Sunday, June 27, 2010

lymph

CHAPTER 14
Chapter Outline
Lymph
Lymph Vessels
Lymphatic Tissue
Lymph Nodes and Nodules
Spleen
Thymus
Immunity
Innate Immunity
Barriers
Defensive cells
Chemical defenses
Adaptive Immunity
Cell-Mediated Immunity
Antibody-Mediated Immunity
Antibody Responses
Types of Immunity
Aging and the Lymphatic System
BOX 14–1 HODGKIN’S DISEASE
BOX 14–2 AIDS
BOX 14–3 DIAGNOSTIC TESTS
BOX 14–4 VACCINES
BOX 14–5 ALLERGIES
BOX 14–6 VACCINES THAT HAVE CHANGED OUR
LIVES
Student Objectives
• Describe the functions of the lymphatic system.
• Describe how lymph is formed.
• Describe the system of lymph vessels, and explain
how lymph is returned to the blood.
• State the locations and functions of the lymph
nodes and nodules.
• State the location and functions of the spleen and
thymus.
• Explain what is meant by immunity.
• Describe the aspects of innate immunity.
• Describe adaptive immunity: cell-mediated and
antibody-mediated.
• Describe the responses to a first and second exposure
to a pathogen.
• Explain the difference between genetic immunity
and acquired immunity.
• Explain the difference between passive acquired
immunity and active acquired immunity.
• Explain how vaccines work.
The Lymphatic System
and Immunity
321
New Terminology
Acquired immunity (uh-KWHY-erd)
Active immunity (AK-tiv)
Antibody-mediated immunity (AN-ti-BAH-dee
ME-dee-ay-ted)
Antigen (AN-ti-jen)
B cells (B SELLS)
Cell-mediated immunity (SELL ME-dee-ay-ted)
Complement (KOM-ple-ment)
Cytokines (SIGH-toh-kines)
Genetic immunity (je-NET-ik)
Humoral immunity (HYOO-mohr-uhl)
Interferon (in-ter-FEER-on)
Lymph (LIMF)
Lymph nodes (LIMF NOHDS)
Lymph nodules (LIMF NAHD-yools)
Opsonization (OP-sah-ni-ZAY-shun)
Passive immunity (PASS-iv)
Plasma cell (PLAZ-mah SELL)
Spleen (SPLEEN)
T cells (T SELLS)
Thymus (THIGH-mus)
Tonsils (TAHN-sills)
Related Clinical Terminology
AIDS (AYDS)
Allergy (AL-er-jee)
Antibody titer (AN-ti-BAH-dee TIGH-ter)
Attenuated (uh-TEN-yoo-AY-ted)
Complement fixation test (KOM-ple-ment
fik-SAY-shun)
Fluorescent antibody test (floor-ESS-ent)
Hodgkin’s disease (HODJ-kinz)
Tonsillectomy (TAHN-si-LEK-toh-mee)
Toxoid (TOK-soyd)
Vaccine (vak-SEEN)
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
Achild falls and scrapes her knee. Is this likely to
be a life-threatening injury? Probably not, even
though the breaks in the skin have permitted the entry
of thousands or even millions of bacteria. Those bacteria,
however, will be quickly destroyed by the cells
and organs of the lymphatic system.
Although the lymphatic system may be considered
part of the circulatory system, we will consider it separately
because its functions are so different from
those of the heart and blood vessels. Keep in mind,
however, that all of these functions are interdependent.
The lymphatic system is responsible for returning
tissue fluid to the blood and for protecting the body
against foreign material. The parts of the lymphatic
system are the lymph, the system of lymph vessels, and
lymphatic tissue, which includes lymph nodes and
nodules, the spleen, and the thymus gland.
LYMPH
Lymph is the name for tissue fluid that enters lymph
capillaries. As you may recall from Chapter 13, filtration
in capillaries creates tissue fluid from blood
plasma, most of which returns almost immediately to
the blood in the capillaries by osmosis. Some tissue
fluid, however, remains in interstitial spaces and must
be returned to the blood by way of the lymphatic vessels.
Without this return, blood volume and blood
pressure would very soon decrease. The relationship
of the lymphatic vessels to the cardiovascular system is
depicted in Fig. 14–1.
LYMPH VESSELS
The system of lymph vessels begins as dead-end
lymph capillaries found in most tissue spaces (Fig.
14–2). Lymph capillaries are very permeable and collect
tissue fluid and proteins. Lacteals are specialized
lymph capillaries in the villi of the small intestine; they
absorb the fat-soluble end products of digestion, such
as fatty acids and vitamins A, D, E, and K.
Lymph capillaries unite to form larger lymph vessels,
whose structure is very much like that of veins.
There is no pump for lymph (as the heart is the pump
for blood), but the lymph is kept moving within lymph
vessels by the same mechanisms that promote venous
return. The smooth muscle layer of the larger lymph
vessels constricts, and the one-way valves (just like
those of veins) prevent backflow of lymph. Lymph vessels
in the extremities, especially the legs, are compressed
by the skeletal muscles that surround them;
this is the skeletal muscle pump. The respiratory
pump alternately expands and compresses the lymph
vessels in the chest cavity and keeps the lymph moving.
Where is the lymph going? Back to the blood to
become plasma again. Refer to Fig. 14–3 as you read
the following. The lymph vessels from the lower body
unite in front of the lumbar vertebrae to form a vessel
called the cisterna chyli, which continues upward in
front of the backbone as the thoracic duct. Lymph
vessels from the upper left quadrant of the body join
the thoracic duct, which empties lymph into the left
subclavian vein. Lymph vessels from the upper right
quadrant of the body unite to form the right lymphatic
duct, which empties lymph into the right subclavian
vein. Flaps in both subclavian veins permit the entry of
lymph but prevent blood from flowing into the lymph
vessels.
LYMPHATIC TISSUE
Lymphatic tissue consists mainly of lymphocytes in a
mesh-like framework of connective tissue. Recall that
most lymphocytes are produced from stem cells in the
red bone marrow, then migrate to the lymph nodes
and nodules, to the spleen, and to the thymus. In these
structures, lymphocytes become activated and proliferate
in response to infection (this is a function of all
lymphatic tissue). The thymus has stem cells that produce
a significant portion of the T lymphocytes.
LYMPH NODES AND NODULES
Lymph nodes and nodules are masses of lymphatic
tissue. Nodes and nodules differ with respect to size
and location. Nodes are usually larger, 10 to 20 mm in
length, and are encapsulated; nodules range from a
fraction of a millimeter to several millimeters in length
and do not have capsules.
Lymph nodes are found in groups along the pathways
of lymph vessels, and lymph flows through these
nodes on its way to the subclavian veins. Lymph enters
a node through several afferent lymph vessels and
322 The Lymphatic System and Immunity
leaves through one or two efferent vessels (Fig. 14–4).
As lymph passes through a lymph node, bacteria and
other foreign materials are phagocytized by fixed (stationary)
macrophages. Plasma cells develop from
lymphocytes exposed to pathogens in the lymph and
produce antibodies. These antibodies will eventually
reach the blood and circulate throughout the body.
There are many groups of lymph nodes along all
the lymph vessels throughout the body, but three
paired groups deserve mention because of their strategic
locations. These are the cervical, axillary, and
inguinal lymph nodes (see Fig. 14–3). Notice that
these are at the junctions of the head and extremities
with the trunk of the body. Breaks in the skin, with
entry of pathogens, are much more likely to occur in
the arms or legs or head rather than in the trunk. If
these pathogens get to the lymph, they will be
destroyed by the lymph nodes before they get to the
trunk, before the lymph is returned to the blood in the
subclavian veins.
The Lymphatic System and Immunity 323
Subclavian vein
Lymphatic
vessel
Valve
Heart
Lymph
flow
Lymph node
Blood
flow
Lymph
capillaries
Blood
capillaries
Figure 14–1. Relationship of lymphatic
vessels to the cardiovascular
system. Lymph capillaries collect tissue
fluid, which is returned to the
blood. The arrows indicate the direction
of flow of the blood and lymph.
QUESTION: To which large veins is
lymph returned, and why is this
return important?
You may be familiar with the expression “swollen
glands,” as when a child has a strep throat (an inflammation
of the pharynx caused by Streptococcus bacteria).
These “glands” are the cervical lymph nodes that have
enlarged as their macrophages attempt to destroy the
bacteria in the lymph from the pharynx (see Box 14–1:
Hodgkin’s Disease).
Lymph nodules are small masses of lymphatic tissue
found just beneath the epithelium of all mucous
membranes. The body systems lined with mucous
membranes are those that have openings to the environment:
the respiratory, digestive, urinary, and reproductive
tracts. You can probably see that these are also
strategic locations for lymph nodules, because any natural
body opening is a possible portal of entry for
pathogens. For example, if bacteria in inhaled air get
through the epithelium of the trachea, lymph nodules
with their macrophages are in position to destroy these
bacteria before they get to the blood.
Some of the lymph nodules have specific names.
Those of the small intestine are called Peyer’s
patches, and those of the pharynx are called tonsils.
The palatine tonsils are on the lateral walls of the
pharynx, the adenoid (pharyngeal tonsil) is on the posterior
wall, and the lingual tonsils are on the base of
the tongue. The tonsils, therefore, form a ring of lymphatic
tissue around the pharynx, which is a common
pathway for food and air and for the pathogens they
contain. A tonsillectomy is the surgical removal of
the palatine tonsils and the adenoid and may be performed
if the tonsils are chronically inflamed and
swollen, as may happen in children. As mentioned earlier,
the body has redundant structures to help ensure
survival if one structure is lost or seriously impaired.
Thus, there are many other lymph nodules in the
pharynx to take over the function of the surgically
removed tonsils.
SPLEEN
The spleen is located in the upper left quadrant of the
abdominal cavity, just below the diaphragm, behind
the stomach. The lower rib cage protects the spleen
from physical trauma (see Fig. 14–3).
In the fetus, the spleen produces red blood cells, a
function assumed by the red bone marrow after birth.
After birth the spleen is very much like a large lymph
node, except that its functions affect the blood that
flows through it rather than lymph.
The functions of the spleen after birth are:
1. Contains plasma cells that produce antibodies to
foreign antigens.
2. Contains fixed macrophages (RE cells) that phagocytize
pathogens or other foreign material in the
blood. The macrophages of the spleen also phagocytize
old red blood cells and form bilirubin. By
way of portal circulation, the bilirubin is sent to the
liver for excretion in bile.
3. Stores platelets and destroys them when they are
no longer useful.
The spleen is not considered a vital organ, because
other organs compensate for its functions if the spleen
must be removed. The liver and red bone marrow
will remove old red blood cells and platelets from circulation.
The many lymph nodes and nodules will
phagocytize pathogens (as will the liver) and have lymphocytes
to be activated and plasma cells to produce
antibodies. Despite this redundancy, a person without
a spleen is somewhat more susceptible to certain bacterial
infections such as pneumonia and meningitis.
THYMUS
The thymus is located inferior to the thyroid gland.
In the fetus and infant, the thymus is large and extends
under the sternum (Fig. 14–5). With increasing age,
the thymus shrinks, and relatively little thymus tissue
is found in adults, though it is still active.
324 The Lymphatic System and Immunity
Tissue fluid
Cells
Lymph
capillary
Venule
Blood capillary
Arteriole
Figure 14–2. Dead-end lymph capillaries found in tissue
spaces. Arrows indicate the movement of plasma,
lymph, and tissue fluid.
QUESTION: Just before water enters lymph capillaries,
what name does it have?
The Lymphatic System and Immunity 325
Cervical nodes
Submaxillary nodes
Left subclavian vein
Thoracic duct
Axillary nodes
Cisterna chyli
Mesenteric nodes
Inguinal nodes
Right lymphatic duct
Mammary plexus
Cubital nodes
Iliac nodes
Popliteal nodes
Spleen
Figure 14–3. System of lymph vessels and the major groups of lymph nodes. Lymph is
returned to the blood in the right and left subclavian veins.
QUESTION: Where are the major paired groups of lymph nodes located?
The stem cells of the thymus produce T lymphocytes
or T cells; their functions are discussed in the
next section. Thymic hormones are necessary for what
may be called “immunological competence.” To be
competent means to be able to do something well.
The thymic hormones enable the T cells to participate
in the recognition of foreign antigens and to provide
immunity. This capability of T cells is established
early in life and then is perpetuated by the lymphocytes
themselves. The newborn’s immune system is
not yet fully mature, and infants are more susceptible
to certain infections than are older children and
adults. Usually by the age of 2 years, the immune system
matures and becomes fully functional. This is why
some vaccines, such as the measles vaccine, are not
recommended for infants younger than 15 to 18
months of age. Their immune systems are not mature
enough to respond strongly to the vaccine, and the
326 The Lymphatic System and Immunity
Afferent lymphatic vessel
Capsule
Cortex
Nodal vein
Nodal artery
Hilus
Valve
Efferent lymphatic
vessel
Bacteria
Lymphocytes
Neutrophil
Plasma cell
Macrophage
Antibody molecule
(enlarged)
Antigen
(enlarged)
A
B
Figure 14–4. Lymph node. (A) Section through a lymph node, showing the flow of
lymph. (B) Microscopic detail of bacteria being destroyed within the lymph node.
QUESTION: What is the function of the plasma cells in a lymph node?
protection provided by the vaccine may be incomplete.
IMMUNITY
Immunity may be defined as the ability to destroy
pathogens or other foreign material and to prevent
further cases of certain infectious diseases. This ability
is of vital importance because the body is exposed to
pathogens from the moment of birth.
Antigens are chemical markers that identify cells.
Human cells have their own antigens that identify all
the cells in an individual as “self ” (recall the HLA
types mentioned in Chapter 11). When antigens are
foreign, or “non-self,” they may be recognized as such
and destroyed. Bacteria, viruses, fungi, and protozoa
are all foreign antigens that activate immune responses,
as are cell products such as bacterial toxins.
Malignant cells, which may be formed within the
body as a result of mutations of normal cells, are also
recognized as foreign and are usually destroyed before
they can establish themselves and cause cancer. Unfortunately,
organ transplants are also foreign tissue, and
the immune system may reject (destroy) a transplanted
kidney or heart. Sometimes the immune system mistakenly
reacts to part of the body itself and causes
an autoimmune disease; several of these were mentioned
in previous chapters. Most often, however, the
immune mechanisms function to protect the body
from the microorganisms around us and within us.
Immunity has two main components: innate immunity
and adaptive immunity. Before we describe each
component, a brief general comparison may be helpful.
Innate immunity may be called nonspecific, does
not create memory, and its responses are always the
same regardless of the target. Adaptive immunity is
very specific as to its target, may involve antibodies,
does create memory, and may become more efficient.
Both kinds of immunity work together to prevent
damage and disease.
INNATE IMMUNITY
Innate immunity has several aspects: anatomic and
physiological barriers, phagocytic and other defensive
cells, and chemical secretions and reactions, including
inflammation. These are not separate and distinct;
rather there is a great deal of overlap among them, as
you will see. The innate immune responses are always
the same, and their degree of efficiency does not
increase with repeated exposure.
Barriers
The stratum corneum of the epidermis of the skin is
non-living, and when unbroken is an excellent barrier
to pathogens of all kinds. The fatty acids in sebum
help limit the growth of bacteria on the skin. The living
cells of the epidermis produce defensins, which are
antimicrobial chemicals. The mucous membranes of
the respiratory, digestive, urinary, and reproductive
tracts are living tissue, yet still a good barrier. The
The Lymphatic System and Immunity 327
BOX 14–1 HODGKIN’S DISEASE
Hodgkin’s disease is a malignant disorder of
the lymph nodes; the cause is not known. The
first symptom is usually a swollen but painless
lymph node, often in the cervical region. The
individual is prompted to seek medical attention
because of other symptoms: chronic fever,
fatigue, and weight loss. The diagnosis involves
biopsy of the lymph node and the finding of
characteristic cells.
Treatment of Hodgkin’s disease requires
chemotherapy, radiation, or both. With early
diagnosis and proper treatment, this malignancy
is very often curable.
Trachea
Clavicle
First
rib
Thymus
gland
Figure 14–5. Location of the thymus in a young child.
QUESTION: Which blood cells mature in the thymus?
ciliated epithelium of the upper respiratory tract is an
especially effective barrier. Dust and pathogens are
trapped on the mucus, the cilia sweep the mucus to the
pharynx, and it is swallowed. The hydrochloric acid of
the gastric juice destroys most pathogens that enter
the stomach, either in mucus or with food and drink.
Lysozyme, an enzyme found in saliva and tears,
inhibits the growth of bacteria in the oral cavity and
on the warm, wet surface of the eye. The subcutaneous
tissue contains many white blood cells (WBCs),
as does the areolar connective tissue below the epithelium
of mucous membranes.
Defensive Cells
Recall from Chapter 11 that many of our defensive
cells are white blood cells. Macrophages, both fixed
and wandering, have receptors for the pathogens
humans are likely to encounter (this probably reflects
millions of years of coexistence) and are very efficient
phagocytes. Other cells capable of phagocytosis of
pathogens or other foreign antigens are the neutrophils
and, to a lesser extent, the eosinophils. Phagocytic
cells use intracellular enzymes and chemicals
such as hydrogen peroxide (H2O2) to destroy ingested
pathogens.
The Langerhans cells of the skin, and other dendritic
cells throughout the body, also phagocytize foreign
material, not merely to destroy it, but to take it to
a lymph node where the lymphocytes of adaptive
immune mechanisms are then activated. The macrophages
are also involved in activating these lymphocytes.
This is a very important link between the two
components of immunity.
Natural killer cells (NK cells) circulate in the
blood but are also found in the red bone marrow,
spleen, and lymph nodes. They are a small portion
(about 10%) of the total lymphocytes, but are able to
destroy many kinds of pathogens and tumor cells. NK
cells make direct contact with foreign cells, and kill
them by rupturing their cell membranes (with chemicals
called perforins) or by inflicting some other kind
of chemical damage.
Basophils and mast cells (a type of connective tissue
cell) are also defensive cells that are found throughout
areolar connective tissue. They produce histamine and
leukotrienes. Histamine causes vasodilation and makes
capillaries more permeable; these are aspects of
inflammation. Leukotrienes also increase capillary
permeability and attract phagocytic cells to the area.
Chemical Defenses
Chemicals that help the body resist infection include
the interferons, complement, and the chemicals
involved in inflammation. The interferons (alpha-,
beta-, and gamma-interferons) are proteins produced
by cells infected with viruses and by T cells. Viruses
must be inside a living cell to reproduce, and although
interferon cannot prevent the entry of viruses into
cells, it does block their reproduction. When viral
reproduction is blocked, the viruses cannot infect new
cells and cause disease. Interferon is probably a factor
in the self-limiting nature of many viral diseases (and
is used in the treatment of some diseases, such as hepatitis
C).
Complement is a group of more than 20 plasma
proteins that circulate in the blood until activated.
They are involved in the lysis of cellular antigens and
the labeling of noncellular antigens. Some stimulate
the release of histamine in inflammation; others
attract WBCs to the site.
Inflammation is a general response to damage of
any kind: microbial, chemical, or physical. Basophils
and mast cells release histamine and leukotrienes,
which affect blood vessels as previously described.
Vasodilation increases blood flow to the damaged area,
and capillaries become more permeable; tissue fluid
and WBCs collect at the site. The purpose of inflammation
is to try to contain the damage, keep it from
spreading, eliminate the cause, and permit repair of the
tissue to begin. From even this brief description you
can see why the four signs of inflammation are redness,
heat, swelling, and pain: redness from greater blood
flow, heat from the blood and greater metabolic activity,
swelling from the accumulation of tissue fluid, and
pain from the damage itself and perhaps the swelling.
As mentioned in Chapter 10, inflammation is a positive
feedback mechanism that may become a vicious
cycle of damage and more damage. The hormone cortisol
is one brake that prevents this, and at least one of
the complement proteins has this function as well.
There are probably other chemical signals (in general
called cytokines and chemokines) that help limit
inflammation to an extent that is useful.
In summary, innate immunity is nonspecific, is
always the same, does not create memory, and does
not become more efficient upon repeated exposures.
Some cells of innate immune mechanisms also activate
the adaptive immune mechanisms. The aspects of
innate immunity are shown in Fig. 14–6.
328 The Lymphatic System and Immunity
The Lymphatic System and Immunity 329

Integumentary system
Mucous membranes and lysozyme
Langerhans cells Phagocytes
Natural killer cells
Basophils and mast cells
Interferon Complement
Inflammation
T cell
Blocks viral
reproduction
Lyses cells,
attracts WBC's
Histamine and leukotrienes
Vasodilation •
Increased
capillary
permeability •
Tissue fluid
and WBCs •
Histamine and leukotrienes
Activate lymphocytes
Perforins

Macrophage


Bacteria

Neutrophils


Antigen


Subcutaneous tissue
and WBC's
Defensins Langerhans cells
Stratum corneum
• •

Hydrochloric
acid
Ciliated epithelium
Lacrimal gland

Salivary glands

• •
A Barriers
B Cells
C Chemicals
Figure 14–6. Innate immunity. (A) Barriers. (B) Defensive cells. (C) Chemical defenses.
See text for description.
QUESTION: The three aspects of innate immunity are interconnected; describe two of
these connections.
ADAPTIVE IMMUNITY
To adapt means to become suitable, and adaptive
immunity can become “suitable” for and respond to
almost any foreign antigen. Adaptive immunity is specific
and is carried out by lymphocytes and macrophages.
The majority of lymphocytes are the T lymphocytes
and B lymphocytes, or, more simply, T cells and
B cells. In the embryo, T cells are produced in the
bone marrow and thymus. They must pass through
the thymus, where the thymic hormones bring about
their maturation. The T cells then migrate to the
spleen, lymph nodes, and lymph nodules, where they
are found after birth.
Produced in the embryonic bone marrow, B cells
migrate directly to the spleen and lymph nodes and
nodules. When activated during an immune response,
some B cells will divide many times and become
plasma cells that produce antibodies to a specific foreign
antigen.
The mechanisms of immunity that involve T cells
and B cells are specific, meaning that one foreign antigen
is the target each time a mechanism is activated. A
macrophage has receptor sites for foreign chemicals
such as those of bacterial cell walls or flagella, and may
phagocytize just about any foreign material it comes
across (as will the Langerhans or dendritic cells). T
cells and B cells, however, become very specific, as you
will see.
The first step in the destruction of a pathogen or
foreign cell is the recognition of its antigens as foreign.
Both T cells and B cells are capable of this, but
the immune mechanisms are activated especially well
when this recognition is accomplished by macrophages
and a specialized group of T lymphocytes
called helper T cells (also called CD4 T cells). The
foreign antigen is first phagocytized by a macrophage,
and parts of it are “presented” on the macrophage’s
cell membrane. Also on the macrophage membrane
are “self” antigens that are representative of the
antigens found on all of the cells of the individual.
Therefore, the helper T cell that encounters this
macrophage is presented not only with the foreign
antigen but also with “self” antigens for comparison.
The helper T cell becomes sensitized to and specific
for the foreign antigen, the one that does not belong
in the body (see Box 14–2: AIDS).
The recognition of an antigen as foreign initiates
one or both of the mechanisms of adaptive immunity.
These are cell-mediated immunity (sometimes
called simply cellular immunity), in which T cells and
macrophages participate, and antibody-mediated
immunity (or humoral immunity), which involves T
cells, B cells, and macrophages.
Cell-Mediated Immunity
This mechanism of immunity does not result in the
production of antibodies, but it is effective against
intracellular pathogens (such as viruses), fungi,
malignant cells, and grafts of foreign tissue. As mentioned
earlier, the first step is the recognition of the
foreign antigen by macrophages and helper T cells,
which become activated and are specific. (You may
find it helpful to refer to Fig. 14–7 as you read the following.)
These activated T cells, which are antigen specific,
divide many times to form memory T cells and cytotoxic
(killer) T cells (also called CD8 T cells). The
memory T cells will remember the specific foreign
antigen and become active if it enters the body again.
Cytotoxic T cells are able to chemically destroy foreign
antigens by disrupting cell membranes. This
is how cytotoxic T cells destroy cells infected with
viruses and prevent the viruses from reproducing.
These T cells also produce cytokines, which are chemicals
that attract macrophages to the area and activate
them to phagocytize the foreign antigen and cellular
debris.
It was once believed that another subset of T cells
served to stop the immune response, but this may not
be so. It seems probable that the CD4 and CD8 T
cells also produce feedback chemicals to limit the
immune response once the foreign antigen has been
destroyed. The memory T cells, however, will quickly
initiate the cell-mediated immune response should
there be a future exposure to the antigen.
Antibody-Mediated Immunity
This mechanism of immunity does involve the production
of antibodies and is also diagrammed in Fig.
14–7. Again, the first step is the recognition of the foreign
antigen, this time by B cells as well as by
macrophages and helper T cells. The sensitized helper
T cell presents the foreign antigen to B cells, which
provides a strong stimulus for the activation of B cells
specific for this antigen. The activated B cells begin to
divide many times, and two types of cells are formed.
Some of the new B cells produced are memory B
cells, which will remember the specific antigen and
330 The Lymphatic System and Immunity
The Lymphatic System and Immunity 331
331
Produces cytokines to
attract macrophages
Chemically destroys
foreign cells
Memory T cell
Helper T cell Cytotoxic T cell
Macrophage

