Essentials of Anatomy and Physiology

Copyright © 2007 by F. A. Davis.

The Digestive System 383

A summary of the digestive secretions and their eral structural modifications of the small intestine;
functions is found in Table 16–1. Regulation of these these are shown in Fig. 16–8. Plica circulares, or cir-
secretions is shown in Table 16–2. cular folds, are macroscopic folds of the mucosa and
submucosa, somewhat like accordion pleats. The
ABSORPTION mucosa is further folded into projections called villi,
which give the inner surface of the intestine a velvet-
Most absorption of the end products of digestion takes like appearance. Each columnar cell (except the
place in the small intestine (although the stomach does mucus-secreting goblet cells) of the villi also has
absorb water and alcohol). The process of absorption microvilli on its free surface. Microvilli are micro-
requires a large surface area, which is provided by sev- scopic folds of the cell membrane, and are collectively

Small intestine
A

Microvilli

Plica Absorptive cell
circulares
Goblet cell
Lacteal

Capillary
network

Intestinal Enteroendocrine cell
gland

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 enteroen-
docrine cells secrete the intestinal hormones.
QUESTION: What is the purpose of the villi? What other structures have the same purpose?

Copyright © 2007 by F. A. Davis.

384 The Digestive System

called the brush border. All of these folds greatly calcium ions requires parathyroid hormone and vita-
increase the surface area of the intestinal lining. It is min D.
estimated that if the intestinal mucosa could be flat-
tened out, it would cover more than 2000 square feet Fat-soluble nutrients are absorbed into the lymph
(half a basketball court). in the lacteals of the villi. Bile salts are necessary for
the efficient absorption of fatty acids and the fat-solu-
The absorption of nutrients takes place from the ble vitamins (A, D, E, and K). Once absorbed, fatty
lumen of the intestine into the vessels within the acids are recombined with glycerol to form triglyc-
villi. Refer to Fig. 16–8 and notice that within each vil- erides. These triglycerides then form globules that
lus is a capillary network and a lacteal, which is a include cholesterol and protein; these lipid–protein
dead-end lymph capillary. Water-soluble nutrients complexes are called chylomicrons. In the form of
are absorbed into the blood in the capillary networks. chylomicrons, most absorbed fat is transported by the
Monosaccharides, amino acids, positive ions, and the lymph and eventually enters the blood in the left sub-
water-soluble vitamins (vitamin C and the B vitamins) clavian vein.
are absorbed by active transport. Negative ions may
be absorbed by either passive or active transport Blood from the capillary networks in the villi does
mechanisms. Water is absorbed by osmosis following not return directly to the heart but first travels
the absorption of minerals, especially sodium. Certain through the portal vein to the liver. You may recall the
nutrients have additional special requirements for importance of portal circulation, discussed in Chapter
their absorption: For example, vitamin B12 requires 13. This pathway enables the liver to regulate the
the intrinsic factor produced by the parietal cells blood levels of glucose and amino acids, store certain
of the gastric mucosa, and the efficient absorption of vitamins, and remove potential poisons from the
blood (see Box 16–3: Disorders of the Intestines).

BOX 16–3 DISORDERS OF THE INTESTINES Salmonella food poisoning is caused by bac-
teria in the genus Salmonella. These are part of the
Duodenal ulcers are erosions of the duodenal intestinal flora of animals, and animal foods such as
wall caused by the gastric juice that enters from the meat and eggs may be sources of infection. These
stomach. The most serious consequences are bleed- bacteria are not normal for people, and they cause
ing and perforation. the intestines to secrete large amounts of fluid.
Symptoms include diarrhea, abdominal cramps,
Paralytic ileus is the cessation of contraction and vomiting and usually last only a few days. For
of the smooth muscle layer of the intestine. This elderly or debilitated people, however, salmonella
is a possible complication of abdominal surgery, food poisoning may be very serious or even fatal.
but it may also be the result of peritonitis or
inflammation elsewhere in the abdominal cavity. Diverticula are small outpouchings through
In the absence of peristalsis, intestinal obstruc- weakened areas of the intestinal wall. They are
tion may occur. Bowel movements cease, and vom- more likely to occur in the colon than in the small
iting occurs to relieve the pressure within the intestine and may exist for years without causing
alimentary tube. Treatment involves suctioning any symptoms. The presence of diverticula is called
the intestinal contents to eliminate any obstruc- diverticulosis. Inflammation of diverticula is
tion and to allow the intestine to regain its normal called diverticulitis, which is usually the result of
motility. entrapment of feces and bacteria. Symptoms
include abdominal pain and tenderness and fever. If
Lactose intolerance is the inability to digest uncomplicated, diverticulitis may be treated with
lactose because of deficiency of the enzyme lactase. antibiotics and modifications in diet. The most seri-
Lactase deficiency may be congenital, a conse- ous complication is perforation of diverticula, allow-
quence of prematurity, or acquired later in life. The ing fecal material into the abdominal cavity, causing
delayed form is quite common among people of peritonitis. A diet high in fiber is believed to be an
African or Asian ancestry, and in part is genetic. important aspect of prevention, to provide bulk in
When lactose, or milk sugar, is not digested, it the colon and prevent weakening of its wall.
undergoes fermentation in the intestine. Symptoms
include diarrhea, abdominal pain, bloating, and
flatulence (gas formation).

Copyright © 2007 by F. A. Davis.

The Digestive System 385

LARGE INTESTINE which may occur if fecal material becomes impacted
within it. This usually necessitates an appendectomy,
The large intestine, also called the colon, is approx- the surgical removal of the appendix.
imately 2.5 inches (6.3 cm) in diameter and 5 feet (1.5
m) in length. It extends from the ileum of the small The remainder of the colon consists of the ascend-
intestine to the anus, the terminal opening. The parts ing, transverse, and descending colon, which encircle
of the colon are shown in Fig. 16–9. The cecum is the the small intestine; the sigmoid colon, which turns
first portion, and at its junction with the ileum is medially and downward; the rectum; and the anal
the ileocecal valve, which is not a sphincter but serves canal. The rectum is about 6 inches long, and the anal
the same purpose. After undigested food (which is canal is the last inch of the colon that surrounds the
now mostly cellulose) and water pass from the ileum anus. Clinically, however, the terminal end of the
into the cecum, closure of the ileocecal valve prevents colon is usually referred to as the rectum.
the backflow of fecal material.
No digestion takes place in the colon. The only
Attached to the cecum is the appendix, a small, secretion of the colonic mucosa is mucus, which lubri-
dead-end tube with abundant lymphatic tissue. The cates the passage of fecal material. The longitudinal
appendix seems to be a vestigial organ, that is, one smooth muscle layer of the colon is in three bands
whose size and function seem to be reduced. Although called taeniae coli. The rest of the colon is “gathered”
there is abundant lymphatic tissue in the wall of the to fit these bands. This gives the colon a puckered
appendix, the possibility that the appendix is con- appearance; the puckers or pockets are called haustra,
cerned with immunity is not known with certainty. which provide for more surface area within the colon.
Appendicitis refers to inflammation of the appendix,
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

Haustra Splenic flexure

Taeniae coli Transverse colon
Hepatic flexure

Figure 16–9. The large intestine shown in Ascending Descending
anterior view. The term flexure means a turn or colon colon
bend.
QUESTION: What is the function of the ileoce- lleum Sigmoid colon
cal valve?
lleocecal valve

Appendix

Cecum Anal canal
Rectum

Anus

Copyright © 2007 by F. A. Davis.

386 The Digestive System

BOX 16–4 INFANT BOTULISM colon pushes feces into it. These waves of peristalsis
tend to occur after eating, especially when food enters
Botulism is most often acquired from food. the duodenum. The wall of the rectum is stretched by
When the spores of the botulism bacteria are in the entry of feces, and this is the stimulus for the defe-
an anaerobic (without oxygen) environment cation reflex.
such as a can of food, they germinate into active
bacteria that produce a neurotoxin. If people Stretch receptors in the smooth muscle layer of the
ingest food containing this toxin, they will rectum generate sensory impulses that travel to the
develop the paralysis that is characteristic of sacral spinal cord. The returning motor impulses
botulism. cause the smooth muscle of the rectum to contract.
Surrounding the anus is the internal anal sphincter,
For infants less than 1 year of age, however, which is made of smooth muscle. As part of the reflex,
ingestion of just the bacterial spores may be this sphincter relaxes, permitting defecation to take
harmful. The infant’s stomach does not produce place.
much HCl, so ingested botulism spores may not
be destroyed. Of equal importance, the infant’s The external anal sphincter is made of skeletal
normal colon flora is not yet established. Without muscle and surrounds the internal anal sphincter (Fig.
the normal population of colon bacteria to pro- 16–10). If defecation must be delayed, the external
vide competition, spores of the botulism bacteria sphincter may be voluntarily contracted to close the
may germinate and produce their toxin. anus. The awareness of the need to defecate passes as
the stretch receptors of the rectum adapt. These
An affected infant becomes lethargic and receptors will be stimulated again when the next wave
weak; paralysis may progress slowly or rapidly. of peristalsis reaches the rectum (see Box 16–5: Fiber).
Treatment (antitoxin) is available, but may be
delayed if botulism is not suspected. Many cases OTHER FUNCTIONS OF THE LIVER
of infant botulism have been traced to honey
that was found to contain botulism spores. Such The liver is a remarkable organ, and only the brain is
spores are not harmful to older children and capable of a greater variety of functions. The liver
adults, who have a normal colon flora that pre- cells (hepatocytes) produce many enzymes that cat-
vents the botulism bacteria from becoming alyze many different chemical reactions. These reac-
established. tions are the functions of the liver. As blood flows
through the sinusoids (capillaries) of the liver (see Fig.
enters the colon is absorbed (400 to 800 mL per day). 16–6), materials are removed by the liver cells, and the
Positive and negative ions are also absorbed. The vita- products of the liver cells are secreted into the blood.
mins absorbed are those produced by the normal Some of the liver functions will already be familiar
flora, the trillions of bacteria that live in the colon. to you. Others are mentioned again and discussed in
Vitamin K is produced and absorbed in amounts usu- more detail in the next chapter. Because the liver
ally sufficient to meet a person’s daily need. Other has such varied effects on so many body systems, we
vitamins produced in smaller amounts include will use the categories below to summarize the liver
riboflavin, thiamin, biotin, and folic acid. Everything functions.
absorbed by the colon circulates first to the liver by
way of portal circulation. Yet another function of the 1. Carbohydrate metabolism—As you know, the
normal colon flora is to inhibit the growth of liver regulates the blood glucose level. Excess glu-
pathogens (see Box 16–4: Infant Botulism). cose is converted to glycogen (glycogenesis) when
blood glucose is high; the hormones insulin and
ELIMINATION OF FECES cortisol facilitate this process. During hypo-
glycemia or stress situations, glycogen is converted
Feces consist of cellulose and other undigestible mate- back to glucose (glycogenolysis) to raise the blood
rial, dead and living bacteria, and water. Elimination glucose level. Epinephrine and glucagon are the
of feces is accomplished by the defecation reflex, a hormones that facilitate this process.
spinal cord reflex that may be controlled voluntarily.
The rectum is usually empty until peristalsis of the

Copyright © 2007 by F. A. Davis.

BOX 16–5 FIBER no protective effect of fiber against colon cancer.
What we can say for sure is that fiber may not be
Fiber is a term we use to refer to the organic mate- the only dietary or environmental factor involved.
rials in the cell walls of plants. These are mainly cel-
lulose and pectins. The role of dietary fiber and Claims that high-fiber diets directly lower blood
possible benefits that a high-fiber diet may provide levels of cholesterol and fats are not supported by
are currently the focus of much research. It is definitive clinical or experimental studies. One pos-
important to differentiate what is known from what sible explanation may be that a person whose diet
is, at present, merely speculation. consists largely of high-fiber foods simply eats less
of the foods high in cholesterol and fats, and this is
Many studies have shown that populations the reason for that person’s lower blood levels of
(large groups of people, especially those of different fats and cholesterol.
cultures) who consume high-fiber diets tend to
have a lower frequency of certain diseases. These Should people try to make great changes in their
include diverticulitis, colon cancer, coronary artery diets? Probably not, not if they are careful to limit
disease, diabetes, and hypertension. Such diseases fat intake and to include significant quantities of
are much more common among populations vegetables and fruits. Besides the possible benefits
whose diets are low in vegetables, fruits, and whole of fiber, unprocessed plant foods provide important
grains, and high in meat, dairy products, and amounts of vitamins and minerals.
processed foods. In contrast, a 2005 study showed

Rectum

Anal canal Longitudinal muscle
Circular muscle
Anal Rectal fold
columns
Levator ani
muscle

B

Internal anal Anus
sphincter

External anal
sphincter

A

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?

387

Copyright © 2007 by F. A. Davis.

388 The Digestive System

The liver also changes other monosaccharides to acids are split into two-carbon molecules called
glucose. Fructose and galactose, for example, are acetyl groups, which are simple carbohydrates.
end products of the digestion of sucrose and lac- These acetyl groups may be used by the liver cells
tose. Because most cells, however, cannot readily to produce ATP or may be combined to form
use fructose and galactose as energy sources, they ketones to be transported in the blood to other
are converted by the liver to glucose, which is eas- cells. These other cells then use the ketones to pro-
ily used by cells. duce ATP in cell respiration.
2. Amino acid metabolism—The liver regulates 4. Synthesis of plasma proteins—This is a liver
blood levels of amino acids based on tissue needs function that you will probably remember from
for protein synthesis. Of the 20 different amino Chapter 11. The liver synthesizes many of the pro-
acids needed for the production of human proteins, teins that circulate in the blood. Albumin, the most
the liver is able to synthesize 12, called the non- abundant plasma protein, helps maintain blood vol-
essential amino acids. The chemical process by ume by pulling tissue fluid into capillaries.
which this is done is called transamination, the
transfer of an amino group (NH2) from an amino The clotting factors are also produced by the
acid present in excess to a free carbon chain that liver. These, as you recall, include prothrombin,
forms a complete, new amino acid molecule. The fibrinogen, and Factor 8, which circulate in the
other eight amino acids, which the liver cannot blood until needed in the chemical clotting mech-
synthesize, are called the essential amino acids. In anism. The liver also synthesizes alpha and beta
this case, “essential” means that the amino acids globulins, which are proteins that serve as carriers
must be supplied by our food, because the liver for other molecules, such as fats, in the blood.
cannot manufacture them. Similarly, “non-essen- 5. Formation of bilirubin—This is another familiar
tial” means that the amino acids do not have to be function: The liver contains fixed macrophages
supplied in our food because the liver can make that phagocytize old red blood cells (RBCs).
them. All 20 amino acids are required in order to Bilirubin is then formed from the heme portion of
make our body proteins. the hemoglobin. The liver also removes from the
blood the bilirubin formed in the spleen and red
Excess amino acids, those not needed right away bone marrow and excretes it into bile to be elimi-
for protein synthesis, cannot be stored. However, nated in feces.
they do serve another useful purpose. By the 6. Phagocytosis by Kupffer cells—The fixed
process of deamination, which also occurs in the macrophages of the liver are called Kupffer cells
liver, the NH2 group is removed from an amino (or stellate reticuloendothelial cells). Besides
acid, and the remaining carbon chain may be con- destroying old RBCs, Kupffer cells phagocytize
verted to a simple carbohydrate molecule or to fat. pathogens or other foreign material that circulate
Thus, excess amino acids are utilized for energy through the liver. Many of the bacteria that get to
production: either for immediate energy or for the the liver come from the colon. These bacteria are
potential energy stored as fat in adipose tissue. The part of the normal flora of the colon but would be
NH2 groups that were detached from the original very harmful elsewhere in the body. The bacteria
amino acids are combined to form urea, a waste that enter the blood with the water absorbed by the
product that will be removed from the blood by the colon are carried to the liver by way of portal cir-
kidneys and excreted in urine. culation. The Kupffer cells in the liver phagocytize
3. Lipid metabolism—The liver forms lipoproteins, and destroy these bacteria, removing them from
which as their name tells us, are molecules of lipids the blood before the blood returns to the heart.
and proteins, for the transport of fats in the blood 7. Storage—The liver stores the fat-soluble vitamins
to other tissues. The liver also synthesizes choles- A, D, E, and K, and the water-soluble vitamin B12.
terol and excretes excess cholesterol into bile to be Up to a 6- to 12-month supply of vitamins A and D
eliminated in feces. may be stored, and beef or chicken liver is an excel-
lent dietary source of these vitamins.
Fatty acids are a potential source of energy, but
in order to be used in cell respiration they must be Also stored by the liver are the minerals iron and
broken down to smaller molecules. In the process copper. You already know that iron is needed for
of beta-oxidation, the long carbon chains of fatty hemoglobin and myoglobin and enables these pro-

Copyright © 2007 by F. A. Davis.

