able to<br />recoil after being stretched. Strength and elasticity are<br />two characteristics of the dermis. With increasing age,<br />however, the deterioration of the elastin fibers causes<br />the skin to lose its elasticity. We can all look forward<br />to at least a few wrinkles as we get older.<br />The uneven junction of the dermis with the epidermis<br />is called the papillary layer (see Fig. 5–1). Capillaries<br />are abundant here to nourish not only the<br />dermis but also the stratum germinativum. The epidermis<br />has no capillaries of its own, and the lower, living<br />cells depend on the blood supply in the dermis for<br />oxygen and nutrients.<br />Within the dermis are the accessory skin structures:<br />hair and nail follicles, sensory receptors, and several<br />types of glands. Some of these project through the<br />epidermis to the skin surface, but their active portions<br />are in the dermis.<br />Hair Follicles<br />Hair follicles are made of epidermal tissue, and the<br />growth process of hair is very similar to growth of the<br />epidermis. At the base of a follicle is the hair root,<br />which contains cells called the matrix, where mitosis<br />takes place (Fig. 5–3). The new cells produce keratin,<br />get their color from melanin, then die and become<br />incorporated into the hair shaft, which is pushed<br />toward the surface of the skin. The hair that we comb<br />and brush every day consists of dead, keratinized cells.<br />The rate of hair growth averages 0.3 to 0.4 in./month<br />(8 to 10 mm).<br />Compared to some other mammals, humans do not<br />have very much hair. The actual functions of human<br />hair are quite few. Eyelashes and eyebrows help to<br />keep dust and perspiration out of the eyes, and the<br />hairs just inside the nostrils help to keep dust out of<br />the nasal cavities. Hair of the scalp does provide insulation<br />from cold for the head. The hair on our bodies,<br />however, no longer serves this function, but we have<br />the evolutionary remnants of it. Attached to each hair<br />follicle is a small, smooth muscle called the pilomotor<br />The Integumentary System 95<br />Hair shaft<br />Follicle<br />Hair root<br />B<br />Capillary<br />Venule<br />Fat cells<br />A<br />Figure 5–3. Structure of a hair follicle. (A) Longitudinal<br />section. (B) Cross-section.<br />QUESTION: What is the hair shaft made of?<br />Table 5–1 EPIDERMIS<br />Part Function<br />Stratum corneum<br />(keratin)<br />Stratum germinativum<br />(stratum<br />basale)<br />Langerhans cells<br />Merkel cells<br />Melanocytes<br />Melanin<br />• Prevents loss or entry of water<br />• If unbroken, prevents entry of<br />pathogens and most chemicals<br />• Continuous mitosis produces<br />new cells to replace worn-off<br />surface cells<br />• Produces antimicrobial<br />defensins<br />• Cholesterol is changed to vitamin<br />D on exposure to UV rays<br />• Phagocytize foreign material<br />and stimulate an immune<br />response by lymphocytes<br />• Receptors for sense of touch<br />• Produce melanin on exposure<br />to UV rays<br />• Protects living skin layers from<br />further exposure to UV rays<br />or arrector pili muscle. When stimulated by cold or<br />emotions such as fear, these muscles pull the hair follicles<br />upright. For an animal with fur, this would trap<br />air and provide greater insulation. Because people do<br />not have thick fur, all this does for us is give us “goose<br />bumps.”<br />Nail Follicles<br />Found on the ends of fingers and toes, nail follicles<br />produce nails just as hair follicles produce hair. Mitosis<br />takes place in the nail root at the base of the nail (Fig.<br />5–4), and the new cells produce keratin (a stronger<br />form of this protein than is found in hair) and then die.<br />Although the nail itself consists of keratinized dead<br />cells, the flat nail bed is living epidermis and dermis.<br />This is why cutting a nail too short can be quite<br />painful. Nails protect the ends of the fingers and toes<br />from mechanical injury and give the fingers greater<br />ability to pick up small objects. Fingernails are also<br />good for scratching. This is more important than it<br />may seem at first. An itch might mean the presence of<br />an arthropod parasite, mosquito, tick, flea, or louse.<br />These parasites (all but the tick are insects) feed on<br />blood, and all are potential vectors of diseases caused<br />by bacteria, viruses, or protozoa. A quick and vigorous<br />scratch may kill or at least dislodge the arthropod and<br />prevent the transmission of the disease. Fingernails<br />grow at the rate of about 0.12 in./month (3 mm), and<br />growth is a little faster during the summer months.<br />Receptors<br />Most sensory receptors for the cutaneous senses<br />are found in the dermis (Merkel cells are in the stratum<br />germinativum, as are some nerve endings). The<br />cutaneous senses are touch, pressure, heat, cold, and<br />pain. For each sensation there is a specific type of<br />receptor, which is a structure that will detect a particular<br />change. For pain, heat, and cold, the receptors are<br />free nerve endings. For touch and pressure, the<br />receptors are called encapsulated nerve endings,<br />which means there is a cellular structure around the<br />sensory nerve ending (see Fig. 5–1). The purpose of<br />these receptors and sensations is to provide the central<br />nervous system with information about the external<br />environment and its effect on the skin. This information<br />may stimulate responses, such as washing a<br />painful cut finger, scratching an insect bite, or<br />responding to a feeling of cold by putting on a sweater.<br />The sensitivity of an area of skin is determined by<br />how many receptors are present. The skin of the fingertips,<br />for example, is very sensitive to touch because<br />there are many receptors per square inch. The skin of<br />the upper arm, with few touch receptors per square<br />inch, is less sensitive.<br />When receptors detect changes, they generate<br />nerve impulses that are carried to the brain, which<br />interprets the impulses as a particular sensation.<br />Sensation, therefore, is actually a function of the brain<br />(we will return to this in Chapters 8 and 9).<br />Glands<br />Glands are made of epithelial tissue. The exocrine<br />glands of the skin have their secretory portions in the<br />dermis. Some of these are shown in Fig. 5–1.<br />Sebaceous Glands. The ducts of sebaceous glands<br />open into hair follicles or directly to the skin surface.<br />Their secretion is sebum, a lipid substance that we<br />commonly refer to as oil. As mentioned previously,<br />sebum inhibits the growth of bacteria on the skin surface.<br />Another function of sebum is to prevent drying of<br />skin and hair. The importance of this may not be readily<br />apparent, but skin that is dry tends to crack more<br />easily. Even very small breaks in the skin are potential<br />entryways for bacteria. Decreased sebum production is<br />another consequence of getting older, and elderly people<br />often have dry and more fragile skin.<br />Adolescents may have the problem of overactive<br />sebaceous glands. Too much sebum may trap bacteria<br />96 The Integumentary System<br />Nail root<br />Nail bed Cuticle<br />Nail body<br />Free edge of nail<br />Lunula<br />Figure 5–4. Structure of a fingernail shown in longitudinal<br />section.<br />QUESTION: The nail bed is which part of the skin?<br />within hair follicles and create small infections.<br />Because sebaceous glands are more numerous around<br />the nose and mouth, these are common sites of pimples<br />in young people (see also Box 5–3: Common Skin<br />Disorders).<br />Ceruminous Glands. These glands are located in the<br />dermis of the ear canals. Their secretion is called<br />cerumen or ear wax (which includes the sebum<br />secreted in the ear canals). Cerumen keeps the outer<br />surface of the eardrum pliable and prevents drying.<br />The Integumentary System 97<br />BOX 5–3 COMMON SKIN DISORDERS<br />Impetigo—a bacterial infection often caused by<br />streptococci or staphylococci. The characteristic<br />pustules (pus-containing lesions) crust as they heal;<br />the infection is contagious to others.<br />Eczema—a symptom of what is more properly<br />called atopic dermatitis. This may be an allergic<br />reaction, and is more common in children than<br />adults; the rash is itchy (pruritus) and may blister<br />or ooze. Eczema may be related to foods such as<br />fish, eggs, or milk products, or to inhaled allergens<br />such as dust, pollens, or animal dander. Prevention<br />depends upon determining what the child is allergic<br />to and eliminating or at least limiting exposure.<br />Warts—caused by a virus that makes epidermal<br />cells divide abnormally, producing a growth on the<br />skin that is often raised and has a rough or pitted<br />surface. Warts are probably most common on the<br />hands, but they may be anywhere on the skin.<br />Plantar warts on the sole of the foot may become<br />quite painful because of the constant pressure of<br />standing and walking.<br />Fever blisters (cold sores)—caused by the herpes<br />simplex virus, to which most people are exposed<br />as children. An active lesion, usually at the edge<br />of the lip (but may be anywhere on the skin), is<br />painful and oozes. If not destroyed by the immune<br />system, the virus “hides out” and becomes dormant<br />in nerves of the face. Another lesion, weeks<br />or months later, may be triggered by stress or<br />another illness.<br />A B<br />C D<br />Box Figure 5–C (A) Impetigo. (B) Eczema of atopic dermatitis. (C) Warts on back<br />of hands. (D) Fever blister on finger, localized but severe. (From Goldsmith, LA,<br />Lazarus, GS, and Tharp, MD: Adult and Pediatric Dermatology: A Color Guide<br />to Diagnosis and Treatment. FA Davis, 1997, pp 80, 241, 306, and 317, with permission.)<br />However, if excess cerumen accumulates in the ear<br />canal, it may become impacted against the eardrum.<br />This might diminish the acuity of hearing by preventing<br />the eardrum from vibrating properly.<br />Sweat Glands. There are two types of sweat glands,<br />apocrine and eccrine. Apocrine glands are most<br />numerous in the axillae (underarm) and genital areas<br />and are most active in stressful and emotional situations.<br />Although their secretion does have an odor, it is<br />barely perceptible to other people. Animals such as<br />dogs, however, can easily distinguish among people<br />because of their individual scents. If the apocrine<br />secretions are allowed to accumulate on the skin, bacteria<br />metabolize the chemicals in the sweat and produce<br />waste products that have distinct odors that<br />many people find unpleasant.<br />Eccrine glands are found all over the body but are<br />especially numerous on the forehead, upper lip, palms,<br />and soles. The secretory portion of these glands is<br />simply a coiled tube in the dermis. The duct of this<br />tube extends to the skin’s surface, where it opens into<br />a pore.<br />The sweat produced by eccrine glands is important<br />in the maintenance of normal body temperature. In a<br />warm environment, or during exercise, more sweat is<br />secreted onto the skin surface, where it is then evaporated<br />by excess body heat. Recall that water has a high<br />heat of vaporization, which means that a great deal of<br />heat can be lost in the evaporation of a relatively small<br />amount of water. Although this is a very effective<br />mechanism of heat loss, it has a potentially serious disadvantage.<br />Loss of too much body water in sweat may<br />lead to dehydration, as in heat exhaustion or even<br />after exercise on a hot and humid day. Increased<br />sweating during exercise or on warm days should<br />always be accompanied by increased fluid intake.<br />Those who exercise regularly know that they must<br />replace salt as well as water. Sodium chloride is also<br />lost in sweat, as are small amounts of urea (a nitrogenous<br />waste product of amino acid metabolism). This<br />excretory function of the skin is very minor, however;<br />the kidneys are primarily responsible for removing<br />waste products from the blood and for maintaining<br />the body’s proper salt-to-water proportion.<br />Blood Vessels<br />Besides the capillaries in the dermis, the other blood<br />vessels of great importance are the arterioles.<br />Arterioles are small arteries, and the smooth muscle<br />in their walls permits them to constrict (close) or<br />dilate (open). This is important in the maintenance of<br />body temperature, because blood carries heat, which is<br />a form of energy.<br />In a warm environment the arterioles dilate<br />(vasodilation), which increases blood flow through the<br />dermis and brings excess heat close to the body surface<br />to be radiated to the environment. In a cold environment,<br />however, body heat must be conserved if possible,<br />so the arterioles constrict. The vasoconstriction<br />decreases the flow of blood through the dermis and<br />keeps heat within the core of the body. This adjusting<br />mechanism is essential for maintaining homeostasis.<br />Regulation of the diameter of the arterioles in<br />response to external temperature changes is controlled<br />by the nervous system. These changes can often be<br />seen in light-skinned people. Flushing, especially in<br />the face, may be observed in hot weather. In cold, the<br />skin of the extremities may become even paler as blood<br />flow through the dermis decreases. In people with dark<br />skin, such changes are not as readily apparent because<br />they are masked by melanin in the epidermis.<br />Vasoconstriction in the dermis may also occur during<br />stressful situations. For our ancestors, stress usually<br />demanded a physical response: Either stand and<br />fight or run away to safety. This is called the “fight or<br />flight response.” Our nervous systems are still programmed<br />to respond as if physical activity were necessary<br />to cope with the stress situation. Vasoconstriction<br />in the dermis will shunt, or redirect, blood to more<br />vital organs such as the muscles, heart, and brain. In<br />times of stress, the skin is a relatively unimportant<br />organ and can function temporarily with a minimal<br />blood flow. You have probably heard the expression<br />“broke out in a cold sweat,” and may even have felt it<br />in a stressful situation. Such sweating feels cold<br />because vasoconstriction in the dermis makes the skin<br />relatively cool.<br />Blood flow in the dermis may be interrupted by<br />prolonged pressure on the skin. For example, a hospital<br />patient who cannot turn over by herself may<br />develop a decubitus ulcer, also called a pressure ulcer<br />or pressure sore. The skin is compressed between the<br />object outside, such as a bed, and a bony prominence<br />within, such as the heel bone or the sacrum in the<br />lower back. Without its blood supply the skin dies,<br />and the dead tissue is a potential site for bacterial<br />infection.<br />The functions of dermal structures are summarized<br />in Table 5–2.<br />98 The Integumentary System<br />SUBCUTANEOUS TISSUE<br />The subcutaneous tissue may also be called the<br />superficial fascia, one of the connective tissue membranes.<br />Made of areolar connective tissue and adipose<br />tissue, the superficial fascia connects the dermis to the<br />underlying muscles. Its other functions are those of its<br />tissues, as you may recall from Chapter 4.<br />Areolar connective tissue, or loose connective tissue,<br />contains collagen and elastin fibers and many<br />white blood cells that have left capillaries to wander<br />around in the tissue fluid between skin and muscles.<br />These migrating white blood cells destroy pathogens<br />that enter the body through breaks in the skin. Mast<br />cells are specialized connective tissue cells found in<br />areolar tissue; they produce histamine, leukotrienes,<br />and other chemicals that help bring about inflammation,<br />the body’s response to damage (inflammation is<br />described in Chapters 10 and 14).<br />The cells (adipocytes) of adipose tissue are specialized<br />to store fat, and our subcutaneous layer of fat<br />stores excess nutrients as a potential energy source.<br />This layer also cushions bony prominences, such as<br />when sitting, and provides some insulation from cold.<br />For people, this last function is relatively minor,<br />because we do not have a thick layer of fat, as do animals<br />such as whales and seals. As mentioned in<br />Chapter 4, adipose tissue is involved in the onset or<br />cessation of eating and in the use of insulin by body<br />cells, and it contributes to inflammation by producing<br />cytokines, chemicals that activate white blood<br />cells.<br />Just as the epidermis forms a continuous sheet that<br />covers the body, the subcutaneous tissue is a continuous<br />layer, though it is internal. If we consider the epidermis<br />as the first line of defense against pathogens,<br />we can consider the subcutaneous tissue a secondary<br />line of defense. There is, however, a significant<br />anatomic difference. The cells of the epidermis are<br />very closely and tightly packed, but the cells and<br />protein fibers of subcutaneous tissue are farther apart,<br />and there is much more tissue fluid. If we imagine the<br />epidermis as a four-lane highway during rush hour<br />with bumper-to-bumper traffic, the superficial fascia<br />would be that same highway at three o’clock in the<br />morning, when it is not crowded and cars can move<br />much faster. This is an obvious benefit for the migrating<br />white blood cells, but may become a disadvantage<br />because some bacterial pathogens, once established,<br />can spread even more rapidly throughout subcutaneous<br />tissue.<br />Group A streptococcus, for example, is a cause of<br />necrotizing fasciitis. Necrotizing means “to cause<br />death,” and fasciitis is the inflammation of a fascia, in<br />this case the superficial fascia and the deep fasciae<br />around muscles. Necrotizing fasciitis is an extremely<br />serious infection and requires surgical removal of the<br />infected tissue, or even amputation of an infected<br />limb, to try to stop the spread of the bacteria. The<br />body’s defenses have been overwhelmed because what<br />is usually an anatomic benefit, the “looseness” of areolar<br />connective tissue, has been turned against us and<br />become a detriment.<br />The functions of subcutaneous tissue are summarized<br />in Table 5–3. Box 5–4: Administering Medications,<br />describes ways in which we give medications<br />through the skin.<br />The Integumentary System 99<br />Table 5–2 DERMIS<br />Part Function<br />Papillary layer<br />Hair (follicles)<br />Nails (follicles)<br />Receptors<br />Sebaceous glands<br />Ceruminous<br />glands<br />Eccrine sweat<br />glands<br />Arterioles<br />• Contains capillaries that nourish<br />the stratum germinativum<br />• Eyelashes and nasal hair keep<br />dust out of eyes and nasal<br />cavities<br />• Scalp hair provides insulation<br />from cold for the head<br />• Protect ends of fingers and<br />toes from mechanical injury<br />• Detect changes that are felt as<br />the cutaneous senses: touch,<br />pressure, heat, cold, and pain<br />• Produce sebum, which prevents<br />drying of skin and hair and<br />inhibits growth of bacteria<br />• Produce cerumen, which prevents<br />drying of the eardrum<br />• Produce watery sweat that is<br />evaporated by excess body heat<br />to cool the body<br />• Dilate in response to warmth<br />to increase heat loss<br />• Constrict in response to cold<br />to conserve body heat<br />• Constrict in stressful situations<br />to shunt blood to more vital<br />organs<br />100 The Integumentary System<br />Table 5–3 SUBCUTANEOUS TISSUE<br />Part Function<br />Areolar connective tissue<br />Adipose tissue<br />• Connects skin to muscles<br />• Contains many WBCs to destroy pathogens that enter breaks in the skin<br />• Contains mast cells that release histamine, leukotrienes, and other<br />chemicals involved in inflammation<br />• Contains stored energy in the form of true fats<br />• Cushions bony prominences<br />• Provides some insulation from cold<br />• Contributes to appetite<br />• Contributes to use of insulin<br />• Produces cytokines that activate WBCs<br />BOX 5–4 ADMINISTERING MEDICATIONS<br />You have probably seen television or print advertisements<br />for skin patches that supply nicotine, and<br />you know that their purpose is to help smokers give<br />up cigarettes. This method of supplying a medication<br />is called transdermal administration. The name<br />is a little misleading because the most difficult part<br />of cutaneous absorption of a drug is absorption<br />through the stratum corneum of the epidermis.<br />Because such absorption is slow, skin patches are<br />useful for medications needed in small but continuous<br />amounts, and over a prolonged period of time.<br />You would expect that such patches should be<br />worn where the epidermis is thin. These sites<br />include the upper arm and the chest. The recommended<br />site for a patch to prevent motion sickness<br />is the skin behind the ear. Also available in patch<br />form are medications for birth control, overactive<br />bladder, high blood pressure, and both systemic<br />and localized pain relief.<br />Medications may also be injected through the<br />skin. Some injections are given subcutaneously, that<br />is, into subcutaneous tissue (see Box Fig. 5–D).<br />Subcutaneous adipose tissue has a moderate blood<br />supply, so the rate of absorption of the drug will be<br />moderate, but predictable. Insulin is given subcutaneously.<br />Other injections are intramuscular, and<br />absorption into the blood is rapid because muscle<br />tissue has a very good blood supply. Most injectable<br />vaccines are given intramuscularly, to promote<br />rapid absorption to stimulate antibody production.<br />Stratum<br />corneum<br />Dermis<br />Adipose<br />tissue<br />Skeletal<br />muscle<br />Patch<br />Subcutaneous<br />Intramuscular<br />Box Figure 5–D The skin, subcutaneous tissue, and muscle sites for the delivery of<br />medications.<br />AGING AND THE<br />INTEGUMENTARY SYSTEM<br />The effects of age on the integumentary system are<br />often quite visible. Both layers of skin become thinner<br />and more fragile as mitosis in the epidermis slows and<br />fibroblasts in the dermis die and are not replaced;<br />repair of even small breaks or cuts is slower. The skin<br />becomes wrinkled as collagen and elastin fibers in the<br />dermis deteriorate. Sebaceous glands and sweat glands<br />become less active; the skin becomes dry, and temperature<br />regulation in hot weather becomes more difficult.<br />Hair follicles become inactive and hair on the<br />scalp and body thins. Melanocytes die and are not<br />replaced; the hair that remains becomes white. There<br />is often less fat in the subcutaneous tissue, which may<br />make an elderly person more sensitive to cold. It is<br />important for elderly people (and those who care for<br />them) to realize that extremes of temperature may be<br />harmful and to take special precautions in very hot or<br />very cold weather.<br />SUMMARY<br />The integumentary system is the outermost organ<br />system of the body. You have probably noticed that<br />many of its functions are related to this location. The<br />skin protects the body against pathogens and chemicals,<br />minimizes loss or entry of water, blocks the<br />harmful effects of sunlight, and produces vitamin D.<br />Sensory receptors in the skin provide information<br />about the external environment, and the skin helps<br />regulate body temperature in response to environmental<br />changes. The subcutaneous tissue is a secondary<br />line of defense against pathogens, a site of fat<br />storage and of the other metabolic functions of adipose<br />tissue.<br />The Integumentary System 101<br />STUDY OUTLINE<br />The integumentary system consists of the<br />skin and its accessory structures and the<br />subcutaneous tissue. The two major layers of<br />the skin are the outer epidermis and the<br />inner dermis.<br />Epidermis—made of stratified squamous<br />epithelium; no capillaries; cells called keratinocytes<br />(see Figs. 5–1 and 5–2 and Table<br />5–1)<br />1. Stratum germinativum—the innermost layer<br />where mitosis takes place; new cells produce keratin<br />and die as they are pushed toward the surface.<br />Defensins are antimicrobial peptides produced<br />when the skin is injured. Vitamin D is formed from<br />cholesterol on exposure to the UV rays of sunlight.<br />2. Stratum corneum—the outermost layers of dead<br />cells; keratin prevents loss and entry of water and<br />resists entry of pathogens and chemicals.<br />3. Langerhans cells—phagocytize foreign material,<br />take it to lymph nodes, and stimulate an immune<br />response by lymphocytes.<br />4. Melanocytes—in the lower epidermis, produce<br />melanin. UV rays stimulate melanin production;<br />melanin prevents further exposure of the stratum<br />germinativum to UV rays by darkening the<br />skin.<br />Dermis—made of irregular fibrous connective<br />tissue; collagen provides strength, and<br />elastin provides elasticity; capillaries in the<br />papillary layer nourish the stratum germinativum<br />(see Fig. 5–1 and Table 5–2)<br />1. Hair follicles—mitosis takes place in the hair root;<br />new cells produce keratin, die, and become the hair<br />shaft. Hair of the scalp provides insulation from<br />cold for the head; eyelashes keep dust out of eyes;<br />nostril hairs keep dust out of nasal cavities (see<br />Figs. 5–1 and 5–3).<br />2. Nail follicles—at the ends of fingers and toes; mitosis<br />takes place in the nail root; the nail itself is dead,<br />keratinized cells. Nails protect the ends of the fingers<br />and toes, enable the fingers to pick up small<br />objects, and provide for efficient scratching (see<br />Fig. 5–4).<br />3. Receptors—detect changes in the skin: touch, pressure,<br />heat, cold, and pain; provide information<br />about the external environment that initiates appropriate<br />responses; sensitivity of the skin depends on<br />the number of receptors present.<br />4. Sebaceous glands—secrete sebum into hair follicles<br />or to the skin surface; sebum inhibits the growth of<br />bacteria and prevents drying of skin and hair.<br />5. Ceruminous glands—secrete cerumen in the ear<br />canals; cerumen prevents drying of the eardrum.<br />6. Apocrine sweat glands—modified scent glands in<br />axillae and genital area; activated by stress and<br />emotions.<br />7. Eccrine sweat glands—most numerous on face,<br />palms, soles. Activated by high external temperature<br />or exercise; sweat on skin surface is evaporated<br />by excess body heat; potential disadvantage is dehydration.<br />Excretion of small amounts of NaCl and<br />urea is a very minor function.<br />8. Arterioles—smooth muscle permits constriction<br />or dilation. Vasoconstriction in cold temperatures<br />decreases dermal blood flow to conserve heat in<br />the body core. Vasodilation in warm temperatures<br />increases dermal blood flow to bring heat to the<br />surface to be lost. Vasoconstriction during stress<br />shunts blood away from the skin to more vital<br />organs, such as muscles, to permit a physical<br />response, if necessary.<br />Subcutaneous Tissue—also called the superficial<br />fascia; connects skin to muscles (see Fig.<br />5–1 and Table 5–3)<br />1. Areolar tissue—also called loose connective tissue;<br />the matrix contains tissue fluid and WBCs that<br />destroy pathogens that get through breaks in the<br />skin; mast cells produce chemicals that bring about<br />inflammation.<br />2. Adipose tissue—stores fat as potential energy;<br />cushions bony prominences; provides some insulation<br />from cold. Other functions: contributes to<br />appetite, the use of insulin, and the activation of<br />WBCs.<br />102 The Integumentary System<br />REVIEW QUESTIONS<br />1. Name the parts of the integumentary system.<br />(p. 90)<br />2. Name the two major layers of skin, the location<br />of each, and the tissue of which each is made.<br />(pp. 90, 95)<br />3. In the epidermis: (pp. 90, 93)<br />a. Where does mitosis take place?<br />b. What protein do the new cells produce?<br />c. What happens to these cells?<br />d. What is the function of Langerhans cells?<br />4. Describe the functions of the stratum corneum.<br />(p. 90)<br />5. Name the cells that produce melanin. What is<br />the stimulus? Describe the function of melanin<br />(p. 94)<br />6. Where, on the body, does human hair have important<br />functions? Describe these functions. (p. 95)<br />7. Describe the functions of nails. (p. 96)<br />8. Name the cutaneous senses. Describe the importance<br />of these senses. (p. 96)<br />9. Explain the functions of sebum and cerumen.<br />(pp. 96–97)<br />10. Explain how sweating helps maintain normal<br />body temperature. (p. 98)<br />11. Explain how the arterioles in the dermis respond<br />to cold or warm external temperatures and to<br />stress situations. (p. 98)<br />12. What vitamin is produced in the skin? What is the<br />stimulus for the production of this vitamin?<br />(p. 90)<br />13. Name the tissues of which the superficial fascia is<br />made. Describe the functions of these tissues.<br />(p. 99)<br />The Integumentary System 103<br />FOR FURTHER THOUGHT<br />1. The epidermis has no capillaries of its own; the<br />stratum corneum is made of dead cells and doesn’t<br />need a blood supply at all. Explain why the epidermis<br />is affected by a decubitus ulcer. Name another<br />group of people, besides hospital or nursing home<br />patients, that is especially susceptible to developing<br />pressure ulcers.<br />2. Going out in the sun stimulates quite a bit of activity<br />in the skin, especially on a hot summer day.<br />Describe what is happening in the skin in response<br />to sunlight.<br />3. Ringworm is a skin condition characterized by<br />scaly red patches, often circular or oval in shape. It<br />is not caused by a worm, but rather by certain<br />fungi. What is the food of these fungi; that is, what<br />are they digesting?<br />4. There are several kinds of cells in the epidermis.<br />Which cells exert their functions when they are<br />dead? Which cells must be living in order to function?<br />5. Wearing a hat in winter is a good idea. What happens<br />to the arterioles in the dermis in a cold environment?<br />How does this affect heat loss? Is the<br />head an exception? Explain.<br />104<br />CHAPTER 6<br />Chapter Outline<br />Functions of the Skeleton<br />Types of Bone Tissue<br />Classification of Bones<br />Embryonic Growth of Bone<br />Factors That Affect Bone Growth and<br />Maintenance<br />The Skeleton<br />Skull<br />Vertebral Column<br />Rib Cage<br />The Shoulder and Arm<br />The Hip and Leg<br />Joints—Articulations<br />The Classification of Joints<br />Synovial Joints<br />Aging and the Skeletal System<br />BOX 6–1 FRACTURES AND THEIR REPAIR<br />BOX 6–2 OSTEOPOROSIS<br />BOX 6–3 HERNIATED DISC<br />BOX 6–4 ABNORMALITIES OF THE CURVES<br />OF THE SPINE<br />BOX 6–5 ARTHRITIS<br />Student Objectives<br />• Describe the functions of the skeleton.<br />• Explain how bones are classified, and give an<br />example of each type.<br />• Describe how the embryonic skeleton model is<br />replaced by bone.<br />• Name the nutrients necessary for bone growth,<br />and explain their functions.<br />• Name the hormones involved in bone growth and<br />maintenance, and explain their functions.<br />• Explain what is meant by “exercise” for bones, and<br />explain its importance.<br />• Name all the bones of the human skeleton (be able<br />to point to each on diagrams, skeleton models, or<br />yourself).<br />• Describe the functions of the skull, vertebral column,<br />rib cage, scapula, and pelvic bone.<br />• Explain how joints are classified. For each type,<br />give an example, and describe the movement possible.<br />• Describe the parts of a synovial joint, and explain<br />their functions.<br />The Skeletal System<br />105<br />New Terminology<br />Appendicular (AP-en-DIK-yoo-lar)<br />Articulation (ar-TIK-yoo-LAY-shun)<br />Axial (AK-see-uhl)<br />Bursa (BURR-sah)<br />Diaphysis (dye-AFF-i-sis)<br />Epiphyseal disc (e-PIFF-i-SEE-al DISK)<br />Epiphysis (e-PIFF-i-sis)<br />Fontanel (FON-tah-NELL)<br />Haversian system (ha-VER-zhun SIS-tem)<br />Ligament (LIG-uh-ment)<br />Ossification (AHS-i-fi-KAY-shun)<br />Osteoblast (AHS-tee-oh-BLAST)<br />Osteoclast (AHS-tee-oh-KLAST)<br />Paranasal sinus (PAR-uh-NAY-zuhl SIGH-nus)<br />Periosteum (PER-ee-AHS-tee-um)<br />Suture (SOO-cher)<br />Symphysis (SIM-fi-sis)<br />Synovial fluid (sin-OH-vee-al FLOO-id)<br />Related Clinical Terminology<br />Autoimmune disease<br />(AW-toh-im-YOON di-ZEEZ)<br />Bursitis (burr-SIGH-tiss)<br />Cleft palate (KLEFT PAL-uht)<br />Fracture (FRAK-chur)<br />Herniated disc (HER-nee-ay-ted DISK)<br />Kyphosis (kye-FOH-sis)<br />Lordosis (lor-DOH-sis)<br />Osteoarthritis (AHS-tee-oh-ar-THRY-tiss)<br />Osteomyelitis (AHS-tee-oh-my-uh-LYE-tiss)<br />Osteoporosis (AHS-tee-oh-por-OH-sis)<br />Rheumatoid arthritis<br />(ROO-muh-toyd ar-THRY-tiss)<br />Rickets (RIK-ets)<br />Scoliosis (SKOH-lee-OH-sis)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />Imagine for a moment that people did not have<br />skeletons. What comes to mind? Probably that each of<br />us would be a little heap on the floor, much like a jellyfish<br />out of water. Such an image is accurate and<br />reflects the most obvious function of the skeleton: to<br />support the body. Although it is a framework for the<br />body, the skeleton is not at all like the wooden beams<br />that support a house. Bones are living organs that<br />actively contribute to the maintenance of the internal<br />environment of the body.<br />The skeletal system consists of bones and other<br />structures that make up the joints of the skeleton. The<br />types of tissue present are bone tissue, cartilage, and<br />fibrous connective tissue, which forms the ligaments<br />that connect bone to bone.<br />FUNCTIONS OF THE SKELETON<br />1. Provides a framework that supports the bo<br /><br /><br />dy; the<br />muscles that are attached to bones move the skeleton.<br />2. Protects some internal organs from mechanical<br />injury; the rib cage protects the heart and lungs, for<br />example.<br />3. Contains and protects the red bone marrow, the<br />primary hemopoietic (blood-forming) tissue.<br />4. Provides a storage site for excess calcium. Calcium<br />may be removed from bone to maintain a normal<br />blood calcium level, which is essential for blood<br />clotting and proper functioning of muscles and<br />nerves.<br />TYPES OF BONE TISSUE<br />Bone was described as a tissue in Chapter 4. Recall<br />that bone cells are called osteocytes, and the matrix<br />of bone is made of calcium salts and collagen. The<br />calcium salts are calcium carbonate (CaCO3) and calcium<br />phosphate (Ca3(PO4)2), which give bone the<br />strength required to perform its supportive and protective<br />functions. Bone matrix is non-living, but it<br />changes constantly, with calcium that is taken from<br />bone into the blood replaced by calcium from the diet.<br />In normal circumstances, the amount of calcium that<br />is removed is replaced by an equal amount of calcium<br />deposited. This is the function of osteocytes, to regulate<br />the amount of calcium that is deposited in, or<br />removed from, the bone matrix.<br />In bone as an organ, two types of bone tissue are<br />present (Fig. 6–1). Compact bone looks solid but is<br />very precisely structured. Compact bone is made of<br />osteons or haversian systems, microscopic cylinders<br />of bone matrix with osteocytes in concentric rings<br />around central haversian canals. In the haversian<br />canals are blood vessels; the osteocytes are in contact<br />with these blood vessels and with one another through<br />microscopic channels (canaliculi) in the matrix.<br />The second type of bone tissue is spongy bone,<br />which does look rather like a sponge with its visible<br />holes or cavities. Osteocytes, matrix, and blood vessels<br />are present but are not arranged in haversian systems.<br />The cavities in spongy bone often contain red bone<br />marrow, which produces red blood cells, platelets,<br />and the five kinds of white blood cells.