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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
mucosa is further folded into projections called villi,
ABSORPTION
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
Absorptive cell
Goblet cell
Lacteal
Capillary
network
Plica
circulares
Enteroendocrine cell
Intestinal
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?
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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- Fat-soluble nutrients are absorbed into the lymph
tened out, it would cover more than 2000 square feet in the lacteals of the villi. Bile salts are necessary for
(half a basketball court). 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 Blood from the capillary networks in the villi does
be absorbed by either passive or active transport not return directly to the heart but first travels
mechanisms. Water is absorbed by osmosis following through the portal vein to the liver. You may recall the
the absorption of minerals, especially sodium. Certain importance of portal circulation, discussed in Chapter
nutrients have additional special requirements for 13. This pathway enables the liver to regulate the
their absorption: For example, vitamin B requires blood levels of glucose and amino acids, store certain
12
the intrinsic factor produced by the parietal cells vitamins, and remove potential poisons from the
of the gastric mucosa, and the efficient absorption of blood (see Box 16–3: Disorders of the Intestines).
BOX 16–3 DISORDERS OF THE INTESTINES
Duodenal ulcers are erosions of the duodenal Salmonella food poisoning is caused by bac-
wall caused by the gastric juice that enters from the teria in the genus Salmonella. These are part of the
stomach. The most serious consequences are bleed- intestinal flora of animals, and animal foods such as
ing and perforation. meat and eggs may be sources of infection. These
Paralytic ileus is the cessation of contraction bacteria are not normal for people, and they cause
of the smooth muscle layer of the intestine. This the intestines to secrete large amounts of fluid.
is a possible complication of abdominal surgery, Symptoms include diarrhea, abdominal cramps,
but it may also be the result of peritonitis or and vomiting and usually last only a few days. For
inflammation elsewhere in the abdominal cavity. elderly or debilitated people, however, salmonella
In the absence of peristalsis, intestinal obstruc- food poisoning may be very serious or even fatal.
tion may occur. Bowel movements cease, and vom- Diverticula are small outpouchings through
iting occurs to relieve the pressure within the weakened areas of the intestinal wall. They are
alimentary tube. Treatment involves suctioning more likely to occur in the colon than in the small
the intestinal contents to eliminate any obstruc- intestine and may exist for years without causing
tion and to allow the intestine to regain its normal any symptoms. The presence of diverticula is called
motility. diverticulosis. Inflammation of diverticula is
Lactose intolerance is the inability to digest called diverticulitis, which is usually the result of
lactose because of deficiency of the enzyme lactase. entrapment of feces and bacteria. Symptoms
Lactase deficiency may be congenital, a conse- include abdominal pain and tenderness and fever. If
quence of prematurity, or acquired later in life. The uncomplicated, diverticulitis may be treated with
delayed form is quite common among people of antibiotics and modifications in diet. The most seri-
African or Asian ancestry, and in part is genetic. ous complication is perforation of diverticula, allow-
When lactose, or milk sugar, is not digested, it ing fecal material into the abdominal cavity, causing
undergoes fermentation in the intestine. Symptoms peritonitis. A diet high in fiber is believed to be an
include diarrhea, abdominal pain, bloating, and important aspect of prevention, to provide bulk in
flatulence (gas formation). the colon and prevent weakening of its wall.
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The Digestive System 385
which may occur if fecal material becomes impacted
LARGE INTESTINE
within it. This usually necessitates an appendectomy,
the surgical removal of the appendix.
The large intestine, also called the colon, is approx- The remainder of the colon consists of the ascend-
imately 2.5 inches (6.3 cm) in diameter and 5 feet (1.5 ing, transverse, and descending colon, which encircle
m) in length. It extends from the ileum of the small the small intestine; the sigmoid colon, which turns
intestine to the anus, the terminal opening. The parts medially and downward; the rectum; and the anal
of the colon are shown in Fig. 16–9. The cecum is the canal. The rectum is about 6 inches long, and the anal
first portion, and at its junction with the ileum is canal is the last inch of the colon that surrounds the
the ileocecal valve, which is not a sphincter but serves anus. Clinically, however, the terminal end of the
the same purpose. After undigested food (which is colon is usually referred to as the rectum.
now mostly cellulose) and water pass from the ileum No digestion takes place in the colon. The only
into the cecum, closure of the ileocecal valve prevents secretion of the colonic mucosa is mucus, which lubri-
the backflow of fecal material. cates the passage of fecal material. The longitudinal
Attached to the cecum is the appendix, a small, smooth muscle layer of the colon is in three bands
dead-end tube with abundant lymphatic tissue. The called taeniae coli. The rest of the colon is “gathered”
appendix seems to be a vestigial organ, that is, one to fit these bands. This gives the colon a puckered
whose size and function seem to be reduced. Although appearance; the puckers or pockets are called haustra,
there is abundant lymphatic tissue in the wall of the which provide for more surface area within the colon.
appendix, the possibility that the appendix is con- The functions of the colon are the absorption of
cerned with immunity is not known with certainty. water, minerals, and vitamins and the elimination of
Appendicitis refers to inflammation of the appendix, undigestible material. About 80% of the water that
Splenic flexure
Haustra
Taeniae coli Transverse colon
Hepatic flexure
Descending
Figure 16–9. The large intestine shown in Ascending colon
colon
anterior view. The term flexure means a turn or
bend.
QUESTION: What is the function of the ileoce- lleum Sigmoid colon
cal valve? lleocecal valve
Appendix
Cecum
Rectum
Anal canal
Anus
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386 The Digestive System
colon pushes feces into it. These waves of peristalsis
BOX 16–4 INFANT BOTULISM
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 Stretch receptors in the smooth muscle layer of the
bacteria that produce a neurotoxin. If people rectum generate sensory impulses that travel to the
ingest food containing this toxin, they will sacral spinal cord. The returning motor impulses
develop the paralysis that is characteristic of cause the smooth muscle of the rectum to contract.
botulism. 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
much HCl, so ingested botulism spores may not place.
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
An affected infant becomes lethargic and the stretch receptors of the rectum adapt. These
weak; paralysis may progress slowly or rapidly. receptors will be stimulated again when the next wave
Treatment (antitoxin) is available, but may be of peristalsis reaches the rectum (see Box 16–5: Fiber).
delayed if botulism is not suspected. Many cases
of infant botulism have been traced to honey
that was found to contain botulism spores. Such
spores are not harmful to older children and OTHER FUNCTIONS OF THE LIVER
adults, who have a normal colon flora that pre-
vents the botulism bacteria from becoming The liver is a remarkable organ, and only the brain is
established. capable of a greater variety of functions. The liver
cells (hepatocytes) produce many enzymes that cat-
alyze many different chemical reactions. These reac-
enters the colon is absorbed (400 to 800 mL per day). tions are the functions of the liver. As blood flows
Positive and negative ions are also absorbed. The vita- through the sinusoids (capillaries) of the liver (see Fig.
mins absorbed are those produced by the normal 16–6), materials are removed by the liver cells, and the
flora, the trillions of bacteria that live in the colon. products of the liver cells are secreted into the blood.
Vitamin K is produced and absorbed in amounts usu- Some of the liver functions will already be familiar
ally sufficient to meet a person’s daily need. Other to you. Others are mentioned again and discussed in
vitamins produced in smaller amounts include more detail in the next chapter. Because the liver
riboflavin, thiamin, biotin, and folic acid. Everything has such varied effects on so many body systems, we
absorbed by the colon circulates first to the liver by will use the categories below to summarize the liver
way of portal circulation. Yet another function of the functions.
normal colon flora is to inhibit the growth of 1. Carbohydrate metabolism—As you know, the
pathogens (see Box 16–4: Infant Botulism).
liver regulates the blood glucose level. Excess glu-
cose is converted to glycogen (glycogenesis) when
ELIMINATION OF FECES
blood glucose is high; the hormones insulin and
Feces consist of cellulose and other undigestible mate- cortisol facilitate this process. During hypo-
rial, dead and living bacteria, and water. Elimination glycemia or stress situations, glycogen is converted
of feces is accomplished by the defecation reflex, a back to glucose (glycogenolysis) to raise the blood
spinal cord reflex that may be controlled voluntarily. glucose level. Epinephrine and glucagon are the
The rectum is usually empty until peristalsis of the hormones that facilitate this process.
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BOX 16–5 FIBER
Fiber is a term we use to refer to the organic mate- no protective effect of fiber against colon cancer.
rials in the cell walls of plants. These are mainly cel- What we can say for sure is that fiber may not be
lulose and pectins. The role of dietary fiber and the only dietary or environmental factor involved.
possible benefits that a high-fiber diet may provide Claims that high-fiber diets directly lower blood
are currently the focus of much research. It is levels of cholesterol and fats are not supported by
important to differentiate what is known from what definitive clinical or experimental studies. One pos-
is, at present, merely speculation. sible explanation may be that a person whose diet
Many studies have shown that populations consists largely of high-fiber foods simply eats less
(large groups of people, especially those of different of the foods high in cholesterol and fats, and this is
cultures) who consume high-fiber diets tend to the reason for that person’s lower blood levels of
have a lower frequency of certain diseases. These fats and cholesterol.
include diverticulitis, colon cancer, coronary artery Should people try to make great changes in their
disease, diabetes, and hypertension. Such diseases diets? Probably not, not if they are careful to limit
are much more common among populations fat intake and to include significant quantities of
whose diets are low in vegetables, fruits, and whole vegetables and fruits. Besides the possible benefits
grains, and high in meat, dairy products, and of fiber, unprocessed plant foods provide important
processed foods. In contrast, a 2005 study showed amounts of vitamins and minerals.
Rectum
Longitudinal muscle
Anal canal Circular muscle
Rectal fold
Anal
columns
Levator ani
muscle
B
Internal anal
sphincter Anus
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
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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 The clotting factors are also produced by the
transfer of an amino group (NH ) from an amino liver. These, as you recall, include prothrombin,
2
acid present in excess to a free carbon chain that fibrinogen, and Factor 8, which circulate in the
forms a complete, new amino acid molecule. The blood until needed in the chemical clotting mech-
other eight amino acids, which the liver cannot anism. The liver also synthesizes alpha and beta
synthesize, are called the essential amino acids. In globulins, which are proteins that serve as carriers
this case, “essential” means that the amino acids for other molecules, such as fats, in the blood.
must be supplied by our food, because the liver 5. Formation of bilirubin—This is another familiar
cannot manufacture them. Similarly, “non-essen- function: The liver contains fixed macrophages
tial” means that the amino acids do not have to be that phagocytize old red blood cells (RBCs).
supplied in our food because the liver can make Bilirubin is then formed from the heme portion of
them. All 20 amino acids are required in order to the hemoglobin. The liver also removes from the
make our body proteins. 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 NH group is removed from an amino (or stellate reticuloendothelial cells). Besides
2
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
NH groups that were detached from the original very harmful elsewhere in the body. The bacteria
2
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 B .