The Digestive System 389

teins to bond to oxygen. Copper (as well as iron) is ineffective, with the result that both the alcohol
part of some of the proteins needed for cell respi- and the medication will remain toxic for a longer
ration, and is part of some of the enzymes neces- time. Barbiturates taken as sleeping pills after con-
sary for hemoglobin synthesis. sumption of alcohol have too often proved fatal for
8. Detoxification—The liver is capable of synthesiz- just this reason.
ing enzymes that will detoxify harmful substances,
that is, change them to less harmful ones. Alcohol, Ammonia is a toxic substance produced by the
for example, is changed to acetate, which is a two- bacteria in the colon. Because it is soluble in water,
carbon molecule (an acetyl group) that can be used some ammonia is absorbed into the blood, but it is
in cell respiration. carried first to the liver by portal circulation. The
liver converts ammonia to urea, a less toxic sub-
Medications are all potentially toxic, but the stance, before the ammonia can circulate and dam-
liver produces enzymes that break them down or age other organs, especially the brain. The urea
change them. When given in a proper dosage, a formed is excreted by the kidneys (see Box 16–6:
medication exerts its therapeutic effect but is then Hepatitis).
changed to less active substances that are usually
excreted by the kidneys. An overdose of a drug AGING AND THE
means that there is too much of it for the liver to DIGESTIVE SYSTEM
detoxify in a given time, and the drug will remain
in the body with possibly harmful effects. This is Many changes can be expected in the aging digestive
why alcohol should never be consumed when tak- system. The sense of taste becomes less acute, less
ing medication. Such a combination may cause the saliva is produced, and there is greater likelihood of
liver’s detoxification ability to be overworked and

BOX 16–6 HEPATITIS include blood and semen. Hepatitis B may be
severe or even fatal, and approximately 10% of
Hepatitis is inflammation of the liver caused those who recover become carriers of the virus.
by any of several viruses. The most common of Possible consequences of the carrier state are
these hepatitis viruses have been designated A, B, chronic hepatitis progressing to cirrhosis or primary
and C, although there are others. Symptoms of hep- liver cancer. Of equal importance, carriers are
atitis include anorexia, nausea, fatigue, and possibly sources of the virus for others, especially their sex-
jaundice. Severity of disease ranges from very mild ual partners.
(even asymptomatic) to fatal. Hundreds of thou-
sands of cases of hepatitis occur in the United States A vaccine is available for hepatitis B, and health-
every year, and although liver inflammation is com- care workers who have contact with blood, even
mon to all of them, the three hepatitis viruses have just occasional contact, should receive it. Other
different modes of transmission and different conse- potential recipients of the vaccine are the sexual
quences for affected people. partners of carriers. Pediatricians now consider this
vaccine one of the standard ones for infants.
Hepatitis A is an intestinal virus that is spread
by the fecal–oral route. Food contaminated by the The hepatitis C virus is also present in body flu-
hands of people with mild cases is the usual vehicle ids and is spread by blood or mucous membrane
of transmission, although shellfish harvested from contact. Most people develop chronic disease, but
water contaminated with human sewage are many may remain asymptomatic for years after
another possible source of this virus. Hepatitis A is being infected. With active disease the virus may
most often mild, recovery provides lifelong immu- cause liver failure. The only therapy then is a liver
nity, and the carrier state is not known to occur. A transplant.
vaccine is available, but people who have been
exposed to hepatitis A may receive gamma globu- It is important for healthcare personnel, and
lin by injection to prevent the disease. their patients, to know that these types of hepatitis
are not spread by blood transfusions. Donated
Hepatitis B is contracted by exposure to the blood is tested for all three viruses.
body fluids of an infected person; these fluids

Copyright © 2007 by F. A. Davis.

390 The Digestive System

periodontal disease and loss of teeth. Secretions are more frequent in older adults. In the absence of spe-
reduced throughout the digestive system, and the cific diseases, the pancreas usually functions well,
effectiveness of peristalsis diminishes. Indigestion may although acute pancreatitis of unknown cause is some-
become more frequent, especially if the LES loses its what more likely in elderly people.
tone, and there is a greater chance of esophageal dam-
age. In the colon, diverticula may form; these are bub- SUMMARY
ble-like outpouchings of the weakened wall of the
colon that may be asymptomatic or become infected. The processes of the digestion of food and the absorp-
Intestinal obstruction, of the large or small bowel, tion of nutrients enable the body to use complex food
occurs with greater frequency among the elderly. molecules for many purposes. Much of the food we eat
Sluggish peristalsis contributes to constipation, which literally becomes part of us. The body synthesizes pro-
in turn may contribute to the formation of hemor- teins and lipids for the growth and repair of tissues and
rhoids. The risk of oral cancer or colon cancer also produces enzymes to catalyze all of the reactions that
increases with age. contribute to homeostasis. Some of our food provides
the energy required for growth, repair, movement,
The liver usually continues to function adequately sensation, and thinking. In the next chapter we will
even well into old age, unless damaged by pathogens discuss the chemical basis of energy production from
such as the hepatitis viruses or by toxins such as alco- food and consider the relationship of energy produc-
hol. There is a greater tendency for gallstones to form, tion to the maintenance of body temperature.
perhaps necessitating removal of the gallbladder.
Inflammation of the gallbladder (cholecystitis) is also

STUDY OUTLINE 3. Proteins are digested to amino acids.
4. Other end products are vitamins, minerals, and
Function of the Digestive System—to break
down food into simple chemicals that can water.
be absorbed into the blood and lymph and
utilized by cells Oral Cavity—food enters by way of the
mouth
Divisions of the Digestive System 1. Teeth and tongue break up food and mix it with
1. Alimentary tube—oral cavity, pharynx, esophagus,
saliva.
stomach, small intestine, large intestine. Digestion 2. Tooth structure (see Fig. 16–2)—enamel covers the
takes place in the oral cavity, stomach, and small
intestine. crown and provides a hard chewing surface; dentin
2. Accessory organs—salivary glands, teeth, tongue, is within the enamel and forms the roots; the pulp
liver, gallbladder, and pancreas. Each contributes to cavity contains blood vessels and endings of the
digestion. trigeminal nerve; the periodontal membrane
produces cement to anchor the tooth in the jaw-
Types of Digestion bone.
1. Mechanical—breaks food into smaller pieces to 3. The tongue is skeletal muscle innervated by the
hypoglossal nerves. Papillae on the upper surface
increase the surface area for the action of enzymes. contain taste buds (facial and glossopharyngeal
2. Chemical—enzymes break down complex organics nerves). Functions: taste, keeps food between the
teeth when chewing, elevates to push food back-
into simpler organics and inorganics; each enzyme ward for swallowing.
is specific for the food it will digest. 4. Salivary glands—parotid, submandibular, and sub-
lingual (see Fig. 16–3); ducts take saliva to the oral
End Products of Digestion cavity.
1. Carbohydrates are digested to monosaccharides.
2. Fats are digested to fatty acids and glycerol.

Copyright © 2007 by F. A. Davis.

The Digestive System 391

5. Saliva—amylase digests starch to maltose; water Liver—consists of two lobes in the upper
dissolves food for tasting and moistens food for right and center of the abdominal cavity
swallowing; lysozyme inhibits the growth of bacte- (see Figs. 16–1 and 16–6)
ria (see Tables 16–1 and 16–2). 1. Functional unit is the hexagonal liver lobule: liver

Pharynx—food passageway from the oral cells, sinusoids, branches of the hepatic artery and
cavity to the esophagus portal vein, and bile ducts.
1. No digestion takes place. 2. The only digestive secretion is bile; the hepatic
2. Contraction of pharyngeal muscles is part of swal- duct takes bile out of the liver and unites with the
cystic duct of the gallbladder to form the common
lowing reflex, regulated by the medulla. bile duct to the duodenum.
3. Bile salts emulsify fats, a type of mechanical diges-
Esophagus—food passageway from pharynx tion (see Table 16–1).
to stomach 4. Excess cholesterol and bilirubin are excreted by the
1. No digestion takes place. liver into bile.
2. Lower esophageal sphincter (LES) at junction with
Gallbladder—on undersurface of right lobe
stomach prevents backup of stomach contents. of liver (see Fig. 16–6)
1. Stores and concentrates bile until needed in the
Structural Layers of the Alimentary Tube
(see Fig. 16–4) duodenum (see Table 16–2).
1. Mucosa (lining)—made of epithelial tissue that 2. The cystic duct joins the hepatic duct to form the

produces the digestive secretions; lymph nodules common bile duct.
contain macrophages to phagocytize pathogens
that penetrate the mucosa; thin layer of smooth Pancreas—in upper left abdominal quadrant
muscle to ripple the epithelium. between the duodenum and the spleen (see
2. Submucosa—areolar connective tissue with blood Fig. 16–1)
vessels and lymphatic vessels; Meissner’s plexus is a 1. Pancreatic juice is secreted by acini, carried by pan-
nerve network that innervates the mucosa, part of
the enteric nervous system that extends the entire creatic duct to the common bile duct to the duode-
length of the alimentary tube. num (see Fig. 16–7).
3. External muscle layer—typically an inner circular 2. Enzyme pancreatic juice contains enzymes for the
layer and an outer longitudinal layer of smooth digestion of all three food types (see Tables 16–1
muscle; function is mechanical digestion and peri- and 16–2).
stalsis; innervated by Auerbach’s plexus, part of the 3. Bicarbonate pancreatic juice neutralizes HCl from
enteric nervous system; sympathetic impulses the stomach in the duodenum.
decrease motility; parasympathetic impulses
increase motility. Small Intestine—coiled within the center of
4. Serosa—outermost layer; above the diaphragm is the abdominal cavity (see Fig. 16–1); extends
fibrous connective tissue; below the diaphragm is from stomach to colon
the mesentery (serous). The peritoneum (serous) 1. Duodenum—first 10 inches; the common bile duct
lines the abdominal cavity; serous fluid prevents
friction between the serous layers. brings in bile and pancreatic juice. Jejunum (8 feet)
and ileum (11 feet).
Stomach—in upper left abdominal quadrant; 2. Enzymes secreted by the intestinal glands complete
a muscular sac that extends from the esoph- digestion (see Tables 16–1 and 16–2). Surface area
agus to the small intestine (see Fig. 16–5) for absorption is increased by plica circulares, villi,
1. Reservoir for food; begins the digestion of protein. and microvilli (see Fig. 16–8); microvilli are the
2. Gastric juice is secreted by gastric pits (see Tables brush border.
3. The villi contain capillary networks for the absorp-
16–1 and 16–2). tion of water-soluble nutrients: monosaccharides,
3. The pyloric sphincter at the junction with the duo-

denum prevents backup of intestinal contents.

Copyright © 2007 by F. A. Davis.

392 The Digestive System

amino acids, vitamin C and the B vitamins, miner- Liver—other functions
als, and water. Blood from the small intestine goes 1. Carbohydrate metabolism—excess glucose is
to the liver first by way of portal circulation.
4. The villi contain lacteals (lymph capillaries) for the stored in the form of glycogen and converted back
absorption of fat-soluble nutrients: vitamins A, D, to glucose during hypoglycemia; fructose and
E, and K, fatty acids, and glycerol, which are com- galactose are changed to glucose.
bined to form chylomicrons. Lymph from the small 2. Amino acid metabolism—the non-essential amino
intestine is carried back to the blood in the left sub- acids are synthesized by transamination; excess
clavian vein. amino acids are changed to carbohydrates or fats by
deamination; the amino groups are converted to
Large Intestine (colon)—extends from the urea and excreted by the kidneys.
small intestine to the anus 3. Lipid metabolism—formation of lipoproteins for
1. Colon—parts (see Fig. 16–9): cecum, ascending transport of fats in the blood; synthesis of choles-
terol; excretion of excess cholesterol into bile; beta-
colon, transverse colon, descending colon, sigmoid oxidation of fatty acids to form two-carbon acetyl
colon, rectum, anal canal. groups for energy use.
2. Ileocecal valve—at the junction of the cecum and 4. Synthesis of plasma proteins—albumin to help
ileum; prevents backup of fecal material into the maintain blood volume; clotting factors for blood
small intestine. clotting; alpha and beta globulins as carrier mole-
3. Colon—functions: absorption of water, minerals, cules.
vitamins; elimination of undigestible material. 5. Formation of bilirubin—old RBCs are phagocy-
4. Normal flora—the bacteria of the colon; produce tized, and bilirubin is formed from the heme and
vitamins, especially vitamin K, and inhibit the put into bile to be eliminated in feces.
growth of pathogens. 6. Phagocytosis by Kupffer cells—fixed macrophages;
5. Defecation reflex—stimulus: stretching of the rec- phagocytize old RBCs and bacteria, especially bac-
tum when peristalsis propels feces into it. Sensory teria absorbed by the colon.
impulses go to the sacral spinal cord, and motor 7. Storage—vitamins: B12, A, D, E, and K, and the
impulses return to the smooth muscle of the rec- minerals iron and copper.
tum, which contracts. The internal anal sphincter 8. Detoxification—liver enzymes change potential
relaxes to permit defecation. Voluntary control is poisons to less harmful substances; examples of
provided by the external anal sphincter, made of toxic substances are alcohol, medications, and
skeletal muscle (see Fig. 16–10). ammonia absorbed by the colon.

REVIEW QUESTIONS 5. Describe the function of the pharynx, esophagus,
and lower esophageal sphincter. (p. 373)
1. Name the organs of the alimentary tube, and
describe the location of each. Name the accessory 6. Name and describe the four layers of the alimen-
digestive organs, and describe the location of each. tary tube. (pp. 373, 376)
(pp. 370, 372, 373, 376, 378, 379, 385)
7. State the two general functions of the stomach and
2. Explain the purpose of mechanical digestion, and the function of the pyloric sphincter. Explain the
give two examples. Explain the purpose of chemical function of pepsin, HCl, and mucus. (pp. 376–378)
digestion, and give two examples. (pp. 370, 374)
8. Describe the general functions of the small intes-
3. Name the end products of digestion, and explain tine, and name the three parts. Describe the struc-
how each is absorbed in the small intestine. tures that increase the surface area of the small
(pp. 370, 384) intestine. (pp. 378, 383–384)

4. Explain the function of teeth and tongue, salivary 9. Explain how the liver, gallbladder, and pancreas
amylase, enamel of teeth, lysozyme, and water of contribute to digestion. (pp. 379, 381)
saliva. (pp. 370–372)

Copyright © 2007 by F. A. Davis.