<br />CLASSIFICATION OF BONES<br />1. Long bones—the bones of the arms, legs, hands,<br />and feet (but not the wrists and ankles). The shaft<br />of a long bone is the diaphysis, and the ends are<br />called epiphyses (see Fig. 6–1). The diaphysis is<br />made of compact bone and is hollow, forming a<br />canal within the shaft. This marrow canal (or<br />medullary cavity) contains yellow bone marrow,<br />which is mostly adipose tissue. The epiphyses are<br />made of spongy bone covered with a thin layer of<br />compact bone. Although red bone marrow is present<br />in the epiphyses of children’s bones, it is largely<br />replaced by yellow bone marrow in adult bones.<br />2. Short bones—the bones of the wrists and ankles.<br />3. Flat bones—the ribs, shoulder blades, hip bones,<br />and cranial bones.<br />4. Irregular bones—the vertebrae and facial bones.<br />Short, flat, and irregular bones are all made of<br />spongy bone covered with a thin layer of compact<br />bone. Red bone marrow is found within the spongy<br />bone.<br />The joint surfaces of bones are covered with articular<br />cartilage, which provides a smooth surface. Covering<br />the rest of the bone is the periosteum, a fibrous<br />connective tissue membrane whose collagen fibers<br />merge with those of the tendons and ligaments that<br />106 The Skeletal System<br />107<br />Diaphysis<br />Proximal<br />epiphysis<br />Distal<br />epiphysis<br />Yellow bone<br />marrow<br />Marrow (medullary)<br />cavity<br />Periosteum<br />Compact bone<br />Spongy bone<br />Marrow<br />Canaliculi<br />Venule<br />Osteocyte<br />Fibrous layer<br />Osteogenic layer<br />(osteoblasts)<br />Periosteum<br />Osteon<br />(haversian system)<br />Haversian canal<br />Concentric rings<br />of osteocytes<br />A B<br />Arteriole<br />Figure 6–1. Bone tissue. (A) Femur with distal end cut in longitudinal section.<br />(B) Compact bone showing haversian systems (osteons).<br />QUESTION: What is the purpose of the blood vessels in bone tissue?<br />are attached to the bone. The periosteum anchors<br />these structures and contains both the blood vessels<br />that enter the bone itself and osteoblasts that will<br />become active if the bone is damaged.<br />EMBRYONIC GROWTH OF BONE<br />During embryonic development, the skeleton is first<br />made of cartilage and fibrous connective tissue, which<br />are gradually replaced by bone. Bone matrix is produced<br />by cells called osteoblasts (a blast cell is a “growing”<br />or “producing” cell, and osteo means “bone”). In<br />the embryonic model of the skeleton, osteoblasts differentiate<br />from the fibroblasts that are present. The<br />production of bone matrix, called ossification, begins<br />in a center of ossification in each bone.<br />The cranial and facial bones are first made of<br />fibrous connective tissue. In the third month of fetal<br />development, fibroblasts (spindle-shaped connective<br />tissue cells) become more specialized and differentiate<br />into osteoblasts, which produce bone matrix. From<br />each center of ossification, bone growth radiates outward<br />as calcium salts are deposited in the collagen of<br />the model of the bone. This process is not complete<br />at birth; a baby has areas of fibrous connective tissue<br />remaining between the bones of the skull. These<br />are called fontanels (Fig. 6–2), which permit compression<br />of the baby’s head during birth without<br />breaking the still thin cranial bones. The fontanels<br />also permit the growth of the brain after birth. You<br />may have heard fontanels referred to as “soft spots,”<br />and indeed they are. A baby’s skull is quite fragile and<br />must be protected from trauma. By the age of 2 years,<br />all the fontanels have become ossified, and the skull<br />becomes a more effective protective covering for the<br />brain.<br />The rest of the embryonic skeleton is first made of<br />cartilage, and ossification begins in the third month of<br />gestation in the long bones. Osteoblasts produce bone<br />matrix in the center of the diaphyses of the long bones<br />and in the center of short, flat, and irregular bones.<br />Bone matrix gradually replaces the original cartilage<br />(Fig. 6–3).<br />The long bones also develop centers of ossification<br />in their epiphyses. At birth, ossification is not yet complete<br />and continues throughout childhood. In long<br />bones, growth occurs in the epiphyseal discs at the<br />junction of the diaphysis with each epiphysis. An epiphyseal<br />disc is still cartilage, and the bone grows in<br />length as more cartilage is produced on the epiphysis<br />side (see Fig. 6–3). On the diaphysis side, osteoblasts<br />produce bone matrix to replace the cartilage. Between<br />the ages of 16 and 25 years (influenced by estrogen or<br />testosterone), all of the cartilage of the epiphyseal<br />discs is replaced by bone. This is called closure of the<br />epiphyseal discs (or we say the discs are closed), and<br />the bone lengthening process stops.<br />Also in bones are specialized cells called osteoclasts<br />(a clast cell is a “destroying” cell), which are able<br />to dissolve and reabsorb the minerals of bone matrix,<br />a process called resorption. Osteoclasts are very<br />active in embryonic long bones, and they reabsorb<br />bone matrix in the center of the diaphysis to form the<br />marrow canal. Blood vessels grow into the marrow<br />canals of embryonic long bones, and red bone marrow<br />is established. After birth, the red bone marrow is<br />replaced by yellow bone marrow. Red bone marrow<br />remains in the spongy bone of short, flat, and irregular<br />bones. For other functions of osteoclasts and<br />osteoblasts, see Box 6–1: Fractures and Their Repair.<br />FACTORS THAT AFFECT BONE<br />GROWTH AND MAINTENANCE<br />1. Heredity—each person has a genetic potential for<br />height, that is, a maximum height, with genes<br />inherited from both parents. Many genes are<br />involved, and their interactions are not well understood.<br />Some of these genes are probably those for<br />the enzymes involved in cartilage and bone production,<br />for this is how bones grow.<br />2. Nutrition—nutrients are the raw materials of<br />which bones are made. Calcium, phosphorus, and<br />protein become part of the bone matrix itself.<br />Vitamin D is needed for the efficient absorption of<br />calcium and phosphorus by the small intestine.<br />Vitamins A and C do not become part of bone but<br />are necessary for the process of bone matrix formation<br />(ossification). Without these and other nutrients,<br />bones cannot grow properly. Children who<br />are malnourished grow very slowly and may not<br />reach their genetic potential for height.<br />3. Hormones—endocrine glands produce hormones<br />that stimulate specific effects in certain cells.<br />108 The Skeletal System<br />(text continued on page 112)<br />Frontal bone<br />Anterior fontanel Parietal bone<br />Posterior fontanel Posterior fontanel<br />Mastoid fontanel<br />Temporal bone<br />Sphenoid fontanel<br />Mandible<br />Sphenoid bone<br />Zygomatic bone<br />Maxilla<br />Occipital bone<br />Occipital bone<br />Frontal bone<br />Parietal bone<br />Anterior fontanel<br />A B<br />C D<br />Figure 6–2. Infant skull with fontanels. (A) Lateral view of left side. (B) Superior view.<br />(C) Fetal skull in anterior superior view. (D) Fetal skull in left lateral view. Try to name the<br />bones; use part A as a guide. The fontanels are translucent connective tissue. (C and D photographs<br />by Dan Kaufman.)<br />QUESTION: What is the difference between the frontal bone of the infant skull and that of<br />the adult skull?<br />109<br />B Epiphyseal disc<br />Chondrocytes producing cartilage<br />Bone<br />Cartilage<br />Epiphyseal<br />disc<br />Cartilaginous<br />model<br />Medullary cavity<br />containing<br />marrow<br />Bone collar<br />and calcifying<br />cartilage in<br />ossification center<br />Secondary<br />ossification<br />center<br />Compact bone<br />Compact bone<br />Spongy bone<br />Articular cartilage<br />Medullary cavity and development<br />A of secondary ossification centers<br />Osteoblasts producing bone<br />Figure 6–3. The ossification process in a long bone. (A) Progression of ossification from<br />the cartilage model of the embryo to the bone of a young adult. (B) Microscopic view of<br />an epiphyseal disc showing cartilage production and bone replacement.<br />QUESTION: The epiphyseal discs of the bone on the far right are closed. What does that<br />mean?<br />110<br />111<br />BOX 6–1 FRACTURES AND THEIR REPAIR<br />Box Figure 6–A Types of fractures. Several types of<br />fractures are depicted in the right arm.<br />A fracture means that a bone has been broken.<br />There are different types of fractures classified as to<br />extent of damage.<br />Simple (closed)—the broken parts are still in normal<br />anatomic position; surrounding tissue damage<br />is minimal (skin is not pierced).<br />Compound (open)—the broken end of a bone<br />has been moved, and it pierces the skin; there may<br />be extensive damage to surrounding blood vessels,<br />nerves, and muscles.<br />Greenstick—the bone splits longitudinally. The<br />bones of children contain more collagen than do<br />adult bones and tend to splinter rather than break<br />completely.<br />Comminuted—two or more intersecting breaks<br />create several bone fragments.<br />Impacted—the broken ends of a bone are forced<br />into one another; many bone fragments may be<br />created.<br />Pathologic (spontaneous)—a bone breaks without<br />apparent trauma; may accompany bone disorders<br />such as osteoporosis.<br />The Repair Process<br />Even a simple fracture involves significant bone<br />damage that must be repaired if the bone is to<br />resume its normal function. Fragments of dead or<br />damaged bone must first be removed. This is<br />accomplished by osteoclasts, which dissolve and<br />reabsorb the calcium salts of bone matrix. Imagine<br />a building that has just collapsed; the rubble must<br />be removed before reconstruction can take place.<br />This is what the osteoclasts do. Then, new bone<br />must be produced. The inner layer of the periosteum<br />contains osteoblasts that are activated when<br />bone is damaged. The osteoblasts produce bone<br />matrix to knit the broken ends of the bone<br />together.<br />Because most bone has a good blood supply, the<br />repair process is usually relatively rapid, and a simple<br />fracture often heals within 6 weeks. Some parts<br />of bones, however, have a poor blood supply, and<br />repair of fractures takes longer. These areas are the<br />neck of the femur (the site of a “fractured hip”) and<br />the lower third of the tibia.<br />Other factors that influence repair include the<br />age of the person, general state of health, and<br />nutrition. The elderly and those in poor health often<br />have slow healing of fractures. A diet with sufficient<br />calcium, phosphorus, vitamin D, and protein is also<br />important. If any of these nutrients is lacking, bone<br />repair will be a slower process.<br />Several hormones make important contributions to<br />bone growth and maintenance. These include<br />growth hormone, thyroxine, parathyroid hormone,<br />and insulin, which help regulate cell division, protein<br />synthesis, calcium metabolism, and energy<br />production. The sex hormones estrogen or testosterone<br />help bring about the cessation of bone<br />growth. The hormones and their specific functions<br />are listed in Table 6–1.<br />4. Exercise or “stress”—for bones, exercise means<br />bearing weight, which is just what bones are specialized<br />to do. Without this stress (which is normal),<br />bones will lose calcium faster than it is<br />replaced. Exercise need not be strenuous; it can be<br />as simple as the walking involved in everyday activities.<br />Bones that do not get this exercise, such as<br />those of patients confined to bed, will become thinner<br />and more fragile. This condition is discussed<br />further in Box 6–2: Osteoporosis.<br />THE SKELETON<br />The human skeleton has two divisions: the axial skeleton,<br />which forms the axis of the body, and the appendicular<br />skeleton, which supports the appendages or<br />limbs. The axial skeleton consists of the skull, vertebral<br />column, and rib cage. The bones of the arms and<br />legs and the shoulder and pelvic girdles make up the<br />appendicular skeleton. Many bones are connected to<br />other bones across joints by ligaments, which are<br />strong cords or sheets of fibrous connective tissue. The<br />importance of ligaments becomes readily apparent<br />when a joint is sprained. A sprain is the stretching or<br />even tearing of the ligaments of a joint, and though the<br />bones are not broken, the joint is weak and unsteady.<br />We do not often think of our ligaments, but they are<br />necessary to keep our bones in the proper positions to<br />keep us upright or to bear weight.<br />There are 206 bones in total, and the complete<br />skeleton is shown in Fig. 6–4.<br />SKULL<br />The skull consists of 8 cranial bones and 14 facial<br />bones. Also in the head are three small bones in each<br />middle ear cavity and the hyoid bone that supports the<br />base of the tongue. The cranial bones form the braincase<br />(lined with the meninges) that encloses and protects<br />the brain, eyes, and ears. The names of some of<br />these bones will be familiar to you; they are the same<br />as the terminology used (see Chapter 1) to describe<br />areas of the head. These are the frontal bone, parietal<br />bones (two), temporal bones (two), and occipital bone.<br />The sphenoid bone and ethmoid bone are part of the<br />floor of the braincase and the orbits (sockets) for<br />the eyes. The frontal bone forms the forehead and<br />the anterior part of the top of the skull. Parietal means<br />“wall,” and the two large parietal bones form the posterior<br />top and much of the side walls of the skull. Each<br />temporal bone on the side of the skull contains an<br />external auditory meatus (ear canal), a middle ear cav-<br />112 The Skeletal System<br />Table 6–1 HORMONES INVOLVED IN BONE GROWTH AND MAINTENANCE<br />Growth hormone (anterior pituitary gland)<br />Thyroxine (thyroid gland)<br />Insulin (pancreas)<br />Parathyroid hormone (parathyroid glands)<br />Calcitonin (thyroid gland)<br />Estrogen (ovaries) or<br />Testosterone (testes)<br />• Increases the rate of mitosis of chondrocytes and osteoblasts<br />• Increases the rate of protein synthesis (collagen, cartilage matrix,<br />and enzymes for cartilage and bone formation)<br />• Increases the rate of protein synthesis<br />• Increases energy production from all food types<br />• Increases energy production from glucose<br />• Increases the reabsorption of calcium from bones to the blood<br />(raises blood calcium level)<br />• Increases the absorption of calcium by the small intestine and kidneys<br />(to the blood)<br />• Decreases the reabsorption of calcium from bones (lowers blood<br />calcium level)<br />• Promotes closure of the epiphyses of long bones (growth stops)<br />• Helps retain calcium in bones to maintain a strong bone matrix<br />ity, and an inner ear labyrinth. The occipital bone<br />forms the lower, posterior part of the braincase. Its<br />foramen magnum is a large opening for the spinal<br />cord, and the two condyles (rounded projections) on<br />either side articulate with the atlas, the first cervical<br />vertebra. The sphenoid bone is said to be shaped like<br />a bat, and the greater wing is visible on the side of the<br />skull between the frontal and temporal bones. The<br />body of the bat has a depression called the sella turcica,<br />which encloses the pituitary gland. The ethmoid<br />bone has a vertical projection called the crista galli<br />(“rooster’s comb”) that anchors the cranial meninges.<br />The rest of the ethmoid bone forms the roof and<br />upper walls of the nasal cavities, and the upper part of<br />the nasal septum.<br />All of the joints between cranial bones are immovable<br />joints called sutures. It may seem strange to refer<br />to a joint without movement, but the term joint (or<br />articulation) is used for any junction of two bones.<br />(The classification of joints will be covered later in<br />The Skeletal System 113<br />BOX 6–2 OSTEOPOROSIS<br />Normal Bone Osteoporosis<br />A B<br />Box Figure 6–B (A) Normal spongy bone, as in the body of a vertebra. (B) Spongy bone thinned<br />by osteoporosis.<br />Bone is an active tissue; calcium is constantly being<br />removed to maintain normal blood calcium levels.<br />Usually, however, calcium is replaced in bones at a<br />rate equal to its removal, and the bone matrix<br />remains strong.<br />Osteoporosis is characterized by excessive loss<br />of calcium from bones without sufficient replacement.<br />Research has suggested that a certain gene<br />for bone buildup in youth is an important factor;<br />less buildup would mean earlier bone thinning.<br />Contributing environmental factors include smoking,<br />insufficient dietary intake of calcium, inactivity,<br />and lack of the sex hormones. Osteoporosis is most<br />common among elderly women, because estrogen<br />secretion decreases sharply at menopause (in older<br />men, testosterone is still secreted in significant<br />amounts). Factors such as bed rest or inability to<br />get even minimal exercise will make calcium loss<br />even more rapid.<br />As bones lose calcium and become thin and brittle,<br />fractures are much more likely to occur. Among<br />elderly women, a fractured hip (the neck of the<br />femur) is an all-too-common consequence of this<br />degenerative bone disorder. Such a serious injury is<br />not inevitable, however, and neither is the thinning<br />of the vertebrae that bows the spines of some elderly<br />people. After menopause, women may wish to<br />have a bone density test to determine the strength<br />of their bone matrix. Several medications are available<br />that diminish the rate of bone loss. A diet high<br />in calcium and vitamin D is essential for both men<br />and women, as is moderate exercise. Young women<br />and teenagers should make sure they get adequate<br />dietary calcium to form strong bone matrix,<br />because this will delay the serious effects of osteoporosis<br />later in life.<br />this chapter.) In a suture, the serrated, or sawtooth,<br />edges of adjacent bones fit into each other. These<br />interlocking projections prevent sliding or shifting of<br />the bones if the skull is subjected to a blow or pressure.<br />In Fig. 6–5 you can see the coronal suture<br />between the frontal and parietal bones, the squamosal<br />suture between the parietal and temporal bones, and<br />the lambdoidal suture between the occipital and parietal<br />bones. Not visible is the sagittal suture, where the<br />two parietal bones articulate along the midline of the<br />top of the skull. All the bones of the skull, as well as<br />the large sutures, are shown in Figs. 6–5 through 6–8.<br />Their anatomically important parts are described in<br />Table 6–2.<br />114 The Skeletal System<br />Phalanges<br />Phalanges<br />Tarsals<br />Metatarsals<br />Humerus<br />Radius<br />Ulna<br />Carpals<br />Metacarpals<br />Maxilla<br />Mandible<br />Skull (cranium)<br />Sternum<br />Clavicle<br />Scapula<br />Sacrum<br />Zygomatic arch<br />Cervical vertebrae<br />Thoracic vertebrae<br />Ribs<br />Lumbar vertebrae<br />Ilium<br />Coccyx<br />Pubis<br />Ischium<br />Femur<br />Patella<br />Tibia<br />Fibula<br />Figure 6–4. Skeleton. Anterior<br />view.<br />QUESTION: Which of the bones<br />shown here would be classified<br />as irregular bones?<br />115<br />Parietal bone<br />Parietal bone<br />Maxillary bone<br />Squamosal suture<br />Squamosal suture<br />Temporal bone<br />Mandible<br />Occipital bone<br />Occipital bone<br />Zygomatic process<br />Mandibular fossa<br />Mastoid process<br />External auditory<br />meatus<br />Coronoid process<br />Body<br />Mandible<br />Mandible<br />Mental foramen<br />Mental foramen<br />Maxilla<br />Maxilla<br />Condyloid process<br />Zygomatic bone<br />Zygomatic bone<br />Zygomatic bone<br />Lambdoidal suture<br />Nasal bone<br />Sphenoid bone<br />Lacrimal canal<br />Lacrimal bone<br />Lacrimal bone<br />Temporal bone<br />Sphenoid bone<br />Sphenoid bone<br />Parietal bone<br />Frontal bone<br />Frontal bone<br />Frontal bone<br />Coronal suture<br />Coronal suture<br />Ethmoid bone<br />Ethmoid bone<br />Nasal bone<br />Inferior nasal concha<br />Vomer<br />Inferior nasal concha<br />Middle nasal concha (ethmoid)<br />Vomer<br />Palatine bone<br />Temporal bone<br />Nasal bone<br />Perpendicular plate (ethmoid)<br />QUESTION: What might be the purpose of the openings at the back of the eye sockets?<br />Figure 6–5. Skull. Lateral view of right side. Figure 6–6. Skull. Anterior view.<br />116 The Skeletal System<br />Palatine process (maxilla)<br />Palatine bone<br />Zygomatic bone<br />Vomer<br />Zygomatic process<br />Temporal bone<br />Styloid process<br />External auditory meatus<br />Mastoid process<br />Occipital condyles<br />Foramen magnum<br />Occipital bone<br />Figure 6–7. Skull. Inferior view with mandible removed.<br />QUESTION: What is the purpose of the foramen magnum?<br />Of the 14 facial bones, only the mandible (lower<br />jaw) is movable; it forms a condyloid joint with each<br />temporal bone. The other joints between facial bones<br />are all sutures. The maxillae are the two upper jaw<br />bones, which also form the anterior portion of the<br />hard palate (roof of the mouth). Sockets for the roots<br />of the teeth are found in the maxillae and the<br />mandible. The two nasal bones form the bridge of<br />the nose where they articulate with the frontal bone<br />(the rest of the nose is supported by cartilage). There<br />is a lacrimal bone at the medial side of each orbit; the<br />lacrimal canal contains the lacrimal sac, a passageway<br />for tears. Each of the two zygomatic bones forms the<br />point of a cheek, and articulates with the maxilla,<br />frontal bone, and temporal bone. The two palatine<br />bones are the posterior portion of the hard palate.<br />The plow-shaped vomer forms the lower part of the<br />nasal septum; it articulates with the ethmoid bone. On<br />either side of the vomer are the conchae, six scrolllike<br />bones that curl downward from the sides of the<br />nasal cavities; they help increase the surface area of the<br />nasal mucosa. These facial bones are included in Table<br />6–2.<br />Paranasal sinuses are air cavities located in the<br />maxillae and frontal, sphenoid, and ethmoid bones<br />(Fig. 6–9). As the name paranasal suggests, they open<br />into the nasal cavities and are lined with ciliated<br />epithelium continuous with the mucosa of the nasal<br />cavities. We are aware of our sinuses only when they<br />become “stuffed up,” which means that the mucus<br />they produce cannot drain into the nasal cavities. This<br />may happen during upper respiratory infections such<br />(text continued on page 119)<br />117<br />Crista galli<br />Cribriform plate<br />Olfactory foramina<br />Greater wing<br />Lambdoidal suture<br />Frontal bone<br />Sphenoid bone<br />Sella turcica<br />Squamosal suture<br />Temporal bone<br />Parietal bone<br />Foramen magnum<br />Occipital bone<br />Ethmoid bone<br />A<br />C<br />B<br />Crista galli<br />Sella turcica<br />Figure 6–8. (A) Skull. Superior view with the top of cranium removed. (B) Sphenoid<br />bone in superior view. (C) Ethmoid bone in superior view. (B and C photographs by Dan<br />Kaufman.)<br />QUESTION: What are the olfactory foramina of the ethmoid bone for?<br />Table 6–2 BONES OF THE SKULL—IMPORTANT PARTS<br />Terminology of Bone Markings<br />Foramen—a hole or opening Meatus—a tunnel-like cavity Condyle—a rounded projection<br />Fossa—a depression Process—a projection Plate—a flat projection<br />Crest—a ridge or edge Facet—a flat projection Tubercle—a round projection<br />Bone Part Description<br />Frontal<br />Parietal (2)<br />Temporal (2)<br />Occipital<br />Sphenoid<br />Ethmoid<br />Mandible<br />Maxilla (2)<br />Nasal (2)<br />Lacrimal (2)<br />Zygomatic (2)<br />Palatine (2)<br />Vomer<br />• Frontal sinus<br />• Coronal suture<br />• Sagittal suture<br />• Squamosal suture<br />• External auditory meatus<br />• Mastoid process<br />• Mastoid sinus<br />• Mandibular fossa<br />• Zygomatic process<br />• Foramen magnum<br />• Condyles<br />• Lambdoidal suture<br />• Greater wing<br />• Sella turcica<br />• Sphenoid sinus<br />• Ethmoid sinus<br />• Crista galli<br />• Cribriform plate and<br />olfactory foramina<br />• Perpendicular plate<br />• Conchae (4 are part of<br />ethmoid; 2 inferior are<br />separate bones)<br />• Body<br />• Condyles<br />• Sockets<br />• Maxillary sinus<br />• Palatine process<br />• Sockets<br />—<br />• Lacrimal canal<br />—<br />—<br />—<br />• Air cavity that opens into nasal cavity<br />• Joint between frontal and parietal bones<br />• Joint between the 2 parietal bones<br />• Joint between temporal and parietal bone<br />• The tunnel-like ear canal<br />• Oval projection behind the ear canal<br />• Air cavity that opens into middle ear<br />• Oval depression anterior to the ear canal; articulates<br />with mandible<br />• Anterior projection that articulates with the<br />zygomatic bone<br />• Large opening for the spinal cord<br />• Oval projections on either side of the foramen<br />magnum; articulate with the atlas<br />• Joint between occipital and parietal bones<br />• Flat, lateral portion between the frontal and<br />temporal bones<br />• Central depression that encloses the pituitary<br />gland<br />• Air cavity that opens into nasal cavity<br />• Air cavity that opens into nasal cavity<br />• Superior projection for attachment of<br />meninges<br />• On either side of base of crista galli; olfactory<br />nerves pass through foramina<br />• Upper part of nasal septum<br />• Shelf-like projections into nasal cavities that<br />increase surface area of nasal mucosa<br />• U-shaped portion with lower teeth<br />• Oval projections that articulate with the temporal<br />bones<br />• Conical depressions that hold roots of lower<br />teeth<br />• Air cavity that opens into nasal cavity<br />• Projection that forms anterior part of hard<br />palate<br />• Conical depressions that hold roots of upper<br />teeth<br />• Form the bridge of the nose<br />• Opening for nasolacrimal duct to take tears to<br />nasal cavity<br />• Form point of cheek; articulate with frontal,<br />temporal, and maxillae<br />• Form the posterior part of hard palate<br />• Lower part of nasal septum<br />118<br />as colds, or with allergies such as hay fever. These<br />sinuses, however, do have functions: They make the<br />skull lighter in weight, because air is lighter than bone,<br />and they provide resonance for the voice, meaning<br />more air to vibrate and thus deepen the pitch of the<br />voice.<br />The mastoid sinuses are air cavities in the mastoid<br />process of each temporal bone; they open into the<br />middle ear. Before the availability of antibiotics, middle<br />ear infections often caused mastoiditis, infection of<br />these sinuses.<br />Within each middle ear cavity are three auditory<br />bones: the malleus, incus, and stapes. As part of the<br />hearing process (discussed in Chapter 9), these bones<br />transmit vibrations from the eardrum to the receptors<br />in the inner ear (see Fig. 9–7).<br />VERTEBRAL COLUMN<br />The vertebral column (spinal column or backbone) is<br />made of individual bones called vertebrae. The names<br />of vertebrae indicate their location along the length of<br />the spinal column. There are 7 cervical vertebrae, 12<br />thoracic, 5 lumbar, 5 sacral fused into 1 sacrum, and<br />4 to 5 small coccygeal vertebrae fused into 1 coccyx<br />(Fig. 6–10).<br />The seven cervical vertebrae are those within the<br />neck. The first vertebra is called the atlas, which articulates<br />with the occipital bone to support the skull and<br />forms a pivot joint with the odontoid process of the<br />axis, the second cervical vertebra. This pivot joint<br />allows us to turn our heads from side to side. The<br />remaining five cervical vertebrae do not have individual<br />names.<br />The thoracic vertebrae articulate (form joints)<br />with the ribs on the posterior side of the trunk. The<br />lumbar vertebrae, the largest and strongest bones of<br />the spine, are found in the small of the back. The<br />sacrum permits the articulation of the two hip bones:<br />the sacroiliac joints. The coccyx is the remnant of<br />tail vertebrae, and some muscles of the perineum<br />(pelvic floor) are anchored to it.<br />The Skeletal System 119<br />Frontal sinus<br />Ethmoid sinus<br />Maxillary sinus<br />Sphenoid sinus<br />Ethmoid sinus<br />A B<br />Figure 6–9. Paranasal sinuses. (A) Anterior view of the skull. (B) Left lateral view of skull.<br />QUESTION: Which of these sinuses often cause the pain of a sinus headache?<br />120 The Skeletal System<br />1<br />2<br />3<br />4<br />5<br />6<br />7<br />1<br />2<br />3<br />4<br />5<br />6<br />7<br />8<br />9<br />10<br />11<br />12<br />1<br />2<br />3<br />4<br />5<br />Coccyx<br />Sacrum<br />Lumbar<br />vertebrae<br />Thoracic<br />vertebrae<br />Cervical<br />vertebrae<br />•<br />•<br />Intervertebral<br />discs<br />•<br />Articular surface<br />for ilium<br />•<br />•<br />Vertebral body<br />Vertebral<br />canal<br />•<br />•<br />Spinous process<br />Facet for rib<br />•<br />Transverse process<br />•<br />•<br />Odontoid process<br />•<br />Vertebral canal<br />Spinous process<br />of axis<br />Facets of atlas for occipital condyles<br />D<br />1st Lumbar<br />C<br />7th Lumbar<br />B<br />Atlas/Axis<br />A<br />Figure 6–10. Vertebral column. (A) Lateral view of left<br />side. (B) Atlas and axis, superior view. (C) 7th thoracic<br />vertebra, left lateral view. (D) 1st lumbar vertebra, left lateral<br />view.<br />QUESTION: Compare the size of the individual thoracic<br />and lumbar vertebrae. What is the reason for this difference?<br />All of the vertebrae articulate with one another in<br />sequence, connected by ligaments, to form a flexible<br />backbone that supports the trunk and head. They also<br />form the vertebral canal, a continuous tunnel (lined<br />with the meninges) within the bones that contains the<br />spinal cord and protects it from mechanical injury.<br />The spinous and transverse processes are projections<br />for the attachment of the muscles that bend the vertebral<br />column. The facets of some vertebrae are small<br />flat surfaces for articulation with other bones, such as<br />the ribs with the facets of the thoracic vertebrae.<br />The supporting part of a vertebra is its body; the<br />bodies of adjacent vertebrae are separated by discs<br />of fibrous cartilage. These discs cushion and absorb<br />shock and permit some movement between vertebrae<br />(symphysis joints). Since there are so many joints,<br />the backbone as a whole is quite flexible (see also Box<br />6–3: Herniated Disc).<br />The normal spine in anatomic position has four<br />natural curves, which are named after the vertebrae<br />that form them. Refer to Fig. 6–10, and notice that the<br />cervical curve is forward, the thoracic curve backward,<br />BOX 6–3 HERNIATED DISC<br />The vertebrae are separated by discs of fibrous cartilage<br />that act as cushions to absorb shock. An intervertebral<br />disc has a tough outer covering and a soft<br />center called the nucleus pulposus. Extreme pressure<br />on a disc may rupture the outer layer and force<br />the nucleus pulposus out. This may occur when a<br />person lifts a heavy object improperly, that is, using<br />the back rather than the legs and jerking upward,<br />which puts sudden, intense pressure on the spine.<br />Most often this affects discs in the lumbar region.<br />Although often called a “slipped disc,” the<br />affected disc is usually not moved out of position.<br />Box Figure 6–C Herniated disc. As a result of compression, a ruptured intervertebral disc puts pressure<br />on a spinal nerve.<br />The terms herniated disc or ruptured disc more<br />accurately describe what happens. The nucleus pulposus<br />is forced out, often posteriorly, where it puts<br />pressure on a spinal nerve. For this reason a herniated<br />disc may be very painful or impair function in<br />the muscles supplied by the nerve.<br />Healing of a herniated disc may occur naturally if<br />the damage is not severe and the person rests and<br />avoids activities that would further compress the<br />disc. Surgery may be required, however, to remove<br />the portion of the nucleus pulposus that is out of<br />place and disrupting nerve functioning.<br />121<br />the lumbar curve forward, and the sacral curve backward.<br />These curves center the skull over the rest of the<br />body, which enables a person to more easily walk<br />upright (see Box 6–4: Abnormalities of the Curves of<br />the Spine).<br />RIB CAGE<br />The rib cage consists of the 12 pairs of ribs and the<br />sternum, or breastbone. The three parts of the sternum<br />are the upper manubrium, the central body, and<br />the lower xiphoid process (Fig. 6–11).<br />All of the ribs articulate posteriorly with the thoracic<br />vertebrae. The first seven pairs of ribs are called<br />true ribs; they articulate directly with the manubrium<br />and body of the sternum by means of costal cartilages.<br />The next three pairs are called false ribs; their cartilages<br />join the 7th rib cartilage. The last two pairs are<br />called floating ribs because they do not articulate<br />with the sternum at all (see Fig. 6–10).<br />An obvious function of the rib cage is that it encloses<br />and protects the heart and lungs. Keep in mind,<br />though, that the rib cage also protects organs in the<br />upper abdominal cavity, such as the liver and spleen.<br />The other important function of the rib cage depends<br />upon its flexibility: The ribs are pulled upward and<br />outward by the external intercostal muscles. This<br />enlarges the chest cavity, which expands the lungs and<br />contributes to inhalation.<br />THE SHOULDER AND ARM<br />The shoulder girdles attach the arms to the axial skeleton.<br />Each consists of a scapula (shoulder blade) and<br />clavicle (collarbone). The scapula is a large, flat bone<br />with several projections (the spine of the scapula, the<br />coracoid process) that anchor some of the muscles that<br />move the upper arm and the forearm. A shallow<br />depression called the glenoid fossa forms a ball-andsocket<br />joint with the humerus, the bone of the upper<br />arm (Fig. 6–12).<br />Each clavicle articulates laterally with a scapula<br />and medially with the manubrium of the sternum. In<br />this position the clavicles act as braces for the scapulae<br />and prevent the shoulders from coming too far forward.<br />Although the shoulder joint is capable of a wide<br />range of movement, the shoulder itself must be relatively<br />stable if these movements are to be effective.<br />The humerus is the long bone of the upper arm. In<br />Fig. 6–12, notice the deltoid tubercle (or tuberosity);<br />the triangular deltoid muscle that caps the shoulder<br />joint is anchored here. Proximally, the humerus forms<br />a ball-and-socket joint with the scapula. Distally, the<br />humerus forms a hinge joint with the ulna of the<br />forearm. This hinge joint, the elbow, permits movement<br />in one plane, that is, back and forth with no lateral<br />movement.<br />The forearm bones are the ulna on the little finger<br />side and the radius on the thumb side. The semilunar<br />notch of the ulna is part of the hinge joint of the<br />elbow; it articulates with the trochlea of the humerus.<br />The radius and ulna articulate proximally to form a<br />pivot joint, which permits turning the hand palm up<br />to palm down. You can demonstrate this yourself by<br />holding your arm palm up in front of you, and noting<br />that the radius and ulna are parallel to each other.<br />Then turn your hand palm down, and notice that your<br />122 The Skeletal System<br />BOX 6–4 ABNORMALITIES OF THE<br />CURVES OF THE SPINE<br />Scoliosis—an abnormal lateral curvature, which<br />may be congenital, the result of having one leg<br />longer than the other, or the result of chronic<br />poor posture during childhood while the vertebrae<br />are still growing. Usually the thoracic vertebrae<br />are affected, which displaces the rib cage to<br />one side. In severe cases, the abdominal organs<br />may be compressed, and the expansion of the<br />rib cage during inhalation may be impaired.<br />Kyphosis*—an exaggerated thoracic curve;<br />sometimes referred to as hunchback.