Foreign antigen


Self antigens


Receptor sites


Self antigens


Receptor sites
Macrophage
Helper T cells Memory B cell
B cell Plasma cell
Antibodies
Opsonization Antigen-antibody complex Complement fixation
lysis of
cellular antigen
Macrophage
A Cell-mediated
B Antibody-mediated
Figure 14–7. Adaptive immunity. (A) Cell-mediated immunity. (B) Antibody-mediated
immunity. See text for description.
QUESTION: Adaptive immunity has memory; which cells provide this? What kind of memory
is it?
332 The Lymphatic System and Immunity
initiate a rapid response upon a second exposure.
Other B cells become plasma cells that produce antibodies
specific for this one foreign antigen.
Antibodies, also called immune globulins (Ig) or
gamma globulins, are proteins shaped somewhat like
the letter Y. Antibodies do not themselves destroy foreign
antigens, but rather become attached to such antigens
to “label” them for destruction. Each antibody
BOX 14–2 AIDS
placental transmission of the virus from mother to
fetus.
In the United States, most of the cases of AIDS
during the 1980s were in homosexual men and IV
drug users who shared syringes contaminated with
their blood. By the 1990s, however, it was clear that
AIDS was becoming more of a heterosexually transmitted
disease, with rapidly increasing case rates
among women and teenagers. In much of the rest
of the world, especially Africa and Asia, the transmission
of AIDS has always been primarily by heterosexual
contact, with equal numbers of women
and men infected. In many of these countries AIDS
is an enormous public health problem, and the
annual number of new cases is still rising.
At present we have no medications that will
eradicate HIV, although certain combinations of
drugs effectively suppress the virus in some people.
For these people, AIDS may become a chronic but
not fatal disease. Unfortunately, the medications do
not work for everyone, and they are very expensive,
beyond the means of most of the world’s AIDS
patients.
Development of an AIDS vaccine has not yet
been successful, although dozens of vaccines are
undergoing clinical trials. A vaccine stimulates antibody
production to a specific pathogen, but everyone
who has died of AIDS had antibodies to HIV.
Those antibodies were not protective because HIV is
a mutating virus; it constantly changes itself, making
previously produced antibodies ineffective. An
AIDS vaccine may not be entirely effective, may not
have the 80% to 90% protection rate we have
come to expect from vaccines.
If we cannot cure AIDS and we cannot yet prevent
it by vaccination, what recourse is left?
Education. Everyone should know how AIDS is
spread. The obvious reason is to be able to avoid
the high-risk behaviors that make acquiring HIV
more likely. Yet another reason, however, is that
everyone should know that they need not fear
casual contact with people with AIDS. Healthcare
personnel have a special responsibility, not only to
educate themselves, but to provide education
about AIDS for their patients and the families of
their patients.
In 1981, young homosexual men in New York and
California were diagnosed with Kaposi’s sarcoma
and Pneumocystis carinii pneumonia. At that time,
Kaposi’s sarcoma was known as a rare, slowly growing
malignancy in elderly men. Pneumocystis pneumonia
was almost unheard of; P. carinii (now P.
jiroveci) is a pathogen that does not cause disease in
healthy people. That in itself was a clue. These
young men were not healthy; their immune systems
were not functioning normally. As the number
of patients increased rapidly, the disease was given
a name (acquired immunodeficiency syndrome—
AIDS) and the pathogen was found. Human
immunodeficiency virus (HIV) is a retrovirus that
infects helper T cells, macrophages, and other
human cells. Once infected, the human cells contain
HIV genes for the rest of their lives. Without sufficient
helper T cells, the immune system is seriously
impaired. Foreign antigens are not recognized, B
cells are not activated, and killer T cells are not
stimulated to proliferate.
The person with AIDS is susceptible to opportunistic
infections, that is, those infections caused
by fungi and protozoa that would not affect average
healthy adults. Some of these infections may be
treated with medications and even temporarily
cured, but the immune system cannot prevent the
next infection, or the next. As of this writing, AIDS
is considered an incurable disease, although with
proper medical treatment, some people with AIDS
may live for many years.
Where did this virus come from? The latest
research suggests that HIV evolved from a harmless
chimpanzee virus in Africa sometime during the
1930s. Spread of the virus was very slow at first,
and only when air travel became commonplace did
the virus spread worldwide.
The incubation period of AIDS is highly variable,
ranging from a few months to several years.
An infected person may unknowingly spread HIV to
others before any symptoms appear. It should
be emphasized that AIDS, although communicable,
is not a contagious disease. It is not spread
by casual contact as is measles or the common
cold. Transmission of AIDS occurs through sexual
contact, by contact with infected blood, or by
produced is specific for only one antigen. Because
there are so many different pathogens, you might
think that the immune system would have to be capable
of producing many different antibodies, and in fact
this is so. It is estimated that millions of different antigen-
specific antibodies can be produced, should there
be a need for them. The structure of antibodies is
shown in Fig. 14–8, and the five classes of antibodies
are described in Table 14–1.
The antibodies produced will bond to the antigen,
forming an antigen–antibody complex. This complex
results in opsonization, which means that the antigen
The Lymphatic System and Immunity 333
Table 14–1 CLASSES OF ANTIBODIES
Name Location Functions
IgG
IgA
IgM
IgD
IgE
Blood
Extracellular fluid
External secretions (tears,
saliva, etc.)
Blood
B lymphocytes
Mast cells or basophils
• Crosses the placenta to provide passive immunity for newborns
• Provides long-term immunity following recovery or a vaccine
• Present in breast milk to provide passive immunity for breast-fed infants
• Found in secretions of all mucous membranes
• Produced first by the maturing immune system of infants
• Produced first during an infection (IgG production follows)
• Part of the ABO blood group
• Receptors on B lymphocytes
• Important in allergic reactions (mast cells release histamine)
A
Antigen binding
site
Complement
binding site
Macrophage
binding site
B
IgG IgD IgE
IgA
IgM
Bacteria Virus Toxin
Agglutination
C
Neutralization
Disulfide
bonds
Figure 14–8. Antibodies.
(A) Structure of one IgG molecule.
Notice how the many
disulfide bonds maintain the
shape of the molecule.
(B) Structure of the five classes
of antibodies. (C) Antibody
activity: Agglutination of bacteria
and neutralization of
viruses or toxins.
QUESTION: In part C, why
does neutralization inactivate
a bacterial toxin?
is now “labeled” for phagocytosis by macrophages or
neutrophils. The antigen–antibody complex also stimulates
the process of complement fixation (see Box
14–3: Diagnostic Tests).
Some of the circulating complement proteins are
activated, or fixed, by an antigen–antibody complex.
Complement fixation may be complete or partial. If
the foreign antigen is cellular, the complement proteins
bond to the antigen–antibody complex, then to
one another, forming an enzymatic ring that punches
a hole in the cell to bring about the death of the cell.
This is complete (or entire) complement fixation and
is what happens to bacterial cells (it is also the cause of
hemolysis in a transfusion reaction).
If the foreign antigen is not a cell—let’s say it’s a
virus for example—partial complement fixation takes
place, in which some of the complement proteins bond
to the antigen–antibody complex. This is a chemotactic
factor. Chemotaxis means “chemical movement” and
is actually another label that attracts macrophages to
engulf and destroy the foreign antigen.
In summary, adaptive immunity is very specific,
does create memory, and because it does, often becomes
more efficient with repeated exposures.
Antibody Responses
The first exposure to a foreign antigen does stimulate
antibody production, but antibodies are produced
slowly and in small amounts (see Fig. 14–9). Let us
take as a specific example the measles virus. On a person’s
first exposure to this virus, antibody production is
usually too slow to prevent the disease itself, and the
person will have clinical measles. Most people who get
measles recover, and upon recovery have antibodies
and memory cells that are specific for the measles
virus.
On a second exposure to this virus, the memory
cells initiate rapid production of large amounts of antibodies,
enough to prevent a second case of measles.
This is the reason why we develop immunity to certain
diseases, and this is also the basis for the protection
given by vaccines (see Box 14–4: Vaccines).
As mentioned previously, antibodies label pathogens
or other foreign antigens for phagocytosis or
complement fixation. More specifically, antibodies
cause agglutination or neutralization of pathogens
before their eventual destruction. Agglutination
means “clumping,” and this is what happens when
antibodies bind to bacterial cells. The bacteria that are
clumped together by attached antibodies are more easily
phagocytized by macrophages (see Fig. 14–8).
The activity of viruses may be neutralized by antibodies.
A virus must get inside a living cell in order to
reproduce itself. However, a virus with antibodies
attached to it is unable to enter a cell, cannot reproduce,
and will soon be phagocytized. Bacterial toxins
may also be neutralized by attached antibodies. The
antibodies change the shape of the toxin, prevent it
from exerting its harmful effects, and promote its
phagocytosis by macrophages.
Allergies are also the result of antibody activity
(see box 14–5: Allergies).
TYPES OF IMMUNITY
If we consider the source of immunity, that is, where
it comes from, we can begin with two major categories:
genetic immunity and acquired immunity.
Genetic immunity is conferred by our DNA, and
acquired immunity is developed or acquired by natural
or artificial means.
Genetic immunity does not involve antibodies or
the immune system; it is the result of our genetic
makeup. What this means is that some pathogens
334 The Lymphatic System and Immunity
BOX 14–3 DIAGNOSTIC TESTS
Several important laboratory tests involve antibodies
and may be very useful to confirm a diagnosis.
Complement fixation test—determines the
presence of a particular antibody in the patient’s
blood, but does not indicate when the infection
occurred.
Antibody titer—determines the level or
amount of a specific antibody in the patient’s
blood. If another titer is done 1 to several weeks
later, an increase in the antibody level shows the
infection to be current.
Fluorescent antibody test—uses antibodies
tagged with fluorescent dyes, which are added
to a clinical specimen such as blood, sputum, or
a biopsy of tissue. If the suspected pathogen is
present, the fluorescent antibodies will bond to it
and the antigen–antibody complex will “glow”
when examined with a fluorescent microscope.
Tests such as these are used in conjunction
with patient history and symptoms to arrive at a
diagnosis.
The Lymphatic System and Immunity 335
Primary and secondary antibody responses
Antibody level
First
exposure
to
antigen
Second
exposure
to
antigen
10 days 20 days some months some years 10 days 20 days
Time after exposure
IgG
IgG
IgM IgM
Figure 14–9. Antibody responses to first and subsequent exposures to a pathogen. See
text for description.
QUESTION: State the two differences in IgG production after a first and a second exposure
to the same antigen.
BOX 14–4 VACCINES
the inactivated toxins of these bacteria. Vaccines for
pneumococcal pneumonia and meningitis contain
bacterial capsules. These vaccines cannot cause disease
because the capsules are non-toxic and nonliving;
there is nothing that can reproduce.
Influenza and rabies vaccines contain killed viruses.
Measles and the oral polio vaccines contain attenuated
(weakened) viruses.
Although attenuated pathogens are usually
strongly antigenic and stimulate a protective
immune response, there is a very small chance that
the pathogen may regain its virulence and cause
the disease. The live-virus oral polio vaccine (still
being used in the quest to eliminate polio throughout
the world) has a risk of 1 in 500,000 of causing
polio. The killed-virus injectable polio vaccine has
no such risk.
The purpose of vaccines is to prevent disease. A vaccine
contains an antigen that the immune system
will respond to, just as it would to the actual
pathogen. The types of vaccine antigens are a killed
or weakened (attenuated) pathogen, part of a
pathogen such as a bacterial capsule, or an inactivated
bacterial toxin called a toxoid.
Because the vaccine itself does not cause disease
(with very rare exceptions), the fact that antibody
production to it is slow is not detrimental to the
person. The vaccine takes the place of the first
exposure to the pathogen and stimulates production
of antibodies and memory cells. On exposure
to the pathogen itself, the memory cells initiate
rapid production of large amounts of antibody,
enough to prevent disease.
We now have vaccines for many diseases. The
tetanus and diphtheria vaccines contain toxoids,
cause disease in certain host species but not in others.
Dogs and cats, for example, have genetic immunity
to the measles virus, which is a pathogen only for people.
Mouse leukemia viruses affect only mice, not
people; we have genetic immunity to them. This is
not because we have antibodies against these mouse
viruses, but rather that we have genes that are the
codes for proteins that make it impossible for such
pathogens to reproduce in our cells and tissues.
Monkeys have similar protective genes and proteins
for the human AIDS virus; HIV does not cause disease
in these monkeys. Because this is a genetic characteristic
programmed in DNA, genetic immunity always
lasts a lifetime.
Acquired immunity does involve antibodies.
Passive immunity means that the antibodies are from
another source, whereas active immunity means that
the individual produces his or her own antibodies.
One type of naturally acquired passive immunity is
the placental transmission of antibodies (IgG) from
maternal blood to fetal circulation. The baby will then
be born temporarily immune to the diseases the
mother is immune to. Such passive immunity may be
prolonged by breast-feeding, because breast milk also
contains maternal antibodies (IgA).
Artificially acquired passive immunity is obtained
by the injection of immune globulins (gamma globulins
or preformed antibodies) after presumed exposure
to a particular pathogen. Such immune globulins are
available for German measles, hepatitis A and B,
tetanus and botulism (anti-toxins), and rabies. These
are not vaccines; they do not stimulate immune mechanisms,
but rather provide immediate antibody protection.
Passive immunity is always temporary, lasting
a few weeks to a few months, because antibodies from
another source eventually break down.
Active immunity is the production of one’s own
antibodies and may be stimulated by natural or artificial
means. Naturally acquired active immunity means
that a person has recovered from a disease and now
has antibodies and memory cells specific for that
pathogen. Artificially acquired active immunity is the
result of a vaccine that has stimulated production of
antibodies and memory cells (see Box 14–6: Vaccines
That Have Changed Our Lives). No general statement
can be made about the duration of active immunity.
Recovering from plague, for example, confers
lifelong immunity, but the plague vaccine does not.
Duration of active immunity, therefore, varies with
the particular disease or vaccine.
The types of immunity are summarized in Table
14–2.
336 The Lymphatic System and Immunity
BOX 14–5 ALLERGIES
In an allergic reaction, the effects of inflammatory
chemicals create symptoms such as watery
eyes and runny nose (hay fever) or the more serious
wheezing and difficult breathing that characterize
asthma. Several medications are available to counteract
these effects (see Chapter 15 for a description
of asthma).
Anaphylactic shock is an extreme allergic
response that may be elicited by exposure to penicillin
or insect venoms. On the first exposure, the
person becomes highly sensitized to the foreign
antigen. On the second exposure, histamine is
released from mast cells throughout the body and
causes a drastic decrease in blood volume. The
resulting drop in blood pressure may be fatal in
only a few minutes. People who know they are
allergic to bee stings, for example, may obtain a
self-contained syringe of epinephrine to carry with
them. Epinephrine can delay the progression of
anaphylactic shock long enough for the person to
seek medical attention.
An allergy is a hypersensitivity to a particular foreign
antigen, called an allergen. Allergens include
plant pollens, foods, chemicals in cosmetics, antibiotics
such as penicillin, dust, and mold spores. Such
allergens are not themselves harmful. Most people,
for example, can inhale pollen, eat peanuts, or take
penicillin with no ill effects.
Hypersensitivity means that the immune system
overresponds to the allergen, and produces tissue
damage by doing so. Allergic responses are characterized
by the production of IgE antibodies, which
bond to mast cells. Mast cells are specialized connective
tissue cells and are numerous in the connective
tissue of the skin and mucous membranes.
Chemicals in mast cells include histamine and
leukotrienes, which are released by the bonding of
IgE antibodies or when tissue damage occurs.
These chemicals contribute to the process of
inflammation by increasing the permeability of capillaries
and venules. Tissue fluid collects and more
WBCs are brought to the damaged area.
AGING AND THE
LYMPHATIC SYSTEM
The aging of the lymphatic system is apparent in the
decreased efficiency of immune responses. Elderly
people are more likely than younger ones to develop
shingles, when an aging immune system cannot keep
the chickenpox virus dormant. They are also more
susceptible to infections such as influenza and to what
are called secondary infections, such as pneumonia
following a case of the flu. Vaccines for both of these
are available, and elderly people should be encouraged
to get them. Elderly people should also be sure to get
a tetanus-diphtheria booster every 10 years.
Autoimmune disorders are also more common
among older people; the immune system mistakenly
perceives a body tissue as foreign and initiates its destruction.
Rheumatoid arthritis and myasthenia gravis
are examples of autoimmune diseases. The incidence
of cancer is also higher. Malignant cells that once
might have been quickly destroyed remain alive and
proliferate.
The Lymphatic System and Immunity 337
BOX 14–6 VACCINES THAT HAVE CHANGED OUR LIVES
they are no longer possible reservoirs or sources of
the pathogen for others, and the spread of disease
may be greatly limited.
Other diseases that have been controlled by the
use of vaccines are tetanus, mumps, influenza,
measles, and German measles. Whooping cough
had been controlled until recently, when the vaccination
rate decreased; the annual number of cases
in the United States has more than doubled. The
vaccine for hepatitis B has significantly decreased
the number of cases of this disease among healthcare
workers, and the vaccine is recommended for
all children. People who have been exposed to
rabies, which is virtually always fatal, can be protected
by a safe vaccine.
Without such vaccines our lives would be very
different. Infant mortality or death in childhood
would be much more frequent, and all of us would
have to be much more aware of infectious diseases.
In many parts of the world this is still true; many of
the developing countries in Africa and Asia still cannot
afford extensive vaccination programs for their
children. Many of the diseases mentioned here,
which we may rarely think of, are still a very significant
part of the lives of millions of people.
In 1797, Edward Jenner (in England) published his
results on the use of the cowpox virus called vaccinia
as the first vaccine for smallpox, a closely
related virus. (He was unaware of the actual pathogens,
because viruses had not yet been discovered,
but he had noticed that milkmaids who got cowpox
rarely got smallpox.) In 1980, the World Health
Organization declared that smallpox had been
eradicated throughout the world. A disease that
had killed or disfigured millions of people throughout
recorded history is now considered part of history
(except for the possible use of the virus as a
biological weapon).
In the 19th century in the northern United
States, thousands of children died of diphtheria
every winter. Today there are fewer than 10 cases
of diphtheria each year in the entire country. In
the early 1950s, 50,000 cases of paralytic polio
were reported in the United States each year.
Today, wild-type polio virus is not found in North
America.
Smallpox, diphtheria, and polio are no longer
the terrible diseases they once were, and this is
because of the development and widespread use of
vaccines. When people are protected by a vaccine,
Table 14–2 TYPES OF IMMUNITY
Type Description
Genetic
Acquired
Passive
NATURAL
ARTIFICIAL
Active
NATURAL
ARTIFICIAL
• Does not involve antibodies; is
programmed in DNA
• Some pathogens affect certain
host species but not others
• Does involve antibodies
• Antibodies from another source
• Placental transmission of antibodies
from mother to fetus
• Transmission of antibodies in
breast milk
• Injection of preformed antibodies
(gamma globulins or immune
globulins) after presumed exposure
• Production of one’s own antibodies
• Recovery from a disease, with production
of antibodies and memory
cells
• A vaccine stimulates production of
antibodies and memory cells
SUMMARY
The preceding discussions of immunity will give you a
small idea of the complexity of the body’s defense system.
However, there is still much more to be learned,
especially about the effects of the nervous system and
endocrine system on immunity. For example, it is
known that people under great stress have immune
systems that may not function as they did when stress
was absent.
At present, much research is being done in this
field. The goal is not to eliminate all disease, for that
would not be possible. Rather, the aim is to enable
people to live healthier lives by preventing certain
diseases.
338 The Lymphatic System and Immunity
STUDY OUTLINE
Functions of the Lymphatic System
1. To return tissue fluid to the blood to maintain
blood volume (see Fig. 14–1).
2. To protect the body against pathogens and other
foreign material.
Parts of the Lymphatic System
1. Lymph and lymph vessels.
2. Lymphatic tissue: lymph nodes and nodules,
spleen, and thymus; lymphocytes mature and proliferate.
Lymph—the tissue fluid that enters lymph
capillaries
1. Similar to plasma, but more WBCs are present,
and has less protein.
2. Must be returned to the blood to maintain blood
volume and blood pressure.
Lymph Vessels
1. Dead-end lymph capillaries are found in most tissue
spaces; collect tissue fluid and proteins (see Fig.
14–2).
2. The structure of larger lymph vessels is like that of
veins; valves prevent the backflow of lymph.
3. Lymph is kept moving in lymph vessels by:
• constriction of the lymph vessels
• the skeletal muscle pump
• the respiratory pump
4. Lymph from the lower body and upper left quadrant
enters the thoracic duct and is returned to the
blood in the left subclavian vein (see Fig. 14–3).
5. Lymph from the upper right quadrant enters the
right lymphatic duct and is returned to the blood in
the right subclavian vein.
Lymph Nodes—encapsulated masses of lymphatic
tissue
1. Found in groups along the pathways of lymph vessels.
2. As lymph flows through the nodes:
• foreign material is phagocytized by fixed macrophages
• lymphocytes are activated and fixed plasma cells
produce antibodies to foreign antigens (see Fig.
14–4)
3. The major paired groups of lymph nodes are the
cervical, axillary, and inguinal groups. These are
at the junctions of the head and extremities with
the trunk; remove pathogens from the lymph from
the extremities before the lymph is returned to the
blood.
Lymph Nodules—small unencapsulated
masses of lymphatic tissue
1. Found beneath the epithelium of all mucous membranes,
that is, the tracts that have natural openings
to the environment.
2. Destroy pathogens that penetrate the epithelium of
the respiratory, digestive, urinary, or reproductive
tracts.
3. Tonsils are the lymph nodules of the pharynx;
Peyer’s patches are those of the small intestine.
Spleen—located in the upper left abdominal
quadrant behind the stomach
1. The fetal spleen produces RBCs.
2. Functions after birth:
• contains lymphocytes to be activated and fixed
plasma cells that produce antibodies
• contains fixed macrophages (RE cells) that
phagocytize pathogens and old RBCs; bilirubin
is formed and sent to the liver for excretion in
bile
• stores platelets and destroys damaged platelets
Thymus—inferior to the thyroid gland; in
the fetus and infant the thymus is large (see
Fig. 14–5); with age the thymus shrinks
1. Produces T lymphocytes (T cells).
2. Produces thymic hormones that make T cells
immunologically competent, that is, able to recognize
foreign antigens and provide immunity.
Immunity—the ability to destroy foreign
antigens and prevent future cases of certain
infectious diseases
1. Antigens are chemical markers that identify cells.
Human cells have “self” antigens—the HLA types.
2. Foreign antigens stimulate antibody production
or other immune responses, and include bacteria,
viruses, fungi, protozoa, and malignant cells.
Innate Immunity (see Fig. 14–6)
1. Is nonspecific, responses are always the same, does
not create memory, and does not become more
efficient. Consists of barriers, defensive cells, and
chemical defenses.
2. Barriers
• Unbroken stratum corneum and sebum; living
epidermal cells secrete defensins
• Subcutaneous tissue with WBCs
• Mucous membranes and areolar CT with WBCs;
upper respiratory epithelium is ciliated
• HCl in gastric juice
• Lysozyme in saliva and tears
3. Defensive cells
• Phagocytes—macrophages, neutrophils, eosinophils;
macrophages also activate the lymphocytes
of adaptive immunity
• Langerhans cells and other dendritic cells—activate
lymphocytes
• Natural killer cells—destroy foreign cells by rupturing
their cell membranes
• Basophils and mast cells—produce histamine and
leukotrienes (inflammation)
4. Chemical defenses
• Interferon blocks viral reproduction
• Complement proteins lyse foreign cells, attract
WBCs, and contribute to inflammation
• Inflammation—the response to any kind of damage;
vasodilation and increased capillary permeability
bring tissue fluid and WBCs to the area.
Purpose: to contain the damage, eliminate the
cause, and make tissue repair possible.
Signs: redness, heat, swelling, and pain
Adaptive Immunity (see Fig. 14–7)
1. Is very specific, may involve antibodies, does create
memory, and responses become more efficient.
Consists of cell-mediated and antibody-mediated
immunity; is carried out by T cells, B cells, and
macrophages.
2. T lymphocytes (T cells)—in the embryo are produced
in the thymus and RBM; they require the
hormones of the thymus for maturation; migrate to
the spleen, lymph nodes, and nodules.
3. B lymphocytes (B cells)—in the embryo are produced
in the RBM; migrate to the spleen, lymph
nodes, and nodules.
4. The antigen must first be recognized as foreign;
this is accomplished by B cells or by helper T cells
that compare the foreign antigen to “self” antigens
present on macrophages.
5. Helper T cells strongly initiate one or both of the
immune mechanisms: cell-mediated immunity and
antibody-mediated immunity.
Cell-Mediated (cellular) Immunity (see Fig.
14–7)
1. Does not involve antibodies; is effective against
intracellular pathogens, malignant cells, and grafts
of foreign tissue.
2. Helper T cells recognize the foreign antigen, are
antigen specific, and begin to divide to form different
groups of T cells.
3. Memory T cells will remember the specific foreign
antigen.
4. Cytotoxic (killer) T cells chemically destroy foreign
cells and produce cytokines to attract macrophages.
Antibody-Mediated (Humoral) Immunity
(see Fig. 14–7)
1. Does involve antibody production; is effective
against pathogens and foreign cells.
The Lymphatic System and Immunity 339
2. B cells and helper T cells recognize the foreign
antigen; the B cells are antigen specific and begin
to divide.
3. Memory B cells will remember the specific foreign
antigen.
4. Other B cells become plasma cells that produce
antigen-specific antibodies.
5. An antigen–antibody complex is formed, which
attracts macrophages (opsonization).
6. Complement fixation is stimulated by antigen–
antibody complexes. The complement proteins
bind to the antigen–antibody complex and lyse cellular
antigens or enhance the phagocytosis of noncellular
antigens.
Antibodies—immune globulins (Ig) or
gamma globulins (see Table 14–1 and
Fig. 14–8)
1. Proteins produced by plasma cells in response to
foreign antigens.
2. Each antibody is specific for only one foreign antigen.
3. Bond to the foreign antigen to label it for phagocytosis
(opsonization).
Antibody Responses and Functions (see Fig.
14–9)
1. On the first exposure to a foreign antigen, antibodies
are produced slowly and in small amounts, and
the person may develop clinical disease.
2. On the second exposure, the memory cells initiate
rapid production of large amounts of antibodies,
and a second case of the disease may be prevented.
This is the basis for the protection given by vaccines,
which take the place of the first exposure.
3. Antibodies cause agglutination (clumping) of bacterial
cells; clumped cells are easier for macrophages
to phagocytize (see Fig. 14–8).
4. Antibodies neutralize viruses by bonding to them
and preventing their entry into cells.
5. Antibodies neutralize bacterial toxins by bonding
to them and changing their shape.
Types of Immunity (see Table 14–2)
340 The Lymphatic System and Immunity
REVIEW QUESTIONS
1. Explain the relationships among plasma, tissue
fluid, and lymph, in terms of movement of water
throughout the body. (p. 322)
2. Describe the system of lymph vessels. Explain how
lymph is kept moving in these vessels. Into which
veins is lymph emptied? (p. 322)
3. State the locations of the major groups of lymph
nodes, and explain their functions. (pp. 322–323)
4. State the locations of lymph nodules, and explain
their functions. (pp. 324)
5. Describe the location of the spleen and explain its
functions. If the spleen is removed, what organs
will compensate for its functions? (p. 324)
6. Explain the function of the thymus, and state when
(age). this function is most important. (pp. 324,
326)
7. Name the different kinds of foreign antigens to
which the immune system responds, and state three
general differences between innate immunity and
adaptive immunity. (p. 327)
8. Innate immunity includes barriers, defensive
cells, and chemicals; give two examples of each.
(p. 328)
9. Explain how a foreign antigen is recognized as
foreign. Which mechanism of adaptive immunity
involves antibody production? Explain what
opsonization means. (pp. 330, 333)
10. State the functions of helper T cells, cytotoxic
T cells, and memory T cells. Plasma cells
differentiate from which type of lymphocyte?
State the function of plasma cells. What other
type of cell comes from B lymphocytes? (pp. 330,
332)
11. What is the stimulus for complement fixation?
How does this process destroy cellular antigens
and non-cellular antigens? (pp. 334)
12. Explain the antibody reactions of agglutination
and neutralization. (p. 334)
13. Explain how a vaccine provides protective immunity
in terms of first and second exposures to a
pathogen. (p. 334)
14. Explain the difference between the following: (pp.
336–337)
a. Genetic immunity and acquired immunity
b. Passive acquired immunity and active acquired
immunity
c. Natural and artificial passive acquired immunity
d. Natural and artificial active acquired immunity
The Lymphatic System and Immunity 341
FOR FURTHER THOUGHT
1. Bubonic plague, also called black plague, is a serious
disease caused by a bacterium and spread by
fleas from rats or rodents to people. It got its
“black” name from “buboes,” dark swellings found
in the groin or armpit of people with plague.
Explain what buboes are, and why they were usually
found in the groin and armpit.
2. In Rh disease of the newborn, maternal antibodies
enter fetal circulation and destroy the red blood
cells of the fetus. A mother with type O blood has
anti-A and anti-B antibodies, but may have a dozen
type A children without any problem at all. Explain
why. (Look at Table 14–1 and Fig. 14–8.)
3. Most vaccines are given by injection. The oral
polio vaccine (OPV), however, is not; it is given by
mouth. Remembering that the purpose of a vaccine
is to expose the individual to the pathogen, what
does this tell you about the polio viruses (there are
three) and their usual site of infection?
4. Everyone should have a tetanus booster shot every
10 years. That is what we often call a “tetanus
shot.” Someone who sustains a soil-contaminated
injury should also receive a tetanus booster (if none
in the past 10 years). But someone who has symptoms
of tetanus should get TIG, tetanus immune
globulin. Explain the difference, and why TIG is so
important.
5. People with AIDS are susceptible to many other
diseases. Which of these would be least likely:
pneumonia, rheumatoid arthritis, yeast infection of
the mouth, or protozoan infection of the intestines?
Explain your answer.
342
CHAPTER 15
Chapter Outline
Divisions of the Respiratory System
Nose and Nasal Cavities
Pharynx
Larynx
Trachea and Bronchial Tree
Lungs and Pleural Membranes
Alveoli
Mechanism of Breathing
Inhalation
Exhalation
Pulmonary Volumes
Exchange of Gases
Diffusion of Gases—Partial Pressures
Transport of Gases in the Blood
Regulation of Respiration
Nervous Regulation
Chemical Regulation
Respiration and Acid–Base Balance
Respiratory Acidosis and Alkalosis
Respiratory Compensation
Aging and the Respiratory System
BOX 15–1 ASTHMA
BOX 15–2 HYALINE MEMBRANE DISEASE
BOX 15–3 PNEUMOTHORAX
BOX 15–4 EMPHYSEMA
BOX 15–5 THE HEIMLICH MANEUVER
BOX 15–6 PULMONARY EDEMA
BOX 15–7 PNEUMONIA
BOX 15–8 CARBON MONOXIDE
Student Objectives
• State the general function of the respiratory system.
• Describe the structure and functions of the nasal
cavities and pharynx.
• Describe the structure of the larynx and explain
the speaking mechanism.
• Describe the structure and functions of the trachea
and bronchial tree.
• State the locations of the pleural membranes, and
explain the functions of serous fluid.
• Describe the structure of the alveoli and pulmonary
capillaries, and explain the importance of
surfactant.
• Name and describe the important air pressures
involved in breathing.
• Describe normal inhalation and exhalation and
forced exhalation.
• Name the pulmonary volumes and define each.
• Explain the diffusion of gases in external respiration
and internal respiration.
• Describe how oxygen and carbon dioxide are
transported in the blood.
• Explain the nervous and chemical mechanisms
that regulate respiration.
• Explain how respiration affects the pH of body
fluids.
The Respiratory System
343
New Terminology
Alveoli (al-VEE-oh-lye)
Bronchial tree (BRONG-kee-uhl TREE)
Epiglottis (ep-i-GLAH-tis)
Glottis (GLAH-tis)
Intrapleural pressure (IN-trah-PLOOR-uhl PRESshur)
Intrapulmonic pressure (IN-trah-pull-MAHN-ik
PRES-shur)
Larynx (LA-rinks)
Partial pressure (PAR-shul PRES-shur)
Phrenic nerves (FREN-ik NURVZ)
Pulmonary surfactant (PULL-muh-ner-ee sir-FAKtent)
Residual air (ree-ZID-yoo-al AYRE)
Respiratory acidosis (RES-pi-rah-TOR-ee ass-i-
DOH-sis)
Respiratory alkalosis (RES-pi-rah-TOR-ee al-kah-
LOH-sis)
Soft palate (SAWFT PAL-uht)
Tidal volume (TIGH-duhl VAHL-yoom)
Ventilation (VEN-ti-LAY-shun)
Vital capacity (VY-tuhl kuh-PASS-i-tee)
Related Clinical Terminology
Cyanosis (SIGH-uh-NOH-sis)
Dyspnea (DISP-nee-ah)
Emphysema (EM-fi-SEE-mah)
Heimlich maneuver (HIGHM-lik ma-NEW-ver)
Hyaline membrane disease (HIGH-e-lin MEMbrain
dis-EEZ)
Pneumonia (new-MOH-nee-ah)
Pneumothorax (NEW-moh-THAW-raks)
Pulmonary edema (PULL-muh-ner-ee
uh-DEE-muh).
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
Sometimes a person will describe a habit as being
“as natural as breathing.” Indeed, what could be more
natural? We rarely think about breathing, and it isn’t
something we look forward to, as we might look forward
to a good dinner. We just breathe, usually at the
rate of 12 to 20 times per minute, and faster when
necessary (such as during exercise). You may have
heard of trained singers “learning how to breathe,”
but they are really learning how to make their breathing
more efficient.
Most of the respiratory system is concerned with
what we think of as breathing: moving air into and out
of the lungs. The lungs are the site of the exchanges of
oxygen and carbon dioxide between the air and the
blood. Both of these exchanges are important. All
of our cells must obtain oxygen to carry out cell respiration
to produce ATP. Just as crucial is the elimination
of the CO2 produced as a waste product of cell
respiration, and, as you already know, the proper functioning
of the circulatory system is essential for the
transport of these gases in the blood.
DIVISIONS OF THE
RESPIRATORY SYSTEM
The respiratory system may be divided into the upper
respiratory tract and the lower respiratory tract. The
upper respiratory tract consists of the parts outside
the chest cavity: the air passages of the nose, nasal cavities,
pharynx, larynx, and upper trachea. The lower
respiratory tract consists of the parts found within
the chest cavity: the lower trachea and the lungs themselves,
which include the bronchial tubes and alveoli.
Also part of the respiratory system are the pleural
membranes and the respiratory muscles that form the
chest cavity: the diaphragm and intercostal muscles.
Have you recognized some familiar organs and
structures thus far? There will be more, because this
chapter includes material from all of the previous
chapters. Even though we are discussing the body
system by system, the respiratory system is an excellent
example of the interdependent functioning of all
the body systems.
NOSE AND NASAL CAVITIES
Air enters and leaves the respiratory system through
the nose, which is made of bone and cartilage covered
with skin. Just inside the nostrils are hairs, which help
block the entry of dust.
The two nasal cavities are within the skull, separated
by the nasal septum, which is a bony plate made
of the ethmoid bone and vomer. The nasal mucosa
(lining) is ciliated epithelium, with goblet cells that
produce mucus. Three shelf-like or scroll-like bones
called conchae project from the lateral wall of each
nasal cavity (Figs. 15–1 and 6–6). Just as shelves in a
cabinet provide more flat space for storage, the conchae
increase the surface area of the nasal mucosa. As
air passes through the nasal cavities it is warmed and
humidified, so that air that reaches the lungs is warm
and moist. Bacteria and particles of air pollution are
trapped on the mucus; the cilia continuously sweep
the mucus toward the pharynx. Most of this mucus is
eventually swallowed, and most bacteria present will
be destroyed by the hydrochloric acid in the gastric
juice.
In the upper nasal cavities are the olfactory receptors,
which detect vaporized chemicals that have been
inhaled. The olfactory nerves pass through the ethmoid
bone to the brain.
You may also recall our earlier discussion of the
paranasal sinuses, air cavities in the maxillae, frontal,
sphenoid, and ethmoid bones (see Figs. 15–1 and 6–9).
These sinuses are lined with ciliated epithelium, and
the mucus produced drains into the nasal cavities. The
functions of the paranasal sinuses are to lighten the
skull and provide resonance (more vibrating air) for
the voice.
PHARYNX
The pharynx is a muscular tube posterior to the nasal
and oral cavities and anterior to the cervical vertebrae.
For descriptive purposes, the pharynx may be divided
into three parts: the nasopharynx, oropharynx, and
laryngopharynx (see Fig. 15–1).
The uppermost portion is the nasopharynx, which
is behind the nasal cavities. The soft palate is elevated
during swallowing to block the nasopharynx and prevent
food or saliva from going up rather than down.
The uvula is the part of the soft palate you can see at
the back of the throat. On the posterior wall of the
nasopharynx is the adenoid or pharyngeal tonsil, a
lymph nodule that contains macrophages. Opening
into the nasopharynx are the two eustachian tubes,
which extend to the middle ear cavities. The purpose
344 The Respiratory System
of the eustachian tubes is to permit air to enter or
leave the middle ears, allowing the eardrums to vibrate
properly.
The nasopharynx is a passageway for air only, but
the remainder of the pharynx serves as both an air and
food passageway, although not for both at the same
time. The oropharynx is behind the mouth; its
mucosa is stratified squamous epithelium, continuous
with that of the oral cavity. On its lateral walls are the
palatine tonsils, also lymph nodules. Together with
The Respiratory System 345
Frontal sinus
Ethmoid bone Olfactory receptors
Conchae
Superior
Middle
Inferior
Nostril
Hard
palate
Maxilla
Palatine
bone
Sphenoid sinus
Opening of
eustachian tube
Pharyngeal tonsil
Nasopharynx
Soft palate
Uvula
Palatine tonsil
Oropharynx
Lingual tonsil
Epiglottis
Laryngopharynx
Esophagus
Hyoid bone
Larynx
Thyroid
cartilage
Cricoid
cartilage
Trachea
Figure 15–1. Midsagittal section of the head and neck showing the structures of the
upper respiratory tract.
QUESTION: Describe the shape of the conchae by using a familiar object. What is the function
of the conchae?
the adenoid and the lingual tonsils on the base of the
tongue, they form a ring of lymphatic tissue around
the pharynx to destroy pathogens that penetrate the
mucosa.
The laryngopharynx is the most inferior portion
of the pharynx. It opens anteriorly into the larynx and
posteriorly into the esophagus. Contraction of the
muscular wall of the oropharynx and laryngopharynx
is part of the swallowing reflex.
LARYNX
The larynx is often called the voice box, a name that
indicates one of its functions, which is speaking. The
other function of the larynx is to be an air passageway
between the pharynx and the trachea. Air passages
must be kept open at all times, and so the larynx is
made of nine pieces of cartilage connected by ligaments.
Cartilage is a firm yet flexible tissue that prevents
collapse of the larynx. In comparison, the
esophagus is a collapsed tube except when food is passing
through it.
The largest cartilage of the larynx is the thyroid
cartilage (Fig. 15–2), which you can feel on the anterior
surface of your neck. The epiglottis is the uppermost
cartilage. During swallowing, the larynx is
elevated, and the epiglottis closes over the top, rather
like a trap door or hinged lid, to prevent the entry of
saliva or food into the larynx.
The mucosa of the larynx is ciliated epithelium,
except for the vocal cords (stratified squamous epithelium).
The cilia of the mucosa sweep upward to
remove mucus and trapped dust and microorganisms.
The vocal cords (or vocal folds) are on either side
of the glottis, the opening between them. During
breathing, the vocal cords are held at the sides of the
glottis, so that air passes freely into and out of the trachea
(Fig. 15–3). During speaking, the intrinsic muscles
of the larynx pull the vocal cords across the glottis,
and exhaled air vibrates the vocal cords to produce
sounds that can be turned into speech. It is also physically
possible to speak while inhaling, but this is not
what we are used to. The cranial nerves that are motor
nerves to the larynx for speaking are the vagus and
accessory nerves. You may also recall that for most
people, the speech areas are in the left cerebral hemisphere.
TRACHEA AND BRONCHIAL TREE
The trachea is about 4 to 5 inches (10 to 13 cm) long
and extends from the larynx to the primary bronchi.
The wall of the trachea contains 16 to 20 C-shaped
pieces of cartilage, which keep the trachea open. The
gaps in these incomplete cartilage rings are posterior,
to permit the expansion of the esophagus when food is
swallowed. The mucosa of the trachea is ciliated
epithelium with goblet cells. As in the larynx, the cilia
sweep upward toward the pharynx.
The right and left primary bronchi (Fig. 15–4) are
the branches of the trachea that enter the lungs. Their
structure is just like that of the trachea, with C-shaped
cartilages and ciliated epithelium. Within the lungs,
each primary bronchus branches into secondary
bronchi leading to the lobes of each lung (three right,
two left). The further branching of the bronchial tubes
is often called the bronchial tree. Imagine the trachea
as the trunk of an upside-down tree with extensive
branches that become smaller and smaller; these
smaller branches are the bronchioles. No cartilage is
present in the walls of the bronchioles; this becomes
clinically important in asthma (see Box 15–1: Asthma).
The smallest bronchioles terminate in clusters of alveoli,
the air sacs of the lungs.
LUNGS AND PLEURAL MEMBRANES
The lungs are located on either side of the heart in
the chest cavity and are encircled and protected by the
346 The Respiratory System
Epiglottis
Hyoid
bone
Thyroid
cartilage
Vocal
cords
Cricoid
cartilage
Tracheal
cartilages
A B
Figure 15–2. Larynx. (A) Anterior view. (B) Midsagittal
section through the larynx, viewed from the left side.
QUESTION: What is the function of the epiglottis?
rib cage. The base of each lung rests on the diaphragm
below; the apex (superior tip) is at the level of the clavicle.
On the medial surface of each lung is an indentation
called the hilus, where the primary bronchus and
the pulmonary artery and veins enter the lung.
The pleural membranes are the serous membranes
of the thoracic cavity. The parietal pleura lines the
chest wall, and the visceral pleura is on the surface of
the lungs. Between the pleural membranes is serous
fluid, which prevents friction and keeps the two membranes
together during breathing.
Alveoli
The functional units of the lungs are the air sacs called
alveoli. The flat alveolar type I cells that form most of
the alveolar walls are simple squamous epithelium. In
the spaces between clusters of alveoli is elastic connective
tissue, which is important for exhalation.
Within the alveoli are macrophages that phagocytize
pathogens or other foreign material that may not have
been swept out by the ciliated epithelium of the
bronchial tree. There are millions of alveoli in each
lung, and their total surface area is estimated to be 700
to 800 square feet (picture a sidewalk two and a half
feet wide that is as long as an American football field,
or a rectangle 25 feet by 30 feet). Each alveolus is surrounded
by a network of pulmonary capillaries (see
Fig 15–4). Recall that capillaries are also made of simple
squamous epithelium, so there are only two cells
between the air in the alveoli and the blood in the pulmonary
capillaries, which permits efficient diffusion of
gases (Fig. 15–5).
Each alveolus is lined with a thin layer of tissue
fluid, which is essential for the diffusion of gases, because
a gas must dissolve in a liquid in order to enter
or leave a cell (the earthworm principle—an earthworm
breathes through its moist skin, and will suffocate
if its skin dries out). Although this tissue fluid is
necessary, it creates a potential problem in that it
would make the walls of an alveolus stick together
internally. Imagine a plastic bag that is wet inside; its
walls would stick together because of the surface tension
of the water. This is just what would happen in
alveoli, and inflation would be very difficult.
This problem is overcome by pulmonary surfactant,
a lipoprotein secreted by alveolar type II cells,
also called septal cells. Surfactant mixes with the tissue
fluid within the alveoli and decreases its surface tension,
permitting inflation of the alveoli (see Box 15–2:
Hyaline Membrane Disease). Normal inflation of the
alveoli in turn permits the exchange of gases, but
before we discuss this process, we will first see how air
gets into and out of the lungs.
MECHANISM OF BREATHING
Ventilation is the term for the movement of air to and
from the alveoli. The two aspects of ventilation are
inhalation and exhalation, which are brought about by
The Respiratory System 347
Epiglottis
Vocal cord
A B
Trachea
Glottis
Figure 15–3. Vocal cords and glottis, superior view. (A) Position of the vocal cords during
breathing. (B) Position of the vocal cords during speaking.
QUESTION: What makes the vocal cords vibrate?
the nervous system and the respiratory muscles. The
respiratory centers are located in the medulla and
pons. Their specific functions will be covered in a later
section, but it is the medulla that generates impulses
to the respiratory muscles.
These muscles are the diaphragm and the external
and internal intercostal muscles (Fig. 15–6). The
diaphragm is a dome-shaped muscle below the lungs;
when it contracts, the diaphragm flattens and moves
downward. The intercostal muscles are found between
the ribs. The external intercostal muscles pull the
ribs upward and outward, and the internal intercostal
muscles pull the ribs downward and inward.
Ventilation is the result of the respiratory muscles producing
changes in the pressure within the alveoli and
bronchial tree.
348 The Respiratory System
Frontal sinuses
Sphenoidal sinuses
Nasal cavity
Nasopharynx
Soft palate
Epiglottis
Larynx and vocal folds
Trachea
Superior lobe
Right lung
Right primary
bronchus
Inferior lobe
Mediastinum
Cardiac notch
Pleural space
Pleural membranes
Inferior lobe
Bronchioles
Superior lobe
Left primary bronchus
Left lung
Venule
Alveolus
Alveolar duct
Arteriole
Pulmonary capillaries
B
A
Middle lobe
Diaphragm
Figure 15–4. Respiratory system. (A) Anterior view of the upper and lower respiratory
tracts. (B) Microscopic view of alveoli and pulmonary capillaries. (The colors represent the
vessels, not the oxygen content of the blood within the vessels.)
QUESTION: What are the first branches of the trachea, and how do they resemble the trachea
in structure?
With respect to breathing, three types of pressure
are important:
1. Atmospheric pressure—the pressure of the air
around us. At sea level, atmospheric pressure is 760
mmHg. At higher altitudes, of course, atmospheric
pressure is lower.
2. Intrapleural pressure—the pressure within the
potential pleural space between the parietal pleura
and visceral pleura. This is a potential rather than a
real space. A thin layer of serous fluid causes the
two pleural membranes to adhere to one another.
Intrapleural pressure is always slightly below
atmospheric pressure (about 756 mmHg), and is
called a negative pressure. The elastic lungs are
always tending to collapse and pull the visceral
pleura away from the parietal pleura. The serous
fluid, however, prevents actual separation of the
pleural membranes (see Box 15–3: Pneumothorax).
3. Intrapulmonic pressure—the pressure within the
bronchial tree and alveoli. This pressure fluctuates
below and above atmospheric pressure during each
cycle of breathing.
INHALATION
Inhalation, also called inspiration, is a precise
sequence of events that may be described as follows:
Motor impulses from the medulla travel along the
phrenic nerves to the diaphragm and along the intercostal
nerves to the external intercostal muscles. The
diaphragm contracts, moves downward, and expands
the chest cavity from top to bottom. The external
intercostal muscles pull the ribs up and out, which
expands the chest cavity from side to side and front to
back.
As the chest cavity is expanded, the parietal pleura
expands with it. Intrapleural pressure becomes even
more negative as a sort of suction is created between
the pleural membranes. The adhesion created by the
serous fluid, however, permits the visceral pleura to be
expanded too, and this expands the lungs as well.
As the lungs expand, intrapulmonic pressure falls
below atmospheric pressure, and air enters the nose
and travels through the respiratory passages to the
alveoli. Entry of air continues until intrapulmonic
pressure is equal to atmospheric pressure; this is a normal
inhalation. Of course, inhalation can be continued
beyond normal, that is, a deep breath. This requires a
more forceful contraction of the respiratory muscles
to further expand the lungs, permitting the entry of
more air.
EXHALATION
Exhalation may also be called expiration and begins
when motor impulses from the medulla decrease and
the diaphragm and external intercostal muscles relax.
As the chest cavity becomes smaller, the lungs are
compressed, and their elastic connective tissue, which
was stretched during inhalation, recoils and also com-
The Respiratory System 349
BOX 15–1 ASTHMA
emphysema. When obstructed bronchioles prevent
ventilation of alveoli, the walls of the alveoli begin
to deteriorate and break down, leaving large cavities
that do not provide much surface area for gas
exchange.
One possible way to prevent such serious lung
damage is to prevent asthma attacks with a medication
that blocks the release of IgE antibodies. An
allergy is an immune overreaction, and blocking
such a reaction would prevent the damaging effects
of inflammation. In the United States the incidence
of asthma is increasing among children; this may be
a result of higher levels of air pollution, though
genetic and immunologic factors may contribute as
well.
Asthma is usually triggered by an infection or allergic
reaction that affects the smooth muscle and
glands of the bronchioles. Allergens include foods
and inhaled substances such as dust and pollen.
Wheezing and dyspnea (difficult breathing) characterize
an asthma attack, which may range from
mild to fatal.
As part of the allergic response, the smooth muscle
of the bronchioles constricts. Because there is no
cartilage present in their walls, the bronchioles may
close completely. The secretion of mucus increases,
perhaps markedly, so the already constricted bronchioles
may become clogged or completely
obstructed with mucus.
Chronic asthma is a predisposing factor for
350 The Respiratory System
Figure 15–5. (A) Alveolar structure showing type I and type II cells, and alveolar
macrophages. The respiratory membrane: the structures and substances through which
gases must pass as they diffuse from air to blood (oxygen) or from blood to air (CO2).
(B) Sections of human lungs embedded in plastic. On the left is a normal adult lung; on
the right is a smoker’s lung. (Photograph by Dan Kaufman.)
QUESTION: Which cells shown here are part of the respiratory membrane? Which cells are
not, and what are their functions?
Elastin fibers
Macrophage
Type I cell
Type II
surfactant
cell
Surfactant
and tissue fluid
Alveolar epithelium
Capillary endothelium
Basement membrane
of capillary endothelium
Capillary
Red blood cells
Interstitial
space
Exhalation
Inhalation
Respiration
Oxygen (O )
Carbon dioxide
(CO )
2
2
Primary
bronchi
B
A
Alveolus
Respiratory
membrane
presses the alveoli. As intrapulmonic pressure rises
above atmospheric pressure, air is forced out of the
lungs until the two pressures are again equal.
Notice that inhalation is an active process that
requires muscle contraction, but normal exhalation is
a passive process, depending to a great extent on the
normal elasticity of healthy lungs. In other words,
under normal circumstances we must expend energy
to inhale but not to exhale (see Box 15–4: Emphysema).
We can, however, go beyond a normal exhalation
and expel more air, such as when talking, singing, or
blowing up a balloon. Such a forced exhalation is an
active process that requires contraction of other muscles.
Contraction of the internal intercostal muscles
pulls the ribs down and in and squeezes even more air
out of the lungs. Contraction of abdominal muscles,
such as the rectus abdominis, compresses the abdominal
organs and pushes the diaphragm upward, which
also forces more air out of the lungs (see Box 15–5:
The Heimlich Maneuver).
PULMONARY VOLUMES
The capacity of the lungs varies with the size and age
of the person. Taller people have larger lungs than do
shorter people. Also, as we get older our lung capacity
diminishes as lungs lose their elasticity and the respiratory
muscles become less efficient. For the following
pulmonary volumes, the values given are those for
healthy young adults. These are also shown in Fig.
15–7.
The Respiratory System 351
External
intercostal
muscles
Sternum
Diaphragm
Lung Ribs
Trachea
Ribs
Inhalation Exhalation
A B
Figure 15–6. Actions of the respiratory
muscles. (A) Inhalation:
diaphragm contracts downward;
external intercostal muscles pull rib
cage upward and outward; lungs
are expanded. (B) Normal exhalation:
diaphragm relaxes upward;
rib cage falls down and in as external
intercostal muscles relax; lungs
are compressed.
QUESTION: Why is a normal exhalation
a passive process?
BOX 15–2 HYALINE MEMBRANE DISEASE
lapse after each breath rather than remain inflated.
Each breath, therefore, is difficult, and the newborn
must expend a great deal of energy just to breathe.
Premature infants may require respiratory assistance
until their lungs are mature enough to produce
surfactant. Use of a synthetic surfactant has
significantly helped some infants, and because they
can breathe more normally, their dependence on
respirators is minimized. Still undergoing evaluation
are the effects of the long-term use of this surfactant
in the most premature babies, who may
require it for much longer periods of time.
Hyaline membrane disease is also called respiratory
distress syndrome (RDS) of the newborn, and
most often affects premature infants whose lungs
have not yet produced sufficient quantities of pulmonary
surfactant.
The first few breaths of a newborn inflate most of
the previously collapsed lungs, and the presence of
surfactant permits the alveoli to remain open. The
following breaths become much easier, and normal
breathing is established.
Without surfactant, the surface tension of the tissue
fluid lining the alveoli causes the air sacs to col-
BOX 15–3 PNEUMOTHORAX
trauma, may result from rupture of weakened alveoli
on the lung surface. Pulmonary diseases such as
emphysema may weaken alveoli.
Puncture wounds of the chest wall also allow air
into the pleural space, with resulting collapse of a
lung. In severe cases, large amounts of air push the
heart, great vessels, trachea, and esophagus toward
the opposite side (mediastinal shift), putting pressure
on the other lung and making breathing difficult.
This is called tension pneumothorax, and
requires rapid medical intervention to remove the
trapped air.
Pneumothorax is the presence of air in the pleural
space, which causes collapse of the lung on that
side. Recall that the pleural space is only a potential
space because the serous fluid keeps the pleural
membranes adhering to one another, and the
intrapleural pressure is always slightly below atmospheric
pressure. Should air at atmospheric pressure
enter the pleural cavity, the suddenly higher pressure
outside the lung will contribute to its collapse
(the other factor is the normal elasticity of the
lungs).
A spontaneous pneumothorax, without apparent
BOX 15–4 EMPHYSEMA
In progressive emphysema, damaged lung tissue
is replaced by fibrous connective tissue (scar tissue),
which further limits the diffusion of gases. Blood
oxygen level decreases, and blood carbon dioxide
level increases. Accumulating carbon dioxide
decreases the pH of body fluids; this is a respiratory
acidosis.
One of the most characteristic signs of emphysema
is that the affected person must make an
effort to exhale. The loss of lung elasticity makes
normal exhalation an active process, rather than the
passive process it usually is. The person must
expend energy to exhale in order to make room in
the lungs for inhaled air. This extra “work” required
for exhalation may be exhausting for the person
and contribute to the debilitating nature of emphysema.
Emphysema, a form of chronic obstructive pulmonary
disease (COPD), is a degenerative disease
in which the alveoli lose their elasticity and cannot
recoil. Perhaps the most common (and avoidable)
cause is cigarette smoking; other causes are longterm
exposure to severe air pollution or industrial
dusts, or chronic asthma. Inhaled irritants damage
the alveolar walls and cause deterioration of the
elastic connective tissue surrounding the alveoli.
Macrophages migrate to the damaged areas and
seem to produce an enzyme that contributes to the
destruction of the protein elastin. This is an instance
of a useful body response (for cleaning up damaged
tissue) becoming damaging when it is excessive. As
the alveoli break down, larger air cavities are created
that are not efficient in gas exchange (see Box
Fig. 15–A).
A Normal Lung B Emphysema
Box Figure 15–A (A) Lung tissue with normal alveoli. (B) Lung tissue in emphysema.
352
1. Tidal volume—the amount of air involved in one
normal inhalation and exhalation. The average tidal
volume is 500 mL, but many people often have
lower tidal volumes because of shallow breathing.
2. Minute respiratory volume (MRV)—the amount
of air inhaled and exhaled in 1 minute. MRV is calculated
by multiplying tidal volume by the number
of respirations per minute (average range: 12 to 20
per minute). If tidal volume is 500 mL and the respiratory
rate is 12 breaths per minute, the MRV is
6000 mL, or 6 liters of air per minute, which is
average. Shallow breathing usually indicates a
smaller than average tidal volume, and would thus
require more respirations per minute to obtain the
necessary MRV.
3. Inspiratory reserve—the amount of air, beyond
tidal volume, that can be taken in with the deepest
possible inhalation. Normal inspiratory reserve
ranges from 2000 to 3000 mL.
4. Expiratory reserve—the amount of air, beyond
tidal volume, that can be expelled with the most
forceful exhalation. Normal expiratory reserve
ranges from 1000 to 1500 mL.
5. Vital capacity—the sum of tidal volume, inspiratory
reserve, and expiratory reserve. Stated another
way, vital capacity is the amount of air involved in
the deepest inhalation followed by the most forceful
exhalation. Average range of vital capacity is
3500 to 5000 mL.
6. Residual air—the amount of air that remains in
the lungs after the most forceful exhalation; the
average range is 1000 to 1500 mL. Residual air is
important to ensure that there is some air in the
lungs at all times, so that exchange of gases is a continuous
process, even between breaths.
Some of the volumes just described can be determined
with instruments called spirometers, which
measure movement of air. Trained singers and musicians
who play wind instruments often have vital
capacities much larger than would be expected for
their height and age, because their respiratory muscles
have become more efficient with “practice.” The same
is true for athletes who exercise regularly. A person
with emphysema, however, must “work” to exhale,
and vital capacity and expiratory reserve volume are
often much lower than average.
Another kind of pulmonary volume is alveolar
The Respiratory System 353
BOX 15–5 THE HEIMLICH MANEUVER
The Heimlich maneuver has received much welldeserved
publicity, and indeed it is a life-saving
technique.
If a person is choking on a foreign object (such
as food) lodged in the pharynx or larynx, the air in
the lungs may be utilized to remove the object. The
physiology of this technique is illustrated in the
accompanying figure.
The person performing the maneuver stands
behind the choking victim and puts both arms
around the victim’s waist. One hand forms a fist
that is placed between the victim’s navel and rib
cage (below the diaphragm), and the other hand
covers the fist. It is important to place hands correctly,
in order to avoid breaking the victim’s ribs.
With both hands, a quick, forceful upward thrust is
made and repeated if necessary. This forces the
diaphragm upward to compress the lungs and force
air out. The forcefully expelled air is often sufficient
to dislodge the foreign object.
Foreign object
Lung
Diaphragm
Box Figure 15–B The Heimlich maneuver.
ventilation, which is the amount of air that actually
reaches the alveoli and participates in gas exchange.
An average tidal volume is 500 mL, of which 350 to
400 mL is in the alveoli at the end of an inhalation.
The remaining 100 to 150 mL of air is anatomic dead
space, the air still within the respiratory passages.
Despite the rather grim name, anatomic dead space is
normal; everyone has it.
Physiological dead space is not normal, and is the
volume of non-functioning alveoli that decrease gas
exchange. Causes of increased physiological dead space
include bronchitis, pneumonia, tuberculosis, emphysema,
asthma, pulmonary edema, and a collapsed lung.
The compliance of the thoracic wall and the lungs,
that is, their normal expansibility, is necessary for sufficient
alveolar ventilation. Thoracic compliance may
be decreased by fractured ribs, scoliosis, pleurisy, or
ascites. Lung compliance will be decreased by any
condition that increases physiologic dead space. Normal
compliance thus promotes sufficient gas exchange
in the alveoli.
EXCHANGE OF GASES
There are two sites of exchange of oxygen and carbon
dioxide: the lungs and the tissues of the body. The
exchange of gases between the air in the alveoli and
the blood in the pulmonary capillaries is called external
respiration. This term may be a bit confusing at
first, because we often think of “external” as being
outside the body. In this case, however, “external”
means the exchange that involves air from the external
environment, though the exchange takes place within
the lungs. Internal respiration is the exchange of
gases between the blood in the systemic capillaries and
the tissue fluid (cells) of the body.
The air we inhale (the earth’s atmosphere) is
approximately 21% oxygen and 0.04% carbon dioxide.
Although most (78%) of the atmosphere is nitrogen,
this gas is not physiologically available to us, and
we simply exhale it. This exhaled air also contains
about 16% oxygen and 4.5% carbon dioxide, so it is
apparent that some oxygen is retained within the body
and the carbon dioxide produced by cells is exhaled.
DIFFUSION OF GASES—
PARTIAL PRESSURES
Within the body, a gas will diffuse from an area of
greater concentration to an area of lesser concentration.
The concentration of each gas in a particular site
(alveolar air, pulmonary blood, and so on) is expressed
in a value called partial pressure. The partial pressure
354 The Respiratory System
Figure 15–7. Pulmonary
volumes. See text for description.
QUESTION: Which volumes
make up vital capacity? Which
volume cannot be measured
with a spirometer?
Liters
6
5
4
3
2.5
2
1
0
Inspiratory
reserve
Expiratory
reserve
Residual volume
Tidal volume
(normal breath)
Total
lung
capacity
Vital
capacity
Time
of a gas, measured in mmHg, is the pressure it exerts
within a mixture of gases, whether the mixture is actually
in a gaseous state or is in a liquid such as blood.
The partial pressures of oxygen and carbon dioxide in
the atmosphere and in the sites of exchange in the
body are listed in Table 15–1. The abbreviation for
partial pressure is “P,” which is used, for example, on
hospital lab slips for blood gases and will be used here.
The partial pressures of oxygen and carbon dioxide
at the sites of external respiration (lungs) and internal
respiration (body) are shown in Fig. 15–8. Because
partial pressure reflects concentration, a gas will diffuse
from an area of higher partial pressure to an area
of lower partial pressure.
The air in the alveoli has a high PO2 and a low PCO2.
The blood in the pulmonary capillaries, which has just
come from the body, has a low PO2 and a high PCO2.
Therefore, in external respiration, oxygen diffuses
from the air in the alveoli to the blood, and carbon
dioxide diffuses from the blood to the air in the alveoli.
The blood that returns to the heart now has a high PO2
and a low PCO2 and is pumped by the left ventricle into
systemic circulation.
The arterial blood that reaches systemic capillaries
has a high PO2 and a low PCO2. The body cells and tissue
fluid have a low PO2 and a high PCO2 because cells
continuously use oxygen in cell respiration (energy
production) and produce carbon dioxide in this
process. Therefore, in internal respiration, oxygen diffuses
from the blood to tissue fluid (cells), and carbon
dioxide diffuses from tissue fluid to the blood. The
blood that enters systemic veins to return to the heart
now has a low PO2 and a high PCO2 and is pumped by
the right ventricle to the lungs to participate in external
respiration.
Disorders of gas exchange often involve the lungs,
that is, external respiration (see Box 15–6: Pulmonary
Edema and Box 15–7: Pneumonia).
TRANSPORT OF GASES
IN THE BLOOD
Although some oxygen is dissolved in blood plasma
and does create the PO2 values, it is only about 1.5%
of the total oxygen transported, not enough to sustain
life. As you already know, most oxygen is carried in the
blood bonded to the hemoglobin in red blood cells
(RBCs). The mineral iron is part of hemoglobin and
gives this protein its oxygen-carrying ability.
The Respiratory System 355
Table 15–1 PARTIAL PRESSURES AND OXYGEN SATURATION
Site PO2 (mmHg) PCO2 (mmHg) Hemoglobin Saturation (SaO2)
Atmosphere 160 0.15 —
Alveolar air 104 40 —
Systemic venous blood 40 45 70–75%
(to pulmonary arteries)
Systemic arterial blood 100 40 95–100%
(from pulmonary veins)
Tissue fluid 40 50 —
Partial pressure is calculated as follows:
% of the gas in the mixture total pressure PGAS
Example: O2 in the atmosphere
21% 760 mmHg 160 mmHg (PO2)
Example: CO2 in the atmosphere
0.04% 760 mmHg 0.15 mmHg (PCO2)
Notice that alveolar partial pressures are not exactly those of the atmosphere. Alveolar air contains significant amounts of
water vapor and the CO2 diffusing in from the blood. Oxygen also diffuses readily from the alveoli into the pulmonary capillaries.
Therefore, alveolar PO2 is lower than atmospheric PO2, and alveolar PCO2 is significantly higher than atmospheric PCO2.
The oxygen–hemoglobin bond is formed in the
lungs where PO2 is high. This bond, however, is relatively
unstable, and when blood passes through tissues
with a low PO2, the bond breaks, and oxygen is
released to the tissues. The lower the oxygen concentration
in a tissue, the more oxygen the hemoglobin
will release. This ensures that active tissues, such as
exercising muscles, receive as much oxygen as possible
to continue cell respiration. Other factors that increase
the release of oxygen from hemoglobin are a high
PCO2 (actually a lower pH) and a high temperature,
both of which are also characteristic of active tissues.
Another measure of blood oxygen is the percent of
oxygen saturation of hemoglobin (SaO2). The higher
the PO2, the higher the SaO2, and as PO2 decreases, so
does SaO2, though not as rapidly. A PO2 of 100 is an
SaO2 of about 97% , as is found in systemic arteries. A
PO2 of 40, as is found in systemic veins, is an SaO2 of
about 75%. Notice that venous blood still has quite a
bit of oxygen. Had this blood flowed through a very
active tissue, more of its oxygen would have been
released from hemoglobin. This venous reserve of
oxygen provides active tissues with the oxygen they
need (see also Box 15–8: Carbon Monoxide).
356 The Respiratory System
Pulmonary
capillaries
Alveoli
Po 40 2
Po 105 2
Po 40 2
Po 40 2
Po 100 2
Po 100 2
Pco 40 2
Pco 45 2
Pco 45 2
Pco 50 2
Pco 40 2
Pco 40 2
Pulmonary
artery
External
respiration
Pulmonary
veins
Aorta
Veins Arteries
Venae
cavae
Right
heart
Left
heart
CO2 to
alveoli
CO2
to blood
O2 to
blood
O2 to tissue
Internal
respiration
Systemic
capillaries
Figure 15–8. External respiration
in the lungs and internal
respiration in the body. The
partial pressures of oxygen and
carbon dioxide are shown at
each site.
QUESTION: In external respiration,
describe the movement
of oxygen. In internal respiration,
describe the movement
of carbon dioxide.
The Respiratory System 357
BOX 15–8 CARBON MONOXIDE
light skin as cyanosis, a bluish cast to the skin, lips,
and nail beds. This is because hemoglobin is dark
red unless something (usually oxygen) is bonded to
it. When hemoglobin bonds to CO, however, it
becomes a bright, cherry red. This color may be
seen in light skin and may be very misleading; the
person with CO poisoning is in a severely hypoxic
state.
Although CO is found in cigarette smoke, it is
present in such minute quantities that it is not
lethal. Heavy smokers, however, may be in a mild
but chronic hypoxic state because much of their
hemoglobin is firmly bonded to CO. As a compensation,
RBC production may increase, and a heavy
smoker may have a hematocrit over 50%.
Carbon monoxide (CO) is a colorless, odorless gas
that is produced during the combustion of fuels
such as gasoline, coal, oil, and wood. As you know,
CO is a poison that may cause death if inhaled in
more than very small quantities or for more than a
short period of time.
The reason CO is so toxic is that it forms a very
strong and stable bond with the hemoglobin in
RBCs (carboxyhemoglobin). Hemoglobin with CO
bonded to it cannot bond to and transport oxygen.
The effect of CO, therefore, is to drastically decrease
the amount of oxygen carried in the blood. As little
as 0.1% CO in inhaled air can saturate half the total
hemoglobin with CO.
Lack of oxygen is often apparent in people with
BOX 15–6 PULMONARY EDEMA
capillaries. As blood pressure increases in the pulmonary
capillaries, filtration creates tissue fluid that
collects in the alveoli.
Fluid-filled alveoli are no longer sites of efficient
gas exchange, and the resulting hypoxia leads to
the symptoms of dyspnea and increased respiratory
rate. The most effective treatment is that which
restores the pumping ability of the heart to normal.
Pulmonary edema is the accumulation of fluid in
the alveoli. This is often a consequence of congestive
heart failure in which the left side of the heart
(or the entire heart) is not pumping efficiently. If the
left ventricle does not pump strongly, the chamber
does not empty as it should and cannot receive all
the blood flowing in from the left atrium. Blood
flow, therefore, is “congested,” and blood backs up
in the pulmonary veins and then in the pulmonary
BOX 15–7 PNEUMONIA
that accumulates in the air sacs. Many neutrophils
migrate to the site of infection and attempt to
phagocytize the bacteria. The alveoli become filled
with fluid, bacteria, and neutrophils (this is called
consolidation); this decreases the exchange of
gases.
Pneumovax is a vaccine for this type of pneumonia.
It contains only the capsules of S. pneumoniae
and cannot cause the disease. The vaccine is recommended
for people over the age of 60 years,
and for those with chronic pulmonary disorders or
any debilitating disease. It has also been approved
for administration to infants.
Pneumonia is a bacterial infection of the lungs.
Although many bacteria can cause pneumonia,
the most common one is probably Streptococcus
pneumoniae. This species is estimated to cause at
least 500,000 cases of pneumonia every year in the
United States, with 50,000 deaths.
S. pneumoniae is a transient inhabitant of the
upper respiratory tract, but in otherwise healthy
people, the ciliated epithelium and the immune system
prevent infection. Most cases of pneumonia
occur in elderly people following a primary infection
such as influenza.
When the bacteria are able to establish themselves
in the alveoli, the alveolar cells secrete fluid
Carbon dioxide transport is a little more complicated.
Some carbon dioxide is dissolved in the plasma,
and some is carried by hemoglobin (carbaminohemoglobin),
but these account for only about 20% of total
CO2 transport. Most carbon dioxide is carried in the
plasma in the form of bicarbonate ions (HCO3
–). Let
us look at the reactions that transform CO2 into a
bicarbonate ion.
When carbon dioxide enters the blood, most diffuses
into red blood cells, which contain the enzyme
carbonic anhydrase. This enzyme (which contains
zinc) catalyzes the reaction of carbon dioxide and
water to form carbonic acid:
CO2 + H2O → H2CO3
The carbonic acid then dissociates:
H2CO3 → H+ + HCO3