The Digestive System 393

10. Describe the internal structure of a villus, and anal sphincter, and the voluntary control possible.
explain how its structure is related to absorption. (p. 386)
(p. 384)
14. Name the vitamins and minerals stored in the
11. Name the parts of the large intestine, and liver. Name the fixed macrophages of the liver,
describe the function of the ileocecal valve. and explain their function. (p. 388)
(p. 385)
15. Describe how the liver regulates blood glucose
12. Describe the functions of the colon and of the level. Explain the purpose of the processes of
normal flora of the colon. (pp. 385–386) deamination and transamination. (pp. 386, 388)

13. With respect to the defecation reflex, explain the 16. Name the plasma proteins produced by the liver,
stimulus, the part of the CNS directly involved, and state the function of each. (p. 388)
the effector muscle, the function of the internal
17. Name the substances excreted by the liver into
bile. (p. 388)

FOR FURTHER THOUGHT purpose. Explain, and state the disadvantages as
well.
1. Many people with GERD take proton-pump
inhibitors, medications that reduce stomach acid. 5. Explain how a spinal cord transection at the level of
Why should these people be especially careful T10 will affect the defecation reflex.
about what they eat or drink?
6. You have seen the word enteric (or entero) several
2. The colon does not have villi as part of its mucosa. times in this chapter. What does it mean? Define
Explain why villi are not necessary. each of these: enteric bacilli, enterovirus, Entero-
coccus.
3. Food remains in the stomach for several hours.
Passage of food through the small intestine also 7. The word symbiosis indicates two different kinds of
requires several hours. These two organs have very living things, and literally means “together-life.”
different shapes. Explain why they are able to Our own alimentary tube is a perfect example.
retain food for so long, for efficient digestion and Explain, and state the advantages to each living
absorption. thing.

4. Diarrhea can be unpleasant, but does have a

Copyright © 2007 by F. A. Davis.

CHAPTER 17

Chapter Outline Student Objectives

Body Temperature • State the normal range of human body tempera-
Heat Production
Heat Loss ture.

Heat loss through the skin • Explain how cell respiration produces heat and the
Heat loss through the respiratory tract
Heat loss through the urinary and digestive tracts factors that affect heat production.
Regulation of Body Temperature
Mechanisms to increase heat loss • Describe the pathways of heat loss through the
Mechanisms to conserve heat
Fever skin and respiratory tract.
Metabolism
Cell Respiration • Explain why the hypothalamus is called the “ther-
Glycolysis
Krebs citric acid cycle mostat” of the body.
Cytochrome transport system
Proteins and fats as energy sources • Describe the mechanisms to increase heat loss.
Energy available from the three nutrient types • Describe the mechanisms to conserve heat.
Synthesis Uses of Foods • Explain how a fever is caused and its advantages
Glucose
Amino acids and disadvantages.
Fatty acids and glycerol
Vitamins and Minerals • Define metabolism, anabolism, and catabolism.
Metabolic Rate • Describe what happens to a glucose molecule dur-
Aging and Metabolism
ing 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 kilo-

calories.

• Describe the factors that affect a person’s meta-

bolic rate.

BOX 17–1 HEAT-RELATED DISORDERS
BOX 17–2 COLD-RELATED DISORDERS
BOX 17–3
BOX 17–4 KETOSIS
BOX 17–5
BOX 17–6 METABOLIC RATE

WEIGHT LOSS

LEPTIN AND BODY-MASS INDEX

394

Copyright © 2007 by F. A. Davis.

Body Temperature
and Metabolism

New Terminology Related Clinical Terminology

Anabolism (an-AB-uh-lizm) Antipyretic (AN-tigh-pye-RET-ik)
Catabolism (kuh-TAB-uh-lizm) Basal metabolic rate (BAY-zuhl met-ah-BAHL-ik
Coenzyme (ko-EN-zime)
Conduction (kon-DUK-shun) RAYT)
Convection (kon-VEK-shun) Frostbite (FRAWST-bite)
Cytochromes (SIGH-toh-krohms) Heat exhaustion (HEET eks-ZAWS-chun)
Endogenous pyrogen (en-DOJ-en-us PYE-roh-jen) Heat stroke (HEET STROHK)
Fever (FEE-ver) Hypothermia (HIGH-poh-THER-mee-ah)
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)

Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.

395

Copyright © 2007 by F. A. Davis.

396 Body Temperature and Metabolism

During every moment of our lives, our cells are 1. The hormone thyroxine (and T3), produced by the
thyroid gland, increases the rate of cell respiration
breaking down food molecules to obtain ATP (adeno- and heat production. The secretion of thyroxine is
sine triphosphate) for energy-requiring cellular regulated by the body’s rate of energy production,
processes. Naturally, we are not aware of the process the metabolic rate itself. (See Chapter 10 for a dis-
of cell respiration, but we may be aware of one of the cussion of the feedback mechanism involving the
products—energy in the form of heat. The human hypothalamus and anterior pituitary gland and
body is indeed warm, and its temperature is regulated Chapter 1 for an illustration.) When the metabolic
very precisely. Though we cannot stand barefoot on rate decreases, the thyroid gland is stimulated to
the ice of Antarctica for months in winter, as penguins secrete more thyroxine. As thyroxine increases the
do, we can adapt to and survive a wide range of envi- rate of cell respiration, a negative feedback mecha-
ronmental temperatures. nism inhibits further secretion until metabolic rate
decreases again. Thus, thyroxine is secreted when-
This chapter discusses the regulation of body tem- ever there is a need for increased cell respiration
perature and also discusses metabolism, which is the and is probably the most important regulator of
total of all reactions that take place within the body. day-to-day energy production.
These reactions include the energy-releasing ones of
cell respiration and energy-requiring ones such as 2. In stress situations, epinephrine and norepineph-
protein synthesis, or DNA synthesis for mitosis. As rine are secreted by the adrenal medulla, and the
you will see, body temperature and metabolism are sympathetic nervous system becomes more active.
inseparable. Epinephrine increases the rate of cell respiration,
especially in organs such as the heart, skeletal mus-
BODY TEMPERATURE cles, and liver. Sympathetic stimulation also in-
creases the activity of these organs. The increased
The normal range of human body temperature is production of ATP to meet the demands of the
96.5° to 99.5°F (36° to 38°C), with an average oral stress situation also means that more heat will be
temperature of 98.6°F (37°C). (A 1992 study sug- produced.
gested a slightly lower average oral temperature: 98.2°
or 36.8°. But everyone seems to prefer the “tradi- 3. Organs that are normally active (producing
tional” average temperature.) Within a 24-hour ATP) are significant sources of heat when the body
period, an individual’s temperature fluctuates 1° to 2°, is at rest. The skeletal muscles, for example, are
with the lowest temperatures occurring during sleep. usually in a state of slight contraction called muscle
tone. Because even slight contraction requires ATP,
At either end of the age spectrum, however, tem- the muscles are also producing heat. This amounts
perature regulation may not be as precise as it is in to about 25% of the total body heat at rest and
older children or younger adults. Infants have more much more during exercise, when more ATP is
surface area (skin) relative to volume and are likely to produced.
lose heat more rapidly. In the elderly, the mechanisms The liver is another organ that is continually
that maintain body temperature may not function as active, producing ATP to supply energy for its
efficiently as they once did, and changes in environ- many functions. As a result, the liver produces as
mental temperature may not be compensated for as much as 20% of the total body heat at rest. The
quickly or effectively. This is especially important to heat produced by these active organs is dispersed
remember when caring for patients who are very throughout the body by the blood. As the relatively
young or very old. cooler blood flows through organs such as the mus-
cles and liver, the heat they produce is transferred
HEAT PRODUCTION to the blood, warming it. The warmed blood circu-
lates to other areas of the body, distributing this
Cell respiration, the process that releases energy from heat.
food to produce ATP, also produces heat as one of its
energy products. Although cell respiration takes place 4. The intake of food also increases heat production,
constantly, many factors influence the rate of this because the metabolic activity of the digestive tract
process: is increased. Heat is generated as the digestive

Copyright © 2007 by F. A. Davis.

Body Temperature and Metabolism 397

organs produce ATP for peristalsis and for the syn- Heat Loss through the Skin
thesis of digestive enzymes.
5. Changes in body temperature also have an effect Because the skin covers the body, most body heat is
on metabolic rate and heat production. This lost from the skin to the environment. When the envi-
becomes clinically important when a person has ronment is cooler than body temperature (as it usually
a fever, an abnormally high body tempera- is), heat loss is unavoidable. The amount of heat that
ture. The higher temperature increases the meta- is lost is determined by blood flow through the skin
bolic rate, which increases heat production and and by the activity of sweat glands.
elevates body temperature further. Thus, a high
fever may trigger a vicious cycle of ever-increasing Blood flow through the skin influences the amount
heat production. Fever is discussed later in this of heat lost by the processes of radiation, conduction,
chapter. and convection. Radiation means that heat from the
body is transferred to cooler objects not touching the
The factors that affect heat production are summa- skin, much as a radiator warms the contents of a room
rized in Table 17–1. (radiation starts to become less effective when the
environmental temperature rises above 88°F). Con-
HEAT LOSS duction is the loss of heat to cooler air or objects, such
as clothing, that touch the skin. Convection means
The pathways of heat loss from the body are the skin, that air currents move the warmer air away from the
the respiratory tract, and, to a lesser extent, the uri- skin surface and facilitate the loss of heat; this is why a
nary and digestive tracts. fan makes us feel cooler on hot days. Loss of heat by
convection also gives us the “wind chill factor” we
Table 17–1 FACTORS THAT AFFECT hear about in winter. A cold day that is windy will feel
HEAT PRODUCTION colder than a cold day when the air is still, because the
wind blows the slightly warmer air surrounding the
Factor Effect body away, replacing it with colder air.
Thyroxine
• The most important regulator As you may recall from Chapter 5, the tempera-
Epinephrine and of day-to-day metabolism; ture of the skin and the subsequent loss of heat are
sympathetic increases use of foods for ATP determined by blood flow through the skin. The arte-
stimulation production, thereby increasing rioles in the dermis may constrict or dilate to decrease
heat production or increase blood flow. In a cold environment, vaso-
Skeletal muscles constriction decreases blood flow through the dermis
• Important in stress situations; and thereby decreases heat loss. In a warm environ-
Liver increases the metabolic activity ment, vasodilation in the dermis increases blood flow
of many organs; increases ATP to the body surface and loss of heat to the environ-
Food intake and heat production ment.

Higher body • Normal muscle tone requires The other mechanism by which heat is lost from
temperature ATP; the heat produced is the skin is sweating. The eccrine sweat glands
about 25% of the total body secrete sweat (water) onto the skin surface, and excess
heat at rest body heat evaporates the sweat. Think of running
water into a hot frying pan; the pan is rapidly cooled
• Always metabolically active; as its heat vaporizes the water. Although sweating is
produces as much as 20% of not quite as dramatic (no visible formation of steam),
total body heat at rest the principle is just the same.

• Increases activity of the GI Sweating is most efficient when the humidity of the
tract; increases ATP and heat surrounding air is low. Humidity is the percentage of
production the maximum amount of water vapor the atmosphere
can contain. A humidity reading of 90% means that
• Increases metabolic rate, which the air is already 90% saturated with water vapor and
increases heat production, can hold little more. In such a situation, sweat does
which further increases meta- not readily evaporate, but instead remains on the skin
bolic rate and heat production; even as more sweat is secreted. If the humidity is 40%,
may become detrimental dur-
ing high fevers

Copyright © 2007 by F. A. Davis.

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
Heat exhaustion is caused by excessive sweating preserve body fluid, and body temperature rises
with loss of water and salts, especially NaCl. The rapidly (over 105°F , possibly as high as 110°F ).
affected person feels very weak, and the skin is usu-
ally cool and clammy (moist). Body temperature is The classic symptom of heat stroke is hot, dry
normal or slightly below normal, the pulse is often skin. The affected person often loses consciousness,
rapid and weak, and blood pressure may be low reflecting the destructive effect of such a high body
because of fluid loss. Other symptoms may include temperature on the brain. Treatment should involve
dizziness, vomiting, and muscle cramps. Treatment hospitalization so that IV fluids may be adminis-
involves rest and consumption of salty fluids or fruit tered and body temperature lowered under med-
juices (in small amounts at frequent intervals). ical supervision. A first-aid measure would be the
application of cool (not ice cold) water to as much
Heat stroke is a life-threatening condition that of the skin as possible. Fluids should never be forced
may affect elderly or chronically ill people on hot, on an unconscious person, because the fluid may
humid days, or otherwise healthy people who exer- be aspirated into the respiratory tract.
cise too strenuously during such weather. High
humidity makes sweating an ineffective mechanism

however, the air can hold a great deal more water Heat Loss through the Urinary
vapor, and sweat evaporates quickly from the skin sur- and Digestive Tracts
face, removing excess body heat. In air that is com-
pletely dry, a person may tolerate a temperature of When excreted, urine and feces are at body tempera-
200°F for nearly 1 hour. ture, and their elimination results in a very small
amount of heat loss.
Although sweating is a very effective mechanism of
heat loss, it does have a disadvantage in that it requires The pathways of heat loss are summarized in Table
the loss of water in order to also lose heat. Water loss 17–2.
during sweating may rapidly lead to dehydration, and
the water lost must be replaced by drinking fluids (see REGULATION OF BODY TEMPERATURE
Box 17–1: Heat-Related Disorders).
The hypothalamus is responsible for the regulation
Small amounts of heat are also lost in what is called of body temperature and is considered the “thermo-
“insensible water loss.” Because the skin is not like a stat” of the body. As the thermostat, the hypothalamus
plastic bag, but is somewhat permeable to water, a maintains the “setting” of body temperature by bal-
small amount of water diffuses through the skin and is ancing heat production and heat loss to keep the body
evaporated by body heat. Compared to sweating, how- at the set temperature.
ever, insensible water loss is a minor source of heat
loss. To do this, the hypothalamus must receive infor-
mation about the temperature within the body and
Heat Loss through about the environmental temperature. Specialized
the Respiratory Tract neurons of the hypothalamus detect changes in the
temperature of the blood that flows through the brain.
Heat is lost from the respiratory tract as the warmth of The temperature receptors in the skin provide infor-
the respiratory mucosa evaporates some water from mation about the external temperature changes to
the living epithelial surface. The water vapor formed which the body is exposed. The hypothalamus then
is exhaled, and a small amount of heat is lost. integrates this sensory information and promotes the
necessary responses to maintain body temperature
Animals such as dogs that do not have numerous within the normal range.
sweat glands often pant in warm weather. Panting is
the rapid movement of air into and out of the upper Mechanisms to Increase Heat Loss
respiratory passages, where the warm surfaces evapo-
rate large amounts of water. In this way the animal In a warm environment or during exercise, the body
may lose large amounts of heat. temperature tends to rise, and greater heat loss is

Copyright © 2007 by F. A. Davis.