<br />Lordosis*—an exaggerated lumbar curve;<br />sometimes referred to as swayback.<br />These abnormal curves are usually the result<br />of degenerative bone diseases such as osteoporosis<br />or tuberculosis of the spine. If osteoporosis,<br />for example, causes the bodies of the thoracic<br />vertebrae to collapse, the normal thoracic curve<br />will be increased. Most often the vertebral body<br />“settles” slowly (rather than collapses suddenly)<br />and there is little, if any, damage to the spinal<br />nerves. The damage to the vertebrae, however,<br />cannot be easily corrected, so these conditions<br />should be thought of in terms of prevention<br />rather than cure.<br />*Although descriptive of normal anatomy, the terms<br />kyphosis and lordosis, respectively, are commonly used to<br />describe the abnormal condition associated with each.<br />upper arm does not move. The radius crosses over the<br />ulna, which permits the hand to perform a great variety<br />of movements without moving the entire arm.<br />The carpals are eight small bones in the wrist;<br />gliding joints between them permit a sliding movement.<br />The carpals also articulate with the distal ends<br />of the ulna and radius, and with the proximal ends of<br />the metacarpals, the five bones of the palm of the<br />hand. All of the joints formed by the carpals and<br />metacarpals make the hand very flexible at the wrist<br />(try this yourself: flexion to extension should be almost<br />180 degrees), but the thumb is more movable than the<br />fingers because of its carpometacarpal joint. This is a<br />saddle joint, which enables the thumb to cross over<br />the palm, and permits gripping.<br />The phalanges are the bones of the fingers. There<br />are two phalanges in each thumb and three in each<br />of the fingers. Between phalanges are hinge joints,<br />which permit movement in one plane. Important<br />parts of the shoulder and arm bones are described<br />in Table 6–3.<br />THE HIP AND LEG<br />The pelvic girdle (or pelvic bone) consists of the two<br />hip bones (coxae or innominate bones), which articulate<br />with the axial skeleton at the sacrum. Each hip<br />bone has three major parts: the ilium, ischium, and<br />pubis, and these are shown in Fig. 6–13, which depicts<br />both a male and a female pelvis. The ilium is the<br />flared, upper portion that forms the sacroiliac joint.<br />The ischium is the lower, posterior part that we sit<br />on. The pubis is the lower, most anterior part. The<br />two pubic bones articulate with one another at the<br />pubic symphysis, with a disc of fibrous cartilage<br />between them. Notice the pubic angle of both the<br />male and female pelvises in Fig. 6–13. The wider<br />female angle is an adaptation for childbirth, in that it<br />helps make the pelvic outlet larger.<br />The acetabulum is the socket in the hip bone that<br />forms a ball-and-socket joint with the femur.<br />Compared to the glenoid fossa of the scapula, the<br />acetabulum is a much deeper socket. This has great<br />The Skeletal System 123<br />Manubrium<br />Body of sternum<br />Costal<br />cartilages<br />Xiphoid process<br />12th thoracic vertebra<br />1st rib<br />1st thoracic vertebra 2nd<br />3rd<br />4th<br />5th<br />6th<br />7th<br />8th<br />9th<br />10th<br />11th<br />12th<br />Figure 6–11. Rib cage. Anterior view.<br />QUESTION: With what bones do all of the ribs articulate?<br />•<br />•<br />•<br />•<br />•<br />•<br />•<br />•<br />•<br />•<br />• • •<br />• •<br />•<br />•<br />•<br />•<br />•<br />•<br />•<br />•<br />• •<br />Clavicle<br />Sternal end<br />Glenoid fossa<br />Scapula<br />Trochlea<br />Semilunar notch<br />Olecranon process<br />On posterior side<br />Ulna<br />Lunate<br />Triquetrum<br />Pisiform<br />Hamate<br />Carpals<br />Metacarpals<br />Phalanges<br />Capitate<br />Trapezoid<br />Trapezium<br />Scaphoid<br />Carpals<br />Radius<br />Radial tuberosity<br />Head<br />Capitulum<br />Humerus<br />Deltoid tubercle<br />Head<br />Coracoid process<br />Acromicon process<br />Acromial end<br />Figure 6–12. Bones of arm and shoulder girdle. Anterior view of right arm.<br />QUESTION: What types of joints are found in the arm? Begin at the shoulder and work<br />downward.<br />124<br />functional importance because the hip is a weightbearing<br />joint, whereas the shoulder is not. Because the<br />acetabulum is deep, the hip joint is not easily dislocated,<br />even by activities such as running and jumping<br />(landing), which put great stress on the joint.<br />The femur is the long bone of the thigh. As mentioned,<br />the femur forms a very movable ball-andsocket<br />joint with the hip bone. At the proximal end of<br />the femur are the greater and lesser trochanters, large<br />projections that are anchors for muscles. At its distal<br />end, the femur forms a hinge joint, the knee, with the<br />tibia of the lower leg. Notice in Fig. 6–14 that each<br />bone has condyles, which are the rounded projections<br />that actually form the joint. The patella, or kneecap,<br />is anterior to the knee joint, enclosed in the tendon of<br />the quadriceps femoris, a large muscle group of the<br />thigh.<br />The tibia is the weight-bearing bone of the lower<br />leg. You can feel the tibial tuberosity (a bump) and<br />anterior crest (a ridge) on the front of your own leg.<br />The medial malleolus, what we may call the “inner<br />ankle bone,” is at the distal end. Notice in Fig. 6–14<br />that the fibula is not part of the knee joint and does<br />not bear much weight. The lateral malleolus of the<br />fibula is the “outer ankle bone” you can find just above<br />your foot. Though not a weight-bearing bone, the<br />fibula is important in that leg muscles are attached and<br />anchored to it, and it helps stabilize the ankle. Two<br />bones on one is a much more stable arrangement than<br />one bone on one, and you can see that the malleoli of<br />the tibia and fibula overlap the sides of the talus. The<br />tibia and fibula do not form a pivot joint as do the<br />radius and ulna in the forearm; this also contributes to<br />the stability of the lower leg and foot and the support<br />of the entire body.<br />The tarsals are the seven bones in the ankle. As<br />you would expect, they are larger and stronger than<br />the carpals of the wrist, and their gliding joints do not<br />provide nearly as much movement. The largest is the<br />calcaneus, or heel bone; the talus transmits weight<br />between the calcaneus and the tibia. Metatarsals are<br />the five long bones of each foot, and phalanges are<br />the bones of the toes. There are two phalanges in the<br />big toe and three in each of the other toes. The phalanges<br />of the toes form hinge joints with each other.<br />Because there is no saddle joint in the foot, the big toe<br />is not as movable as the thumb. The foot has two<br />major arches, longitudinal and transverse, that are<br />supported by ligaments. These are adaptations for<br />walking completely upright, in that arches provide for<br />spring or bounce in our steps. Important parts of hip<br />and leg bones are described in Table 6–4.<br />The Skeletal System 125<br />Table 6–3 BONES OF THE SHOULDER AND ARM—IMPORTANT PARTS<br />Bone Part Description<br />Scapula<br />Clavicle<br />Humerus<br />Radius<br />Ulna<br />Carpals (8)<br />• Glenoid fossa<br />• Spine<br />• Acromion process<br />• Acromial end<br />• Sternal end<br />• Head<br />• Deltoid tubercle<br />• Olecranon fossa<br />• Capitulum<br />• Trochlea<br />• Head<br />• Olecranon process<br />• Semilunar notch<br />• Scaphoid • Lunate<br />• Triquetrum • Pisiform<br />• Trapezium • Trapezoid<br />• Capitate • Hamate<br />• Depression that articulates with humerus<br />• Long, posterior process for muscle attachment<br />• Articulates with clavicle<br />• Articulates with scapula<br />• Articulates with manubrium of sternum<br />• Round process that articulates with scapula<br />• Round process for the deltoid muscle<br />• Posterior, oval depression for the olecranon process<br />of the ulna<br />• Round process superior to radius<br />• Concave surface that articulates with ulna<br />• Articulates with the ulna<br />• Fits into olecranon fossa of humerus<br />• “Half-moon” depression that articulates with the<br />trochlea of ulna<br />• Proximal row<br />• Distal row<br />(text continued on page 128)<br />Figure 6–13. Hip bones and sacrum. (A) Male pelvis, anterior view. (B) Male pelvis, lateral<br />view of right side. (C) Female pelvis, anterior view. (D) Female pelvis, lateral view of<br />right side.<br />QUESTION: Compare the male and female pelvic inlets. What is the reason for this difference?<br />126<br />Ischium<br />Medial condyle<br />Medial condyle<br />Tibial tuberosity<br />Pubis<br />Tibia<br />Medial malleolus<br />Talus<br />Navicular<br />Tarsals<br />Cuneiforms<br />First<br />Second<br />Third<br />Acetabulum<br />Femur<br />Patella<br />Lateral condyle<br />Lateral condyle<br />Head<br />Greater trochanter<br />Neck<br />Lesser trochanter<br />Fibula<br />Lateral malleolus<br />Calcaneus<br />Cuboid<br />Tarsals<br />Metatarsals<br />Phalanges<br />A<br />Fibula<br />Talus<br />Calcaneus<br />Cuboid<br />Tibia<br />Phalanges Metatarsals<br />B<br />Anterior crest<br />Head<br />Figure 6–14. (A) Bones of the leg and portion of hip bone, anterior view of left leg.<br />(B) Lateral view of left foot.<br />QUESTION: What types of joints found in the arm do not have counterparts in the leg?<br />127<br />128 The Skeletal System<br />Table 6–4 BONES OF THE HIP AND LEG—IMPORTANT PARTS<br />Bone Part Description<br />Pelvic (2 hip bones)<br />Femur<br />Tibia<br />Fibula<br />Tarsals (7)<br />• Ilium<br />• Iliac crest<br />• Posterior superior iliac spine<br />• Ischium<br />• Pubis<br />• Pubic symphysis<br />• Acetabulum<br />• Head<br />• Neck<br />• Greater trochanter<br />• Lesser trochanter<br />• Condyles<br />• Condyles<br />• Tibial tuberosity<br />• Anterior crest<br />• Medial malleolus<br />• Head<br />• Lateral malleolus<br />• Calcaneus<br />• Talus<br />• Cuboid, navicular<br />• Cuneiform: 1st, 2nd, 3rd<br />• Flared, upper portion<br />• Upper edge of ilium<br />• Posterior continuation of iliac crest<br />• Lower, posterior portion<br />• Anterior, medial portion<br />• Joint between the 2 pubic bones<br />• Deep depression that articulates with femur<br />• Round process that articulates with hip bone<br />• Constricted portion distal to head<br />• Large lateral process for muscle attachment<br />• Medial process for muscle attachment<br />• Rounded processes that articulate with tibia<br />• Articulate with the femur<br />• Round process for the patellar ligament<br />• Vertical ridge<br />• Distal process; medial “ankle bone”<br />• Articulates with tibia<br />• Distal process; lateral “ankle bone”<br />• Heel bone<br />• Articulates with calcaneus and tibia<br />—— JOINTS—ARTICULATIONS<br />A joint is where two bones meet, or articulate.<br />THE CLASSIFICATION OF JOINTS<br />The classification of joints is based on the amount<br />of movement possible. A synarthrosis is an immovable<br />joint, such as a suture between two cranial bones.<br />An amphiarthrosis is a slightly movable joint, such<br />as the symphysis joint between adjacent vertebrae. A<br />diarthrosis is a freely movable joint. This is the largest<br />category of joints and includes the ball-and-socket<br />joint, the pivot, hinge, and others. Examples of each<br />type of joint are described in Table 6–5, and many of<br />these are illustrated in Fig. 6–15.<br />SYNOVIAL JOINTS<br />All diarthroses, or freely movable joints, are synovial<br />joints because they share similarities of structure. A<br />typical synovial joint is shown in Fig. 6–16. On the<br />joint surface of each bone is the articular cartilage,<br />which provides a smooth surface. The joint capsule,<br />made of fibrous connective tissue, encloses the joint in<br />a strong sheath, like a sleeve. Lining the joint capsule<br />is the synovial membrane, which secretes synovial<br />fluid into the joint cavity. Synovial fluid is thick and<br />slippery and prevents friction as the bones move.<br />Many synovial joints also have bursae (or bursas),<br />which are small sacs of synovial fluid between the joint<br />and the tendons that cross over the joint. Bursae permit<br />the tendons to slide easily as the bones are moved.<br />If a joint is used excessively, the bursae may become<br />inflamed and painful; this condition is called bursitis.<br />Some other disorders of joints are described in Box<br />6–5: Arthritis.<br />AGING AND THE SKELETAL SYSTEM<br />With age, bone tissue tends to lose more calcium than<br />is replaced. Bone matrix becomes thinner, the bones<br />themselves more brittle, and fractures are more likely<br />to occur with mild trauma.<br />Erosion of the articular cartilages of joints is also a<br />common consequence of aging. Joints affected include<br />weight-bearing joints such as the knees, and active,<br />small joints such as those of the fingers.<br />(text continued on page 131)<br />•<br />•<br />•<br />•<br />•<br />•<br />Carpals<br />•<br />•<br />Head of<br />femur<br />Acetabulum of<br />hip bone<br />•<br />•<br />Trochlea<br />of humerus<br />Semilunar notch<br />of ulna<br />Bodies of<br />vertebra<br />Intervertebral<br />disc<br />•<br />•<br />Metacarpal<br />of thumb<br />Trapezium<br />(carpal)<br />•<br />•<br />Atlas<br />Odontoid process<br />of axis<br />D Gliding<br />E Symphysis<br />F Saddle<br />A Ball and socket B Hinge C Pivot<br />Figure 6–15. Types of joints. For each type, a specific joint is depicted, and a simple<br />diagram shows the position of the joint surfaces. (A) Ball and socket. (B) Hinge. (C) Pivot.<br />(D) Gliding. (E) Symphysis. (F) Saddle.<br />QUESTION: Which of these types of joints is most movable? Which is least movable?<br />Table 6–5 TYPES OF JOINTS<br />Category Type and Description Examples<br />Synarthrosis (immovable)<br />Amphiarthrosis (slightly<br />movable)<br />Diarthrosis (freely movable)<br />Suture—fibrous connective tissue<br />between bone surfaces<br />Symphysis—disc of fibrous cartilage<br />between bones<br />Ball and socket—movement in all<br />planes<br />Hinge—movement in one plane<br />Condyloid—movement in one plane<br />with some lateral movement<br />Pivot—rotation<br />Gliding—side-to-side movement<br />Saddle—movement in several planes<br />• Between cranial bones; between<br />facial bones<br />• Between vertebrae; between pubic<br />bones<br />• Scapula and humerus; pelvic bone<br />and femur<br />• Humerus and ulna; femur and tibia;<br />between phalanges<br />• Temporal bone and mandible<br />• Atlas and axis; radius and ulna<br />• Between carpals<br />• Carpometacarpal of thumb<br />129<br />130<br />Bone<br />Synovial<br />membrane<br />Joint<br />capsule<br />Joint cavity<br />(synovial fluid)<br />Bursa<br />Articular<br />cartilage<br />Tendon<br />Bone<br />Figure 6–16. Structure of a synovial joint. See<br />text for description.<br />QUESTION: How can you tell that this is a joint<br />between two long bones? Give an example.<br />BOX 6–5 ARTHRITIS<br />bacterial and viral infections have been suggested<br />as possibilities.<br />Rheumatoid arthritis often begins in joints of the<br />extremities, such as those of the fingers. The<br />autoimmune activity seems to affect the synovial<br />membrane, and joints become painful and stiff.<br />Sometimes the disease progresses to total destruction<br />of the synovial membrane and calcification of<br />the joint. Such a joint is then fused and has no<br />mobility at all. Autoimmune damage may also<br />occur in the heart and blood vessels, and those with<br />RA are more prone to heart attacks and strokes (RA<br />is a systemic, not a localized, disease).<br />Treatment of rheumatoid arthritis is directed at<br />reducing inflammation as much as possible, for it is<br />the inflammatory process that causes the damage.<br />Therapies being investigated involve selectively<br />blocking specific aspects of the immune response,<br />such as antibody production. At present there is no<br />cure for autoimmune diseases.<br />The term arthritis means “inflammation of a<br />joint.” Of the many types of arthritis, we will consider<br />two: osteoarthritis and rheumatoid arthritis.<br />Osteoarthritis is a natural consequence of getting<br />older. In joints that have borne weight for<br />many years, the articular cartilage is gradually worn<br />away. The once smooth joint surface becomes<br />rough, and the affected joint is stiff and painful. As<br />you might guess, the large, weight-bearing joints<br />are most often subjected to this form of arthritis. If<br />we live long enough, most of us can expect some<br />osteoarthritis in knees, hips, or ankles.<br />Rheumatoid arthritis (RA) can be a truly crippling<br />disease that may begin in early middle age<br />or, less commonly, during adolescence. It is an<br />autoimmune disease, which means that the<br />immune system mistakenly directs its destructive<br />capability against part of the body. Exactly what<br />triggers this abnormal response by the immune<br />system is not known with certainty, but certain<br />Although the normal wear and tear of joints cannot<br />be prevented, elderly people can preserve their bone<br />matrix with exercise (dancing counts) and diets high in<br />calcium and vitamin D.<br />SUMMARY<br />Your knowledge of the bones and joints will be useful<br />in the next chapter as you learn the actions of the muscles<br />that move the skeleton. It is important to remember,<br />however, that bones have other functions as well.<br />As a storage site for excess calcium, bones contribute<br />to the maintenance of a normal blood calcium level.<br />The red bone marrow found in flat and irregular<br />bones produces the blood cells: red blood cells, white<br />blood cells, and platelets. Some bones protect vital<br />organs such as the brain, heart, and lungs. As you can<br />see, bones themselves may also be considered vital<br />organs.<br />The Skeletal System 131<br />STUDY OUTLINE<br />The skeleton is made of bone and cartilage<br />and has these functions:<br />1. Is a framework for support, connected by ligaments,<br />moved by muscles.<br />2. Protects internal organs from mechanical injury.<br />3. Contains and protects red bone marrow.<br />4. Stores excess calcium; important to regulate blood<br />calcium level.<br />Bone Tissue (see Fig. 6–1)<br />1. Osteocytes (cells) are found in the matrix of calcium<br />phosphate, calcium carbonate, and collagen.<br />2. Compact bone—haversian systems are present.<br />3. Spongy bone—no haversian systems; red bone<br />marrow present.<br />4. Articular cartilage—smooth, on joint surfaces.<br />5. Periosteum—fibrous connective tissue membrane;<br />anchors tendons and ligaments; has blood vessels<br />that enter the bone.<br />Classification of Bones<br />1. Long—arms, legs; shaft is the diaphysis (compact<br />bone) with a marrow cavity containing yellow bone<br />marrow (fat); ends are epiphyses (spongy bone) (see<br />Fig. 6–1).<br />2. Short—wrists, ankles (spongy bone covered with<br />compact bone).<br />3. Flat—ribs, pelvic bone, cranial bones (spongy bone<br />covered with compact bone).<br />4. Irregular—vertebrae, facial bones (spongy bone<br />covered with compact bone).<br />Embryonic Growth of Bone<br />1. The embryonic skeleton is first made of other tissues<br />that are gradually replaced by bone. Ossification<br />begins in the third month of gestation;<br />osteoblasts differentiate from fibroblasts and produce<br />bone matrix.<br />2. Cranial and facial bones are first made of fibrous<br />connective tissue; osteoblasts produce bone matrix<br />in a center of ossification in each bone; bone growth<br />radiates outward; fontanels remain at birth, permit<br />compression of infant skull during birth; fontanels<br />are calcified by age 2 (see Fig. 6–2).<br />3. All other bones are first made of cartilage; in a long<br />bone the first center of ossification is in the diaphysis,<br />other centers develop in the epiphyses. After<br />birth a long bone grows at the epiphyseal discs:<br />Cartilage is produced on the epiphysis side, and<br />bone replaces cartilage on the diaphysis side.<br />Osteoclasts form the marrow cavity by reabsorbing<br />bone matrix in the center of the diaphysis (see<br />Fig. 6–3).<br />Factors That Affect Bone Growth and<br />Maintenance<br />1. Heredity—many pairs of genes contribute to<br />genetic potential for height.<br />2. Nutrition—calcium, phosphorus, and protein become<br />part of the bone matrix; vitamin D is needed<br />for absorption of calcium in the small intestine;<br />vitamins C and A are needed for bone matrix production<br />(calcification).<br />3. Hormones—produced by endocrine glands; concerned<br />with cell division, protein synthesis, calcium<br />metabolism, and energy production (see Table 6–1).<br />4. Exercise or stress—weight-bearing bones must<br />bear weight or they will lose calcium and become<br />brittle.<br />The Skeleton—206 bones (see Fig. 6–4);<br />bones are connected by ligaments<br />1. Axial—skull, vertebrae, rib cage.<br />• Skull—see Figs. 6–5 through 6–8 and Table 6–2.<br />Eight cranial bones form the braincase, which<br />also protects the eyes and ears; 14 facial bones<br />make up the face; the immovable joints<br />between these bones are called sutures.<br />Paranasal sinuses are air cavities in the maxillae,<br />frontal, sphenoid, and ethmoid bones; they<br />lighten the skull and provide resonance for<br />voice (see Fig. 6–9).<br />Three auditory bones in each middle ear cavity<br />transmit vibrations for the hearing process.<br />• Vertebral column—see Fig. 6–10.<br />Individual bones are called vertebrae: 7 cervical,<br />12 thoracic, 5 lumbar, 5 sacral (fused into<br />one sacrum), 4 to 5 coccygeal (fused into one<br />coccyx). Supports trunk and head, encloses<br />and protects the spinal cord in the vertebral<br />canal. Discs of fibrous cartilage absorb shock<br />between the bodies of adjacent vertebrae, also<br />permit slight movement. Four natural curves<br />center head over body for walking upright (see<br />Table 6–5 for joints).<br />• Rib cage—see Fig. 6–11.<br />Sternum and 12 pairs of ribs; protects thoracic<br />and upper abdominal organs from mechanical<br />injury and is expanded to contribute to inhalation.<br />Sternum consists of manubrium, body,<br />and xiphoid process. All ribs articulate with<br />thoracic vertebrae; true ribs (first seven pairs)<br />articulate directly with sternum by means of<br />costal cartilages; false ribs (next three pairs)<br />articulate with 7th costal cartilage; floating<br />ribs (last two pairs) do not articulate with the<br />sternum.<br />2. Appendicular—bones of the arms and legs and the<br />shoulder and pelvic girdles.<br />• Shoulder and arm—see Fig. 6–12 and Table 6–3.<br />Scapula—shoulder muscles are attached; glenoid<br />fossa articulates with humerus.<br />Clavicle—braces the scapula.<br />Humerus—upper arm; articulates with the<br />scapula and the ulna (elbow).<br />Radius and ulna—forearm—articulate with<br />one another and with carpals.<br />Carpals—eight—wrist; metacarpals—five—<br />hand; phalanges—14—fingers (for joints, see<br />Table 6–5).<br />• Hip and leg—see Figs. 6–13 and 6–14 and Table<br />6–4.<br />Pelvic bone—two hip bones; ilium, ischium,<br />pubis; acetabulum articulates with femur.<br />Femur—thigh; articulates with pelvic bone<br />and tibia (knee).<br />Patella—kneecap; in tendon of quadriceps<br />femoris muscle.<br />Tibia and fibula—lower leg; tibia bears weight;<br />fibula does not bear weight, but does anchor<br />muscles and stabilizes ankle.<br />Tarsals—seven—ankle; calcaneus is heel bone.<br />Metatarsals—five—foot; phalanges—14—toes<br />(see Table 6–5 for joints).<br />Joints—Articulations<br />1. Classification based on amount of movement:<br />• Synarthrosis—immovable.<br />• Amphiarthrosis—slightly movable.<br />• Diarthrosis—freely movable (see Table 6–5 for<br />examples; see also Fig. 6–15).<br />2. Synovial joints—all diarthroses have similar structure<br />(see Fig. 6–16):<br />• Articular cartilage—smooth on joint surfaces.<br />• Joint capsule—strong fibrous connective tissue<br />sheath that encloses the joint.<br />• Synovial membrane—lines the joint capsule;<br />secretes synovial fluid that prevents friction.<br />• Bursae—sacs of synovial fluid that permit tendons<br />to slide easily across joints.<br />132 The Skeletal System<br />The Skeletal System 133<br />REVIEW QUESTIONS<br />1. Explain the differences between compact bone<br />and spongy bone, and state where each type is<br />found. (p. 106)<br />2. State the locations of red bone marrow, and name<br />the blood cells it produces. (p. 106)<br />3. Name the tissue of which the embryonic skull is<br />first made. Explain how ossification of cranial<br />bones occurs. (p. 108)<br />4. State what fontanels are, and explain their function.<br />(p. 108)<br />5. Name the tissue of which the embryonic femur<br />is first made. Explain how ossification of this<br />bone occurs. Describe what happens in epiphyseal<br />discs to produce growth of long bones.<br />(p. 108)<br />6. Explain what is meant by “genetic potential” for<br />height, and name the nutrients a child must have<br />in order to attain genetic potential. (p. 108)<br />7. Explain the functions of calcitonin and parathyroid<br />hormone with respect to bone matrix and to<br />blood calcium level. (p. 112)<br />8. Explain how estrogen or testosterone affects bone<br />growth, and when. (p. 112)<br />9. State one way each of the following hormones<br />helps promote bone growth: insulin, thyroxine,<br />growth hormone. (p. 112)<br />10. Name the bones that make up the braincase.<br />(p. 112)<br />11. Name the bones that contain paranasal sinuses<br />and explain the functions of these sinuses.<br />(pp. 116, 119)<br />12. Name the bones that make up the rib cage, and<br />describe two functions of the rib cage. (p. 122)<br />13. Describe the functions of the vertebral column.<br />State the number of each type of vertebra.<br />(pp. 119–120)<br />14. Explain how the shoulder and hip joints are similar<br />and how they differ. (pp. 122, 125)<br />15. Give a specific example (name two bones) for each<br />of the following types of joints: (p. 129)<br />a. Hinge<br />b. Symphysis<br />c. Pivot<br />d. Saddle<br />e. Suture<br />f. Ball and socket<br />16. Name the part of a synovial joint with each of the<br />following functions: (p. 128)<br />a. Fluid within the joint cavity that prevents friction<br />b. Encloses the joint in a strong sheath<br />c. Provides a smooth surface on bone surfaces<br />d. Lines the joint capsule and secretes synovial<br />fluid<br />17. Refer to the diagram (Fig. 6–4) of the full skeleton,<br />and point to each bone on yourself. (p. 114)<br />FOR FURTHER THOUGHT<br />1. Following a severe spinal cord injury in the lumbar<br />region, the voluntary muscles of the legs and hips<br />will be paralyzed. Describe the effects of paralysis<br />on the skeleton.<br />2. The sutures of the adult skull are joints that do not<br />allow movement. Why have joints at all if no movement<br />is permitted? Explain.<br />3. Without looking at any of the illustrations, try to<br />name all the bones that form the orbits, the sockets<br />for the eyes. Check your list with Figs. 6–5 and<br />6–6.<br />4. In an effort to prevent sudden infant death syndrome<br />(SIDS), parents were advised to put their<br />infants to sleep lying on their backs, not their<br />stomachs. Since then (1994), the number of SIDS<br />deaths has decreased markedly. What do you think<br />has happened to the skulls of many of those<br />infants? Explain.<br />5. A 5-month-old infant is brought to a clinic after<br />having diarrhea for 2 days. The nurse checks the<br />baby’s anterior fontanel and notices that it appears<br />sunken. What has caused this?<br />6. Look at the lateral view of the adult skull in Fig.<br />6–5 and notice the size of the face part in proportion<br />to the braincase part. Compare the infant skull<br />in Fig. 6–2, and notice how small the infant face is,<br />relative to the size of the braincase. There is a very<br />good reason for this. What do you think it is?<br />7. Look at the photograph here, Question Fig. 6–A.<br />Name the bone and the type of section in which it<br />is cut. What are the large arrows indicating? What<br />are the smaller arrows indicating? Look carefully<br />at this site, describe it, and explain possible consequences.<br />134 The Skeletal System<br />Question Figure 6–A (Photograph by Dan Kaufman.internet fast worldhttp://www.blogger.com/profile/13869077830569899582noreply@blogger.com1tag:blogger.com,1999:blog-135611804747902727.post-40764030856227707382010-06-27T05:45:00.000-07:002010-06-27T06:01:14.918-07:00chemistryCHAPTER 2<br />Some Basic Chemistry<br />21<br />CHAPTER 2<br />Chapter Outline<br />Elements<br />Atoms<br />Chemical Bonds<br />Ionic Bonds<br />Covalent Bonds<br />Disulfide Bonds and Hydrogen Bonds<br />Chemical Reactions<br />Inorganic Compounds of Importance<br />Water<br />Water Compartments<br />Oxygen<br />Carbon Dioxide<br />Cell Respiration<br />Trace Elements<br />Acids, Bases, and pH<br />Buffer systems<br />Organic Compounds of Importance<br />Carbohydrates<br />Lipids<br />Proteins<br />Enzymes<br />Nucleic Acids<br />DNA and RNA<br />ATP<br />BOX 2–1 BLOOD GASES<br />BOX 2–2 NITRIC OXIDE<br />BOX 2–3 LIPIDS IN THE BLOOD<br />BOX 2–4 A PROTEIN MYSTERY: PRIONS<br />Student Objectives<br />• Define the terms element, atom, proton, neutron, and<br />electron.<br />• Describe the formation and purpose of ionic<br />bonds, covalent bonds, disulfide bonds, and hydrogen<br />bonds.<br />• Describe what happens in synthesis and decomposition<br />reactions.<br />• Explain the importance of water to the functioning<br />of the human body.<br />• Name and describe the water compartments.<br />• Explain the roles of oxygen and carbon dioxide in<br />cell respiration.<br />• State what trace elements are, and name some,<br />with their functions.<br />• Explain the pH scale. State the normal pH ranges<br />of body fluids.<br />• Explain how a buffer system limits great changes<br />in pH.<br />• Describe the functions of monosaccharides, disaccharides,<br />oligosaccharides, and polysaccharides.<br />• Describe the functions of true fats, phospholipids,<br />and steroids.<br />• Describe the functions of proteins, and explain<br />how enzymes function as catalysts.<br />• Describe the functions of DNA, RNA, and ATP.<br />22<br />Some Basic Chemistry<br />23<br />New Terminology<br />Acid (ASS-sid)<br />Amino acid (ah-MEE-noh ASS-sid)<br />Atom (A-tum)<br />Base (BAYSE)<br />Buffer system (BUFF-er SIS-tem)<br />Carbohydrates (KAR-boh-HIGH-drayts)<br />Catalyst (KAT-ah-list)<br />Cell respiration (SELL RES-pi-RAY-shun)<br />Covalent bond (ko-VAY-lent)<br />Dissociation/ionization (dih-SEW-see-AYshun/<br />EYE-uh-nih-ZAY-shun)<br />Element (EL-uh-ment)<br />Enzyme (EN-zime)<br />Extracellular fluid (EKS-trah-SELL-yoo-ler)<br />Intracellular fluid (IN-trah-SELL-yoo-ler)<br />Ion (EYE-on)<br />Ionic bond (eye-ON-ik)<br />Lipids (LIP-ids)<br />Matter (MAT-ter)<br />Molecule (MAHL-e-kuhl)<br />Nucleic acids (new-KLEE-ik ASS-sids)<br />pH and pH scale (pee-h SKALE)<br />Protein (PROH-teen)<br />Salt (SAWLT)<br />Solvent/solution (SAHL-vent/suh-LOO-shun)<br />Steroid (STEER-oyd)<br />Trace elements (TRAYSE EL-uh-ments)<br />Related Clinical Terminology<br />Acidosis (ASS-i-DOH-sis)<br />Atherosclerosis (ATH-er-oh-skle-ROH-sis)<br />Hypoxia (high-POK-see-ah)<br />Saturated fats (SAT-uhr-ay-ted)<br />Unsaturated (un-SAT-uhr-ay-ted) fats<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />When you hear or see the word chemistry, you may<br />think of test tubes and Bunsen burners in a laboratory<br />experiment. However, literally everything in our physical<br />world is made of chemicals. The paper used for<br />this book, which was once the wood of a tree, is made<br />of chemicals. The air we breathe is a mixture of chemicals<br />in the form of gases. Water, gasoline, and diet<br />soda are chemicals in liquid form. Our foods are<br />chemicals, and our bodies are complex arrangements<br />of thousands of chemicals. Recall from Chapter 1 that<br />the simplest level of organization of the body is the<br />chemical level.<br />This chapter covers some very basic aspects of<br />chemistry as they are related to living organisms, and<br />most especially as they are related to our understanding<br />of the human body. So try to think of chemistry<br />not as a complicated science, but as the air, water, and<br />food we need, and every substance that is part of us.<br />ELEMENTS<br />All matter, both living and not living, is made of elements,<br />the simplest chemicals. An element is a substance<br />made of only one type of atom (therefore, an<br />atom is the smallest part of an element). There are 92<br />naturally occurring elements in the world around us.<br />Examples are hydrogen (H), iron (Fe), oxygen (O),<br />calcium (Ca), nitrogen (N), and carbon (C). In nature,<br />an element does not usually exist by itself but rather<br />combines with the atoms of other elements to form<br />compounds. Examples of some compounds important<br />to our study of the human body are water (H2O), in<br />which two atoms of hydrogen combine with one atom<br />of oxygen; carbon dioxide (CO2), in which an atom of<br />carbon combines with two atoms of oxygen; and glucose<br />(C6H12O6), in which six carbon atoms and six<br />oxygen atoms combine with 12 hydrogen atoms.<br />The elements carbon, hydrogen, oxygen, nitrogen,<br />phosphorus, and sulfur are found in all living things. If<br />calcium is included, these seven elements make up<br />approximately 99% of the human body (weight).<br />More than 20 different elements are found, in varying<br />amounts, in the human body. Some of these are<br />listed in Table 2–1. As you can see, each element has a<br />standard chemical symbol. This is simply the first (and<br />sometimes the second) letter of the element’s English<br />or Latin name. You should know the symbols of the<br />elements in this table, because they are used in text-<br />24 Some Basic Chemistry<br />Table 2–1 ELEMENTS IN THE<br />HUMAN BODY<br />Percent of<br />Atomic the Body<br />Elements Symbol Number* by Weight<br />Hydrogen H 1 9.5<br />Carbon C 6 18.5<br />Nitrogen N 7 3.3<br />Oxygen O 8 65.0<br />Fluorine F 9 Trace<br />Sodium Na 11 0.2<br />Magnesium Mg 12 0.1<br />Phosphorus P 15 1.0<br />Sulfur S 16 0.3<br />Chlorine Cl 17 0.2<br />Potassium K 19 0.4<br />Calcium Ca 20 1.5<br />Manganese Mn 25 Trace<br />Iron Fe 26 Trace<br />Cobalt Co 27 Trace<br />Copper Cu 29 Trace<br />Zinc Zn 30 Trace<br />Iodine I 53 Trace<br />*Atomic number is the number of protons in the nucleus of<br />the atom. It also represents the number of electrons that<br />orbit the nucleus.<br />books, articles, hospital lab reports, and so on. Notice<br />that if a two-letter symbol is used for an element, the<br />second letter is always lowercase, not a capital. For<br />example, the symbol for calcium is Ca, not CA. CA is<br />an abbreviation often used for cancer.<br />ATOMS<br />Atoms are the smallest parts of an element that have<br />the characteristics of that element. An atom consists of<br />three major subunits or particles: protons, neutrons,<br />and electrons (Fig. 2–1). A proton has a positive electrical<br />charge and is found in the nucleus (or center) of<br />the atom. A neutron is electrically neutral (has no<br />charge) and is also found in the nucleus. An electron<br />has a negative electrical charge and is found outside<br />the nucleus orbiting in what may be called an electron<br />cloud or shell around the nucleus.<br />The number of protons in an atom gives it its<br />atomic number. Protons and neutrons have mass and<br />weight; they give an atom its atomic weight. In an<br />atom, the number of protons ( ) equals the number of<br />electrons ( ); therefore, an atom is electrically neutral.<br />The electrons, however, are important in that<br />they may enable an atom to connect, or bond, to other<br />atoms to form molecules. A molecule is a combination<br />of atoms (usually of more than one element) that<br />are so tightly bound together that the molecule<br />behaves as a single unit.<br />Each atom is capable of bonding in only very specific<br />ways. This capability depends on the number and<br />the arrangement of the electrons of the atom.