12
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-
Fatty acids are a potential source of energy, but lent dietary source of these vitamins.
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-
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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, Ammonia is a toxic substance produced by the
that is, change them to less harmful ones. Alcohol, bacteria in the colon. Because it is soluble in water,
for example, is changed to acetate, which is a two- some ammonia is absorbed into the blood, but it is
carbon molecule (an acetyl group) that can be used carried first to the liver by portal circulation. The
in cell respiration. 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
means that there is too much of it for the liver to AGING AND THE
detoxify in a given time, and the drug will remain DIGESTIVE SYSTEM
in the body with possibly harmful effects. This is
why alcohol should never be consumed when tak- Many changes can be expected in the aging digestive
ing medication. Such a combination may cause the system. The sense of taste becomes less acute, less
liver’s detoxification ability to be overworked and saliva is produced, and there is greater likelihood of
BOX 16–6 HEPATITIS
Hepatitis is inflammation of the liver caused include blood and semen. Hepatitis B may be
by any of several viruses. The most common of severe or even fatal, and approximately 10% of
these hepatitis viruses have been designated A, B, those who recover become carriers of the virus.
and C, although there are others. Symptoms of hep- Possible consequences of the carrier state are
atitis include anorexia, nausea, fatigue, and possibly chronic hepatitis progressing to cirrhosis or primary
jaundice. Severity of disease ranges from very mild liver cancer. Of equal importance, carriers are
(even asymptomatic) to fatal. Hundreds of thou- sources of the virus for others, especially their sex-
sands of cases of hepatitis occur in the United States ual partners.
every year, and although liver inflammation is com- A vaccine is available for hepatitis B, and health-
mon to all of them, the three hepatitis viruses have care workers who have contact with blood, even
different modes of transmission and different conse- just occasional contact, should receive it. Other
quences for affected people. potential recipients of the vaccine are the sexual
Hepatitis A is an intestinal virus that is spread partners of carriers. Pediatricians now consider this
by the fecal–oral route. Food contaminated by the vaccine one of the standard ones for infants.
hands of people with mild cases is the usual vehicle The hepatitis C virus is also present in body flu-
of transmission, although shellfish harvested from ids and is spread by blood or mucous membrane
water contaminated with human sewage are contact. Most people develop chronic disease, but
another possible source of this virus. Hepatitis A is many may remain asymptomatic for years after
most often mild, recovery provides lifelong immu- being infected. With active disease the virus may
nity, and the carrier state is not known to occur. A cause liver failure. The only therapy then is a liver
vaccine is available, but people who have been transplant.
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
Hepatitis B is contracted by exposure to the are not spread by blood transfusions. Donated
body fluids of an infected person; these fluids blood is tested for all three viruses.
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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-
ble-like outpouchings of the weakened wall of the SUMMARY
colon that may be asymptomatic or become infected.
Intestinal obstruction, of the large or small bowel, The processes of the digestion of food and the absorp-
occurs with greater frequency among the elderly. tion of nutrients enable the body to use complex food
Sluggish peristalsis contributes to constipation, which molecules for many purposes. Much of the food we eat
in turn may contribute to the formation of hemor- literally becomes part of us. The body synthesizes pro-
rhoids. The risk of oral cancer or colon cancer also teins and lipids for the growth and repair of tissues and
increases with age. produces enzymes to catalyze all of the reactions that
The liver usually continues to function adequately contribute to homeostasis. Some of our food provides
even well into old age, unless damaged by pathogens the energy required for growth, repair, movement,
such as the hepatitis viruses or by toxins such as alco- sensation, and thinking. In the next chapter we will
hol. There is a greater tendency for gallstones to form, discuss the chemical basis of energy production from
perhaps necessitating removal of the gallbladder. food and consider the relationship of energy produc-
Inflammation of the gallbladder (cholecystitis) is also tion to the maintenance of body temperature.
STUDY OUTLINE
Function of the Digestive System—to break 3. Proteins are digested to amino acids.
down food into simple chemicals that can 4. Other end products are vitamins, minerals, and
be absorbed into the blood and lymph and water.
utilized by cells
Oral Cavity—food enters by way of the
Divisions of the Digestive System mouth
1. Alimentary tube—oral cavity, pharynx, esophagus, 1. Teeth and tongue break up food and mix it with
stomach, small intestine, large intestine. Digestion saliva.
takes place in the oral cavity, stomach, and small 2. Tooth structure (see Fig. 16–2)—enamel covers the
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
increase the surface area for the action of enzymes. hypoglossal nerves. Papillae on the upper surface
2. Chemical—enzymes break down complex organics contain taste buds (facial and glossopharyngeal
into simpler organics and inorganics; each enzyme nerves). Functions: taste, keeps food between the
is specific for the food it will digest. teeth when chewing, elevates to push food back-
ward for swallowing.
End Products of Digestion 4. Salivary glands—parotid, submandibular, and sub-
1. Carbohydrates are digested to monosaccharides. lingual (see Fig. 16–3); ducts take saliva to the oral
2. Fats are digested to fatty acids and glycerol. cavity.
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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
cells, sinusoids, branches of the hepatic artery and
Pharynx—food passageway from the oral portal vein, and bile ducts.
cavity to the esophagus 2. The only digestive secretion is bile; the hepatic
1. No digestion takes place. duct takes bile out of the liver and unites with the
2. Contraction of pharyngeal muscles is part of swal- 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
to stomach tion (see Table 16–1).
1. No digestion takes place. 4. Excess cholesterol and bilirubin are excreted by the
2. Lower esophageal sphincter (LES) at junction with liver into bile.
stomach prevents backup of stomach contents.
Gallbladder—on undersurface of right lobe
Structural Layers of the Alimentary Tube
(see Fig. 16–4) of liver (see Fig. 16–6)
1. Mucosa (lining)—made of epithelial tissue that 1. Stores and concentrates bile until needed in the
produces the digestive secretions; lymph nodules duodenum (see Table 16–2).
contain macrophages to phagocytize pathogens 2. The cystic duct joins the hepatic duct to form the
that penetrate the mucosa; thin layer of smooth common bile duct.
muscle to ripple the epithelium.
2. Submucosa—areolar connective tissue with blood Pancreas—in upper left abdominal quadrant
vessels and lymphatic vessels; Meissner’s plexus is a between the duodenum and the spleen (see
nerve network that innervates the mucosa, part of Fig. 16–1)
the enteric nervous system that extends the entire 1. Pancreatic juice is secreted by acini, carried by pan-
length of the alimentary tube. creatic duct to the common bile duct to the duode-
3. External muscle layer—typically an inner circular num (see Fig. 16–7).
layer and an outer longitudinal layer of smooth 2. Enzyme pancreatic juice contains enzymes for the
muscle; function is mechanical digestion and peri- digestion of all three food types (see Tables 16–1
stalsis; innervated by Auerbach’s plexus, part of the and 16–2).
enteric nervous system; sympathetic impulses 3. Bicarbonate pancreatic juice neutralizes HCl from
decrease motility; parasympathetic impulses the stomach in the duodenum.
increase motility.
4. Serosa—outermost layer; above the diaphragm is
fibrous connective tissue; below the diaphragm is Small Intestine—coiled within the center of
the mesentery (serous). The peritoneum (serous) the abdominal cavity (see Fig. 16–1); extends
lines the abdominal cavity; serous fluid prevents from stomach to colon
friction between the serous layers. 1. Duodenum—first 10 inches; the common bile duct
brings in bile and pancreatic juice. Jejunum (8 feet)
Stomach—in upper left abdominal quadrant; and ileum (11 feet).
a muscular sac that extends from the esoph- 2. Enzymes secreted by the intestinal glands complete
agus to the small intestine (see Fig. 16–5) digestion (see Tables 16–1 and 16–2). Surface area
1. Reservoir for food; begins the digestion of protein. for absorption is increased by plica circulares, villi,
2. Gastric juice is secreted by gastric pits (see Tables and microvilli (see Fig. 16–8); microvilli are the
16–1 and 16–2). brush border.
3. The pyloric sphincter at the junction with the duo- 3. The villi contain capillary networks for the absorp-
denum prevents backup of intestinal contents. tion of water-soluble nutrients: monosaccharides,
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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. stored in the form of glycogen and converted back
4. The villi contain lacteals (lymph capillaries) for the to glucose during hypoglycemia; fructose and
absorption of fat-soluble nutrients: vitamins A, D, galactose are changed to glucose.
E, and K, fatty acids, and glycerol, which are com- 2. Amino acid metabolism—the non-essential amino
bined to form chylomicrons. Lymph from the small acids are synthesized by transamination; excess
intestine is carried back to the blood in the left sub- amino acids are changed to carbohydrates or fats by
clavian vein. deamination; the amino groups are converted to
urea and excreted by the kidneys.
Large Intestine (colon)—extends from the 3. Lipid metabolism—formation of lipoproteins for
small intestine to the anus transport of fats in the blood; synthesis of choles-
1. Colon—parts (see Fig. 16–9): cecum, ascending 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: B , A, D, E, and K, and the
12
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
1. Name the organs of the alimentary tube, and 5. Describe the function of the pharynx, esophagus,
describe the location of each. Name the accessory and lower esophageal sphincter. (p. 373)
digestive organs, and describe the location of each. 6. Name and describe the four layers of the alimen-
(pp. 370, 372, 373, 376, 378, 379, 385)
tary tube. (pp. 373, 376)
2. Explain the purpose of mechanical digestion, and 7. State the two general functions of the stomach and
give two examples. Explain the purpose of chemical the function of the pyloric sphincter. Explain the
digestion, and give two examples. (pp. 370, 374) function of pepsin, HCl, and mucus. (pp. 376–378)
3. Name the end products of digestion, and explain 8. Describe the general functions of the small intes-
how each is absorbed in the small intestine. tine, and name the three parts. Describe the struc-
(pp. 370, 384) tures that increase the surface area of the small
intestine. (pp. 378, 383–384)
4. Explain the function of teeth and tongue, salivary
amylase, enamel of teeth, lysozyme, and water of 9. Explain how the liver, gallbladder, and pancreas
saliva. (pp. 370–372) contribute to digestion. (pp. 379, 381)
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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
level. Explain the purpose of the processes of
12. Describe the functions of the colon and of the deamination and transamination. (pp. 386, 388)
normal flora of the colon. (pp. 385–386)
16. Name the plasma proteins produced by the liver,
13. With respect to the defecation reflex, explain the and state the function of each. (p. 388)
stimulus, the part of the CNS directly involved, 17. Name the substances excreted by the liver into
the effector muscle, the function of the internal bile. (p. 388)
FOR FURTHER THOUGHT
1. Many people with GERD take proton-pump purpose. Explain, and state the disadvantages as
inhibitors, medications that reduce stomach acid. well.