Body Temperature and Metabolism 399

Table 17–2 PATHWAYS OF HEAT LOSS If these mechanisms are not sufficient to prevent
the body temperature from dropping, more heat may
Pathway Mechanism be produced by increasing muscle tone. When this
Skin (major greater muscle tone becomes noticeable and rhythmic,
• Radiation and conduction— it is called shivering and may increase heat production
pathway) heat is lost from the body to by as much as five times the normal.
cooler air or objects.
Respiratory tract People also have behavioral responses to cold, and
(secondary • Convection—air currents move these too are important to prevent heat loss. Such
pathway) warm air away from the skin. things as putting on a sweater or going indoors reflect
our awareness of the discomfort of being cold. For
Urinary tract • Sweating—excess body heat people (we do not have thick fur as do some other
(minor pathway) evaporates sweat on the skin mammals), these voluntary activities are of critical
surface. importance to the prevention of excessive heat loss
Digestive tract when it is very cold (see Box 17–2: Cold-Related
(minor pathway) • Evaporation—body heat evap- Disorders).
orates water from the respira-
tory mucosa, and water vapor FEVER
is exhaled.
A fever is an abnormally high body temperature
• Urination—urine is at body and may accompany infectious diseases, extensive
temperature when eliminated. physical trauma, cancer, or damage to the CNS. The
substances that may cause a fever are called pyrogens.
• Defecation—feces are at body Pyrogens include bacteria, foreign proteins, and
temperature when eliminated. chemicals released during inflammation. These
inflammatory chemicals are called endogenous pyro-
needed. This is accomplished by vasodilation in the gens. Endogenous means “generated from within.” It
dermis and an increase in sweating. Vasodilation is believed that pyrogens chemically affect the hypo-
brings more warm blood close to the body surface, thalamus and “raise the setting” of the hypothalamic
and heat is lost to the environment. However, if the thermostat. The hypothalamus will then stimulate
environmental temperature is close to or higher than responses by the body to raise body temperature to
body temperature, this mechanism becomes ineffec- this higher setting.
tive. The second mechanism is increased sweating, in
which excess body heat evaporates the sweat on the Let us use as a specific example a child who has a
skin surface. As mentioned previously, sweating strep throat. The bacterial and endogenous pyrogens
becomes inefficient when the atmospheric humidity is reset the hypothalamic thermostat upward, to 102°F.
high. At first, the body is “colder” than the setting of the
hypothalamus, and the heat conservation and produc-
On hot days, heat production may also be tion mechanisms are activated. The child feels cold
decreased by a decrease in muscle tone. This is why we and begins to shiver (chills). Eventually, sufficient heat
may feel very sluggish on hot days; our muscles are is produced to raise the body temperature to the hypo-
even slightly less contracted than usual and are slower thalamic setting of 102°F. At this time, the child will
to respond. feel neither too warm nor too cold, because the body
temperature is what the hypothalamus wants.
Mechanisms to Conserve Heat
As the effects of the pyrogens diminish, the hypo-
In a cold environment, heat loss from the body is thalamic setting decreases, perhaps close to normal
unavoidable but may be reduced to some extent. again, 99°F. Now the child will feel warm, and the heat
Vasoconstriction in the dermis shunts blood away loss mechanisms will be activated. Vasodilation in the
from the body surface, so that more heat is kept in the skin and sweating will occur until the body tempera-
core of the body. Sweating decreases, and will stop ture drops to the new hypothalamic setting. This
completely if the temperature of the hypothalamus is sometimes referred to as the “crisis,” but actually
falls below about 98.6°F. (Remember that the internal the crisis has passed, because sweating indicates that
temperature of the brain is higher than an oral tem-
perature, and is less subject to any changes in environ-
mental temperature.)

Copyright © 2007 by F. A. Davis.

400 Body Temperature and Metabolism

BOX 17–2 COLD-RELATED DISORDERS slurred speech, drowsiness, and lack of coordina-
tion. At this stage, people often do not realize the
Frostbite is the freezing of part of the body. seriousness of their condition, and if outdoors (ice
Fingers, toes, the nose, and ears are most often skating or skiing) may not seek a warmer environ-
affected by prolonged exposure to cold, because ment. In progressive hypothermia, breathing and
these areas have little volume in proportion to their heart rate slow, and coma and death follow.
surface.
Other people at greater risk for hypothermia
At first the skin tingles, then becomes numb. If include the elderly, whose temperature-regulating
body fluids freeze, ice crystals may destroy capillar- mechanisms are no longer effective, and quadriple-
ies and tissues (because water expands when it gics, who have no sensation of cold in the body. For
freezes), and blisters form. In the most severe cases both of these groups, heat production is or may be
gangrene develops; that is, tissue dies because of low because of inactivity of skeletal muscles.
lack of oxygen.
Artificial hypothermia may be induced during
Treatment of frostbite includes rewarming the some types of cardiovascular or neurologic surgery.
affected area. If skin damage is apparent, it should This carefully controlled lowering of body tempera-
be treated as if it were a burn injury. ture decreases the metabolic rate and need for
oxygen and makes possible prolonged surgery
Hypothermia is an abnormally low body tem- without causing extensive tissue death in the
perature (below 95°F) that is most often the result patient.
of prolonged exposure to cold. Although the
affected person certainly feels cold at first, this sen-
sation may pass and be replaced by confusion,

the body temperature is returning to normal. The cause of brain damage that may follow a prolonged
sequence of temperature changes during a fever is high fever. The effects of changes in body temperature
shown in Fig. 17–1. on the hypothalamus are shown in Fig. 17–2.

You may be wondering if a fever serves a useful pur- A medication such as aspirin is called an anti-
pose. For low fevers that are the result of infection, the pyretic because it lowers a fever, probably by affecting
answer is yes. White blood cells increase their activity the hypothalamic thermostat. To help lower a high
at moderately elevated temperatures, and the metabo- fever, the body may be cooled by sponging it with cool
lism of some pathogens is inhibited. Thus, a fever may water. The excessive body heat will cause this external
be beneficial in that it may shorten the duration of an water to evaporate, thus reducing temperature. A very
infection by accelerating the destruction of the high fever requires medical attention.
pathogen.
METABOLISM
High fevers, however, may have serious conse-
quences. A fever increases the metabolic rate, which The term metabolism encompasses all of the reac-
increases heat production, which in turn raises body tions that take place in the body. Everything that hap-
temperature even more. This is a positive feedback pens within us is part of our metabolism. The reactions
mechanism that will continue until an external event of metabolism may be divided into two major cate-
(such as aspirin or death of the pathogens) acts as a gories: anabolism and catabolism.
brake (see Fig. 1–3). When the body temperature rises
above 106°F, the hypothalamus begins to lose its abil- Anabolism means synthesis or “formation” reac-
ity to regulate temperature. The proteins of cells, tions, the bonding together of smaller molecules to
especially the enzymes, are also damaged by such high form larger ones. The synthesis of hemoglobin by
temperatures. Enzymes become denatured, that is, cells of the red bone marrow, synthesis of glycogen by
lose their shape and do not catalyze the reactions nec- liver cells, and synthesis of fat to be stored in adipose
essary within cells (see Fig. 2–9). As a result, cells tissue are all examples of anabolism. Such reactions
begin to die. This is most serious in the brain, because require energy, usually in the form of ATP.
neurons cannot be replaced, and cellular death is the

Copyright © 2007 by F. A. Davis.

Body Temperature and Metabolism 401

Hypothalamic Actual body
thermostat temperature

105˚ Pyrogen affects hypothalamus
104˚ Vasoconstriction, shivering
103˚
102˚ Effect of pyrogen diminishes

Sweating, vasodilation

Body temperature 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
Time

Figure 17–1. Changes in body temperature during an episode of fever. The body tem-
perature 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?

Catabolism means decomposition, the breaking of CELL RESPIRATION
bonds of larger molecules to form smaller molecules.
Cell respiration is a series of catabolic reactions that You are already familiar with the summary reaction of
break down food molecules to carbon dioxide and cell respiration,
water. During catabolism, energy is often released and
used to synthesize ATP (the heat energy released was C6H12O6 ϩ O2 → CO2 ϩ H2O ϩ ATP ϩ Heat,
discussed in the previous section). The ATP formed (glucose)
during catabolism is then used for energy-requiring
anabolic reactions. the purpose of which is to produce ATP. Glucose con-
tains potential energy, and when it is broken down to
Most of our anabolic and catabolic reactions are CO2 and H2O, this energy is released in the forms of
catalyzed by enzymes. Enzymes are proteins that ATP and heat. The oxygen that is required comes
enable reactions to take place rapidly at body temper- from breathing, and the CO2 formed is circulated to
ature (see Chapter 2 to review the active site theory of the lungs to be exhaled. The water formed is called
enzyme functioning). The body has thousands of metabolic water, and helps to meet our daily need for
enzymes, and each is specific, that is, will catalyze only water. Energy in the form of heat gives us a body tem-
one type of reaction. As you read the discussions that perature, and the ATP formed is used for energy-
follow, keep in mind the essential role of enzymes. requiring reactions. Synthesis of ATP means that

Copyright © 2007 by F. A. Davis.

402 Body Temperature and Metabolism

Body temperature Temperature regulation by
the hypothalamus
Upper limit of survival? 114˚ 44˚
Heat stroke or high fever 112˚ 42˚ Temperature regulation is
Strenuous exercise or fever 110˚ 40˚ seriously impaired
108˚ 38˚
Usual range of normal 106˚ 37˚ Temperature regulation is
104˚ 36˚ efficient
Hypothermia 102˚
(cold weather or 100˚ 34˚ Temperature regulation is
immersion in 98˚ impaired
cold water) 96˚ 32˚
30˚ Temperature regulation is
Lower limit of survival? 94˚ 28˚ lost
92˚
90˚ 26˚
88˚
24˚
86˚
84˚
82˚
80˚
78˚

76˚
74˚
72˚
70˚
68˚
66˚

˚F ˚C

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.

energy is used to bond a free phosphate molecule to piece,” with the removal of hydrogens and the split-
ADP (adenosine diphosphate). ADP and free phos- ting of carbon–carbon bonds. This releases the energy
phates are present in cells after ATP has been broken of glucose gradually, so that a significant portion
down for energy-requiring processes. (about 40%) is available to synthesize ATP.

The breakdown of glucose summarized here is not Cell respiration of glucose involves three major
quite that simple, however, and involves a complex stages: glycolysis, the Krebs citric acid cycle, and the
series of reactions. Glucose is taken apart “piece by cytochrome (or electron) transport system. Although

Copyright © 2007 by F. A. Davis.

Body Temperature and Metabolism 403

the details of each stage are beyond the scope of this (two per glucose). Notice also that a four-carbon mol-
book, we will summarize the most important aspects ecule (oxaloacetic acid) is regenerated after the forma-
of each, and then relate to them the use of amino acids tion of CO2. This molecule will react with the next
and fats for energy. This simple summary is depicted acetyl CoA, which is what makes the Krebs cycle truly
in Fig. 17–3. a self-perpetuating cycle. The results of the stages of
cell respiration are listed in Table 17–3. Before you
Glycolysis continue, you may wish to look at that table to see just
where the process has gotten thus far.
The enzymes for the reactions of glycolysis are found
in the cytoplasm of cells, and oxygen is not required Cytochrome Transport System
(glycolysis is an anaerobic process). Refer now to Fig.
17–3 as you read the following. In glycolysis, a six- Cytochromes are proteins that contain either iron or
carbon glucose molecule is broken down to two three- copper and are found in the mitochondria of cells.
carbon molecules of pyruvic acid. Two molecules of The pairs of hydrogens that were once part of glucose
ATP are necessary to start the process. The energy are brought to the cytochromes by the carrier mole-
they supply is called energy of activation and is neces- cules NAD and FAD. Each hydrogen atom is then
sary to make glucose unstable enough to begin to break split into its proton (H+ ion) and its electron. The
down. As a result of these reactions, enough energy is electrons of the hydrogens are passed from one
released to synthesize four molecules of ATP, for a net cytochrome to the next, and finally to oxygen. The
gain of two ATP molecules per glucose molecule. Also reactions of the electrons with the cytochromes
during glycolysis, two pairs of hydrogens are removed release most of the energy that was contained in the
by NAD, a carrier molecule that contains the vitamin glucose molecule, enough to synthesize 34 molecules
niacin. Two NAD molecules thus become 2NADH2, of ATP. As you can see, most of the ATP produced in
and these attached hydrogen pairs will be transported cell respiration comes from this third stage.
to the cytochrome transport system (stage 3).
Finally, and very importantly, each oxygen atom
If no oxygen is present in the cell, as may happen in that has gained two electrons (from the cytochromes)
muscle cells during exercise, pyruvic acid is converted reacts with two of the H+ ions (protons) to form water.
to lactic acid, which causes muscle fatigue. If oxygen is The formation of metabolic water contributes to the
present, however, pyruvic acid continues into the next necessary intracellular fluid, and also prevents acido-
stage, the Krebs citric acid cycle (or, more simply, the sis. If H+ ions accumulated, they would rapidly lower
Krebs cycle). the pH of the cell. This does not happen, however,
because the H+ ions react with oxygen to form water,
Krebs Citric Acid Cycle and a decrease in pH is prevented.

The enzymes for the Krebs cycle (or citric acid The summary of the three stages of cell respiration
cycle) are located in the mitochondria of cells. This in Table 17–3 also includes the vitamins and minerals
second stage of cell respiration is aerobic, meaning that are essential for this process. An important over-
that oxygen is required. In a series of reactions, a pyru- all concept is the relationship between eating and
vic acid molecule is “taken apart,” and its carbons are breathing. Eating provides us with a potential energy
converted to CO2. The first CO2 molecule is removed source (often glucose) and with necessary vitamins and
by an enzyme that contains the vitamin thiamine. minerals. However, to release the energy from food,
This leaves a two-carbon molecule called an acetyl we must breathe. This is why we breathe. The oxygen
group, which combines with a molecule called coen- we inhale is essential for the completion of cell respi-
zyme A to form acetyl coenzyme A (acetyl CoA). As ration, and the CO2 produced is exhaled.
acetyl CoA continues in the Krebs cycle, two more
carbons are removed as CO2, and more pairs of hydro- Proteins and Fats as Energy Sources
gens are picked up by NAD and FAD (another carrier
molecule that contains the vitamin riboflavin). Although glucose is the preferred energy source for
NADH2 and FADH2 will carry their hydrogens to the cells, proteins and fats also contain potential energy
cytochrome transport system. and are alternative energy sources in certain situations.

During the Krebs cycle, a small amount of energy As you know, proteins are made of the smaller mol-
is released, enough to synthesize one molecule of ATP ecules called amino acids, and the primary use for the
amino acids we obtain from food is the synthesis of

Copyright © 2007 by F. A. Davis.

404 Body Temperature and Metabolism

Proteins Carbohydrates Fats

(Digestion) (Digestion) (Digestion)

Glucose Glycerol
CCC
Amino NH2 C C C C C CCC
acids C

C 2 ATP Fatty acid
NH2 C CC
C C CC CC CC C
C NH2 C C CC C C C C C

C

2 NAD H2 4 ATP (Beta
oxidation
CC C CC in the liver)
Pyruvic acid
CCC C
CCC C CO2 Coenzyme A
C
CC
CC

Acetyl
groups

NH2 CC
NH2
Acetyl CoA
NH2
Oxaloacetic C C
Urea acid C C Citric acid
C
C
C
3 NAD H2 C

FAD H2 C
C

C CO2 C CO2
ATP

He

H+ Cytochromes 34 ATP
H+ H+

H2O =
Metabolic water Oxygen

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 cit-
ric acid cycle?

Copyright © 2007 by F. A. Davis.