<br />Electrons orbit the nucleus of an atom in shells or<br />energy levels. The first, or innermost, energy level<br />can contain a maximum of two electrons and is then<br />considered stable. The second energy level is stable<br />when it contains its maximum of eight electrons. The<br />remaining energy levels, more distant from the<br />nucleus, are also most stable when they contain eight<br />electrons, or a multiple of eight.<br />A few atoms (elements) are naturally stable, or<br />uninterested in reacting, because their outermost<br />energy level already contains the maximum number of<br />electrons. The gases helium and neon are examples of<br />these stable atoms, which do not usually react with<br />other atoms. Most atoms are not stable, however, and<br />tend to gain, lose, or share electrons in order to fill<br />their outermost shell. By doing so, an atom is capable<br />of forming one or more chemical bonds with other<br />atoms. In this way, the atom becomes stable, because<br />its outermost shell of electrons has been filled. It is<br />these reactive atoms that are of interest in our study of<br />anatomy and physiology.<br />CHEMICAL BONDS<br />A chemical bond is not a structure, but rather a force<br />or attraction between positive and negative electrical<br />charges that keeps two or more atoms closely associated<br />with each other to form a molecule. By way of<br />comparison, think of gravity. We know that gravity is<br />not a “thing,” but rather the force that keeps our feet<br />on the floor and allows us to pour coffee with consistent<br />success. Molecules formed by chemical bonding<br />often have physical characteristics different from those<br />of the atoms of the original elements. For example,<br />the elements hydrogen and oxygen are gases, but<br />atoms of each may chemically bond to form molecules<br />of water, which is a liquid.<br />The type of chemical bonding depends upon the<br />tendencies of the electrons of atoms involved, as you<br />will see. Four kinds of bonds are very important to the<br />chemistry of the body: ionic bonds, covalent bonds,<br />disulfide bonds, and hydrogen bonds.<br />IONIC BONDS<br />An ionic bond involves the loss of one or more electrons<br />by one atom and the gain of the electron(s) by<br />another atom or atoms. Refer to Fig. 2–2 as you read<br />the following.<br />An atom of sodium (Na) has one electron in its outermost<br />shell, and in order to become stable, it tends to<br />lose that electron. When it does so, the sodium atom<br />has one more proton than it has electrons. Therefore,<br />it now has an electrical charge (or valence) of 1 and<br />is called a sodium ion (Na ). An atom of chlorine has<br />seven electrons in its outermost shell, and in order to<br />become stable tends to gain one electron. When it<br />does so, the chlorine atom has one more electron than<br />it has protons, and now has a charge (valence) of 1.<br />It is called a chloride ion (Cl ).<br />When an atom of sodium loses an electron to an<br />atom of chlorine, their ions have unlike charges (positive<br />and negative) and are thus attracted to one<br />another. The result is the formation of a molecule of<br />sodium chloride: NaCl, or common table salt. The<br />bond that holds these ions together is called an ionic<br />bond.<br />Some Basic Chemistry 25<br />Second energy level<br />First energy level<br />Proton [+]<br />Neutron<br />Nucleus<br />Electrons [--]<br />Figure 2–1. An atom of carbon. The nucleus contains<br />six protons and six neutrons (not all are visible here). Six<br />electrons orbit the nucleus, two in the first energy level<br />and four in the second energy level.<br />QUESTION: What is the electrical charge of this atom as<br />a whole?<br />Another example is the bonding of chlorine to calcium.<br />An atom of calcium has two electrons in its outermost<br />shell and tends to lose those electrons in order<br />to become stable. If two atoms of chlorine each gain<br />one of those electrons, they become chloride ions.<br />The positive and negative ions are then attracted to<br />one another, forming a molecule of calcium chloride,<br />CaCl2, which is also a salt. A salt is a molecule made<br />of ions other than hydrogen (H ) ions or hydroxyl<br />(OH ) ions.<br />Ions with positive charges are called cations. These<br />include Na , Ca 2, K , Fe 2, and Mg 2. Ions with<br />negative charges are called anions, which include Cl ,<br />SO4<br /> 2 (sulfate), and HCO3<br /> (bicarbonate). The types<br />of compounds formed by ionic bonding are salts,<br />acids, and bases. (Acids and bases are discussed later in<br />this chapter.)<br />In the solid state, ionic bonds are relatively strong.<br />Our bones, for example, contain the salt calcium carbonate<br />(CaCO3), which helps give bone its strength.<br />However, in an aqueous (water) solution, many ionic<br />bonds are weakened. The bonds may become so weak<br />that the bound ions of a molecule separate, creating a<br />solution of free positive and negative ions. For example,<br />if sodium chloride is put in water, it dissolves, then<br />ionizes. The water now contains Na ions and Cl <br />ions. Ionization, also called dissociation, is important<br />to living organisms because once dissociated, the ions<br />are free to take part in other chemical reactions within<br />the body. Cells in the stomach lining produce<br />hydrochloric acid (HCl) and must have Cl ions to do<br />so. The chloride in NaCl would not be free to take<br />part in another reaction since it is tightly bound to the<br />sodium atom. However, the Cl ions available from<br />ionized NaCl in the cellular water can be used for the<br />synthesis, or chemical manufacture, of HCl in the<br />stomach.<br />COVALENT BONDS<br />Covalent bonds involve the sharing of electrons<br />between atoms. As shown in Fig. 2–3, an atom of oxygen<br />needs two electrons to become stable. It may<br />share two of its electrons with another atom of oxygen,<br />also sharing two electrons. Together they form a<br />molecule of oxygen gas (O2), which is the form in<br />which oxygen exists in the atmosphere.<br />An atom of oxygen may also share two of its electrons<br />with two atoms of hydrogen, each sharing its<br />single electron (see Fig. 2–3). Together they form a<br />molecule of water (H2O). When writing structural<br />formulas for chemical molecules, a pair of shared electrons<br />is indicated by a single line, as shown in the formula<br />for water; this is a single covalent bond. A double<br />covalent bond is indicated by two lines, as in the formula<br />for oxygen; this represents two pairs of shared<br />electrons.<br />The element carbon always forms covalent bonds;<br />an atom of carbon has four electrons to share with<br />other atoms. If these four electrons are shared with<br />four atoms of hydrogen, each sharing its one electron,<br />a molecule of methane gas (CH4) is formed. Carbon<br />may form covalent bonds with other carbons, hydrogen,<br />oxygen, nitrogen, or other elements. Organic<br />26 Some Basic Chemistry<br />Na + Cl = NaCl<br />+ –<br />Figure 2–2. Formation of an ionic bond. An atom of sodium loses an electron to an<br />atom of chlorine. The two ions formed have unlike charges, are attracted to one another,<br />and form a molecule of sodium chloride.<br />QUESTION: Why is the charge of a sodium ion 1?<br />compounds such as proteins and carbohydrates are<br />complex and precise arrangements of these atoms<br />covalently bonded to one another. Covalent bonds are<br />relatively strong and are not weakened in an aqueous<br />solution. This is important because the proteins produced<br />by the body, for example, must remain intact in<br />order to function properly in the water of our cells and<br />blood. The functions of organic compounds will be<br />considered later in this chapter.<br />DISULFIDE BONDS AND<br />HYDROGEN BONDS<br />Two other types of bonds that are important to<br />the chemistry of the body are disulfide bonds and<br />hydrogen bonds. Disulfide bonds are found in some<br />proteins. Hydrogen bonds are part of many different<br />molecules.<br />A disulfide bond (also called a disulfide bridge) is a<br />covalent bond formed between two atoms of sulfur,<br />usually within the same large protein molecule. The<br />hormone insulin, for example, is a protein that must<br />have a very specific three-dimensional shape in order<br />to function properly to regulate the blood glucose<br />level. Each molecule of insulin has two disulfide bonds<br />that help maintain its proper shape and function (see<br />Box Fig. 10–A). Other proteins with shapes that<br />depend upon disulfide bonds are antibodies of the<br />immune system (see Fig. 14–8) and keratin of the skin<br />and hair.<br />Some Basic Chemistry 27<br />Figure 2–3. Formation of covalent bonds. (A) Two atoms of oxygen share two electrons<br />each, forming a molecule of oxygen gas. (B) An atom of oxygen shares one electron with<br />each of two hydrogen atoms, each sharing its electron. A molecule of water is formed.<br />QUESTION: Which of the bonds shown here is a double covalent bond?<br />O + O = O2 O=O<br />8+ + 8+ = 8+ 8+<br />A<br />O + H+H = H2O O<br />H H<br />8+ + =<br />1+<br />1+<br />1+ 1+<br />8+<br />B<br />A strand of hair maintains its shape (a genetic characteristic)<br />because of disulfide bonds. When naturally<br />curly hair is straightened, the disulfide bonds in the<br />keratin molecules are broken. When naturally straight<br />hair is “permed” or curled, the disulfide bonds in the<br />keratin are first broken, then re-formed in the curled<br />hair. Neither process affects the living part of the hair,<br />the hair root, so the hair will grow out in its original<br />shape. We would not want such a process affecting our<br />insulin or antibody molecules, for that would destroy<br />their functioning.<br />A hydrogen bond does not involve the sharing or<br />exchange of electrons, but rather results because of a<br />property of hydrogen atoms. When a hydrogen atom<br />shares its one electron in a covalent bond with another<br />atom, its proton has a slight positive charge and may<br />then be attracted to a nearby oxygen or nitrogen atom,<br />which has a slight negative charge.<br />Although they are weak bonds, hydrogen bonds are<br />important in several ways. Large organic molecules<br />such as proteins and DNA have very specific functions<br />that depend upon their three-dimensional shapes. The<br />shapes of these molecules, so crucial to their proper<br />functioning, are often maintained by hydrogen bonds.<br />Hydrogen bonds also make water cohesive; that is,<br />each water molecule is attracted to nearby water molecules.<br />Such cohesiveness can be seen if water is<br />dropped onto clean glass; the surface tension created<br />by the hydrogen bonds makes the water form threedimensional<br />beads. Within the body, the cohesiveness<br />of water helps keep blood a continuous stream as it<br />flows within the blood vessels, and keeps tissue fluid<br />continuous around cells. These hydrogen bonds are<br />also responsible for the other important characteristics<br />of water, which are discussed in a later section.<br />CHEMICAL REACTIONS<br />A chemical reaction is a change brought about by the<br />formation or breaking of chemical bonds. Two general<br />types of reactions are synthesis reactions and decomposition<br />reactions.<br />In a synthesis reaction, bonds are formed to join<br />two or more atoms or molecules to make a new compound.<br />The production of the protein hemoglobin in<br />potential red blood cells is an example of a synthesis<br />reaction. Proteins are synthesized by the bonding of<br />many amino acids, their smaller subunits. Synthesis<br />reactions require energy for the formation of bonds.<br />In a decomposition reaction, bonds are broken,<br />and a large molecule is changed to two or more<br />smaller ones. One example is the digestion of large<br />molecules of starch into many smaller glucose molecules.<br />Some decomposition reactions release energy;<br />this is described in a later section on cell respiration.<br />In this and future chapters, keep in mind that the<br />term reaction refers to the making or breaking of<br />chemical bonds and thus to changes in the physical<br />and chemical characteristics of the molecules<br />involved.<br />INORGANIC COMPOUNDS<br />OF IMPORTANCE<br />Inorganic compounds are usually simple molecules<br />that often consist of only one or two different elements.<br />Despite their simplicity, however, some inorganic<br />compounds are essential to normal structure and<br />functioning of the body.<br />WATER<br />Water makes up 60% to 75% of the human body, and<br />is essential to life for several reasons:<br />1. Water is a solvent; that is, many substances (called<br />solutes) can dissolve in water. Nutrients such as<br />glucose are dissolved in blood plasma (which is<br />largely water) to be transported to cells throughout<br />the body. The sense of taste depends upon the solvent<br />ability of saliva; dissolved food stimulates the<br />receptors in taste buds. The excretion of waste<br />products is possible because they are dissolved in<br />the water of urine.<br />2. Water is a lubricant, which prevents friction where<br />surfaces meet and move. In the digestive tract,<br />swallowing depends upon the presence of saliva,<br />and mucus is a slippery fluid that permits the<br />smooth passage of food through the intestines.<br />Synovial fluid within joint cavities prevents friction<br />as bones move.<br />3. Water changes temperature slowly. Water has a high<br />heat capacity, which means that it will absorb a<br />great deal of heat before its temperature rises significantly,<br />or it must lose a great deal of heat before<br />its temperature drops significantly. This is one of<br />the factors that helps the body maintain a constant<br />temperature. Water also has a high heat of vaporization,<br />which is important for the process of<br />28 Some Basic Chemistry<br />sweating. Excess body heat evaporates sweat on the<br />skin surfaces, rather than overheating the body’s<br />cells, and because of water’s high heat of vaporization,<br />a great deal of heat can be given off with the<br />loss of a relatively small amount of water.<br />WATER COMPARTMENTS<br />All water within the body is continually moving, but<br />water is given different names when it is in specific<br />body locations, which are called compartments (Fig.<br />2–4).<br />Intracellular fluid (ICF)—the water within cells;<br />about 65% of the total body water<br />Extracellular fluid (ECF)—all the rest of the water<br />in the body; about 35% of the total. More specific<br />compartments of extracellular fluid include:<br />Plasma—water found in blood vessels<br />Lymph—water found in lymphatic vessels<br />Tissue fluid or interstitial fluid—water found in<br />the small spaces between cells<br />Specialized fluids—synovial fluid, cerebrospinal<br />fluid, aqueous humor in the eye, and others<br />The movement of water between compartments in<br />the body and the functions of the specialized fluids<br />will be discussed in later chapters.<br />OXYGEN<br />Oxygen in the form of a gas (O2) is approximately<br />21% of the atmosphere, which we inhale. We all know<br />that without oxygen we wouldn’t survive very long,<br />but exactly what does it do? Oxygen is important to us<br />because it is essential for a process called cell respiration,<br />in which cells break down simple nutrients such<br />as glucose in order to release energy. The reason we<br />breathe is to obtain oxygen for cell respiration and to<br />exhale the carbon dioxide produced in cell respiration<br />(this will be discussed in the next section). Biologically<br />useful energy that is released by the reactions of cell<br />respiration is trapped in a molecule called ATP<br />(adenosine triphosphate). ATP can then be used for<br />cellular processes that require energy.<br />CARBON DIOXIDE<br />Carbon dioxide (CO2) is produced by cells as a waste<br />product of cell respiration. You may ask why a waste<br />product is considered important. Keep in mind that<br />“important” does not always mean “beneficial,” but it<br />Some Basic Chemistry 29<br />Figure 2–4. Water compartments, showing the names water is given in its different locations<br />and the ways in which water moves between compartments.<br />QUESTION: Which of the fluids shown are extracellular fluids?<br />Fluid<br />movement<br />Intracellular fluid Cell<br />Capillary<br />Lymph<br />capillary<br />Lymph<br />Interstitial<br />(tissue) fluid<br />Plasma<br />does mean “significant.” If the amount of carbon dioxide<br />in the body fluids increases, it causes these fluids<br />to become too acidic. Therefore, carbon dioxide must<br />be exhaled as rapidly as it is formed to keep the<br />amount in the body within normal limits. Normally<br />this is just what happens, but severe pulmonary diseases<br />such as pneumonia or emphysema decrease gas<br />exchange in the lungs and permit carbon dioxide to<br />accumulate in the blood. When this happens, a person<br />is said to be in a state of acidosis, which may seriously<br />disrupt body functioning (see the sections on pH and<br />enzymes later in this chapter; see also Box 2–1: Blood<br />Gases).<br />CELL RESPIRATION<br />Cell respiration is the name for energy production<br />within cells and involves both respiratory gases, oxygen<br />and carbon dioxide. Many chemical reactions are<br />involved, but in its simplest form, cell respiration may<br />be summarized by the following equation:<br />Glucose (C6H12O6) 6O2 → 6CO2 6H2O ATP heat<br />This reaction shows us that glucose and oxygen<br />combine to yield carbon dioxide, water, ATP, and heat.<br />Food, represented here by glucose, in the presence of<br />oxygen is broken down into the simpler molecules<br />carbon dioxide and water. The potential energy in the<br />glucose molecule is released in two forms: ATP and<br />heat. Each of the four products of this process has a<br />purpose or significance in the body. The carbon dioxide<br />is a waste product that moves from the cells into<br />the blood to be carried to the lungs and eventually<br />exhaled. The water formed is useful and becomes part<br />of the intracellular fluid. The heat produced contributes<br />to normal body temperature. ATP is used for<br />cell processes such as mitosis, protein synthesis, and<br />muscle contraction, all of which require energy and<br />will be discussed a bit further on in the text.<br />We will also return to cell respiration in later chapters.<br />For now, the brief description just given will suffice<br />to show that eating and breathing are interrelated;<br />both are essential for energy production.<br />TRACE ELEMENTS<br />Trace elements are those that are needed by the body<br />in very small amounts. When they are present in food<br />or nutritional supplements, we often call them minerals,<br />and examples are iron, cobalt, and zinc. Although<br />they may not be as abundant in the body as are carbon,<br />hydrogen, or oxygen, they are nonetheless essential.<br />Table 2–2 lists some of these trace elements and their<br />functions (see also Box 2–2: Nitric Oxide).<br />30 Some Basic Chemistry<br />BOX 2–1 BLOOD GASES<br />oxygen falls below the normal range, oxygen will<br />be administered; if blood carbon dioxide rises<br />above the normal range, blood pH will be corrected<br />to prevent serious acidosis.<br />Damage to the heart may also bring about a<br />change in blood gases, especially oxygen. Oxygen<br />is picked up by red blood cells as they circulate<br />through lung capillaries; as red blood cells circulate<br />through the body, they release oxygen to tissues.<br />What keeps the blood circulating or moving? The<br />pumping of the heart.<br />A mild heart attack, when heart failure is unlikely,<br />is often characterized by a blood oxygen level that<br />is low but still within normal limits. A more severe<br />heart attack that seriously impairs the pumping of<br />the heart will decrease the blood oxygen level to<br />less than normal. This condition is called hypoxia,<br />which means that too little oxygen is reaching tissues.<br />When this is determined by measurement of<br />blood gases, appropriate oxygen therapy can be<br />started to correct the hypoxia.<br />A patient is admitted to the emergency room with<br />a possible heart attack, and the doctor in charge<br />orders “blood gases.” Another patient hospitalized<br />with pneumonia has “blood gases” monitored at<br />frequent intervals. What are blood gases, and what<br />does measurement of them tell us? The blood gases<br />are oxygen and carbon dioxide, and their levels<br />in arterial blood provide information about the<br />functioning of the respiratory and circulatory<br />systems. Arterial blood normally has a high concentration<br />of oxygen and a low concentration of<br />carbon dioxide. These levels are maintained by<br />gas exchange in the lungs and by the proper circulation<br />of blood.<br />A pulmonary disease such as pneumonia interferes<br />with efficient gas exchange in the lungs. As a<br />result, blood oxygen concentration may decrease,<br />and blood carbon dioxide concentration may<br />increase. Either of these changes in blood gases<br />may become life threatening for the patient, so<br />monitoring of blood gases is important. If blood<br />ACIDS, BASES, AND pH<br />An acid may be defined as a substance that increases<br />the concentration of hydrogen ions (H ) in a water<br />solution. A base is a substance that decreases the concentration<br />of H ions, which, in the case of water, has<br />the same effect as increasing the concentration of<br />hydroxyl ions (OH ).<br />The acidity or alkalinity (basicity) of a solution is<br />measured on a scale of values called pH (parts hydrogen).<br />The values on the pH scale range from 0 to 14,<br />with 0 indicating the most acidic level and 14 the most<br />alkaline. A solution with a pH of 7 is neutral because<br />it contains the same number of H ions and OH <br />ions. Pure water has a pH of 7. A solution with a<br />higher concentration of H ions than OH ions is an<br />acidic solution with a pH below 7. An alkaline solution,<br />therefore, has a higher concentration of OH <br />ions than H ions and has a pH above 7.<br />The pH scale, with the relative concentrations<br />of H ions and OH ions, is shown in Fig. 2–5. A<br />change of one pH unit is a 10-fold change in H ion<br />concentration. This means that a solution with a pH<br />of 4 has 10 times as many H ions as a solution with a<br />pH of 5, and 100 times as many H ions as a solution<br />with a pH of 6. Figure 2–5 also shows the pH of some<br />body fluids and other familiar solutions. Notice that<br />gastric juice has a pH of 1 and coffee has a pH of 5.<br />This means that gastric juice has 10,000 times as many<br />H ions as does coffee. Although coffee is acidic, it is<br />a weak acid and does not have the corrosive effect of<br />gastric juice, a strong acid.<br />The cells and internal fluids of the human body<br />have a pH close to neutral. The pH of intracellular<br />fluid is around 6.8, and the normal pH range of blood<br />is 7.35 to 7.45. Fluids such as gastric juice and urine<br />are technically external fluids, because they are in<br />body tracts that open to the environment. The pH of<br />these fluids may be more strongly acidic or alkaline<br />without harm to the body.<br />The pH of blood, however, must be maintained<br />within its very narrow, slightly alkaline range. A<br />decrease of only one pH unit, which is 10 times as<br />many H ions, would disrupt the chemical reactions<br />of the blood and cause the death of the individual.<br />Normal metabolism tends to make body fluids more<br />Some Basic Chemistry 31<br />Table 2–2 TRACE ELEMENTS<br />Element Function<br />Calcium<br />Phosphorus<br />Iron<br />Copper<br />Sodium and<br />potassium<br />Sulfur<br />Cobalt<br />Iodine<br />• Provides strength in bones and teeth<br />• Necessary for blood clotting<br />• Necessary for muscle contraction<br />• Provides strength in bones and teeth<br />• Part of DNA, RNA, and ATP<br />• Part of cell membranes<br />• Part of hemoglobin in red blood<br />cells; transports oxygen<br />• Part of myoglobin in muscles; stores<br />oxygen<br />• Necessary for cell respiration<br />• Necessary for cell respiration<br />• Necessary for hemoglobin synthesis<br />• Necessary for muscle contraction<br />• Necessary for nerve impulse transmission<br />• Part of some proteins such as insulin<br />and keratin<br />• Part of vitamin B12<br />• Part of thyroid hormones—thyroxine<br />BOX 2–2 NITRIC OXIDE<br />Nitric oxide is a gas with the molecular formula<br />NO. You have probably heard of it as a component<br />of air pollution and cigarette smoke, but it<br />is synthesized by several human tissues, and<br />this deceptively simple molecule has important<br />functions.<br />Nitric oxide is produced by the endothelium<br />(lining) of blood vessels and promotes vasodilation<br />of arterioles, permitting greater blood flow<br />and oxygen delivery to tissues. It is involved in<br />nerve impulse transmission in the brain, and may<br />contribute to memory storage. Some immune<br />system cells produce nitric oxide as a cytotoxic<br />(cell-poisoning) agent to help destroy foreign<br />cells such as bacteria.<br />Nitric oxide is also being used therapeutically<br />in clinical trials. It has been found useful in the<br />treatment of pulmonary hypertension to relax<br />abnormally constricted arteries in the lungs to<br />permit normal gas exchange. Other studies<br />show that nitric oxide helps some premature<br />babies breathe more easily and efficiently.<br />Much more research is needed, including a<br />determination of possible harmful side effects of<br />greater than normal amounts of nitric oxide, but<br />the results of some clinical trials thus far are<br />promising.<br />acidic, and this tendency to acidosis must be continually<br />corrected. Normal pH of internal fluids is maintained<br />by the kidneys, respiratory system, and buffer<br />systems. Although acid–base balance will be a major<br />topic of Chapter 19, we will briefly mention buffer<br />systems here.<br />Buffer Systems<br />A buffer system is a chemical or pair of chemicals<br />that minimizes changes in pH by reacting with strong<br />acids or strong bases to transform them into substances<br />that will not drastically change pH. Expressed<br />in another way, a buffer may bond to H ions when a<br />body fluid is becoming too acidic, or release H ions<br />when a fluid is becoming too alkaline.<br />As a specific example, we will use the bicarbonate<br />buffer system, which consists of carbonic acid<br />(H2CO3), a weak acid, and sodium bicarbonate<br />(NaHCO3), a weak base. This pair of chemicals is<br />present in all body fluids but is especially important to<br />buffer blood and tissue fluid.<br />Carbonic acid ionizes as follows (but remember,<br />because it is a weak acid it does not contribute many<br />H ions to a solution):<br />H2CO3 → H HCO3<br /> <br />Sodium bicarbonate ionizes as follows:<br />NaHCO3 → Na HCO3<br /> <br />If a strong acid, such as HCl, is added to extracellular<br />fluid, this reaction will occur:<br />32 Some Basic Chemistry<br />Figure 2–5. The pH scale. The pH values of several body fluids are indicated above the<br />scale. The pH values of some familiar solutions are indicated below the scale.<br />QUESTION: Describe the pH range of blood compared to the pH range of urine.<br />HCl NaHCO3 → NaCl H2CO3<br />What has happened here? Hydrochloric acid, a<br />strong acid that would greatly lower pH, has reacted<br />with sodium bicarbonate. The products of this reaction<br />are NaCl, a salt that has no effect on pH, and<br />H2CO3, a weak acid that lowers pH only slightly. This<br />prevents a drastic change in the pH of the extracellular<br />fluid.<br />If a strong base, such as sodium hydroxide, is added<br />to the extracellular fluid, this reaction will occur:<br />NaOH H2CO3 → H2O NaHCO3<br />Sodium hydroxide, a strong base that would greatly<br />raise pH, has reacted with carbonic acid. The products<br />of this reaction are water, which has no effect on pH,<br />and sodium bicarbonate, a weak base that raises pH<br />only slightly. Again, this prevents a drastic change in<br />the pH of the extracellular fluid.<br />In the body, such reactions take place in less than a<br />second whenever acids or bases are formed that would<br />greatly change pH. Because of the body’s tendency to<br />become more acidic, the need to correct acidosis is<br />more frequent. With respect to the bicarbonate buffer<br />system, this means that more NaHCO3 than H2CO3 is<br />needed. For this reason, the usual ratio of these<br />buffers is 20:1 (NaHCO3:H2CO3).<br />ORGANIC COMPOUNDS<br />OF IMPORTANCE<br />Organic compounds all contain covalently bonded<br />carbon and hydrogen atoms and perhaps other elements<br />as well. In the human body there are four major<br />groups of organic compounds: carbohydrates, lipids,<br />proteins, and nucleic acids.<br />CARBOHYDRATES<br />A primary function of carbohydrates is to serve as<br />sources of energy in cell respiration. All carbohydrates<br />contain carbon, hydrogen, and oxygen and are classified<br />as monosaccharides, disaccharides, oligosaccharides,<br />and polysaccharides. Saccharide means sugar, and<br />the prefix indicates how many are present.<br />Monosaccharides, or single-sugar compounds,<br />are the simplest sugars. Glucose is a hexose, or sixcarbon,<br />sugar with the formula C6H12O6 (Fig. 2–6).<br />Fructose and galactose also have the same formula,<br />Some Basic Chemistry 33<br />Figure 2–6. Carbohydrates. (A) Glucose, depicting its structural formula. (B) A disaccharide<br />such as sucrose. (C) Cellulose, a polysaccharide. (D) Starch, a polysaccharide.<br />(E) Glycogen, a polysaccharide. Each hexagon represents a hexose sugar such as glucose.<br />QUESTION: What is the chemical formula of glucose?<br />H<br />C<br />OH<br />CH2OH<br />C<br />H<br />OH<br />C<br />H<br />H<br />C<br />OH<br />H<br />C<br />OH<br />A Glucose<br />E Glycogen<br />B Disaccharide<br />C Cellulose<br />D Starch<br />o<br />but the physical arrangement of the carbon, hydrogen,<br />and oxygen atoms in each differs from that of glucose.<br />This gives each hexose sugar a different threedimensional<br />shape. The liver is able to change fructose<br />and galactose to glucose, which is then used by cells in<br />the process of cell respiration to produce ATP.<br />Another type of monosaccharide is the pentose, or<br />five-carbon, sugar. These are not involved in energy<br />production but rather are structural components of<br />the nucleic acids. Deoxyribose (C5H10O4) is part of<br />DNA, which is the genetic material of chromosomes.<br />Ribose (C5H10O5) is part of RNA, which is essential<br />for protein synthesis. We will return to the nucleic<br />acids later in this chapter.<br />Disaccharides are double sugars, made of two<br />monosaccharides linked by a covalent bond. Sucrose,<br />or cane sugar, for example, is made of one glucose and<br />one fructose. Others are lactose (glucose and galactose)<br />and maltose (two glucose), which are also present<br />in food. Disaccharides are digested into monosaccharides<br />and then used for energy production.<br />The prefix oligo means “few”; oligosaccharides<br />consist of from 3 to 20 monosaccharides. In human<br />cells, oligosaccharides are found on the outer surface<br />of cell membranes. Here they serve as antigens,<br />which are chemical markers (or “signposts”) that identify<br />cells. The A, B, and AB blood types, for example,<br />are the result of oligosaccharide antigens on the outer<br />surface of red blood cell membranes. All of our cells<br />have “self” antigens, which identify the cells that<br />belong in an individual. The presence of “self” antigens<br />on our own cells enables the immune system to<br />recognize antigens that are “non-self.” Such foreign<br />antigens include bacteria and viruses, and immunity<br />will be a major topic of Chapter 14.<br />Polysaccharides are made of thousands of glucose<br />molecules, bonded in different ways, resulting in different<br />shapes (see Fig. 2–6). Starches are branched<br />chains of glucose and are produced by plant cells to<br />store energy. We have digestive enzymes that split the<br />bonds of starch molecules, releasing glucose. The glucose<br />is then absorbed and used by cells to produce<br />ATP.<br />Glycogen, a highly branched chain of glucose molecules,<br />is our own storage form for glucose. After a<br />meal high in carbohydrates, the blood glucose level<br />rises. Excess glucose is then changed to glycogen and<br />stored in the liver and skeletal muscles. When the<br />blood glucose level decreases between meals, the<br />glycogen is converted back to glucose, which is<br />released into the blood (these reactions are regulated<br />by insulin and other hormones). The blood glucose<br />level is kept within normal limits, and cells can take in<br />this glucose to produce energy.<br />Cellulose is a nearly straight chain of glucose molecules<br />produced by plant cells as part of their cell<br />walls. We have no enzyme to digest the cellulose we<br />consume as part of vegetables and grains, and it passes<br />through the digestive tract unchanged. Another name<br />for dietary cellulose is “fiber,” and although we cannot<br />use its glucose for energy, it does have a function.<br />Fiber provides bulk within the cavity of the large<br />intestine. This promotes efficient peristalsis, the<br />waves of contraction that propel undigested material<br />through the colon. A diet low in fiber does not give<br />the colon much exercise, and the muscle tissue of the<br />colon will contract weakly, just as our skeletal muscles<br />will become flabby without exercise. A diet high in<br />fiber provides exercise for the colon muscle and may<br />help prevent chronic constipation.<br />The structure and functions of the carbohydrates<br />are summarized in Table 2–3.<br />LIPIDS<br />Lipids contain the elements carbon, hydrogen, and<br />oxygen; some also contain phosphorus. In this group<br />of organic compounds are different types of substances<br />with very different functions. We will consider<br />three types: true fats, phospholipids, and steroids<br />(Fig. 2–7).<br />True fats (also called neutral fats) are made of one<br />molecule of glycerol and one, two, or three fatty acid<br />molecules. If three fatty acid molecules are bonded to<br />a single glycerol, a triglyceride is formed. Two fatty<br />acids and a glycerol form a diglyceride, and one fatty<br />acid and a glycerol form a monoglyceride.<br />The fatty acids in a true fat may be saturated or<br />unsaturated. Refer to Fig. 2–7 and notice that one of<br />the fatty acids has single covalent bonds between all its<br />carbon atoms. Each of these carbons is then bonded to<br />the maximum number of hydrogens; this is a saturated<br />fatty acid, meaning saturated with hydrogen. The<br />other fatty acids shown have one or more (poly) double<br />covalent bonds between their carbons and less<br />than the maximum number of hydrogens; these are<br />unsaturated fatty acids. Many triglycerides contain<br />both saturated and unsaturated fatty acids, and though<br />34 Some Basic Chemistry<br />it is not as precise, it is often easier to speak of saturated<br />and unsaturated fats, indicating the predominance<br />of one or the other type of fatty acid.<br />At room temperature, saturated fats are often in<br />solid form, while unsaturated fats are often (not<br />always) in liquid form. Saturated fats tend to be found<br />in animal foods such as beef, pork, eggs, and cheese,<br />but palm oil and coconut oil are also saturated.<br />Unsaturated fats are found in other plant oils such as<br />corn oil, sunflower oil, and safflower oil, but certain<br />fish oils are also unsaturated, and even pork contains<br />unsaturated fatty acids.<br />Unsaturated fats may be changed to saturated fats<br />in order to give packaged foods a more pleasing texture<br />or taste, or to allow them to be stored longer<br />without refrigeration (a longer shelf life). These are<br />hydrogenated fats (meaning that hydrogens have been<br />added), also called trans fats. Trans fats contribute significantly<br />to atherosclerosis of arteries, that is,<br />abnormal cholesterol deposits in the lining that may<br />clog arteries, especially the coronary arteries of the<br />heart. (See also Box 2–3: Lipids in the Blood.)