The bicarbonate ions diffuse out of the red blood
cells into the plasma, leaving the hydrogen ions (H+)
in the red blood cells. The many H+ ions would tend
to make the red blood cells too acidic, but hemoglobin
acts as a buffer to prevent acidosis. To maintain an
ionic equilibrium, chloride ions (Cl–) from the plasma
enter the red blood cells; this is called the chloride
shift. Where is the CO2? It is in the plasma as part of
HCO3
– ions. When the blood reaches the lungs, an
area of lower PCO2, these reactions are reversed, and
CO2 is re-formed and diffuses into the alveoli to be
exhaled.
REGULATION OF RESPIRATION
Two types of mechanisms regulate breathing: nervous
mechanisms and chemical mechanisms. Because any
changes in the rate or depth of breathing are ultimately
brought about by nerve impulses, we will consider
nervous mechanisms first.
NERVOUS REGULATION
The respiratory centers are located in the medulla
and pons, which are parts of the brain stem (see Fig.
15–9). Within the medulla are the inspiration center
and expiration center.
The inspiration center automatically generates
impulses in rhythmic spurts. These impulses travel
along nerves to the respiratory muscles to stimulate
their contraction. The result is inhalation. As the
lungs inflate, baroreceptors in lung tissue detect this
stretching and generate sensory impulses to the
medulla; these impulses begin to depress the inspiration
center. This is called the Hering-Breuer inflation
reflex, which also helps prevent overinflation of the
lungs.
As the inspiration center is depressed, the result is
a decrease in impulses to the respiratory muscles,
which relax to bring about exhalation. Then the inspiration
center becomes active again to begin another
cycle of breathing. When there is a need for more
forceful exhalations, such as during exercise, the inspiration
center activates the expiration center, which
generates impulses to the internal intercostal and
abdominal muscles.
The two respiratory centers in the pons work with
the inspiration center to produce a normal rhythm of
breathing. The apneustic center prolongs inhalation,
and is then interrupted by impulses from the pneumotaxic
center, which contributes to exhalation. In
normal breathing, inhalation lasts 1 to 2 seconds, followed
by a slightly longer (2 to 3 seconds) exhalation,
producing the normal respiratory rate range of 12 to
20 breaths per minute.
What has just been described is normal breathing,
but variations are possible and quite common. Emotions
often affect respiration; a sudden fright may bring
about a gasp or a scream, and anger usually increases
the respiratory rate. In these situations, impulses from
the hypothalamus modify the output from the
medulla. The cerebral cortex enables us to voluntarily
change our breathing rate or rhythm to talk, sing,
breathe faster or slower, or even to stop breathing for
1 or 2 minutes. Such changes cannot be continued
indefinitely, however, and the medulla will eventually
resume control.
Coughing and sneezing are reflexes that remove
irritants from the respiratory passages; the medulla
contains the centers for both of these reflexes. Sneezing
is stimulated by an irritation of the nasal mucosa,
and coughing is stimulated by irritation of the mucosa
of the pharynx, larynx, or trachea. The reflex action is
essentially the same for both: An inhalation is followed
by exhalation beginning with the glottis closed to
build up pressure. Then the glottis opens suddenly,
and the exhalation is explosive. A cough directs the
exhalation out the mouth, while a sneeze directs the
exhalation out the nose.
358 The Respiratory System
Hiccups, also a reflex, are spasms of the diaphragm.
The result is a quick inhalation that is stopped when
the glottis snaps shut, causing the “hic” sound. The
stimulus may be irritation of the phrenic nerves
or nerves of the stomach. Excessive alcohol is an irritant
that can cause hiccups. Some causes are simply
unknown.
Yet another respiratory reflex is yawning. Most of
us yawn when we are tired, but the stimulus for and
purpose of yawning are not known with certainty.
There are several possibilities, such as lack of oxygen
or accumulation of carbon dioxide, but we really do
not know. Nor do we know why yawning is contagious,
but seeing someone yawn is almost sure to elicit
a yawn of one’s own. You may even have yawned while
reading this paragraph about yawning.
CHEMICAL REGULATION
Chemical regulation refers to the effect on breathing
of blood pH and blood levels of oxygen and carbon
dioxide. This is shown in Fig. 15–10. Chemorecep-
The Respiratory System 359
Hypothalamus
Pneumotaxic
center
Apneustic
center
Stimulatory
Inhibitory
Cerebral cortex
External intercostal muscles
Baroreceptors in
lungs
Inspiration center Diaphragm
Expiration center
Medulla
Pons
A
B
C
Figure 15–9. Nervous regulation of respiration. (A) Midsagittal section of brain.
(B) Respiratory centers in medulla and pons. (C) Respiratory muscles. See text for description.
QUESTION: Which center directly stimulates inhalation? How can you tell from this
picture?
tors that detect changes in blood gases and pH are
located in the carotid and aortic bodies and in the
medulla itself.
A decrease in the blood level of oxygen (hypoxia) is
detected by the chemoreceptors in the carotid and
aortic bodies. The sensory impulses generated by
these receptors travel along the glossopharyngeal and
vagus nerves to the medulla, which responds by
increasing respiratory rate or depth (or both). This
response will bring more air into the lungs so that
more oxygen can diffuse into the blood to correct the
hypoxic state.
Carbon dioxide becomes a problem when it is present
in excess in the blood, because excess CO2 (hypercapnia)
lowers the pH when it reacts with water to
form carbonic acid (a source of H+ ions). That is,
excess CO2 makes the blood or other body fluids less
alkaline (or more acidic). The medulla contains
chemoreceptors that are very sensitive to changes
in pH, especially decreases. If accumulating CO2 lowers
blood pH, the medulla responds by increasing respiration.
This is not for the purpose of inhaling, but
rather to exhale more CO2 to raise the pH back to
normal.
Of the two respiratory gases, which is the more
important as a regulator of respiration? Our guess
might be oxygen, because it is essential for energy production
in cell respiration. However, the respiratory
system can maintain a normal blood level of oxygen
even if breathing decreases to half the normal rate or
stops for a few moments. Recall that exhaled air is
16% oxygen. This oxygen did not enter the blood but
was available to do so if needed. Also, the residual air
in the lungs supplies oxygen to the blood even if
breathing rate slows.
Therefore, carbon dioxide must be the major regulator
of respiration, and the reason is that carbon dioxide
affects the pH of the blood. As was just mentioned,
an excess of CO2 causes the blood pH to decrease, a
process that must not be allowed to continue.
Therefore, any increase in the blood CO2 level is
quickly compensated for by increased breathing to
exhale more CO2. If, for example, you hold your
breath, what is it that makes you breathe again? Have
you run out of oxygen? Probably not, for the reasons
mentioned. What has happened is that accumulating
CO2 has lowered blood pH enough to stimulate the
medulla to start the breathing cycle again.
In some situations, oxygen does become the major
regulator of respiration. People with severe, chronic
pulmonary diseases such as emphysema have decreased
exchange of both oxygen and carbon dioxide
in the lungs. The decrease in pH caused by accumulating
CO2 is corrected by the kidneys, but the blood
oxygen level keeps decreasing. Eventually, the oxygen
level may fall so low that it does provide a very strong
stimulus to increase the rate and depth of respiration.
RESPIRATION AND
ACID–BASE BALANCE
As you have just seen, respiration affects the pH of
body fluids because it regulates the amount of carbon
360 The Respiratory System
Chemoreceptors
in carotid and
aortic bodies
Chemoreceptors
in medulla
O2
CO2 or
pH
Medulla;
inspiration
center
Increase rate
and depth of
respiration
More O2 available
to enter blood
More CO2 exhaled
pH
Figure 15–10. Chemical regulation of respiration. See text for description.
QUESTION: The body’s response to two very different changes (less O2 or more CO2)
is the same. Explain why.
dioxide in these fluids. Remember that CO2 reacts
with water to form carbonic acid (H2CO3), which ionizes
into H+ ions and HCO3
– ions. The more hydrogen
ions present in a body fluid, the lower the pH, and
the fewer hydrogen ions present, the higher the pH.
The respiratory system may be the cause of a pH
imbalance, or it may help correct a pH imbalance created
by some other cause.
RESPIRATORY ACIDOSIS
AND ALKALOSIS
Respiratory acidosis occurs when the rate or efficiency
of respiration decreases, permitting carbon
dioxide to accumulate in body fluids. The excess CO2
results in the formation of more H+ ions, which
decrease the pH. Holding one’s breath can bring about
a mild respiratory acidosis, which will soon stimulate
the medulla to initiate breathing again. More serious
causes of respiratory acidosis are pulmonary diseases
such as pneumonia and emphysema, or severe asthma.
Each of these impairs gas exchange and allows excess
CO2 to remain in body fluids.
Respiratory alkalosis occurs when the rate of respiration
increases, and CO2 is very rapidly exhaled.
Less CO2 decreases H+ ion formation, which increases
the pH. Breathing faster for a few minutes can bring
about a mild state of respiratory alkalosis. Babies who
cry for extended periods (crying is a noisy exhalation)
put themselves in this condition. In general, however,
respiratory alkalosis is not a common occurrence.
Severe physical trauma and shock, or certain states
of mental or emotional anxiety, may be accompanied
by hyperventilation and also result in respiratory
alkalosis. In addition, traveling to a higher altitude
(less oxygen in the atmosphere) may cause a temporary
increase in breathing rate before compensation
occurs (increased rate of RBC production—see
Chapter 11).
RESPIRATORY COMPENSATION
If a pH imbalance is caused by something other than a
change in respiration, it is called metabolic acidosis or
alkalosis. In either case, the change in pH stimulates a
change in respiration that may help restore the pH of
body fluids to normal.
Metabolic acidosis may be caused by untreated
diabetes mellitus (ketoacidosis), kidney disease, or
severe diarrhea. In such situations, the H+ ion concentration
of body fluids is increased. Respiratory compensation
involves an increase in the rate and depth of
respiration to exhale more CO2 to decrease H+ ion
formation, which will raise the pH toward the normal
range.
Metabolic alkalosis is not a common occurrence
but may be caused by ingestion of excessive amounts
of alkaline medications such as those used to relieve
gastric disturbances. Another possible cause is vomiting
of stomach contents only. In such situations, the
H+ ion concentration of body fluids is decreased.
Respiratory compensation involves a decrease in respiration
to retain CO2 in the body to increase H+ ion
formation, which will lower the pH toward the normal
range.
Respiratory compensation for an ongoing metabolic
pH imbalance cannot be complete, because there
are limits to the amounts of CO2 that may be exhaled
or retained. At most, respiratory compensation is only
about 75% effective. A complete discussion of acid–
base balance is found in Chapter 19.
AGING AND THE
RESPIRATORY SYSTEM
Perhaps the most important way to help your respiratory
system age gracefully is not to smoke. In the
absence of chemical assault, respiratory function does
diminish but usually remains adequate. The respiratory
muscles, like all skeletal muscles, weaken with
age. Lung tissue loses its elasticity and alveoli are
lost as their walls deteriorate. All of this results in decreased
ventilation and lung capacity, but the remaining
capacity is usually sufficient for ordinary activities.
The cilia of the respiratory mucosa deteriorate with
age, and the alveolar macrophages are not as efficient,
which make elderly people more prone to pneumonia,
a serious pulmonary infection.
Chronic alveolar hypoxia from diseases such as
emphysema or chronic bronchitis may lead to pulmonary
hypertension, which in turn overworks the
right ventricle of the heart. Systemic hypertension
often weakens the left ventricle of the heart, leading to
congestive heart failure and pulmonary edema, in
which excess tissue fluid collects in the alveoli and
decreases gas exchange. Though true at any age, the
interdependence of the respiratory and circulatory
systems is particularly apparent in elderly people.
The Respiratory System 361
SUMMARY
As you have learned, respiration is much more than
the simple mechanical actions of breathing. Inhalation
provides the body with the oxygen that is necessary for
the production of ATP in the process of cell respiration.
Exhalation removes the CO2 that is a product of
cell respiration. Breathing also regulates the level of
CO2 within the body, and this contributes to the
maintenance of the acid–base balance of body fluids.
Although the respiratory gases do not form structural
components of the body, their contributions to the
chemical level of organization are essential to the
functioning of the body at every level.
362 The Respiratory System
STUDY OUTLINE
The respiratory system moves air into and
out of the lungs, which are the site of
exchange for O2 and CO2 between the air
and the blood. The functioning of the respiratory
system depends directly on the
proper functioning of the circulatory system.
1. The upper respiratory tract consists of those parts
outside the chest cavity.
2. The lower respiratory tract consists of those parts
within the chest cavity.
Nose—made of bone and cartilage covered
with skin
1. Hairs inside the nostrils block the entry of dust.
Nasal Cavities—within the skull; separated
by the nasal septum (see Fig. 15–1)
1. Nasal mucosa is ciliated epithelium with goblet
cells; surface area is increased by the conchae.
2. Nasal mucosa warms and moistens the incoming
air; dust and microorganisms are trapped on mucus
and swept by the cilia to the pharynx.
3. Olfactory receptors respond to vapors in inhaled
air.
4. Paranasal sinuses in the maxillae, frontal, sphenoid,
and ethmoid bones open into the nasal cavities:
functions are to lighten the skull and provide resonance
for the voice.
Pharynx—posterior to nasal and oral cavities
(see Fig. 15–1)
1. Nasopharynx—above the level of the soft palate,
which blocks it during swallowing; a passageway
for air only. The eustachian tubes from the middle
ears open into it. The adenoid is a lymph nodule on
the posterior wall.
2. Oropharynx—behind the mouth; a passageway for
both air and food. Palatine tonsils are on the lateral
walls.
3. Laryngopharynx—a passageway for both air and
food; opens anteriorly into the larynx and posteriorly
into the esophagus.
Larynx—the voice box and the airway
between the pharynx and trachea (see Fig.
15–2)
1. Made of nine cartilages; the thyroid cartilage is the
largest and most anterior.
2. The epiglottis is the uppermost cartilage; covers
the larynx during swallowing.
3. The vocal cords are lateral to the glottis, the opening
for air (see Fig. 15–3).
4. During speaking, the vocal cords are pulled across
the glottis and vibrated by exhaled air, producing
sounds that may be turned into speech.
5. The cranial nerves for speaking are the vagus and
accessory.
Trachea—extends from the larynx to the primary
bronchi (see Fig. 15–4)
1. Sixteen to 20 C-shaped cartilages in the tracheal
wall keep the trachea open.
2. Mucosa is ciliated epithelium with goblet cells; cilia
sweep mucus, trapped dust, and microorganisms
upward to the pharynx.
Bronchial Tree—extends from the trachea to
the alveoli (see Fig. 15–4)
1. The right and left primary bronchi are branches of
the trachea; one to each lung; same structure as the
trachea.
2. Secondary bronchi: to the lobes of each lung (three
right, two left)
3. Bronchioles—no cartilage in their walls.
Pleural Membranes—serous membranes of
the thoracic cavity
1. Parietal pleura lines the chest wall.
2. Visceral pleura covers the lungs.
3. Serous fluid between the two layers prevents friction
and keeps the membranes together during
breathing.
Lungs—on either side of the heart in the
chest cavity; extend from the diaphragm
below up to the level of the clavicles
1. The rib cage protects the lungs from mechanical
injury.
2. Hilus—indentation on the medial side: primary
bronchus and pulmonary artery and veins enter
(also bronchial vessels).
Alveoli—the sites of gas exchange in the
lungs
1. Made of alveolar type I cells, simple squamous
epithelium; thin to permit diffusion of gases.
2. Surrounded by pulmonary capillaries, which are
also made of simple squamous epithelium (see Fig.
15–4).
3. Elastic connective tissue between alveoli is important
for normal exhalation.
4. A thin layer of tissue fluid lines each alveolus; essential
to permit diffusion of gases (see Fig. 15–5).
5. Alveolar type II cells produce pulmonary surfactant
that mixes with the tissue fluid lining to decrease
surface tension to permit inflation of the alveoli.
6. Alveolar macrophages phagocytize foreign material.
Mechanism of Breathing
1. Ventilation is the movement of air into and out of
the lungs: inhalation and exhalation.
2. Respiratory centers are in the medulla and pons.
3. Respiratory muscles are the diaphragm and external
and internal intercostal muscles (see Fig. 15–6).
• Atmospheric pressure is air pressure: 760 mmHg
at sea level.
• Intrapleural pressure is within the potential pleural
space; always slightly below atmospheric
pressure (“negative”).
• Intrapulmonic pressure is within the bronchial
tree and alveoli; fluctuates during breathing.
Inhalation (inspiration)
1. Motor impulses from medulla travel along phrenic
nerves to diaphragm, which contracts and moves
down. Impulses are sent along intercostal nerves to
external intercostal muscles, which pull ribs up and
out.
2. The chest cavity is expanded and expands the parietal
pleura.
3. The visceral pleura adheres to the parietal pleura
and is also expanded and in turn expands the lungs.
4. Intrapulmonic pressure decreases, and air rushes
into the lungs.
Exhalation (expiration)
1. Motor impulses from the medulla decrease, and the
diaphragm and external intercostal muscles relax.
2. The chest cavity becomes smaller and compresses
the lungs.
3. The elastic lungs recoil and further compress the
alveoli.
4. Intrapulmonic pressure increases, and air is forced
out of the lungs. Normal exhalation is passive.
5. Forced exhalation: contraction of the internal
intercostal muscles pulls the ribs down and in; contraction
of the abdominal muscles forces the
diaphragm upward.
Pulmonary Volumes (see Fig. 15–7)
1. Tidal volume—the amount of air in one normal
inhalation and exhalation.
2. Minute respiratory volume—the amount of air
inhaled and exhaled in 1 minute.
3. Inspiratory reserve—the amount of air beyond
tidal in a maximal inhalation.
4. Expiratory reserve—the amount of air beyond tidal
in the most forceful exhalation.
5. Vital capacity—the sum of tidal volume, inspiratory
and expiratory reserves.
6. Residual volume—the amount of air that remains
in the lungs after the most forceful exhalation; provides
for continuous exchange of gases.
7. Alveolar ventilation—air that reaches the alveoli
for gas exchange; depends on normal thoracic and
lung compliance.
• Anatomic dead space—air still in the respiratory
passages at the end of inhalation (is normal).
The Respiratory System 363
• Physiological dead space—the volume of nonfunctional
alveoli; decreases compliance.
Exchange of Gases
1. External respiration is the exchange of gases
between the air in the alveoli and the blood in the
pulmonary capillaries.
2. Internal respiration is the exchange of gases
between blood in the systemic capillaries and tissue
fluid (cells).
3. Inhaled air (atmosphere) is 21% O2 and 0.04%
CO2. Exhaled air is 16% O2 and 4.5% CO2.
4. Diffusion of O2 and CO2 in the body occurs
because of pressure gradients (see Table 15–1). A
gas will diffuse from an area of higher partial pressure
to an area of lower partial pressure.
5. External respiration: PO2 in the alveoli is high, and
PO2in the pulmonary capillaries is low, so O2 diffuses
from the air to the blood. PCO2 in the alveoli
is low, and PCO2 in the pulmonary capillaries is
high, so CO2 diffuses from the blood to the air and
is exhaled (see Fig. 15–8).
6. Internal respiration: PO2 in the systemic capillaries
is high, and PO2 in the tissue fluid is low, so O2 diffuses
from the blood to the tissue fluid and cells.
PCO2 in the systemic capillaries is low, and PCO2 in
the tissue fluid is high, so CO2 diffuses from the tissue
fluid to the blood (see Fig. 15–8).
Transport of Gases in the Blood
1. Oxygen is carried by the iron of hemoglobin (Hb)
in the RBCs. The O2–Hb bond is formed in the
lungs where the PO2 is high.
2. In tissues, Hb releases much of its O2; the important
factors are low PO2 in tissues, high PCO2 in tissues,
and a high temperature in tissues.
3. Oxygen saturation of hemoglobin (SaO2) is 95% to
97% in systemic arteries and averages 70% to 75%
in systemic veins.
4. Most CO2 is carried as HCO3
– ions in blood
plasma. CO2 enters the RBCs and reacts with H2O
to form carbonic acid (H2CO3). Carbonic anhydrase
is the enzyme that catalyzes this reaction.
H2CO3 dissociates to H+ ions and HCO3
– ions.
The HCO3
– ions leave the RBCs and enter the
plasma; Hb buffers the H+ ions that remain in the
RBCs. Cl– ions from the plasma enter the RBCs to
maintain ionic equilibrium (the chloride shift).
5. When blood reaches the lungs, CO2 is re-formed,
diffuses into the alveoli, and is exhaled.
Nervous Regulation of Respiration
(see Fig. 15–9)
1. The medulla contains the inspiration center and
expiration center.
2. Impulses from the inspiration center to the respiratory
muscles cause their contraction; the chest cavity
is expanded.
3. Baroreceptors in lung tissue detect stretching and
send impulses to the medulla to depress the inspiration
center. This is the Hering-Breuer inflation
reflex, which also prevents overinflation of the
lungs.
4. The expiration center is stimulated by the inspiration
center when forceful exhalations are needed.
5. In the pons: the apneustic center prolongs inhalation,
and the pneumotaxic center helps bring about
exhalation. These centers work with the inspiration
center in the medulla to produce a normal breathing
rhythm.
6. The hypothalamus influences changes in breathing
in emotional situations. The cerebral cortex permits
voluntary changes in breathing.
7. Coughing and sneezing remove irritants from the
upper respiratory tract; the centers for these
reflexes are in the medulla.
Chemical Regulation of Respiration
(see Fig. 15–10)
1. Decreased blood O2 is detected by chemoreceptors
in the carotid body and aortic body. Response:
increased respiration to take more air into the
lungs.
2. Increased blood CO2 level is detected by chemoreceptors
in the medulla. Response: increased respiration
to exhale more CO2.
3. CO2 is the major regulator of respiration because
excess CO2 decreases the pH of body fluids (CO2 +
H2O → H2CO3 → H+ + HCO3
–). Excess H+ ions
lower pH.
4. Oxygen becomes a major regulator of respiration
when blood level is very low, as may occur with
severe, chronic pulmonary disease.
Respiration and Acid–Base Balance
1. Respiratory acidosis: a decrease in the rate or efficiency
of respiration permits excess CO2 to accumulate
in body fluids, resulting in the formation of
excess H+ ions, which lower pH. Occurs in severe
pulmonary disease.
2. Respiratory alkalosis: an increase in the rate of res-
364 The Respiratory System
The Respiratory System 365
REVIEW QUESTIONS
1. State the three functions of the nasal mucosa.
(p. 344)
2. Name the three parts of the pharynx; state whether
each is an air passage only or an air and food passage.
(pp. 344–346)
3. Name the tissue that lines the larynx and trachea,
and describe its function. State the function of the
cartilage of the larynx and trachea. (p. 346)
4. Name the pleural membranes, state the location
of each, and describe the functions of serous fluid.
(pp. 347)
5. Name the tissue of which the alveoli and pulmonary
capillaries are made, and explain the
importance of this tissue in these locations. Explain
the function of pulmonary surfactant. (p. 347)
6. Name the respiratory muscles, and describe how
they are involved in normal inhalation and exhalation.
Define these pressures and relate them to a
cycle of breathing: atmospheric pressure, intrapulmonic
pressure. (pp. 348–349)
7. Describe external respiration in terms of partial
pressures of oxygen and carbon dioxide. (p. 355)
8. Describe internal respiration in terms of partial
pressures of oxygen and carbon dioxide. (p. 355)
9. Name the cell, protein, and mineral that transport
oxygen in the blood. State the three factors
that increase the release of oxygen in tissues.
(pp. 355–356)
10. Most carbon dioxide is transported in what part of
the blood, and in what form? Explain the function
of hemoglobin with respect to carbon dioxide
transport. (p. 358)
11. Name the respiratory centers in the medulla and
pons, and explain how each is involved in a
breathing cycle. (p. 358)
12. State the location of chemoreceptors affected by a
low blood oxygen level; describe the body’s
response to hypoxia and its purpose. State the
location of chemoreceptors affected by a high
blood CO2 level; describe the body’s response and
its purpose. (p. 360)
13. For respiratory acidosis and alkalosis: state a cause
and explain what happens to the pH of body fluids.
(p. 361)
14. Explain how the respiratory system may compensate
for metabolic acidosis or alkalosis. For an
ongoing pH imbalance, what is the limit of respiratory
compensation? (pp. 361)
FOR FURTHER THOUGHT
1. The success of an organ transplant depends on
many factors. What factor would diminish the
chance of success of a lung transplant, but is not a
factor at all in a heart transplant?
2. Name four types of tissues that contribute to the
functioning of the lungs, and describe the physical
characteristics of each that are important. Name
two types of cells that are also important to the
functioning of the lungs.
3. As recently as 45 years ago (the early 1960s)
it was believed that mouth-to-mouth resuscitation
was not really helpful to another person.
What mistaken belief about the air we exhale
contributed to that thinking, and what are the
facts?
4. You are making a list of vital organs, organs we cannot
live without. Should you include the larynx on
your list? Explain why or why not.
piration increases the CO2 exhaled, which decreases
the formation of H+ ions and raises pH.
Occurs during hyperventilation or when first at a
high altitude.
3. Respiratory compensation for metabolic acidosis:
increased respiration to exhale CO2 to decrease H+
ion formation to raise pH to normal.
4. Respiratory compensation for metabolic alkalosis:
decreased respiration to retain CO2 to increase H+
ion formation to lower pH to normal.
5. At a construction site, a hole caved in and buried a
workman up to his shoulders in wet sand. The foreman
told the trapped man that a crane would be
there in 20 minutes to pull him out, but another
worker said they couldn’t wait, and had to dig now.
He was right. Why is this a life-threatening emergency?
6. A patient’s blood pH is 7.34, and his respirations
are 32 per minute. What acid–base situation is this
patient in? State your reasoning step-by-step.
7. Mrs. D is in the emergency room because of severe
abdominal pain that may be appendicitis. Her
blood pH is 7.47 and her respirations are 34 per
minute. What acid–base situation is Mrs. D in?
State your reasoning step-by-step.
366 The Respiratory System
CHAPTER 16
The Digestive System
367
368
CHAPTER 16
Chapter Outline
Divisions of the Digestive System
Types of Digestion
End Products of Digestion
Oral Cavity
Teeth
Tongue
Salivary Glands
Pharynx
Esophagus
Structural Layers of the Alimentary Tube
Mucosa
Submucosa
External Muscle Layer
Serosa
Stomach
Small Intestine
Liver
Gallbladder
Pancreas
Completion of Digestion and Absorption
Small Intestine
Absorption
Large Intestine
Elimination of Feces
Other Functions of the Liver
Aging and the Digestive System
BOX 16–1 DISORDERS OF THE STOMACH
BOX 16–2 GALLSTONES
BOX 16–3 DISORDERS OF THE INTESTINES
BOX 16–4 INFANT BOTULISM
BOX 16–5 FIBER
BOX 16–6 HEPATITIS
Student Objectives
• Describe the general functions of the digestive
system, and name its major divisions.
• Explain the difference between mechanical and
chemical digestion, and name the end products of
digestion.
• Describe the structure and functions of the teeth
and tongue.
• Explain the functions of saliva.
• Describe the location and function of the pharynx
and esophagus.
• Describe the structure and function of each of the
four layers of the alimentary tube.
• Describe the location, structure, and function of
the stomach, small intestine, liver, gallbladder, and
pancreas.
• Describe absorption in the small intestine.
• Describe the location and functions of the large
intestine.
• Explain the functions of the normal flora of the
colon.
• Describe the functions of the liver.
The Digestive System
369
New Terminology
Alimentary tube (AL-i-MEN-tah-ree TOOB)
Chemical digestion (KEM-i-kuhl dye-JES-chun)
Common bile duct (KOM-mon BYL DUKT)
Defecation reflex (DEF-e-KAY-shun)
Duodenum (dew-AH-den-um)
Emulsify (e-MULL-si-fye)
Enamel (e-NAM-uhl)
Essential amino acids (e-SEN-shul ah-ME-noh
ASS-ids)
External anal sphincter (eks-TER-nuhl AY-nuhl
SFINK-ter)
Ileocecal valve (ILL-ee-oh-SEE-kuhl VALV)
Internal anal sphincter (in-TER-nuhl AY-nuhl
SFINK-ter)
Lower esophageal sphincter (e-SOF-uh-JEE-uhl
SFINK-ter)
Mechanical digestion (muh-KAN-i-kuhl dye-JESchun)
Non-essential amino acids (NON-e-SEN-shul ah-
ME-noh ASS-ids)
Normal flora (NOR-muhl FLOOR-ah)
Periodontal membrane (PER-ee-oh-DON-tal)
Pyloric sphincter (pye-LOR-ik SFINK-ter)
Rugae (ROO-gay)
Villi (VILL-eye)
Related Clinical Terminology
Appendicitis (uh-PEN-di-SIGH-tis)
Diverticulitis (DYE-ver-TIK-yoo-LYE-tis)
Gastric ulcer (GAS-trik UL-ser)
Hepatitis (HEP-uh-TIGH-tis)
Lactose intolerance (LAK-tohs in-TAHL-er-ense)
Lithotripsy (LITH-oh-TRIP-see)
Paralytic ileus (PAR-uh-LIT-ik ILL-ee-us)
Peritonitis (per-i-toh-NIGH-tis)
Pyloric stenosis (pye-LOR-ik ste-NOH-sis)
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
Ahurried breakfast when you are late for work or
school . . . Thanksgiving dinner . . . going on a diet to
lose 5 pounds . . . what do these experiences all have in
common? Food. We may take food for granted, celebrate
with it, or wish we wouldn’t eat quite so much of
it. Although food is not as immediate a need for
human beings as is oxygen, it is a very important part
of our lives. Food provides the raw materials or nutrients
that cells use to reproduce and to build new tissue.
The energy needed for cell reproduction and
tissue building is released from food in the process of
cell respiration. In fact, a supply of nutrients from regular
food intake is so important that the body can even
store any excess for use later. Those “extra 5 pounds”
are often stored fat in adipose tissue.
The food we eat, however, is not in a form that our
body cells can use. A turkey sandwich, for example,
consists of complex proteins, fats, and carbohydrates.
The function of the digestive system is to change
these complex organic nutrient molecules into simple
organic and inorganic molecules that can then be
absorbed into the blood or lymph to be transported to
cells. In this chapter we will discuss the organs of
digestion and the contribution each makes to digestion
and absorption.
DIVISIONS OF THE
DIGESTIVE SYSTEM
The two divisions of the digestive system are the alimentary
tube and the accessory organs (Fig. 16–1).
The alimentary tube extends from the mouth to the
anus. It consists of the oral cavity, pharynx, esophagus,
stomach, small intestine, and large intestine.
Digestion takes place within the oral cavity, stomach,
and small intestine; most absorption of nutrients takes
place in the small intestine. Undigestible material, primarily
cellulose, is eliminated by the large intestine
(also called the colon).
The accessory organs of digestion are the teeth,
tongue, salivary glands, liver, gallbladder, and pancreas.
Digestion does not take place within these organs, but
each contributes something to the digestive process.
TYPES OF DIGESTION
The food we eat is broken down in two complementary
processes: mechanical digestion and chemical
digestion. Mechanical digestion is the physical breaking
up of food into smaller pieces. Chewing is an
example of this. As food is broken up, more of its surface
area is exposed for the action of digestive enzymes.
Enzymes are discussed in Chapter 2. The work of the
digestive enzymes is the chemical digestion of broken-
up food particles, in which complex chemical molecules
are changed into much simpler chemicals that
the body can utilize. Such enzymes are specific with
respect to the fat, protein, or carbohydrate food molecules
each can digest. For example, protein-digesting
enzymes work only on proteins, not on carbohydrates
or fats. Each enzyme is produced by a particular digestive
organ and functions at a specific site. However, the
enzyme’s site of action may or may not be its site of
production. These digestive enzymes and their functions
are discussed in later sections.
END PRODUCTS OF DIGESTION
Before we describe the organs of digestion, let us see
where the process of digestion will take us, or rather,
will take our food. The three types of complex organic
molecules found in food are carbohydrates, proteins,
and fats. Each of these complex molecules is digested
to a much more simple substance that the body can
then use. Carbohydrates, such as starches and disaccharides,
are digested to monosaccharides such as glucose,
fructose, and galactose. Proteins are digested to
amino acids, and fats are digested to fatty acids and
glycerol. Also part of food, and released during digestion,
are vitamins, minerals, and water.
We will now return to the beginning of the alimentary
tube and consider the digestive organs and the
process of digestion.
ORAL CAVITY
Food enters the oral cavity (or buccal cavity) by way
of the mouth. The boundaries of the oral cavity are
the hard and soft palates superiorly; the cheeks laterally;
and the floor of the mouth inferiorly. Within the
oral cavity are the teeth and tongue and the openings
of the ducts of the salivary glands.
TEETH
The function of the teeth is, of course, chewing. This
is the process that mechanically breaks food into
smaller pieces and mixes it with saliva. An individual
370 The Digestive System
The Digestive System 371
Tongue
Teeth
Parotid gland
Sublingual gland
Submandibular gland
Esophagus
Liver Left lobe
Stomach (cut)
Right lobe
Gallbladder
Bile duct
Transverse colon
(cut)
Ascending colon
Cecum
Vermiform appendix
Spleen
Duodenum
Pancreas
Descending colon
Small intestine
Rectum
Anal canal
Pharynx
Figure 16–1. The digestive organs
shown in anterior view of the trunk
and left lateral view of the head. (The
spleen is not a digestive organ but is
included to show its location relative
to the stomach, pancreas, and colon.)
QUESTION: In which parts of the
digestive system does digestion actually
take place?
develops two sets of teeth: deciduous and permanent.
The deciduous teeth begin to erupt through the
gums at about 6 months of age, and the set of 20 teeth
is usually complete by the age of 2 years. These teeth
are gradually lost throughout childhood and replaced
by the permanent teeth, the first of which are molars
that emerge around the age of 6 years. A complete set
of permanent teeth consists of 32 teeth; the types of
teeth are incisors, canines, premolars, and molars. The
wisdom teeth are the third molars on either side of
each jawbone. In some people, the wisdom teeth may
not emerge from the jawbone because there is no
room for them along the gum line. These wisdom
teeth are said to be impacted and may put pressure on
the roots of the second molars. In such cases, extraction
of a wisdom tooth may be necessary to prevent
damage to other teeth.
The structure of a tooth is shown in Fig. 16–2. The
crown is visible above the gum (gingiva). The root is
enclosed in a socket in the mandible or maxillae. The
periodontal membrane lines the socket and produces
a bone-like cement that anchors the tooth. The outermost
layer of the crown is enamel, which is made by
cells called ameloblasts. Enamel provides a hard chewing
surface and is more resistant to decay than are
other parts of the tooth. Within the enamel is dentin,
which is very similar to bone and is produced by cells
called odontoblasts. Dentin also forms the roots of a
tooth. The innermost portion of a tooth is the pulp
cavity, which contains blood vessels and nerve endings
of the trigeminal nerve (5th cranial). Erosion of
the enamel and dentin layers by bacterial acids (dental
caries or cavities) may result in bacterial invasion of
the pulp cavity and a very painful toothache.
TONGUE
The tongue is made of skeletal muscle that is innervated
by the hypoglossal nerves (12th cranial). On the
upper surface of the tongue are small projections
called papillae, many of which contain taste buds (see
also Chapter 9). The sensory nerves for taste are also
cranial nerves: the facial (7th) and glossopharyngeal
(9th). As you know, the sense of taste is important
because it makes eating enjoyable, but the tongue has
other functions as well.
Chewing is efficient because of the action of the
tongue in keeping the food between the teeth and
mixing it with saliva. Elevation of the tongue is the
first step in swallowing. This is a voluntary action, in
which the tongue contracts and meets the resistance of
the hard palate. The mass of food, called a bolus, is
thus pushed backward toward the pharynx. The
remainder of swallowing is a reflex, which is described
in the section on the pharynx.
SALIVARY GLANDS
The digestive secretion in the oral cavity is saliva,
produced by three pairs of salivary glands, which are
shown in Fig. 16–3. The parotid glands are just
below and in front of the ears. The submandibular
(also called submaxillary) glands are at the posterior
corners of the mandible, and the sublingual glands
are below the floor of the mouth. Each gland has at
least one duct that takes saliva to the oral cavity.
Secretion of saliva is continuous, but the amount
varies in different situations. The presence of food
(or anything else) in the mouth increases saliva secretion.
This is a parasympathetic response mediated
by the facial and glossopharyngeal nerves. The sight
or smell of food also increases secretion of saliva.
Sympathetic stimulation in stress situations decreases
secretion, making the mouth dry and swallowing
difficult.
Saliva is mostly water, which is important to dissolve
food for tasting and to moisten food for swallowing.
The digestive enzyme in saliva is salivary
amylase, which breaks down starch molecules to
shorter chains of glucose molecules, or to maltose, a
disaccharide. Most of us, however, do not chew our
food long enough for the action of salivary amylase to
be truly effective. As you will see, another amylase
from the pancreas is also available to digest starch.
Table 16–1 summarizes the functions of digestive
secretions.
Saliva is made from blood plasma and thus contains
many of the chemicals that are found in plasma.
Considerable research is focused on detecting in saliva
chemical markers for diseases such as cancer, with the
372 The Digestive System
Enamel
Dentin
Pulp
cavity
Gingiva
(gum)
Cementum
Periodontal
membrane
Blood vessels
Crown
Neck
Root
Nerve
Figure 16–2. Tooth structure. Longitudinal section of a
tooth showing internal structure.
QUESTION: Which parts of a tooth are living? How do
you know?
goal of using saliva rather than blood for diagnostic
tests.