Body Temperature and Metabolism 405

Table 17–3 SUMMARY OF CELL RESPIRATION

Stage Molecules That Results Vitamins or
Glycolysis Enter the Process Minerals Needed
• 2 ATP (net) • Niacin (part of NAD)
(cytoplasm) Glucose—ATP needed as • 2 NADH2 (to cytochrome
energy of activation • Thiamine (for removal
Krebs citric acid cycle transport system) of CO2)
(mitochondria) Pyruvic acid—from glucose • 2 pyruvic acid (aerobic:
or glycerol or excess • Niacin (part of NAD)
Cytochrome amino acids to Krebs cycle; anaerobic: • Riboflavin (part of FAD)
transport system lactic acid formation) • Pantothenic acid (part
(mitochondria) or
Acetyl CoA—from fatty acids • CO2 (exhaled) of coenzyme A)
• ATP (2 per glucose) • Iron and copper (part
or excess amino acids • 3 NADH2 and 1 FADH2 (to
NADH2 and FADH2—from of some cytochromes)
cytochrome transport system)
glycolysis or the Krebs • A 4-carbon molecule is regen-
cycle
erated for the next cycle

• 34 ATP
• Metabolic water

new proteins. Excess amino acids, however, those not converted to glucose; this is important to supply the
needed immediately for protein synthesis, may be brain when dietary intake of carbohydrates is low. The
used for energy production. In the liver, excess amino effects of hormones on the metabolism of food are
acids are deaminated, that is, the amino group (NH2) summarized in Table 17–4.
is removed. The remaining portion is converted to a
molecule that will fit into the Krebs cycle. For exam- Energy Available from
ple, a deaminated amino acid may be changed to a the Three Nutrient Types
three-carbon pyruvic acid or to a two-carbon acetyl
group. When these molecules enter the Krebs cycle, The potential energy in food is measured in units
the results are just the same as if they had come from called Calories or kilocalories. A calorie (lowercase
glucose. This is diagrammed in Fig. 17–3. “c”) is the amount of energy needed to raise the tem-
perature of 1 gram of water 1°C. A kilocalorie or
Fats are made of glycerol and fatty acids, which are Calorie (capital “C”) is 1000 times that amount of
the end products of fat digestion. These molecules may energy.
also be changed to ones that will take part in the Krebs
cycle, and the reactions that change them usually take One gram of carbohydrate yields about 4 kilocalo-
place in the liver. Glycerol is a three-carbon molecule ries. A gram of protein also yields about 4 kilocalories.
that can be converted to the three-carbon pyruvic acid, A gram of fat, however, yields 9 kilocalories, and
which enters the Krebs cycle. In the process of beta- a gram of alcohol yields 7 kilocalories. This is why a
oxidation, the long carbon chains of fatty acids are split diet high in fat is more likely to result in weight gain
into two-carbon acetyl groups, which enter a later step if the calories are not expended in energy-requiring
in the Krebs cycle (see Fig. 17–3). activities.

Both amino acids and fatty acids may be converted You may have noticed that calorie content is part of
by the liver to ketones, which are two- or four-carbon the nutritional information on food labels. On such
molecules such as acetone and acetoacetic acid. labels the term calorie actually means Calorie or kilo-
Although body cells can use ketones in cell respira- calories but is used for the sake of simplicity.
tion, they do so slowly. In situations in which fats or
amino acids have become the primary energy sources, SYNTHESIS USES OF FOODS
a state called ketosis may develop; this is described in
Box 17–3: Ketosis. Excess amino acids may also be Besides being available for energy production, each of
the three food types is used in anabolic reactions to

Copyright © 2007 by F. A. Davis.

406 Body Temperature and Metabolism

Table 17–4 HORMONES THAT BOX 17–3 KETOSIS
REGULATE METABOLISM
When fats and amino acids are to be used for
Hormone (Gland) Effects energy, they are often converted by the liver to
Thyroxine (thyroid ketones. Ketones are organic molecules such as
• Increases use of all three acetone that may be changed to acetyl CoA and
gland) food types for energy (glu- enter the Krebs cycle. Other cells are able to use
Growth hormone cose, fats, amino acids) ketones as an energy source, but they do so
slowly. When ketones are produced in small
(anterior pituitary) • Increases protein synthesis amounts, as they usually are between meals, the
blood level does not rise sharply.
Insulin (pancreas) • Increases amino acid trans-
port into cells A state of ketosis exists when fats and pro-
Glucagon (pancreas) teins become the primary energy sources, and
• Increases protein synthesis ketones accumulate in the blood faster than cells
Cortisol (adrenal • Increases use of fats for can utilize them. Because ketones are organic
cortex) acids, they lower the pH of the blood. As the
energy blood ketone level rises, the kidneys excrete
Epinephrine ketones, but they must also excrete more water
(adrenal medulla) • Increases glucose transport as a solvent, which leads to dehydration.
into cells and use for energy
Ketosis is clinically important in diabetes mel-
• Increases conversion of glu- litus, starvation, and eating disorders such as
cose to glycogen in liver anorexia nervosa. Diabetics whose disease is
and muscles poorly controlled may progress to ketoacido-
sis, a form of metabolic acidosis that may lead to
• Increases transport of amino confusion, coma, and death. Reversal of this
acids and fatty acids into state requires a carbohydrate energy source and
cells to be used for synthe- the insulin necessary to utilize it.
sis (not energy production)
in RNA. This function of glucose is very important,
• Increases conversion of for without the pentose sugars our cells could neither
glycogen to glucose produce new chromosomes for cell division nor carry
out the process of protein synthesis.
• Increases use of amino acids
and fats for energy Any glucose in excess of immediate energy needs or
the need for pentose sugars is converted to glycogen
• Increases conversion of glu- in the liver and skeletal muscles. Glycogen is then an
cose to glycogen in liver energy source during states of hypoglycemia or during
exercise. If still more glucose is present, it will be
• Increases use of amino acids changed to fat and stored in adipose tissue.
and fats for energy
Amino Acids
• Decreases protein synthesis
except in liver and GI tract As mentioned previously, the primary uses for amino
acids are the synthesis of the non-essential amino
• Increases conversion of acids by the liver and the synthesis of new proteins in
glycogen to glucose all tissues. By way of review, we can mention some
proteins with which you are already familiar: keratin
• Increases use of fats for and melanin in the epidermis; collagen in the dermis,
energy tendons, and ligaments; myosin, actin, and myoglobin
in muscle cells; hemoglobin in RBCs; antibodies pro-
synthesize necessary materials for cells and tissues. A duced by WBCs; prothrombin and fibrinogen for
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 sug-
ars that are part of nucleic acids. Deoxyribose is the
five-carbon sugar found in DNA, and ribose is found

Copyright © 2007 by F. A. Davis.

Body Temperature and Metabolism 407

True fats
(adipose tissue)

Proteins Glycogen
(enzymes, structural)

Excess Cholesterol
(deamination) and other
steroids
Pentose sugars
Non-essential Excess
amino acids
Phospholipids
(cell membranes)

Transamination

Proteins Carbohydrates Fats
(Digestion) (Digestion)

(Digestion) C C CCCC
Glucose
Amino NH2 C C CCC Fatty acid
Glycerol

acids C

NH2 C C C C CCC CC
C C CC C C C C
C
C NH2 C
C

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?

clotting; albumin to maintain blood volume; pepsin when the body’s needs for new proteins have been
and amylase for digestion; growth hormone and met are amino acids used for energy production. But
insulin; and the thousands of enzymes needed to cat- notice in Fig. 17–4 what happens to excess amino
alyze reactions within the body. acids; they will be deaminated and converted to simple
carbohydrates and contribute to glycogen storage
The amino acids we obtain from the proteins in or they may be changed to fat and stored in adipose
our food are used by our cells to synthesize all of these tissue.
proteins in the amounts needed by the body. Only

Copyright © 2007 by F. A. Davis.

408 Body Temperature and Metabolism

Fatty Acids and Glycerol increase their formation. Antioxidant vitamins com-
bine with free radicals before they can react with cel-
The end products of fat digestion that are not needed lular components. Plant foods are good sources of
immediately for energy production may be stored as these vitamins. Table 17–5 summarizes some impor-
fat (triglycerides) in adipose tissue. Most adipose tis- tant metabolic and nutritional aspects of the vitamins
sue is found subcutaneously and is potential energy for we need.
times when food intake decreases. Notice in Table
17–4 that insulin promotes fat synthesis and storage. Deficiencies of vitamins often result in disease:
One theory of weight gain proposes that a diet high in vitamin C deficiency and scurvy, for example (see Box
sugars and starches stimulates the secretion of so 4–2). Other deficiency diseases that have been known
much insulin that fat can only be stored, not taken out for decades include pellagra (lack of niacin), beri-beri
of storage and used for energy. (riboflavin), pernicious anemia (B12), and rickets (D).
More recently the importance of folic acid (folacin)
Fatty acids and glycerol are also used for the syn- for the development of the fetal central nervous sys-
thesis of phospholipids, which are essential compo- tem has been recognized. Adequate folic acid during
nents of all cell membranes. Myelin, for example, is a pregnancy can significantly decrease the chance of
phospholipid of the membranes of Schwann cells, spina bifida (open spinal column) and anencephaly
which form the myelin sheath of peripheral neurons. (absence of the cerebrum, always fatal) in a fetus. All
women should be aware of the need for extra (400
The liver can synthesize most of the fatty acids micrograms) folic acid during pregnancy.
needed by the body. Two exceptions are linoleic acid
and linolenic acid, which are essential fatty acids and Minerals are simple inorganic chemicals and have
must be obtained from the diet. Linoleic acid is part of a variety of functions, many of which you are already
lecithin, which in turn is part of all cell membranes. familiar with. Table 17–6 lists some important aspects
Vegetable oils are good sources of these essential fatty of minerals. We will return to the minerals as part of
acids. our study of fluid–electrolyte balance in Chapter 19.

When fatty acids are broken down in the process of METABOLIC RATE
beta-oxidation, the resulting acetyl groups may also be
used for the synthesis of cholesterol, a steroid. This Although the term metabolism is used to describe all
takes place primarily in the liver, although all cells are of the chemical reactions that take place within the
capable of synthesizing cholesterol for their cell mem- body, metabolic rate is usually expressed as an
branes. The liver uses cholesterol to synthesize bile amount of heat production. This is because many
salts for the emulsification of fats in digestion. The body processes that utilize ATP also produce heat.
steroid hormones are also synthesized from choles- These processes include the contraction of skeletal
terol. Cortisol and aldosterone are produced by the muscle, the pumping of the heart, and the normal
adrenal cortex, estrogen and progesterone by the breakdown of cellular components. Therefore, it is
ovaries, and testosterone by the testes. possible to quantify heat production as a measure of
metabolic activity.
VITAMINS AND MINERALS
As mentioned previously, the energy available from
Vitamins are organic molecules needed in very small food is measured in kilocalories (kcal). Kilocalories are
amounts for normal body functioning. Some vitamins also the units used to measure the energy expended by
are coenzymes; that is, they are necessary for the the body. During sleep, for example, energy expended
functioning of certain enzymes. Others are antioxi- by a 150-pound person is about 60 to 70 kcal per hour.
dant vitamins, including vitamins C, E, and beta- Getting up and preparing breakfast increases energy
carotene (a precursor for vitamin A). Antioxidants expenditure to 80 to 90 kcal per hour. For mothers
prevent damage from free radicals, which are mole- with several small children, this value may be signifi-
cules that contain an unpaired electron and are highly cantly higher. Clearly, greater activity results in
reactive. The reactions of free radicals can damage greater energy expenditure.
DNA, cell membranes, and the cell organelles. Free
radicals are formed during some normal body reac- The energy required for merely living (lying qui-
tions, but smoking and exposure to pollution will etly in bed) is the basal metabolic rate (BMR). See
Box 17–4: Metabolic Rate for a formula to estimate

Copyright © 2007 by F. A. Davis.

Table 17–5 VITAMINS

Vitamin Functions Food Sources Comment
Water Soluble
Thiamine (B1) • Conversion of pyruvic acid to acetyl • Meat, eggs, Rapidly destroyed by heat
CoA in cell respiration legumes, green
Riboflavin (B2) leafy vegetables, Small amounts produced
Niacin (nicotinamide) • Synthesis of pentose sugars grains by GI bacteria
Pyridoxine (B6) • Synthesis of acetylcholine
• Meat, milk, cheese, Small amounts produced
B12 (cyanocobalamin) • Part of FAD in cell respiration grains by GI bacteria
Biotin
Folic acid (folacin) • Part of NAD in cell respiration • Meat, fish, grains, Contains cobalt; intrinsic
• Metabolism of fat for energy legumes factor required for
Pantothenic acid absorption
Vitamin C (ascorbic • Part of enzymes needed for amino • Meat, fish, grains,
acid metabolism and protein synthe- yeast, yogurt Small amounts produced
acid) sis, nucleic acid synthesis, synthesis of by GI bacteria
antibodies • Liver, meat, fish,
Fat Soluble eggs, milk, cheese Small amounts produced
Vitamin A • Synthesis of DNA, especially in RBC by GI bacteria
Vitamin D production • Yeast, liver, eggs
Small amounts produced
Vitamin E • Metabolism of amino acids for energy • Liver, grains, by GI bacteria
legumes, leafy
Vitamin K • Synthesis of nucleic acids green vegetables Rapidly destroyed by
• Metabolism of fatty acids and amino heat
• Meat, fish, grains,
acids legumes, vegetables Stored in liver; bile salts
required for absorp-
• Synthesis of DNA, especially in blood • Citrus fruits, toma- tion
cell production toes, potatoes
Produced in skin exposed
• Contributes to development of fetal • Yellow and green to UV rays; stored
CNS vegetables, liver, in liver; bile salts
milk, eggs required for absorp-
• Part of coenzyme A in cell respiration, tion
use of amino acids and fats for energy • Fortified milk, egg
yolks, fish liver oils Stored in liver and adi-
• Synthesis of collagen, especially for pose tissue; bile salts
wound healing • Nuts, wheat germ, required for absorp-
seed oils tion
• Metabolism of amino acids
• Absorption of iron • Liver, spinach, Large amounts produced
• An antioxidant—prevents cellular cabbage by GI bacteria; bile salts
required for absorption;
damage from free radicals stored in liver

• Synthesis of rhodopsin
• Calcification of growing bones
• Maintenance of epithelial tissues

• Absorption of calcium and phospho-
rus 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

409

Copyright © 2007 by F. A. Davis.

410 Body Temperature and Metabolism

Table 17–6 MINERALS

Mineral Functions Food Sources Comment
Calcium • Milk, cheese, yogurt, Vitamin D required for
Phosphorus • Formation of bones and teeth
Sodium • Neuron and muscle functioning shellfish, leafy green absorption; stored in
• Blood clotting vegetables bones
Potassium • Milk, cheese, fish, meat Vitamin D required for
• Formation of bones and teeth absorption; stored in
Chlorine • Part of DNA, RNA, and ATP • Table salt, almost all bones
Iron • Part of phosphate buffer system foods Most abundant cation (ϩ)
Iodine in extracellular fluid
• Contributes to osmotic pressure • Virtually all foods
of body fluids Most abundant cation (ϩ)
• Table salt in intracellular fluid
• Nerve impulse transmission and
muscle contraction • Meat, shellfish, dried Most abundant anion (Ϫ)
apricots, legumes, eggs in extracellular fluid
• Part of bicarbonate buffer system
• Iodized salt, seafood Stored in liver
• Contributes to osmotic pressure of
body fluids Insulin and keratin require
sulfur
• Nerve impulse transmission and
muscle contraction Part of chlorophyll in green
plants
• Contributes to osmotic pressure of
body fluids Some stored in liver

• Part of HCI in gastric juice Stored in liver

• Part of hemoglobin and myoglobin Vitamin B12 stored in liver
• Part of some cytochromes in cell

respiration

• Part of thyroxine and T3

Sulfur • Part of some amino acids • Meat, eggs
Magnesium • Part of thiamine and biotin
Manganese • Green vegetables,
• Formation of bone legumes, seafood, milk
Copper • Metabolism of ATP–ADP
• Legumes, grains, nuts,
Cobalt • Formation of urea leafy green vegetables
Zinc • Synthesis of fatty acids and choles-
• Liver, seafood, grains,
terol nuts, legumes

• Synthesis of hemoglobin • Liver, meat, fish
• Part of some cytochromes in cell • Meat, seafood, grains,

respiration legumes
• 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

your own metabolic rate. A number of factors affect 2. Age—Metabolic rate is highest in young children
the metabolic rate of an active person: and decreases with age. The energy require-
ments for growth and the greater heat loss by a
1. Exercise—Contraction of skeletal muscle increases smaller body contribute to the higher rate in chil-
energy expenditure and raises metabolic rate (see dren. After growth has stopped, metabolic rate
Box 17–5: Weight Loss). decreases about 2% per decade. If a person

Copyright © 2007 by F. A. Davis.