Why should these people be especially careful 5. Explain how a spinal cord transection at the level of
about what they eat or drink?
T10 will affect the defecation reflex.
2. The colon does not have villi as part of its mucosa. 6. You have seen the word enteric (or entero) several
Explain why villi are not necessary.
times in this chapter. What does it mean? Define
3. Food remains in the stomach for several hours. each of these: enteric bacilli, enterovirus, Entero-
Passage of food through the small intestine also coccus.
requires several hours. These two organs have very 7. The word symbiosis indicates two different kinds of
different shapes. Explain why they are able to living things, and literally means “together-life.”
retain food for so long, for efficient digestion and Our own alimentary tube is a perfect example.
absorption.
Explain, and state the advantages to each living
4. Diarrhea can be unpleasant, but does have a thing.
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Copyright © 2007 by F. A. Davis.
CHAPTER 17
Chapter Outline Student Objectives
Body Temperature • State the normal range of human body tempera-
Heat Production ture.
Heat Loss • Explain how cell respiration produces heat and the
Heat loss through the skin factors that affect heat production.
Heat loss through the respiratory tract • Describe the pathways of heat loss through the
Heat loss through the urinary and digestive tracts skin and respiratory tract.
Regulation of Body Temperature • Explain why the hypothalamus is called the “ther-
Mechanisms to increase heat loss mostat” of the body.
Mechanisms to conserve heat • Describe the mechanisms to increase heat loss.
Fever • Describe the mechanisms to conserve heat.
Metabolism • Explain how a fever is caused and its advantages
Cell Respiration and disadvantages.
Glycolysis • Define metabolism, anabolism, and catabolism.
Krebs citric acid cycle • Describe what happens to a glucose molecule dur-
Cytochrome transport system ing the three stages of cell respiration.
Proteins and fats as energy sources • State what happens to each of the products of cell
Energy available from the three nutrient types respiration.
Synthesis Uses of Foods • Explain how amino acids and fats may be used for
Glucose energy production.
Amino acids • Describe the synthesis uses for glucose, amino
Fatty acids and glycerol acids, and fats.
Vitamins and Minerals • Explain what is meant by metabolic rate and kilo-
Metabolic Rate calories.
Aging and Metabolism • 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 KETOSIS
BOX 17–4 METABOLIC RATE
BOX 17–5 WEIGHT LOSS
BOX 17–6 LEPTIN AND BODY-MASS INDEX
394
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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) RAYT)
Conduction (kon-DUK-shun) Frostbite (FRAWST-bite)
Convection (kon-VEK-shun) Heat exhaustion (HEET eks-ZAWS-chun)
Cytochromes (SIGH-toh-krohms) Heat stroke (HEET STROHK)
Endogenous pyrogen (en-DOJ-en-us PYE-roh-jen) Hypothermia (HIGH-poh-THER-mee-ah)
Fever (FEE-ver)
Glycolysis (gly-KAHL-ah-sis)
Kilocalorie (KILL-oh-KAL-oh-ree)
Krebs cycle (KREBS SIGH-kuhl)
Pyrogen (PYE-roh-jen)
Radiation (RAY-dee-AY-shun)
Vitamins (VY-tah-mins)
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
395
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396 Body Temperature and Metabolism
During every moment of our lives, our cells are 1. The hormone thyroxine (and T ), produced by the
3
breaking down food molecules to obtain ATP (adeno- thyroid gland, increases the rate of cell respiration
sine triphosphate) for energy-requiring cellular and heat production. The secretion of thyroxine is
processes. Naturally, we are not aware of the process regulated by the body’s rate of energy production,
of cell respiration, but we may be aware of one of the the metabolic rate itself. (See Chapter 10 for a dis-
products—energy in the form of heat. The human cussion of the feedback mechanism involving the
body is indeed warm, and its temperature is regulated hypothalamus and anterior pituitary gland and
very precisely. Though we cannot stand barefoot on Chapter 1 for an illustration.) When the metabolic
the ice of Antarctica for months in winter, as penguins rate decreases, the thyroid gland is stimulated to
do, we can adapt to and survive a wide range of envi- secrete more thyroxine. As thyroxine increases the
ronmental temperatures. rate of cell respiration, a negative feedback mecha-
This chapter discusses the regulation of body tem- nism inhibits further secretion until metabolic rate
perature and also discusses metabolism, which is the decreases again. Thus, thyroxine is secreted when-
total of all reactions that take place within the body. ever there is a need for increased cell respiration
These reactions include the energy-releasing ones of and is probably the most important regulator of
cell respiration and energy-requiring ones such as day-to-day energy production.
protein synthesis, or DNA synthesis for mitosis. As 2. In stress situations, epinephrine and norepineph-
you will see, body temperature and metabolism are rine are secreted by the adrenal medulla, and the
inseparable. sympathetic nervous system becomes more active.
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° 3. Organs that are normally active (producing
or 36.8°. But everyone seems to prefer the “tradi- ATP) are significant sources of heat when the body
tional” average temperature.) Within a 24-hour is at rest. The skeletal muscles, for example, are
period, an individual’s temperature fluctuates 1° to 2°, usually in a state of slight contraction called muscle
with the lowest temperatures occurring during sleep. 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-
Cell respiration, the process that releases energy from lates to other areas of the body, distributing this
food to produce ATP, also produces heat as one of its heat.
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
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Body Temperature and Metabolism 397
organs produce ATP for peristalsis and for the syn- Heat Loss through the Skin
thesis of digestive enzymes. Because the skin covers the body, most body heat is
5. Changes in body temperature also have an effect lost from the skin to the environment. When the envi-
on metabolic rate and heat production. This ronment is cooler than body temperature (as it usually
becomes clinically important when a person has is), heat loss is unavoidable. The amount of heat that
a fever, an abnormally high body tempera- is lost is determined by blood flow through the skin
ture. The higher temperature increases the meta- and by the activity of sweat glands.
bolic rate, which increases heat production and Blood flow through the skin influences the amount
elevates body temperature further. Thus, a high of heat lost by the processes of radiation, conduction,
fever may trigger a vicious cycle of ever-increasing and convection. Radiation means that heat from the
heat production. Fever is discussed later in this body is transferred to cooler objects not touching the
chapter. skin, much as a radiator warms the contents of a room
The factors that affect heat production are summa- (radiation starts to become less effective when the
rized in Table 17–1. environmental temperature rises above 88°F). Con-
duction is the loss of heat to cooler air or objects, such
as clothing, that touch the skin. Convection means
HEAT LOSS
that air currents move the warmer air away from the
The pathways of heat loss from the body are the skin, skin surface and facilitate the loss of heat; this is why a
the respiratory tract, and, to a lesser extent, the uri- fan makes us feel cooler on hot days. Loss of heat by
nary and digestive tracts. convection also gives us the “wind chill factor” we
hear about in winter. A cold day that is windy will feel
colder than a cold day when the air is still, because the
Table 17–1 FACTORS THAT AFFECT wind blows the slightly warmer air surrounding the
HEAT PRODUCTION body away, replacing it with colder air.
As you may recall from Chapter 5, the tempera-
Factor Effect ture of the skin and the subsequent loss of heat are
determined by blood flow through the skin. The arte-
Thyroxine • The most important regulator
of day-to-day metabolism; rioles in the dermis may constrict or dilate to decrease
increases use of foods for ATP or increase blood flow. In a cold environment, vaso-
production, thereby increasing constriction decreases blood flow through the dermis
heat production and thereby decreases heat loss. In a warm environ-
Epinephrine and • Important in stress situations; ment, vasodilation in the dermis increases blood flow
sympathetic increases the metabolic activity to the body surface and loss of heat to the environ-
stimulation of many organs; increases ATP ment.
and heat production
The other mechanism by which heat is lost from
Skeletal muscles • Normal muscle tone requires the skin is sweating. The eccrine sweat glands
ATP; the heat produced is
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
Liver • Always metabolically active; water into a hot frying pan; the pan is rapidly cooled
produces as much as 20% of as its heat vaporizes the water. Although sweating is
total body heat at rest not quite as dramatic (no visible formation of steam),
Food intake • Increases activity of the GI the principle is just the same.
tract; increases ATP and heat Sweating is most efficient when the humidity of the
production surrounding air is low. Humidity is the percentage of
Higher body • Increases metabolic rate, which the maximum amount of water vapor the atmosphere
temperature increases heat production, can contain. A humidity reading of 90% means that
which further increases meta- the air is already 90% saturated with water vapor and
bolic rate and heat production;
may become detrimental dur- can hold little more. In such a situation, sweat does
ing high fevers not readily evaporate, but instead remains on the skin
even as more sweat is secreted. If the humidity is 40%,
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398 Body Temperature and Metabolism
BOX 17–1 HEAT-RELATED DISORDERS
Heat exhaustion is caused by excessive sweating of heat loss, but in high heat the sweating process
with loss of water and salts, especially NaCl. The continues. As fluid loss increases, sweating stops to
affected person feels very weak, and the skin is usu- preserve body fluid, and body temperature rises
ally cool and clammy (moist). Body temperature is rapidly (over 105°F , possibly as high as 110°F ).
normal or slightly below normal, the pulse is often The classic symptom of heat stroke is hot, dry
rapid and weak, and blood pressure may be low skin. The affected person often loses consciousness,
because of fluid loss. Other symptoms may include reflecting the destructive effect of such a high body
dizziness, vomiting, and muscle cramps. Treatment temperature on the brain. Treatment should involve
involves rest and consumption of salty fluids or fruit hospitalization so that IV fluids may be adminis-
juices (in small amounts at frequent intervals). tered and body temperature lowered under med-
Heat stroke is a life-threatening condition that ical supervision. A first-aid measure would be the
may affect elderly or chronically ill people on hot, application of cool (not ice cold) water to as much
humid days, or otherwise healthy people who exer- of the skin as possible. Fluids should never be forced
cise too strenuously during such weather. High on an unconscious person, because the fluid may
humidity makes sweating an ineffective mechanism be aspirated into the respiratory tract.