PHARYNX
As described in the preceding chapter, the oropharynx
and laryngopharynx are food passageways connecting
the oral cavity to the esophagus. No digestion takes
place in the pharynx. Its only related function is swallowing,
the mechanical movement of food. When the
bolus of food is pushed backward by the tongue, the
constrictor muscles of the pharynx contract as part of
the swallowing reflex. The reflex center for swallowing
is in the medulla, which coordinates the many
actions that take place: constriction of the pharynx,
cessation of breathing, elevation of the soft palate to
block the nasopharynx, elevation of the larynx and closure
of the epiglottis, and peristalsis of the esophagus.
As you can see, swallowing is rather complicated, but
because it is a reflex we don’t have to think about making
it happen correctly. Talking or laughing while eating,
however, may interfere with the reflex and cause
food to go into the “wrong pipe,” the larynx. When
that happens, the cough reflex is usually effective in
clearing the airway.
ESOPHAGUS
The esophagus is a muscular tube that takes food
from the pharynx to the stomach; no digestion takes
place here. Peristalsis of the esophagus propels food in
one direction and ensures that food gets to the stomach
even if the body is horizontal or upside down. At
the junction with the stomach, the lumen (cavity) of
the esophagus is surrounded by the lower esophageal
sphincter (LES or cardiac sphincter), a circular
smooth muscle. The LES relaxes to permit food to
enter the stomach, then contracts to prevent the
backup of stomach contents. If the LES does not close
completely, gastric juice may splash up into the esophagus;
this is a painful condition we call heartburn, or
gastroesophageal reflux disease (GERD). Most people
experience heartburn once in a while, and it is merely
uncomfortable, but chronic GERD is more serious.
The lining of the esophagus cannot withstand the corrosive
action of gastric acid and will be damaged, perhaps
resulting in bleeding or even perforation.
Medications are available to treat this condition.
STRUCTURAL LAYERS
OF THE ALIMENTARY TUBE
Before we continue with our discussion of the organs
of digestion, we will first examine the typical structure
of the alimentary tube. When viewed in cross-section,
the alimentary tube has four layers (Fig. 16–4): the
mucosa, submucosa, external muscle layer, and serosa.
Each layer has a specific structure, and its functions
contribute to the functioning of the organs of which it
is a part.
MUCOSA
The mucosa, or lining, of the alimentary tube is made
of epithelial tissue, areolar connective tissue, and two
The Digestive System 373
Parotid duct
Sublingual
ducts
Parotid
gland
Submandibular
gland
Sublingual gland
Submandibular duct
Figure 16–3. The salivary glands shown in left lateral
view.
QUESTION: Why are these exocrine glands? What is
saliva made from?
374 The Digestive System
Table 16–1 THE PROCESS OF DIGESTION
Enzyme or
Organ Other Secretion Function Site of Action
Salivary glands
Stomach
Liver
Pancreas
Small intestine
Amylase
Amylase
Sucrase, Maltase, Lactase
Lipase
Bile
Pepsin
Trypsin
Peptidases
Proteins
Fats
Salivary Carbohydrates
glands
Liver
Stomach
Pancreas
Small
intestine
Amylase
Pepsin
HCI
Bile salts
Amylase
Trypsin
Lipase
Peptidases
Sucrase
Maltase
Lactase
• Converts starch to maltose
• Converts proteins to polypeptides
• Changes pepsinogen to pepsin; maintains pH
1–2; destroys pathogens
• Emulsify fats
• Converts starch to maltose
• Converts polypeptides to peptides
• Converts emulsified fats to fatty acids and glycerol
• Convert peptides to amino acids
• Converts sucrose to glucose and fructose
• Converts maltose to glucose (2)
• Converts lactose to glucose and galactose
Oral cavity
Stomach
Stomach
Small intestine
Small intestine
Small intestine
Small intestine
Small intestine
Small intestine
Small intestine
Small intestine
Table Figure 16–A Functions of digestive secretions.
QUESTION: Proteins are digested by secretions from which organs? How did you decide?
375
Small intestine
Mucosa
Submucosa
External
muscle
layer
Serosa
Epithelium
Lacteal
Capillary
network
Lymph
nodule
Lymphatic
vessel
Smooth
muscle
Venule
Arteriole
Meissner's
plexus
Circular
smooth muscle
Longitudinal
smooth muscle
Auerbach's
plexus
Parietal
peritoneum
Transverse
colon
Greater
omentum
Uterus
Bladder
Diaphragm
Liver
Pancreas
Stomach
Duodenum
Mesentery
Small intestine
Sigmoid colon
Rectum
A
B
Figure 16–4. (A) The four layers of the wall of the
alimentary tube. A small part of the wall of the small
intestine has been magnified to show the four layers
typical of the alimentary tube. (B) Sagittal section
through the abdomen showing the relationship of the
peritoneum and mesentery to the abdominal organs.
QUESTION: What is the function of the external muscle
layer?
thin layers of smooth muscle. In the esophagus the
epithelium is stratified squamous epithelium; in the
stomach and intestines it is simple columnar epithelium.
The epithelium secretes mucus, which lubricates
the passage of food, and also secretes the digestive
enzymes of the stomach and small intestine. Just
below the epithelium, within the areolar connective
tissue, are lymph nodules that contain lymphocytes to
produce antibodies, and macrophages to phagocytize
bacteria or other foreign material that get through the
epithelium. The thin layers of smooth muscle create
folds in the mucosa, and ripples, so that all of the
epithelial cells are in touch with the contents of the
organ. In the stomach and small intestine this is
important for absorption.
SUBMUCOSA
The submucosa is made of areolar connective tissue
with many blood vessels and lymphatic vessels. Many
millions of nerve fibers are also present, part of what
is called the enteric nervous system, or the “brain of
the gut,” which extends the entire length of the alimentary
tube. The nerve networks in the submucosa
are called Meissner’s plexus (or submucosal plexus),
and they innervate the mucosa to regulate secretions.
Parasympathetic impulses increase secretions, whereas
sympathetic impulses decrease secretions. Sensory
neurons are also present to the smooth muscle (a
stretched or cramping gut is painful), as are motor
neurons to blood vessels, to regulate vessel diameter
and blood flow.
EXTERNAL MUSCLE LAYER
The external muscle layer typically contains two layers
of smooth muscle: an inner, circular layer and an outer,
longitudinal layer. Variations from the typical do
occur, however. In the esophagus, this layer is striated
muscle in the upper third, which gradually changes to
smooth muscle in the lower portions. The stomach has
three layers of smooth muscle, rather than two.
Contractions of this muscle layer help break up
food and mix it with digestive juices. The one-way
contractions of peristalsis move the food toward the
anus. Auerbach’s plexus (or myenteric plexus) is the
portion of the enteric nervous system in this layer, and
some of its millions of neurons are autonomic. Sympathetic
impulses decrease contractions and peristalsis,
whereas parasympathetic impulses increase contractions
and peristalsis, promoting normal digestion. The
parasympathetic nerves are the vagus (10th cranial)
nerves; they truly live up to the meaning of vagus,
which is “wanderer.”
SEROSA
Above the diaphragm, for the esophagus, the serosa,
the outermost layer, is fibrous connective tissue. Below
the diaphragm, the serosa is the mesentery or visceral
peritoneum, a serous membrane. Lining the abdominal
cavity is the parietal peritoneum, usually simply
called the peritoneum. The peritoneum-mesentery is
actually one continuous membrane (see Fig. 16–4).
The serous fluid between the peritoneum and mesentery
prevents friction when the alimentary tube contracts
and the organs slide against one another.
The preceding descriptions are typical of the layers
of the alimentary tube. As noted, variations are possible,
and any important differences are mentioned in
the sections that follow on specific organs.
STOMACH
The stomach is located in the upper left quadrant of
the abdominal cavity, to the left of the liver and in
front of the spleen. Although part of the alimentary
tube, the stomach is not a tube, but rather a sac that
extends from the esophagus to the small intestine.
Because it is a sac, the stomach is a reservoir for food,
so that digestion proceeds gradually and we do not
have to eat constantly. Both mechanical and chemical
digestion take place in the stomach.
The parts of the stomach are shown in Fig. 16–5.
The cardiac orifice is the opening of the esophagus,
and the fundus is the portion above the level of this
opening. The body of the stomach is the large central
portion, bounded laterally by the greater curvature and
medially by the lesser curvature. The pylorus is adjacent
to the duodenum of the small intestine, and the
pyloric sphincter surrounds the junction of the two
organs. The fundus and body are mainly storage areas,
whereas most digestion takes place in the pylorus.
When the stomach is empty, the mucosa appears
wrinkled or folded. These folds are called rugae; they
flatten out as the stomach is filled and permit expansion
of the lining without tearing it. The gastric pits
are the glands of the stomach and consist of several
376 The Digestive System
types of cells; their collective secretions are called gastric
juice. Mucous cells secrete mucus, which coats
the stomach lining and helps prevent erosion by the
gastric juice. Chief cells secrete pepsinogen, an inactive
form of the enzyme pepsin. Parietal cells produce
hydrochloric acid (HCl); these cells have
enzymes called proton pumps, which secrete H+ ions
into the stomach cavity. The H+ ions unite with Cl–
ions that have diffused from the parietal cells to form
HCl in the lumen of the stomach. HCl converts
pepsinogen to pepsin, which then begins the digestion
of proteins to polypeptides, and also gives gastric juice
its pH of 1 to 2. This very acidic pH is necessary for
pepsin to function and also kills most microorganisms
that enter the stomach. The parietal cells also secrete
intrinsic factor, which is necessary for the absorption
of vitamin B12. Enteroendocrine cells called G cells
secrete the hormone gastrin.
Gastric juice is secreted in small amounts at the
sight or smell of food. This is a parasympathetic
response that ensures that some gastric juice will be
present in the stomach when food arrives. The pres-
The Digestive System 377
Esophagus Fundus of stomach
Longitudinal muscle
layer
Circular muscle layer
Oblique muscle layer
Body
Greater curvature
Rugae
Pylorus
Duodenum
Pyloric sphincter
Lesser curvature
Cardiac orifice
Mucous cell
Parietal cell
Chief cell
G cell
B
A
Figure 16–5. (A) The stomach in anterior view. The stomach wall has been sectioned to
show the muscle layers and the rugae of the mucosa. (B) Gastric pits (glands) showing the
types of cells present. See text for functions.
QUESTION: What is the function of the pyloric sphincter?
ence of food in the stomach causes the G cells to
secrete gastrin, a hormone that stimulates the secretion
of greater amounts of gastric juice.
The external muscle layer of the stomach consists
of three layers of smooth muscle: circular, longitudinal,
and oblique layers. These three layers are innervated
by the myenteric plexuses of the enteric nervous
system. Stimulatory impulses are carried from the
CNS by the vagus nerves (10th cranial) and provide
for very efficient mechanical digestion to change food
into a thick liquid called chyme. The pyloric sphincter
is usually contracted when the stomach is churning
food; it relaxes at intervals to permit small amounts of
chyme to pass into the duodenum. This sphincter then
contracts again to prevent the backup of intestinal
contents into the stomach (see Box 16–1: Disorders of
the Stomach).
SMALL INTESTINE
The small intestine is about 1 inch (2.5 cm) in diameter
and approximately 20 feet (6 m) long and extends
from the stomach to the cecum of the large intestine.
Within the abdominal cavity, the large intestine encircles
the coils of the small intestine (see Fig. 16–1).
The duodenum is the first 10 inches (25 cm) of the
small intestine. The common bile duct enters the duodenum
at the ampulla of Vater (or hepatopancreatic
ampulla). The jejunum is about 8 feet long, and the
ileum is about 11 feet in length. In a living person,
however, the small intestine is always contracted and is
therefore somewhat shorter.
Digestion is completed in the small intestine, and
the end products of digestion are absorbed into the
blood and lymph. The mucosa (see Fig. 16–4) has
simple columnar epithelium that includes cells with
microvilli and goblet cells that secrete mucus.
Enteroendocrine cells secrete the hormones of the
small intestine. Lymph nodules called Peyer’s patches
are especially abundant in the ileum to destroy
absorbed pathogens. The external muscle layer has the
typical circular and longitudinal smooth muscle layers
that mix the chyme with digestive secretions and propel
the chyme toward the colon. Stimulatory impulses
to the enteric nerves of these muscle layers are carried
by the vagus nerves. The waves of peristalsis, however,
can take place without stimulation by the central nerv-
378 The Digestive System
BOX 16–1 DISORDERS OF THE STOMACH
pyloric stenosis. Correcting this condition requires
surgery to widen the opening in the sphincter.
A gastric ulcer is an erosion of the mucosa of
the stomach. Because the normal stomach lining is
adapted to resist the corrosive action of gastric
juice, ulcer formation is the result of oversecretion
of HCl or undersecretion of mucus.
As erosion reaches the submucosa, small blood
vessels are ruptured and bleed. If vomiting occurs,
the vomitus has a “coffee-ground” appearance due
to the presence of blood acted on by gastric juice.
A more serious complication is perforation of the
stomach wall, with leakage of gastric contents into
the abdominal cavity, and peritonitis.
The bacterium called Helicobacter pylori is the
cause of most gastric ulcers. For many patients, a
few weeks of antibiotic therapy to eradicate this
bacterium has produced rapid healing of their
ulcers. This bacterium also seems to be responsible
for virtually all cases of stomach cancer.
The medications that decrease the secretion of
HCl are useful for ulcer patients not helped by
antibiotics.
Vomiting is the expulsion of stomach and intestinal
contents through the esophagus and mouth.
Stimuli include irritation of the stomach, motion
sickness, food poisoning, or diseases such as meningitis.
The vomiting center is in the medulla, which
coordinates the simultaneous contraction of the
diaphragm and the abdominal muscles. This
squeezes the stomach and upper intestine, expelling
their contents. As part of the reflex, the lower
esophageal sphincter relaxes, and the glottis closes.
If the glottis fails to close, as may happen in alcohol
or drug intoxication, aspiration of vomitus may
occur and result in fatal obstruction of the respiratory
passages.
Pyloric stenosis means that the opening of the
pyloric sphincter is narrowed, and emptying of the
stomach is impaired. This is most often a congenital
disorder caused by hypertrophy of the pyloric
sphincter. For reasons unknown, this condition is
more common in male infants than in female
infants. When the stomach does not empty efficiently,
its internal pressure increases. Vomiting
relieves the pressure; this is a classic symptom of
ous system; the enteric nervous system can function
independently and promote normal peristalsis.
There are three sources of digestive secretions that
function within the small intestine: the liver, the pancreas,
and the small intestine itself. We will return to
the small intestine after considering these other
organs.
LIVER
The liver (Fig. 16–6) consists of two large lobes, right
and left, and fills the upper right and center of the
abdominal cavity, just below the diaphragm. The
structural unit of the liver is the liver lobule, a
roughly hexagonal column of liver cells (hepatocytes).
Between adjacent lobules are branches of the hepatic
artery and portal vein. The capillaries of a lobule are
sinusoids, large and very permeable vessels between
the rows of liver cells. The sinusoids receive blood
from both the hepatic artery and portal vein, and it is
with this mixture of blood that the liver cells carry out
their functions. The hepatic artery brings oxygenated
blood, and the portal vein brings blood from the
digestive organs and spleen (see Fig. 13–7). Each lobule
has a central vein. The central veins of all the lobules
unite to form the hepatic veins, which take blood
out of the liver to the inferior vena cava.
The cells of the liver have many functions (which
are discussed in a later section), but their only digestive
function is the production of bile. Bile enters the
small bile ducts, called bile canaliculi, on the liver
cells, which unite to form larger ducts and finally
merge to form the hepatic duct, which takes bile out
of the liver (see Fig. 16–6). The hepatic duct unites
with the cystic duct of the gallbladder to form the
common bile duct, which takes bile to the duodenum.
Bile is mostly water and has an excretory function
in that it carries bilirubin and excess cholesterol to the
intestines for elimination in feces. The digestive function
of bile is accomplished by bile salts, which emulsify
fats in the small intestine. Emulsification means
that large fat globules are broken into smaller globules.
This is mechanical, not chemical, digestion; the
fat is still fat but now has more surface area to facilitate
chemical digestion.
Production of bile is stimulated by the hormone
secretin, which is produced by the duodenum when
food enters the small intestine. Table 16–2 summa-
The Digestive System 379
rizes the regulation of secretion of all digestive secretions.
GALLBLADDER
The gallbladder is a sac about 3 to 4 inches (7.5 to 10
cm) long located on the undersurface of the right lobe
of the liver. Bile in the hepatic duct of the liver flows
through the cystic duct into the gallbladder (see Fig.
16–6), which stores bile until it is needed in the small
intestine. The gallbladder also concentrates bile by
absorbing water (see Box 16–2: Gallstones).
When fatty foods enter the duodenum, the
enteroendocrine cells of the duodenal mucosa secrete
the hormone cholecystokinin. This hormone stimulates
contraction of the smooth muscle in the wall of
the gallbladder, which forces bile into the cystic duct,
then into the common bile duct, and on into the duodenum.
PANCREAS
The pancreas is located in the upper left abdominal
quadrant between the curve of the duodenum and the
spleen and is about 6 inches (15 cm) in length. The
endocrine functions of the pancreas were discussed in
Chapter 10, so only the exocrine functions will be
considered here. The exocrine glands of the pancreas
are called acini (singular: acinus) (Fig. 16–7). They
produce enzymes that are involved in the digestion of
all three types of complex food molecules.
The pancreatic enzyme amylase digests starch to
maltose. You may recall that this is the “backup”
enzyme for salivary amylase, though pancreatic amylase
is responsible for most digestion of starch. Lipase
converts emulsified fats to fatty acids and glycerol.
The emulsifying or fat-separating action of bile salts
increases the surface area of fats so that lipase works
effectively. Trypsinogen is an inactive enzyme that is
changed to active trypsin in the duodenum. Trypsin
digests polypeptides to shorter chains of amino acids.
The pancreatic enzyme juice is carried by small
ducts that unite to form larger ducts, then finally the
main pancreatic duct. An accessory duct may also be
present. The main pancreatic duct emerges from the
medial side of the pancreas and joins the common bile
duct to the duodenum (see Fig. 16–7).
The pancreas also produces a bicarbonate juice
(containing sodium bicarbonate), which is alkaline.
380
Right hepatic vein
Liver
Cystic
duct
Inferior vena cava
Left hepatic vein
Hepatic duct
Common bile duct
Hepatic artery
Portal vein
Central vein
(branch of
hepatic vein)
A
Gallbladder
Sinusoids
Bile duct
Liver lobule
Branch of portal vein
Branch of hepatic artery
Bile duct
Hepatocytes
B
Figure 16–6. (A) The liver and gallbladder with blood vessels and bile ducts.
(B) Magnified view of one liver lobule. See text for description.
QUESTION: In part B, trace the pathway of blood flow through a liver lobule.
Because the gastric juice that enters the duodenum
is very acidic, it must be neutralized to prevent damage
to the duodenal mucosa. This neutralizing is
accomplished by the sodium bicarbonate in pancreatic
juice, and the pH of the duodenal chyme is raised to
about 7.5.
Secretion of pancreatic juice is stimulated by the
hormones secretin and cholecystokinin, which are
produced by the duodenal mucosa when chyme enters
the small intestine. Secretin stimulates the production
of bicarbonate juice by the pancreas, and cholecystokinin
stimulates the secretion of the pancreatic
enzymes.
COMPLETION OF DIGESTION
AND ABSORPTION
SMALL INTESTINE
The secretion of the epithelium of the intestinal
glands (or crypts of Lieberk├╝hn) is stimulated by the
The Digestive System 381
Table 16–2 REGULATION OF DIGESTIVE SECRETIONS
Secretion Nervous Regulation Chemical Regulation
Saliva
Gastric juice
Bile
Secretion by
the liver
Contraction of
the gallbladder
Enzyme pancreatic juice
Bicarbonate pancreatic juice
Intestinal juice
Presence of food in mouth or sight of food;
parasympathetic impulses along 7th and
9th cranial nerves
Sight or smell of food; parasympathetic
impulses along 10th cranial nerves
None
None
None
None
Presence of chyme in the duodenum; parasympathetic
impulses along 10th cranial nerves
None
Gastrin—produced by the G cells of
the gastric mucosa when food is
present in the stomach
Secretin—produced by the enteroendocrine
cells of the duodenum
when chyme enters
Cholecystokinin—produced by the
enteroendocrine cells of the duodenum
when chyme enters
Cholecystokinin—from the duodenum
Secretin—from the duodenum
None
BOX 16–2 GALLSTONES
Several treatments are available for gallstones.
Medications that dissolve gallstones work slowly,
over the course of several months, and are useful if
biliary obstruction is not severe. An instrument that
generates shock waves (called a lithotripter) may be
used to pulverize the stones into smaller pieces that
may easily pass into the duodenum; this procedure
is called lithotripsy. Surgery to remove the gallbladder
(cholecystectomy) is required in some
cases. The hepatic duct is then connected directly
to the common bile duct, and dilute bile flows into
the duodenum. Following such surgery, the patient
should avoid meals high in fats.
One of the functions of the gallbladder is to concentrate
bile by absorbing water. If the bile contains
a high concentration of cholesterol, absorption of
water may lead to precipitation and the formation of
cholesterol crystals. These crystals are gallstones.
If the gallstones are small, they will pass through
the cystic duct and common bile duct to the duodenum
without causing symptoms. If large, however,
the gallstones cannot pass out of the
gallbladder, and may cause mild to severe pain that
often radiates to the right shoulder. Obstructive
jaundice may occur if bile backs up into the liver
and bilirubin is reabsorbed into the blood.
presence of food in the duodenum. The intestinal
enzymes are the peptidases and sucrase, maltase, and
lactase. Peptidases complete the digestion of protein
by breaking down short polypeptide chains to amino
acids. Sucrase, maltase, and lactase, respectively,
digest the disaccharides sucrose, maltose, and lactose
to monosaccharides.
The enteroendocrine cells of the intestinal glands
secrete the hormones of the small intestine. Secretion
is stimulated by food entering the duodenum.
382 The Digestive System
Pyloric sphincter
Hepatic duct
Cystic duct
Duodenum
Main
pancreatic
duct
Pancreas
Superior mesenteric
artery and vein
Islets of
Langerhans
Ducts
Acini
Capillaries
Delta cell
Beta cell
Alpha cell
A
B
Common
bile duct
Ampulla of Vater
Figure 16–7. (A) The pancreas, sectioned to show the pancreatic ducts. The main pancreatic
duct joins the common bile duct. (B) Microscopic section showing acini with their
ducts and several islets of Langerhans.
QUESTION: In part B, what do the acini secrete?
A summary of the digestive secretions and their
functions is found in Table 16–1. Regulation of these
secretions is shown in Table 16–2.
ABSORPTION
Most absorption of the end products of digestion takes
place in the small intestine (although the stomach does
absorb water and alcohol). The process of absorption
requires a large surface area, which is provided by several
structural modifications of the small intestine;
these are shown in Fig. 16–8. Plica circulares, or circular
folds, are macroscopic folds of the mucosa and
submucosa, somewhat like accordion pleats. The
mucosa is further folded into projections called villi,
which give the inner surface of the intestine a velvetlike
appearance. Each columnar cell (except the
mucus-secreting goblet cells) of the villi also has
microvilli on its free surface. Microvilli are microscopic
folds of the cell membrane, and are collectively
The Digestive System 383
Small intestine
Plica
circulares
Intestinal
gland
Microvilli
Absorptive cell
Goblet cell
Lacteal
Capillary
network
Enteroendocrine cell
A
B
Figure 16–8. The small intestine. (A) Section through the small intestine showing plica
circulares. (B) Microscopic view of a villus showing the internal structure. The enteroendocrine
cells secrete the intestinal hormones.
QUESTION: What is the purpose of the villi? What other structures have the same purpose?
called the brush border. All of these folds greatly
increase the surface area of the intestinal lining. It is
estimated that if the intestinal mucosa could be flattened
out, it would cover more than 2000 square feet
(half a basketball court).
The absorption of nutrients takes place from the
lumen of the intestine into the vessels within the
villi. Refer to Fig. 16–8 and notice that within each villus
is a capillary network and a lacteal, which is a
dead-end lymph capillary. Water-soluble nutrients
are absorbed into the blood in the capillary networks.
Monosaccharides, amino acids, positive ions, and the
water-soluble vitamins (vitamin C and the B vitamins)
are absorbed by active transport. Negative ions may
be absorbed by either passive or active transport
mechanisms. Water is absorbed by osmosis following
the absorption of minerals, especially sodium. Certain
nutrients have additional special requirements for
their absorption: For example, vitamin B12 requires
the intrinsic factor produced by the parietal cells
of the gastric mucosa, and the efficient absorption of
calcium ions requires parathyroid hormone and vitamin
D.
Fat-soluble nutrients are absorbed into the lymph
in the lacteals of the villi. Bile salts are necessary for
the efficient absorption of fatty acids and the fat-soluble
vitamins (A, D, E, and K). Once absorbed, fatty
acids are recombined with glycerol to form triglycerides.
These triglycerides then form globules that
include cholesterol and protein; these lipid–protein
complexes are called chylomicrons. In the form of
chylomicrons, most absorbed fat is transported by the
lymph and eventually enters the blood in the left subclavian
vein.
Blood from the capillary networks in the villi does
not return directly to the heart but first travels
through the portal vein to the liver. You may recall the
importance of portal circulation, discussed in Chapter
13. This pathway enables the liver to regulate the
blood levels of glucose and amino acids, store certain
vitamins, and remove potential poisons from the
blood (see Box 16–3: Disorders of the Intestines).
384 The Digestive System
BOX 16–3 DISORDERS OF THE INTESTINES
Salmonella food poisoning is caused by bacteria
in the genus Salmonella. These are part of the
intestinal flora of animals, and animal foods such as
meat and eggs may be sources of infection. These
bacteria are not normal for people, and they cause
the intestines to secrete large amounts of fluid.
Symptoms include diarrhea, abdominal cramps,
and vomiting and usually last only a few days. For
elderly or debilitated people, however, salmonella
food poisoning may be very serious or even fatal.
Diverticula are small outpouchings through
weakened areas of the intestinal wall. They are
more likely to occur in the colon than in the small
intestine and may exist for years without causing
any symptoms. The presence of diverticula is called
diverticulosis. Inflammation of diverticula is
called diverticulitis, which is usually the result of
entrapment of feces and bacteria. Symptoms
include abdominal pain and tenderness and fever. If
uncomplicated, diverticulitis may be treated with
antibiotics and modifications in diet. The most serious
complication is perforation of diverticula, allowing
fecal material into the abdominal cavity, causing
peritonitis. A diet high in fiber is believed to be an
important aspect of prevention, to provide bulk in
the colon and prevent weakening of its wall.
Duodenal ulcers are erosions of the duodenal
wall caused by the gastric juice that enters from the
stomach. The most serious consequences are bleeding
and perforation.
Paralytic ileus is the cessation of contraction
of the smooth muscle layer of the intestine. This
is a possible complication of abdominal surgery,
but it may also be the result of peritonitis or
inflammation elsewhere in the abdominal cavity.
In the absence of peristalsis, intestinal obstruction
may occur. Bowel movements cease, and vomiting
occurs to relieve the pressure within the
alimentary tube. Treatment involves suctioning
the intestinal contents to eliminate any obstruction
and to allow the intestine to regain its normal
motility.
Lactose intolerance is the inability to digest
lactose because of deficiency of the enzyme lactase.
Lactase deficiency may be congenital, a consequence
of prematurity, or acquired later in life. The
delayed form is quite common among people of
African or Asian ancestry, and in part is genetic.
When lactose, or milk sugar, is not digested, it
undergoes fermentation in the intestine. Symptoms
include diarrhea, abdominal pain, bloating, and
flatulence (gas formation).
LARGE INTESTINE
The large intestine, also called the colon, is approximately
2.5 inches (6.3 cm) in diameter and 5 feet (1.5
m) in length. It extends from the ileum of the small
intestine to the anus, the terminal opening. The parts
of the colon are shown in Fig. 16–9. The cecum is the
first portion, and at its junction with the ileum is
the ileocecal valve, which is not a sphincter but serves
the same purpose. After undigested food (which is
now mostly cellulose) and water pass from the ileum
into the cecum, closure of the ileocecal valve prevents
the backflow of fecal material.
Attached to the cecum is the appendix, a small,
dead-end tube with abundant lymphatic tissue. The
appendix seems to be a vestigial organ, that is, one
whose size and function seem to be reduced. Although
there is abundant lymphatic tissue in the wall of the
appendix, the possibility that the appendix is concerned
with immunity is not known with certainty.
Appendicitis refers to inflammation of the appendix,
which may occur if fecal material becomes impacted
within it. This usually necessitates an appendectomy,
the surgical removal of the appendix.
The remainder of the colon consists of the ascending,
transverse, and descending colon, which encircle
the small intestine; the sigmoid colon, which turns
medially and downward; the rectum; and the anal
canal. The rectum is about 6 inches long, and the anal
canal is the last inch of the colon that surrounds the
anus. Clinically, however, the terminal end of the
colon is usually referred to as the rectum.
No digestion takes place in the colon. The only
secretion of the colonic mucosa is mucus, which lubricates
the passage of fecal material. The longitudinal
smooth muscle layer of the colon is in three bands
called taeniae coli. The rest of the colon is “gathered”
to fit these bands. This gives the colon a puckered
appearance; the puckers or pockets are called haustra,
which provide for more surface area within the colon.
The functions of the colon are the absorption of
water, minerals, and vitamins and the elimination of
undigestible material. About 80% of the water that
The Digestive System 385
Hepatic flexure
Haustra
Taeniae coli
Splenic flexure
Transverse colon
Ascending
colon
lleum
Descending
colon
lleocecal valve
Appendix
Cecum
Sigmoid colon
Rectum
Anus
Anal canal
Figure 16–9. The large intestine shown in
anterior view. The term flexure means a turn or
bend.
QUESTION: What is the function of the ileocecal
valve?
enters the colon is absorbed (400 to 800 mL per day).
Positive and negative ions are also absorbed. The vitamins
absorbed are those produced by the normal
flora, the trillions of bacteria that live in the colon.
Vitamin K is produced and absorbed in amounts usually
sufficient to meet a person’s daily need. Other
vitamins produced in smaller amounts include
riboflavin, thiamin, biotin, and folic acid. Everything
absorbed by the colon circulates first to the liver by
way of portal circulation. Yet another function of the
normal colon flora is to inhibit the growth of
pathogens (see Box 16–4: Infant Botulism).
ELIMINATION OF FECES
Feces consist of cellulose and other undigestible material,
dead and living bacteria, and water. Elimination
of feces is accomplished by the defecation reflex, a
spinal cord reflex that may be controlled voluntarily.
The rectum is usually empty until peristalsis of the
colon pushes feces into it. These waves of peristalsis
tend to occur after eating, especially when food enters
the duodenum. The wall of the rectum is stretched by
the entry of feces, and this is the stimulus for the defecation
reflex.
Stretch receptors in the smooth muscle layer of the
rectum generate sensory impulses that travel to the
sacral spinal cord. The returning motor impulses
cause the smooth muscle of the rectum to contract.
Surrounding the anus is the internal anal sphincter,
which is made of smooth muscle. As part of the reflex,
this sphincter relaxes, permitting defecation to take
place.
The external anal sphincter is made of skeletal
muscle and surrounds the internal anal sphincter (Fig.
16–10). If defecation must be delayed, the external
sphincter may be voluntarily contracted to close the
anus. The awareness of the need to defecate passes as
the stretch receptors of the rectum adapt. These
receptors will be stimulated again when the next wave
of peristalsis reaches the rectum (see Box 16–5: Fiber).
OTHER FUNCTIONS OF THE LIVER
The liver is a remarkable organ, and only the brain is
capable of a greater variety of functions. The liver
cells (hepatocytes) produce many enzymes that catalyze
many different chemical reactions. These reactions
are the functions of the liver. As blood flows
through the sinusoids (capillaries) of the liver (see Fig.
16–6), materials are removed by the liver cells, and the
products of the liver cells are secreted into the blood.
Some of the liver functions will already be familiar
to you. Others are mentioned again and discussed in
more detail in the next chapter. Because the liver
has such varied effects on so many body systems, we
will use the categories below to summarize the liver
functions.
1. Carbohydrate metabolism—As you know, the
liver regulates the blood glucose level. Excess glucose
is converted to glycogen (glycogenesis) when
blood glucose is high; the hormones insulin and
cortisol facilitate this process. During hypoglycemia
or stress situations, glycogen is converted
back to glucose (glycogenolysis) to raise the blood
glucose level. Epinephrine and glucagon are the
hormones that facilitate this process.
386 The Digestive System
BOX 16–4 INFANT BOTULISM
Botulism is most often acquired from food.
When the spores of the botulism bacteria are in
an anaerobic (without oxygen) environment
such as a can of food, they germinate into active
bacteria that produce a neurotoxin. If people
ingest food containing this toxin, they will
develop the paralysis that is characteristic of
botulism.
For infants less than 1 year of age, however,
ingestion of just the bacterial spores may be
harmful. The infant’s stomach does not produce
much HCl, so ingested botulism spores may not
be destroyed. Of equal importance, the infant’s
normal colon flora is not yet established. Without
the normal population of colon bacteria to provide
competition, spores of the botulism bacteria
may germinate and produce their toxin.
An affected infant becomes lethargic and
weak; paralysis may progress slowly or rapidly.
Treatment (antitoxin) is available, but may be
delayed if botulism is not suspected. Many cases
of infant botulism have been traced to honey
that was found to contain botulism spores. Such
spores are not harmful to older children and
adults, who have a normal colon flora that prevents
the botulism bacteria from becoming
established.
387
BOX 16–5 FIBER
no protective effect of fiber against colon cancer.
What we can say for sure is that fiber may not be
the only dietary or environmental factor involved.
Claims that high-fiber diets directly lower blood
levels of cholesterol and fats are not supported by
definitive clinical or experimental studies. One possible
explanation may be that a person whose diet
consists largely of high-fiber foods simply eats less
of the foods high in cholesterol and fats, and this is
the reason for that person’s lower blood levels of
fats and cholesterol.
Should people try to make great changes in their
diets? Probably not, not if they are careful to limit
fat intake and to include significant quantities of
vegetables and fruits. Besides the possible benefits
of fiber, unprocessed plant foods provide important
amounts of vitamins and minerals.
Fiber is a term we use to refer to the organic materials
in the cell walls of plants. These are mainly cellulose
and pectins. The role of dietary fiber and
possible benefits that a high-fiber diet may provide
are currently the focus of much research. It is
important to differentiate what is known from what
is, at present, merely speculation.
Many studies have shown that populations
(large groups of people, especially those of different
cultures) who consume high-fiber diets tend to
have a lower frequency of certain diseases. These
include diverticulitis, colon cancer, coronary artery
disease, diabetes, and hypertension. Such diseases
are much more common among populations
whose diets are low in vegetables, fruits, and whole
grains, and high in meat, dairy products, and
processed foods. In contrast, a 2005 study showed
Rectum
Anal canal
Anal
columns
Longitudinal muscle
Circular muscle
Rectal fold
Levator ani
muscle
Internal anal
sphincter
External anal
sphincter
Anus
A
B
Figure 16–10. (A) Internal and external anal sphincters shown in a frontal section
through the lower rectum and anal canal. (B) Position of rectum and anal canal relative to
pelvic bone.
QUESTION: The internal anal sphincter is a continuation of which part of the rectum?
The liver also changes other monosaccharides to
glucose. Fructose and galactose, for example, are
end products of the digestion of sucrose and lactose.
Because most cells, however, cannot readily
use fructose and galactose as energy sources, they
are converted by the liver to glucose, which is easily
used by cells.
2. Amino acid metabolism—The liver regulates
blood levels of amino acids based on tissue needs
for protein synthesis. Of the 20 different amino
acids needed for the production of human proteins,
the liver is able to synthesize 12, called the nonessential
amino acids. The chemical process by
which this is done is called transamination, the
transfer of an amino group (NH2) from an amino
acid present in excess to a free carbon chain that
forms a complete, new amino acid molecule. The
other eight amino acids, which the liver cannot
synthesize, are called the essential amino acids. In