Body Temperature and Metabolism 411

BOX 17–4 METABOLIC RATE

To estimate your own basal metabolic rate Example: A 160-pound man:
(BMR), calculate kilocalories (kcal) used per hour as
follows: 1. 160 lb at 2.2 lb/kg = 73 kg
2. 73 kg x 1.0 kcal/kg/hr = 73 kcal/hr
For women: use the factor of 0.9 kcal per kilogram 3. 73 kcal/hr x 24 = 1752 kcal/day
(kg) of body weight
To approximate the amount of energy actually
For men: use the factor of 1.0 kcal per kg of body expended during an average day (24 hours), the
weight following percentages may be used:

Then multiply kcal/hour by 24 hours to determine Sedentary activity: add 40% to 50% of the BMR to
kcal per day. the BMR

Example: A 120-pound woman: Light activity: add 50% to 65% of the BMR to the
BMR
1. Change pounds to kilograms:
120 lb at 2.2 lb/kg = 55 kg Moderate activity: add 65% to 75% of the BMR to
the BMR
2. Multiply kg weight by the BMR factor:
55 kg x 0.9 kcal/kg/hr = 49.5 kcal/hr Strenuous activity: add 75% to 100% of the BMR to
the BMR
3. Multiply kcal/hr by 24: 1200
Using our example of the 120-pound woman
49.5 kcal/hr x 24 = 1188 kcal/day with a BMR of 1200 kcal/day:
(An approximate BMR, about
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

BOX 17–5 WEIGHT LOSS

Although diet books are often found on the best- food. Keeping track of daily caloric intake is an
seller lists, there is no magic method that will result important part of a decision to try to lose weight. It
in weight loss. Losing weight depends on one sim- is also important to remember that sustained loss of
ple fact: calorie expenditure in activity must exceed fat usually does not exceed 1 to 2 pounds per week.
calorie intake in food (the term calorie here will be In part this is so because as calorie intake decreases,
used to mean kilocalorie). the metabolic rate decreases. There will also be loss
of some body protein so that amino acids can be
To lose 1 pound of body fat, which consists of fat, converted to carbohydrates to supply the brain.
water, and protein, 3500 calories of energy must be
expended. Although any form of exercise requires A sensible weight-loss diet will include carbohy-
calories, the more strenuous the exercise, the more drate to supply energy needs, will have sufficient
calories expended. Some examples are shown in the protein (40 to 45 grams per day), and will be low in
accompanying table. animal fat. Including vegetables and fruits will sup-
ply vitamins, minerals, and fiber.
Most food packaging contains nutritional infor-
mation, including the calories per serving of the

Activity Calories per 10 Activity Calories per 10
minutes (average minutes (average
for a 150-lb person) Running (8 mph) for a 150-lb person)
Cycling (10 mph)
Walking slowly 30 Cycling (15 mph) 120
Walking briskly 45 Swimming 70
Walking up stairs 170
Dancing (slow) 40 115
Dancing (fast) 65 100

Copyright © 2007 by F. A. Davis.

Box 17–6 LEPTIN AND BODY-MASS INDEX directly decreases fat storage in cells, and improves
the efficiency of the pancreatic cells that produce
The 1994 discovery of the hormone leptin was insulin. What was first believed to be a simple chem-
reported to the general public in 1995, along with ical signal has proved to be much more complex.
speculation that leptin could become an anti-obe-
sity medication, which it has not. Leptin is a protein A good measure of leanness or fatness is the
produced by fat cells, and signals the hypothalamus body-mass index.
to release a chemical that acts as an appetite sup-
pressant. It seems to inform the brain of how much To calculate: Multiple weight in pounds by 703.
stored fat the body has, and is therefore involved in Divide by height in inches.
the regulation of body weight (along with many Divide again by height in inches = body-mass
other chemicals, some still unknown). index

Another likely role for leptin is as a contributor to Example: A person five foot six weighing 130
the onset of puberty, especially in females. Girls pounds.
who are very thin, with little body fat, tend to have
a later first menstrual period than girls with average 130 x 703 = 91,390
body fat, and a certain level of body fat is necessary 91,390 Ϭ 66 = 1385
for continued ovulation. Leptin may be the chemi- 1385 Ϭ 66 = 20.98
cal mediator of this information.
The optimal body-mass index is considered to be
The most recent research indicates that leptin 21. Any index over 25 is considered overweight.

becomes less active, the total decrease is almost 5% the tall, thin person has a larger surface area (pro-
per decade. portional to weight) through which heat is con-
3. Body configuration of adults—Tall, thin people tinuously lost. The metabolic rate, therefore, is
usually have higher metabolic rates than do short, slightly higher to compensate for the greater heat
stocky people of the same weight. This is so because loss. The variance of surface-to-weight ratios for
different body configurations is illustrated in
Figure 17–5. Surface-to-weight ratios. Imagine that Fig. 17–5.
the three shapes are people who all weigh the same 4. Sex hormones—Testosterone increases metabolic
amount. The “tall, thin person” on the right has about activity to a greater degree than does estrogen, giv-
50% more surface area than does the “short, stocky per- ing men a slightly higher metabolic rate than
son” on the left. The more surface area (where heat is women. Also, men tend to have more muscle, an
lost), the higher the metabolic rate. active tissue, whereas women tend to have more
QUESTION: Which of these ratios best represents an fat, a relatively inactive tissue.
infant? (Rather than weight, think of inside-outside 5. Sympathetic stimulation—In stress situations, the
proportion.) metabolism of many body cells is increased. Also
contributing to this are the hormones epinephrine
412 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 what-
ever 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 peo-
ple who live in tropical regions. This is believed to
be due to the variations in the secretion of thyrox-
ine, the hormone most responsible for regulation
of metabolic rate. In a cold climate, the necessity
for greater heat production brings about an

Copyright © 2007 by F. A. Davis.

Body Temperature and Metabolism 413

increased secretion of thyroxine and a higher meta- active, and prolonged high environmental tempera-
bolic rate. tures are a real danger for elderly people. In August
2003, in Europe, an unusually long and severe heat
AGING AND METABOLISM wave was the cause of at least 25,000 deaths. Most of
those who died were elderly.
As mentioned in the previous section, metabolic rate
decreases with age. Elderly people who remain active, SUMMARY
however, can easily maintain a metabolic rate (energy
production) adequate for their needs as long as their Food is needed for the synthesis of new cells and tis-
general health is good. Some elderly people subject to sues, or is utilized to produce the energy required for
physical or emotional disability, however, may be at such synthesis reactions. As a consequence of metabo-
risk for malnutrition. Caregivers may assess such a risk lism, heat energy is released to provide a constant
by asking how often the person eats every day; if body temperature and permit the continuation of
appetite is good, fair, or poor; and how the food tastes. metabolic activity. The metabolic pathways described
These simple questions may help ensure adequate in this chapter are only a small portion of the body’s
nutrition. total metabolism. Even this simple presentation, how-
ever, suggests the great chemical complexity of the
Sensitivity to external temperature changes may functioning human being.
decrease with age, and the regulation of body temper-
ature is no longer as precise. Sweat glands are not as

STUDY OUTLINE A fever increases the metabolic rate, and more heat
is produced; this may become detrimental during
Body Temperature very high fevers.
1. Normal range is 96.5° to 99.5°F (36° to 38°C),
Heat Loss (see Table 17–2)
with an average of 98.6°F (37°C). 1. Most heat is lost through the skin.
2. Normal fluctuation in 24 hours is 1° to 2°F. 2. Blood flow through the dermis determines the
3. Temperature regulation in infants and the elderly is
amount of heat that is lost by radiation, conduc-
not as precise as it is at other ages. tion, and convection.
3. Vasodilation in the dermis increases blood flow and
Heat Production heat loss; radiation and conduction are effective
Heat is one of the energy products of cell respiration. only if the environment is cooler than the body.
Many factors affect the total heat actually produced 4. Vasoconstriction in the dermis decreases blood
(see Table 17–1). flow and conserves heat in the core of the body.
1. Thyroxine from the thyroid gland—the most im- 5. Sweating is a very effective heat loss mechanism;
excess body heat evaporates sweat on the skin sur-
portant regulator of daily heat production. As meta- face; sweating is most effective when the atmos-
bolic rate decreases, more thyroxine is secreted to pheric humidity is low.
increase the rate of cell respiration. 6. Sweating also has a disadvantage in that water is
2. Stress—sympathetic impulses and epinephrine and lost and must be replaced to prevent serious dehy-
norepinephrine increase the metabolic activity of dration.
many organs, increasing the production of ATP 7. Heat is lost from the respiratory tract by the evap-
and heat. oration of water from the warm respiratory
3. Active organs continuously produce heat. Skeletal mucosa; water vapor is part of exhaled air.
muscle tone produces 25% of the total body heat at 8. A very small amount of heat is lost as urine and
rest. The liver provides up to 20% of the resting feces are excreted at body temperature.
body heat.
4. Food intake increases the activity of the digestive
organs and increases heat production.
5. Changes in body temperature affect metabolic rate.

Copyright © 2007 by F. A. Davis.

414 Body Temperature and Metabolism

Regulation of Heat Loss glycolysis, the Krebs cycle, and the cytochrome
1. The hypothalamus is the thermostat of the body (electron) transport system (see also Table 17–3).
3. The oxygen necessary comes from breathing.
and regulates body temperature by balancing heat 4. The water formed becomes part of intracellular
production and heat loss. fluid; CO2 is exhaled; ATP is used for energy-
2. The hypothalamus receives information from its requiring reactions; heat provides a body tempera-
own neurons (blood temperature) and from the ture.
temperature receptors in the dermis.
3. Mechanisms to increase heat loss are vasodilation Proteins and Fats—as energy sources (see
in the dermis and increased sweating. Decreased Table 17–4 for hormonal regulation)
muscle tone will decrease heat production. 1. Excess amino acids are deaminated in the liver and
4. Mechanisms to conserve heat are vasoconstriction
in the dermis and decreased sweating. Increased converted to pyruvic acid or acetyl groups to enter
muscle tone (shivering) will increase heat produc- the Krebs cycle. Amino acids may also be converted
tion. to glucose to supply the brain (Fig. 17–3).
2. Glycerol is converted to pyruvic acid to enter the
Fever—an abnormally elevated body temper- Krebs cycle.
ature 3. Fatty acids, in the process of beta-oxidation in the
1. Pyrogens are substances that cause a fever: bacteria, liver, are split into acetyl groups to enter the Krebs
cycle; ketones are formed for transport to other
foreign proteins, or chemicals released during cells (see Fig. 17–3).
inflammation (endogenous pyrogens).
2. Pyrogens raise the setting of the hypothalamic Energy Available from Food
thermostat; the person feels cold and begins to 1. Energy is measured in kilocalories (Calories):
shiver to produce heat.
3. When the pyrogen has been eliminated, the hypo- kcal.
thalamic setting returns to normal; the person feels 2. There are 4 kcal per gram of carbohydrate, 4 kcal
warm, and sweating begins to lose heat to lower the
body temperature. per gram of protein, 9 kcal per gram of fat. With
4. A low fever may be beneficial because it increases reference to food, kilocalories may be called calo-
the activity of WBCs and inhibits the activity of ries.
some pathogens.
5. A high fever may be detrimental because enzymes Synthesis Uses of Foods (Fig. 17–4)
are denatured at high temperatures. This is most 1. Glucose—used to synthesize the pentose sugars for
critical in the brain, where cells that die cannot be
replaced. DNA and RNA; used to synthesize glycogen to
store energy in liver and muscles.
Metabolism—all the reactions within the 2. Amino acids—used to synthesize new proteins and
body the non-essential amino acids; essential amino
1. Anabolism—synthesis reactions that usually acids must be obtained in the diet.
3. Fatty acids and glycerol—used to synthesize phos-
require energy in the form of ATP. pholipids for cell membranes, triglycerides for fat
2. Catabolism—decomposition reactions that often storage in adipose tissue, and cholesterol and other
steroids; essential fatty acids must be obtained in
release energy in the form of ATP. the diet.
3. Enzymes catalyze most anabolic and catabolic reac- 4. Any food eaten in excess will be changed to fat and
stored.
tions. 5. Vitamins and minerals—see Tables 17–5 and 17–6.

Cell Respiration—the breakdown of food Metabolic Rate—heat production by the
molecules to release their potential energy body; measured in kcal
and synthesize ATP (Fig. 17–3) 1. Basal metabolic rate (BMR) is the energy required
1. Glucose ϩ oxygen yields CO2 ϩ H2O ϩ ATP ϩ
to maintain life (see Box 17–4); several factors
heat. influence the metabolic rate of an active person.
2. The breakdown of glucose involves three stages:

Copyright © 2007 by F. A. Davis.

Body Temperature and Metabolism 415

2. Age—metabolic rate is highest in young children 5. Sympathetic stimulation—metabolic activity in-
and decreases with age. creases in stress situations.

3. Body configuration—more surface area propor- 6. Decreased food intake—metabolic rate decreases
tional to weight (tall and thin) means a higher to conserve available energy sources.
metabolic rate.
7. Climate—people who live in cold climates usually
4. Sex hormones—men usually have a higher meta- have higher metabolic rates because of a greater
bolic rate than do women; men have more muscle need for heat production.
proportional to fat than do women.

REVIEW QUESTIONS 9. Define metabolism, anabolism, catabolism, kilo-
calorie, and metabolic rate. (pp. 400, 401, 405,
1. State the normal range of human body temperature 408)
in °F and °C. (p. 396)
10. Name the three stages of the cell respiration
2. State the summary equation of cell respiration, and of glucose and state where in the cell each takes
state what happens to (or the purpose of) each of place and whether or not oxygen is required.
the products. (p. 401) (pp. 403)

3. Describe the role of each in heat production: thy- 11. For each, state the molecules that enter the
roxine, skeletal muscles, stress situations, and the process and the results of the process: glycolysis,
liver. (p. 396) Krebs cycle, and cytochrome transport system.
(pp. 403–405)
4. Describe the two mechanisms of heat loss through
the skin, and explain the role of blood flow. 12. Explain how fatty acids, glycerol, and excess
Describe how heat is lost through the respiratory amino acids are used for energy production in cell
tract. (pp. 397–398) respiration. (pp. 403, 405)

5. Explain the circumstances that exist when sweating 13. Describe the synthesis uses for glucose, amino
and vasodilation in the dermis are not effective acids, and fatty acids. (pp. 406–408)
mechanisms of heat loss. (p. 397)
14. Describe four factors that affect the metabolic
6. Name the part of the brain that regulates body rate of an active person. (pp. 410, 412)
temperature, and explain what is meant by a ther-
mostat. (p. 398) 15. If lunch consists of 60 grams of carbohydrate,
15 grams of protein, and 10 grams of fat, how
7. Describe the responses by the body to a warm envi- many kilocalories are provided by this meal?
ronment and to a cold environment. (pp. 399) (p. 405)

8. Explain how pyrogens are believed to cause a fever,
and give two examples of pyrogens. (p. 399)

FOR FURTHER THOUGHT

1. For many people, iceberg lettuce is the vegetable that is, what nutrients may be lacking? How may
eaten most often. What does lettuce provide? What they be obtained?
does lettuce lack, compared to vegetables such as
broccoli? 3. Studies with animals have shown that caloric
restriction may prolong life by protecting the brain
2. Fourteen-year-old Donna has just decided that eat- from some effects of aging. The animals’ diet was
ing meat is “gross,” and that she will be a vegetar- about half the usual calories they would consume.
ian. What difficulties are there with such a diet; For people, 1250 to 1500 calories per day would be

Copyright © 2007 by F. A. Davis.

416 Body Temperature and Metabolism

restrictive (compared to the 2000 calories or more 5. Remember the Titanic, which sank in April of
that many of us in North America consume). 1912? There were not enough lifeboats for every-
Would it be worth it for a life span of 110 years? one, and many people were in the water of the
Describe the problems with such a diet. North Atlantic. They did have life jackets, and did
not drown, but many were dead within half an
4. Every summer small children are left alone in cars hour. Explain why.
“just for a few minutes,” while a parent does an
errand. The result may be tragic—severe brain 6. An elderly person and a quadriplegic person may
damage or death of the child from heat stroke. each have difficulties during cold weather. Explain
Explain why small children are so susceptible to how the problem is a little different for each.
heat.