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
Although sweating is a very effective mechanism of amount of heat loss.
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 To do this, the hypothalamus must receive infor-
loss. mation about the temperature within the body and
about the environmental temperature. Specialized
Heat Loss through neurons of the hypothalamus detect changes in the
the Respiratory Tract temperature of the blood that flows through the brain.
The temperature receptors in the skin provide infor-
Heat is lost from the respiratory tract as the warmth of mation about the external temperature changes to
the respiratory mucosa evaporates some water from which the body is exposed. The hypothalamus then
the living epithelial surface. The water vapor formed integrates this sensory information and promotes the
is exhaled, and a small amount of heat is lost. necessary responses to maintain body temperature
Animals such as dogs that do not have numerous
sweat glands often pant in warm weather. Panting is within the normal range.
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
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Body Temperature and Metabolism 399
If these mechanisms are not sufficient to prevent
Table 17–2 PATHWAYS OF HEAT LOSS
the body temperature from dropping, more heat may
be produced by increasing muscle tone. When this
Pathway Mechanism
greater muscle tone becomes noticeable and rhythmic,
Skin (major • 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.
• Convection—air currents move People also have behavioral responses to cold, and
warm air away from the skin. these too are important to prevent heat loss. Such
• Sweating—excess body heat things as putting on a sweater or going indoors reflect
evaporates sweat on the skin our awareness of the discomfort of being cold. For
surface. people (we do not have thick fur as do some other
Respiratory tract • Evaporation—body heat evap- mammals), these voluntary activities are of critical
(secondary orates water from the respira- importance to the prevention of excessive heat loss
pathway) tory mucosa, and water vapor when it is very cold (see Box 17–2: Cold-Related
is exhaled.
Disorders).
Urinary tract • Urination—urine is at body
(minor pathway) temperature when eliminated.
Digestive tract • Defecation—feces are at body FEVER
(minor pathway) temperature when eliminated.
A fever is an abnormally high body temperature
and may accompany infectious diseases, extensive
physical trauma, cancer, or damage to the CNS. The
substances that may cause a fever are called pyrogens.
needed. This is accomplished by vasodilation in the
dermis and an increase in sweating. Vasodilation Pyrogens include bacteria, foreign proteins, and
brings more warm blood close to the body surface, chemicals released during inflammation. These
and heat is lost to the environment. However, if the inflammatory chemicals are called endogenous pyro-
environmental temperature is close to or higher than gens. Endogenous means “generated from within.” It
body temperature, this mechanism becomes ineffec- is believed that pyrogens chemically affect the hypo-
tive. The second mechanism is increased sweating, in thalamus and “raise the setting” of the hypothalamic
which excess body heat evaporates the sweat on the thermostat. The hypothalamus will then stimulate
skin surface. As mentioned previously, sweating responses by the body to raise body temperature to
becomes inefficient when the atmospheric humidity is this higher setting.
high. Let us use as a specific example a child who has a
On hot days, heat production may also be strep throat. The bacterial and endogenous pyrogens
decreased by a decrease in muscle tone. This is why we reset the hypothalamic thermostat upward, to 102°F.
may feel very sluggish on hot days; our muscles are At first, the body is “colder” than the setting of the
even slightly less contracted than usual and are slower hypothalamus, and the heat conservation and produc-
to respond. tion mechanisms are activated. The child feels cold
and begins to shiver (chills). Eventually, sufficient heat
is produced to raise the body temperature to the hypo-
Mechanisms to Conserve Heat
thalamic setting of 102°F. At this time, the child will
In a cold environment, heat loss from the body is feel neither too warm nor too cold, because the body
unavoidable but may be reduced to some extent. temperature is what the hypothalamus wants.
Vasoconstriction in the dermis shunts blood away As the effects of the pyrogens diminish, the hypo-
from the body surface, so that more heat is kept in the thalamic setting decreases, perhaps close to normal
core of the body. Sweating decreases, and will stop again, 99°F. Now the child will feel warm, and the heat
completely if the temperature of the hypothalamus loss mechanisms will be activated. Vasodilation in the
falls below about 98.6°F. (Remember that the internal skin and sweating will occur until the body tempera-
temperature of the brain is higher than an oral tem- ture drops to the new hypothalamic setting. This
perature, and is less subject to any changes in environ- is sometimes referred to as the “crisis,” but actually
mental temperature.) the crisis has passed, because sweating indicates that
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400 Body Temperature and Metabolism
BOX 17–2 COLD-RELATED DISORDERS
Frostbite is the freezing of part of the body. slurred speech, drowsiness, and lack of coordina-
Fingers, toes, the nose, and ears are most often tion. At this stage, people often do not realize the
affected by prolonged exposure to cold, because seriousness of their condition, and if outdoors (ice
these areas have little volume in proportion to their skating or skiing) may not seek a warmer environ-
surface. ment. In progressive hypothermia, breathing and
At first the skin tingles, then becomes numb. If heart rate slow, and coma and death follow.
body fluids freeze, ice crystals may destroy capillar- Other people at greater risk for hypothermia
ies and tissues (because water expands when it include the elderly, whose temperature-regulating
freezes), and blisters form. In the most severe cases mechanisms are no longer effective, and quadriple-
gangrene develops; that is, tissue dies because of gics, who have no sensation of cold in the body. For
lack of oxygen. both of these groups, heat production is or may be
Treatment of frostbite includes rewarming the low because of inactivity of skeletal muscles.
affected area. If skin damage is apparent, it should Artificial hypothermia may be induced during
be treated as if it were a burn injury. some types of cardiovascular or neurologic surgery.
Hypothermia is an abnormally low body tem- This carefully controlled lowering of body tempera-
perature (below 95°F) that is most often the result ture decreases the metabolic rate and need for
of prolonged exposure to cold. Although the oxygen and makes possible prolonged surgery
affected person certainly feels cold at first, this sen- without causing extensive tissue death in the
sation may pass and be replaced by confusion, patient.
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.
High fevers, however, may have serious conse-
quences. A fever increases the metabolic rate, which METABOLISM
increases heat production, which in turn raises body
temperature even more. This is a positive feedback The term metabolism encompasses all of the reac-
mechanism that will continue until an external event tions that take place in the body. Everything that hap-
(such as aspirin or death of the pathogens) acts as a pens within us is part of our metabolism. The reactions
brake (see Fig. 1–3). When the body temperature rises of metabolism may be divided into two major cate-
above 106°F, the hypothalamus begins to lose its abil- gories: anabolism and catabolism.
ity to regulate temperature. The proteins of cells, Anabolism means synthesis or “formation” reac-
especially the enzymes, are also damaged by such high tions, the bonding together of smaller molecules to
temperatures. Enzymes become denatured, that is, form larger ones. The synthesis of hemoglobin by
lose their shape and do not catalyze the reactions nec- cells of the red bone marrow, synthesis of glycogen by
essary within cells (see Fig. 2–9). As a result, cells liver cells, and synthesis of fat to be stored in adipose
begin to die. This is most serious in the brain, because tissue are all examples of anabolism. Such reactions
neurons cannot be replaced, and cellular death is the require energy, usually in the form of ATP.
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Body Temperature and Metabolism 401
Hypothalamic Actual body
thermostat temperature
105˚
Pyrogen affects hypothalamus
Vasoconstriction, shivering
104˚
Effect of pyrogen diminishes
103˚
Sweating, vasodilation
Body temperature 101˚
102˚
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. You are already familiar with the summary reaction of
Cell respiration is a series of catabolic reactions that cell respiration,
break down food molecules to carbon dioxide and
water. During catabolism, energy is often released and C H O O → CO H O ATP Heat,
12
6
2
6
2
2
used to synthesize ATP (the heat energy released was
(glucose)
discussed in the previous section). The ATP formed
during catabolism is then used for energy-requiring the purpose of which is to produce ATP. Glucose con-
anabolic reactions. tains potential energy, and when it is broken down to
Most of our anabolic and catabolic reactions are CO and H O, this energy is released in the forms of
2 2
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 CO formed is circulated to
2
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
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Copyright © 2007 by F. A. Davis.
402 Body Temperature and Metabolism
Temperature regulation by
Body temperature
the hypothalamus
Upper limit of survival? 114˚
112˚
44˚ Temperature regulation is
110˚
seriously impaired
108˚
Heat stroke or high fever 42˚
106˚
104˚ 40˚
Strenuous exercise or fever 102˚
38˚ Temperature regulation is
100˚ efficient
37˚
Usual range of normal 98˚
36˚
96˚
94˚
34˚
92˚
90˚ 32˚ Temperature regulation is
Hypothermia 88˚ impaired
(cold weather or
immersion in 86˚ 30˚
cold water) 84˚
28˚
82˚
Temperature regulation is
80˚
26˚ lost
78˚
76˚ 24˚
74˚
72˚
70˚
68˚
Lower limit of survival? 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
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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 CO . This molecule will react with the next
2
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 2NADH , of ATP. As you can see, most of the ATP produced in
2
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 summary of the three stages of cell respiration
The enzymes for the Krebs cycle (or citric acid in Table 17–3 also includes the vitamins and minerals
cycle) are located in the mitochondria of cells. This that are essential for this process. An important over-
second stage of cell respiration is aerobic, meaning all concept is the relationship between eating and
that oxygen is required. In a series of reactions, a pyru- breathing. Eating provides us with a potential energy
vic acid molecule is “taken apart,” and its carbons are source (often glucose) and with necessary vitamins and
converted to CO . The first CO molecule is removed
2 2 minerals. However, to release the energy from food,
by an enzyme that contains the vitamin thiamine. we must breathe. This is why we breathe. The oxygen
This leaves a two-carbon molecule called an acetyl we inhale is essential for the completion of cell respi-
group, which combines with a molecule called coen- ration, and the CO produced is exhaled.
zyme A to form acetyl coenzyme A (acetyl CoA). As 2
acetyl CoA continues in the Krebs cycle, two more Proteins and Fats as Energy Sources
carbons are removed as CO , and more pairs of hydro-
2
gens are picked up by NAD and FAD (another carrier Although glucose is the preferred energy source for
molecule that contains the vitamin riboflavin). cells, proteins and fats also contain potential energy
NADH and FADH will carry their hydrogens to the and are alternative energy sources in certain situations.