this case, “essential” means that the amino acids
must be supplied by our food, because the liver
cannot manufacture them. Similarly, “non-essential”
means that the amino acids do not have to be
supplied in our food because the liver can make
them. All 20 amino acids are required in order to
make our body proteins.
Excess amino acids, those not needed right away
for protein synthesis, cannot be stored. However,
they do serve another useful purpose. By the
process of deamination, which also occurs in the
liver, the NH2 group is removed from an amino
acid, and the remaining carbon chain may be converted
to a simple carbohydrate molecule or to fat.
Thus, excess amino acids are utilized for energy
production: either for immediate energy or for the
potential energy stored as fat in adipose tissue. The
NH2 groups that were detached from the original
amino acids are combined to form urea, a waste
product that will be removed from the blood by the
kidneys and excreted in urine.
3. Lipid metabolism—The liver forms lipoproteins,
which as their name tells us, are molecules of lipids
and proteins, for the transport of fats in the blood
to other tissues. The liver also synthesizes cholesterol
and excretes excess cholesterol into bile to be
eliminated in feces.
Fatty acids are a potential source of energy, but
in order to be used in cell respiration they must be
broken down to smaller molecules. In the process
of beta-oxidation, the long carbon chains of fatty
acids are split into two-carbon molecules called
acetyl groups, which are simple carbohydrates.
These acetyl groups may be used by the liver cells
to produce ATP or may be combined to form
ketones to be transported in the blood to other
cells. These other cells then use the ketones to produce
ATP in cell respiration.
4. Synthesis of plasma proteins—This is a liver
function that you will probably remember from
Chapter 11. The liver synthesizes many of the proteins
that circulate in the blood. Albumin, the most
abundant plasma protein, helps maintain blood volume
by pulling tissue fluid into capillaries.
The clotting factors are also produced by the
liver. These, as you recall, include prothrombin,
fibrinogen, and Factor 8, which circulate in the
blood until needed in the chemical clotting mechanism.
The liver also synthesizes alpha and beta
globulins, which are proteins that serve as carriers
for other molecules, such as fats, in the blood.
5. Formation of bilirubin—This is another familiar
function: The liver contains fixed macrophages
that phagocytize old red blood cells (RBCs).
Bilirubin is then formed from the heme portion of
the hemoglobin. The liver also removes from the
blood the bilirubin formed in the spleen and red
bone marrow and excretes it into bile to be eliminated
in feces.
6. Phagocytosis by Kupffer cells—The fixed
macrophages of the liver are called Kupffer cells
(or stellate reticuloendothelial cells). Besides
destroying old RBCs, Kupffer cells phagocytize
pathogens or other foreign material that circulate
through the liver. Many of the bacteria that get to
the liver come from the colon. These bacteria are
part of the normal flora of the colon but would be
very harmful elsewhere in the body. The bacteria
that enter the blood with the water absorbed by the
colon are carried to the liver by way of portal circulation.
The Kupffer cells in the liver phagocytize
and destroy these bacteria, removing them from
the blood before the blood returns to the heart.
7. Storage—The liver stores the fat-soluble vitamins
A, D, E, and K, and the water-soluble vitamin B12.
Up to a 6- to 12-month supply of vitamins A and D
may be stored, and beef or chicken liver is an excellent
dietary source of these vitamins.
Also stored by the liver are the minerals iron and
copper. You already know that iron is needed for
hemoglobin and myoglobin and enables these pro-
388 The Digestive System
teins to bond to oxygen. Copper (as well as iron) is
part of some of the proteins needed for cell respiration,
and is part of some of the enzymes necessary
for hemoglobin synthesis.
8. Detoxification—The liver is capable of synthesizing
enzymes that will detoxify harmful substances,
that is, change them to less harmful ones. Alcohol,
for example, is changed to acetate, which is a twocarbon
molecule (an acetyl group) that can be used
in cell respiration.
Medications are all potentially toxic, but the
liver produces enzymes that break them down or
change them. When given in a proper dosage, a
medication exerts its therapeutic effect but is then
changed to less active substances that are usually
excreted by the kidneys. An overdose of a drug
means that there is too much of it for the liver to
detoxify in a given time, and the drug will remain
in the body with possibly harmful effects. This is
why alcohol should never be consumed when taking
medication. Such a combination may cause the
liver’s detoxification ability to be overworked and
ineffective, with the result that both the alcohol
and the medication will remain toxic for a longer
time. Barbiturates taken as sleeping pills after consumption
of alcohol have too often proved fatal for
just this reason.
Ammonia is a toxic substance produced by the
bacteria in the colon. Because it is soluble in water,
some ammonia is absorbed into the blood, but it is
carried first to the liver by portal circulation. The
liver converts ammonia to urea, a less toxic substance,
before the ammonia can circulate and damage
other organs, especially the brain. The urea
formed is excreted by the kidneys (see Box 16–6:
Hepatitis).
AGING AND THE
DIGESTIVE SYSTEM
Many changes can be expected in the aging digestive
system. The sense of taste becomes less acute, less
saliva is produced, and there is greater likelihood of
The Digestive System 389
BOX 16–6 HEPATITIS
include blood and semen. Hepatitis B may be
severe or even fatal, and approximately 10% of
those who recover become carriers of the virus.
Possible consequences of the carrier state are
chronic hepatitis progressing to cirrhosis or primary
liver cancer. Of equal importance, carriers are
sources of the virus for others, especially their sexual
partners.
A vaccine is available for hepatitis B, and healthcare
workers who have contact with blood, even
just occasional contact, should receive it. Other
potential recipients of the vaccine are the sexual
partners of carriers. Pediatricians now consider this
vaccine one of the standard ones for infants.
The hepatitis C virus is also present in body fluids
and is spread by blood or mucous membrane
contact. Most people develop chronic disease, but
many may remain asymptomatic for years after
being infected. With active disease the virus may
cause liver failure. The only therapy then is a liver
transplant.
It is important for healthcare personnel, and
their patients, to know that these types of hepatitis
are not spread by blood transfusions. Donated
blood is tested for all three viruses.
Hepatitis is inflammation of the liver caused
by any of several viruses. The most common of
these hepatitis viruses have been designated A, B,
and C, although there are others. Symptoms of hepatitis
include anorexia, nausea, fatigue, and possibly
jaundice. Severity of disease ranges from very mild
(even asymptomatic) to fatal. Hundreds of thousands
of cases of hepatitis occur in the United States
every year, and although liver inflammation is common
to all of them, the three hepatitis viruses have
different modes of transmission and different consequences
for affected people.
Hepatitis A is an intestinal virus that is spread
by the fecal–oral route. Food contaminated by the
hands of people with mild cases is the usual vehicle
of transmission, although shellfish harvested from
water contaminated with human sewage are
another possible source of this virus. Hepatitis A is
most often mild, recovery provides lifelong immunity,
and the carrier state is not known to occur. A
vaccine is available, but people who have been
exposed to hepatitis A may receive gamma globulin
by injection to prevent the disease.
Hepatitis B is contracted by exposure to the
body fluids of an infected person; these fluids
Function of the Digestive System—to break
down food into simple chemicals that can
be absorbed into the blood and lymph and
utilized by cells
Divisions of the Digestive System
1. Alimentary tube—oral cavity, pharynx, esophagus,
stomach, small intestine, large intestine. Digestion
takes place in the oral cavity, stomach, and small
intestine.
2. Accessory organs—salivary glands, teeth, tongue,
liver, gallbladder, and pancreas. Each contributes to
digestion.
Types of Digestion
1. Mechanical—breaks food into smaller pieces to
increase the surface area for the action of enzymes.
2. Chemical—enzymes break down complex organics
into simpler organics and inorganics; each enzyme
is specific for the food it will digest.
End Products of Digestion
1. Carbohydrates are digested to monosaccharides.
2. Fats are digested to fatty acids and glycerol.
3. Proteins are digested to amino acids.
4. Other end products are vitamins, minerals, and
water.
Oral Cavity—food enters by way of the
mouth
1. Teeth and tongue break up food and mix it with
saliva.
2. Tooth structure (see Fig. 16–2)—enamel covers the
crown and provides a hard chewing surface; dentin
is within the enamel and forms the roots; the pulp
cavity contains blood vessels and endings of the
trigeminal nerve; the periodontal membrane
produces cement to anchor the tooth in the jawbone.
3. The tongue is skeletal muscle innervated by the
hypoglossal nerves. Papillae on the upper surface
contain taste buds (facial and glossopharyngeal
nerves). Functions: taste, keeps food between the
teeth when chewing, elevates to push food backward
for swallowing.
4. Salivary glands—parotid, submandibular, and sublingual
(see Fig. 16–3); ducts take saliva to the oral
cavity.
390 The Digestive System
STUDY OUTLINE
periodontal disease and loss of teeth. Secretions are
reduced throughout the digestive system, and the
effectiveness of peristalsis diminishes. Indigestion may
become more frequent, especially if the LES loses its
tone, and there is a greater chance of esophageal damage.
In the colon, diverticula may form; these are bubble-
like outpouchings of the weakened wall of the
colon that may be asymptomatic or become infected.
Intestinal obstruction, of the large or small bowel,
occurs with greater frequency among the elderly.
Sluggish peristalsis contributes to constipation, which
in turn may contribute to the formation of hemorrhoids.
The risk of oral cancer or colon cancer also
increases with age.
The liver usually continues to function adequately
even well into old age, unless damaged by pathogens
such as the hepatitis viruses or by toxins such as alcohol.
There is a greater tendency for gallstones to form,
perhaps necessitating removal of the gallbladder.
Inflammation of the gallbladder (cholecystitis) is also
more frequent in older adults. In the absence of specific
diseases, the pancreas usually functions well,
although acute pancreatitis of unknown cause is somewhat
more likely in elderly people.
SUMMARY
The processes of the digestion of food and the absorption
of nutrients enable the body to use complex food
molecules for many purposes. Much of the food we eat
literally becomes part of us. The body synthesizes proteins
and lipids for the growth and repair of tissues and
produces enzymes to catalyze all of the reactions that
contribute to homeostasis. Some of our food provides
the energy required for growth, repair, movement,
sensation, and thinking. In the next chapter we will
discuss the chemical basis of energy production from
food and consider the relationship of energy production
to the maintenance of body temperature.
5. Saliva—amylase digests starch to maltose; water
dissolves food for tasting and moistens food for
swallowing; lysozyme inhibits the growth of bacteria
(see Tables 16–1 and 16–2).
Pharynx—food passageway from the oral
cavity to the esophagus
1. No digestion takes place.
2. Contraction of pharyngeal muscles is part of swallowing
reflex, regulated by the medulla.
Esophagus—food passageway from pharynx
to stomach
1. No digestion takes place.
2. Lower esophageal sphincter (LES) at junction with
stomach prevents backup of stomach contents.
Structural Layers of the Alimentary Tube
(see Fig. 16–4)
1. Mucosa (lining)—made of epithelial tissue that
produces the digestive secretions; lymph nodules
contain macrophages to phagocytize pathogens
that penetrate the mucosa; thin layer of smooth
muscle to ripple the epithelium.
2. Submucosa—areolar connective tissue with blood
vessels and lymphatic vessels; Meissner’s plexus is a
nerve network that innervates the mucosa, part of
the enteric nervous system that extends the entire
length of the alimentary tube.
3. External muscle layer—typically an inner circular
layer and an outer longitudinal layer of smooth
muscle; function is mechanical digestion and peristalsis;
innervated by Auerbach’s plexus, part of the
enteric nervous system; sympathetic impulses
decrease motility; parasympathetic impulses
increase motility.
4. Serosa—outermost layer; above the diaphragm is
fibrous connective tissue; below the diaphragm is
the mesentery (serous). The peritoneum (serous)
lines the abdominal cavity; serous fluid prevents
friction between the serous layers.
Stomach—in upper left abdominal quadrant;
a muscular sac that extends from the esophagus
to the small intestine (see Fig. 16–5)
1. Reservoir for food; begins the digestion of protein.
2. Gastric juice is secreted by gastric pits (see Tables
16–1 and 16–2).
3. The pyloric sphincter at the junction with the duodenum
prevents backup of intestinal contents.
Liver—consists of two lobes in the upper
right and center of the abdominal cavity
(see Figs. 16–1 and 16–6)
1. Functional unit is the hexagonal liver lobule: liver
cells, sinusoids, branches of the hepatic artery and
portal vein, and bile ducts.
2. The only digestive secretion is bile; the hepatic
duct takes bile out of the liver and unites with the
cystic duct of the gallbladder to form the common
bile duct to the duodenum.
3. Bile salts emulsify fats, a type of mechanical digestion
(see Table 16–1).
4. Excess cholesterol and bilirubin are excreted by the
liver into bile.
Gallbladder—on undersurface of right lobe
of liver (see Fig. 16–6)
1. Stores and concentrates bile until needed in the
duodenum (see Table 16–2).
2. The cystic duct joins the hepatic duct to form the
common bile duct.
Pancreas—in upper left abdominal quadrant
between the duodenum and the spleen (see
Fig. 16–1)
1. Pancreatic juice is secreted by acini, carried by pancreatic
duct to the common bile duct to the duodenum
(see Fig. 16–7).
2. Enzyme pancreatic juice contains enzymes for the
digestion of all three food types (see Tables 16–1
and 16–2).
3. Bicarbonate pancreatic juice neutralizes HCl from
the stomach in the duodenum.
Small Intestine—coiled within the center of
the abdominal cavity (see Fig. 16–1); extends
from stomach to colon
1. Duodenum—first 10 inches; the common bile duct
brings in bile and pancreatic juice. Jejunum (8 feet)
and ileum (11 feet).
2. Enzymes secreted by the intestinal glands complete
digestion (see Tables 16–1 and 16–2). Surface area
for absorption is increased by plica circulares, villi,
and microvilli (see Fig. 16–8); microvilli are the
brush border.
3. The villi contain capillary networks for the absorption
of water-soluble nutrients: monosaccharides,
The Digestive System 391
amino acids, vitamin C and the B vitamins, minerals,
and water. Blood from the small intestine goes
to the liver first by way of portal circulation.
4. The villi contain lacteals (lymph capillaries) for the
absorption of fat-soluble nutrients: vitamins A, D,
E, and K, fatty acids, and glycerol, which are combined
to form chylomicrons. Lymph from the small
intestine is carried back to the blood in the left subclavian
vein.
Large Intestine (colon)—extends from the
small intestine to the anus
1. Colon—parts (see Fig. 16–9): cecum, ascending
colon, transverse colon, descending colon, sigmoid
colon, rectum, anal canal.
2. Ileocecal valve—at the junction of the cecum and
ileum; prevents backup of fecal material into the
small intestine.
3. Colon—functions: absorption of water, minerals,
vitamins; elimination of undigestible material.
4. Normal flora—the bacteria of the colon; produce
vitamins, especially vitamin K, and inhibit the
growth of pathogens.
5. Defecation reflex—stimulus: stretching of the rectum
when peristalsis propels feces into it. Sensory
impulses go to the sacral spinal cord, and motor
impulses return to the smooth muscle of the rectum,
which contracts. The internal anal sphincter
relaxes to permit defecation. Voluntary control is
provided by the external anal sphincter, made of
skeletal muscle (see Fig. 16–10).
Liver—other functions
1. Carbohydrate metabolism—excess glucose is
stored in the form of glycogen and converted back
to glucose during hypoglycemia; fructose and
galactose are changed to glucose.
2. Amino acid metabolism—the non-essential amino
acids are synthesized by transamination; excess
amino acids are changed to carbohydrates or fats by
deamination; the amino groups are converted to
urea and excreted by the kidneys.
3. Lipid metabolism—formation of lipoproteins for
transport of fats in the blood; synthesis of cholesterol;
excretion of excess cholesterol into bile; betaoxidation
of fatty acids to form two-carbon acetyl
groups for energy use.
4. Synthesis of plasma proteins—albumin to help
maintain blood volume; clotting factors for blood
clotting; alpha and beta globulins as carrier molecules.
5. Formation of bilirubin—old RBCs are phagocytized,
and bilirubin is formed from the heme and
put into bile to be eliminated in feces.
6. Phagocytosis by Kupffer cells—fixed macrophages;
phagocytize old RBCs and bacteria, especially bacteria
absorbed by the colon.
7. Storage—vitamins: B12, A, D, E, and K, and the
minerals iron and copper.
8. Detoxification—liver enzymes change potential
poisons to less harmful substances; examples of
toxic substances are alcohol, medications, and
ammonia absorbed by the colon.
392 The Digestive System
REVIEW QUESTIONS
1. Name the organs of the alimentary tube, and
describe the location of each. Name the accessory
digestive organs, and describe the location of each.
(pp. 370, 372, 373, 376, 378, 379, 385)
2. Explain the purpose of mechanical digestion, and
give two examples. Explain the purpose of chemical
digestion, and give two examples. (pp. 370, 374)
3. Name the end products of digestion, and explain
how each is absorbed in the small intestine.
(pp. 370, 384)
4. Explain the function of teeth and tongue, salivary
amylase, enamel of teeth, lysozyme, and water of
saliva. (pp. 370–372)
5. Describe the function of the pharynx, esophagus,
and lower esophageal sphincter. (p. 373)
6. Name and describe the four layers of the alimentary
tube. (pp. 373, 376)
7. State the two general functions of the stomach and
the function of the pyloric sphincter. Explain the
function of pepsin, HCl, and mucus. (pp. 376–378)
8. Describe the general functions of the small intestine,
and name the three parts. Describe the structures
that increase the surface area of the small
intestine. (pp. 378, 383–384)
9. Explain how the liver, gallbladder, and pancreas
contribute to digestion. (pp. 379, 381)
1. Many people with GERD take proton-pump
inhibitors, medications that reduce stomach acid.
Why should these people be especially careful
about what they eat or drink?
2. The colon does not have villi as part of its mucosa.
Explain why villi are not necessary.
3. Food remains in the stomach for several hours.
Passage of food through the small intestine also
requires several hours. These two organs have very
different shapes. Explain why they are able to
retain food for so long, for efficient digestion and
absorption.
4. Diarrhea can be unpleasant, but does have a
purpose. Explain, and state the disadvantages as
well.
5. Explain how a spinal cord transection at the level of
T10 will affect the defecation reflex.
6. You have seen the word enteric (or entero) several
times in this chapter. What does it mean? Define
each of these: enteric bacilli, enterovirus, Enterococcus.
7. The word symbiosis indicates two different kinds of
living things, and literally means “together-life.”
Our own alimentary tube is a perfect example.
Explain, and state the advantages to each living
thing.
The Digestive System 393
FOR FURTHER THOUGHT
10. Describe the internal structure of a villus, and
explain how its structure is related to absorption.
(p. 384)
11. Name the parts of the large intestine, and
describe the function of the ileocecal valve.
(p. 385)
12. Describe the functions of the colon and of the
normal flora of the colon. (pp. 385–386)
13. With respect to the defecation reflex, explain the
stimulus, the part of the CNS directly involved,
the effector muscle, the function of the internal
anal sphincter, and the voluntary control possible.
(p. 386)
14. Name the vitamins and minerals stored in the
liver. Name the fixed macrophages of the liver,
and explain their function. (p. 388)
15. Describe how the liver regulates blood glucose
level. Explain the purpose of the processes of
deamination and transamination. (pp. 386, 388)
16. Name the plasma proteins produced by the liver,
and state the function of each. (p. 388)
17. Name the substances excreted by the liver into
bile. (p. 388)
394
CHAPTER 17
Chapter Outline
Body Temperature
Heat Production
Heat Loss
Heat loss through the skin
Heat loss through the respiratory tract
Heat loss through the urinary and digestive tracts
Regulation of Body Temperature
Mechanisms to increase heat loss
Mechanisms to conserve heat
Fever
Metabolism
Cell Respiration
Glycolysis
Krebs citric acid cycle
Cytochrome transport system
Proteins and fats as energy sources
Energy available from the three nutrient types
Synthesis Uses of Foods
Glucose
Amino acids
Fatty acids and glycerol
Vitamins and Minerals
Metabolic Rate
Aging and Metabolism
BOX 17–1 HEAT-RELATED DISORDERS
BOX 17–2 COLD-RELATED DISORDERS
BOX 17–3 KETOSIS
BOX 17–4 METABOLIC RATE
BOX 17–5 WEIGHT LOSS
BOX 17–6 LEPTIN AND BODY-MASS INDEX
Student Objectives
• State the normal range of human body temperature.
• Explain how cell respiration produces heat and the
factors that affect heat production.
• Describe the pathways of heat loss through the
skin and respiratory tract.
• Explain why the hypothalamus is called the “thermostat”
of the body.
• Describe the mechanisms to increase heat loss.
• Describe the mechanisms to conserve heat.
• Explain how a fever is caused and its advantages
and disadvantages.
• Define metabolism, anabolism, and catabolism.
• Describe what happens to a glucose molecule during
the three stages of cell respiration.
• State what happens to each of the products of cell
respiration.
• Explain how amino acids and fats may be used for
energy production.
• Describe the synthesis uses for glucose, amino
acids, and fats.
• Explain what is meant by metabolic rate and kilocalories.
• Describe the factors that affect a person’s metabolic
rate.
Body Temperature
and Metabolism
395
New Terminology
Anabolism (an-AB-uh-lizm)
Catabolism (kuh-TAB-uh-lizm)
Coenzyme (ko-EN-zime)
Conduction (kon-DUK-shun)
Convection (kon-VEK-shun)
Cytochromes (SIGH-toh-krohms)
Endogenous pyrogen (en-DOJ-en-us PYE-roh-jen)
Fever (FEE-ver)
Glycolysis (gly-KAHL-ah-sis)
Kilocalorie (KILL-oh-KAL-oh-ree)
Krebs cycle (KREBS SIGH-kuhl)
Pyrogen (PYE-roh-jen)
Radiation (RAY-dee-AY-shun)
Vitamins (VY-tah-mins)
Related Clinical Terminology
Antipyretic (AN-tigh-pye-RET-ik)
Basal metabolic rate (BAY-zuhl met-ah-BAHL-ik
RAYT)
Frostbite (FRAWST-bite)
Heat exhaustion (HEET eks-ZAWS-chun)
Heat stroke (HEET STROHK)
Hypothermia (HIGH-poh-THER-mee-ah)
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
During every moment of our lives, our cells are
breaking down food molecules to obtain ATP (adenosine
triphosphate) for energy-requiring cellular
processes. Naturally, we are not aware of the process
of cell respiration, but we may be aware of one of the
products—energy in the form of heat. The human
body is indeed warm, and its temperature is regulated
very precisely. Though we cannot stand barefoot on
the ice of Antarctica for months in winter, as penguins
do, we can adapt to and survive a wide range of environmental
temperatures.
This chapter discusses the regulation of body temperature
and also discusses metabolism, which is the
total of all reactions that take place within the body.
These reactions include the energy-releasing ones of
cell respiration and energy-requiring ones such as
protein synthesis, or DNA synthesis for mitosis. As
you will see, body temperature and metabolism are
inseparable.
BODY TEMPERATURE
The normal range of human body temperature is
96.5° to 99.5°F (36° to 38°C), with an average oral
temperature of 98.6°F (37°C). (A 1992 study suggested
a slightly lower average oral temperature: 98.2°
or 36.8°. But everyone seems to prefer the “traditional”
average temperature.) Within a 24-hour
period, an individual’s temperature fluctuates 1° to 2°,
with the lowest temperatures occurring during sleep.
At either end of the age spectrum, however, temperature
regulation may not be as precise as it is in
older children or younger adults. Infants have more
surface area (skin) relative to volume and are likely to
lose heat more rapidly. In the elderly, the mechanisms
that maintain body temperature may not function as
efficiently as they once did, and changes in environmental
temperature may not be compensated for as
quickly or effectively. This is especially important to
remember when caring for patients who are very
young or very old.
HEAT PRODUCTION
Cell respiration, the process that releases energy from
food to produce ATP, also produces heat as one of its
energy products. Although cell respiration takes place
constantly, many factors influence the rate of this
process:
1. The hormone thyroxine (and T3), produced by the
thyroid gland, increases the rate of cell respiration
and heat production. The secretion of thyroxine is
regulated by the body’s rate of energy production,
the metabolic rate itself. (See Chapter 10 for a discussion
of the feedback mechanism involving the
hypothalamus and anterior pituitary gland and
Chapter 1 for an illustration.) When the metabolic
rate decreases, the thyroid gland is stimulated to
secrete more thyroxine. As thyroxine increases the
rate of cell respiration, a negative feedback mechanism
inhibits further secretion until metabolic rate
decreases again. Thus, thyroxine is secreted whenever
there is a need for increased cell respiration
and is probably the most important regulator of
day-to-day energy production.
2. In stress situations, epinephrine and norepinephrine
are secreted by the adrenal medulla, and the
sympathetic nervous system becomes more active.
Epinephrine increases the rate of cell respiration,
especially in organs such as the heart, skeletal muscles,
and liver. Sympathetic stimulation also increases
the activity of these organs. The increased
production of ATP to meet the demands of the
stress situation also means that more heat will be
produced.
3. Organs that are normally active (producing
ATP) are significant sources of heat when the body
is at rest. The skeletal muscles, for example, are
usually in a state of slight contraction called muscle
tone. Because even slight contraction requires ATP,
the muscles are also producing heat. This amounts
to about 25% of the total body heat at rest and
much more during exercise, when more ATP is
produced.
The liver is another organ that is continually
active, producing ATP to supply energy for its
many functions. As a result, the liver produces as
much as 20% of the total body heat at rest. The
heat produced by these active organs is dispersed
throughout the body by the blood. As the relatively
cooler blood flows through organs such as the muscles
and liver, the heat they produce is transferred
to the blood, warming it. The warmed blood circulates
to other areas of the body, distributing this
heat.
4. The intake of food also increases heat production,
because the metabolic activity of the digestive tract
is increased. Heat is generated as the digestive
396 Body Temperature and Metabolism
organs produce ATP for peristalsis and for the synthesis
of digestive enzymes.
5. Changes in body temperature also have an effect
on metabolic rate and heat production. This
becomes clinically important when a person has
a fever, an abnormally high body temperature.
The higher temperature increases the metabolic
rate, which increases heat production and
elevates body temperature further. Thus, a high
fever may trigger a vicious cycle of ever-increasing
heat production. Fever is discussed later in this
chapter.
The factors that affect heat production are summarized
in Table 17–1.
HEAT LOSS
The pathways of heat loss from the body are the skin,
the respiratory tract, and, to a lesser extent, the urinary
and digestive tracts.
Heat Loss through the Skin
Because the skin covers the body, most body heat is
lost from the skin to the environment. When the environment
is cooler than body temperature (as it usually
is), heat loss is unavoidable. The amount of heat that
is lost is determined by blood flow through the skin
and by the activity of sweat glands.
Blood flow through the skin influences the amount
of heat lost by the processes of radiation, conduction,
and convection. Radiation means that heat from the
body is transferred to cooler objects not touching the
skin, much as a radiator warms the contents of a room
(radiation starts to become less effective when the
environmental temperature rises above 88°F). Conduction
is the loss of heat to cooler air or objects, such
as clothing, that touch the skin. Convection means
that air currents move the warmer air away from the
skin surface and facilitate the loss of heat; this is why a
fan makes us feel cooler on hot days. Loss of heat by
convection also gives us the “wind chill factor” we
hear about in winter. A cold day that is windy will feel
colder than a cold day when the air is still, because the
wind blows the slightly warmer air surrounding the
body away, replacing it with colder air.
As you may recall from Chapter 5, the temperature
of the skin and the subsequent loss of heat are
determined by blood flow through the skin. The arterioles
in the dermis may constrict or dilate to decrease
or increase blood flow. In a cold environment, vasoconstriction
decreases blood flow through the dermis
and thereby decreases heat loss. In a warm environment,
vasodilation in the dermis increases blood flow
to the body surface and loss of heat to the environment.
The other mechanism by which heat is lost from
the skin is sweating. The eccrine sweat glands
secrete sweat (water) onto the skin surface, and excess
body heat evaporates the sweat. Think of running
water into a hot frying pan; the pan is rapidly cooled
as its heat vaporizes the water. Although sweating is
not quite as dramatic (no visible formation of steam),
the principle is just the same.
Sweating is most efficient when the humidity of the
surrounding air is low. Humidity is the percentage of
the maximum amount of water vapor the atmosphere
can contain. A humidity reading of 90% means that
the air is already 90% saturated with water vapor and
can hold little more. In such a situation, sweat does
not readily evaporate, but instead remains on the skin
even as more sweat is secreted. If the humidity is 40%,
Body Temperature and Metabolism 397
Table 17–1 FACTORS THAT AFFECT
HEAT PRODUCTION
Factor Effect
Thyroxine
Epinephrine and
sympathetic
stimulation
Skeletal muscles
Liver
Food intake
Higher body
temperature
• The most important regulator
of day-to-day metabolism;
increases use of foods for ATP
production, thereby increasing
heat production
• Important in stress situations;
increases the metabolic activity
of many organs; increases ATP
and heat production
• Normal muscle tone requires
ATP; the heat produced is
about 25% of the total body
heat at rest
• Always metabolically active;
produces as much as 20% of
total body heat at rest
• Increases activity of the GI
tract; increases ATP and heat
production
• Increases metabolic rate, which
increases heat production,
which further increases metabolic
rate and heat production;
may become detrimental during
high fevers
however, the air can hold a great deal more water
vapor, and sweat evaporates quickly from the skin surface,
removing excess body heat. In air that is completely
dry, a person may tolerate a temperature of
200°F for nearly 1 hour.
Although sweating is a very effective mechanism of
heat loss, it does have a disadvantage in that it requires
the loss of water in order to also lose heat. Water loss
during sweating may rapidly lead to dehydration, and
the water lost must be replaced by drinking fluids (see
Box 17–1: Heat-Related Disorders).
Small amounts of heat are also lost in what is called
“insensible water loss.” Because the skin is not like a
plastic bag, but is somewhat permeable to water, a
small amount of water diffuses through the skin and is
evaporated by body heat. Compared to sweating, however,
insensible water loss is a minor source of heat
loss.
Heat Loss through
the Respiratory Tract
Heat is lost from the respiratory tract as the warmth of
the respiratory mucosa evaporates some water from
the living epithelial surface. The water vapor formed
is exhaled, and a small amount of heat is lost.
Animals such as dogs that do not have numerous
sweat glands often pant in warm weather. Panting is
the rapid movement of air into and out of the upper
respiratory passages, where the warm surfaces evaporate
large amounts of water. In this way the animal
may lose large amounts of heat.
Heat Loss through the Urinary
and Digestive Tracts
When excreted, urine and feces are at body temperature,
and their elimination results in a very small
amount of heat loss.
The pathways of heat loss are summarized in Table
17–2.
REGULATION OF BODY TEMPERATURE
The hypothalamus is responsible for the regulation
of body temperature and is considered the “thermostat”
of the body. As the thermostat, the hypothalamus
maintains the “setting” of body temperature by balancing
heat production and heat loss to keep the body
at the set temperature.
To do this, the hypothalamus must receive information
about the temperature within the body and
about the environmental temperature. Specialized
neurons of the hypothalamus detect changes in the
temperature of the blood that flows through the brain.
The temperature receptors in the skin provide information
about the external temperature changes to
which the body is exposed. The hypothalamus then
integrates this sensory information and promotes the
necessary responses to maintain body temperature
within the normal range.
Mechanisms to Increase Heat Loss
In a warm environment or during exercise, the body
temperature tends to rise, and greater heat loss is
398 Body Temperature and Metabolism
BOX 17–1 HEAT-RELATED DISORDERS
of heat loss, but in high heat the sweating process
continues. As fluid loss increases, sweating stops to
preserve body fluid, and body temperature rises
rapidly (over 105°F , possibly as high as 110°F ).
The classic symptom of heat stroke is hot, dry
skin. The affected person often loses consciousness,
reflecting the destructive effect of such a high body
temperature on the brain. Treatment should involve
hospitalization so that IV fluids may be administered
and body temperature lowered under medical
supervision. A first-aid measure would be the
application of cool (not ice cold) water to as much
of the skin as possible. Fluids should never be forced
on an unconscious person, because the fluid may
be aspirated into the respiratory tract.
Heat exhaustion is caused by excessive sweating
with loss of water and salts, especially NaCl. The
affected person feels very weak, and the skin is usually
cool and clammy (moist). Body temperature is
normal or slightly below normal, the pulse is often
rapid and weak, and blood pressure may be low
because of fluid loss. Other symptoms may include
dizziness, vomiting, and muscle cramps. Treatment
involves rest and consumption of salty fluids or fruit
juices (in small amounts at frequent intervals).
Heat stroke is a life-threatening condition that
may affect elderly or chronically ill people on hot,
humid days, or otherwise healthy people who exercise
too strenuously during such weather. High
humidity makes sweating an ineffective mechanism
needed. This is accomplished by vasodilation in the
dermis and an increase in sweating. Vasodilation
brings more warm blood close to the body surface,
and heat is lost to the environment. However, if the
environmental temperature is close to or higher than
body temperature, this mechanism becomes ineffective.
The second mechanism is increased sweating, in
which excess body heat evaporates the sweat on the
skin surface. As mentioned previously, sweating
becomes inefficient when the atmospheric humidity is
high.
On hot days, heat production may also be
decreased by a decrease in muscle tone. This is why we
may feel very sluggish on hot days; our muscles are
even slightly less contracted than usual and are slower
to respond.
Mechanisms to Conserve Heat
In a cold environment, heat loss from the body is
unavoidable but may be reduced to some extent.
Vasoconstriction in the dermis shunts blood away
from the body surface, so that more heat is kept in the
core of the body. Sweating decreases, and will stop
completely if the temperature of the hypothalamus
falls below about 98.6°F. (Remember that the internal
temperature of the brain is higher than an oral temperature,
and is less subject to any changes in environmental
temperature.)
If these mechanisms are not sufficient to prevent
the body temperature from dropping, more heat may
be produced by increasing muscle tone. When this
greater muscle tone becomes noticeable and rhythmic,
it is called shivering and may increase heat production
by as much as five times the normal.
People also have behavioral responses to cold, and
these too are important to prevent heat loss. Such
things as putting on a sweater or going indoors reflect
our awareness of the discomfort of being cold. For
people (we do not have thick fur as do some other
mammals), these voluntary activities are of critical
importance to the prevention of excessive heat loss
when it is very cold (see Box 17–2: Cold-Related
Disorders).
FEVER
A fever is an abnormally high body temperature
and may accompany infectious diseases, extensive
physical trauma, cancer, or damage to the CNS. The
substances that may cause a fever are called pyrogens.
Pyrogens include bacteria, foreign proteins, and
chemicals released during inflammation. These
inflammatory chemicals are called endogenous pyrogens.
Endogenous means “generated from within.” It
is believed that pyrogens chemically affect the hypothalamus
and “raise the setting” of the hypothalamic
thermostat. The hypothalamus will then stimulate
responses by the body to raise body temperature to
this higher setting.
Let us use as a specific example a child who has a
strep throat. The bacterial and endogenous pyrogens
reset the hypothalamic thermostat upward, to 102°F.
At first, the body is “colder” than the setting of the
hypothalamus, and the heat conservation and production
mechanisms are activated. The child feels cold
and begins to shiver (chills). Eventually, sufficient heat
is produced to raise the body temperature to the hypothalamic
setting of 102°F. At this time, the child will
feel neither too warm nor too cold, because the body
temperature is what the hypothalamus wants.
As the effects of the pyrogens diminish, the hypothalamic
setting decreases, perhaps close to normal
again, 99°F. Now the child will feel warm, and the heat
loss mechanisms will be activated. Vasodilation in the
skin and sweating will occur until the body temperature
drops to the new hypothalamic setting. This
is sometimes referred to as the “crisis,” but actually