Copyright © 2007 by F. A. Davis.

CHAPTER 18

The Urinary System

417

Copyright © 2007 by F. A. Davis.

CHAPTER 18

Chapter Outline Student Objectives

Kidneys • Describe the location and general function of each
Internal Structure of the Kidney
The Nephron organ of the urinary system.

Renal corpuscle • Name the parts of a nephron and the important
Renal tubule
Blood Vessels of the Kidney blood vessels associated with them.
Formation of Urine
Glomerular Filtration • Explain how the following are involved in urine
Tubular Reabsorption
Mechanisms of reabsorption formation: glomerular filtration, tubular reabsorp-
Tubular Secretion tion, tubular secretion, and blood flow through the
Hormones That Influence Reabsorption of Water kidney.
Summary of Urine Formation
The Kidneys and Acid–Base Balance • Describe the mechanisms of tubular reabsorption,
Other Functions of the Kidneys
Elimination of Urine and explain the importance of tubular secretion.
Ureters
Urinary Bladder • Describe how the kidneys help maintain normal
Urethra
The Urination Reflex blood volume and blood pressure.
Characteristics of Urine
Aging and the Urinary System • 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 vol-

untary control is possible.

• Describe the characteristics of normal urine.

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

418

Copyright © 2007 by F. A. Davis.

The Urinary System

New Terminology Related Clinical Terminology

Bowman’s capsule (BOW-manz KAP-suhl) Cystitis (sis-TIGH-tis)
Detrusor muscle (de-TROO-ser) Dysuria (dis-YOO-ree-ah)
External urethral sphincter (yoo-REE-thruhl Hemodialysis (HEE-moh-dye-AL-i-sis)
Nephritis (ne-FRY-tis)
SFINK-ter) Oliguria (AH-li-GYOO-ree-ah)
Glomerular filtration rate (gloh-MER-yoo-ler fill- Polyuria (PAH-li-YOO-ree-ah)
Renal calculi (REE-nuhl KAL-kew-lye)
TRAY-shun RAYT) Renal failure (REE-nuhl FAYL-yer)
Glomerulus (gloh-MER-yoo-lus) Uremia (yoo-REE-me-ah)
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)

Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.

419

Copyright © 2007 by F. A. Davis.

420 The Urinary System

The first successful human organ transplant was a is sufficient to carry out the complex work required to
maintain homeostasis of the body fluids.
kidney transplant performed in 1953. Because the
donor and recipient were identical twins, rejection was The urinary system consists of two kidneys, two
not a problem. Thousands of kidney transplants have ureters, the urinary bladder, and the urethra (Fig.
been performed since then, and the development of 18–1). The formation of urine is the function of the
immunosuppressive medications has permitted many kidneys, and the rest of the system is responsible for
people to live normal lives with donated kidneys. eliminating the urine.
Although a person usually has two kidneys, one kidney
Body cells produce waste products such as urea,

Diaphragm Ribs
Aorta
Inferior vena cava

Left adrenal gland
Superior mesenteric artery
Left renal artery and vein

Left kidney

Right kidney Left ureter

Psoas major muscle Left common iliac
lliacus muscle artery and vein

Lumbar vertebra
Pelvis
Sacrum

Right ureter

Urinary bladder Opening of ureter
Urethra Trigone of bladder
Symphysis pubis

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?

Copyright © 2007 by F. A. Davis.

The Urinary System 421

creatinine, and ammonia, which must be removed is in turn covered by a fibrous connective tissue mem-
from the blood before they accumulate to toxic levels. brane called the renal fascia, which helps hold the
As the kidneys form urine to excrete these waste prod- kidneys in place (see Box 18–1: Floating Kidney).
ucts, they also accomplish several other important
functions: Each kidney has an indentation called the hilus
on its medial side. At the hilus, the renal artery enters
1. Regulation of the volume of blood by excretion or the kidney, and the renal vein and ureter emerge. The
conservation of water renal artery is a branch of the abdominal aorta, and the
renal vein returns blood to the inferior vena cava (see
2. Regulation of the electrolyte content of the blood Fig. 18–1). The ureter carries urine from the kidney to
by the excretion or conservation of minerals the urinary bladder.

3. Regulation of the acid–base balance of the blood by INTERNAL STRUCTURE
excretion or conservation of ions such as Hϩ ions OF THE KIDNEY
or HCO3Ϫ ions
In a coronal or frontal section of the kidney, three
4. Regulation of all of the above in tissue fluid areas can be distinguished (Fig. 18–2). The lateral and
middle areas are tissue layers, and the medial area at
The process of urine formation, therefore, helps the hilus is a cavity. The outer tissue layer is called the
maintain the normal composition, volume, and pH of renal cortex; it is made of renal corpuscles and convo-
both blood and tissue fluid by removing those sub- luted tubules. These are parts of the nephron and are
stances that would upset the normal constancy and described in the next section. The inner tissue layer is
balance of these extracellular fluids. the renal medulla, which is made of loops of Henle
and collecting tubules (also parts of the nephron). The
KIDNEYS renal medulla consists of wedge-shaped pieces called
renal pyramids. The tip of each pyramid is its apex or
The two kidneys are located in the upper abdominal papilla.
cavity on either side of the vertebral column, behind
the peritoneum (retroperitoneal). The upper por- The third area is the renal pelvis; this is not a layer
tions of the kidneys rest on the lower surface of the of tissues, but rather a cavity formed by the expansion
diaphragm and are enclosed and protected by the of the ureter within the kidney at the hilus. Funnel-
lower rib cage (see Fig. 18–1). The kidneys are shaped extensions of the renal pelvis, called calyces
embedded in adipose tissue that acts as a cushion and (singular: calyx), enclose the papillae of the renal pyr-
amids. Urine flows from the renal pyramids into the
BOX 18–1 FLOATING KIDNEY calyces, then to the renal pelvis and out into the ureter.

A floating kidney is one that has moved out of its THE NEPHRON
normal position. This may happen in very thin
people whose renal cushion of adipose tissue is The nephron is the structural and functional unit of
thin, or it may be the result of a sharp blow to the kidney. Each kidney contains approximately 1 mil-
the back that dislodges a kidney. lion nephrons. It is in the nephrons, with their associ-
ated blood vessels, that urine is formed. Each nephron
A kidney can function in any position; the has two major portions: a renal corpuscle and a renal
problem with a floating kidney is that the ureter tubule. Each of these major parts has further subdivi-
may become twisted or kinked. If urine cannot sions, which are shown with their blood vessels in Fig.
flow through the ureter, the urine backs up and 18–3.
collects in the renal pelvis. Incoming urine
from the renal tubules then backs up as well. If Renal Corpuscle
the renal filtrate cannot flow out of Bowman’s
capsules, the pressure within Bowman’s capsules A renal corpuscle consists of a glomerulus surroun-
increases, opposing the blood pressure in the ded by a Bowman’s capsule. The glomerulus is a cap-
glomeruli. Glomerular filtration then cannot take illary network that arises from an afferent arteriole
place efficiently. If uncorrected, this may lead to and empties into an efferent arteriole. The diameter
permanent kidney damage.

Copyright © 2007 by F. A. Davis.

422 The Urinary System

Nephron Renal cortex
Renal medulla
(pyramids)

Papilla of pyramid Renal corpuscle
Calyx Renal cortex
Renal pelvis Renal tubule

Renal artery Renal medulla
Renal vein Papillary duct

Interlobar
artery

Arcuate artery

Ureter

A
B

CD

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?

Copyright © 2007 by F. A. Davis. Collecting tubule

Podocyte Distal
convoluted tubule

Glomerulus

Renal
cortex

Bowman's capsule
(inner layer)

Bowman's capsule
(outer layer)

Efferent arteriole

Juxtaglomerular
cells

Afferent arteriole

Peritubular capillaries

Proximal convoluted
tubule

Renal
medulla

Loop of Henle

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 lin-
ing of the proximal convoluted tubule specialized?

423

Copyright © 2007 by F. A. Davis.

424 The Urinary System

of the efferent arteriole is smaller than that of the Notice that in this pathway there are two sets of cap-
afferent arteriole, which helps maintain a fairly high illaries, and recall that it is in capillaries that exchanges
blood pressure in the glomerulus. take place between the blood and surrounding tissues.
Therefore, in the kidneys there are two sites of ex-
Bowman’s capsule (or glomerular capsule) is the change. The exchanges that take place between the
expanded end of a renal tubule; it encloses the nephrons and the capillaries of the kidneys will form
glomerulus. The inner layer of Bowman’s capsule is urine from blood plasma.
made of podocytes; the name means “foot cells,” and
the “feet” of the podocytes are on the surface of the Figure 18–2 shows two views of a vascular cast of a
glomerular capillaries. The arrangement of podocytes kidney; the shape of the blood vessels has been pre-
creates pores, spaces between adjacent “feet,” which served in red plastic. You can see how dense the vas-
make this layer very permeable. The outer layer of culature of a kidney is, and most of these vessels are
Bowman’s capsule has no pores and is not permeable. capillaries.
The space between the inner and outer layers of
Bowman’s capsule contains renal filtrate, the fluid that FORMATION OF URINE
is formed from the blood in the glomerulus and will
eventually become urine. The formation of urine involves three major pro-
cesses. The first is glomerular filtration, which takes
Renal Tubule place in the renal corpuscles. The second and third are
tubular reabsorption and tubular secretion, which take
The renal tubule continues from Bowman’s capsule place in the renal tubules.
and consists of the following parts: proximal convo-
luted tubule (in the renal cortex), loop of Henle (or GLOMERULAR FILTRATION
loop of the nephron, in the renal medulla), and distal
convoluted tubule (in the renal cortex). The distal You may recall that filtration is the process in which
convoluted tubules from several nephrons empty into blood pressure forces plasma and dissolved material
a collecting tubule. Several collecting tubules then out of capillaries. In glomerular filtration, blood
unite to form a papillary duct that empties urine into pressure forces plasma, dissolved substances, and small
a calyx of the renal pelvis. proteins out of the glomeruli and into Bowman’s cap-
sules. This fluid is no longer plasma but is called renal
Cross-sections of the parts of the renal tubule are filtrate.
shown in Fig. 18–3. Notice how thin the walls of the
tubule are, and also the microvilli in the proximal con- The blood pressure in the glomeruli, compared
voluted tubule. These anatomic characteristics provide with that in other capillaries, is relatively high, about
for efficient exchanges of materials, as you will see. 60 mmHg. The pressure in Bowman’s capsule is very
low, and its inner, podocyte layer is very permeable, so
All parts of the renal tubule are surrounded by that approximately 20% to 25% of the blood that
peritubular capillaries, which arise from the efferent enters glomeruli becomes renal filtrate in Bowman’s
arteriole. The peritubular capillaries will receive the capsules. The blood cells and larger proteins are too
materials reabsorbed by the renal tubules; this is large to be forced out of the glomeruli, so they remain
described in the section on urine formation. in the blood. Waste products are dissolved in blood
plasma, so they pass into the renal filtrate. Useful
BLOOD VESSELS OF THE KIDNEY materials such as nutrients and minerals are also dis-
solved in plasma and are also present in renal filtrate.
The pathway of blood flow through the kidney is an Filtration is not selective with respect to usefulness; it
essential part of the process of urine formation. Blood is selective only with respect to size. Therefore, renal
from the abdominal aorta enters the renal artery, filtrate is very much like blood plasma, except that
which branches extensively within the kidney into there is far less protein and no blood cells are present.
smaller arteries (see Fig. 18–2). The smallest arteries
give rise to afferent arterioles in the renal cortex (see The glomerular filtration rate (GFR) is the
Fig. 18–3). From the afferent arterioles, blood flows amount of renal filtrate formed by the kidneys in 1
into the glomeruli (capillaries), to efferent arterioles, minute, and averages 100 to 125 mL per minute. GFR
to peritubular capillaries, to veins within the kidney, to
the renal vein, and finally to the inferior vena cava.

Copyright © 2007 by F. A. Davis.

The Urinary System 425

BOX 18–2 RENAL FAILURE AND HEMODIALYSIS

Renal failure, the inability of the kidneys to func- artificial kidney machine to do what the patient’s
tion properly, may be the result of three general nephrons can no longer do. The patient’s blood is
causes, which may be called prerenal, intrinsic passed through minute tubes surrounded by fluid
renal, and postrenal. (dialysate) with the same chemical composition as
plasma. Waste products and excess minerals diffuse
“Prerenal” means that the problem is “before” out of the patient’s blood into the fluid of the
the kidneys, that is, in the blood flow to the kid- machine.
neys. Any condition that decreases blood flow to
the kidneys may result in renal damage and failure. Although hemodialysis does prolong life for those
Examples are severe hemorrhage or very low blood with chronic renal failure, it does not fully take the
pressure following a heart attack (MI). place of functioning kidneys. The increasing success
rate of kidney transplants, however, does indeed
“Intrinsic renal” means that the problem is in the provide the possibility of a normal life for people
kidneys themselves. Diabetes and hypertension with chronic renal failure.
damage the blood vessels of the kidneys, and are
the causes of 70% of all cases of end-stage renal AB
failure. Bacterial infections of the kidneys or expo-
sure to chemicals (certain antibiotics) may cause C
damage to the nephrons. Polycystic kidney disease
is a genetic disorder in which the kidney tubules Box Figure 18–A Causes of renal failure. (A) Prerenal.
dilate and become nonfunctional. Severe damage (B) Intrinsic renal. (C) Postrenal.
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

may be altered if the rate of blood flow through the Most reabsorption and secretion (about 65%) take
kidney changes. If blood flow increases, the GFR place in the proximal convoluted tubules, whose cells
increases, and more filtrate is formed. If blood flow have microvilli that greatly increase their surface area.
decreases (as may happen following a severe hemor- The distal convoluted tubules and collecting tubules
rhage), the GFR decreases, less filtrate is formed, and are also important sites for the reabsorption of water
urinary output decreases (see Box 18–2: Renal Failure (Fig. 18–4).
and Hemodialysis).
Mechanisms of Reabsorption
TUBULAR REABSORPTION
1. Active transport—the cells of the renal tubule use
Tubular reabsorption takes place from the renal ATP to transport most of the useful materials from
tubules into the peritubular capillaries. In a 24-hour the filtrate to the blood. These useful materials
period, the kidneys form 150 to 180 liters of filtrate, include glucose, amino acids, vitamins, and positive
and normal urinary output in that time is 1 to 2 liters. ions.
Therefore, it becomes apparent that most of the renal For many of these substances, the renal tubules
filtrate does not become urine. Approximately 99% of have a threshold level of reabsorption. This means
the filtrate is reabsorbed back into the blood in the that there is a limit to how much the tubules can
peritubular capillaries. Only about 1% of the filtrate remove from the filtrate. For example, if the filtrate
will enter the renal pelvis as urine. level of glucose is normal (reflecting a normal

Copyright © 2007 by F. A. Davis.

426 The Urinary System

Proximal convoluted tubule

H2O H2O Bowman's capsule
Glucose Glucose
Amino acids Amino acids Glomerulus
Small proteins Small proteins Efferent arteriole
Minerals Minerals Afferent arteriole
Wastes

H+ Distal Collecting
Ammonia convoluted tubule
tubule H2O
Creatinine H2O
Medications H2O Wastes
H+
Peritubular Artery Na+
capillaries Vein K+

Loop of Henle Urine
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 sub-
stances may be secreted?

Copyright © 2007 by F. A. Davis.