<br />The triglyceride forms of true fats are a storage<br />form for excess food, that is, they are stored energy<br />(potential energy). Any type of food consumed in<br />excess of the body’s caloric needs will be converted to<br />fat and stored in adipose tissue. Most adipose tissue is<br />subcutaneous, between the skin and muscles. Some<br />organs, however, such as the eyes and kidneys, are<br />enclosed in a layer of fat that acts as a cushion to<br />absorb shock.<br />Phospholipids are diglycerides with a phosphate<br />group (PO4) in the third bonding site of glycerol.<br />Although similar in structure to the true fats, phospholipids<br />are not stored energy but rather structural<br />components of cells. Lecithin is a phospholipid that is<br />part of our cell membranes (see Fig. 3–1; each phospholipid<br />molecule looks like a sphere with two tails;<br />the sphere is the glycerol and phosphate, the tails are<br />the two fatty acids). Another phospholipid is myelin,<br />Some Basic Chemistry 35<br />Table 2–3 CARBOHYDRATES<br />Name Structure Function<br />Monosaccharides—“Single” Sugars<br />Glucose<br />Fructose and<br />galactose<br />Deoxyribose<br />Ribose<br />Disaccharides—“Double” Sugars<br />Sucrose, lactose,<br />and maltose<br />Oligosaccharides—“Few” Sugars (3–20)<br />Polysaccharides—“Many” Sugars (Thousands)<br />Starches<br />Glycogen<br />Cellulose<br />Hexose sugar<br />Hexose sugar<br />Pentose sugar<br />Pentose sugar<br />• Most important energy source for cells<br />• Converted to glucose by the liver, then used for energy<br />production<br />• Part of DNA, the genetic code in the chromosomes of cells<br />• Part of RNA, needed for protein synthesis within cells<br />Two hexose sugars • Present in food; digested to monosaccharides, which are<br />then used for energy production<br />• Form “self” antigens on cell membranes; important to<br />permit the immune system to distinguish “self” from<br />foreign antigens (pathogens)<br />Branched chains of<br />glucose molecules<br />Highly branched chains<br />of glucose molecules<br />Straight chains of<br />glucose molecules<br />• Found in plant foods; digested to monosaccharides and<br />used for energy production<br />• Storage form for excess glucose in the liver and skeletal<br />muscles<br />• Part of plant cell walls; provides fiber to promote peristalsis,<br />especially by the colon<br />BOX 2–3 LIPIDS IN THE BLOOD<br />cause in this form cholesterol is more easily<br />removed from the blood by the liver and excreted<br />in bile.<br />A diet low in total fat, with most of it unsaturated<br />fat, tends to raise HDL levels and lower LDL levels.<br />The benefit is the delaying of atherosclerosis and<br />coronary artery disease. A simple blood test called a<br />lipid profile (or lipid panel) can determine levels of<br />total cholesterol, triglycerides, HDLs, and LDLs. A<br />high HDL level, above 50 mg/dL, is considered<br />good, but some researchers now believe that the<br />LDL level is more important and should be as low as<br />possible, below 100 mg/dL.<br />Other factors contribute to coronary artery disease,<br />such as heredity, smoking, being overweight,<br />and lack of exercise. Diet alone cannot prevent atherosclerosis.<br />However, a diet low in total fat and<br />high in polyunsaturated fats is a good start.<br />Triglycerides and cholesterol are transported in the<br />blood in combination with proteins. Such molecules<br />made by the small intestine are called chylomicrons.<br />Those made by the liver are called<br />lipoproteins and are categorized by their density,<br />which reflects the proportion of protein to cholesterol.<br />Low-density lipoproteins (LDLs, which are low in<br />protein and high in cholesterol) transport cholesterol<br />to the tissues, where it is used to synthesize<br />cell membranes or secretions. LDLs are also called<br />“bad cholesterol,” because in this form the cholesterol<br />is more likely to be deposited in the walls of<br />blood vessels, leading to atherosclerosis.<br />High-density lipoproteins (HDLs, which are<br />higher in protein and lower in cholesterol than<br />LDLs) transport cholesterol from the tissues to the<br />liver. HDLs are also called “good cholesterol,” be-<br />B<br />A<br />Glycerol<br />Triglyceride<br />3 Fatty acids<br />Cholesterol<br />Figure 2–7. Lipids. (A) A triglyceride made of one glycerol and three fatty acids. (B) The<br />steroid cholesterol. The hexagons and pentagon represent rings of carbon and hydrogen.<br />QUESTION: What would a diglyceride look like?<br />36<br />which forms the myelin sheath around nerve cells and<br />provides electrical insulation for nerve impulse transmission.<br />The structure of steroids is very different from<br />that of the other lipids. Cholesterol is an important<br />steroid; it is made of four rings of carbon and hydrogen<br />(not fatty acids and glycerol) and is shown in Fig.<br />2–7. The liver synthesizes cholesterol, in addition to<br />the cholesterol we eat in food as part of our diet.<br />Cholesterol is another component of cell membranes<br />and is the precursor (raw material) for the synthesis of<br />other steroids. In the ovaries or testes, cholesterol is<br />used to synthesize the steroid hormones estrogen or<br />testosterone, respectively. A form of cholesterol in the<br />skin is changed to vitamin D on exposure to sunlight.<br />Liver cells use cholesterol for the synthesis of bile<br />salts, which emulsify fats in digestion. Despite its link<br />to coronary artery disease and heart attacks, cholesterol<br />is an essential substance for human beings.<br />The structure and functions of lipids are summarized<br />in Table 2–4.<br />PROTEINS<br />Proteins are made of smaller subunits or building<br />blocks called amino acids, which all contain the elements<br />carbon, hydrogen, oxygen, and nitrogen. Some<br />amino acids contain sulfur, which permits the formation<br />of disulfide bonds. There are about 20 amino<br />acids that make up human proteins. The structure of<br />amino acids is shown in Fig. 2–8. Each amino acid has<br />a central carbon atom covalently bonded to an atom of<br />hydrogen, an amino group (NH2), and a carboxyl<br />group (COOH). At the fourth bond of the central carbon<br />is the variable portion of the amino acid, represented<br />by R. The R group may be a single hydrogen<br />atom, or a CH3 group, or a more complex configuration<br />of carbon and hydrogen. This gives each of the 20<br />amino acids a slightly different physical shape. A bond<br />between two amino acids is called a peptide bond,<br />and a short chain of amino acids linked by peptide<br />bonds is a polypeptide.<br />A protein may consist of from 50 to thousands of<br />amino acids. The sequence of the amino acids is<br />specific and unique for each protein, and is called<br />its primary structure. This unique sequence, and the<br />hydrogen bonds and disulfide bonds formed within<br />the amino acid chain, determines how the protein will<br />be folded to complete its synthesis. The folding may<br />be simple, a helix (coil) or pleated sheet, called the secondary<br />structure, or a more complex folding may<br />occur to form a globular protein, called the tertiary<br />structure. Myoglobin, found in muscles, is a globular<br />protein (Fig. 2–8). When complete, each protein has a<br />characteristic three-dimensional shape, which in turn<br />determines its function. Some proteins consist of<br />more than one amino acid chain (quaternary structure).<br />Hemoglobin, for example, has four amino acid<br />chains (see Box 3–2). Notice that myoglobin contains<br />an atom of iron (a hemoglobin molecule has four iron<br />Some Basic Chemistry 37<br />Table 2–4 LIPIDS<br />Name Structure Function<br />True fats<br />Phospholipids<br />Steroids (cholesterol)<br />A triglyceride consists of three<br />fatty acid molecules bonded to<br />a glycerol molecule (some are<br />monoglycerides or diglycerides)<br />Diglycerides with a phosphate<br />group bonded to the glycerol<br />molecule<br />Four carbon–hydrogen rings<br />• Storage form for excess food molecules in<br />subcutaneous tissue<br />• Cushion organs such as the eyes and kidneys<br />• Part of cell membranes (lecithin)<br />• Form the myelin sheath to provide electrical<br />insulation for neurons<br />• Part of cell membranes<br />• Converted to vitamin D in the skin on exposure to<br />UV rays of the sun<br />• Converted by the liver to bile salts, which emulsify<br />fats during digestion<br />• Precursor for the steroid hormones such as estrogen<br />in women (ovaries) or testosterone in men (testes)<br />atoms). Some proteins require a trace element such as<br />iron or zinc to complete their structure and permit<br />them to function properly.<br />Our body proteins have many functions; some of<br />these are listed in Table 2–5 and will be mentioned<br />again in later chapters. And though we usually do not<br />think of protein as an energy food, if our diet includes<br />more amino acids than are necessary for our protein<br />synthesis, those excess amino acids will be converted<br />to simple carbohydrates or even to fat, to be stored as<br />potential energy. (See Box 2–4: A Protein Mystery:<br />Prions, for a discussion of disease-causing proteins.)<br />One very important function of proteins will be discussed<br />further here: the role of proteins as enzymes.<br />Enzymes<br />Enzymes are catalysts, which means that they speed<br />up chemical reactions without the need for an external<br />source of energy such as heat. The many reactions<br />that take place within the body are catalyzed by specific<br />enzymes; all of these reactions must take place at<br />body temperature.<br />The way in which enzymes function as catalysts is<br />called the active site theory, and is based on the<br />shape of the enzyme and the shapes of the reacting<br />molecules, called substrates. A simple synthesis reaction<br />is depicted in Fig. 2–9A. Notice that the enzyme<br />has a specific shape, as do the substrate molecules.<br />38 Some Basic Chemistry<br />NH2 C COOH<br />H<br />R<br />•<br />Amino<br />group<br />•<br />Carboxyl<br />group<br />•<br />Variable portion<br />• •<br />Peptide bonds<br />•<br />Iron in heme<br />A Amino acid<br />B Polypeptide<br />C Primary structure<br />D Secondary structurealpha<br />helix E Tertiary structuremyoglobin<br />Figure 2–8. Amino acid and protein structure. (A) The structural formula of an amino<br />acid. The “R” represents the variable portion of the molecule. (B) A polypeptide. Several<br />amino acids, represented by different shapes, are linked by peptide bonds. (C) The primary<br />structure of a protein. (D) The secondary structure of a protein. (E) The tertiary structure<br />of the protein myoglobin. See text for further description.<br />QUESTION: What mineral is part of myoglobin, and what is its function?<br />tween them, creating a new compound. The product<br />of the reaction, the new compound, is then released,<br />leaving the enzyme itself unchanged and able to catalyze<br />another reaction of the same type.<br />The reaction shown in Fig. 2–9B is a decomposition<br />reaction. As the substrate molecule bonds to the active<br />site of the enzyme, strain is put on its internal bonds,<br />which break, forming two product molecules and<br />again leaving the enzyme unchanged. Each enzyme is<br />specific in that it will catalyze only one type of reaction.<br />An enzyme that digests the protein in food, for<br />example, has the proper shape for that reaction but<br />cannot digest starches. For starch digestion, another<br />enzyme with a differently shaped active site is needed.<br />Thousands of chemical reactions take place within the<br />body, and therefore we have thousands of enzymes,<br />each with its own shape and active site.<br />The ability of enzymes to function may be limited<br />or destroyed by changes in the intracellular or extracellular<br />fluids in which they are found. Changes in pH<br />and temperature are especially crucial. Recall that the<br />pH of intracellular fluid is approximately 6.8, and that<br />a decrease in pH means that more H ions are present.<br />If pH decreases significantly, the excess H ions<br />will react with the active sites of cellular enzymes,<br />change their shapes, and prevent them from catalyzing<br />reactions. This is why a state of acidosis may cause the<br />death of cells—the cells’ enzymes are unable to function<br />properly.<br />Some Basic Chemistry 39<br />Table 2–5 FUNCTIONS OF PROTEINS<br />Type of Protein Function<br />Structural<br />proteins<br />Hormones<br />Hemoglobin<br />Myoglobin<br />Antibodies<br />Myosin and actin<br />Enzymes<br />• Form pores and receptor sites in<br />cell membranes<br />• Keratin—part of skin and hair<br />• Collagen—part of tendons and<br />ligaments<br />• Insulin—enables cells to take in<br />glucose; lowers blood glucose level<br />• Growth hormone—increases<br />protein synthesis and cell division<br />• Enables red blood cells to carry<br />oxygen<br />• Stores oxygen in muscle cells<br />• Produced by lymphocytes (white<br />blood cells); label pathogens for<br />destruction<br />• Muscle structure and contraction<br />• Catalyze reactions<br />BOX 2–4 A PROTEIN MYSTERY: PRIONS<br />ing misfolding, which brings about deterioration of<br />brain tissue. We do not know how to destroy prions.<br />Prions are not living; they do not contain genetic<br />material or carry out processes that might be disrupted<br />by antibiotics or antiviral medications.<br />Standard sterilization practices that kill bacteria and<br />viruses do not seem to inactivate prions.<br />Prevention of prion disease depends upon<br />keeping animal brain tissue from contaminating<br />meat destined for human or animal consumption.<br />In Great Britain, where the human form of madcow<br />disease emerged and killed nearly 100 people,<br />butchering practices are now stringently regulated.<br />The first cases of mad-cow disease in Canada<br />and the United States were found in 2003, in cattle.<br />As of this writing, people have not yet been<br />affected.<br />Prions are proteinaceous infectious particles, the<br />cause of lethal diseases of the nervous system in<br />people and animals. Mad-cow disease is perhaps<br />the best known; its formal name is bovine spongiform<br />encephalopathy (BSE). The name tells us about<br />the disease: Encephalopathy means that the brain is<br />affected, and spongiform indicates that brain tissue<br />becomes spongy, full of holes. People may acquire<br />BSE by eating beef contaminated with infected cow<br />brain tissue. They develop what is called variant<br />Creutzfeldt-Jakob disease (CJD). CJD is characterized<br />by loss of coordination, loss of memory and personality,<br />and death within a few months. There is no<br />treatment.<br />How do prions cause this disease? We do not yet<br />have the entire answer. We do know that prions<br />change the structure of other brain proteins, caus-<br />The active site of the enzyme is the part that matches<br />the shapes of the substrates. The substrates must “fit”<br />into the active site of the enzyme, and temporary<br />bonds may form between the enzyme and the substrate.<br />This is called the enzyme–substrate complex. In<br />this case, two substrate molecules are thus brought<br />close together so that chemical bonds are formed be-<br />With respect to temperature, most human enzymes<br />have their optimum functioning in the normal range<br />of body temperature: 97 to 99 F (36 to 38 C). A temperature<br />of 106 F, a high fever, may break the chemical<br />bonds that maintain the shapes of enzymes (see<br />Fig. 2–9C). If an enzyme loses its shape, it is said to be<br />denatured, and a denatured enzyme is unable to function<br />as a catalyst. Some human enzymes, when denatured<br />by a high fever, may revert to their original<br />shapes if the fever is lowered quickly. Others, however,<br />will not. (An example of irreversible denaturation is a<br />hard-boiled egg; the proteins in the egg white and<br />yolk will never revert to what they were in the original<br />egg.) A high fever may cause brain damage or death<br />because enzymes in the brain have become permanently<br />denatured.<br />You already know that metals such as lead and mercury<br />are harmful to humans and that both may cause<br />serious damage to the nervous system and other body<br />tissues. These heavy metals are harmful to us because<br />they are very reactive and block the actions of our<br />enzymes. Fig. 2–9D depicts what happens. Notice that<br />the heavy metal ion bonds with part of the active site<br />of the enzyme and changes its shape. The substrate<br />molecule cannot fit, and the enzyme is useless. Many<br />other chemicals are poisonous to us for the very same<br />reason: They destroy the functioning of our enzymes,<br />and essential reactions cannot take place.<br />40 Some Basic Chemistry<br />Active site<br />Enzyme<br />Substrates Enzyme-substrate<br />complex<br />Enzyme<br />Product<br />Enzyme<br />Substrate Enzyme-substrate<br />complex<br />Enzyme<br />Products<br />Enzyme<br />Enzyme<br />Denatured enzyme<br />Heavy-metal ion<br />or toxin<br />Nonfunctional<br />enzyme<br />•<br />A<br />B<br />C<br />D<br />Figure 2–9. Active site theory. (A) Synthesis reaction. (B) Decomposition reaction.<br />(C) The effect of heat. (D) The effect of poisons. See text for description.<br />QUESTION: Which of these four pictures best represents the effect of an acidic pH on an<br />enzyme, and why?<br />NUCLEIC ACIDS<br />DNA and RNA<br />The nucleic acids, DNA (deoxyribonucleic acid) and<br />RNA (ribonucleic acid), are large molecules made of<br />smaller subunits called nucleotides. A nucleotide consists<br />of a pentose sugar, a phosphate group, and one of<br />several nitrogenous bases. In DNA nucleotides, the<br />sugar is deoxyribose, and the bases are adenine, guanine,<br />cytosine, or thymine. In RNA nucleotides, the<br />sugar is ribose, and the bases are adenine, guanine,<br />cytosine, or uracil. DNA and RNA molecules are<br />shown in Fig. 2–10. Notice that DNA looks somewhat<br />like a twisted ladder; this ladder is two strands of<br />Some Basic Chemistry 41<br />•<br />Deoxyribose (DNA)<br />Ribose (RNA)<br />Phosphate<br />Adenine<br />Guanine<br />Thymine (DNA)<br />Uracil (RNA)<br />Cytosine<br />Chromatin in<br />the nucleus<br />•<br />Cell<br />Double helix<br />DNA strands<br />Hydrogen<br />bonds<br />RNA strand<br />Figure 2–10. DNA and RNA. Both molecules are shown, with each part of a nucleotide<br />represented by its shape and in a different color. Note the complementary base pairing of<br />DNA (A–T and G–C). When RNA is synthesized, it is a complementary copy of half the DNA<br />molecule (with U in place of T).<br />QUESTION: Why can’t adenine pair with guanine to form a rung of the DNA ladder?<br />nucleotides called a double helix (two coils). Alternating<br />phosphate and sugar molecules form the uprights<br />of the ladder, and pairs of nitrogenous bases form the<br />rungs. The size of the bases and the number of hydrogen<br />bonds each can form the complementary base<br />pairing of the nucleic acids. In DNA, adenine is always<br />paired with thymine (with two hydrogen bonds), and<br />guanine is always paired with cytosine (with three<br />hydrogen bonds).<br />DNA makes up the chromosomes of cells and is,<br />therefore, the genetic code for hereditary characteristics.<br />The sequence of bases in the DNA strands is actually<br />a code for the many kinds of proteins living things<br />produce; the code is the same in plants, other animals,<br />and microbes. The sequence of bases for one protein is<br />called a gene. Human genes are the codes for the proteins<br />produced by human cells (though many of these<br />genes are also found in all other forms of life—we are<br />all very much related). The functioning of DNA will<br />be covered in more detail in the next chapter.<br />RNA is often a single strand of nucleotides (see Fig.<br />2–10), with uracil nucleotides in place of thymine<br />nucleotides. RNA is synthesized from DNA in the<br />nucleus of a cell but carries out a major function in the<br />cytoplasm. This function is protein synthesis, which<br />will also be discussed in the following chapter.<br />ATP<br />ATP (adenosine triphosphate) is a specialized<br />nucleotide that consists of the base adenine, the sugar<br />ribose, and three phosphate groups. Mention has<br />already been made of ATP as a product of cell respiration<br />that contains biologically useful energy. ATP is<br />one of several “energy transfer” molecules within<br />cells, transferring the potential energy in food molecules<br />to cell processes. When a molecule of glucose is<br />broken down into carbon dioxide and water with the<br />release of energy, the cell uses some of this energy to<br />synthesize ATP. Present in cells are molecules of ADP<br />(adenosine diphosphate) and phosphate. The energy<br />released from glucose is used to loosely bond a third<br />phosphate to ADP, forming ATP. When the bond of<br />this third phosphate is again broken and energy is<br />released, ATP then becomes the energy source for cell<br />processes such as mitosis.<br />All cells have enzymes that can remove the third<br />phosphate group from ATP to release its energy,<br />forming ADP and phosphate. As cell respiration continues,<br />ATP is resynthesized from ADP and phosphate.<br />ATP formation to trap energy from food and<br />breakdown to release energy for cell processes is a<br />continuing cycle in cells.<br />The structure and functions of the nucleic acids are<br />summarized in Table 2–6.<br />SUMMARY<br />All of the chemicals we have just described are considered<br />to be non-living, even though they are essential<br />parts of all living organisms. The cells of our bodies<br />are precise arrangements of these non-living chemicals<br />and yet are considered living matter. The cellular<br />level, therefore, is the next level of organization we<br />will examine.<br />42 Some Basic Chemistry<br />Table 2–6 NUCLEIC ACIDS<br />Name Structure Function<br />DNA (deoxyribonucleic acid)<br />RNA (ribonucleic acid)<br />ATP (adenosine triphosphate)<br />A double helix of nucleotides;<br />adenine paired with<br />thymine, and guanine<br />paired with cytosine<br />A single strand of nucleotides;<br />adenine, guanine, cytosine,<br />and uracil<br />A single adenine nucleotide<br />with three phosphate<br />groups<br />• Found in the chromosomes in the nucleus of a cell<br />• Is the genetic code for hereditary characteristics<br />• Copies the genetic code of DNA to direct protein<br />synthesis in the cytoplasm of cells<br />• An energy-transferring molecule<br />• Formed when cell respiration releases energy from<br />food molecules<br />• Used for energy-requiring cellular processes<br />Elements<br />1. Elements are the simplest chemicals, which make<br />up all matter.<br />2. Carbon, hydrogen, oxygen, nitrogen, phosphorus,<br />sulfur, and calcium make up 99% of the human<br />body.<br />3. Elements combine in many ways to form molecules.<br />Atoms (see Fig. 2–1)<br />1. Atoms are the smallest part of an element that still<br />retains the characteristics of the element.<br />2. Atoms consist of positively and negatively charged<br />particles and neutral (or uncharged) particles.<br />• Protons have a positive charge and are found in<br />the nucleus of the atom.<br />• Neutrons have no charge and are found in the<br />nucleus of the atom.<br />• Electrons have a negative charge and orbit the<br />nucleus.<br />3. The number and arrangement of electrons give an<br />atom its bonding capabilities.<br />Chemical Bonds<br />1. An ionic bond involves the loss of electrons by one<br />atom and the gain of these electrons by another<br />atom: Ions are formed that attract one another (see<br />Fig. 2–2).<br />• Cations are ions with positive charges: Na ,<br />Ca 2.<br />• Anions are ions with negative charges: Cl ,<br />HCO3<br /> .<br />• Salts, acids, and bases are formed by ionic bonding.<br />• In water, many ionic bonds break; dissociation<br />releases ions for other reactions.<br />2. A covalent bond involves the sharing of electrons<br />between two atoms (see Fig. 2–3).<br />• Oxygen gas (O2) and water (H2O) are covalently<br />bonded molecules.<br />• Carbon always forms covalent bonds; these are<br />the basis for the organic compounds.<br />• Covalent bonds are not weakened in an aqueous<br />solution.<br />3. A disulfide bond is a covalent bond between two<br />sulfur atoms in a protein; it helps maintain the<br />three-dimensional shape of some proteins.<br />4. A hydrogen bond is the attraction of a covalently<br />bonded hydrogen to a nearby oxygen or nitrogen<br />atom.<br />• The three-dimensional shape of proteins and<br />nucleic acids is maintained by hydrogen bonds.<br />• Water is cohesive because of hydrogen bonds.<br />Chemical Reactions<br />1. A change brought about by the formation or breaking<br />of chemical bonds.<br />2. Synthesis—bonds are formed to join two or more<br />molecules.<br />3. Decomposition—bonds are broken within a molecule.<br />Inorganic Compounds of Importance<br />1. Water—makes up 60% to 75% of the body.<br />• Solvent—for transport of nutrients in the blood<br />and excretion of wastes in urine.<br />• Lubricant—mucus in the digestive tract.<br />• Changes temperature slowly, and prevents sudden<br />changes in body temperature; absorbs body<br />heat in evaporation of sweat.<br />• Water compartments—the locations of water<br />within the body (see Fig. 2–4).<br />Intracellular—within cells; 65% of total body<br />water.<br />Extracellular—35% of total body water<br />— Plasma—in blood vessels.<br />— Lymph—in lymphatic vessels.<br />— Tissue fluid—in tissue spaces between<br />cells.<br />2. Oxygen—21% of the atmosphere.<br />• Essential for cell respiration: the breakdown of<br />food molecules to release energy.<br />3. Carbon dioxide<br />• Produced as a waste product of cell respiration.<br />• Must be exhaled; excess CO2 causes acidosis.<br />4. Cell respiration—the energy-producing processes<br />of cells.<br />• Glucose O2 → CO2 H2O ATP heat<br />• This is why we breathe: to take in oxygen to<br />break down food to produce energy, and to<br />exhale the CO2 produced.<br />5. Trace elements—minerals needed in small amounts<br />(see Table 2–2).<br />Some Basic Chemistry 43<br />STUDY OUTLINE<br />6. Acids, bases, and pH<br />• The pH scale ranges from 0 to 14; 7 is neutral;<br />below 7 is acidic; above 7 is alkaline.<br />• An acid increases the H ion concentration of a<br />solution; a base decreases the H ion concentration<br />(or increases the OH– ion concentration)<br />(see Fig. 2–5).<br />• The pH of cells is about 6.8. The pH range of<br />blood is 7.35 to 7.45.<br />• Buffer systems maintain normal pH by reacting<br />with strong acids or strong bases to change<br />them to substances that do not greatly change<br />pH.<br />• The bicarbonate buffer system consists of H2CO3<br />and NaHCO3.<br />Organic Compounds of Importance<br />1. Carbohydrates (see Table 2–3 and Fig. 2–6).<br />• Monosaccharides are simple sugars. Glucose, a<br />hexose sugar (C6H12O6), is the primary energy<br />source for cell respiration.<br />Pentose sugars are part of the nucleic acids<br />DNA and RNA.<br />• Disaccharides are made of two hexose sugars.<br />Sucrose, lactose, and maltose are digested to<br />monosaccharides and used for cell respiration.<br />• Oligosaccharides consist of from 3 to 20 monosaccharides;<br />they are antigens on the cell membrane<br />that identify cells as “self.”<br />• Polysaccharides are made of thousands of glucose<br />molecules.<br />Starches are plant products broken down in<br />digestion to glucose.<br />Glycogen is the form in which glucose is<br />stored in the liver and muscles.<br />Cellulose, the fiber portion of plant cells, cannot<br />be digested but promotes efficient peristalsis<br />in the colon.<br />2. Lipids (see Table 2–4 and Fig. 2–7).<br />• True fats are made of fatty acids and glycerol;<br />triglycerides are a storage form for potential<br />energy in adipose tissue. The eyes and kidneys<br />are cushioned by fat. Fatty acids may be saturated<br />or unsaturated. Saturated fats and hydrogenated<br />or trans fats contribute to atherosclerosis.<br />• Phospholipids are diglycerides such as lecithin<br />that are part of cell membranes. Myelin is a<br />phospholipid that provides electrical insulation<br />for nerve cells.<br />• Steroids consist of four rings of carbon and<br />hydrogen. Cholesterol, produced by the liver<br />and consumed in food, is the basic steroid from<br />which the body manufactures others: steroid<br />hormones, vitamin D, and bile salts.<br />3. Proteins<br />• Amino acids are the subunits of proteins; 20<br />amino acids make up human proteins. Peptide<br />bonds join amino acids to one another (see Fig.<br />2–8).<br />• A protein consists of from 50 to thousands of<br />amino acids in a specific sequence (primary<br />structure) that is folded into a specific shape (secondary<br />and tertiary structures). Some proteins<br />are made of two or more amino acid chains;<br />some proteins contain trace elements.<br />• Protein functions—see Table 2–5.<br />• Amino acids in excess of the need for protein<br />synthesis are converted to simple carbohydrates<br />or to fat, for energy production.<br />• Enzymes are catalysts, which speed up reactions<br />without additional energy. The active site theory<br />is based on the shapes of the enzyme and the substrate<br />molecules: These must “fit” (see Fig. 2–9).<br />The enzyme remains unchanged after the product<br />of the reaction is released. Each enzyme is<br />specific for one type of reaction. The functioning<br />of enzymes may be disrupted by changes in pH<br />or body temperature or by the presence of a poison,<br />which changes the shape of the active sites<br />of enzymes.<br />4. Nucleic acids (see Table 2–6 and Fig. 2–10).<br />• Nucleotides are the subunits of nucleic acids. A<br />nucleotide consists of a pentose sugar, a phosphate<br />group, and a nitrogenous base.<br />• DNA is a double strand of nucleotides, coiled<br />into a double helix, with complementary base<br />pairing: A–T and G–C. DNA makes up the<br />chromosomes of cells and is the genetic code for<br />the synthesis of proteins.<br />• RNA is a single strand of nucleotides, synthesized<br />from DNA, with U in place of T. RNA<br />functions in protein synthesis.<br />• ATP is a nucleotide that is specialized to trap<br />and release energy. Energy released from food in<br />cell respiration is used to synthesize ATP from<br />ADP P. When cells need energy, ATP is broken<br />down to ADP P and the energy is released<br />for cell processes.<br />44 Some Basic Chemistry<br />1. State the chemical symbol for each of the following<br />elements: sodium, potassium, iron, calcium, oxygen,<br />carbon, hydrogen, copper, and chlorine.<br />(p. 24)<br />2. Explain, in terms of their electrons, how an atom of<br />sodium and an atom of chlorine form a molecule of<br />sodium chloride. (p. 25)<br />3. a. Explain, in terms of their electrons, how an<br />atom of carbon and two atoms of oxygen form a<br />molecule of carbon dioxide. (pp. 26–28)<br />b. Explain the functions of hydrogen bonds<br />c. Explain the function of disulfide bonds<br />4. Name the subunits (smaller molecules) of which<br />each of the following is made: DNA, glycogen, a<br />true fat, and a protein. (pp. 34, 37, 41)<br />5. State precisely where in the body each of these fluids<br />is found: plasma, intracellular water, lymph, and<br />tissue fluid. (p. 29)<br />6. Explain the importance of the fact that water<br />changes temperature slowly. (pp. 28–29)<br />7. Describe two ways in which the solvent ability of<br />water is important to the body. (p. 28)<br />8. Name the organic molecule with each of the following<br />functions: (pp. 34–36, 42)<br />a. The genetic code in chromosomes<br />b. “Self” antigens in our cell membranes<br />c. The storage form for glucose in the liver<br />d. The storage form for excess food in adipose<br />tissue<br />e. The precursor molecule for the steroid hormones<br />f. The undigested part of food that promotes<br />peristalsis<br />g. The sugars that are part of the nucleic acids<br />9. State the summary equation of cell respiration.<br />(p. 30)<br />10. State the role or function of each of the following<br />in cell respiration: CO2, glucose, O2, heat, and<br />ATP. (p. 30)<br />11. State a specific function of each of the following<br />in the human body: Ca, Fe, Na, I, and Co. (p. 31)<br />12. Explain, in terms of relative concentrations of H <br />ions and OH ions, each of the following: acid,<br />base, and neutral substance. (p. 31)<br />13. State the normal pH range of blood. (p. 31)<br />14. Complete the following equation, and state how<br />each of the products affects pH: (p. 33)<br />HCl NaHCO3 → _______ _______.<br />15. Explain the active site theory of enzyme functioning.<br />(p. 38–39)<br />16. Explain the difference between a synthesis reaction<br />and a decomposition reaction. (p. 28)<br />Some Basic Chemistry 45<br />REVIEW QUESTIONS<br />FOR FURTHER THOUGHT<br />1. Orange juice usually has a pH of around 4. How<br />does this compare with the pH of the blood? Why<br />is it possible for us to drink orange juice without<br />disrupting the pH of our blood?<br />2. Estrela, age 7, has cereal with milk and sugar for<br />breakfast, then walks to school. Explain the relationship<br />between eating and walking, and remember<br />that Estrela is breathing.<br />3. The body is able to store certain nutrients. Name<br />the storage forms, and state an advantage and a disadvantage.<br />4. Many “vitamin pills” also contain minerals. Which<br />minerals are likely to be found in such dietary supplements?<br />What purpose do they have; that is, what<br />are their functions?<br />CHAPTER 3<br />Chapter Outline<br />Cell Structure<br />Cell Membrane<br />Nucleus<br />Cytoplasm and Cell Organelles<br />Cellular Transport Mechanisms<br />Diffusion<br />Osmosis<br />Facilitated Diffusion<br />Active Transport<br />Filtration<br />Phagocytosis and Pinocytosis<br />The Genetic Code and Protein Synthesis<br />DNA and the Genetic Code<br />RNA and Protein Synthesis<br />Cell Division<br />Mitosis<br />Meiosis<br />Aging and Cells<br />BOX 3–1 TERMINOLOGY OF SOLUTIONS<br />BOX 3–2 GENETIC DISEASE—SICKLE-CELL ANEMIA<br />BOX 3–3 ABNORMAL CELLULAR FUNCTIONING—<br />CANCER<br />Student Objectives<br />• Name the organic molecules that make up cell<br />membranes and state their functions.<br />• State the functions of the nucleus and chromosomes.<br />• Describe the functions of the cell organelles.<br />• Define each of these cellular transport mechanisms<br />and give an example of the role of each in<br />the body: diffusion, osmosis, facilitated diffusion,<br />active transport, filtration, phagocytosis, and<br />pinocytosis.<br />• Describe the triplet code of DNA.<br />• Explain how the triplet code of DNA is transcribed<br />and translated in the synthesis of proteins.<br />• Describe what happens in mitosis and in meiosis.<br />• Use examples to explain the importance of mitosis.<br />• Explain the importance of meiosis.<br />46<br />Cells<br />47<br />New Terminology<br />Absorption (ab-ZORB-shun)<br />Active transport (AK-tiv TRANS-port)<br />Aerobic (air-ROH-bik)<br />Cell membrane (SELL MEM-brayn)<br />Chromosomes (KROH-muh-sohms)<br />Cytoplasm (SIGH-toh-plazm)<br />Diffusion (di-FEW-zhun)<br />Diploid number (DIH-ployd)<br />Filtration (fill-TRAY-shun)<br />Gametes (GAM-eets)<br />Gene (JEEN)<br />Haploid number (HA-ployd)<br />Meiosis (my-OH-sis)<br />Microvilli (MY-kro-VILL-eye)<br />Mitochondria (MY-toh-KAHN-dree-ah)<br />Mitosis (my-TOH-sis)<br />Nucleus (NEW-klee-us)<br />Organelles (OR-gan-ELLS)<br />Osmosis (ahs-MOH-sis)<br />Phagocytosis (FAG-oh-sigh-TOH-sis)<br />Pinocytosis (PIN-oh-sigh-TOH-sis)<br />Selectively permeable (se-LEK-tiv-lee PER-me-uhbuhl)<br />Theory (THEER-ree)<br />Related Clinical Terminology<br />Benign (bee-NINE)<br />Carcinogen (kar-SIN-oh-jen)<br />Chemotherapy (KEE-moh-THER-uh-pee)<br />Genetic disease (je-NET-ik di-ZEEZ)<br />Hypertonic (HIGH-per-TAHN-ik)<br />Hypotonic (HIGH-poh-TAHN-ik)<br />Isotonic (EYE-soh-TAHN-ik)<br />Malignant (muh-LIG-nunt)<br />Metastasis (muh-TASS-tuh-sis)<br />Mutation (mew-TAY-shun)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />All living organisms are made of cells and cell<br />products. This simple statement, called the cell theory,<br />was first proposed more than 150 years ago. You<br />may think of a theory as a guess or hypothesis, and<br />sometimes this is so. A scientific theory, however, is<br />actually the best explanation of all available evidence.