2 2
cytochrome transport system. As you know, proteins are made of the smaller mol-
During the Krebs cycle, a small amount of energy ecules called amino acids, and the primary use for the
is released, enough to synthesize one molecule of ATP amino acids we obtain from food is the synthesis of
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404 Body Temperature and Metabolism
Proteins Carbohydrates Fats
(Digestion) (Digestion) (Digestion)
Glucose Glycerol
Amino C C C C C C C CC C
acids NH 2 C
C Fatty acid
C 2 ATP
NH 2 C C C C C C C C C
C NH 2 C
C C C C C C C CC
(Beta
oxidation
2 NAD H 2 4 ATP in the liver)
C
C C
C
C C C C C C C C C
Pyruvic acid
C C C C C
Acetyl
CC C CO 2 groups
Coenzyme A
NH 2
NH 2
C C
NH 2
Acetyl CoA
Oxaloacetic C C
acid C Citric acid
C
C C
C
Urea 3 NAD H 2 C
C
C
FAD H 2
C
CO 2 C
ATP CO 2
H e
H+
H+ H+ Cytochromes 34 ATP
=
H 2 O Oxygen
Metabolic water
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?
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Body Temperature and Metabolism 405
Table 17–3 SUMMARY OF CELL RESPIRATION
Molecules That Vitamins or
Stage Enter the Process Results Minerals Needed
Glycolysis Glucose—ATP needed as • 2 ATP (net) • Niacin (part of NAD)
(cytoplasm) energy of activation • 2 NADH (to cytochrome
2
transport system)
• 2 pyruvic acid (aerobic:
to Krebs cycle; anaerobic:
lactic acid formation)
Krebs citric acid cycle Pyruvic acid—from glucose •CO (exhaled) • Thiamine (for removal
2
(mitochondria) or glycerol or excess • ATP (2 per glucose) of CO )
2
amino acids • 3 NADH and 1 FADH (to • Niacin (part of NAD)
2
2
or cytochrome transport system) • Riboflavin (part of FAD)
Acetyl CoA—from fatty acids • A 4-carbon molecule is regen- • Pantothenic acid (part
or excess amino acids erated for the next cycle of coenzyme A)
Cytochrome NADH and FADH —from • 34 ATP • Iron and copper (part
2
2
transport system glycolysis or the Krebs • Metabolic water of some cytochromes)
(mitochondria) cycle
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 (NH ) summarized in Table 17–4.
2
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 The potential energy in food is measured in units
group. When these molecules enter the Krebs cycle, called Calories or kilocalories. A calorie (lowercase
the results are just the same as if they had come from “c”) is the amount of energy needed to raise the tem-
glucose. This is diagrammed in Fig. 17–3. perature of 1 gram of water 1°C. A kilocalorie or
Fats are made of glycerol and fatty acids, which are
the end products of fat digestion. These molecules may Calorie (capital “C”) is 1000 times that amount of
energy.
also be changed to ones that will take part in the Krebs One gram of carbohydrate yields about 4 kilocalo-
cycle, and the reactions that change them usually take ries. A gram of protein also yields about 4 kilocalories.
place in the liver. Glycerol is a three-carbon molecule A gram of fat, however, yields 9 kilocalories, and
that can be converted to the three-carbon pyruvic acid, a gram of alcohol yields 7 kilocalories. This is why a
which enters the Krebs cycle. In the process of beta- diet high in fat is more likely to result in weight gain
oxidation, the long carbon chains of fatty acids are split if the calories are not expended in energy-requiring
into two-carbon acetyl groups, which enter a later step activities.
in the Krebs cycle (see Fig. 17–3). You may have noticed that calorie content is part of
Both amino acids and fatty acids may be converted
by the liver to ketones, which are two- or four-carbon the nutritional information on food labels. On such
labels the term calorie actually means Calorie or kilo-
molecules such as acetone and acetoacetic acid. calories but is used for the sake of simplicity.
Although body cells can use ketones in cell respira-
tion, they do so slowly. In situations in which fats or SYNTHESIS USES OF FOODS
amino acids have become the primary energy sources,
a state called ketosis may develop; this is described in Besides being available for energy production, each of
Box 17–3: Ketosis. Excess amino acids may also be the three food types is used in anabolic reactions to
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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
ketones. Ketones are organic molecules such as
Thyroxine (thyroid • 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
cose, fats, amino acids)
• Increases protein synthesis ketones as an energy source, but they do so
slowly. When ketones are produced in small
Growth hormone • Increases amino acid trans- amounts, as they usually are between meals, the
(anterior pituitary) port into cells blood level does not rise sharply.
• Increases protein synthesis A state of ketosis exists when fats and pro-
• Increases use of fats for
energy teins become the primary energy sources, and
ketones accumulate in the blood faster than cells
Insulin (pancreas) • Increases glucose transport can utilize them. Because ketones are organic
into cells and use for energy
• Increases conversion of glu- acids, they lower the pH of the blood. As the
cose to glycogen in liver blood ketone level rises, the kidneys excrete
and muscles ketones, but they must also excrete more water
• Increases transport of amino as a solvent, which leads to dehydration.
acids and fatty acids into Ketosis is clinically important in diabetes mel-
cells to be used for synthe- litus, starvation, and eating disorders such as
sis (not energy production) anorexia nervosa. Diabetics whose disease is
Glucagon (pancreas) • Increases conversion of poorly controlled may progress to ketoacido-
glycogen to glucose sis, a form of metabolic acidosis that may lead to
• Increases use of amino acids confusion, coma, and death. Reversal of this
and fats for energy state requires a carbohydrate energy source and
Cortisol (adrenal • Increases conversion of glu- the insulin necessary to utilize it.
cortex) cose to glycogen in liver
• Increases use of amino acids
and fats for energy
• Decreases protein synthesis in RNA. This function of glucose is very important,
except in liver and GI tract for without the pentose sugars our cells could neither
Epinephrine • Increases conversion of produce new chromosomes for cell division nor carry
(adrenal medulla) glycogen to glucose out the process of protein synthesis.
• Increases use of fats for Any glucose in excess of immediate energy needs or
energy
the need for pentose sugars is converted to glycogen
in the liver and skeletal muscles. Glycogen is then an
energy source during states of hypoglycemia or during
synthesize necessary materials for cells and tissues. A exercise. If still more glucose is present, it will be
simple summary of these reactions is shown in Fig. changed to fat and stored in adipose tissue.
17–4. The three food types and their end products of
digestion are at the bottom of the picture, and the Amino Acids
arrows going upward indicate synthesis and lead to the
products formed. You may wish to refer to Fig. 17–4 As mentioned previously, the primary uses for amino
as you read the next sections. acids are the synthesis of the non-essential amino
acids by the liver and the synthesis of new proteins in
all tissues. By way of review, we can mention some
Glucose
proteins with which you are already familiar: keratin
Glucose is the raw material for the synthesis of and melanin in the epidermis; collagen in the dermis,
another important monosaccharide, the pentose sug- tendons, and ligaments; myosin, actin, and myoglobin
ars that are part of nucleic acids. Deoxyribose is the in muscle cells; hemoglobin in RBCs; antibodies pro-
five-carbon sugar found in DNA, and ribose is found duced by WBCs; prothrombin and fibrinogen for
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Body Temperature and Metabolism 407
True fats
(adipose tissue)
Proteins
(enzymes, structural)
Glycogen
Cholesterol
and other
steroids
Excess
(deamination)
Excess
Phospholipids
Non-essential (cell membranes)
amino acids Pentose sugars
Transamination
Proteins Carbohydrates Fats
(Digestion) (Digestion)
(Digestion)
C C C
C C C CCC C
Amino NH 2 C Glucose Glycerol
acids C Fatty acid
C
NH 2 C C C C C C C CC
C NH 2 C C
C C C C C C 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
The amino acids we obtain from the proteins in carbohydrates and contribute to glycogen storage
our food are used by our cells to synthesize all of these or they may be changed to fat and stored in adipose
proteins in the amounts needed by the body. Only tissue.
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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 Deficiencies of vitamins often result in disease:
17–4 that insulin promotes fat synthesis and storage. vitamin C deficiency and scurvy, for example (see Box
One theory of weight gain proposes that a diet high in 4–2). Other deficiency diseases that have been known
sugars and starches stimulates the secretion of so for decades include pellagra (lack of niacin), beri-beri
much insulin that fat can only be stored, not taken out (riboflavin), pernicious anemia (B ), and rickets (D).
12
of storage and used for energy. 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
The liver can synthesize most of the fatty acids women should be aware of the need for extra (400
needed by the body. Two exceptions are linoleic acid micrograms) folic acid during pregnancy.
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
beta-oxidation, the resulting acetyl groups may also be
used for the synthesis of cholesterol, a steroid. This METABOLIC RATE
takes place primarily in the liver, although all cells are Although the term metabolism is used to describe all
capable of synthesizing cholesterol for their cell mem- of the chemical reactions that take place within the
branes. The liver uses cholesterol to synthesize bile body, metabolic rate is usually expressed as an
salts for the emulsification of fats in digestion. The amount of heat production. This is because many
steroid hormones are also synthesized from choles- body processes that utilize ATP also produce heat.
terol. Cortisol and aldosterone are produced by the These processes include the contraction of skeletal
adrenal cortex, estrogen and progesterone by the muscle, the pumping of the heart, and the normal
ovaries, and testosterone by the testes. breakdown of cellular components. Therefore, it is
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 The energy required for merely living (lying qui-
radicals are formed during some normal body reac- etly in bed) is the basal metabolic rate (BMR). See
tions, but smoking and exposure to pollution will Box 17–4: Metabolic Rate for a formula to estimate
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Copyright © 2007 by F. A. Davis.