the crisis has passed, because sweating indicates that
Body Temperature and Metabolism 399
Table 17–2 PATHWAYS OF HEAT LOSS
Pathway Mechanism
Skin (major
pathway)
Respiratory tract
(secondary
pathway)
Urinary tract
(minor pathway)
Digestive tract
(minor pathway)
• Radiation and conduction—
heat is lost from the body to
cooler air or objects.
• Convection—air currents move
warm air away from the skin.
• Sweating—excess body heat
evaporates sweat on the skin
surface.
• Evaporation—body heat evaporates
water from the respiratory
mucosa, and water vapor
is exhaled.
• Urination—urine is at body
temperature when eliminated.
• Defecation—feces are at body
temperature when eliminated.
the body temperature is returning to normal. The
sequence of temperature changes during a fever is
shown in Fig. 17–1.
You may be wondering if a fever serves a useful purpose.
For low fevers that are the result of infection, the
answer is yes. White blood cells increase their activity
at moderately elevated temperatures, and the metabolism
of some pathogens is inhibited. Thus, a fever may
be beneficial in that it may shorten the duration of an
infection by accelerating the destruction of the
pathogen.
High fevers, however, may have serious consequences.
A fever increases the metabolic rate, which
increases heat production, which in turn raises body
temperature even more. This is a positive feedback
mechanism that will continue until an external event
(such as aspirin or death of the pathogens) acts as a
brake (see Fig. 1–3). When the body temperature rises
above 106°F, the hypothalamus begins to lose its ability
to regulate temperature. The proteins of cells,
especially the enzymes, are also damaged by such high
temperatures. Enzymes become denatured, that is,
lose their shape and do not catalyze the reactions necessary
within cells (see Fig. 2–9). As a result, cells
begin to die. This is most serious in the brain, because
neurons cannot be replaced, and cellular death is the
cause of brain damage that may follow a prolonged
high fever. The effects of changes in body temperature
on the hypothalamus are shown in Fig. 17–2.
A medication such as aspirin is called an antipyretic
because it lowers a fever, probably by affecting
the hypothalamic thermostat. To help lower a high
fever, the body may be cooled by sponging it with cool
water. The excessive body heat will cause this external
water to evaporate, thus reducing temperature. A very
high fever requires medical attention.
METABOLISM
The term metabolism encompasses all of the reactions
that take place in the body. Everything that happens
within us is part of our metabolism. The reactions
of metabolism may be divided into two major categories:
anabolism and catabolism.
Anabolism means synthesis or “formation” reactions,
the bonding together of smaller molecules to
form larger ones. The synthesis of hemoglobin by
cells of the red bone marrow, synthesis of glycogen by
liver cells, and synthesis of fat to be stored in adipose
tissue are all examples of anabolism. Such reactions
require energy, usually in the form of ATP.
400 Body Temperature and Metabolism
BOX 17–2 COLD-RELATED DISORDERS
slurred speech, drowsiness, and lack of coordination.
At this stage, people often do not realize the
seriousness of their condition, and if outdoors (ice
skating or skiing) may not seek a warmer environment.
In progressive hypothermia, breathing and
heart rate slow, and coma and death follow.
Other people at greater risk for hypothermia
include the elderly, whose temperature-regulating
mechanisms are no longer effective, and quadriplegics,
who have no sensation of cold in the body. For
both of these groups, heat production is or may be
low because of inactivity of skeletal muscles.
Artificial hypothermia may be induced during
some types of cardiovascular or neurologic surgery.
This carefully controlled lowering of body temperature
decreases the metabolic rate and need for
oxygen and makes possible prolonged surgery
without causing extensive tissue death in the
patient.
Frostbite is the freezing of part of the body.
Fingers, toes, the nose, and ears are most often
affected by prolonged exposure to cold, because
these areas have little volume in proportion to their
surface.
At first the skin tingles, then becomes numb. If
body fluids freeze, ice crystals may destroy capillaries
and tissues (because water expands when it
freezes), and blisters form. In the most severe cases
gangrene develops; that is, tissue dies because of
lack of oxygen.
Treatment of frostbite includes rewarming the
affected area. If skin damage is apparent, it should
be treated as if it were a burn injury.
Hypothermia is an abnormally low body temperature
(below 95°F) that is most often the result
of prolonged exposure to cold. Although the
affected person certainly feels cold at first, this sensation
may pass and be replaced by confusion,
Catabolism means decomposition, the breaking of
bonds of larger molecules to form smaller molecules.
Cell respiration is a series of catabolic reactions that
break down food molecules to carbon dioxide and
water. During catabolism, energy is often released and
used to synthesize ATP (the heat energy released was
discussed in the previous section). The ATP formed
during catabolism is then used for energy-requiring
anabolic reactions.
Most of our anabolic and catabolic reactions are
catalyzed by enzymes. Enzymes are proteins that
enable reactions to take place rapidly at body temperature
(see Chapter 2 to review the active site theory of
enzyme functioning). The body has thousands of
enzymes, and each is specific, that is, will catalyze only
one type of reaction. As you read the discussions that
follow, keep in mind the essential role of enzymes.
CELL RESPIRATION
You are already familiar with the summary reaction of
cell respiration,
C6H12O6 O2 → CO2 H2O ATP Heat,
(glucose)
the purpose of which is to produce ATP. Glucose contains
potential energy, and when it is broken down to
CO2 and H2O, this energy is released in the forms of
ATP and heat. The oxygen that is required comes
from breathing, and the CO2 formed is circulated to
the lungs to be exhaled. The water formed is called
metabolic water, and helps to meet our daily need for
water. Energy in the form of heat gives us a body temperature,
and the ATP formed is used for energyrequiring
reactions. Synthesis of ATP means that
Body Temperature and Metabolism 401
105˚
104˚
103˚
102˚
101˚
100˚
99˚
98˚
2 hr 4 hr 6 hr 8 hr 10 hr 12 hr 14 hr 16 hr 18 hr 20 hr 22 hr 24 hr
Body temperature
Time
Hypothalamic
thermostat
Actual body
temperature
Pyrogen affects hypothalamus
Vasoconstriction, shivering
Effect of pyrogen diminishes
Sweating, vasodilation
Figure 17–1. Changes in body temperature during an episode of fever. The body temperature
changes (purple line) lag behind the changes in the hypothalamic thermostat
(blue line) but eventually reach whatever the thermostat has called for.
QUESTION: In this cycle of fever, why do sweating and vasodilation occur when they do?
energy is used to bond a free phosphate molecule to
ADP (adenosine diphosphate). ADP and free phosphates
are present in cells after ATP has been broken
down for energy-requiring processes.
The breakdown of glucose summarized here is not
quite that simple, however, and involves a complex
series of reactions. Glucose is taken apart “piece by
piece,” with the removal of hydrogens and the splitting
of carbon–carbon bonds. This releases the energy
of glucose gradually, so that a significant portion
(about 40%) is available to synthesize ATP.
Cell respiration of glucose involves three major
stages: glycolysis, the Krebs citric acid cycle, and the
cytochrome (or electron) transport system. Although
402 Body Temperature and Metabolism
114˚
112˚
110˚
108˚
106˚
104˚
102˚
100˚
98˚
96˚
94˚
92˚
90˚
88˚
86˚
84˚
82˚
80˚
78˚
76˚
74˚
72˚
70˚
68˚
66˚
44˚
42˚
40˚
38˚
37˚
36˚
34˚
32˚
30˚
28˚
26˚
24˚
Upper limit of survival?
Body temperature Temperature regulation by
the hypothalamus
Heat stroke or high fever
Strenuous exercise or fever
Usual range of normal
Hypothermia
(cold weather or
immersion in
cold water)
Lower limit of survival?
˚F ˚C
Temperature regulation is
lost
Temperature regulation is
impaired
Temperature regulation is
efficient
Temperature regulation is
seriously impaired
Figure 17–2. Effects of changes in body temperature on the temperature-regulating
ability of the hypothalamus. Body temperature is shown in degrees Fahrenheit and degrees
Celsius.
QUESTION: Give a range of temperature that an average person would probably survive.
the details of each stage are beyond the scope of this
book, we will summarize the most important aspects
of each, and then relate to them the use of amino acids
and fats for energy. This simple summary is depicted
in Fig. 17–3.
Glycolysis
The enzymes for the reactions of glycolysis are found
in the cytoplasm of cells, and oxygen is not required
(glycolysis is an anaerobic process). Refer now to Fig.
17–3 as you read the following. In glycolysis, a sixcarbon
glucose molecule is broken down to two threecarbon
molecules of pyruvic acid. Two molecules of
ATP are necessary to start the process. The energy
they supply is called energy of activation and is necessary
to make glucose unstable enough to begin to break
down. As a result of these reactions, enough energy is
released to synthesize four molecules of ATP, for a net
gain of two ATP molecules per glucose molecule. Also
during glycolysis, two pairs of hydrogens are removed
by NAD, a carrier molecule that contains the vitamin
niacin. Two NAD molecules thus become 2NADH2,
and these attached hydrogen pairs will be transported
to the cytochrome transport system (stage 3).
If no oxygen is present in the cell, as may happen in
muscle cells during exercise, pyruvic acid is converted
to lactic acid, which causes muscle fatigue. If oxygen is
present, however, pyruvic acid continues into the next
stage, the Krebs citric acid cycle (or, more simply, the
Krebs cycle).
Krebs Citric Acid Cycle
The enzymes for the Krebs cycle (or citric acid
cycle) are located in the mitochondria of cells. This
second stage of cell respiration is aerobic, meaning
that oxygen is required. In a series of reactions, a pyruvic
acid molecule is “taken apart,” and its carbons are
converted to CO2. The first CO2 molecule is removed
by an enzyme that contains the vitamin thiamine.
This leaves a two-carbon molecule called an acetyl
group, which combines with a molecule called coenzyme
A to form acetyl coenzyme A (acetyl CoA). As
acetyl CoA continues in the Krebs cycle, two more
carbons are removed as CO2, and more pairs of hydrogens
are picked up by NAD and FAD (another carrier
molecule that contains the vitamin riboflavin).
NADH2 and FADH2 will carry their hydrogens to the
cytochrome transport system.
During the Krebs cycle, a small amount of energy
is released, enough to synthesize one molecule of ATP
(two per glucose). Notice also that a four-carbon molecule
(oxaloacetic acid) is regenerated after the formation
of CO2. This molecule will react with the next
acetyl CoA, which is what makes the Krebs cycle truly
a self-perpetuating cycle. The results of the stages of
cell respiration are listed in Table 17–3. Before you
continue, you may wish to look at that table to see just
where the process has gotten thus far.
Cytochrome Transport System
Cytochromes are proteins that contain either iron or
copper and are found in the mitochondria of cells.
The pairs of hydrogens that were once part of glucose
are brought to the cytochromes by the carrier molecules
NAD and FAD. Each hydrogen atom is then
split into its proton (H+ ion) and its electron. The
electrons of the hydrogens are passed from one
cytochrome to the next, and finally to oxygen. The
reactions of the electrons with the cytochromes
release most of the energy that was contained in the
glucose molecule, enough to synthesize 34 molecules
of ATP. As you can see, most of the ATP produced in
cell respiration comes from this third stage.
Finally, and very importantly, each oxygen atom
that has gained two electrons (from the cytochromes)
reacts with two of the H+ ions (protons) to form water.
The formation of metabolic water contributes to the
necessary intracellular fluid, and also prevents acidosis.
If H+ ions accumulated, they would rapidly lower
the pH of the cell. This does not happen, however,
because the H+ ions react with oxygen to form water,
and a decrease in pH is prevented.
The summary of the three stages of cell respiration
in Table 17–3 also includes the vitamins and minerals
that are essential for this process. An important overall
concept is the relationship between eating and
breathing. Eating provides us with a potential energy
source (often glucose) and with necessary vitamins and
minerals. However, to release the energy from food,
we must breathe. This is why we breathe. The oxygen
we inhale is essential for the completion of cell respiration,
and the CO2 produced is exhaled.
Proteins and Fats as Energy Sources
Although glucose is the preferred energy source for
cells, proteins and fats also contain potential energy
and are alternative energy sources in certain situations.
As you know, proteins are made of the smaller molecules
called amino acids, and the primary use for the
amino acids we obtain from food is the synthesis of
Body Temperature and Metabolism 403
404 Body Temperature and Metabolism
Carbohydrates Fats
Fatty acid
Pyruvic acid
Acetyl CoA
Citric acid
(Beta
oxidation
in the liver)
Glucose Glycerol
ATP
ATP
ATP
H+
H e
H+ H+ Cytochromes 34 ATP
Metabolic water
H2O
CO2
H2
H2
FAD
NH2
NH2
NH2
NH2
NH2
NH2
H2
NAD
NAD
Urea 3
4
C
C
C
C
C
C
C
C
C C
C
C C C C C C C C
C
C
C
C
C
C
C
C
C C C C C C C C C
C C C C C C C C
C C C C C C
C C
C
C
C C
C C
C C C
C C
C
C
C
C
C
CO2
CO2
=
Oxygen
Proteins
Amino
acids
(Digestion) (Digestion) (Digestion)
2
Coenzyme A
Acetyl
groups
Oxaloacetic
acid
2
Figure 17–3. Schematic representation of cell respiration. The breakdown of glucose is
shown in the center, amino acids on the left, and fatty acids and glycerol on the right. See
text for description.
QUESTION: To which two molecules can all three food types be converted to enter the citric
acid cycle?
new proteins. Excess amino acids, however, those not
needed immediately for protein synthesis, may be
used for energy production. In the liver, excess amino
acids are deaminated, that is, the amino group (NH2)
is removed. The remaining portion is converted to a
molecule that will fit into the Krebs cycle. For example,
a deaminated amino acid may be changed to a
three-carbon pyruvic acid or to a two-carbon acetyl
group. When these molecules enter the Krebs cycle,
the results are just the same as if they had come from
glucose. This is diagrammed in Fig. 17–3.
Fats are made of glycerol and fatty acids, which are
the end products of fat digestion. These molecules may
also be changed to ones that will take part in the Krebs
cycle, and the reactions that change them usually take
place in the liver. Glycerol is a three-carbon molecule
that can be converted to the three-carbon pyruvic acid,
which enters the Krebs cycle. In the process of betaoxidation,
the long carbon chains of fatty acids are split
into two-carbon acetyl groups, which enter a later step
in the Krebs cycle (see Fig. 17–3).
Both amino acids and fatty acids may be converted
by the liver to ketones, which are two- or four-carbon
molecules such as acetone and acetoacetic acid.
Although body cells can use ketones in cell respiration,
they do so slowly. In situations in which fats or
amino acids have become the primary energy sources,
a state called ketosis may develop; this is described in
Box 17–3: Ketosis. Excess amino acids may also be
converted to glucose; this is important to supply the
brain when dietary intake of carbohydrates is low. The
effects of hormones on the metabolism of food are
summarized in Table 17–4.
Energy Available from
the Three Nutrient Types
The potential energy in food is measured in units
called Calories or kilocalories. A calorie (lowercase
“c”) is the amount of energy needed to raise the temperature
of 1 gram of water 1°C. A kilocalorie or
Calorie (capital “C”) is 1000 times that amount of
energy.
One gram of carbohydrate yields about 4 kilocalories.
A gram of protein also yields about 4 kilocalories.
A gram of fat, however, yields 9 kilocalories, and
a gram of alcohol yields 7 kilocalories. This is why a
diet high in fat is more likely to result in weight gain
if the calories are not expended in energy-requiring
activities.
You may have noticed that calorie content is part of
the nutritional information on food labels. On such
labels the term calorie actually means Calorie or kilocalories
but is used for the sake of simplicity.
SYNTHESIS USES OF FOODS
Besides being available for energy production, each of
the three food types is used in anabolic reactions to
Body Temperature and Metabolism 405
Table 17–3 SUMMARY OF CELL RESPIRATION
Molecules That Vitamins or
Stage Enter the Process Results Minerals Needed
Glycolysis
(cytoplasm)
Krebs citric acid cycle
(mitochondria)
Cytochrome
transport system
(mitochondria)
• 2 ATP (net)
• 2 NADH2 (to cytochrome
transport system)
• 2 pyruvic acid (aerobic:
to Krebs cycle; anaerobic:
lactic acid formation)
• CO2 (exhaled)
• ATP (2 per glucose)
• 3 NADH2 and 1 FADH2 (to
cytochrome transport system)
• A 4-carbon molecule is regenerated
for the next cycle
• 34 ATP
• Metabolic water
• Niacin (part of NAD)
• Thiamine (for removal
of CO2)
• Niacin (part of NAD)
• Riboflavin (part of FAD)
• Pantothenic acid (part
of coenzyme A)
• Iron and copper (part
of some cytochromes)
Glucose—ATP needed as
energy of activation
Pyruvic acid—from glucose
or glycerol or excess
amino acids
or
Acetyl CoA—from fatty acids
or excess amino acids
NADH2 and FADH2—from
glycolysis or the Krebs
cycle
synthesize necessary materials for cells and tissues. A
simple summary of these reactions is shown in Fig.
17–4. The three food types and their end products of
digestion are at the bottom of the picture, and the
arrows going upward indicate synthesis and lead to the
products formed. You may wish to refer to Fig. 17–4
as you read the next sections.
Glucose
Glucose is the raw material for the synthesis of
another important monosaccharide, the pentose sugars
that are part of nucleic acids. Deoxyribose is the
five-carbon sugar found in DNA, and ribose is found
in RNA. This function of glucose is very important,
for without the pentose sugars our cells could neither
produce new chromosomes for cell division nor carry
out the process of protein synthesis.
Any glucose in excess of immediate energy needs or
the need for pentose sugars is converted to glycogen
in the liver and skeletal muscles. Glycogen is then an
energy source during states of hypoglycemia or during
exercise. If still more glucose is present, it will be
changed to fat and stored in adipose tissue.
Amino Acids
As mentioned previously, the primary uses for amino
acids are the synthesis of the non-essential amino
acids by the liver and the synthesis of new proteins in
all tissues. By way of review, we can mention some
proteins with which you are already familiar: keratin
and melanin in the epidermis; collagen in the dermis,
tendons, and ligaments; myosin, actin, and myoglobin
in muscle cells; hemoglobin in RBCs; antibodies produced
by WBCs; prothrombin and fibrinogen for
406 Body Temperature and Metabolism
Table 17–4 HORMONES THAT
REGULATE METABOLISM
Hormone (Gland) Effects
Thyroxine (thyroid
gland)
Growth hormone
(anterior pituitary)
Insulin (pancreas)
Glucagon (pancreas)
Cortisol (adrenal
cortex)
Epinephrine
(adrenal medulla)
• Increases use of all three
food types for energy (glucose,
fats, amino acids)
• Increases protein synthesis
• Increases amino acid transport
into cells
• Increases protein synthesis
• Increases use of fats for
energy
• Increases glucose transport
into cells and use for energy
• Increases conversion of glucose
to glycogen in liver
and muscles
• Increases transport of amino
acids and fatty acids into
cells to be used for synthesis
(not energy production)
• Increases conversion of
glycogen to glucose
• Increases use of amino acids
and fats for energy
• Increases conversion of glucose
to glycogen in liver
• Increases use of amino acids
and fats for energy
• Decreases protein synthesis
except in liver and GI tract
• Increases conversion of
glycogen to glucose
• Increases use of fats for
energy
BOX 17–3 KETOSIS
When fats and amino acids are to be used for
energy, they are often converted by the liver to
ketones. Ketones are organic molecules such as
acetone that may be changed to acetyl CoA and
enter the Krebs cycle. Other cells are able to use
ketones as an energy source, but they do so
slowly. When ketones are produced in small
amounts, as they usually are between meals, the
blood level does not rise sharply.
A state of ketosis exists when fats and proteins
become the primary energy sources, and
ketones accumulate in the blood faster than cells
can utilize them. Because ketones are organic
acids, they lower the pH of the blood. As the
blood ketone level rises, the kidneys excrete
ketones, but they must also excrete more water
as a solvent, which leads to dehydration.
Ketosis is clinically important in diabetes mellitus,
starvation, and eating disorders such as
anorexia nervosa. Diabetics whose disease is
poorly controlled may progress to ketoacidosis,
a form of metabolic acidosis that may lead to
confusion, coma, and death. Reversal of this
state requires a carbohydrate energy source and
the insulin necessary to utilize it.
clotting; albumin to maintain blood volume; pepsin
and amylase for digestion; growth hormone and
insulin; and the thousands of enzymes needed to catalyze
reactions within the body.
The amino acids we obtain from the proteins in
our food are used by our cells to synthesize all of these
proteins in the amounts needed by the body. Only
when the body’s needs for new proteins have been
met are amino acids used for energy production. But
notice in Fig. 17–4 what happens to excess amino
acids; they will be deaminated and converted to simple
carbohydrates and contribute to glycogen storage
or they may be changed to fat and stored in adipose
tissue.
Body Temperature and Metabolism 407
Proteins
(enzymes, structural)
Non-essential
amino acids
Transamination
Proteins
(Digestion)
Amino
acids
NH2
C
C
NH2
NH2
C
C
C
C
C
C
C C C C C C
C C C
C C C C C C C C
C C C C C C C C
(Digestion) (Digestion)
Glucose
Glycerol
Fatty acid
Carbohydrates Fats
Phospholipids
(cell membranes)
Pentose sugars
Excess
(deamination)
Excess
Glycogen
True fats
(adipose tissue)
Cholesterol
and other
steroids
Figure 17–4. Synthesis uses of foods. See text for description.
QUESTION: Excess amino acids can be used to synthesize carbohydrates or fats. Can any
other food be used to synthesize proteins?
Fatty Acids and Glycerol
The end products of fat digestion that are not needed
immediately for energy production may be stored as
fat (triglycerides) in adipose tissue. Most adipose tissue
is found subcutaneously and is potential energy for
times when food intake decreases. Notice in Table
17–4 that insulin promotes fat synthesis and storage.
One theory of weight gain proposes that a diet high in
sugars and starches stimulates the secretion of so
much insulin that fat can only be stored, not taken out
of storage and used for energy.
Fatty acids and glycerol are also used for the synthesis
of phospholipids, which are essential components
of all cell membranes. Myelin, for example, is a
phospholipid of the membranes of Schwann cells,
which form the myelin sheath of peripheral neurons.
The liver can synthesize most of the fatty acids
needed by the body. Two exceptions are linoleic acid
and linolenic acid, which are essential fatty acids and
must be obtained from the diet. Linoleic acid is part of
lecithin, which in turn is part of all cell membranes.
Vegetable oils are good sources of these essential fatty
acids.
When fatty acids are broken down in the process of
beta-oxidation, the resulting acetyl groups may also be
used for the synthesis of cholesterol, a steroid. This
takes place primarily in the liver, although all cells are
capable of synthesizing cholesterol for their cell membranes.
The liver uses cholesterol to synthesize bile
salts for the emulsification of fats in digestion. The
steroid hormones are also synthesized from cholesterol.
Cortisol and aldosterone are produced by the
adrenal cortex, estrogen and progesterone by the
ovaries, and testosterone by the testes.
VITAMINS AND MINERALS
Vitamins are organic molecules needed in very small
amounts for normal body functioning. Some vitamins
are coenzymes; that is, they are necessary for the
functioning of certain enzymes. Others are antioxidant
vitamins, including vitamins C, E, and betacarotene
(a precursor for vitamin A). Antioxidants
prevent damage from free radicals, which are molecules
that contain an unpaired electron and are highly
reactive. The reactions of free radicals can damage
DNA, cell membranes, and the cell organelles. Free
radicals are formed during some normal body reactions,
but smoking and exposure to pollution will
increase their formation. Antioxidant vitamins combine
with free radicals before they can react with cellular
components. Plant foods are good sources of
these vitamins. Table 17–5 summarizes some important
metabolic and nutritional aspects of the vitamins
we need.
Deficiencies of vitamins often result in disease:
vitamin C deficiency and scurvy, for example (see Box
4–2). Other deficiency diseases that have been known
for decades include pellagra (lack of niacin), beri-beri
(riboflavin), pernicious anemia (B12), and rickets (D).
More recently the importance of folic acid (folacin)
for the development of the fetal central nervous system
has been recognized. Adequate folic acid during
pregnancy can significantly decrease the chance of
spina bifida (open spinal column) and anencephaly
(absence of the cerebrum, always fatal) in a fetus. All
women should be aware of the need for extra (400
micrograms) folic acid during pregnancy.
Minerals are simple inorganic chemicals and have
a variety of functions, many of which you are already
familiar with. Table 17–6 lists some important aspects
of minerals. We will return to the minerals as part of
our study of fluid–electrolyte balance in Chapter 19.
METABOLIC RATE
Although the term metabolism is used to describe all
of the chemical reactions that take place within the
body, metabolic rate is usually expressed as an
amount of heat production. This is because many
body processes that utilize ATP also produce heat.
These processes include the contraction of skeletal
muscle, the pumping of the heart, and the normal
breakdown of cellular components. Therefore, it is
possible to quantify heat production as a measure of
metabolic activity.
As mentioned previously, the energy available from
food is measured in kilocalories (kcal). Kilocalories are
also the units used to measure the energy expended by
the body. During sleep, for example, energy expended
by a 150-pound person is about 60 to 70 kcal per hour.
Getting up and preparing breakfast increases energy
expenditure to 80 to 90 kcal per hour. For mothers
with several small children, this value may be significantly
higher. Clearly, greater activity results in
greater energy expenditure.
The energy required for merely living (lying quietly
in bed) is the basal metabolic rate (BMR). See
Box 17–4: Metabolic Rate for a formula to estimate
408 Body Temperature and Metabolism
409
Table 17–5 VITAMINS
Vitamin Functions Food Sources Comment
Water Soluble
Thiamine (B1)
Riboflavin (B2)
Niacin (nicotinamide)
Pyridoxine (B6)
B12 (cyanocobalamin)
Biotin
Folic acid (folacin)
Pantothenic acid
Vitamin C (ascorbic
acid)
Fat Soluble
Vitamin A
Vitamin D
Vitamin E
Vitamin K
• Conversion of pyruvic acid to acetyl
CoA in cell respiration
• Synthesis of pentose sugars
• Synthesis of acetylcholine
• Part of FAD in cell respiration
• Part of NAD in cell respiration
• Metabolism of fat for energy
• Part of enzymes needed for amino
acid metabolism and protein synthesis,
nucleic acid synthesis, synthesis of
antibodies
• Synthesis of DNA, especially in RBC
production
• Metabolism of amino acids for energy
• Synthesis of nucleic acids
• Metabolism of fatty acids and amino
acids
• Synthesis of DNA, especially in blood
cell production
• Contributes to development of fetal
CNS
• Part of coenzyme A in cell respiration,
use of amino acids and fats for energy
• Synthesis of collagen, especially for
wound healing
• Metabolism of amino acids
• Absorption of iron
• An antioxidant—prevents cellular
damage from free radicals
• Synthesis of rhodopsin
• Calcification of growing bones
• Maintenance of epithelial tissues
• Absorption of calcium and phosphorus
in the small intestine
• Contributes to immune responses,
action of insulin, and preservation of
muscle mass and strength
• An antioxidant—prevents destruction
of cell membranes
• Contributes to wound healing and
detoxifying ability of the liver
• Synthesis of prothrombin and
other clotting factors
• Meat, eggs,
legumes, green
leafy vegetables,
grains
• Meat, milk, cheese,
grains
• Meat, fish, grains,
legumes
• Meat, fish, grains,
yeast, yogurt
• Liver, meat, fish,
eggs, milk, cheese
• Yeast, liver, eggs
• Liver, grains,
legumes, leafy
green vegetables
• Meat, fish, grains,
legumes, vegetables
• Citrus fruits, tomatoes,
potatoes
• Yellow and green
vegetables, liver,
milk, eggs
• Fortified milk, egg
yolks, fish liver oils
• Nuts, wheat germ,
seed oils
• Liver, spinach,
cabbage
Rapidly destroyed by heat
Small amounts produced
by GI bacteria
Small amounts produced
by GI bacteria
Contains cobalt; intrinsic
factor required for
absorption
Small amounts produced
by GI bacteria
Small amounts produced
by GI bacteria
Small amounts produced
by GI bacteria
Rapidly destroyed by
heat
Stored in liver; bile salts
required for absorption
Produced in skin exposed
to UV rays; stored
in liver; bile salts
required for absorption
Stored in liver and adipose
tissue; bile salts
required for absorption
Large amounts produced
by GI bacteria; bile salts
required for absorption;
stored in liver
your own metabolic rate. A number of factors affect
the metabolic rate of an active person:
1. Exercise—Contraction of skeletal muscle increases
energy expenditure and raises metabolic rate (see
Box 17–5: Weight Loss).
2. Age—Metabolic rate is highest in young children
and decreases with age. The energy requirements
for growth and the greater heat loss by a
smaller body contribute to the higher rate in children.
After growth has stopped, metabolic rate
decreases about 2% per decade. If a person
410 Body Temperature and Metabolism
Table 17–6 MINERALS
Mineral Functions Food Sources Comment
Calcium
Phosphorus
Sodium
Potassium
Chlorine
Iron
Iodine
Sulfur
Magnesium
Manganese
Copper
Cobalt
Zinc
• Formation of bones and teeth
• Neuron and muscle functioning
• Blood clotting
• Formation of bones and teeth
• Part of DNA, RNA, and ATP
• Part of phosphate buffer system
• Contributes to osmotic pressure
of body fluids
• Nerve impulse transmission and
muscle contraction
• Part of bicarbonate buffer system
• Contributes to osmotic pressure of
body fluids
• Nerve impulse transmission and
muscle contraction
• Contributes to osmotic pressure of
body fluids
• Part of HCI in gastric juice
• Part of hemoglobin and myoglobin
• Part of some cytochromes in cell
respiration
• Part of thyroxine and T3
• Part of some amino acids
• Part of thiamine and biotin
• Formation of bone
• Metabolism of ATP–ADP
• Formation of urea
• Synthesis of fatty acids and cholesterol
• Synthesis of hemoglobin
• Part of some cytochromes in cell
respiration
• Synthesis of melanin
• Part of vitamin B12
• Part of carbonic anhydrase needed
for CO2 transport
• Part of peptidases needed for protein
digestion
• Necessary for normal taste sensation
• Involved in wound healing
• Milk, cheese, yogurt,
shellfish, leafy green
vegetables
• Milk, cheese, fish, meat
• Table salt, almost all
foods
• Virtually all foods
• Table salt
• Meat, shellfish, dried
apricots, legumes, eggs
• Iodized salt, seafood
• Meat, eggs
• Green vegetables,
legumes, seafood, milk
• Legumes, grains, nuts,
leafy green vegetables
• Liver, seafood, grains,
nuts, legumes
• Liver, meat, fish
• Meat, seafood, grains,
legumes
Vitamin D required for
absorption; stored in
bones
Vitamin D required for
absorption; stored in
bones
Most abundant cation ( )
in extracellular fluid
Most abundant cation ( )
in intracellular fluid
Most abundant anion ( )
in extracellular fluid
Stored in liver
Insulin and keratin require
sulfur
Part of chlorophyll in green
plants
Some stored in liver
Stored in liver
Vitamin B12 stored in liver
Body Temperature and Metabolism 411
BOX 17–4 METABOLIC RATE
Example: A 160-pound man:
1. 160 lb at 2.2 lb/kg = 73 kg
2. 73 kg x 1.0 kcal/kg/hr = 73 kcal/hr
3. 73 kcal/hr x 24 = 1752 kcal/day
To approximate the amount of energy actually
expended during an average day (24 hours), the
following percentages may be used:
Sedentary activity: add 40% to 50% of the BMR to
the BMR
Light activity: add 50% to 65% of the BMR to the
BMR
Moderate activity: add 65% to 75% of the BMR to
the BMR
Strenuous activity: add 75% to 100% of the BMR to
the BMR
Using our example of the 120-pound woman
with a BMR of 1200 kcal/day:
Sedentary: 1680 to 1800 kcal/day
Light: 1800 to 1980 kcal/day
Moderate: 1980 to 2100 kcal/day
Strenuous: 2100 to 2400 kcal/day
To estimate your own basal metabolic rate
(BMR), calculate kilocalories (kcal) used per hour as
follows:
For women: use the factor of 0.9 kcal per kilogram
(kg) of body weight
For men: use the factor of 1.0 kcal per kg of body
weight
Then multiply kcal/hour by 24 hours to determine
kcal per day.
Example: A 120-pound woman:
1. Change pounds to kilograms:
120 lb at 2.2 lb/kg = 55 kg
2. Multiply kg weight by the BMR factor:
55 kg x 0.9 kcal/kg/hr = 49.5 kcal/hr
3. Multiply kcal/hr by 24:
49.5 kcal/hr x 24 = 1188 kcal/day
(An approximate BMR, about 1200
kcal/day)
BOX 17–5 WEIGHT LOSS
food. Keeping track of daily caloric intake is an
important part of a decision to try to lose weight. It
is also important to remember that sustained loss of
fat usually does not exceed 1 to 2 pounds per week.
In part this is so because as calorie intake decreases,
the metabolic rate decreases. There will also be loss
of some body protein so that amino acids can be
converted to carbohydrates to supply the brain.
A sensible weight-loss diet will include carbohydrate
to supply energy needs, will have sufficient
protein (40 to 45 grams per day), and will be low in
animal fat. Including vegetables and fruits will supply
vitamins, minerals, and fiber.
Although diet books are often found on the bestseller
lists, there is no magic method that will result
in weight loss. Losing weight depends on one simple
fact: calorie expenditure in activity must exceed
calorie intake in food (the term calorie here will be
used to mean kilocalorie).
To lose 1 pound of body fat, which consists of fat,
water, and protein, 3500 calories of energy must be
expended. Although any form of exercise requires
calories, the more strenuous the exercise, the more
calories expended. Some examples are shown in the
accompanying table.
Most food packaging contains nutritional information,
including the calories per serving of the
Calories per 10 Calories per 10
minutes (average minutes (average
Activity for a 150-lb person) Activity for a 150-lb person)
Walking slowly
Walking briskly
Walking up stairs
Dancing (slow)
Dancing (fast)
30
45
170
40
65
Running (8 mph)
Cycling (10 mph)
Cycling (15 mph)
Swimming
120
70
115
100
becomes less active, the total decrease is almost 5%
per decade.
3. Body configuration of adults—Tall, thin people
usually have higher metabolic rates than do short,
stocky people of the same weight. This is so because
the tall, thin person has a larger surface area (proportional
to weight) through which heat is continuously
lost. The metabolic rate, therefore, is
slightly higher to compensate for the greater heat
loss. The variance of surface-to-weight ratios for
different body configurations is illustrated in
Fig. 17–5.
4. Sex hormones—Testosterone increases metabolic
activity to a greater degree than does estrogen, giving
men a slightly higher metabolic rate than
women. Also, men tend to have more muscle, an
active tissue, whereas women tend to have more
fat, a relatively inactive tissue.
5. Sympathetic stimulation—In stress situations, the
metabolism of many body cells is increased. Also
contributing to this are the hormones epinephrine
and norepinephrine. As a result, metabolic rate
increases.
6. Decreased food intake—If the intake of food
decreases for a prolonged period of time, metabolic
rate also begins to decrease. It is as if the body’s
metabolism is “slowing down” to conserve whatever
energy sources may still be available. (See also
Box 17–6: Leptin and Body-Mass Index.)
7. Climate—People who live in cold climates may
have metabolic rates 10% to 20% higher than people
who live in tropical regions. This is believed to
be due to the variations in the secretion of thyroxine,
the hormone most responsible for regulation
of metabolic rate. In a cold climate, the necessity
for greater heat production brings about an
412
Figure 17–5. Surface-to-weight ratios. Imagine that
the three shapes are people who all weigh the same
amount. The “tall, thin person” on the right has about
50% more surface area than does the “short, stocky person”
on the left. The more surface area (where heat is
lost), the higher the metabolic rate.
QUESTION: Which of these ratios best represents an
infant? (Rather than weight, think of inside-outside
proportion.)
Box 17–6 LEPTIN AND BODY-MASS INDEX
directly decreases fat storage in cells, and improves
the efficiency of the pancreatic cells that produce
insulin. What was first believed to be a simple chemical
signal has proved to be much more complex.
A good measure of leanness or fatness is the
body-mass index.
To calculate: Multiple weight in pounds by 703.
Divide by height in inches.
Divide again by height in inches = body-mass
index
Example: A person five foot six weighing 130
pounds.
130 x 703 = 91,390
91,390 66 = 1385
1385 66 = 20.98
The optimal body-mass index is considered to be
21. Any index over 25 is considered overweight.
The 1994 discovery of the hormone leptin was
reported to the general public in 1995, along with
speculation that leptin could become an anti-obesity
medication, which it has not. Leptin is a protein
produced by fat cells, and signals the hypothalamus
to release a chemical that acts as an appetite suppressant.
It seems to inform the brain of how much
stored fat the body has, and is therefore involved in
the regulation of body weight (along with many
other chemicals, some still unknown).
Another likely role for leptin is as a contributor to
the onset of puberty, especially in females. Girls
who are very thin, with little body fat, tend to have
a later first menstrual period than girls with average
body fat, and a certain level of body fat is necessary
for continued ovulation. Leptin may be the chemical
mediator of this information.
The most recent research indicates that leptin
Body Temperature
1. Normal range is 96.5° to 99.5°F (36° to 38°C),
with an average of 98.6°F (37°C).
2. Normal fluctuation in 24 hours is 1° to 2°F.
3. Temperature regulation in infants and the elderly is
not as precise as it is at other ages.
Heat Production
Heat is one of the energy products of cell respiration.
Many factors affect the total heat actually produced
(see Table 17–1).
1. Thyroxine from the thyroid gland—the most important
regulator of daily heat production. As metabolic
rate decreases, more thyroxine is secreted to
increase the rate of cell respiration.
2. Stress—sympathetic impulses and epinephrine and
norepinephrine increase the metabolic activity of
many organs, increasing the production of ATP
and heat.
3. Active organs continuously produce heat. Skeletal
muscle tone produces 25% of the total body heat at
rest. The liver provides up to 20% of the resting