<br />All of the evidence science has gathered so far supports<br />the validity of the cell theory.<br />Cells are the smallest living subunits of a multicellular<br />organism such as a human being. A cell is a complex<br />arrangement of the chemicals discussed in the<br />previous chapter, is living, and carries out specific<br />activities. Microorganisms, such as amoebas and bacteria,<br />are single cells that function independently.<br />Human cells, however, must work together and function<br />interdependently. Homeostasis depends upon the<br />contributions of all of the different kinds of cells.<br />Human cells vary in size, shape, and function. Most<br />human cells are so small they can only be seen with<br />the aid of a microscope and are measured in units<br />called micrometers (formerly called microns). One<br />micrometer 1/1,000,000 of a meter or 1/25,000 of<br />an inch (see Appendix A: Units of Measure). One<br />exception is the human ovum or egg cell, which is<br />about 1 millimeter in diameter, just visible to the<br />unaided eye. Some nerve cells, although microscopic<br />in diameter, may be quite long. Those in our arms and<br />legs, for example, are at least 2 feet (60 cm) long.<br />With respect to shape, human cells vary greatly.<br />Some are round or spherical, others rectangular, still<br />others irregular. White blood cells even change shape<br />as they move.<br />Cell functions also vary, and since our cells do not<br />act independently, we will cover specialized cell functions<br />as part of tissue functions in Chapter 4. Based on<br />function, there are more than 200 different kinds of<br />human cells. This chapter is concerned with the basic<br />structure of cells and the cellular activities common to<br />all our cells.<br />CELL STRUCTURE<br />Despite their many differences, human cells have several<br />similar structural features: a cell membrane, a<br />nucleus, and cytoplasm and cell organelles. Red blood<br />cells are an exception because they have no nuclei<br />when mature. The cell membrane forms the outer<br />boundary of the cell and surrounds the cytoplasm,<br />organelles, and nucleus.<br />CELL MEMBRANE<br />Also called the plasma membrane, the cell membrane<br />is made of phospholipids, cholesterol, and proteins.<br />The arrangement of these organic molecules is<br />shown in Fig. 3–1. The phospholipids are diglycerides,<br />and form a bilayer, or double layer, which<br />makes up most of the membrane. Phospholipids<br />permit lipid-soluble materials to easily enter or leave<br />the cell by diffusion through the cell membrane. The<br />presence of cholesterol decreases the fluidity of<br />the membrane, thus making it more stable. The proteins<br />have several functions: Some form channels or<br />pores to permit passage of materials such as water or<br />ions; others are carrier enzymes or transporters that<br />also help substances enter the cell. Still other proteins,<br />with oligosaccharides on their outer surface, are antigens,<br />markers that identify the cells of an individual as<br />“self.” Yet another group of proteins serves as receptor<br />sites for hormones. Many hormones bring about<br />their specific effects by first bonding to a particular<br />receptor on the cell membrane, a receptor with the<br />proper shape. This bonding, or fit, then triggers<br />chemical reactions within the cell membrane or the<br />interior of the cell (see Box 10–3 for an illustration<br />involving the hormone insulin).<br />Many receptors for other molecules are also part of<br />cell membranes. These molecules are part of the<br />chemical communication networks our cells have. An<br />unavoidable consequence of having so many receptors<br />for chemical communication is that some pathogens<br />have evolved shapes to match certain receptors. For<br />example, HIV, the virus that causes AIDS, just happens<br />to fit a particular surface receptor on our white<br />blood cells. When the virus fits in, the receptor<br />becomes a gateway into the cell, which begins the<br />takeover of the cell by the virus.<br />Most often, however, the cell membrane is a beneficial<br />structure. Although the cell membrane is the<br />outer boundary of the cell, it should already be apparent<br />to you that it is not a static or wall-like boundary,<br />but rather an active, dynamic one. It is much more like<br />a line of tollbooths than a concrete barrier. The cell<br />membrane is selectively permeable; that is, certain<br />substances are permitted to pass through and others<br />48 Cells<br />are not. These mechanisms of cellular transport will<br />be covered later in this chapter. The cell membrane is<br />of particular importance for muscle cells and nerve<br />cells because it carries electrical impulses. This will be<br />covered in Chapters 7 and 8.<br />NUCLEUS<br />With the exception of mature red blood cells, all<br />human cells have a nucleus. The nucleus is within the<br />cytoplasm and is bounded by a double-layered nuclear<br />membrane with many pores. It contains one or more<br />nucleoli and the chromosomes of the cell (Fig. 3–2).<br />A nucleolus is a small sphere made of DNA, RNA,<br />and protein. The nucleoli form a type of RNA called<br />ribosomal RNA, which becomes part of ribosomes<br />(a cell organelle) and is involved in protein synthesis.<br />The nucleus is the control center of the cell<br />because it contains the chromosomes. Even with a<br />microscope, the 46 chromosomes of a human cell are<br />usually not visible; they are long threads called chromatin.<br />Before a cell divides, however, the chromatin<br />coils extensively into visible chromosomes. Chromosomes<br />are made of DNA and protein. Some chromosomal<br />proteins provide the structural framework for<br />the coiling of the chromatin into chromosomes so<br />that cell division can take place. Other chromosomal<br />proteins help regulate the activity of the DNA.<br />Remember from our earlier discussion that the DNA<br />is the genetic code for the characteristics and activities<br />of the cell. Although the DNA in the nucleus of each<br />cell contains all of the genetic information for all<br />human traits, only a small number of genes (a gene is<br />the genetic code for one protein) are actually active, or<br />Cells 49<br />Receptor site<br />Outside of cell Oligosaccharide antigens<br />Cholesterol<br />Inside of cell<br />Protein forming a pore<br />Phospholipid<br />bilayer<br />Figure 3–1. The cell (plasma) membrane depicting the types of molecules present.<br />QUESTION: The receptor site shown is probably what type of organic molecule?<br />“switched on,” in a particular cell. These active genes<br />are the codes for the proteins necessary for the specific<br />cell type and its functions. For example, the gene for<br />insulin is present in all human cells, but only in certain<br />islet cells of the pancreas is the gene active or switched<br />on. Only in these cells is insulin produced. How the<br />genetic code in chromosomes is translated into proteins<br />will be covered in a later section.<br />CYTOPLASM AND CELL ORGANELLES<br />Cytoplasm is a watery solution of minerals, gases,<br />organic molecules, and cell organelles that is found<br />between the cell membrane and the nucleus. Cytosol<br />is the water portion of cytoplasm, and many chemical<br />reactions take place within it. Cell organelles are<br />intracellular structures, often bounded by their own<br />50 Cells<br />Nuclear<br />membrane<br />Nucleolus<br />Nucleus<br />Chromatin<br />Golgi apparatus<br />Cilia<br />Microvilli<br />Smooth endoplasmic<br />reticulum<br />Centrioles<br />Cell membrane<br />Rough endoplasmic<br />reticulum<br />Ribosomes<br />Cytoplasm<br />Mitochondrion<br />Lysosome<br />Proteasome<br />Figure 3–2. Generalized human cell depicting the structural components. See text and<br />Table 3–1 for descriptions.<br />QUESTION: How do cilia differ in structure from microvilli?<br />membranes, that have specific functions in cellular<br />metabolism. They are also shown in Fig. 3–2.<br />The endoplasmic reticulum (ER) is an extensive<br />network of membranous tubules that extend from<br />the nuclear membrane to the cell membrane. Rough<br />ER has numerous ribosomes on its surface, whereas<br />smooth ER has no ribosomes. As a network of interconnected<br />tunnels, the ER is a passageway for the<br />transport of the materials necessary for cell function<br />within the cell. These include proteins synthesized by<br />the ribosomes on the rough ER, and lipids synthesized<br />by the smooth ER.<br />Ribosomes are very small structures made of protein<br />and ribosomal RNA. Some are found on the surface<br />of rough ER, while others float freely within the<br />cytoplasm. Ribosomes are the site of protein synthesis.<br />The proteins produced may be structural proteins<br />such as collagen in the skin, enzymes, or hormones<br />such as insulin that regulate cellular processes. These<br />proteins may function within the cell or be secreted<br />from the cell to be used elsewhere in the body.<br />Our protein molecules are subject to damage, and<br />some cellular proteins, especially regulatory proteins,<br />may be needed just for a very short time. All such<br />proteins must be destroyed, and this is the function<br />of proteasomes. A proteasome is a barrel-shaped<br />organelle made of enzymes that cut protein molecules<br />apart (protease enzymes). Proteins that are to be destroyed,<br />that is, those no longer needed or those that<br />are damaged or misfolded, are tagged by a protein<br />called ubiquitin (sort of a cellular mop or broom) and<br />carried into a proteasome. The protein is snipped<br />into peptides or amino acids, which may be used again<br />for protein synthesis on ribosomes. Proteasomes are<br />particularly important during cell division and during<br />embryonic development, when great changes are taking<br />place very rapidly as cells become specialized.<br />Many of our cells have secretory functions, that is,<br />they produce specific products to be used elsewhere in<br />tissues. Secretion is one task of the Golgi apparatus,<br />a series of flat, membranous sacs, somewhat like<br />a stack of saucers. Carbohydrates are synthesized<br />within the Golgi apparatus, and are packaged, along<br />with other materials, for secretion from the cell.<br />Proteins from the ribosomes or lipids from the<br />smooth endoplasmic reticulum may also be secreted in<br />this way. To secrete a substance, small sacs of the<br />Golgi membrane break off and fuse with the cell<br />membrane, releasing the substance to the exterior of<br />the cell. This is exocytosis, exo meaning “to go out”<br />of the cell.<br />Mitochondria are oval or spherical organelles<br />bounded by a double membrane. The inner membrane<br />has folds called cristae. Within the mitochondria,<br />the aerobic (oxygen-requiring) reactions of cell<br />respiration take place. Therefore, mitochondria are<br />the site of ATP (and hence energy) production. Cells<br />that require large amounts of ATP, such as muscle<br />cells, have many mitochondria to meet their need for<br />energy. Mitochondria contain their own genes in a<br />single DNA molecule and duplicate themselves when<br />a cell divides. An individual’s mitochondrial DNA<br />(mDNA) is of maternal origin, that is, from the mitochondria<br />that were present in the ovum, or egg cell,<br />which was then fertilized by a sperm cell. The mitochondria<br />of the sperm cell usually do not enter the<br />ovum during fertilization, because they are not found<br />in the head of the sperm with the chromosomes (see<br />Fig. 20–1).<br />Lysosomes are single-membrane structures that<br />contain digestive enzymes. When certain white blood<br />cells engulf bacteria, the bacteria are digested and<br />destroyed by these lysosomal enzymes. Worn-out cell<br />parts and dead cells are also digested by these enzymes.<br />This is a beneficial process, and is necessary before tissue<br />repair can begin. But it does have a disadvantage in<br />that lysosomal digestion contributes to inflammation<br />in damaged tissues. An excess of inflammation can<br />start a vicious cycle, actually a positive feedback mechanism,<br />that results in extensive tissue damage.<br />Many of our cells are capable of dividing, or reproducing,<br />themselves. Centrioles are a pair of rodshaped<br />structures perpendicular to one another,<br />located just outside the nucleus. Their function is to<br />organize the spindle fibers during cell division. The<br />spindle fibers are contracting proteins that pull the<br />two sets of chromosomes apart, toward the ends of the<br />original cell as it divides into two new cells. Each new<br />cell then has a full set of chromosomes.<br />Cilia and flagella are mobile thread-like projections<br />through the cell membrane; each is anchored by<br />a basal body just within the membrane. Cilia are usually<br />shorter than flagella, and an individual cell has<br />many of them on its free surface. The cilia of a cell<br />beat in unison and sweep materials across the cell surface.<br />Cells lining the fallopian tubes, for example, have<br />cilia to sweep the egg cell toward the uterus. The only<br />human cell with a flagellum is the sperm cell. The flagellum<br />provides motility, or movement, for the sperm<br />cell.<br />Microvilli are folds of the cell membrane on the<br />free surface of a cell. These folds greatly increase the<br />Cells 51<br />surface area of the membrane, and are part of the cells<br />lining organs that absorb materials. The small intestine,<br />for example, requires a large surface area for the<br />absorption of nutrients, and many of its lining cells<br />have microvilli. Some cells of the kidney tubules also<br />have microvilli (see Fig. 1–1) that provide for the efficient<br />reabsorption of useful materials back to the<br />blood.<br />The functions of the cell organelles are summarized<br />in Table 3–1.<br />CELLULAR TRANSPORT<br />MECHANISMS<br />Living cells constantly interact with the blood or tissue<br />fluid around them, taking in some substances and<br />secreting or excreting others. There are several mechanisms<br />of transport that enable cells to move materials<br />into or out of the cell: diffusion, osmosis, facilitated<br />diffusion, active transport, filtration, phagocytosis,<br />and pinocytosis. Some of these take place without the<br />52 Cells<br />Table 3–1 FUNCTIONS OF CELL<br />ORGANELLES<br />Organelle Function<br />Endoplasmic<br />reticulum (ER)<br />Ribosomes<br />Proteasomes<br />Golgi apparatus<br />Mitochondria<br />Lysosomes<br />Centrioles<br />Cilia<br />Flagellum<br />Microvilli<br />• Passageway for transport of<br />materials within the cell<br />• Synthesis of lipids<br />• Site of protein synthesis<br />• Site of destruction of old<br />or damaged proteins<br />• Synthesis of carbohydrates<br />• Packaging of materials for secretion<br />from the cell<br />• Site of aerobic cell respiration—ATP<br />production<br />• Contain enzymes to digest ingested<br />material or damaged tissue<br />• Organize the spindle fibers during<br />cell division<br />• Sweep materials across the cell<br />surface<br />• Enables a cell to move<br />• Increase a cell’s surface area for<br />absorption<br />expenditure of energy by the cells. But others do<br />require energy, often in the form of ATP. Each of<br />these mechanisms is described in the following sections<br />and an example is included to show how each is<br />important to the body.<br />DIFFUSION<br />Diffusion is the movement of molecules from an area<br />of greater concentration to an area of lesser concentration<br />(that is, with or along a concentration gradient).<br />Diffusion occurs because molecules have free<br />energy; that is, they are always in motion. The molecules<br />in a solid move very slowly; those in a liquid<br />move faster; and those in a gas move faster still, such<br />as when ice absorbs heat energy, melts, and then evaporates.<br />Imagine a green sugar cube at the bottom of a<br />glass of water (green so that we can see it). As the<br />sugar dissolves, the sugar molecules collide with one<br />another or the water molecules, and the green color<br />seems to rise in the glass. These collisions spread out<br />the sugar molecules until they are evenly dispersed<br />among the water molecules (this would take a very<br />long time), and the water eventually becomes entirely<br />green. The molecules are still moving, but as some go<br />to the top, others go to the bottom, and so on. Thus,<br />an equilibrium (or steady state) is reached.<br />Diffusion is a very slow process, but may be an<br />effective transport mechanism across microscopic distances.<br />Within the body, the gases oxygen and carbon<br />dioxide move by diffusion. In the lungs, for example,<br />there is a high concentration of oxygen in the alveoli<br />(air sacs) and a low concentration of oxygen in the<br />blood in the surrounding pulmonary capillaries (see<br />Fig. 3–3). The opposite is true for carbon dioxide: a<br />low concentration in the air in the alveoli and a high<br />concentration in the blood in the pulmonary capillaries.<br />These gases diffuse in opposite directions, each<br />moving from where there is more to where there is<br />less. Oxygen diffuses from the air to the blood to be<br />circulated throughout the body. Carbon dioxide diffuses<br />from the blood to the air to be exhaled.<br />OSMOSIS<br />Osmosis may be simply defined as the diffusion of<br />water through a selectively permeable membrane.<br />That is, water will move from an area with more water<br />present to an area with less water. Another way to say<br />this is that water will naturally tend to move to an area<br />where there is more dissolved material, such as salt or<br />sugar. If a 2% salt solution and a 6% salt solution are<br />separated by a membrane allowing water but not salt<br />to pass through it, water will diffuse from the 2% salt<br />solution to the 6% salt solution. The result is that the<br />2% solution will become more concentrated, and the<br />6% solution will become more dilute.<br />In the body, the cells lining the small intestine<br />absorb water from digested food by osmosis. These<br />cells have first absorbed salts, have become more<br />“salty,” and water follows salt into the cells (see Fig.<br />3–3). The process of osmosis also takes place in the<br />kidneys, which reabsorb large amounts of water (many<br />gallons each day) to prevent its loss in urine. Box 3–1:<br />Terminology of Solutions lists some terminology we<br />use when discussing solutions and the effects of various<br />solutions on cells.<br />Cells 53<br />BOX 3–1 TERMINOLOGY OF SOLUTIONS<br />Human cells or other body fluids contain many dissolved<br />substances (called solutes) such as salts,<br />sugars, acids, and bases. The concentration of<br />solutes in a fluid creates the osmotic pressure of<br />the solution, which in turn determines the movement<br />of water through membranes.<br />As an example here, we will use sodium chloride<br />(NaCl). Human cells have an NaCl concentration of<br />0.9%. With human cells as a reference point, the<br />relative NaCl concentrations of other solutions may<br />be described with the following terms:<br />Isotonic—a solution with the same salt concentration<br />as in cells.<br />The blood plasma is isotonic to red blood cells.<br />Hypotonic—a solution with a lower salt concentration<br />than in cells.<br />Distilled water (0% salt) is hypotonic to human<br />cells.<br />Hypertonic—a solution with a higher salt concentration<br />than in cells.<br />Seawater (3% salt) is hypertonic to human cells.<br />Refer now to the diagrams shown in Box<br />Figure 3–A of red blood cells (RBCs) in each<br />of these different types of solutions, and<br />note the effect of each on osmosis:<br />• When RBCs are in plasma, water moves into<br />and out of them at equal rates, and the cells<br />remain normal in size and water content.<br />• If RBCs are placed in distilled water, more<br />water will enter the cells than leave, and the<br />cells will swell and eventually burst.<br />• If RBCs are placed in seawater, more water will<br />leave the cells than enter, and the cells will<br />shrivel and die.<br />This knowledge of osmotic pressure is used<br />when replacement fluids are needed for a patient<br />who has become dehydrated. Isotonic solutions<br />are usually used; normal saline and Ringer’s solution<br />are examples. These will provide rehydration<br />without causing osmotic damage to cells or<br />extensive shifts of fluid between the blood and<br />tissues.<br />Box Figure 3–A Red blood cells in different solutions and the effect of osmosis in each.<br />FACILITATED DIFFUSION<br />The word facilitate means to help or assist. In facilitated<br />diffusion, molecules move through a membrane<br />from an area of greater concentration to an area of<br />lesser concentration, but they need some help to do<br />this.<br />In the body, our cells must take in glucose to use for<br />ATP production. Glucose, however, will not diffuse<br />through most cell membranes by itself, even if there is<br />more outside the cell than inside. Diffusion of glucose<br />into most cells requires a glucose transporter, which<br />may also be called a carrier enzyme. These transporters<br />are proteins that are part of the cell membrane.<br />Glucose bonds to the transporter (see Fig. 3–3),<br />and by doing so changes the shape of the protein. This<br />physical change propels the glucose into the interior<br />of the cell. Other transporters are specific for other<br />organic molecules such as amino acids.<br />ACTIVE TRANSPORT<br />Active transport requires the energy of ATP to move<br />molecules from an area of lesser concentration to an<br />area of greater concentration. Notice that this is the<br />opposite of diffusion, in which the free energy of molecules<br />causes them to move to where there are fewer<br />of them. Active transport is therefore said to be movement<br />against a concentration gradient.<br />In the body, nerve cells and muscle cells have<br />“sodium pumps” to move sodium ions (Na ) out of<br />the cells. Sodium ions are more abundant outside the<br />cells, and they constantly diffuse into the cell (through<br />specific diffusion channels), their area of lesser concentration<br />(see Fig. 3–3). Without the sodium pumps<br />to return them outside, the incoming sodium ions<br />would bring about an unwanted nerve impulse or<br />muscle contraction. Nerve and muscle cells constantly<br />produce ATP to keep their sodium pumps (and similar<br />potassium pumps) working and prevent spontaneous<br />impulses.<br />Another example of active transport is the absorption<br />of glucose and amino acids by the cells lining<br />the small intestine. The cells use ATP to absorb these<br />nutrients from digested food, even when their intracellular<br />concentration becomes greater than their<br />extracellular concentration.<br />FILTRATION<br />The process of filtration also requires energy, but the<br />energy needed does not come directly from ATP. It is<br />the energy of mechanical pressure. Filtration means<br />that water and dissolved materials are forced through<br />a membrane from an area of higher pressure to an area<br />of lower pressure.<br />In the body, blood pressure is created by the<br />54 Cells<br />Table 3–2 CELLULAR TRANSPORT MECHANISMS<br />Mechanism Definition Example in the Body<br />Diffusion<br />Osmosis<br />Facilitated diffusion<br />Active transport<br />Filtration<br />Phagocytosis<br />Pinocytosis<br />Movement of molecules from an area of<br />greater concentration to an area of<br />lesser concentration.<br />The diffusion of water.<br />Carrier and transporter enzymes move molecules<br />across cell membranes.<br />Movement of molecules from an area of<br />lesser concentration to an area of greater<br />concentration (requires ATP).<br />Movement of water and dissolved substances<br />from an area of higher pressure to an area<br />of lower pressure (blood pressure).<br />A moving cell engulfs something.<br />A stationary cell engulfs something.<br />Exchange of gases in the lungs or body tissues.<br />Absorption of water by the small intestine<br />or kidneys.<br />Intake of glucose by most cells.<br />Absorption of amino acids and glucose from food<br />by the cells of the small intestine.<br />Sodium and potassium pumps in muscle and<br />nerve cells.<br />Formation of tissue fluid; the first step in the formation<br />of urine.<br />White blood cells engulf bacteria.<br />Cells of the kidney tubules reabsorb small proteins.<br />55<br />•<br />A Diffusion<br />B Osmosis<br />C Facilitated Diffusion<br />D Active transport<br />E Filtration<br />F Phagocytosis<br />G Pinocytosis<br />Small protein<br />Cell of<br />kidney<br />tubule<br />•<br />•<br />•<br />Lysosome Bacterium<br />White blood cell<br />•<br />• •<br />•<br />•<br />H2O<br />BP<br />Glucose<br />Amino<br />acid<br />RBC Capillary in<br />tissues<br />•<br />•<br />•<br />•<br />Cell<br />membrane<br />Na+<br />Tissue fluid<br />ATP Active<br />transport<br />Cytoplasm channel<br />Diffusion<br />channel<br />•<br />•<br />•<br />Cell membrane of intestinal cell<br />H2O<br />Na+<br />Cytoplasm<br />•<br />Alveolus of lung<br />•<br />Capillary<br />O2<br />O2<br />O2<br />O2<br />O2<br />O2<br />O2<br />O2<br />O2<br />O2<br />O2<br />CO2<br />CO2 CO2<br />CO2<br />CO2<br />CO2<br />•<br />Glucose<br />•<br />Transporter<br />Cytoplasm<br />Tissue fluid<br />•<br />Cell membrane<br />Figure 3–3. Cellular transport mechanisms. (A) Diffusion in an alveolus in the lung.<br />(B) Osmosis in the small intestine. (C) Facilitated diffusion in a muscle cell. (D) Active transport<br />in a muscle cell. (E) Filtration in a capillary. (F) Phagocytosis by a white blood cell.<br />(G) Pinocytosis by a cell of the kidney tubules. See text for description.<br />QUESTION: Which mechanism depends on blood pressure? Which depends on the movement<br />of a cell?<br />pumping of the heart. Filtration occurs when blood<br />flows through capillaries, whose walls are only one<br />cell thick and very permeable. The blood pressure in<br />capillaries is higher than the pressure of the surrounding<br />tissue fluid. In capillaries throughout the body,<br />blood pressure forces plasma (water) and dissolved<br />materials through the capillary membranes into the<br />surrounding tissue spaces (see Fig. 3–3). This creates<br />more tissue fluid and is how cells receive glucose,<br />amino acids, and other nutrients. Blood pressure<br />in the capillaries of the kidneys also brings about<br />filtration, which is the first step in the formation of<br />urine.<br />PHAGOCYTOSIS AND PINOCYTOSIS<br />These two processes are similar in that both involve a<br />cell engulfing something, and both are forms of endocytosis,<br />endo meaning “to take into” a cell. An example<br />of phagocytosis is a white blood cell engulfing<br />bacteria. The white blood cell flows around the bacterium<br />(see Fig. 3–3), taking it in and eventually<br />digesting it. Digestion is accomplished by the enzymes<br />in the cell’s lysosomes.<br />Other cells that are stationary may take in small<br />molecules that become adsorbed or attached to their<br />membranes. The cells of the kidney tubules reabsorb<br />small proteins by pinocytosis (see Fig. 3–3) so that<br />the protein is not lost in urine.<br />Table 3–2 summarizes the cellular transport mechanisms.<br />THE GENETIC CODE<br />AND PROTEIN SYNTHESIS<br />The structure of DNA, RNA, and protein was<br />described in Chapter 2. We will review some of the<br />essentials here, and go a step further with a simple<br />description of how all of these organic molecules are<br />involved in the process of protein synthesis.<br />DNA AND THE GENETIC CODE<br />DNA is a double strand of nucleotides in the form of<br />a double helix, very much like a spiral ladder. The<br />uprights of the ladder are made of alternating phosphate<br />groups and deoxyribose sugar molecules. The<br />rungs of the ladder are made of the four nitrogenous<br />bases, always found in complementary pairs: adenine<br />with thymine (A–T) and guanine with cytosine (G–C).<br />Although DNA contains just these four bases, the<br />bases may be arranged in many different sequences<br />(reading up or down the ladder). It is the sequence of<br />bases, the sequence of A, T, C, and G, that is the<br />genetic code. The DNA of our 46 chromosomes may<br />also be called our genome, which is the term for the<br />total genetic information in a particular species. The<br />human genome is believed to contain about 3 billion<br />base pairs, and the number of our genes is now estimated<br />to be between 20,000 and 25,000 (or perhaps as<br />many as 30,000, but much lower than previously<br />thought).<br />56 Cells<br />Table 3–3 PROTEIN SYNTHESIS<br />Molecule or Organelle Function<br />DNA<br />mRNA (messenger RNA)<br />Ribosomes<br />tRNA (transfer RNA)<br />• A double strand (helix) of nucleotides that is the genetic code in the chromosomes<br />of cells.<br />• A gene is the sequence of bases (segment of DNA) that is the code for one protein.<br />• A single strand of nucleotides formed as a complementary copy of a gene in the DNA.<br />• Now contains the triplet code: three bases is the code for one amino acid (a codon).<br />• Leaves the DNA in the nucleus, enters the cytoplasm of the cell, and becomes<br />attached to ribosomes.<br />• The cell organelles that are the site of protein synthesis.<br />• Attach the mRNA molecule.<br />• Contain enzymes to form peptide bonds between amino acids.<br />• Picks up amino acids (from food) in the cytoplasm and transports them to their proper<br />sites (triplets) along the mRNA molecule; has anticodons to match mRNA codons.<br />57<br />Cell nucleus<br />Nuclear pore<br />Nuclear membrane<br />A<br />A Codon<br />A<br />DNA<br />Amino acids<br />Peptide bonds<br />mRNA<br />tRNA<br />Ribosome<br />Anticodon<br />A A G C A U A A A G U C U U U<br />G<br />U A U U U<br />Figure 3–4. Protein synthesis. The mRNA is formed as a copy of a portion of the DNA in<br />the nucleus of a cell. In the cytoplasm, the mRNA becomes attached to ribosomes. See text<br />for further description.<br />QUESTION: A tRNA molecule has two attachment sites; what is each for?<br />Recall that in Chapter 2 you read that a gene is the<br />genetic code for one protein. This is a simplification,<br />and the functioning of genes is often much more complex.<br />We have genes with segments that may be shuffled<br />or associated in many combinations, with the<br />potential for coding for many more proteins. A full<br />explanation is beyond the scope of our book, so for the<br />sake of simplicity, and in the following discussion, we<br />will say that a gene is the code for one protein. Recall<br />too that a protein is a specific sequence of amino acids.<br />Therefore, a gene, or segment of DNA, is the code for<br />the sequence of amino acids in a particular protein.<br />The code for a single amino acid consists of three<br />bases in the DNA molecule; this triplet of bases may<br />be called a codon (see Fig. 3–4). There is a triplet of<br />bases in the DNA for each amino acid in the protein.<br />If a protein consists of 100 amino acids, the gene for<br />that protein would consist of 100 triplets, or 300 bases.<br />Some of the triplets will be the same, since the same<br />amino acid may be present in several places within the<br />protein. Also part of the gene are other triplets that<br />start and stop the process of making the protein,<br />rather like capital letters or punctuation marks start<br />and stop sentences.<br />RNA AND PROTEIN SYNTHESIS<br />RNA, the other nucleic acid, has become a surprising<br />molecule, in that it has been found to have quite a few<br />functions. It may be involved in the repair of DNA,<br />and it is certainly involved in gene expression. The<br />expression of a gene means that the product of the<br />gene is somehow apparent to us, in a way we can see<br />or measure, or is not apparent when it should be.<br />Examples would be having brown eyes or blue eyes, or<br />having or not having the intestinal enzyme lactase to<br />digest milk sugar. Although these functions of RNA<br />are essential for us, they too are beyond the scope of<br />our book, so the roles of RNA in the process of protein<br />synthesis will be our focus.<br />The transcription and translation of the genetic<br />code in DNA into proteins require RNA. DNA is<br />found in the chromosomes in the nucleus of the cell,<br />but protein synthesis takes place on the ribosomes in<br />the cytoplasm. Messenger RNA (mRNA) is the<br />intermediary molecule between these two sites.<br />When a protein is to be made, the segment of DNA<br />that is its gene uncoils, and the hydrogen bonds<br />between the base pairs break (see Fig. 3–4). Within<br />the nucleus are RNA nucleotides (A, C, G, U) and<br />enzymes to construct a single strand of nucleotides<br />that is a complementary copy of half the DNA gene<br />(with uracil in place of thymine). This process is transcription,<br />or copying, and the copy of the gene is<br />mRNA, which now has the codons for the amino acids<br />of the protein, and then separates from the DNA. The<br />gene coils back into the double helix, and the mRNA<br />leaves the nucleus, enters the cytoplasm, and becomes<br />attached to ribosomes.<br />As the copy of the gene, mRNA is a series of triplets<br />of bases; each triplet is a codon, the code for one<br />amino acid. Another type of RNA, called transfer<br />RNA (tRNA), is also found in the cytoplasm. Each<br />tRNA molecule has an anticodon, a triplet complementary<br />to a triplet on the mRNA. The tRNA<br />molecules pick up specific amino acids (which have<br />come from protein in our food) and bring them to<br />their proper triplets on the mRNA. This process is<br />translation; that is, it is as if we are translating from<br />one language to another—the language of nucleotide<br />bases to that of amino acids. The ribosomes contain<br />enzymes to catalyze the formation of peptide bonds<br />between the amino acids. When an amino acid has<br />been brought to each triplet on the mRNA, and<br />all peptide bonds have been formed, the protein is<br />finished.<br />The protein then leaves the ribosomes and may be<br />transported by the endoplasmic reticulum to wherever<br />it is needed in the cell, or it may be packaged by the<br />Golgi apparatus for secretion from the cell. A summary<br />of the process of protein synthesis is found in<br />Table 3–3.<br />Thus, the expression of the genetic code may be<br />described by the following sequence:<br />Each of us is the sum total of our genetic characteristics.<br />Blood type, hair color, muscle proteins, nerve<br />cells, and thousands of other aspects of our structure<br />and functioning have their basis in the genetic code of<br />DNA.<br />If there is a “mistake” in the DNA, that is, incorrect<br />bases or triplets of bases, this mistake will be copied by<br />the mRNA. The result is the formation of a malfunctioning<br />or non-functioning protein. This is called a<br />genetic or hereditary disease, and a specific example<br />is described in Box 3–2: Genetic Disease—Sickle-Cell<br />Anemia.<br />DNA RNA Proteins:<br />Structural Enzymes<br />Catalyze Reactions<br />Hereditary Characteristics<br />Proteins<br />58 Cells<br />59<br />BOX 3–2 GENETIC DISEASE—SICKLE-CELL ANEMIA<br />Normal hemoglobin<br />Normal red blood cells (RBCs)<br />Sickle red blood cells<br />Sickle<br />hemoglobin<br />(HbS)<br />Deoxygenation<br />α<br />β<br />β<br />α<br />Iron in heme<br />Box Figure 3–B Structure of hemoglobin<br />A and sickle-cell hemoglobin and<br />their effect on red blood cells.<br />A genetic disease is a hereditary disorder, one<br />that may be passed from generation to generation.<br />Although there are hundreds of genetic diseases,<br />they all have the same basis: a mistake in DNA.<br />Because DNA makes up the chromosomes that are<br />found in eggs and sperm, this mistake may be<br />passed from parents to children.<br />Sickle-cell anemia is the most common genetic<br />disorder among people of African descent and<br />affects the hemoglobin in red blood cells. Normal<br />hemoglobin, called hemoglobin A (HbA), is a protein<br />made of two alpha chains (141 amino acids<br />each) and two beta chains (146 amino acids each).