Table 17–5 VITAMINS
Vitamin Functions Food Sources Comment
Water Soluble
Thiamine (B ) • Conversion of pyruvic acid to acetyl • Meat, eggs, Rapidly destroyed by heat
1
CoA in cell respiration legumes, green
• Synthesis of pentose sugars leafy vegetables,
• Synthesis of acetylcholine grains
Riboflavin (B ) • Part of FAD in cell respiration • Meat, milk, cheese, Small amounts produced
2
grains by GI bacteria
Niacin (nicotinamide) • Part of NAD in cell respiration • Meat, fish, grains,
• Metabolism of fat for energy legumes
Pyridoxine (B ) • Part of enzymes needed for amino • Meat, fish, grains, Small amounts produced
6
acid metabolism and protein synthe- yeast, yogurt by GI bacteria
sis, nucleic acid synthesis, synthesis of
antibodies
B (cyanocobalamin) • Synthesis of DNA, especially in RBC • Liver, meat, fish, Contains cobalt; intrinsic
12
production eggs, milk, cheese factor required for
• Metabolism of amino acids for energy absorption
Biotin • Synthesis of nucleic acids • Yeast, liver, eggs Small amounts produced
• Metabolism of fatty acids and amino by GI bacteria
acids
Folic acid (folacin) • Synthesis of DNA, especially in blood • Liver, grains, Small amounts produced
cell production legumes, leafy by GI bacteria
• Contributes to development of fetal green vegetables
CNS
Pantothenic acid • Part of coenzyme A in cell respiration, • Meat, fish, grains, Small amounts produced
use of amino acids and fats for energy legumes, vegetables by GI bacteria
Vitamin C (ascorbic • Synthesis of collagen, especially for • Citrus fruits, toma- Rapidly destroyed by
acid) wound healing toes, potatoes heat
• Metabolism of amino acids
• Absorption of iron
• An antioxidant—prevents cellular
damage from free radicals
Fat Soluble
Vitamin A • Synthesis of rhodopsin • Yellow and green Stored in liver; bile salts
• Calcification of growing bones vegetables, liver, required for absorp-
• Maintenance of epithelial tissues milk, eggs tion
Vitamin D • Absorption of calcium and phospho- • Fortified milk, egg Produced in skin exposed
rus in the small intestine yolks, fish liver oils to UV rays; stored
• Contributes to immune responses, in liver; bile salts
action of insulin, and preservation of required for absorp-
muscle mass and strength tion
Vitamin E • An antioxidant—prevents destruction • Nuts, wheat germ, Stored in liver and adi-
of cell membranes seed oils pose tissue; bile salts
• Contributes to wound healing and required for absorp-
detoxifying ability of the liver tion
Vitamin K • Synthesis of prothrombin and • Liver, spinach, Large amounts produced
other clotting factors cabbage by GI bacteria; bile salts
required for absorption;
stored in liver
409
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410 Body Temperature and Metabolism
Table 17–6 MINERALS
Mineral Functions Food Sources Comment
Calcium • Formation of bones and teeth • Milk, cheese, yogurt, Vitamin D required for
• Neuron and muscle functioning shellfish, leafy green absorption; stored in
• Blood clotting vegetables bones
Phosphorus • Formation of bones and teeth • Milk, cheese, fish, meat Vitamin D required for
• Part of DNA, RNA, and ATP absorption; stored in
• Part of phosphate buffer system bones
Sodium • Contributes to osmotic pressure • Table salt, almost all Most abundant cation ( )
of body fluids foods in extracellular fluid
• Nerve impulse transmission and
muscle contraction
• Part of bicarbonate buffer system
Potassium • Contributes to osmotic pressure of • Virtually all foods Most abundant cation ( )
body fluids in intracellular fluid
• Nerve impulse transmission and
muscle contraction
Chlorine • Contributes to osmotic pressure of • Table salt Most abundant anion ( )
body fluids in extracellular fluid
• Part of HCI in gastric juice
Iron • Part of hemoglobin and myoglobin • Meat, shellfish, dried Stored in liver
• Part of some cytochromes in cell apricots, legumes, eggs
respiration
Iodine • Part of thyroxine and T 3 • Iodized salt, seafood
Sulfur • Part of some amino acids • Meat, eggs Insulin and keratin require
• Part of thiamine and biotin sulfur
Magnesium • Formation of bone • Green vegetables, Part of chlorophyll in green
• Metabolism of ATP–ADP legumes, seafood, milk plants
Manganese • Formation of urea • Legumes, grains, nuts, Some stored in liver
• Synthesis of fatty acids and choles- leafy green vegetables
terol
Copper • Synthesis of hemoglobin • Liver, seafood, grains, Stored in liver
• Part of some cytochromes in cell nuts, legumes
respiration
• Synthesis of melanin
Cobalt • Part of vitamin B 12 • Liver, meat, fish Vitamin B 12 stored in liver
Zinc • Part of carbonic anhydrase needed • Meat, seafood, grains,
for CO transport legumes
2
• 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
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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 1. 160 lb at 2.2 lb/kg = 73 kg
follows: 2. 73 kg x 1.0 kcal/kg/hr = 73 kcal/hr
3. 73 kcal/hr x 24 = 1752 kcal/day
For women: use the factor of 0.9 kcal per kilogram
(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
Light activity: add 50% to 65% of the BMR to the
Example: A 120-pound woman: BMR
Moderate activity: add 65% to 75% of the BMR to
1. Change pounds to kilograms: the BMR
120 lb at 2.2 lb/kg = 55 kg
Strenuous activity: add 75% to 100% 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 Using our example of the 120-pound woman
with a BMR of 1200 kcal/day:
3. Multiply kcal/hr by 24:
Sedentary: 1680 to 1800 kcal/day
49.5 kcal/hr x 24 = 1188 kcal/day Light: 1800 to 1980 kcal/day
(An approximate BMR, about 1200 Moderate: 1980 to 2100 kcal/day
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
To lose 1 pound of body fat, which consists of fat, of some body protein so that amino acids can be
water, and protein, 3500 calories of energy must be converted to carbohydrates to supply the brain.
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-
Most food packaging contains nutritional infor- ply vitamins, minerals, and fiber.
mation, including the calories per serving of the
Calories per 10 Calories per 10
minutes (average minutes (average
Activity for a 150-lb person) Activity for a 150-lb person)
Walking slowly 30 Running (8 mph) 120
Walking briskly 45 Cycling (10 mph) 70
Walking up stairs 170 Cycling (15 mph) 115
Dancing (slow) 40 Swimming 100
Dancing (fast) 65
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Box 17–6 LEPTIN AND BODY-MASS INDEX
The 1994 discovery of the hormone leptin was directly decreases fat storage in cells, and improves
reported to the general public in 1995, along with the efficiency of the pancreatic cells that produce
speculation that leptin could become an anti-obe- insulin. What was first believed to be a simple chem-
sity medication, which it has not. Leptin is a protein ical signal has proved to be much more complex.
produced by fat cells, and signals the hypothalamus A good measure of leanness or fatness is the
to release a chemical that acts as an appetite sup- body-mass index.
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
the onset of puberty, especially in females. Girls Example: A person five foot six weighing 130
who are very thin, with little body fat, tend to have pounds.
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 66 = 1385
for continued ovulation. Leptin may be the chemi- 1385 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
Fig. 17–5.
4. Sex hormones—Testosterone increases metabolic
activity to a greater degree than does estrogen, giv-
ing men a slightly higher metabolic rate than
women. Also, men tend to have more muscle, an
active tissue, whereas women tend to have more
fat, a relatively inactive tissue.
5. Sympathetic stimulation—In stress situations, the
metabolism of many body cells is increased. Also
contributing to this are the hormones epinephrine
and norepinephrine. As a result, metabolic rate
increases.
6. Decreased food intake—If the intake of food
decreases for a prolonged period of time, metabolic
rate also begins to decrease. It is as if the body’s
metabolism is “slowing down” to conserve what-
ever energy sources may still be available. (See also
Figure 17–5. Surface-to-weight ratios. Imagine that
Box 17–6: Leptin and Body-Mass Index.)
the three shapes are people who all weigh the same
7. Climate—People who live in cold climates may
amount. The “tall, thin person” on the right has about
have metabolic rates 10% to 20% higher than peo-
50% more surface area than does the “short, stocky per-
ple who live in tropical regions. This is believed to
son” on the left. The more surface area (where heat is
be due to the variations in the secretion of thyrox-
lost), the higher the metabolic rate.
QUESTION: Which of these ratios best represents an ine, the hormone most responsible for regulation
infant? (Rather than weight, think of inside-outside of metabolic rate. In a cold climate, the necessity
proportion.) for greater heat production brings about an
412
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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
wave was the cause of at least 25,000 deaths. Most of
AGING AND METABOLISM those who died were elderly.
As mentioned in the previous section, metabolic rate
decreases with age. Elderly people who remain active,
however, can easily maintain a metabolic rate (energy SUMMARY
production) adequate for their needs as long as their
general health is good. Some elderly people subject to Food is needed for the synthesis of new cells and tis-
physical or emotional disability, however, may be at sues, or is utilized to produce the energy required for
risk for malnutrition. Caregivers may assess such a risk such synthesis reactions. As a consequence of metabo-
by asking how often the person eats every day; if lism, heat energy is released to provide a constant
appetite is good, fair, or poor; and how the food tastes. body temperature and permit the continuation of
These simple questions may help ensure adequate metabolic activity. The metabolic pathways described
nutrition. in this chapter are only a small portion of the body’s
Sensitivity to external temperature changes may total metabolism. Even this simple presentation, how-
decrease with age, and the regulation of body temper- ever, suggests the great chemical complexity of the
ature is no longer as precise. Sweat glands are not as functioning human being.
STUDY OUTLINE
Body Temperature A fever increases the metabolic rate, and more heat
1. Normal range is 96.5° to 99.5°F (36° to 38°C), is produced; this may become detrimental during
with an average of 98.6°F (37°C). very high fevers.
2. Normal fluctuation in 24 hours is 1° to 2°F.
3. Temperature regulation in infants and the elderly is Heat Loss (see Table 17–2)
not as precise as it is at other ages. 1. Most heat is lost through the skin.
2. Blood flow through the dermis determines the
Heat Production amount of heat that is lost by radiation, conduc-
Heat is one of the energy products of cell respiration. tion, and convection.
Many factors affect the total heat actually produced 3. Vasodilation in the dermis increases blood flow and
(see Table 17–1). heat loss; radiation and conduction are effective
1. Thyroxine from the thyroid gland—the most im- only if the environment is cooler than the body.
portant regulator of daily heat production. As meta- 4. Vasoconstriction in the dermis decreases blood
bolic rate decreases, more thyroxine is secreted to flow and conserves heat in the core of the body.
increase the rate of cell respiration. 5. Sweating is a very effective heat loss mechanism;
2. Stress—sympathetic impulses and epinephrine and excess body heat evaporates sweat on the skin sur-
norepinephrine increase the metabolic activity of face; sweating is most effective when the atmos-
many organs, increasing the production of ATP pheric humidity is low.
and heat. 6. Sweating also has a disadvantage in that water is
3. Active organs continuously produce heat. Skeletal lost and must be replaced to prevent serious dehy-
muscle tone produces 25% of the total body heat at dration.
rest. The liver provides up to 20% of the resting 7. Heat is lost from the respiratory tract by the evap-
body heat. oration of water from the warm respiratory
4. Food intake increases the activity of the digestive mucosa; water vapor is part of exhaled air.
organs and increases heat production. 8. A very small amount of heat is lost as urine and
5. Changes in body temperature affect metabolic rate. feces are excreted at body temperature.
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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).
and regulates body temperature by balancing heat 3. The oxygen necessary comes from breathing.
production and heat loss. 4. The water formed becomes part of intracellular
2. The hypothalamus receives information from its fluid; CO is exhaled; ATP is used for energy-
2
own neurons (blood temperature) and from the requiring reactions; heat provides a body tempera-
temperature receptors in the dermis. ture.