body heat.
4. Food intake increases the activity of the digestive
organs and increases heat production.
5. Changes in body temperature affect metabolic rate.
A fever increases the metabolic rate, and more heat
is produced; this may become detrimental during
very high fevers.
Heat Loss (see Table 17–2)
1. Most heat is lost through the skin.
2. Blood flow through the dermis determines the
amount of heat that is lost by radiation, conduction,
and convection.
3. Vasodilation in the dermis increases blood flow and
heat loss; radiation and conduction are effective
only if the environment is cooler than the body.
4. Vasoconstriction in the dermis decreases blood
flow and conserves heat in the core of the body.
5. Sweating is a very effective heat loss mechanism;
excess body heat evaporates sweat on the skin surface;
sweating is most effective when the atmospheric
humidity is low.
6. Sweating also has a disadvantage in that water is
lost and must be replaced to prevent serious dehydration.
7. Heat is lost from the respiratory tract by the evaporation
of water from the warm respiratory
mucosa; water vapor is part of exhaled air.
8. A very small amount of heat is lost as urine and
feces are excreted at body temperature.
Body Temperature and Metabolism 413
STUDY OUTLINE
increased secretion of thyroxine and a higher metabolic
rate.
AGING AND METABOLISM
As mentioned in the previous section, metabolic rate
decreases with age. Elderly people who remain active,
however, can easily maintain a metabolic rate (energy
production) adequate for their needs as long as their
general health is good. Some elderly people subject to
physical or emotional disability, however, may be at
risk for malnutrition. Caregivers may assess such a risk
by asking how often the person eats every day; if
appetite is good, fair, or poor; and how the food tastes.
These simple questions may help ensure adequate
nutrition.
Sensitivity to external temperature changes may
decrease with age, and the regulation of body temperature
is no longer as precise. Sweat glands are not as
active, and prolonged high environmental temperatures
are a real danger for elderly people. In August
2003, in Europe, an unusually long and severe heat
wave was the cause of at least 25,000 deaths. Most of
those who died were elderly.
SUMMARY
Food is needed for the synthesis of new cells and tissues,
or is utilized to produce the energy required for
such synthesis reactions. As a consequence of metabolism,
heat energy is released to provide a constant
body temperature and permit the continuation of
metabolic activity. The metabolic pathways described
in this chapter are only a small portion of the body’s
total metabolism. Even this simple presentation, however,
suggests the great chemical complexity of the
functioning human being.
Regulation of Heat Loss
1. The hypothalamus is the thermostat of the body
and regulates body temperature by balancing heat
production and heat loss.
2. The hypothalamus receives information from its
own neurons (blood temperature) and from the
temperature receptors in the dermis.
3. Mechanisms to increase heat loss are vasodilation
in the dermis and increased sweating. Decreased
muscle tone will decrease heat production.
4. Mechanisms to conserve heat are vasoconstriction
in the dermis and decreased sweating. Increased
muscle tone (shivering) will increase heat production.
Fever—an abnormally elevated body temperature
1. Pyrogens are substances that cause a fever: bacteria,
foreign proteins, or chemicals released during
inflammation (endogenous pyrogens).
2. Pyrogens raise the setting of the hypothalamic
thermostat; the person feels cold and begins to
shiver to produce heat.
3. When the pyrogen has been eliminated, the hypothalamic
setting returns to normal; the person feels
warm, and sweating begins to lose heat to lower the
body temperature.
4. A low fever may be beneficial because it increases
the activity of WBCs and inhibits the activity of
some pathogens.
5. A high fever may be detrimental because enzymes
are denatured at high temperatures. This is most
critical in the brain, where cells that die cannot be
replaced.
Metabolism—all the reactions within the
body
1. Anabolism—synthesis reactions that usually
require energy in the form of ATP.
2. Catabolism—decomposition reactions that often
release energy in the form of ATP.
3. Enzymes catalyze most anabolic and catabolic reactions.
Cell Respiration—the breakdown of food
molecules to release their potential energy
and synthesize ATP (Fig. 17–3)
1. Glucose oxygen yields CO2 H2O ATP
heat.
2. The breakdown of glucose involves three stages:
glycolysis, the Krebs cycle, and the cytochrome
(electron) transport system (see also Table 17–3).
3. The oxygen necessary comes from breathing.
4. The water formed becomes part of intracellular
fluid; CO2 is exhaled; ATP is used for energyrequiring
reactions; heat provides a body temperature.
Proteins and Fats—as energy sources (see
Table 17–4 for hormonal regulation)
1. Excess amino acids are deaminated in the liver and
converted to pyruvic acid or acetyl groups to enter
the Krebs cycle. Amino acids may also be converted
to glucose to supply the brain (Fig. 17–3).
2. Glycerol is converted to pyruvic acid to enter the
Krebs cycle.
3. Fatty acids, in the process of beta-oxidation in the
liver, are split into acetyl groups to enter the Krebs
cycle; ketones are formed for transport to other
cells (see Fig. 17–3).
Energy Available from Food
1. Energy is measured in kilocalories (Calories):
kcal.
2. There are 4 kcal per gram of carbohydrate, 4 kcal
per gram of protein, 9 kcal per gram of fat. With
reference to food, kilocalories may be called calories.
Synthesis Uses of Foods (Fig. 17–4)
1. Glucose—used to synthesize the pentose sugars for
DNA and RNA; used to synthesize glycogen to
store energy in liver and muscles.
2. Amino acids—used to synthesize new proteins and
the non-essential amino acids; essential amino
acids must be obtained in the diet.
3. Fatty acids and glycerol—used to synthesize phospholipids
for cell membranes, triglycerides for fat
storage in adipose tissue, and cholesterol and other
steroids; essential fatty acids must be obtained in
the diet.
4. Any food eaten in excess will be changed to fat and
stored.
5. Vitamins and minerals—see Tables 17–5 and 17–6.
Metabolic Rate—heat production by the
body; measured in kcal
1. Basal metabolic rate (BMR) is the energy required
to maintain life (see Box 17–4); several factors
influence the metabolic rate of an active person.
414 Body Temperature and Metabolism
2. Age—metabolic rate is highest in young children
and decreases with age.
3. Body configuration—more surface area proportional
to weight (tall and thin) means a higher
metabolic rate.
4. Sex hormones—men usually have a higher metabolic
rate than do women; men have more muscle
proportional to fat than do women.
5. Sympathetic stimulation—metabolic activity increases
in stress situations.
6. Decreased food intake—metabolic rate decreases
to conserve available energy sources.
7. Climate—people who live in cold climates usually
have higher metabolic rates because of a greater
need for heat production.
Body Temperature and Metabolism 415
REVIEW QUESTIONS
1. State the normal range of human body temperature
in °F and °C. (p. 396)
2. State the summary equation of cell respiration, and
state what happens to (or the purpose of) each of
the products. (p. 401)
3. Describe the role of each in heat production: thyroxine,
skeletal muscles, stress situations, and the
liver. (p. 396)
4. Describe the two mechanisms of heat loss through
the skin, and explain the role of blood flow.
Describe how heat is lost through the respiratory
tract. (pp. 397–398)
5. Explain the circumstances that exist when sweating
and vasodilation in the dermis are not effective
mechanisms of heat loss. (p. 397)
6. Name the part of the brain that regulates body
temperature, and explain what is meant by a thermostat.
(p. 398)
7. Describe the responses by the body to a warm environment
and to a cold environment. (pp. 399)
8. Explain how pyrogens are believed to cause a fever,
and give two examples of pyrogens. (p. 399)
9. Define metabolism, anabolism, catabolism, kilocalorie,
and metabolic rate. (pp. 400, 401, 405,
408)
10. Name the three stages of the cell respiration
of glucose and state where in the cell each takes
place and whether or not oxygen is required.
(pp. 403)
11. For each, state the molecules that enter the
process and the results of the process: glycolysis,
Krebs cycle, and cytochrome transport system.
(pp. 403–405)
12. Explain how fatty acids, glycerol, and excess
amino acids are used for energy production in cell
respiration. (pp. 403, 405)
13. Describe the synthesis uses for glucose, amino
acids, and fatty acids. (pp. 406–408)
14. Describe four factors that affect the metabolic
rate of an active person. (pp. 410, 412)
15. If lunch consists of 60 grams of carbohydrate,
15 grams of protein, and 10 grams of fat, how
many kilocalories are provided by this meal?
(p. 405)
FOR FURTHER THOUGHT
1. For many people, iceberg lettuce is the vegetable
eaten most often. What does lettuce provide? What
does lettuce lack, compared to vegetables such as
broccoli?
2. Fourteen-year-old Donna has just decided that eating
meat is “gross,” and that she will be a vegetarian.
What difficulties are there with such a diet;
that is, what nutrients may be lacking? How may
they be obtained?
3. Studies with animals have shown that caloric
restriction may prolong life by protecting the brain
from some effects of aging. The animals’ diet was
about half the usual calories they would consume.
For people, 1250 to 1500 calories per day would be
restrictive (compared to the 2000 calories or more
that many of us in North America consume).
Would it be worth it for a life span of 110 years?
Describe the problems with such a diet.
4. Every summer small children are left alone in cars
“just for a few minutes,” while a parent does an
errand. The result may be tragic—severe brain
damage or death of the child from heat stroke.
Explain why small children are so susceptible to
heat.
5. Remember the Titanic, which sank in April of
1912? There were not enough lifeboats for everyone,
and many people were in the water of the
North Atlantic. They did have life jackets, and did
not drown, but many were dead within half an
hour. Explain why.
6. An elderly person and a quadriplegic person may
each have difficulties during cold weather. Explain
how the problem is a little different for each.
416 Body Temperature and Metabolism
CHAPTER 18
The Urinary System
417
418
CHAPTER 18
Chapter Outline
Kidneys
Internal Structure of the Kidney
The Nephron
Renal corpuscle
Renal tubule
Blood Vessels of the Kidney
Formation of Urine
Glomerular Filtration
Tubular Reabsorption
Mechanisms of reabsorption
Tubular Secretion
Hormones That Influence Reabsorption of Water
Summary of Urine Formation
The Kidneys and Acid–Base Balance
Other Functions of the Kidneys
Elimination of Urine
Ureters
Urinary Bladder
Urethra
The Urination Reflex
Characteristics of Urine
Aging and the Urinary System
BOX 18–1 FLOATING KIDNEY
BOX 18–2 RENAL FAILURE AND HEMODIALYSIS
BOX 18–3 ERYTHROPOIETIN
BOX 18–4 KIDNEY STONES
BOX 18–5 BLOOD TESTS AND KIDNEY FUNCTION
BOX 18–6 URINARY TRACT INFECTIONS
Student Objectives
• Describe the location and general function of each
organ of the urinary system.
• Name the parts of a nephron and the important
blood vessels associated with them.
• Explain how the following are involved in urine
formation: glomerular filtration, tubular reabsorption,
tubular secretion, and blood flow through the
kidney.
• Describe the mechanisms of tubular reabsorption,
and explain the importance of tubular secretion.
• Describe how the kidneys help maintain normal
blood volume and blood pressure.
• Name and state the functions of the hormones
that affect the kidneys.
• Describe how the kidneys help maintain normal
pH of blood and tissue fluid.
• Describe the urination reflex, and explain how voluntary
control is possible.
• Describe the characteristics of normal urine.
The Urinary System
419
New Terminology
Bowman’s capsule (BOW-manz KAP-suhl)
Detrusor muscle (de-TROO-ser)
External urethral sphincter (yoo-REE-thruhl
SFINK-ter)
Glomerular filtration rate (gloh-MER-yoo-ler fill-
TRAY-shun RAYT)
Glomerulus (gloh-MER-yoo-lus)
Internal urethral sphincter (yoo-REE-thruhl
SFINK-ter)
Juxtaglomerular cells ( JUKS-tah-gloh-MER-yoo-ler
SELLS)
Micturition (MIK-tyoo-RISH-un)
Nephron (NEFF-ron)
Nitrogenous wastes (nigh-TRAH-jen-us)
Peritubular capillaries (PER-ee-TOO-byoo-ler)
Renal corpuscle (REE-nuhl KOR-pus’l)
Renal filtrate (REE-nuhl FILL-trayt)
Renal tubule (REE-nuhl TOO-byoo’l)
Retroperitoneal (RE-troh-PER-i-toh-NEE-uhl)
Specific gravity (spe-SIF-ik GRA-vi-tee)
Threshold level (THRESH-hold LE-vuhl)
Trigone (TRY-gohn)
Ureter (YOOR-uh-ter)
Urethra (yoo-REE-thrah)
Urinary bladder (YOOR-i-NAR-ee BLA-der)
Related Clinical Terminology
Cystitis (sis-TIGH-tis)
Dysuria (dis-YOO-ree-ah)
Hemodialysis (HEE-moh-dye-AL-i-sis)
Nephritis (ne-FRY-tis)
Oliguria (AH-li-GYOO-ree-ah)
Polyuria (PAH-li-YOO-ree-ah)
Renal calculi (REE-nuhl KAL-kew-lye)
Renal failure (REE-nuhl FAYL-yer)
Uremia (yoo-REE-me-ah)
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
The first successful human organ transplant was a
kidney transplant performed in 1953. Because the
donor and recipient were identical twins, rejection was
not a problem. Thousands of kidney transplants have
been performed since then, and the development of
immunosuppressive medications has permitted many
people to live normal lives with donated kidneys.
Although a person usually has two kidneys, one kidney
is sufficient to carry out the complex work required to
maintain homeostasis of the body fluids.
The urinary system consists of two kidneys, two
ureters, the urinary bladder, and the urethra (Fig.
18–1). The formation of urine is the function of the
kidneys, and the rest of the system is responsible for
eliminating the urine.
Body cells produce waste products such as urea,
420 The Urinary System
Diaphragm
Right kidney
Psoas major muscle
lliacus muscle
Right ureter
Urinary bladder
Urethra
Symphysis pubis
Trigone of bladder
Opening of ureter
Sacrum
Pelvis
Lumbar vertebra
Left common iliac
artery and vein
Left ureter
Left kidney
Left renal artery and vein
Superior mesenteric artery
Left adrenal gland
Inferior vena cava
Aorta
Ribs
Figure 18–1. The urinary system shown in anterior view.
QUESTION: Why is blood pressure relatively high in the kidneys? What do you see that
would suggest this?
creatinine, and ammonia, which must be removed
from the blood before they accumulate to toxic levels.
As the kidneys form urine to excrete these waste products,
they also accomplish several other important
functions:
1. Regulation of the volume of blood by excretion or
conservation of water
2. Regulation of the electrolyte content of the blood
by the excretion or conservation of minerals
3. Regulation of the acid–base balance of the blood by
excretion or conservation of ions such as H ions
or HCO3
ions
4. Regulation of all of the above in tissue fluid
The process of urine formation, therefore, helps
maintain the normal composition, volume, and pH of
both blood and tissue fluid by removing those substances
that would upset the normal constancy and
balance of these extracellular fluids.
KIDNEYS
The two kidneys are located in the upper abdominal
cavity on either side of the vertebral column, behind
the peritoneum (retroperitoneal). The upper portions
of the kidneys rest on the lower surface of the
diaphragm and are enclosed and protected by the
lower rib cage (see Fig. 18–1). The kidneys are
embedded in adipose tissue that acts as a cushion and
is in turn covered by a fibrous connective tissue membrane
called the renal fascia, which helps hold the
kidneys in place (see Box 18–1: Floating Kidney).
Each kidney has an indentation called the hilus
on its medial side. At the hilus, the renal artery enters
the kidney, and the renal vein and ureter emerge. The
renal artery is a branch of the abdominal aorta, and the
renal vein returns blood to the inferior vena cava (see
Fig. 18–1). The ureter carries urine from the kidney to
the urinary bladder.
INTERNAL STRUCTURE
OF THE KIDNEY
In a coronal or frontal section of the kidney, three
areas can be distinguished (Fig. 18–2). The lateral and
middle areas are tissue layers, and the medial area at
the hilus is a cavity. The outer tissue layer is called the
renal cortex; it is made of renal corpuscles and convoluted
tubules. These are parts of the nephron and are
described in the next section. The inner tissue layer is
the renal medulla, which is made of loops of Henle
and collecting tubules (also parts of the nephron). The
renal medulla consists of wedge-shaped pieces called
renal pyramids. The tip of each pyramid is its apex or
papilla.
The third area is the renal pelvis; this is not a layer
of tissues, but rather a cavity formed by the expansion
of the ureter within the kidney at the hilus. Funnelshaped
extensions of the renal pelvis, called calyces
(singular: calyx), enclose the papillae of the renal pyramids.
Urine flows from the renal pyramids into the
calyces, then to the renal pelvis and out into the ureter.
THE NEPHRON
The nephron is the structural and functional unit of
the kidney. Each kidney contains approximately 1 million
nephrons. It is in the nephrons, with their associated
blood vessels, that urine is formed. Each nephron
has two major portions: a renal corpuscle and a renal
tubule. Each of these major parts has further subdivisions,
which are shown with their blood vessels in Fig.
18–3.
Renal Corpuscle
A renal corpuscle consists of a glomerulus surrounded
by a Bowman’s capsule. The glomerulus is a capillary
network that arises from an afferent arteriole
and empties into an efferent arteriole. The diameter
The Urinary System 421
BOX 18–1 FLOATING KIDNEY
A floating kidney is one that has moved out of its
normal position. This may happen in very thin
people whose renal cushion of adipose tissue is
thin, or it may be the result of a sharp blow to
the back that dislodges a kidney.
A kidney can function in any position; the
problem with a floating kidney is that the ureter
may become twisted or kinked. If urine cannot
flow through the ureter, the urine backs up and
collects in the renal pelvis. Incoming urine
from the renal tubules then backs up as well. If
the renal filtrate cannot flow out of Bowman’s
capsules, the pressure within Bowman’s capsules
increases, opposing the blood pressure in the
glomeruli. Glomerular filtration then cannot take
place efficiently. If uncorrected, this may lead to
permanent kidney damage.
422 The Urinary System
Renal corpuscle
Renal cortex
Renal tubule
Renal medulla
Papillary duct
B
D
Papilla of pyramid
Calyx
Renal pelvis
Renal artery
Renal vein
Interlobar
artery
Arcuate artery
Ureter
A
C
Nephron
Renal cortex
Renal medulla
(pyramids)
Figure 18–2. (A) Frontal section of the right kidney showing internal structure and
blood vessels. (B) The magnified section of the kidney shows several nephrons. (C) Vascular
cast of a kidney in lateral view. Red plastic fills the blood vessels. (D) Vascular cast in medial
view. Blood vessels have been removed; yellow plastic fills the renal pelvis. (Photographs C
and D by Dan Kaufman.)
QUESTION: Which main parts of a nephron are found in the renal cortex? Which areas of
a kidney have many blood vessels?
423
Bowman's capsule
(inner layer)
Bowman's capsule
(outer layer)
Efferent arteriole
Juxtaglomerular
cells
Afferent arteriole
Collecting tubule
Distal
convoluted tubule
Podocyte
Glomerulus
Proximal convoluted
tubule
Loop of Henle
Peritubular capillaries
Renal
cortex
Renal
medulla
Figure 18–3. A nephron with its associated blood vessels. Portions of the nephron have
been magnified. The arrows indicate the direction of blood flow and flow of renal filtrate.
See text for description.
QUESTION: How does the shape of a podocyte contribute to its function? How is the lining
of the proximal convoluted tubule specialized?
of the efferent arteriole is smaller than that of the
afferent arteriole, which helps maintain a fairly high
blood pressure in the glomerulus.
Bowman’s capsule (or glomerular capsule) is the
expanded end of a renal tubule; it encloses the
glomerulus. The inner layer of Bowman’s capsule is
made of podocytes; the name means “foot cells,” and
the “feet” of the podocytes are on the surface of the
glomerular capillaries. The arrangement of podocytes
creates pores, spaces between adjacent “feet,” which
make this layer very permeable. The outer layer of
Bowman’s capsule has no pores and is not permeable.
The space between the inner and outer layers of
Bowman’s capsule contains renal filtrate, the fluid that
is formed from the blood in the glomerulus and will
eventually become urine.
Renal Tubule
The renal tubule continues from Bowman’s capsule
and consists of the following parts: proximal convoluted
tubule (in the renal cortex), loop of Henle (or
loop of the nephron, in the renal medulla), and distal
convoluted tubule (in the renal cortex). The distal
convoluted tubules from several nephrons empty into
a collecting tubule. Several collecting tubules then
unite to form a papillary duct that empties urine into
a calyx of the renal pelvis.
Cross-sections of the parts of the renal tubule are
shown in Fig. 18–3. Notice how thin the walls of the
tubule are, and also the microvilli in the proximal convoluted
tubule. These anatomic characteristics provide
for efficient exchanges of materials, as you will see.
All parts of the renal tubule are surrounded by
peritubular capillaries, which arise from the efferent
arteriole. The peritubular capillaries will receive the
materials reabsorbed by the renal tubules; this is
described in the section on urine formation.
BLOOD VESSELS OF THE KIDNEY
The pathway of blood flow through the kidney is an
essential part of the process of urine formation. Blood
from the abdominal aorta enters the renal artery,
which branches extensively within the kidney into
smaller arteries (see Fig. 18–2). The smallest arteries
give rise to afferent arterioles in the renal cortex (see
Fig. 18–3). From the afferent arterioles, blood flows
into the glomeruli (capillaries), to efferent arterioles,
to peritubular capillaries, to veins within the kidney, to
the renal vein, and finally to the inferior vena cava.
Notice that in this pathway there are two sets of capillaries,
and recall that it is in capillaries that exchanges
take place between the blood and surrounding tissues.
Therefore, in the kidneys there are two sites of exchange.
The exchanges that take place between the
nephrons and the capillaries of the kidneys will form
urine from blood plasma.
Figure 18–2 shows two views of a vascular cast of a
kidney; the shape of the blood vessels has been preserved
in red plastic. You can see how dense the vasculature
of a kidney is, and most of these vessels are
capillaries.
FORMATION OF URINE
The formation of urine involves three major processes.
The first is glomerular filtration, which takes
place in the renal corpuscles. The second and third are
tubular reabsorption and tubular secretion, which take
place in the renal tubules.
GLOMERULAR FILTRATION
You may recall that filtration is the process in which
blood pressure forces plasma and dissolved material
out of capillaries. In glomerular filtration, blood
pressure forces plasma, dissolved substances, and small
proteins out of the glomeruli and into Bowman’s capsules.
This fluid is no longer plasma but is called renal
filtrate.
The blood pressure in the glomeruli, compared
with that in other capillaries, is relatively high, about
60 mmHg. The pressure in Bowman’s capsule is very
low, and its inner, podocyte layer is very permeable, so
that approximately 20% to 25% of the blood that
enters glomeruli becomes renal filtrate in Bowman’s
capsules. The blood cells and larger proteins are too
large to be forced out of the glomeruli, so they remain
in the blood. Waste products are dissolved in blood
plasma, so they pass into the renal filtrate. Useful
materials such as nutrients and minerals are also dissolved
in plasma and are also present in renal filtrate.
Filtration is not selective with respect to usefulness; it
is selective only with respect to size. Therefore, renal
filtrate is very much like blood plasma, except that
there is far less protein and no blood cells are present.
The glomerular filtration rate (GFR) is the
amount of renal filtrate formed by the kidneys in 1
minute, and averages 100 to 125 mL per minute. GFR
424 The Urinary System
may be altered if the rate of blood flow through the
kidney changes. If blood flow increases, the GFR
increases, and more filtrate is formed. If blood flow
decreases (as may happen following a severe hemorrhage),
the GFR decreases, less filtrate is formed, and
urinary output decreases (see Box 18–2: Renal Failure
and Hemodialysis).
TUBULAR REABSORPTION
Tubular reabsorption takes place from the renal
tubules into the peritubular capillaries. In a 24-hour
period, the kidneys form 150 to 180 liters of filtrate,
and normal urinary output in that time is 1 to 2 liters.
Therefore, it becomes apparent that most of the renal
filtrate does not become urine. Approximately 99% of
the filtrate is reabsorbed back into the blood in the
peritubular capillaries. Only about 1% of the filtrate
will enter the renal pelvis as urine.
Most reabsorption and secretion (about 65%) take
place in the proximal convoluted tubules, whose cells
have microvilli that greatly increase their surface area.
The distal convoluted tubules and collecting tubules
are also important sites for the reabsorption of water
(Fig. 18–4).
Mechanisms of Reabsorption
1. Active transport—the cells of the renal tubule use
ATP to transport most of the useful materials from
the filtrate to the blood. These useful materials
include glucose, amino acids, vitamins, and positive
ions.
For many of these substances, the renal tubules
have a threshold level of reabsorption. This means
that there is a limit to how much the tubules can
remove from the filtrate. For example, if the filtrate
level of glucose is normal (reflecting a normal
The Urinary System 425
BOX 18–2 RENAL FAILURE AND HEMODIALYSIS
artificial kidney machine to do what the patient’s
nephrons can no longer do. The patient’s blood is
passed through minute tubes surrounded by fluid
(dialysate) with the same chemical composition as
plasma. Waste products and excess minerals diffuse
out of the patient’s blood into the fluid of the
machine.
Although hemodialysis does prolong life for those
with chronic renal failure, it does not fully take the
place of functioning kidneys. The increasing success
rate of kidney transplants, however, does indeed
provide the possibility of a normal life for people
with chronic renal failure.
Renal failure, the inability of the kidneys to function
properly, may be the result of three general
causes, which may be called prerenal, intrinsic
renal, and postrenal.
“Prerenal” means that the problem is “before”
the kidneys, that is, in the blood flow to the kidneys.
Any condition that decreases blood flow to
the kidneys may result in renal damage and failure.
Examples are severe hemorrhage or very low blood
pressure following a heart attack (MI).
“Intrinsic renal” means that the problem is in the
kidneys themselves. Diabetes and hypertension
damage the blood vessels of the kidneys, and are
the causes of 70% of all cases of end-stage renal
failure. Bacterial infections of the kidneys or exposure
to chemicals (certain antibiotics) may cause
damage to the nephrons. Polycystic kidney disease
is a genetic disorder in which the kidney tubules
dilate and become nonfunctional. Severe damage
may not be apparent until age 40 to 60 years but
may then progress to renal failure.
“Postrenal” means that the problem is “after” the
kidneys, somewhere in the rest of the urinary tract.
Obstruction of urine flow may be caused by kidney
stones, a twisted ureter, or prostatic hypertrophy.
Treatment of renal failure involves correcting the
specific cause, if possible. If not possible, and kidney
damage is permanent, the person is said to have
chronic renal failure. Hemodialysis is the use of an
A
C
B
Box Figure 18–A Causes of renal failure. (A) Prerenal.
(B) Intrinsic renal. (C) Postrenal.
426 The Urinary System
Proximal convoluted tubule
H2O
Glucose
Amino acids
Small proteins
Minerals
Wastes
Bowman's capsule
Glomerulus
Efferent arteriole
Afferent arteriole
Distal
convoluted
tubule
Collecting
tubule
H2O
H+
Ammonia
H2O
Glucose
Amino acids
Small proteins
Minerals
H2O
H2O
Wastes
H+
Na+
K+
Urine
Artery
Vein
Creatinine
Medications
Peritubular
capillaries
Loop of Henle
Glomerular
filtration
Tubular
reabsorption
Tubular secretion
Figure 18–4. Schematic representation of glomerular filtration, tubular reabsorption,
and tubular secretion. The renal tubule has been uncoiled, and the peritubular capillaries
are shown adjacent to the tubule.
QUESTION: Describe tubular secretion; that is, it goes from where to where? What substances
may be secreted?
blood glucose level), the tubules will reabsorb all of
the glucose, and none will be found in the urine.
What happens is this: The number of glucose
transporter molecules in the membranes of the
tubule cells is sufficient to take in the number of
glucose molecules passing by in the filtrate. If,
however, the blood glucose level is above normal,
the amount of glucose in the filtrate will also be
above normal and will exceed the threshold level of
reabsorption. The number of glucose molecules to
be reabsorbed is more than the number of the
transporter molecules available to do so. In this situation,
therefore, some glucose will remain in the
filtrate and be present in urine.
The reabsorption of Ca 2 ions is increased by
parathyroid hormone (PTH). The parathyroid
glands secrete PTH when the blood calcium level
decreases. The reabsorption of Ca 2 ions by the
kidneys is one of the mechanisms by which the
blood calcium level is raised back to normal.
The hormone aldosterone, secreted by the adrenal
cortex, increases the reabsorption of Na ions
and the excretion of K ions. Besides regulating the
blood levels of sodium and potassium, aldosterone
also affects the volume of blood.
2. Passive transport—many of the negative ions that
are returned to the blood are reabsorbed following
the reabsorption of positive ions, because unlike
charges attract.
3. Osmosis—the reabsorption of water follows the
reabsorption of minerals, especially sodium ions.
The hormones that affect reabsorption of water are
discussed in the next section.
4. Pinocytosis—small proteins are too large to be
reabsorbed by active transport. They become
adsorbed to the membranes of the cells of the proximal
convoluted tubules. The cell membrane then
sinks inward and folds around the protein to take it
in (see Fig. 3–3 for depictions of this and the other
transport mechanisms). Normally all proteins in
the filtrate are reabsorbed; none is found in urine.
TUBULAR SECRETION
This mechanism also changes the composition of
urine. In tubular secretion, substances are actively
secreted from the blood in the peritubular capillaries
into the filtrate in the renal tubules. Waste products,
such as ammonia and some creatinine, and the metabolic
products of medications may be secreted into the
filtrate to be eliminated in urine. Hydrogen ions (H )
may be secreted by the tubule cells to help maintain
the normal pH of blood.
HORMONES THAT INFLUENCE
REABSORPTION OF WATER
Aldosterone is secreted by the adrenal cortex in response
to a high blood potassium level, to a low blood
sodium level, or to a decrease in blood pressure. When
aldosterone stimulates the reabsorption of Na ions,
water follows from the filtrate back to the blood. This
helps maintain normal blood volume and blood pressure.
You may recall that the antagonist to aldosterone is
atrial natriuretic peptide (ANP), which is secreted
by the atria of the heart when the atrial walls are
stretched by high blood pressure or greater blood volume.
ANP decreases the reabsorption of Na ions by
the kidneys; these remain in the filtrate, as does water,
and are excreted. By increasing the elimination of
sodium and water, ANP lowers blood volume and
blood pressure.
Antidiuretic hormone (ADH) is released by the
posterior pituitary gland when the amount of water in
the body decreases. Under the influence of ADH, the
distal convoluted tubules and collecting tubules are
able to reabsorb more water from the renal filtrate.
This helps maintain normal blood volume and blood
pressure, and also permits the kidneys to produce
urine that is more concentrated than body fluids.
Producing a concentrated urine is essential to prevent
excessive water loss while still excreting all the substances
that must be eliminated.
If the amount of water in the body increases, however,
the secretion of ADH diminishes, and the kidneys
will reabsorb less water. Urine then becomes
dilute, and water is eliminated until its concentration
in the body returns to normal. This may occur following
ingestion of excessive quantities of fluids.
SUMMARY OF URINE FORMATION
1. The kidneys form urine from blood plasma. Blood
flow through the kidneys is a major factor in determining
urinary output.
2. Glomerular filtration is the first step in urine formation.
Filtration is not selective in terms of usefulness
of materials; it is selective only in terms of
size. High blood pressure in the glomeruli forces
The Urinary System 427
plasma, dissolved materials, and small proteins into
Bowman’s capsules; the fluid is now called renal
filtrate.
3. Tubular reabsorption is selective in terms of usefulness.
Nutrients such as glucose, amino acids, and
vitamins are reabsorbed by active transport and
may have renal threshold levels. Positive ions are
reabsorbed by active transport and negative
ions are reabsorbed most often by passive transport.
Water is reabsorbed by osmosis, and small
proteins are reabsorbed by pinocytosis.
Reabsorption takes place from the filtrate in the
renal tubules to the blood in the peritubular capillaries.
4. Tubular secretion takes place from the blood in the
peritubular capillaries to the filtrate in the renal
tubules and can ensure that wastes such as creatinine
or excess H ions are actively put into the filtrate
to be excreted.
5. Hormones such as aldosterone, ANP, and ADH
influence the reabsorption of water and help maintain
normal blood volume and blood pressure. The
secretion of ADH determines whether a concentrated
or dilute urine will be formed.
6. Waste products remain in the renal filtrate and are
excreted in urine. The effects of hormones on the
kidneys are summarized in Table 18–1 and illustrated
in Fig. 18–5.
THE KIDNEYS AND
ACID–BASE BALANCE
The kidneys are the organs most responsible for maintaining
the pH of blood and tissue fluid within normal
ranges. They have the greatest ability to compensate
for the pH changes that are a normal part of body
metabolism or the result of disease, and to make the
necessary corrections.
This regulatory function of the kidneys is complex,
but at its simplest it may be described as follows. If
body fluids are becoming too acidic, the kidneys will
secrete more H ions into the renal filtrate and will
return more HCO3
ions to the blood. This will help
raise the pH of the blood back to normal. The reactions
involved in such a mechanism are shown in Fig.
18–6, to which we will return later. First, however, let
us briefly consider how the kidneys will compensate
for body fluids that are becoming too alkaline. You
might expect the kidneys to do just the opposite of
what was just described, and that is just what happens.
The kidneys will return H ions to the blood and
excrete HCO3
ions in urine. This will help lower the
pH of the blood back to normal.
Because the natural tendency is for body fluids to
become more acidic, let us look at the pH-raising
mechanism in more detail (see Fig. 18–6). The cells
of the renal tubules can secrete H ions or ammonia
in exchange for Na ions and, by doing so, influence
the reabsorption of other ions. Hydrogen ions are
obtained from the reaction of CO2 and water (or other
processes). An amine group from an amino acid is
combined with an H ion to form ammonia.
The tubule cell secretes the H ion and the ammonia
into the renal filtrate, and two Na ions are reabsorbed
in exchange. In the filtrate, the H ion and
ammonia form NH4
(an ammonium radical), which
reacts with a chloride ion (Cl ) to form NH4Cl
(ammonium chloride) that is excreted in urine.
As the Na ions are returned to the blood in the
428 The Urinary System
Table 18–1 EFFECTS OF HORMONES ON THE KIDNEYS
Hormone (gland) Function
Antidiuretic hormone (ADH)
(posterior pituitary)
Parathyroid hormone (PTH)
(parathyroid glands)
Aldosterone
(adrenal cortex)
Atrial natriuretic peptide
(ANP) (atria of heart)
• Increases reabsorption of water from the filtrate to the blood.
• Increases reabsorption of Ca 2 ions from filtrate to the blood and excretion of
phosphate ions into the filtrate.
• Increases reabsorption of Na ions from the filtrate to the blood and excretion of
K ions into the filtrate. Water is reabsorbed following the reabsorption of sodium.
• Decreases reabsorption of Na ions, which remain in the filtrate. More sodium
and water are eliminated in urine.
peritubular capillaries, HCO3
ions follow. Notice
what has happened: Two H ions have been excreted
in urine, and two Na ions and two HCO3
ions have
been returned to the blood. As reactions like these
take place, the body fluids are prevented from becoming
too acidic.
Another mechanism used by the cells of the kidney
tubules to regulate pH is the phosphate buffer system,
which is described in Chapter 19.
OTHER FUNCTIONS
OF THE KIDNEYS
In addition to the functions described thus far, the kidneys
have other functions, some of which are not
directly related to the formation of urine. These functions
are secretion of renin (which does influence
urine formation), production of erythropoietin, and
activation of vitamin D.
1. Secretion of renin—When blood pressure decreases,
the juxtaglomerular ( juxta means “next
to”) cells in the walls of the afferent arterioles
secrete the enzyme renin. Renin then initiates the
renin-angiotensin mechanism to raise blood pressure.
This was first described in Chapter 13, and
the sequence of events is presented in Table 18–2.
The end product of this mechanism is angiotensin
II, which causes vasoconstriction and increases the
secretion of aldosterone, both of which help raise
blood pressure.
The Urinary System 429
ADH Increases reabsorption of H2O
ANP
PTH
Aldosterone
Blood
Urine
Increases reabsorption of Ca+2
Increases reabsorption of Na+
and excretion of K+
Decreases reabsorption of Na+
Figure 18–5. Effects of hormones on the kidneys.
QUESTION: Do any of these hormones affect both reabsorption and secretion? If so, how?
A normal blood pressure is essential to normal
body functioning. Perhaps the most serious change
is a sudden, drastic decrease in blood pressure, such
as would follow a severe hemorrhage. In response to
such a decrease, the kidneys will decrease filtration
and urinary output and will initiate the formation of
angiotensin II. In these ways the kidneys help
ensure that the heart has enough blood to pump to
maintain cardiac output and blood pressure.
2. Secretion of erythropoietin—This hormone is
secreted whenever the blood oxygen level decreases
(a state of hypoxia). Erythropoietin stimulates
the red bone marrow to increase the rate of RBC
production. With more RBCs in circulation, the
oxygen-carrying capacity of the blood is greater,
and the hypoxic state may be corrected (see also
Box 18–3: Erythropoietin).
3. Activation of vitamin D—This vitamin exists in
several structural forms that are converted to calcitriol
(D2) by the kidneys. Calcitriol is the most
active form of vitamin D, which increases the
absorption of calcium and phosphate in the small
intestine.
430 The Urinary System
Peritubular
capillary Filtrate
Renal tubule
Na+ Na+ Na+
HCO
3