<br />In sickle-cell hemoglobin (HbS), the sixth amino<br />acid in each beta chain is incorrect; valine instead of<br />the glutamic acid found in HbA. This difference<br />seems minor—only 2 incorrect amino acids out of<br />more than 500—but the consequences for the person<br />are very serious.<br />HbS has a great tendency to crystallize when<br />oxygen levels are low, as is true in capillaries. When<br />HbS crystallizes, the red blood cells are deformed<br />into crescents (sickles) and other irregular shapes.<br />These irregular, rigid red blood cells clog and rupture<br />capillaries, causing internal bleeding and<br />severe pain. These cells are also fragile and break up<br />easily, leading to anemia and hypoxia (lack of oxygen).<br />Treatment of this disease has improved<br />greatly, but it is still incurable.<br />What has happened to cause the formation of<br />HbS rather than HbA? Hemoglobin is a protein; the<br />gene for its beta chain is in DNA (chromosome 11).<br />One amino acid in the beta chains is incorrect,<br />therefore, one triplet in its DNA gene must be, and<br />is, incorrect. This mistake is copied by mRNA in the<br />cells of the red bone marrow, and HbS is synthesized<br />in red blood cells.<br />Sickle-cell anemia is a recessive genetic disease,<br />which means that a person with one gene for HbS<br />and one gene for HbA will have “sickle-cell trait.”<br />Such a person usually will not have the severe<br />effects of sickle-cell anemia, but may pass the gene<br />for HbS to children. It is estimated that 9% of<br />African-Americans have sickle-cell trait and about<br />1% have sickle-cell anemia.<br />CELL DIVISION<br />Cell division is the process by which a cell reproduces<br />itself. There are two types of cell division, mitosis and<br />meiosis. Although both types involve cell reproduction,<br />their purposes are very different.<br />MITOSIS<br />Each of us began life as one cell, a fertilized egg. Each<br />of us now consists of billions of cells produced by the<br />process of mitosis. In mitosis, one cell with the<br />diploid number of chromosomes (the usual number,<br />46 for people) divides into two identical cells, each<br />with the diploid number of chromosomes. This production<br />of identical cells is necessary for the growth of<br />the organism and for repair of tissues (see also Box<br />3–3: Abnormal Cellular Functioning—Cancer).<br />Before mitosis can take place, a cell must have two<br />complete sets of chromosomes, because each new cell<br />must have the diploid number. The process of DNA<br />replication enables each chromosome (in the form of<br />chromatin) to make a copy of itself. The time during<br />which this takes place is called interphase, the time<br />between mitotic divisions. Although interphase is<br />sometimes referred to as the resting stage, resting<br />means “not dividing” rather than “inactive.” The cell<br />is quite actively producing a second set of chromosomes<br />and storing energy in ATP.<br />The long, thin, and invisible chromatin molecules<br />then begin to coil very precisely and extensively, and if<br />we were looking at the nucleus of a living cell under a<br />microscope, we would see the duplicated chromosomes<br />appear. Each would look somewhat like the letter<br />X, because the original DNA molecule and its copy<br />(now called chromatids) are still attached.<br />The stages of mitosis are prophase, metaphase,<br />anaphase, and telophase. What happens in each of<br />these stages is described in Table 3–4. As you read the<br />events of each stage, refer to Fig. 3–5, which depicts<br />mitosis in a cell with a diploid number of four.<br />As mentioned previously, mitosis is essential for<br />repair of tissues, to replace damaged or dead cells.<br />Some examples may help illustrate this. In several<br />areas of the body, mitosis takes place constantly. These<br />sites include the epidermis of the skin, the stomach<br />lining, and the red bone marrow. For each of these<br />sites, there is a specific reason why this constant mitosis<br />is necessary.<br />What happens to the surface of the skin? The dead,<br />outer cells are worn off by contact with the environment.<br />Mitosis of the epidermal cells in the lower living<br />layer replaces these cells, and the epidermis<br />maintains its normal thickness.<br />The stomach lining, although internal, is also constantly<br />worn away. Gastric juice, especially hydrochloric<br />acid, is very damaging to cells. Rapid mitosis of the<br />several kinds of lining cells replaces damaged cells and<br />keeps the stomach lining intact.<br />One of the functions of red bone marrow is the<br />production of red blood cells. Because red blood cells<br />have a life span of only about 120 days, new ones are<br />needed to replace the older ones that die. Very rapid<br />mitosis in the red bone marrow produces approximately<br />2 million new red blood cells every second.<br />These dividing cells in the red bone marrow are<br />among the stem cells present in the body. A stem cell<br />is an unspecialized cell that may develop into several<br />different kinds of cells. Stem cells in the red bone marrow<br />may become red blood cells, white blood cells, or<br />platelets. These marrow stem cells are often called<br />adult stem cells, and many, if not all, of the body’s<br />organs have such cells. Embryonic stem cells will be<br />described in a later chapter; these are cells in which all<br />of the DNA still has the potential to be active. They<br />may become any of the more than 200 different kinds<br />of human cells. The stem cells found in the umbilical<br />cords of newborns are between the adult and embryonic<br />cells in terms of their potential.<br />It is also important to be aware of the areas of the<br />body where mitosis does not take place. In an adult,<br />most muscle cells and neurons (nerve cells) do not<br />reproduce themselves. If they die, their functions are<br />also lost. Someone whose spinal cord has been severed<br />will have paralysis and loss of sensation below the level<br />of the injury. The spinal cord neurons do not undergo<br />mitosis to replace the ones that were lost, and such an<br />injury is permanent.<br />Skeletal muscle cells are capable of limited mitosis<br />for repair. The heart is made of cardiac muscle cells,<br />which, like neurons, seem to be incapable of mitosis. A<br />heart attack (myocardial infarction) means that a portion<br />of cardiac muscle dies because of lack of oxygen.<br />These cells are not replaced, and the heart will be a<br />less effective pump. If a large enough area of the heart<br />muscle dies, the heart attack may be fatal.<br />Some research has found evidence for the potential<br />for mitosis after damage in both the central nervous<br />system and the heart. Such cell division may be that of<br />neurons or muscle cells that were stimulated to divide<br />60 Cells<br />by chemicals from the adjacent damaged tissue. Or the<br />dividing cells may be stem cells that are among the<br />specialized cells. (A region of the brain called the hippocampus,<br />which is necessary to form new memories,<br />seems to have cells capable of division.) At present we<br />do not have definitive knowledge, but we do know that<br />for most people with heart damage or central nervous<br />system injury, mitosis does not take place, or not sufficiently<br />enough to replace the cells that have died and<br />preserve or restore normal functioning of the organ.<br />Research is continuing, and may eventually find the<br />stimulus necessary to produce extended mitosis that<br />would bring about true tissue repair.<br />MEIOSIS<br />Meiosis is a more complex process of cell division that<br />results in the formation of gametes, which are egg<br />and sperm cells. In meiosis, one cell with the diploid<br />number of chromosomes divides twice to form four<br />Cells 61<br />BOX 3–3 ABNORMAL CELLULAR FUNCTIONING—CANCER<br />the trigger is believed to be infection with certain<br />viruses that cause cellular mutations. Carriers of<br />hepatitis B virus, for example, are more likely to<br />develop primary liver cancer than are people who<br />have never been exposed to this virus. Research has<br />discovered two genes, one on chromosome 2 and<br />the other on chromosome 3, that contribute to a<br />certain form of colon cancer. Both of these genes<br />are the codes for proteins that correct the “mistakes”<br />that may occur when the new DNA is synthesized.<br />When these repair proteins do not<br />function properly, the mistakes (mutations) in the<br />DNA lead to the synthesis of yet other faulty proteins<br />that impair the functioning of the cell and predispose<br />it to becoming malignant.<br />Once cells have become malignant, their functioning<br />cannot return to normal, and though the<br />immune system will often destroy such cells, sometimes<br />it does not, especially as we get older.<br />Therefore, the treatments for cancer are directed at<br />removing or destroying the abnormal cells. Surgery<br />to remove tumors, radiation to destroy cells, and<br />chemotherapy to stop cell division or interfere<br />with other aspects of cell metabolism are all aspects<br />of cancer treatment.<br />New chemotherapy drugs are becoming more<br />specific, with very precise targets. For example, the<br />cells of several types of solid-tumor cancers have<br />been found to have mutations in the gene for the<br />cell membrane receptor for a natural growth factor<br />(epidermal growth factor receptor, or EGFR). These<br />altered receptors, when triggered by their usual<br />growth factor, then cause the cell to divide uncontrollably,<br />an abnormal response. Medications that<br />target only these altered receptors have already<br />been developed for some forms of lung cancer and<br />breast cancer. Not only do they show promise in<br />treating the cancer, they do not have the side<br />effects of other forms of chemotherapy.<br />There are more than 200 different types of cancer,<br />all of which are characterized by abnormal cellular<br />functioning. Normally, our cells undergo mitosis<br />only when necessary and stop when appropriate. A<br />cut in the skin, for example, is repaired by mitosis,<br />usually without formation of excess tissue. The new<br />cells fill in the damaged area, and mitosis slows<br />when the cells make contact with surrounding cells.<br />This is called contact inhibition, which limits the<br />new tissue to just what is needed. Malignant<br />(cancer) cells, however, are characterized by uncontrolled<br />cell division. Our cells are genetically programmed<br />to have particular life spans and to divide<br />or die. One gene is known to act as a brake on cell<br />division; another gene enables cells to live indefinitely,<br />beyond their normal life span, and to keep<br />dividing. Any imbalance in the activity of these<br />genes may lead to abnormal cell division. Such cells<br />are not inhibited by contact with other cells, keep<br />dividing, and tend to spread.<br />A malignant tumor begins in a primary site such<br />as the colon, then may spread or metastasize. Often<br />the malignant cells are carried by the lymph or<br />blood to other organs such as the liver, where secondary<br />tumors develop. Metastasis is characteristic<br />only of malignant cells; benign tumors do not<br />metastasize but remain localized in their primary<br />site.<br />What causes normal cells to become malignant?<br />At present, we have only partial answers. A malignant<br />cell is created by a mutation, a genetic<br />change that brings about abnormal cell functions<br />or responses and often leads to a series of mutations.<br />Environmental substances that cause mutations<br />are called carcinogens. One example is the<br />tar found in cigarette smoke, which is definitely a<br />cause of lung cancer. Ultraviolet light may also<br />cause mutations, especially in skin that is overexposed<br />to sunlight. For a few specific kinds of cancer,<br />cells, each with the haploid number (half the usual<br />number) of chromosomes. In women, meiosis takes<br />place in the ovaries and is called oogenesis. In men,<br />meiosis takes place in the testes and is called spermatogenesis.<br />The differences between oogenesis and<br />spermatogenesis will be discussed in Chapter 20, The<br />Reproductive Systems.<br />The egg and sperm cells produced by meiosis have<br />the haploid number of chromosomes, which is 23 for<br />humans. Meiosis is sometimes called reduction division<br />because the division process reduces the chromosome<br />number in the egg or sperm. Then, during<br />fertilization, in which the egg unites with the sperm,<br />the 23 chromosomes of the sperm plus the 23 chromosomes<br />of the egg will restore the diploid number<br />of 46 in the fertilized egg. Thus, the proper chromo-<br />62 Cells<br />Interphase<br />Nucleus<br />Prophase<br />Centrioles<br />Pair of chromatids<br />Centromere<br />Equator of cell<br />Spindle<br />fibers<br />Metaphase<br />Anaphase<br />Nuclear membrane re-forms<br />Telophase<br />Cytoplasm divides<br />Figure 3–5. Stages of mitosis in a cell with the diploid number of four. See Table 3–4 for<br />description.<br />QUESTION: In prophase, what is a pair of chromatids made of?<br />some number is maintained in the cells of the new<br />individual.<br />AGING AND CELLS<br />Multicellular organisms, including people, age and<br />eventually die; our cells do not have infinite life spans.<br />It has been proposed that some cells capable of mitosis<br />are limited to a certain number of divisions; that is,<br />every division is sort of a tick-tock off a biological<br />clock. We do not yet know exactly what this cellular<br />biological clock is. There is evidence that the ends of<br />chromosomes, called telomeres, may be an aspect of it.<br />With each cell division, part of the telomeres is lost<br />(rather like a piece of rope fraying at both ends), and<br />eventually the telomeres are gone. With the next division,<br />the ends of the chromosomes, actual genes,<br />begin to be lost. This may be one signal that a cell’s life<br />span has come to an end (there are probably many different<br />kinds of signals).<br />Cellular aging also involves the inevitable deterioration<br />of membranes and cell organelles. Just as the<br />parts of a car break down in time, so too will cells.<br />Unlike cars or machines, however, cells can often<br />repair themselves, but they do have limits. As cells<br />age, structural proteins break down and are not<br />replaced, or necessary enzymes are not synthesized.<br />Proteins called chaperones, which are responsible<br />for the proper folding of many other proteins and<br />for the repair or disposal of damaged proteins, no<br />longer function as well as cells age. Without chaperones,<br />damaged proteins accumulate within cells and<br />disrupt normal cellular processes. Clinical manifestations<br />of impaired chaperones include cataracts (see<br />Box 9–1) and neurodegenerative diseases such as<br />Alzheimer’s disease (see Box 8–6), Parkinson’s disease<br />(see Box 8–7), and Huntington’s disease (see<br />Table 21–4).<br />Much about the chemistry of the aging process<br />remains a mystery, though we can describe what happens<br />to organs and to the body as a whole. Each of the<br />following chapters on body systems provides a brief<br />discussion of how aging affects the system. Keep in<br />mind that a system is the sum of its cells, in tissues and<br />organs, and that all aging is ultimately at the cellular<br />level.<br />SUMMARY<br />As mentioned at the beginning of this chapter, human<br />cells work closely together and function interdependently.<br />Each type of human cell makes a contribution to<br />the body as a whole. Usually, however, cells do not<br />function as individuals, but rather in groups. Groups<br />of cells with similar structure and function form a tissue,<br />which is the next level of organization.<br />Cells 63<br />Table 3–4 STAGES OF MITOSIS<br />Stage Events<br />Prophase<br />Metaphase<br />Anaphase<br />Telophase<br />Cytokinesis<br />1. The chromosomes coil up and become visible as short rods. Each chromosome is really two<br />chromatids (original DNA plus its copy) still attached at a region called the centromere.<br />2. The nuclear membrane disappears.<br />3. The centrioles move toward opposite poles of the cell and organize the spindle fibers,<br />which extend across the equator of the cell.<br />1. The pairs of chromatids line up along the equator of the cell. The centromere of each pair<br />is attached to a spindle fiber.<br />2. The centromeres now divide.<br />1. Each chromatid is now considered a separate chromosome; there are two complete and<br />separate sets.<br />2. The spindle fibers contract and pull the chromosomes, one set toward each pole of the cell.<br />1. The sets of chromosomes reach the poles of the cell and become indistinct as their DNA<br />uncoils to form chromatin.<br />2. A nuclear membrane re-forms around each set of chromosomes.<br />1. The cytoplasm divides; new cell membrane is formed.<br />Human cells vary in size, shape, and function.<br />Our cells function interdependently to maintain<br />homeostasis.<br />Cell Structure—the major parts of a cell are<br />the cell membrane, nucleus (except mature<br />RBCs), cytoplasm, and cell organelles<br />1. Cell membrane—the selectively permeable boundary<br />of the cell (see Fig. 3–1).<br />• Phospholipids permit diffusion of lipid-soluble<br />materials.<br />• Cholesterol provides stability.<br />• Proteins form channels, transporters, “self” antigens,<br />and receptor sites for hormones or other<br />signaling molecules.<br />2. Nucleus—the control center of the cell; has a<br />double-layer membrane.<br />• Nucleolus—forms ribosomal RNA.<br />• Chromosomes—made of DNA and protein;<br />DNA is the genetic code for the structure and<br />functioning of the cell. A gene is a segment of<br />DNA that is the code for one protein. Human<br />cells have 46 chromosomes, and their genetic<br />information is called the genome.<br />3. Cytoplasm—a watery solution of minerals, gases,<br />and organic molecules; contains the cell organelles;<br />site for many chemical reactions.<br />4. Cell organelles—intracellular structures with specific<br />functions (see Table 3–1 and Fig. 3–2).<br />Cellular Transport Mechanisms—the processes<br />by which cells take in or secrete or<br />excrete materials through the selectively<br />permeable cell membrane (see Fig. 3–3 and<br />Table 3–2).<br />1. Diffusion—movement of molecules from an area<br />of greater concentration to an area of lesser concentration;<br />occurs because molecules have free<br />energy: They are constantly in motion. Oxygen<br />and carbon dioxide are exchanged by diffusion in<br />the lungs and tissues.<br />2. Osmosis—the diffusion of water. Water diffuses to<br />an area of less water, that is, to an area of more dissolved<br />material. The small intestine absorbs water<br />from digested food by osmosis. Isotonic, hypertonic,<br />and hypotonic (see Box 3–1).<br />3. Facilitated diffusion—transporters (carrier<br />enzymes) that are part of the cell membrane permit<br />cells to take in materials that would not diffuse by<br />themselves. Most cells take in glucose by facilitated<br />diffusion.<br />4. Active transport—a cell uses ATP to move substances<br />from an area of lesser concentration to an<br />area of greater concentration. Nerve cells and muscle<br />cells have sodium pumps to return Na ions to<br />the exterior of the cells; this prevents spontaneous<br />impulses. Cells of the small intestine absorb glucose<br />and amino acids from digested food by active<br />transport.<br />5. Filtration—pressure forces water and dissolved<br />materials through a membrane from an area of<br />higher pressure to an area of lower pressure. Tissue<br />fluid is formed by filtration: Blood pressure forces<br />plasma and dissolved nutrients out of capillaries<br />and into tissues. Blood pressure in the kidney capillaries<br />creates filtration, which is the first step in<br />the formation of urine.<br />6. Phagocytosis—(a form of endocytosis) a moving<br />cell engulfs something; white blood cells phagocytize<br />bacteria to destroy them.<br />7. Pinocytosis—(a form of endocytosis) a stationary<br />cell engulfs small molecules; kidney tubule cells<br />reabsorb small proteins by pinocytosis.<br />The Genetic Code and Protein Synthesis (see<br />Fig. 3–4 and Table 3–3)<br />1. DNA and the genetic code<br />• DNA is a double helix with complementary base<br />pairing: A–T and G–C.<br />• The sequence of bases in the DNA is the genetic<br />code for proteins.<br />• The triplet code: three bases (a codon) is the<br />code for one amino acid.<br />• A gene consists of all the triplets that code for a<br />single protein.<br />2. RNA and protein synthesis<br />• Transcription—mRNA is formed as a complementary<br />copy of the sequence of bases in a gene<br />(DNA).<br />• mRNA moves from the nucleus to the ribosomes<br />in the cytoplasm.<br />• tRNA molecules (in the cytoplasm) have anticodons<br />for the triplets on the mRNA.<br />• Translation—tRNA molecules bring amino acids<br />to their proper triplets on the mRNA.<br />• Ribosomes contain enzymes to form peptide<br />bonds between the amino acids.<br />3. Expression of the genetic code<br />64 Cells<br />STUDY OUTLINE<br />• DNA → RNA → proteins (structural proteins<br />and enzymes that catalyze reactions) → hereditary<br />characteristics.<br />• A genetic disease is a “mistake” in the DNA,<br />which is copied by mRNA and results in a malfunctioning<br />protein.<br />Cell Division<br />1. Mitosis—one cell with the diploid number of chromosomes<br />divides once to form two cells, each with<br />the diploid number of chromosomes (46 for<br />humans).<br />• DNA replication forms two sets of chromosomes<br />during interphase.<br />• Stages of mitosis (see Fig. 3–5 and Table 3–4):<br />prophase, metaphase, anaphase, and telophase.<br />Cytokinesis is the division of the cytoplasm following<br />telophase.<br />• Mitosis is essential for growth and for repair and<br />replacement of damaged cells.<br />• Most adult nerve and muscle cells seem unable to<br />divide; their loss may involve permanent loss of<br />function.<br />2. Meiosis—one cell with the diploid number of chromosomes<br />divides twice to form four cells, each with<br />the haploid number of chromosomes (23 for<br />humans).<br />• Oogenesis in the ovaries forms egg cells.<br />• Spermatogenesis in the testes forms sperm cells.<br />• Fertilization of an egg by a sperm restores the<br />diploid number in the fertilized egg.<br />Cells 65<br />REVIEW QUESTIONS<br />1. State the functions of the organic molecules of<br />cell membranes: cholesterol, proteins, and phospholipids.<br />(p. 48)<br />2. Describe the function of each of these cell<br />organelles: mitochondria, lysosomes, Golgi apparatus,<br />ribosomes, proteasomes, and endoplasmic<br />reticulum. (p. 51)<br />3. Explain why the nucleus is the control center of<br />the cell. (p. 49)<br />4. What part of the cell membrane is necessary for<br />facilitated diffusion? Describe one way this<br />process is important within the body. (p. 54)<br />5. What provides the energy for filtration? Describe<br />one way this process is important within the body.<br />(p. 54)<br />6. What provides the energy for diffusion? Describe<br />one way this process is important within the body.<br />(p. 52)<br />7. What provides the energy for active transport?<br />Describe one way this process is important within<br />the body. (p. 54)<br />8. Define osmosis, and describe one way this process<br />is important within the body. (p. 52–53)<br />9. Explain the difference between hypertonic and<br />hypotonic, using human cells as a reference point.<br />(p. 53)<br />10. In what way are phagocytosis and pinocytosis similar?<br />Describe one way each process is important<br />within the body. (p. 56)<br />11. How many chromosomes does a human cell have?<br />What are these chromosomes made of? (p. 56)<br />12. Name the stage of mitosis in which each of the<br />following takes place: (p. 63)<br />a. The two sets of chromosomes are pulled<br />toward opposite poles of the cell<br />b. The chromosomes become visible as short rods<br />c. A nuclear membrane re-forms around each<br />complete set of chromosomes<br />d. The pairs of chromatids line up along the<br />equator of the cell<br />e. The centrioles organize the spindle fibers<br />f. Cytokinesis takes place after this stage<br />13. Describe two specific ways mitosis is important<br />within the body. Explain why meiosis is important.<br />(pp. 60, 62)<br />14. Compare mitosis and meiosis in terms of: (pp.<br />60–62)<br />a. Number of divisions<br />b. Number of cells formed<br />c. Chromosome number of the cells formed<br />15. Explain the triplet code of DNA. Name the molecule<br />that copies the triplet code of DNA. Name<br />the organelle that is the site of protein synthesis.<br />What other function does this organelle have in<br />protein formation? (pp. 56–58)<br />1. Antibiotics are drugs used to treat bacterial infections.<br />Some antibiotics disrupt the process of protein<br />synthesis within bacteria. Others block DNA<br />synthesis and cell division by the bacteria. Still others<br />inhibit cell wall synthesis by the bacteria. If all<br />antibiotics worked equally well against bacteria,<br />would any of those mentioned here be better than<br />the others, from the patient’s perspective? Explain<br />your answer.<br />2. A new lab instructor wants his students to see living<br />cells. He puts a drop of his own blood on a glass<br />slide, adds two drops of distilled water “so the cells<br />will be spread out and easier to see,” puts on a<br />cover glass, and places the slide under a microscope<br />on high power. He invites his students to see living<br />red blood cells. The students claim that they cannot<br />see any cells. Explain what has happened. How<br />could this have been prevented?<br />3. A friend asks you how DNA can be used to identify<br />someone, and why it is called a “DNA fingerprint.”<br />What simple explanation can you give?<br />4. A cell has extensive rough ER and Golgi apparatus.<br />Give a brief explanation of its function. A second<br />cell has microvilli and many mitochondria. Give a<br />brief explanation of its function.<br />5. A bacterial toxin is found to cause harm by first fitting<br />into a receptor on human cell membranes;<br />once the toxin fits, the cell will be destroyed. A<br />medication is going to be made to stop this toxin,<br />and can work in one of two ways: The drug can<br />block the receptors to prevent the toxin from fitting<br />in, or the drug can act as decoy molecules<br />shaped like the receptors. Which one of these<br />might be better, and why?<br />66 Cellsinternet fast worldhttp://www.blogger.com/profile/13869077830569899582noreply@blogger.com0tag:blogger.com,1999:blog-135611804747902727.post-57761983855252713012010-06-27T05:39:00.000-07:002010-06-27T11:00:17.212-07:00home<span style="font-size:180%;"><span style="color: rgb(102, 0, 204);">Organization and General</span><br /><span style="color: rgb(102, 0, 204);">Plan of the Body</span><br /></span><br />Levels of Organization<br />Chemicals<br />Cells<br />Tissues<br />Organs<br />Organ Systems<br />Metabolism and Homeostasis<br />Terminology and General Plan of the Body<br />Body Parts and Areas<br />Terms of Location and Position<br />Body Cavities and Their Membranes<br />Dorsal cavity<br />Ventral cavity<br />Planes and Sections<br />Areas of the Abdomen<br />BOX 1–1 REPLACING TISSUES AND ORGANS<br />BOX 1–2 VISUALIZING THE INTERIOR OF THE BODY<br />Student Objectives<br />• Define the terms anatomy, physiology, and pathophysiology.<br />Use an example to explain how they are<br />related.<br />• Name the levels of organization of the body from<br />simplest to most complex, and explain each.<br />• Define the terms metabolism, metabolic rate, and<br />homeostasis, and use examples to explain.<br />• Explain how a negative feedback mechanism<br />works, and how a positive feedback mechanism<br />differs.<br />• Describe the anatomic position.<br />• State the anatomic terms for the parts of the body.<br />• Use proper terminology to describe the location<br />of body parts with respect to one another.<br />• Name the body cavities, their membranes, and<br />some organs within each cavity.<br />• Describe the possible sections through the body or<br />an organ.<br />• Explain how and why the abdomen is divided into<br />smaller areas. Be able to name organs in these<br />areas.<br />2<br />Organization and General<br />Plan of the Body<br />3<br />New Terminology<br />Anatomy (uh-NAT-uh-mee)<br />Body cavity (BAH-dee KAV-i-tee)<br />Cell (SELL)<br />Homeostasis (HOH-me-oh-STAY-sis)<br />Inorganic chemicals (IN-or-GAN-ik KEM-i-kuls)<br />Meninges (me-NIN-jeez)<br />Metabolism (muh-TAB-uh-lizm)<br />Negative feedback (NEG-ah-tiv FEED-bak)<br />Organ (OR-gan)<br />Organ system (OR-gan SIS-tem)<br />Organic chemicals (or-GAN-ik KEM-i-kuls)<br />Pathophysiology (PATH-oh-FIZZ-ee-AH-luh-jee)<br />Pericardial membranes (PER-ee-KAR-dee-uhl<br />MEM-brayns)<br />Peritoneum/Mesentery (PER-i-toh-NEE-um/MEZen-<br />TER-ee)<br />Physiology (FIZZ-ee-AH-luh-jee)<br />Plane (PLAYN)<br />Pleural membranes (PLOOR-uhl MEM-brayns)<br />Positive feedback (PAHS-ah-tiv FEED-bak)<br />Section (SEK-shun)<br />Tissue (TISH-yoo)<br />Related Clinical Terminology<br />Computed tomography (CT) scan<br />(kom-PEW-ted toh-MAH-grah-fee SKAN)<br />Diagnosis (DYE-ag-NO-sis)<br />Disease (di-ZEEZ)<br />Magnetic resonance imaging (MRI)<br />(mag-NET-ik REZ-ah-nanse IM-ah-jing)<br />Positron emission tomography (PET)<br />(PAHZ-i-tron e-MISH-un toh-MAH-grah-fee)<br />Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.<br />The human body is a precisely structured container<br />of chemical reactions. Have you ever thought of yourself<br />in this way? Probably not, and yet, in the strictly<br />physical sense, that is what each of us is. The body<br />consists of trillions of atoms in specific arrangements<br />and thousands of chemical reactions proceeding in<br />a very orderly manner. That literally describes<br />us, and yet it is clearly not the whole story. The keys<br />to understanding human consciousness and selfawareness<br />are still beyond our grasp. We do not yet<br />know what enables us to study ourselves—no other<br />animals do, as far as we know—but we have accumulated<br />a great deal of knowledge about what we are<br />made of and how it all works. Some of this knowledge<br />makes up the course you are about to take, a course in<br />basic human anatomy and physiology.<br />Anatomy is the study of body structure, which<br />includes size, shape, composition, and perhaps even<br />coloration. Physiology is the study of how the body<br />functions. The physiology of red blood cells, for example,<br />includes what these cells do, how they do it, and<br />how this is related to the functioning of the rest of the<br />body. Physiology is directly related to anatomy. For<br />example, red blood cells contain the mineral iron in<br />molecules of the protein called hemoglobin; this is an<br />aspect of their anatomy. The presence of iron enables<br />red blood cells to carry oxygen, which is their function.<br />All cells in the body must receive oxygen in order to<br />function properly, so the physiology of red blood cells<br />is essential to the physiology of the body as a whole.<br />Pathophysiology is the study of disorders of functioning,<br />and a knowledge of normal physiology makes<br />such disorders easier to understand. For example, you<br />are probably familiar with the anemia called irondeficiency<br />anemia. With insufficient iron in the diet,<br />there will not be enough iron in the hemoglobin of<br />red blood cells, and hence less oxygen will be transported<br />throughout the body, resulting in the symptoms<br />of the iron-deficiency disorder. This example<br />shows the relationship between anatomy, physiology,<br />and pathophysiology.<br />The purpose of this text is to enable you to gain<br />an understanding of anatomy and physiology with<br />the emphasis on normal structure and function. Many<br />examples of pathophysiology have been included,<br />however, to illustrate the relationship of disease to<br />normal physiology and to describe some of the procedures<br />used in the diagnosis of disease. Many of the<br />examples are clinical applications that will help you<br />begin to apply what you have learned and demonstrate<br />that your knowledge of anatomy and physiology will<br />become the basis for your further study in the health<br />professions.<br />LEVELS OF ORGANIZATION<br />The human body is organized into structural and<br />functional levels of increasing complexity. Each higher<br />level incorporates the structures and functions of the<br />previous level, as you will see. We will begin with the<br />simplest level, which is the chemical level, and proceed<br />to cells, tissues, organs, and organ systems. All of<br />the levels of organization are depicted in Fig. 1–1.<br />CHEMICALS<br />The chemicals that make up the body may be divided<br />into two major categories: inorganic and organic.<br />Inorganic chemicals are usually simple molecules<br />made of one or two elements other than carbon (with<br />a few exceptions). Examples of inorganic chemicals are<br />water (H2O); oxygen (O2); one of the exceptions, carbon<br />dioxide (CO2); and minerals such as iron (Fe), calcium<br />(Ca), and sodium (Na). Organic chemicals are<br />often very complex and always contain the elements<br />carbon and hydrogen. In this category of organic<br />chemicals are carbohydrates, fats, proteins, and<br />nucleic acids. The chemical organization of the body<br />is the subject of Chapter 2.<br />CELLS<br />The smallest living units of structure and function are<br />cells. There are many different types of human cells,<br />though they all have certain similarities. Each type of<br />cell is made of chemicals and carries out specific<br />chemical reactions. Cell structure and function are<br />discussed in Chapter 3.<br />TISSUES<br />A tissue is a group of cells with similar structure and<br />function. There are four groups of tissues:<br />Epithelial tissues—cover or line body surfaces; some<br />are capable of producing secretions with specific<br />functions. The outer layer of the skin and sweat<br />glands are examples of epithelial tissues. Internal<br />epithelial tissues include the walls of capillaries<br />(squamous epithelium) and the kidney tubules<br />(cuboidal epithelium), as shown in Fig. 1–1.<br />4 Organization and General Plan of the Body<br />1. Chemical Level<br />2. Cellular Level<br />3. Tissue Level<br />4. Organ Level<br />5. Organ System<br />Level<br />6. Organism Level<br />Cuboidal epithelium<br />Squamous epithelium<br />Smooth muscle<br />Kidney<br />Urinary<br />bladder<br />Urinary<br />system<br />Figure 1–1. Levels of structural organization of the human body, depicted from the<br />simplest (chemical) to the most complex (organism). The organ system shown here is the<br />urinary system.<br />QUESTION: What other organ system seems to work directly with the urinary system?<br />5<br />Connective tissues—connect and support parts of<br />the body; some transport or store materials. Blood,<br />bone, cartilage, and adipose tissue are examples of<br />this group.<br />Muscle tissues—specialized for contraction, which<br />brings about movement. Our skeletal muscles and<br />the heart are examples of muscle tissue. In Fig. 1–1,<br />you see smooth muscle tissue, which is found in<br />organs such as the urinary bladder and stomach.<br />Nerve tissue—specialized to generate and transmit<br />electrochemical impulses that regulate body functions.<br />The brain and optic nerves are examples of<br />nerve tissue.<br />The types of tissues in these four groups, as well as<br />their specific functions, are the subject of Chapter 4.<br />ORGANS<br />An organ is a group of tissues precisely arranged so as<br />to accomplish specific functions. Examples of organs<br />are the kidneys, individual bones, the liver, lungs,<br />and stomach. The kidneys contain several kinds of<br />epithelial, or surface tissues, for their work of absorption.