3. Mechanisms to increase heat loss are vasodilation
in the dermis and increased sweating. Decreased Proteins and Fats—as energy sources (see
muscle tone will decrease heat production. Table 17–4 for hormonal regulation)
4. Mechanisms to conserve heat are vasoconstriction 1. Excess amino acids are deaminated in the liver and
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
foreign proteins, or chemicals released during cycle; ketones are formed for transport to other
inflammation (endogenous pyrogens). cells (see Fig. 17–3).
2. Pyrogens raise the setting of the hypothalamic
thermostat; the person feels cold and begins to Energy Available from Food
shiver to produce heat. 1. Energy is measured in kilocalories (Calories):
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 per gram of protein, 9 kcal per gram of fat. With
body temperature. reference to food, kilocalories may be called calo-
4. A low fever may be beneficial because it increases ries.
the activity of WBCs and inhibits the activity of
some pathogens. Synthesis Uses of Foods (Fig. 17–4)
5. A high fever may be detrimental because enzymes 1. Glucose—used to synthesize the pentose sugars for
are denatured at high temperatures. This is most DNA and RNA; used to synthesize glycogen to
critical in the brain, where cells that die cannot be store energy in liver and muscles.
replaced. 2. Amino acids—used to synthesize new proteins and
the non-essential amino acids; essential amino
Metabolism—all the reactions within the acids must be obtained in the diet.
body 3. Fatty acids and glycerol—used to synthesize phos-
1. Anabolism—synthesis reactions that usually pholipids for cell membranes, triglycerides for fat
require energy in the form of ATP. storage in adipose tissue, and cholesterol and other
2. Catabolism—decomposition reactions that often 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
tions. stored.
5. Vitamins and minerals—see Tables 17–5 and 17–6.
Cell Respiration—the breakdown of food
molecules to release their potential energy Metabolic Rate—heat production by the
and synthesize ATP (Fig. 17–3) body; measured in kcal
1. Glucose oxygen yields CO H O ATP 1. Basal metabolic rate (BMR) is the energy required
2 2
heat. to maintain life (see Box 17–4); several factors
2. The breakdown of glucose involves three stages: influence the metabolic rate of an active person.
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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
1. State the normal range of human body temperature 9. Define metabolism, anabolism, catabolism, kilo-
in °F and °C. (p. 396) calorie, and metabolic rate. (pp. 400, 401, 405,
408)
2. State the summary equation of cell respiration, and
state what happens to (or the purpose of) each of 10. Name the three stages of the cell respiration
the products. (p. 401) of glucose and state where in the cell each takes
place and whether or not oxygen is required.
3. Describe the role of each in heat production: thy-
roxine, skeletal muscles, stress situations, and the (pp. 403)
liver. (p. 396) 11. For each, state the molecules that enter the
4. Describe the two mechanisms of heat loss through process and the results of the process: glycolysis,
the skin, and explain the role of blood flow. Krebs cycle, and cytochrome transport system.
Describe how heat is lost through the respiratory (pp. 403–405)
tract. (pp. 397–398) 12. Explain how fatty acids, glycerol, and excess
5. Explain the circumstances that exist when sweating amino acids are used for energy production in cell
and vasodilation in the dermis are not effective respiration. (pp. 403, 405)
mechanisms of heat loss. (p. 397) 13. Describe the synthesis uses for glucose, amino
6. Name the part of the brain that regulates body acids, and fatty acids. (pp. 406–408)
temperature, and explain what is meant by a ther- 14. Describe four factors that affect the metabolic
mostat. (p. 398)
rate of an active person. (pp. 410, 412)
7. Describe the responses by the body to a warm envi- 15. If lunch consists of 60 grams of carbohydrate,
ronment and to a cold environment. (pp. 399)
15 grams of protein, and 10 grams of fat, how
8. Explain how pyrogens are believed to cause a fever, many kilocalories are provided by this meal?
and give two examples of pyrogens. (p. 399) (p. 405)
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
3. Studies with animals have shown that caloric
broccoli?
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
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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.
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CHAPTER 18
The Urinary System
417
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CHAPTER 18
Chapter Outline Student Objectives
Kidneys • Describe the location and general function of each
Internal Structure of the Kidney organ of the urinary system.
The Nephron • Name the parts of a nephron and the important
Renal corpuscle blood vessels associated with them.
Renal tubule • Explain how the following are involved in urine
Blood Vessels of the Kidney formation: glomerular filtration, tubular reabsorp-
Formation of Urine tion, tubular secretion, and blood flow through the
Glomerular Filtration kidney.
Tubular Reabsorption • Describe the mechanisms of tubular reabsorption,
Mechanisms of reabsorption and explain the importance of tubular secretion.
Tubular Secretion • Describe how the kidneys help maintain normal
Hormones That Influence Reabsorption of Water blood volume and blood pressure.
Summary of Urine Formation • Name and state the functions of the hormones
The Kidneys and Acid–Base Balance that affect the kidneys.
Other Functions of the Kidneys • Describe how the kidneys help maintain normal
Elimination of Urine pH of blood and tissue fluid.
Ureters • Describe the urination reflex, and explain how vol-
Urinary Bladder untary control is possible.
Urethra • Describe the characteristics of normal urine.
The Urination Reflex
Characteristics of Urine
Aging and the Urinary System
BOX 18–1 FLOATING KIDNEY
BOX 18–2 RENAL FAILURE AND HEMODIALYSIS
BOX 18–3 ERYTHROPOIETIN
BOX 18–4 KIDNEY STONES
BOX 18–5 BLOOD TESTS AND KIDNEY FUNCTION
BOX 18–6 URINARY TRACT INFECTIONS
418
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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)
SFINK-ter) Nephritis (ne-FRY-tis)
Glomerular filtration rate (gloh-MER-yoo-ler fill- Oliguria (AH-li-GYOO-ree-ah)
TRAY-shun RAYT) Polyuria (PAH-li-YOO-ree-ah)
Glomerulus (gloh-MER-yoo-lus) Renal calculi (REE-nuhl KAL-kew-lye)
Internal urethral sphincter (yoo-REE-thruhl Renal failure (REE-nuhl FAYL-yer)
SFINK-ter) Uremia (yoo-REE-me-ah)
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
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420 The Urinary System
The first successful human organ transplant was a is sufficient to carry out the complex work required to
kidney transplant performed in 1953. Because the maintain homeostasis of the body fluids.
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,
Ribs
Aorta
Inferior vena cava
Left adrenal gland
Diaphragm Superior mesenteric artery
Left renal artery and vein
Left kidney
Left ureter
Right kidney
Left common iliac
artery and vein
Lumbar vertebra
Psoas major muscle Pelvis
lliacus muscle Sacrum
Right ureter
Opening of ureter
Urinary bladder Trigone of bladder
Symphysis pubis
Urethra
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?
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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 Each kidney has an indentation called the hilus
functions: on its medial side. At the hilus, the renal artery enters
the kidney, and the renal vein and ureter emerge. The
1. Regulation of the volume of blood by excretion or
conservation of water renal artery is a branch of the abdominal aorta, and the
2. Regulation of the electrolyte content of the blood renal vein returns blood to the inferior vena cava (see
by the excretion or conservation of minerals Fig. 18–1). The ureter carries urine from the kidney to
3. Regulation of the acid–base balance of the blood by the urinary bladder.
excretion or conservation of ions such as H ions
or HCO ions INTERNAL STRUCTURE
3
4. Regulation of all of the above in tissue fluid OF THE KIDNEY
The process of urine formation, therefore, helps In a coronal or frontal section of the kidney, three
maintain the normal composition, volume, and pH of areas can be distinguished (Fig. 18–2). The lateral and
both blood and tissue fluid by removing those sub- middle areas are tissue layers, and the medial area at
stances that would upset the normal constancy and the hilus is a cavity. The outer tissue layer is called the
balance of these extracellular fluids. renal cortex; it is made of renal corpuscles and convo-
luted tubules. These are parts of the nephron and are
described in the next section. The inner tissue layer is
KIDNEYS the renal medulla, which is made of loops of Henle
and collecting tubules (also parts of the nephron). The
The two kidneys are located in the upper abdominal renal medulla consists of wedge-shaped pieces called
cavity on either side of the vertebral column, behind renal pyramids. The tip of each pyramid is its apex or
the peritoneum (retroperitoneal). The upper por- papilla.
tions of the kidneys rest on the lower surface of the The third area is the renal pelvis; this is not a layer
diaphragm and are enclosed and protected by the of tissues, but rather a cavity formed by the expansion
lower rib cage (see Fig. 18–1). The kidneys are of the ureter within the kidney at the hilus. Funnel-
embedded in adipose tissue that acts as a cushion and shaped extensions of the renal pelvis, called calyces
(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-
A kidney can function in any position; the ated blood vessels, that urine is formed. Each nephron
problem with a floating kidney is that the ureter has two major portions: a renal corpuscle and a renal
may become twisted or kinked. If urine cannot tubule. Each of these major parts has further subdivi-
flow through the ureter, the urine backs up and sions, which are shown with their blood vessels in Fig.
collects in the renal pelvis. Incoming urine 18–3.
from the renal tubules then backs up as well. If
the renal filtrate cannot flow out of Bowman’s
capsules, the pressure within Bowman’s capsules Renal Corpuscle
increases, opposing the blood pressure in the A renal corpuscle consists of a glomerulus surroun-
glomeruli. Glomerular filtration then cannot take
place efficiently. If uncorrected, this may lead to ded by a Bowman’s capsule. The glomerulus is a cap-
permanent kidney damage. illary network that arises from an afferent arteriole
and empties into an efferent arteriole. The diameter
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422 The Urinary System
Renal cortex
Nephron
Renal medulla
(pyramids)
Renal corpuscle
Papilla of pyramid Renal cortex
Calyx
Renal pelvis
Renal artery
Renal tubule
Renal vein
Interlobar
artery
Renal medulla
Arcuate artery
Ureter Papillary duct
A
B
C D
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?
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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
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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.