Na+
HCO
3

HCO
3

HCO
3

CI CI
Na+
Na+
H CI +
H+
Na+
Na+
CO
2
H
2
O H
2
CO
3

CO
2
H
2
O H
2
CO
3

H+
H+



+
NH
3
NH
2
NH
NH 4
3
NH
4
Cl
Blood Urine
Figure 18–6. Renal regulation of acid–base
balance. The cells of the renal tubule secrete H
ions and ammonia into the filtrate and return
Na ions and HCO3
ions to the blood in the
peritubular capillaries. See text for further description.
QUESTION: The cells of the renal tubule make
good use of CO2. What do the cells use CO2 for?
Table 18–2 THE RENIN-ANGIOTENSIN
MECHANISM
Sequence
1. Decreased blood pressure stimulates the kidneys to
secrete renin.
2. Renin splits the plasma protein angiotensinogen
(synthesized by the liver) to angiotensin I.
3. Angiotensin I is converted to angiotensin II by an
enzyme found in lung tissue and vascular endothelium.
4. Angiotensin II causes vasoconstriction and stimulates
the adrenal cortex to secrete aldosterone.
ELIMINATION OF URINE
The ureters, urinary bladder, and urethra do not
change the composition or amount of urine, but are
responsible for the periodic elimination of urine.
URETERS
Each ureter extends from the hilus of a kidney to the
lower, posterior side of the urinary bladder (see Fig.
18–1). Like the kidneys, the ureters are retroperitoneal,
that is, behind the peritoneum of the dorsal
abdominal cavity.
The smooth muscle in the wall of the ureter contracts
in peristaltic waves to propel urine toward the
urinary bladder. As the bladder fills, it expands and
compresses the lower ends of the ureters to prevent
backflow of urine.
URINARY BLADDER
The urinary bladder is a muscular sac below the peritoneum
and behind the pubic bones. In women, the
bladder is inferior to the uterus; in men, the bladder is
superior to the prostate gland. The bladder is a reservoir
for accumulating urine, and it contracts to eliminate
urine.
The mucosa of the bladder is transitional epithelium,
which permits expansion without tearing the
lining. When the bladder is empty, the mucosa
appears wrinkled; these folds are rugae, which also
permit expansion. On the floor of the bladder is a triangular
area called the trigone, which has no rugae
and does not expand. The points of the triangle are
the openings of the two ureters and that of the urethra
(Fig. 18–7).
The smooth muscle layer in the wall of the bladder
is called the detrusor muscle. It is a muscle in the
form of a sphere; when it contracts it becomes a
smaller sphere, and its volume diminishes. Around the
opening of the urethra the muscle fibers of the detrusor
form the internal urethral sphincter (or sphincter
of the bladder), which is involuntary.
URETHRA
The urethra (see Fig. 18–7) carries urine from the
bladder to the exterior. The external urethral
sphincter is made of the surrounding skeletal muscle
of the pelvic floor, and is under voluntary control.
In women, the urethra is 1 to 1.5 inches (2.5 to 4
cm) long and is anterior to the vagina. In men, the
urethra is 7 to 8 inches (17 to 20 cm) long. The first
part just outside the bladder is called the prostatic urethra
because it is surrounded by the prostate gland.
The next inch is the membranous urethra, around
which is the external urethral sphincter. The longest
portion is the cavernous urethra (or spongy or penile
urethra), which passes through the cavernous (or erectile)
tissue of the penis. The male urethra carries
semen as well as urine.
THE URINATION REFLEX
Urination may also be called micturition or voiding.
This reflex is a spinal cord reflex over which voluntary
control may be exerted. The stimulus for the reflex is
stretching of the detrusor muscle of the bladder. The
bladder can hold as much as 800 mL of urine, or even
more, but the reflex is activated long before the maximum
is reached.
When urine volume reaches 200 to 400 mL, the
stretching is sufficient to generate sensory impulses
that travel to the sacral spinal cord. Motor impulses
return along parasympathetic nerves to the detrusor
muscle, causing contraction. At the same time, the
internal urethral sphincter relaxes. If the external urethral
sphincter is voluntarily relaxed, urine flows into
the urethra, and the bladder is emptied.
Urination can be prevented by voluntary contraction
of the external urethral sphincter. However, if the
bladder continues to fill and be stretched, voluntary
control is eventually no longer possible.
The Urinary System 431
BOX 18–3 ERYTHROPOIETIN
Anemia is one of the most debilitating consequences
of renal failure, one that hemodialysis
cannot reverse. Diseased kidneys stop producing
erythropoietin, a natural stimulus for RBC
production. Erythropoietin can be produced by
genetic engineering and is available for hemodialysis
patients. In the past, their anemia could
only be treated with transfusions, which exposed
these patients to possible immunologic complications
of repeated exposure to donated blood
or to viral diseases. The synthetic erythropoietin
eliminates such risks. Others who benefit from
this medication are cancer patients and AIDS
patients with severe anemia.
CHARACTERISTICS OF URINE
The characteristics of urine include the physical and
chemical aspects that are often evaluated as part of a
urinalysis. Some of these are described in this section,
and others are included in Appendix D: Normal
Values for Some Commonly Used Urine Tests.
Amount—normal urinary output per 24 hours is 1
to 2 liters. Many factors can significantly change
output. Excessive sweating or loss of fluid through
432 The Urinary System
Internal
urethral sphincter
External
urethral sphincter
Urethra
Urethral orifice
Trigone
Ureter
Trigone
Rugae
Openings of
ureters
Detrusor muscle
Parietal peritoneum Ureter
Prostate gland
Prostatic urethra
Membranous
urethra
Cavernous (spongy)
urethra
Cavernous (erectile)
tissue of penis
A
B
Figure 18–7. (A) Frontal section of female urinary bladder and urethra. (B) Frontal section
of male urinary bladder and urethra.
QUESTION: Name the sphincters of the urinary system and state whether each is voluntary
or involuntary.
diarrhea will decrease urinary output (oliguria) to
conserve body water. Excessive fluid intake will
increase urinary output (polyuria). Consumption of
alcohol will also increase output because alcohol
inhibits the secretion of ADH, and the kidneys will
reabsorb less water.
Color—the typical yellow color of urine (from
urochrome, a breakdown product of bile) is often
referred to as “straw” or “amber.” Concentrated
urine is a deeper yellow (amber) than is dilute urine.
Freshly voided urine is also clear rather than cloudy.
Specific gravity—the normal range is 1.010 to 1.025;
this is a measure of the dissolved materials in urine.
The specific gravity of distilled water is 1.000, meaning
that there are no solutes present. Therefore, the
higher the specific gravity number, the more dissolved
material is present. Someone who has been
exercising strenuously and has lost body water in
sweat will usually produce less urine, which will
be more concentrated and have a higher specific
gravity.
The specific gravity of the urine is an indicator of
the concentrating ability of the kidneys: The kidneys
must excrete the waste products that are constantly
formed in as little water as possible.
pH—the pH range of urine is between 4.6 and 8.0,
with an average value of 6.0. Diet has the greatest
influence on urine pH. A vegetarian diet will result
in a more alkaline urine, whereas a high-protein
diet will result in a more acidic urine.
Constituents—urine is approximately 95% water,
which is the solvent for waste products and salts.
Salts are not considered true waste products because
they may well be utilized by the body when needed,
but excess amounts will be excreted in urine (see
Box 18–4: Kidney Stones).
Nitrogenous wastes—as their name indicates, all of
these wastes contain nitrogen. Urea is formed by
liver cells when excess amino acids are deaminated
to be used for energy production. Creatinine comes
from the metabolism of creatine phosphate, an
energy source in muscles. Uric acid comes from the
metabolism of nucleic acids, that is, the breakdown
of DNA and RNA. Although these are waste products,
there is always a certain amount of each in the
blood. Box 18–5: Blood Tests and Kidney Function
describes the relationship between blood levels of
these waste products and kidney function.
Other non-nitrogenous waste products include
small amounts of urobilin from the hemoglobin of
old RBCs (see Fig. 11–4), and may include the
metabolic products of medications. Table 18–3
summarizes the characteristics of urine.
When a substance not normally found in urine
does appear there, there is a reason for it. The reason
may be quite specific or more general. Table
18–4 lists some abnormal constituents of urine and
possible reasons for each (see also Box 18–6:
Urinary Tract Infections).
AGING AND THE URINARY SYSTEM
With age, the number of nephrons in the kidneys
decreases, often to half the original number by the age
of 70 to 80, and the kidneys lose some of their con-
The Urinary System 433
BOX 18–4 KIDNEY STONES
The entry of a kidney stone into a ureter may
cause intense pain (renal colic) and bleeding.
Obstruction of a ureter by a stone may cause
backup of urine and possible kidney damage.
Treatments include surgery to remove the stone, or
lithotripsy, the use of shock waves to crush the
stone into pieces small enough to be eliminated
without damage to the urinary tract. A recent study
links lithotripsy with an increased risk of diabetes or
hypertension later in life, though the mechanisms
that would bring about these conditions have not
yet been discovered.
Kidney stones, or renal calculi, are crystals of the
salts that are normally present in urine. A very high
concentration of salts in urine may trigger precipitation
of the salt and formation of crystals, which
can range in size from microscopic to 10 to 20 mm
in diameter. The most common type of kidney
stone is made of calcium salts; a less common type
is made of uric acid.
Kidney stones are most likely to form in the renal
pelvis. Predisposing factors include decreased fluid
intake or overingestion of minerals (as in mineral
supplements), both of which lead to the formation
of a very concentrated urine.
434 The Urinary System
Table 18–3 CHARACTERISTICS OF NORMAL URINE
Characteristic Description
Amount
Color
Specific gravity
pH
Composition
Nitrogenous wastes
1–2 liters per 24 hours; highly variable depending on fluid intake and water loss through the
skin and GI tract
Straw or amber; darker means more concentrated; should be clear, not cloudy
1.010–1.025; a measure of the dissolved material in urine; the lower the value, the more dilute
the urine
Average 6; range 4.6–8.0; diet has the greatest effect on urine pH
95% water; 5% salts and waste products
Urea—from amino acid metabolism
Creatinine—from muscle metabolism
Uric acid—from nucleic acid metabolism
Table 18–4 ABNORMAL CONSTITUENTS IN URINE
Characteristic Reason(s)
Glycosuria
(presence of
glucose)
Proteinuria
(presence of
protein)
Hematuria
(presence of
blood—RBCs)
Bacteriuria
(presence
of bacteria)
Ketonuria
(presence of
ketones)
As long as blood glucose levels are within normal limits, filtrate levels will also be normal and will
not exceed the threshold level for reabsorption. In an untreated diabetic, for example, blood
glucose is too high; therefore the filtrate glucose level is too high. The kidneys reabsorb glucose
up to their threshold level, but the excess remains in the filtrate and is excreted in urine.
Most plasma proteins are too large to be forced out of the glomeruli, and the small proteins that
enter the filtrate are reabsorbed by pinocytosis. The presence of protein in the urine indicates
that the glomeruli have become too permeable, as occurs in some types of kidney disease.
The presence of RBCs in urine may also indicate that the glomeruli have become too permeable.
Another possible cause might be bleeding somewhere in the urinary tract. Pinpointing the site
of bleeding would require specific diagnostic tests.
Bacteria give urine a cloudy rather than clear appearance; WBCs may be present also. The presence
of bacteria means that there is an infection somewhere in the urinary tract. Further diagnostic
tests would be needed to determine the precise location.
Ketones are formed from fats and proteins that are used for energy production. A trace of ketones
in urine is normal. Higher levels of ketones indicate an increased use of fats and proteins for
energy. This may be the result of malfunctioning carbohydrate metabolism (as in diabetes mellitus)
or simply the result of a high-protein diet.
BOX 18–5 BLOOD TESTS AND KIDNEY FUNCTION
impaired. Of the three, the creatinine level is probably
the most reliable indicator of kidney functioning.
Blood urea nitrogen (BUN) may vary considerably in
certain situations not directly related to the kidneys.
For example, BUN may be elevated as a consequence
of a high-protein diet or of starvation when
body protein is being broken down at a faster rate
than normal. Uric acid levels may also vary according
to diet. However, elevated blood levels of all
three nitrogenous wastes usually indicate impaired
glomerular filtration.
Waste products are normally present in the blood,
and the concentration of each varies within a normal
range. As part of the standard lab work called
blood chemistry, the levels of the three nitrogenous
waste products are determined (urea, creatinine,
and uric acid).
If blood levels of these three substances are
within normal ranges, it may be concluded that the
kidneys are excreting these wastes at normal rates.
If, however, these blood levels are elevated, one
possible cause is that kidney function has been
centrating ability. The glomerular filtration rate also
decreases, partly as a consequence of arteriosclerosis
and diminished renal blood flow. Despite these changes,
excretion of nitrogenous wastes usually remains
adequate.
The urinary bladder decreases in size, and the tone
of the detrusor muscle decreases. These changes may
lead to a need to urinate more frequently. Urinary
incontinence (the inability to control voiding) is not an
inevitable consequence of aging, and can be prevented
or minimized. Elderly people are, however, more at
risk for infections of the urinary tract, especially if
voiding leaves residual urine in the bladder.
SUMMARY
The kidneys are the principal regulators of the internal
environment of the body. The composition of all
body fluids is either directly or indirectly regulated by
the kidneys as they form urine from blood plasma.
The kidneys are also of great importance in the regulation
of the pH of the body fluids. These topics are
the subject of the next chapter.
The Urinary System 435
BOX 18–6 URINARY TRACT INFECTIONS
Symptoms include frequency of urination, painful
voiding, and low back pain. Nephritis (or pyelonephritis)
is inflammation of the kidneys. Although
this may be the result of a systemic bacterial infection,
nephritis is a common complication of
untreated lower urinary tract infections such as cystitis.
Possible symptoms are fever and flank pain (in
the area of the kidneys). Untreated nephritis may
result in severe damage to nephrons and progress
to renal failure.
Infections may occur anywhere in the urinary tract
and are most often caused by the microbial agents
of sexually transmitted diseases (see Chapter 20) or
by the bacteria that are part of the normal flora of
the colon. In women especially, the urinary and
anal openings are in close proximity, and colon bacteria
on the skin of the perineum may invade the
urinary tract. The use of urinary catheters in hospitalized
or bedridden patients may also be a factor if
sterile technique is not carefully followed.
Cystitis is inflammation of the urinary bladder.
STUDY OUTLINE
The urinary system consists of two kidneys,
two ureters, the urinary bladder, and the
urethra.
1. The kidneys form urine to excrete waste products
and to regulate the volume, electrolytes, and pH of
blood and tissue fluid.
2. The other organs of the system are concerned with
elimination of urine.
Kidneys (see Fig. 18–1)
1. Retroperitoneal on either side of the backbone in
the upper abdominal cavity; partially protected by
the lower rib cage.
2. Adipose tissue and the renal fascia cushion the kidneys
and help hold them in place.
3. Hilus—an indentation on the medial side; renal
artery enters, renal vein and ureter emerge.
Kidney—internal structure (see Fig. 18–2)
1. Renal cortex—outer tissue layer, made of renal corpuscles
and convoluted tubules.
2. Renal medulla (pyramids)—inner tissue layer,
made of loops of Henle and collecting tubules.
3. Renal pelvis—a cavity formed by the expanded end
of the ureter within the kidney at the hilus; extensions
around the papillae of the pyramids are called
calyces, which collect urine.
The Nephron—the functional unit of the kidney
(see Fig. 18–3); 1 million per kidney
1. Renal corpuscle—consists of a glomerulus surrounded
by a Bowman’s capsule.
• Glomerulus—a capillary network between an
afferent arteriole and an efferent arteriole.
• Bowman’s capsule—the expanded end of a renal
tubule that encloses the glomerulus; inner layer
is made of podocytes, has pores, and is very permeable;
contains renal filtrate (potential urine).
2. Renal tubule—consists of the proximal convoluted
tubule, loop of Henle, distal convoluted tubule,
and collecting tubule. Collecting tubules unite to
form papillary ducts that empty urine into the
calyces of the renal pelvis.
• Peritubular capillaries—arise from the efferent
arteriole and surround all parts of the renal
tubule.
Blood Vessels of the Kidney (see Figs. 18–1,
18–2, and 18–3)
1. Pathway: abdominal aorta → renal artery → small
arteries in the kidney → afferent arterioles →
glomeruli → efferent arterioles → peritubular capillaries
→ small veins in the kidney → renal vein →
inferior vena cava.
2. Two sets of capillaries provide for two sites of
exchanges between the blood and tissues in the
process of urine formation.
Formation of Urine (see Fig. 18–4)
1. Glomerular filtration—takes place from the
glomerulus to Bowman’s capsule. High blood pressure
(60 mmHg) in the glomerulus forces plasma,
dissolved materials, and small proteins out of the
blood and into Bowman’s capsule. The fluid is now
called filtrate. Filtration is selective only in terms of
size; blood cells and large proteins remain in the
blood.
2. GFR is 100 to 125 mL per minute. Increased blood
flow to the kidney increases GFR; decreased blood
flow decreases GFR.
3. Tubular reabsorption—takes place from the filtrate
in the renal tubule to the blood in the peritubular
capillaries; 99% of the filtrate is reabsorbed; only
1% becomes urine.
• Active transport—reabsorption of glucose,
amino acids, vitamins, and positive ions; threshold
level is a limit to the quantity that can be
reabsorbed.
• Passive transport—most negative ions follow the
reabsorption of positive ions.
• Osmosis—water follows the reabsorption of
minerals, especially sodium.
• Pinocytosis—small proteins are engulfed by
proximal tubule cells.
4. Tubular secretion—takes place from the blood in
the peritubular capillaries to the filtrate in the renal
tubule; creatinine and other waste products may be
secreted into the filtrate to be excreted in urine;
secretion of H ions helps maintain pH of blood.
5. Hormones that affect reabsorption—aldosterone,
atrial natriuretic peptide, antidiuretic hormone,
and parathyroid hormone—see Table 18–1 and
Fig. 18–5.
The Kidneys and Acid–Base Balance
1. The kidneys have the greatest capacity to compensate
for normal and abnormal pH changes.
2. If the body fluids are becoming too acidic, the kidneys
excrete H ions and return HCO3
ions to
the blood (see Fig. 18–6).
3. If the body fluids are becoming too alkaline, the
kidneys return H ions to the blood and excrete
HCO3
ions.
Other Functions of the Kidneys
1. Secretion of renin by juxtaglomerular cells when
blood pressure decreases (see Table 18–2). Angiotensin
II causes vasoconstriction and increases
secretion of aldosterone.
2. Secretion of erythropoietin in response to hypoxia;
stimulates red bone marrow to increase rate of
RBC production.
3. Activation of vitamin D—conversion of inactive
forms to the active form.
Elimination of Urine—the function of the
ureters, urinary bladder, and urethra
Ureters (see Figs. 18–1 and 18–7)
1. Each extends from the hilus of a kidney to the
lower posterior side of the urinary bladder.
2. Peristalsis of smooth muscle layer propels urine
toward bladder.
Urinary Bladder (see Figs. 18–1 and 18–7)
1. A muscular sac below the peritoneum and behind
the pubic bones; in women, below the uterus; in
men, above the prostate gland.
2. Mucosa—transitional epithelial tissue folded into
rugae; permit expansion without tearing.
3. Trigone—triangular area on bladder floor; no
rugae, does not expand; bounded by openings of
ureters and urethra.
436 The Urinary System
4. Detrusor muscle—the smooth muscle layer, a
spherical muscle; contracts to expel urine (reflex).
5. Internal urethral sphincter—involuntary; formed
by detrusor muscle fibers around the opening of
the urethra.
Urethra—takes urine from the bladder to
the exterior (see Fig. 18–7)
1. In women—1 to 1.5 inches long; anterior to vagina.
2. In men—7 to 8 inches long; passes through the
prostate gland and penis.
3. Has the external urethral sphincter: skeletal muscle
of pelvic floor (voluntary).
The Urination Reflex—also called micturition
or voiding
1. Stimulus: stretching of the detrusor muscle by
accumulating urine.
2. Sensory impulses to spinal cord, motor impulses
(parasympathetic) return to detrusor muscle, which
contracts; internal urethral sphincter relaxes.
3. The external urethral sphincter provides voluntary
control.
Characteristics of Urine (see Table 18–3)
Abnormal Constituents of Urine (see
Table 18–4)
The Urinary System 437
REVIEW QUESTIONS
1. Describe the location of the kidneys, ureters, urinary
bladder, and urethra. (pp. 421, 431)
2. Name the three areas of the kidney, and state what
each consists of. (p. 421)
3. Name the two major parts of a nephron. State the
general function of nephrons. (p. 421)
4. Name the parts of a renal corpuscle. What process
takes place here? Name the parts of a renal tubule.
What processes take place here? (pp. 421, 424)
5. State the mechanism of tubular reabsorption of
each of the following: (pp. 425, 427)
a. Water
b. Glucose
c. Small proteins
d. Positive ions
e. Negative ions
f. Amino acids
g. Vitamins
Also explain what is meant by a threshold level of
reabsorption.
6. Explain the importance of tubular secretion. (p.
427)
7. Describe the pathway of blood flow through the
kidney from the abdominal aorta to the inferior
vena cava. (p. 424)
8. Name the two sets of capillaries in the kidney, and
state the processes that take place in each. (pp. 424,
425)
9. Name the hormone that has each of these effects
on the kidneys: (pp. 428–429)
a. Promotes reabsorption of Na ions
b. Promotes direct reabsorption of water
c. Promotes reabsorption of Ca 2 ions
d. Promotes excretion of K ions
e. Decreases reabsorption of Na ions
10. In what circumstances will the kidneys excrete H
ions? What ions will be returned to the blood?
How will this affect the pH of blood? (p. 428)
11. In what circumstances do the kidneys secrete
renin, and what is its purpose? (p. 429)
12. In what circumstances do the kidneys secrete erythropoietin,
and what is its purpose? (p. 430)
13. Describe the function of the ureters and that of
the urethra. (p. 431)
14. With respect to the urinary bladder, describe
the function of rugae and the detrusor muscle.
(p. 431)
15. Describe the urination reflex in terms of stimulus,
part of the CNS involved, effector muscle, internal
urethral sphincter, and voluntary control.
(pp. 431)
16. Describe the characteristics of normal urine in
terms of appearance, amount, pH, specific gravity,
and composition. (pp. 432–433)
17. State the source of each of the nitrogenous waste
products: creatinine, uric acid, and urea. (p. 433)
1. The functioning of the kidneys may be likened to
cleaning your room by throwing everything out the
window, then going outside to retrieve what you
wish to keep, such as jammies and slippers. Imagine
the contents of a room, liken them to the materials
in the blood (you yourself are a kidney), and
describe what happens to each, and why.
2. Explain why fatty acids are not found in urine.
Under what circumstances are water-soluble vitamins
(such as vitamin C) found in urine?
3. Explain how a spinal cord transection at the level of
T11 will affect the urination reflex.
4. As part of his yearly physical for the college football
team, 20-year-old Patrick has a urinalysis,
which shows a high level of ketones. He is not
diabetic, and is not ill. What might cause the
high urine level of ketones? What blood chemistry
test (for nitrogenous wastes) would help confirm
this?
5. A patient being evaluated for food poisoning has a
blood pH of 7.33, a urine pH of 4.5, and a respiratory
rate of 28 per minute. What kind of pH imbalance
is this? Explain your reasoning step by step.
6. Erythropoietin, called EPO, has become a drug
used illegally by some athletes. Which athletes use
EPO, that is, in what kind of sports? What benefits
are they hoping for? What part of a CBC would
indicate that an athlete is taking EPO? Explain.
7. After a 4-hour workout on a hot June day, the high
school track coach tells her group to keep drinking
plenty of water. The girls assure their coach that
they will know just how to determine if they are sufficiently
hydrated that evening, that they have their
color scheme memorized. What do they mean?
438 The Urinary System
FOR FURTHER THOUGHT

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