<br />The stomach is lined with epithelial tissue that<br />secretes gastric juice for digestion. Smooth muscle<br />tissue in the wall of the stomach contracts to mix<br />food with gastric juice and propel it to the small intestine.<br />Nerve tissue carries impulses that increase or<br />decrease the contractions of the stomach (see Box 1–1:<br />Replacing Tissues and Organs).<br />ORGAN SYSTEMS<br />An organ system is a group of organs that all contribute<br />to a particular function. Examples are the urinary<br />system, digestive system, and respiratory system.<br />In Fig. 1–1 you see the urinary system, which consists<br />of the kidneys, ureters, urinary bladder, and urethra.<br />These organs all contribute to the formation and<br />elimination of urine.<br />As a starting point, Table 1–1 lists the organ systems<br />of the human body with their general functions,<br />and some representative organs, and Fig. 1–2 depicts<br />6 Organization and General Plan of the Body<br />BOX 1–1 REPLACING TISSUES AND ORGANS<br />eventually be used to cover a large surface. Other<br />cells grown in culture include cartilage, bone, pancreas,<br />and liver. Much research is being done on<br />liver implants (not transplants), clusters of functional<br />liver cells grown in a lab. Such implants<br />would reduce or eliminate the need for human<br />donors. Tissue engineering is also being used to create<br />arteries and urinary bladders.<br />Many artificial replacement parts have also been<br />developed. These are made of plastic or metal and<br />are not rejected as foreign by the recipient’s<br />immune system. Damaged heart valves, for example,<br />may be replaced by artificial ones, and sections<br />of arteries may be replaced by tubular grafts made<br />of synthetic materials. Artificial joints are available<br />for every joint in the body, as is artificial bone for<br />reconstructive surgery. Cochlear implants are tiny<br />instruments that convert sound waves to electrical<br />impulses the brain can learn to interpret, and have<br />provided some sense of hearing for people with certain<br />types of deafness. Work is also progressing on<br />the use of a featherweight computer chip as an artificial<br />retina, on devices that help damaged hearts<br />pump blood more efficiently, and on small, selfcontained<br />artificial hearts.<br />Blood transfusions are probably the most familiar<br />and frequent form of “replacement parts” for people.<br />Blood is a tissue, and when properly typed and<br />cross-matched (blood types will be discussed in<br />Chapter 11) may safely be given to someone with<br />the same or a compatible blood type.<br />Organs, however, are much more complex structures.<br />When a patient receives an organ transplant,<br />there is always the possibility of rejection (destruction)<br />of the organ by the recipient’s immune system<br />(Chapter 14). With the discovery and use of<br />more effective immune-suppressing medications,<br />however, the success rate for many types of organ<br />transplants has increased. Organs that may be transplanted<br />include corneas, kidneys, the heart, the<br />liver, and the lungs.<br />The skin is also an organ, but skin transplanted<br />from another person will not survive very long.<br />Several kinds of artificial skin are now available to<br />temporarily cover large areas of damaged skin.<br />Patients with severe burns, for example, will eventually<br />need skin grafts from their own unburned<br />skin to form permanent new skin over the burn<br />sites. It is possible to “grow” a patient’s skin in laboratory<br />culture, so that a small patch of skin may<br />all of the organ systems. Some organs are part of two<br />organ systems; the pancreas, for example, is both a<br />digestive and an endocrine organ, and the diaphragm<br />is part of both the muscular and respiratory systems.<br />All of the organ systems make up an individual person.<br />The balance of this text discusses each system in more<br />detail.<br />METABOLISM AND HOMEOSTASIS<br />Metabolism is a collective noun; it is all of the chemical<br />reactions and physical processes that take place<br />within the body. Metabolism includes growing, repairing,<br />reacting, and reproducing—all the characteristics<br />of life. The pumping of the heart, the digestion of<br />food in the stomach, the diffusion of gases in the lungs<br />and tissues, and the production of energy in each cell<br />of the body are just a few of the thousands of aspects<br />of metabolism. Metabolism comes from a Greek word<br />meaning “change,” and the body is always changing in<br />visible ways (walking down the street), microscopic<br />ways (cells dividing in the skin to produce new epidermis),<br />and submicroscopic or molecular ways (RNA<br />and enzymes constructing new proteins). A related<br />concept, metabolic rate, is most often used to mean<br />the speed at which the body produces energy and heat,<br />or, put another way, energy production per unit of<br />time, such as 24 hours. Metabolic rate, therefore, is<br />one aspect of metabolism.<br />Organization and General Plan of the Body 7<br />Table 1–1 THE ORGAN SYSTEMS<br />System Functions Organs*<br />Integumentary<br />Skeletal<br />Muscular<br />Nervous<br />Endocrine<br />Circulatory<br />Lymphatic<br />Respiratory<br />Digestive<br />Urinary<br />Reproductive<br />*These are simply representative organs, not an all-inclusive list.<br />• Is a barrier to pathogens and chemicals<br />• Prevents excessive water loss<br />• Supports the body<br />• Protects internal organs and red bone marrow<br />• Provides a framework to be moved by muscles<br />• Moves the skeleton<br />• Produces heat<br />• Interprets sensory information<br />• Regulates body functions such as movement by means<br />of electrochemical impulses<br />• Regulates body functions such as growth and reproduction<br />by means of hormones<br />• Regulates day-to-day metabolism by means of hormones<br />• Transports oxygen and nutrients to tissues and removes<br />waste products<br />• Returns tissue fluid to the blood<br />• Destroys pathogens that enter the body and provides<br />immunity<br />• Exchanges oxygen and carbon dioxide between the air<br />and blood<br />• Changes food to simple chemicals that can be absorbed<br />and used by the body<br />• Removes waste products from the blood<br />• Regulates volume and pH of blood and tissue fluid<br />• Produces eggs or sperm<br />• In women, provides a site for the developing<br />embryo-fetus<br />skin, subcutaneous tissue<br />bones, ligaments<br />muscles, tendons<br />brain, nerves, eyes, ears<br />thyroid gland, pituitary<br />gland, pancreas<br />heart, blood, arteries<br />spleen, lymph nodes<br />lungs, trachea, larynx,<br />diaphragm<br />stomach, colon, liver,<br />pancreas<br />kidneys, urinary bladder,<br />urethra<br />Female: ovaries, uterus<br />Male: testes, prostate gland<br />Circulatory system<br />Skeletal<br />system<br />Integumentary<br />system<br />Muscular<br />system<br />Nervous<br />system<br />Figure 1–2. Organ systems. Compare the depiction of each system to its description in<br />Table 1–1.<br />QUESTION: Name at least one organ shown in each system.<br />8<br />Respiratory<br />system<br />Urinary system Endocrine system<br />Digestive<br />system<br />Lymphatic system<br />Reproductive system<br />Figure 1–2. (Continued)<br />9<br />A person who is in good health may be said to be in<br />a state of homeostasis. Homeostasis reflects the ability<br />of the body to maintain a relatively stable metabolism<br />and to function normally despite many constant<br />changes. The changes that are part of normal metabolism<br />may be internal or external, and the body must<br />respond appropriately.<br />Eating breakfast, for example, brings about an<br />internal change. Suddenly there is food in the stomach,<br />and something must be done with it. What happens?<br />The food is digested or broken down into<br />simple chemicals that the body can use. The protein in<br />a hard-boiled egg is digested into amino acids, its basic<br />chemical building blocks; these amino acids can then<br />be used by the cells of the body to produce their own<br />specialized proteins.<br />An example of an external change is a rise in environmental<br />temperature. On a hot day, the body temperature<br />would also tend to rise. However, body<br />temperature must be kept within its normal range of<br />about 97 to 99 F (36 to 38 C) in order to support<br />normal functioning. What happens? One of the body’s<br />responses to the external temperature rise is to<br />increase sweating so that excess body heat can be lost<br />by the evaporation of sweat on the surface of the skin.<br />This response, however, may bring about an undesirable<br />internal change, dehydration. What happens? As<br />body water decreases, we feel the sensation of thirst<br />and drink fluids to replace the water lost in sweating.<br />Notice that when certain body responses occur, they<br />reverse the event that triggered them. In the preceding<br />example a rising body temperature stimulates<br />increased sweating, which lowers body temperature,<br />which in turn decreases sweating. Unnecessary sweating<br />that would be wasteful of water is prevented. This<br />is an example of a negative feedback mechanism, in<br />which the body’s response reverses the stimulus (in<br />effect, turning it off for a while) and keeps some aspect<br />of the body metabolism within its normal range.<br />Look at Fig. 1–3 for another negative feedback<br />mechanism, one in which the hormone thyroxine regulates<br />the metabolic rate of the body. As metabolic<br />rate decreases, the hypothalamus (part of the brain)<br />and pituitary gland detect this decrease and secrete<br />hormones to stimulate the thyroid gland (on the front<br />of the neck just below the larynx) to secrete the hormone<br />thyroxine. Thyroxine stimulates the cellular<br />enzyme systems that produce energy from food, which<br />increases the metabolic rate. The rise in energy and<br />heat production is detected by the brain and pituitary<br />gland. They then decrease secretion of their hormones,<br />which in turn inhibits any further secretion of<br />thyroxine until the metabolic rate decreases again.<br />Metabolic rate does rise and fall, but is kept within<br />normal limits.<br />You may be wondering if there is such a thing as a<br />positive feedback mechanism. There is, but they are<br />rare in the body and quite different from a negative<br />feedback mechanism. In a positive feedback mechanism,<br />the response to the stimulus does not stop or<br />reverse the stimulus, but instead keeps the sequence of<br />events going. A good example is childbirth, in which<br />the sequence of events, simply stated, is as follows:<br />Stretching of the uterine cervix stimulates secretion of<br />the hormone oxytocin by the posterior pituitary gland.<br />Oxytocin stimulates contraction of the uterine muscle,<br />which causes more stretching, which stimulates more<br />oxytocin and, hence, more contractions. The mechanism<br />stops with the delivery of the baby and the placenta.<br />This is the “brake,” the interrupting event.<br />Any positive feedback mechanism requires an<br />external “brake,” something to interrupt it. Blood<br />clotting is such a mechanism, and without external<br />controls, clotting may become a vicious cycle of clotting<br />and more clotting, doing far more harm than<br />good (discussed in Chapter 11). Inflammation following<br />an injury is beneficial and necessary for repair to<br />begin, but the process may evolve into a cycle of damage<br />and more damage. The rise of a fever may also<br />trigger a positive feedback mechanism. Notice in Fig.<br />1–3 that bacteria have affected the body’s thermostat<br />in the hypothalamus and caused a fever. The rising<br />body temperature increases the metabolic rate, which<br />increases body temperature even more, becoming a<br />cycle. Where is the inhibition, the brake? For this<br />infection, the brake is white blood cells destroying the<br />bacteria that caused the fever. An interruption from<br />outside the cycle is necessary. It is for this reason,<br />because positive feedback mechanisms have the potential<br />to be self-perpetuating and cause harm, that they<br />are rare in the body.<br />Negative feedback mechanisms, however, contain<br />their own brakes in that inhibition is a natural part of<br />the cycle, and the body has many of them. The secretion<br />of most hormones (Chapter 10) is regulated by<br />negative feedback mechanisms. The regulation of<br />heart rate (Chapter 12) and blood pressure (Chapter<br />13) involves several negative feedback mechanisms.<br />10 Organization and General Plan of the Body<br />The result of all of these mechanisms working together<br />is that all aspects of body functioning, that is, of<br />metabolism, are kept within normal limits, a steady<br />state or equilibrium. This is homeostasis.<br />In the chapters to come, you will find many more<br />examples of homeostasis. As you continue your study<br />of the human body, keep in mind that the proper functioning<br />of each organ and organ system contributes to<br />homeostasis. Keep in mind as well that what we call<br />the normal values of metabolism are often ranges, not<br />single numbers. Recall that normal body temperature<br />is a range: 97 to 99 F (36 to 38 C). Normal pulse<br />Organization and General Plan of the Body 11<br />Cells decrease<br />energy<br />production<br />Metabolic<br />rate<br />decreases<br />Bacteria<br />White blood cells<br />Hypothalamus<br />Heat gain<br />mechanisms<br />Key:<br />Stimulates Inhibits Leads to<br />Cells increase<br />heat<br />production<br />Fever<br />Metabolic<br />rate<br />increases<br />Cells increase<br />energy<br />production<br />Thyroid gland<br />Thyroxine<br />increases<br />Stimulates<br />thyroid<br />gland<br />Thyroxine<br />decreases<br />Thyroid gland<br />No longer<br />stimulates<br />thyroid gland<br />Metabolic<br />rate<br />increases<br />Hypothalamus<br />and pituitary gland<br />A<br />B<br />Hypothalamus<br />and pituitary gland<br />Figure 1–3. Feedback mechanisms. (A) The negative feedback mechanism of regulation<br />of metabolic rate by thyroxine. (B) The positive feedback mechanism triggered by a fever.<br />See text for description.<br />QUESTION: For each mechanism, where is the source of the “brake” or inhibition?<br />rate, another example, is 60 to 80 beats per minute; a<br />normal respiratory rate is 12 to 20 breaths per minute.<br />Variations within the normal range are part of normal<br />metabolism.<br />TERMINOLOGY AND GENERAL<br />PLAN OF THE BODY<br />As part of your course in anatomy and physiology,<br />you will learn many new words or terms. At times you<br />may feel that you are learning a second language, and<br />indeed you are. Each term has a precise meaning,<br />which is understood by everyone else who has learned<br />the language. Mastering the terminology of your profession<br />is essential to enable you to communicate effectively<br />with your coworkers and your future patients.<br />Although the number of new terms may seem a bit<br />overwhelming at first, you will find that their use soon<br />becomes second nature to you.<br />The terminology presented in this chapter will be<br />used throughout the text in the discussion of the organ<br />systems. This will help to reinforce the meanings of<br />these terms and will transform these new words into<br />knowledge.<br />BODY PARTS AND AREAS<br />Each of the terms listed in Table 1–2 and shown in<br />Fig. 1–4 refers to a specific part or area of the body.<br />For example, the term femoral always refers to the<br />thigh. The femoral artery is a blood vessel that passes<br />through the thigh, and the quadriceps femoris is a<br />large muscle group of the thigh.<br />Another example is pulmonary, which always refers<br />to the lungs, as in pulmonary artery, pulmonary edema,<br />and pulmonary embolism. Although you may not<br />know the exact meaning of each of these terms now,<br />you do know that each has something to do with the<br />lungs.<br />TERMS OF LOCATION AND POSITION<br />When describing relative locations, the body is always<br />assumed to be in anatomic position: standing upright<br />facing forward, arms at the sides with palms forward,<br />and the feet slightly apart. The terms of location are<br />listed in Table 1–3, with a definition and example for<br />each. As you read each term, find the body parts used<br />as examples in Figs. 1–4 and 1–5. Notice also that<br />these are pairs of terms and that each pair is a set of<br />opposites. This will help you recall the terms and their<br />meanings.<br />BODY CAVITIES AND<br />THEIR MEMBRANES<br />The body has two major cavities: the dorsal cavity<br />(posterior) and the ventral cavity (anterior). Each of<br />these cavities has further subdivisions, which are<br />shown in Fig. 1–5.<br />12 Organization and General Plan of the Body<br />Table 1–2 DESCRIPTIVE TERMS FOR<br />BODY PARTS AND AREAS<br />Term Definition (Refers to)<br />Antebrachial forearm<br />Antecubital front of elbow<br />Axillary armpit<br />Brachial upper arm<br />Buccal (oral) mouth<br />Cardiac heart<br />Cervical neck<br />Cranial head<br />Cutaneous skin<br />Deltoid shoulder<br />Femoral thigh<br />Frontal forehead<br />Gastric stomach<br />Gluteal buttocks<br />Hepatic liver<br />Iliac hip<br />Inguinal groin<br />Lumbar small of back<br />Mammary breast<br />Nasal nose<br />Occipital back of head<br />Orbital eye<br />Parietal crown of head<br />Patellar kneecap<br />Pectoral chest<br />Pedal foot<br />Perineal pelvic floor<br />Plantar sole of foot<br />Popliteal back of knee<br />Pulmonary lungs<br />Renal kidney<br />Sacral base of spine<br />Scapular shoulder blade<br />Sternal breastbone<br />Temporal side of head<br />Umbilical navel<br />Volar (palmar) palm<br />Dorsal Cavity<br />The dorsal cavity contains the central nervous system,<br />and consists of the cranial cavity and the vertebral or<br />spinal cavity. The dorsal cavity is a continuous one;<br />that is, no wall or boundary separates its subdivisions.<br />The cranial cavity is formed by the skull and contains<br />the brain. The spinal cavity is formed by the backbone<br />(spine) and contains the spinal cord. The membranes<br />that line these cavities and cover the brain and spinal<br />cord are called the meninges.<br />Ventral Cavity<br />The ventral cavity consists of two compartments, the<br />thoracic cavity and the abdominal cavity, which are<br />separated by the diaphragm. The diaphragm is a large,<br />dome-shaped respiratory muscle. It has openings for<br />the esophagus and for large blood vessels, but otherwise<br />is a wall between the thoracic and abdominal cavities.<br />The pelvic cavity may be considered a<br />subdivision of the abdominal cavity (there is no wall<br />between them) or as a separate cavity.<br />Organization and General Plan of the Body 13<br />Body Parts and Areas<br />Anatomic position<br />Cranial<br />Orbital<br />Nasal<br />Buccal<br />Axillary<br />Umbilical<br />Volar<br />Patellar<br />Plantar<br />Popliteal<br />Femoral<br />Inguinal<br />Iliac<br />Brachial<br />Mammary<br />Pectoral<br />Deltoid<br />Cervical<br />Parietal<br />Occipital<br />Lumbar<br />Sacral<br />Gluteal<br />Perineal<br />A B<br />Frontal<br />Temporal<br />Sternal<br />Antecubital<br />Antebrachial<br />Pedal<br />Scapular<br />Figure 1–4. Body parts and areas. The body is shown in anatomic position. (A) Anterior<br />view. (B) Posterior view. (Compare with Table 1–2.)<br />QUESTION: Name a body area that contains a bone with a similar name. Can you name<br />two more?<br />Cranial cavity<br />Foramen magnum<br />Spinal cavity<br />Dorsal<br />cavity<br />Sacral promontory<br />Symphysis pubis<br />Pelvic cavity<br />Abdominal cavity<br />Diaphragm<br />Thoracic cavity<br />Ventral<br />cavity<br />Figure 1–5. Body cavities (lateral view<br />from the left side).<br />QUESTION: Which of these cavities are<br />surrounded by bone?<br />Table 1–3 TERMS OF LOCATION AND POSITION<br />Term Definition Example<br />Superior<br />Inferior<br />Anterior<br />Posterior<br />Ventral<br />Dorsal<br />Medial<br />Lateral<br />Internal<br />External<br />Superficial<br />Deep<br />Central<br />Peripheral<br />Proximal<br />Distal<br />Parietal<br />Visceral<br />above, or higher<br />below, or lower<br />toward the front<br />toward the back<br />toward the front<br />toward the back<br />toward the midline<br />away from the midline<br />within, or interior to<br />outside, or exterior to<br />toward the surface<br />within, or interior to<br />the main part<br />extending from the main part<br />closer to the origin<br />farther from the origin<br />pertaining to the wall of a cavity<br />pertaining to the organs within a cavity<br />The heart is superior to the liver.<br />The liver is inferior to the lungs.<br />The chest is on the anterior side of the body.<br />The lumbar area is posterior to the umbilical area.<br />The mammary area is on the ventral side of the body.<br />The buttocks are on the dorsal side of the body.<br />The heart is medial to the lungs.<br />The shoulders are lateral to the neck.<br />The brain is internal to the skull.<br />The ribs are external to the lungs.<br />The skin is the most superficial organ.<br />The deep veins of the legs are surrounded by muscles.<br />The brain is part of the central nervous system.<br />Nerves in the arm are part of the peripheral nervous system.<br />The knee is proximal to the foot.<br />The palm is distal to the elbow.<br />The parietal pleura lines the chest cavity.<br />The visceral pleura covers the lungs.<br />14<br />Organization and General Plan of the Body 15<br />Organs in the thoracic cavity include the heart and<br />lungs. The membranes of the thoracic cavity are<br />serous membranes called the pleural membranes.<br />The parietal pleura lines the chest wall, and the visceral<br />pleura covers the lungs. The heart has its own set<br />of serous membranes called the pericardial membranes.<br />The parietal pericardium lines the fibrous<br />pericardial sac, and the visceral pericardium covers the<br />heart muscle.<br />Organs in the abdominal cavity include the liver,<br />stomach, and intestines. The membranes of the<br />abdominal cavity are also serous membranes called the<br />peritoneum and mesentery. The peritoneum is the<br />membrane that lines the entire abdominal wall, and<br />the mesentery is the continuation of this membrane,<br />folded around and covering the outer surfaces of the<br />abdominal organs.<br />The pelvic cavity is inferior to the abdominal cavity.<br />Although the peritoneum does not line the pelvic<br />cavity, it covers the free surfaces of several pelvic<br />organs. Within the pelvic cavity are the urinary bladder<br />and reproductive organs such as the uterus in<br />women and the prostate gland in men.<br />PLANES AND SECTIONS<br />When internal anatomy is described, the body or an<br />organ is often cut or sectioned in a specific way so as<br />to make particular structures easily visible. A plane is<br />an imaginary flat surface that separates two portions of<br />-<br />A<br />B<br />Figure 1–6. (A) Planes and sections of the body. (B) Cross-section and longitudinal section<br />of the small intestine.<br />QUESTION: What other organs would have sections that look like those of the small intestine?<br />16 Organization and General Plan of the Body<br />Stomach<br />Pancreas<br />Colon<br />Spleen<br />Aorta<br />Left kidney<br />Vertebra<br />Spinal cord<br />Liver<br />Gallbladder<br />Duodenum<br />Ribs<br />Inferior vena cava<br />Right kidney<br />Back Muscle<br />Front<br />C<br />Figure 1–6. (Continued) (C) Transverse section through the upper abdomen.<br />the body or an organ. These planes and sections are<br />shown in Fig. 1–6 (see Box 1–2: Visualizing the<br />Interior of the Body).<br />Frontal (coronal) section—a plane from side to side<br />separates the body into front and back portions.<br />Sagittal section—a plane from front to back separates<br />the body into right and left portions. A midsagittal<br />section creates equal right and left halves.<br />Transverse section—a horizontal plane separates the<br />body into upper and lower portions.<br />Cross-section—a plane perpendicular to the long<br />axis of an organ. A cross-section of the small intestine<br />(which is a tube) would look like a circle with<br />the cavity of the intestine in the center.<br />Longitudinal section—a plane along the long axis of<br />an organ. A longitudinal section of the intestine is<br />shown in Fig. 1–6, and a frontal section of the<br />femur (thigh bone) would also be a longitudinal<br />section (see Fig. 6–1 in Chapter 6).<br />AREAS OF THE ABDOMEN<br />The abdomen is a large area of the lower trunk of the<br />body. If a patient reported abdominal pain, the physician<br />or nurse would want to know more precisely<br />where the pain was. To determine this, the abdomen<br />may be divided into smaller regions or areas, which<br />are shown in Fig. 1–7.<br />Quadrants—a transverse plane and a midsagittal<br />plane that cross at the umbilicus will divide the<br />abdomen into four quadrants. Clinically, this is<br />probably the division used more frequently. The<br />pain of gallstones might then be described as in the<br />right upper quadrant.<br />Nine areas—two transverse planes and two sagittal<br />planes divide the abdomen into nine areas:<br />Upper areas—above the level of the rib cartilages are<br />the left hypochondriac, epigastric, and right<br />hypochondriac.<br />Middle areas—the left lumbar, umbilical, and right<br />lumbar.<br />Lower areas—below the level of the top of the pelvic<br />bone are the left iliac, hypogastric, and right<br />iliac.<br />These divisions are often used in anatomic studies<br />to describe the location of organs. The liver, for example,<br />is located in the epigastric and right hypochondriac<br />areas.<br />Organization and General Plan of the Body 17<br />SUMMARY<br />As you will see, the terminology presented in this<br />chapter is used throughout the text to describe the<br />anatomy of organs and the names of their parts. All<br />organs of the body contribute to homeostasis, the<br />healthy state of the body that is maintained by constant<br />and appropriate responses to internal and external<br />changes. In the chapters that follow, you will find<br />detailed descriptions of the physiology of each organ<br />and organ system, and how the metabolism of each is<br />necessary to homeostasis. We will now return to a<br />consideration of the structural organization of the<br />body and to more extensive descriptions of its levels of<br />organization. The first of these, the chemical level, is<br />the subject of the next chapter.<br />A B<br />Figure 1–7. Areas of the abdomen. (A) Four quadrants. (B) Nine regions.<br />QUESTION: Are there any organs found in all four abdominal quadrants?<br />Introduction<br />1. Anatomy—the study of structure.<br />2. Physiology—the study of function.<br />3. Pathophysiology—the study of disorders of functioning.<br />Levels of Organization<br />1. Chemical—inorganic and organic chemicals make<br />up all matter, both living and non-living.<br />2. Cells—the smallest living units of the body.<br />18 Organization and General Plan of the Body<br />STUDY OUTLINE<br />BOX 1–2 VISUALIZING THE INTERIOR OF THE BODY<br />A B C<br />Box Figure 1–A Imaging techniques. (A) CT scan of eye in lateral view showing a tumor (arrow)<br />below the optic nerve. (B) MRI of midsagittal section of head (compare with Figs. 8–6 and 15–1).<br />(C) PET scan of brain in transverse section (frontal lobes at top) showing glucose metabolism. (From<br />Mazziotta, JC, and Gilman, S: Clinical Brain Imaging: Principles and Applications. FA Davis,<br />Philadelphia, 1992, pp 27 and 298, with permission.)<br />In the past, the need for exploratory surgery<br />brought with it hospitalization, risk of infection, and<br />discomfort and pain for the patient. Today, however,<br />several technologies and the extensive use of<br />computers permit us to see the interior of the body<br />without surgery.<br />Computed tomography (CT) scanning uses a<br />narrowly focused x-ray beam that circles rapidly<br />around the body. A detector then measures how<br />much radiation passes through different tissues,<br />and a computer constructs an image of a thin<br />slice through the body. Several images may be<br />made at different levels—each takes only a few<br />seconds—to provide a more complete picture of<br />an organ or part of the body. The images are<br />much more detailed than are those produced by<br />conventional x-rays.<br />Magnetic resonance imaging (MRI) is another<br />diagnostic tool that is especially useful for visualizing<br />soft tissues, including the brain and spinal<br />cord. Recent refinements have produced images<br />of individual nerve bundles, which had not been<br />possible using any other technique. The patient<br />is placed inside a strong magnetic field, and the<br />tissues are pulsed with radio waves. Because<br />each tissue has different proportions of various<br />atoms, which resonate or respond differently,<br />each tissue emits a characteristic signal. A computer<br />then translates these signals into an image;<br />the entire procedure takes 30 to 45 minutes.<br />Positron emission tomography (PET) scanning<br />creates images that depict the rates of physiological<br />processes such as blood flow, oxygen<br />usage, or glucose metabolism. The comparative<br />rates are depicted by colors: Red represents the<br />highest rate, followed by yellow, then green, and<br />finally blue representing the lowest rate.<br />One drawback of these technologies is their cost;<br />they are expensive. However, the benefits to<br />patients are great: Highly detailed images of the<br />body are obtained without the risks of surgery and<br />with virtually no discomfort in the procedures themselves.<br />3. Tissues—groups of cells with similar structure and<br />function.<br />4. Organs—groups of tissues that contribute to specific<br />functions.<br />5. Organ systems—groups of organs that work<br />together to perform specific functions (see Table<br />1–1 and Fig. 1–2).<br />6. Person—all the organ systems functioning properly.<br />Metabolism and Homeostasis<br />1. Metabolism is the sum of all of the chemical and<br />physical changes that take place in the body.<br />Metabolic rate is the amount of energy and heat<br />production per unit of time.<br />2. Homeostasis is a state of good health maintained<br />by the normal metabolism (functioning) of the<br />organ systems.<br />3. The body constantly responds to internal and<br />external changes, yet remains stable; its many<br />aspects of metabolism are kept within normal limits<br />(usually a range of values, not a single value).<br />4. Negative feedback mechanism—a control system<br />in which a stimulus initiates a response that<br />reverses or reduces the stimulus, thereby stopping<br />the response until the stimulus occurs again and<br />there is a need for the response (see Fig. 1–3).<br />5. Positive feedback mechanism—a control system<br />that requires an external interruption or brake. Has<br />the potential to become a self-perpetuating and<br />harmful cycle, therefore is rare in the body (see Fig.<br />1–3).<br />Terminology and General Plan of the Body<br />1. Body parts and areas—see Table 1–2 and Fig. 1–4.<br />2. Terms of location and position—used to describe<br />relationships of position (see Table 1–3 and Figs.<br />1–4 and 1–5).<br />3. Body cavities and their membranes (see Fig. 1–5).<br />• Dorsal cavity—lined with membranes called<br />meninges; consists of the cranial and vertebral<br />cavities.<br />Cranial cavity contains the brain.<br />Vertebral cavity contains the spinal cord.<br />• Ventral cavity—the diaphragm separates the thoracic<br />and abdominal cavities; the pelvic cavity is<br />inferior to the abdominal cavity.<br />Thoracic cavity—contains the lungs and heart.<br />— Pleural membranes line the chest wall and<br />cover the lungs.<br />— Pericardial membranes surround the<br />heart.<br />Abdominal cavity—contains many organs<br />including the stomach, liver, and intestines.<br />— The peritoneum lines the abdominal cavity;<br />the mesentery covers the abdominal<br />organs.<br />Pelvic cavity—contains the urinary bladder<br />and reproductive organs.<br />4. Planes and sections—cutting the body or an organ<br />in a specific way (see Fig. 1–6).<br />• Frontal or coronal—separates front and back<br />parts.<br />• Sagittal—separates right and left parts.<br />• Transverse—separates upper and lower parts.<br />• Cross—a section perpendicular to the long axis.<br />• Longitudinal—a section along the long axis.<br />5. Areas of the abdomen—permits easier description<br />of locations:<br />• Quadrants—see Fig. 1–7.<br />• Nine areas—see Fig. 1–7.<br />Organization and General Plan of the Body 19<br />REVIEW QUESTIONS<br />1. Explain how the physiology of a bone is related to<br />its anatomy. Explain how the physiology of the<br />hand is related to its anatomy. (p. 4)<br />2. Describe anatomic position. Why is this knowledge<br />important? (p. 12)<br />3. Name the organ system with each of the following<br />functions: (p. 7)<br />a. Moves the skeleton<br />b. Regulates body functions by means of hormones<br />c. Covers the body and prevents entry of<br />pathogens<br />d. Destroys pathogens that enter the body<br />e. Exchanges oxygen and carbon dioxide between<br />the air and blood<br />1. The human foot is similar to the human hand, but<br />does have anatomic differences. Describe two of<br />these differences, and explain how they are related<br />to the physiology of the hand and the foot.<br />2. Complete each statement using the everyday term<br />for the body part.<br />a. The distal femoral area is immediately superior<br />to the ____.<br />b. The proximal brachial area is immediately inferior<br />to the ____.<br />c. The patellar area is directly proximal to the<br />____.<br />d. The volar area is directly distal to the ____.<br />3. Name a structure or organ that is both superior and<br />inferior to the brain. Name one that is both anterior<br />and posterior.<br />4. If a person has appendicitis (inflammation of the<br />appendix caused by bacteria), pain is felt in which<br />abdominal quadrant? (If you’re not sure, take a<br />look at Fig. 16–1 in Chapter 16.) Surgery is usually<br />necessary to remove an inflamed appendix before it<br />ruptures and causes peritonitis. Using your knowledge<br />of the location of the peritoneum, explain why<br />peritonitis is a very serious condition.<br />5. Keep in mind your answer to Question 4, and<br />explain why bacterial meningitis can be a very serious<br />infection.<br />6. Use a mental picture to cut the following sections.<br />Then describe in simple words what each section<br />looks like, and give each a proper anatomic name.<br />First: a tree trunk cut top to bottom, then cut side<br />to side.<br />Second: a grapefruit cut top to bottom (straight<br />down from where the stem was attached), then<br />sliced through its equator.<br />20 Organization and General Plan of the Body<br />FOR FURTHER THOUGHT<br />4. Name the two major body cavities and their subdivisions.<br />Name the cavity lined by the peritoneum,<br />meninges, and parietal pleura. (pp. 13, 15)<br />5. Name the four quadrants of the abdomen. Name at<br />least one organ in each quadrant. (p. 17)<br />6. Name the section through the body that would<br />result in each of the following: equal right and left<br />halves, anterior and posterior parts, superior and<br />inferior parts. (pp. 15–16)<br />7. Review Table 1–2, and try to find each external area<br />on your own body. (pp. 12–13)<br />8. Define cell. When similar cells work together, what<br />name are they given? (p. 4)<br />9. Define organ. When a group of organs works<br />together, what name is it given? (p. 6)<br />10. Define metabolism, metabolic rate, and homeostasis.<br />(pp. 7, 10)<br />a. Give an example of an external change and<br />explain how the body responds to maintain<br />homeostasis<br />b. Give an example of an internal change and<br />explain how the body responds to maintain<br />homeostasis<br />c. Briefly explain how a negative feedback mechanism<br />works, and how a positive feedback<br />mechanism differsinternet fast worldhttp://www.blogger.com/profile/13869077830569899582noreply@blogger.com0