The Urinary System 427

blood glucose level), the tubules will reabsorb all of filtrate to be eliminated in urine. Hydrogen ions (Hϩ)
the glucose, and none will be found in the urine. may be secreted by the tubule cells to help maintain
What happens is this: The number of glucose the normal pH of blood.
transporter molecules in the membranes of the
tubule cells is sufficient to take in the number of HORMONES THAT INFLUENCE
glucose molecules passing by in the filtrate. If, REABSORPTION OF WATER
however, the blood glucose level is above normal,
the amount of glucose in the filtrate will also be Aldosterone is secreted by the adrenal cortex in re-
above normal and will exceed the threshold level of sponse to a high blood potassium level, to a low blood
reabsorption. The number of glucose molecules to sodium level, or to a decrease in blood pressure. When
be reabsorbed is more than the number of the aldosterone stimulates the reabsorption of Naϩ ions,
transporter molecules available to do so. In this sit- water follows from the filtrate back to the blood. This
uation, therefore, some glucose will remain in the helps maintain normal blood volume and blood pres-
filtrate and be present in urine. sure.

The reabsorption of Caϩ2 ions is increased by You may recall that the antagonist to aldosterone is
parathyroid hormone (PTH). The parathyroid atrial natriuretic peptide (ANP), which is secreted
glands secrete PTH when the blood calcium level by the atria of the heart when the atrial walls are
decreases. The reabsorption of Caϩ2 ions by the stretched by high blood pressure or greater blood vol-
kidneys is one of the mechanisms by which the ume. ANP decreases the reabsorption of Naϩ ions by
blood calcium level is raised back to normal. the kidneys; these remain in the filtrate, as does water,
and are excreted. By increasing the elimination of
The hormone aldosterone, secreted by the adre- sodium and water, ANP lowers blood volume and
nal cortex, increases the reabsorption of Naϩ ions blood pressure.
and the excretion of Kϩ ions. Besides regulating the
blood levels of sodium and potassium, aldosterone Antidiuretic hormone (ADH) is released by the
also affects the volume of blood. posterior pituitary gland when the amount of water in
2. Passive transport—many of the negative ions that the body decreases. Under the influence of ADH, the
are returned to the blood are reabsorbed following distal convoluted tubules and collecting tubules are
the reabsorption of positive ions, because unlike able to reabsorb more water from the renal filtrate.
charges attract. This helps maintain normal blood volume and blood
3. Osmosis—the reabsorption of water follows the pressure, and also permits the kidneys to produce
reabsorption of minerals, especially sodium ions. urine that is more concentrated than body fluids.
The hormones that affect reabsorption of water are Producing a concentrated urine is essential to prevent
discussed in the next section. excessive water loss while still excreting all the sub-
4. Pinocytosis—small proteins are too large to be stances that must be eliminated.
reabsorbed by active transport. They become
adsorbed to the membranes of the cells of the prox- If the amount of water in the body increases, how-
imal convoluted tubules. The cell membrane then ever, the secretion of ADH diminishes, and the kid-
sinks inward and folds around the protein to take it neys will reabsorb less water. Urine then becomes
in (see Fig. 3–3 for depictions of this and the other dilute, and water is eliminated until its concentration
transport mechanisms). Normally all proteins in in the body returns to normal. This may occur follow-
the filtrate are reabsorbed; none is found in urine. ing ingestion of excessive quantities of fluids.

TUBULAR SECRETION SUMMARY OF URINE FORMATION

This mechanism also changes the composition of 1. The kidneys form urine from blood plasma. Blood
urine. In tubular secretion, substances are actively flow through the kidneys is a major factor in deter-
secreted from the blood in the peritubular capillaries mining urinary output.
into the filtrate in the renal tubules. Waste products,
such as ammonia and some creatinine, and the meta- 2. Glomerular filtration is the first step in urine for-
bolic products of medications may be secreted into the mation. Filtration is not selective in terms of use-
fulness of materials; it is selective only in terms of
size. High blood pressure in the glomeruli forces

Copyright © 2007 by F. A. Davis.

428 The Urinary System

plasma, dissolved materials, and small proteins into ranges. They have the greatest ability to compensate
Bowman’s capsules; the fluid is now called renal for the pH changes that are a normal part of body
filtrate. metabolism or the result of disease, and to make the
3. Tubular reabsorption is selective in terms of useful- necessary corrections.
ness. Nutrients such as glucose, amino acids, and
vitamins are reabsorbed by active transport and This regulatory function of the kidneys is complex,
may have renal threshold levels. Positive ions are but at its simplest it may be described as follows. If
reabsorbed by active transport and negative body fluids are becoming too acidic, the kidneys will
ions are reabsorbed most often by passive trans- secrete more Hϩ ions into the renal filtrate and will
port. Water is reabsorbed by osmosis, and small return more HCO3Ϫ ions to the blood. This will help
proteins are reabsorbed by pinocytosis. raise the pH of the blood back to normal. The reac-
tions involved in such a mechanism are shown in Fig.
Reabsorption takes place from the filtrate in the 18–6, to which we will return later. First, however, let
renal tubules to the blood in the peritubular capil- us briefly consider how the kidneys will compensate
laries. for body fluids that are becoming too alkaline. You
4. Tubular secretion takes place from the blood in the might expect the kidneys to do just the opposite of
peritubular capillaries to the filtrate in the renal what was just described, and that is just what happens.
tubules and can ensure that wastes such as creati- The kidneys will return Hϩ ions to the blood and
nine or excess Hϩ ions are actively put into the fil- excrete HCO3Ϫ ions in urine. This will help lower the
trate to be excreted. pH of the blood back to normal.
5. Hormones such as aldosterone, ANP, and ADH
influence the reabsorption of water and help main- Because the natural tendency is for body fluids to
tain normal blood volume and blood pressure. The become more acidic, let us look at the pH-raising
secretion of ADH determines whether a concen- mechanism in more detail (see Fig. 18–6). The cells
trated or dilute urine will be formed. of the renal tubules can secrete Hϩ ions or ammonia
6. Waste products remain in the renal filtrate and are in exchange for Naϩ ions and, by doing so, influence
excreted in urine. The effects of hormones on the the reabsorption of other ions. Hydrogen ions are
kidneys are summarized in Table 18–1 and illus- obtained from the reaction of CO2 and water (or other
trated in Fig. 18–5. processes). An amine group from an amino acid is
combined with an Hϩ ion to form ammonia.
THE KIDNEYS AND
ACID–BASE BALANCE The tubule cell secretes the Hϩ ion and the ammo-
nia into the renal filtrate, and two Naϩ ions are reab-
The kidneys are the organs most responsible for main- sorbed in exchange. In the filtrate, the Hϩ ion and
taining the pH of blood and tissue fluid within normal ammonia form NH4ϩ (an ammonium radical), which
reacts with a chloride ion (ClϪ) to form NH4Cl
(ammonium chloride) that is excreted in urine.

As the Naϩ ions are returned to the blood in the

Table 18–1 EFFECTS OF HORMONES ON THE KIDNEYS

Hormone (gland) Function

Antidiuretic hormone (ADH) • Increases reabsorption of water from the filtrate to the blood.
(posterior pituitary)
• Increases reabsorption of Caϩ2 ions from filtrate to the blood and excretion of
Parathyroid hormone (PTH) phosphate ions into the filtrate.
(parathyroid glands)
• Increases reabsorption of Naϩ ions from the filtrate to the blood and excretion of
Aldosterone Kϩ ions into the filtrate. Water is reabsorbed following the reabsorption of sodium.
(adrenal cortex)
• Decreases reabsorption of Naϩ ions, which remain in the filtrate. More sodium
Atrial natriuretic peptide and water are eliminated in urine.
(ANP) (atria of heart)

Copyright © 2007 by F. A. Davis. The Urinary System 429
ADH
Blood

Increases reabsorption of H2O

PTH Increases reabsorption of Ca+2 Urine
Aldosterone
Increases reabsorption of Na+
and excretion of K+

ANP 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?

peritubular capillaries, HCO3Ϫ ions follow. Notice directly related to the formation of urine. These func-
what has happened: Two Hϩ ions have been excreted tions are secretion of renin (which does influence
in urine, and two Naϩ ions and two HCO3Ϫ ions have urine formation), production of erythropoietin, and
been returned to the blood. As reactions like these activation of vitamin D.
take place, the body fluids are prevented from becom-
ing too acidic. 1. Secretion of renin—When blood pressure de-
creases, the juxtaglomerular ( juxta means “next
Another mechanism used by the cells of the kidney to”) cells in the walls of the afferent arterioles
tubules to regulate pH is the phosphate buffer system, secrete the enzyme renin. Renin then initiates the
which is described in Chapter 19. renin-angiotensin mechanism to raise blood pres-
sure. This was first described in Chapter 13, and
OTHER FUNCTIONS the sequence of events is presented in Table 18–2.
OF THE KIDNEYS The end product of this mechanism is angiotensin
II, which causes vasoconstriction and increases the
In addition to the functions described thus far, the kid- secretion of aldosterone, both of which help raise
neys have other functions, some of which are not blood pressure.

Copyright © 2007 by F. A. Davis. Renal tubule

430 The Urinary System Filtrate

Peritubular
capillary

Na+ Na+ Na+
HCO Ϫ
CO2ϩ HO H2CO3 CIϪ CIϪ Figure 18–6. Renal regulation of acid–base
3 2 H+ Na+ balance. The cells of the renal tubule secrete Hϩ
HCO Ϫ ions and ammonia into the filtrate and return
HCO Ϫ ϩ NH Na+ Naϩ ions and HCO3Ϫ ions to the blood in the
3 3 3 CIϪ peritubular capillaries. See text for further des-
Na+ cription.
NH2ϩ H+ H+ QUESTION: The cells of the renal tubule make
NH + good use of CO2. What do the cells use CO2 for?

NH 4
3

HCO3Ϫ ϩ H+

CO ϩ H O H CO Na+
22 23 NH Cl
Na+
4

Blood Urine

A normal blood pressure is essential to normal Table 18–2 THE RENIN-ANGIOTENSIN
body functioning. Perhaps the most serious change MECHANISM
is a sudden, drastic decrease in blood pressure, such
as would follow a severe hemorrhage. In response to Sequence
such a decrease, the kidneys will decrease filtration
and urinary output and will initiate the formation of 1. Decreased blood pressure stimulates the kidneys to
angiotensin II. In these ways the kidneys help secrete renin.
ensure that the heart has enough blood to pump to
maintain cardiac output and blood pressure. 2. Renin splits the plasma protein angiotensinogen
2. Secretion of erythropoietin—This hormone is (synthesized by the liver) to angiotensin I.
secreted whenever the blood oxygen level decreases
(a state of hypoxia). Erythropoietin stimulates 3. Angiotensin I is converted to angiotensin II by an
the red bone marrow to increase the rate of RBC enzyme found in lung tissue and vascular endothe-
production. With more RBCs in circulation, the lium.
oxygen-carrying capacity of the blood is greater,
and the hypoxic state may be corrected (see also 4. Angiotensin II causes vasoconstriction and stimulates
Box 18–3: Erythropoietin). the adrenal cortex to secrete aldosterone.
3. Activation of vitamin D—This vitamin exists in
several structural forms that are converted to cal- citriol (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.

Copyright © 2007 by F. A. Davis.

The Urinary System 431

BOX 18–3 ERYTHROPOIETIN angular area called the trigone, which has no rugae
and does not expand. The points of the triangle are
Anemia is one of the most debilitating conse- the openings of the two ureters and that of the urethra
quences of renal failure, one that hemodialysis (Fig. 18–7).
cannot reverse. Diseased kidneys stop produc-
ing erythropoietin, a natural stimulus for RBC The smooth muscle layer in the wall of the bladder
production. Erythropoietin can be produced by is called the detrusor muscle. It is a muscle in the
genetic engineering and is available for hemo- form of a sphere; when it contracts it becomes a
dialysis patients. In the past, their anemia could smaller sphere, and its volume diminishes. Around the
only be treated with transfusions, which exposed opening of the urethra the muscle fibers of the detru-
these patients to possible immunologic compli- sor form the internal urethral sphincter (or sphinc-
cations of repeated exposure to donated blood ter of the bladder), which is involuntary.
or to viral diseases. The synthetic erythropoietin
eliminates such risks. Others who benefit from URETHRA
this medication are cancer patients and AIDS
patients with severe anemia. The urethra (see Fig. 18–7) carries urine from the
bladder to the exterior. The external urethral
ELIMINATION OF URINE sphincter is made of the surrounding skeletal muscle
of the pelvic floor, and is under voluntary control.
The ureters, urinary bladder, and urethra do not
change the composition or amount of urine, but are In women, the urethra is 1 to 1.5 inches (2.5 to 4
responsible for the periodic elimination of urine. cm) long and is anterior to the vagina. In men, the
urethra is 7 to 8 inches (17 to 20 cm) long. The first
URETERS part just outside the bladder is called the prostatic ure-
thra because it is surrounded by the prostate gland.
Each ureter extends from the hilus of a kidney to the The next inch is the membranous urethra, around
lower, posterior side of the urinary bladder (see Fig. which is the external urethral sphincter. The longest
18–1). Like the kidneys, the ureters are retroperi- portion is the cavernous urethra (or spongy or penile
toneal, that is, behind the peritoneum of the dorsal urethra), which passes through the cavernous (or erec-
abdominal cavity. tile) tissue of the penis. The male urethra carries
semen as well as urine.
The smooth muscle in the wall of the ureter con-
tracts in peristaltic waves to propel urine toward the THE URINATION REFLEX
urinary bladder. As the bladder fills, it expands and
compresses the lower ends of the ureters to prevent Urination may also be called micturition or voiding.
backflow of urine. This reflex is a spinal cord reflex over which voluntary
control may be exerted. The stimulus for the reflex is
URINARY BLADDER stretching of the detrusor muscle of the bladder. The
bladder can hold as much as 800 mL of urine, or even
The urinary bladder is a muscular sac below the peri- more, but the reflex is activated long before the maxi-
toneum and behind the pubic bones. In women, the mum is reached.
bladder is inferior to the uterus; in men, the bladder is
superior to the prostate gland. The bladder is a reser- When urine volume reaches 200 to 400 mL, the
voir for accumulating urine, and it contracts to elimi- stretching is sufficient to generate sensory impulses
nate urine. that travel to the sacral spinal cord. Motor impulses
return along parasympathetic nerves to the detrusor
The mucosa of the bladder is transitional epithe- muscle, causing contraction. At the same time, the
lium, which permits expansion without tearing the internal urethral sphincter relaxes. If the external ure-
lining. When the bladder is empty, the mucosa thral sphincter is voluntarily relaxed, urine flows into
appears wrinkled; these folds are rugae, which also the urethra, and the bladder is emptied.
permit expansion. On the floor of the bladder is a tri-
Urination can be prevented by voluntary contrac-
tion of the external urethral sphincter. However, if the
bladder continues to fill and be stretched, voluntary
control is eventually no longer possible.

Copyright © 2007 by F. A. Davis. Ureter

432 The Urinary System

Parietal peritoneum
Detrusor muscle

Ureter Openings of
ureters

Rugae

Trigone B

Prostate gland
Prostatic urethra

Trigone Internal Membranous
A urethral sphincter urethra

External
urethral sphincter

Urethra Cavernous (spongy)
urethra
Urethral orifice
Cavernous (erectile)
tissue of penis

Figure 18–7. (A) Frontal section of female urinary bladder and urethra. (B) Frontal sec-
tion of male urinary bladder and urethra.
QUESTION: Name the sphincters of the urinary system and state whether each is volun-
tary or involuntary.

CHARACTERISTICS OF URINE and others are included in Appendix D: Normal
Values for Some Commonly Used Urine Tests.
The characteristics of urine include the physical and
chemical aspects that are often evaluated as part of a Amount—normal urinary output per 24 hours is 1
urinalysis. Some of these are described in this section, to 2 liters. Many factors can significantly change
output. Excessive sweating or loss of fluid through