Bowman’s capsule (or glomerular capsule) is the Therefore, in the kidneys there are two sites of ex-
expanded end of a renal tubule; it encloses the change. The exchanges that take place between the
glomerulus. The inner layer of Bowman’s capsule is nephrons and the capillaries of the kidneys will form
made of podocytes; the name means “foot cells,” and urine from blood plasma.
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
is formed from the blood in the glomerulus and will FORMATION OF URINE
eventually become urine.
The formation of urine involves three major pro-
Renal Tubule cesses. The first is glomerular filtration, which takes
The renal tubule continues from Bowman’s capsule place in the renal corpuscles. The second and third are
and consists of the following parts: proximal convo- tubular reabsorption and tubular secretion, which take
luted tubule (in the renal cortex), loop of Henle (or place in the renal tubules.
loop of the nephron, in the renal medulla), and distal
convoluted tubule (in the renal cortex). The distal GLOMERULAR FILTRATION
convoluted tubules from several nephrons empty into You may recall that filtration is the process in which
a collecting tubule. Several collecting tubules then blood pressure forces plasma and dissolved material
unite to form a papillary duct that empties urine into out of capillaries. In glomerular filtration, blood
a calyx of the renal pelvis. pressure forces plasma, dissolved substances, and small
Cross-sections of the parts of the renal tubule are
shown in Fig. 18–3. Notice how thin the walls of the proteins out of the glomeruli and into Bowman’s cap-
sules. This fluid is no longer plasma but is called renal
tubule are, and also the microvilli in the proximal con- filtrate.
voluted tubule. These anatomic characteristics provide The blood pressure in the glomeruli, compared
for efficient exchanges of materials, as you will see. with that in other capillaries, is relatively high, about
All parts of the renal tubule are surrounded by
peritubular capillaries, which arise from the efferent 60 mmHg. The pressure in Bowman’s capsule is very
low, and its inner, podocyte layer is very permeable, so
arteriole. The peritubular capillaries will receive the that approximately 20% to 25% of the blood that
materials reabsorbed by the renal tubules; this is enters glomeruli becomes renal filtrate in Bowman’s
described in the section on urine formation.
capsules. The blood cells and larger proteins are too
large to be forced out of the glomeruli, so they remain
BLOOD VESSELS OF THE KIDNEY
in the blood. Waste products are dissolved in blood
The pathway of blood flow through the kidney is an plasma, so they pass into the renal filtrate. Useful
essential part of the process of urine formation. Blood materials such as nutrients and minerals are also dis-
from the abdominal aorta enters the renal artery, solved in plasma and are also present in renal filtrate.
which branches extensively within the kidney into Filtration is not selective with respect to usefulness; it
smaller arteries (see Fig. 18–2). The smallest arteries is selective only with respect to size. Therefore, renal
give rise to afferent arterioles in the renal cortex (see filtrate is very much like blood plasma, except that
Fig. 18–3). From the afferent arterioles, blood flows there is far less protein and no blood cells are present.
into the glomeruli (capillaries), to efferent arterioles, The glomerular filtration rate (GFR) is the
to peritubular capillaries, to veins within the kidney, to amount of renal filtrate formed by the kidneys in 1
the renal vein, and finally to the inferior vena cava. minute, and averages 100 to 125 mL per minute. GFR
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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
“Prerenal” means that the problem is “before” plasma. Waste products and excess minerals diffuse
the kidneys, that is, in the blood flow to the kid- out of the patient’s blood into the fluid of the
neys. Any condition that decreases blood flow to machine.
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
“Intrinsic renal” means that the problem is in the rate of kidney transplants, however, does indeed
kidneys themselves. Diabetes and hypertension provide the possibility of a normal life for people
damage the blood vessels of the kidneys, and are with chronic renal failure.
the causes of 70% of all cases of end-stage renal
failure. Bacterial infections of the kidneys or expo-
sure to chemicals (certain antibiotics) may cause A B
damage to the nephrons. Polycystic kidney disease
is a genetic disorder in which the kidney tubules
dilate and become nonfunctional. Severe damage C
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 Box Figure 18–A Causes of renal failure. (A) Prerenal.
chronic renal failure. Hemodialysis is the use of an (B) Intrinsic renal. (C) Postrenal.
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
18Scanlon(p3)-ch18 8/17/06 11:01 AM Page 426
Copyright © 2007 by F. A. Davis.
426 The Urinary System
Proximal convoluted tubule
Bowman's capsule
H O
2
Glucose
Amino acids
Small proteins Glomerulus
Minerals
H O Wastes
2
Glucose
Amino acids Efferent arteriole
Small proteins
Minerals Afferent arteriole
Distal
convoluted
tubule
Collecting
H + tubule
Ammonia
Creatinine H O
Medications 2 H O
2
Peritubular H O
2
capillaries Wastes
H +
Na +
K +
Loop of Henle
Artery
Glomerular
filtration
Tubular
reabsorption
Vein Urine
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?
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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 (H )
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 Na 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 Ca 2 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 Ca 2 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 Na ions by
blood calcium level is raised back to normal. the kidneys; these remain in the filtrate, as does water,
The hormone aldosterone, secreted by the adre- and are excreted. By increasing the elimination of
nal cortex, increases the reabsorption of Na ions sodium and water, ANP lowers blood volume and
and the excretion of K ions. Besides regulating the blood pressure.
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 If the amount of water in the body increases, how-
adsorbed to the membranes of the cells of the prox- ever, the secretion of ADH diminishes, and the kid-
imal convoluted tubules. The cell membrane then neys will reabsorb less water. Urine then becomes
sinks inward and folds around the protein to take it dilute, and water is eliminated until its concentration
in (see Fig. 3–3 for depictions of this and the other in the body returns to normal. This may occur follow-
transport mechanisms). Normally all proteins in ing ingestion of excessive quantities of fluids.
the filtrate are reabsorbed; none is found in urine.
SUMMARY OF URINE FORMATION
TUBULAR SECRETION
1. The kidneys form urine from blood plasma. Blood
This mechanism also changes the composition of flow through the kidneys is a major factor in deter-
urine. In tubular secretion, substances are actively mining urinary output.
secreted from the blood in the peritubular capillaries 2. Glomerular filtration is the first step in urine for-
into the filtrate in the renal tubules. Waste products, mation. Filtration is not selective in terms of use-
such as ammonia and some creatinine, and the meta- fulness of materials; it is selective only in terms of
bolic products of medications may be secreted into the size. High blood pressure in the glomeruli forces
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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 This regulatory function of the kidneys is complex,
vitamins are reabsorbed by active transport and but at its simplest it may be described as follows. If
may have renal threshold levels. Positive ions are body fluids are becoming too acidic, the kidneys will
reabsorbed by active transport and negative secrete more H ions into the renal filtrate and will
ions are reabsorbed most often by passive trans- return more HCO ions to the blood. This will help
3
port. Water is reabsorbed by osmosis, and small raise the pH of the blood back to normal. The reac-
proteins are reabsorbed by pinocytosis. 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 H ions to the blood and
nine or excess H ions are actively put into the fil- excrete HCO ions in urine. This will help lower the
3
trate to be excreted. pH of the blood back to normal.
5. Hormones such as aldosterone, ANP, and ADH Because the natural tendency is for body fluids to
influence the reabsorption of water and help main- become more acidic, let us look at the pH-raising
tain normal blood volume and blood pressure. The mechanism in more detail (see Fig. 18–6). The cells
secretion of ADH determines whether a concen- of the renal tubules can secrete H ions or ammonia
trated or dilute urine will be formed. in exchange for Na ions and, by doing so, influence
6. Waste products remain in the renal filtrate and are the reabsorption of other ions. Hydrogen ions are
excreted in urine. The effects of hormones on the obtained from the reaction of CO and water (or other
2
kidneys are summarized in Table 18–1 and illus- processes). An amine group from an amino acid is
trated in Fig. 18–5. combined with an H ion to form ammonia.
The tubule cell secretes the H ion and the ammo-
nia into the renal filtrate, and two Na ions are reab-
THE KIDNEYS AND sorbed in exchange. In the filtrate, the H ion and
ACID–BASE BALANCE ammonia form NH (an ammonium radical), which
4
reacts with a chloride ion (Cl ) to form NH Cl
4
The kidneys are the organs most responsible for main- (ammonium chloride) that is excreted in urine.
taining the pH of blood and tissue fluid within normal As the Na 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)
Parathyroid hormone (PTH) • Increases reabsorption of Ca 2 ions from filtrate to the blood and excretion of
(parathyroid glands) phosphate ions into the filtrate.
Aldosterone • Increases reabsorption of Na ions from the filtrate to the blood and excretion of
(adrenal cortex) K ions into the filtrate. Water is reabsorbed following the reabsorption of sodium.
Atrial natriuretic peptide • Decreases reabsorption of Na ions, which remain in the filtrate. More sodium
(ANP) (atria of heart) and water are eliminated in urine.
18Scanlon(p3)-ch18 8/17/06 11:01 AM Page 429
Copyright © 2007 by F. A. Davis.
The Urinary System 429
Blood
ADH Increases reabsorption of H 2 O
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, HCO 3 ions follow. Notice directly related to the formation of urine. These func-
what has happened: Two H ions have been excreted tions are secretion of renin (which does influence
in urine, and two Na ions and two HCO ions have urine formation), production of erythropoietin, and
3
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-
Another mechanism used by the cells of the kidney creases, the juxtaglomerular ( juxta means “next
tubules to regulate pH is the phosphate buffer system, to”) cells in the walls of the afferent arterioles
which is described in Chapter 19. secrete the enzyme renin. Renin then initiates the
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.
18Scanlon(p3)-ch18 8/17/06 11:01 AM Page 430
Copyright © 2007 by F. A. Davis.
430 The Urinary System
Peritubular Renal tubule
capillary
Filtrate
+ + +
Na Na Na
CI
CO H O H CO CI
2 2 2 3 + Figure 18–6. Renal regulation of acid–base
Na
+
Na balance. The cells of the renal tubule secrete H
+
HCO HCO H ions and ammonia into the filtrate and return
3 3 CI
+ Na ions and HCO ions to the blood in the
H 3
+ NH + peritubular capillaries. See text for further des-
NH H NH NH 4
2 3 3 cription.
+ QUESTION: The cells of the renal tubule make
HCO HCO H
3 3 good use of CO . What do the cells use CO for?
2 2
CO H O H CO
2 2 2 3
Na +
+ +
Na Na
NH Cl
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
secrete renin.
angiotensin II. In these ways the kidneys help 2. Renin splits the plasma protein angiotensinogen
ensure that the heart has enough blood to pump to (synthesized by the liver) to angiotensin I.
maintain cardiac output and blood pressure. 3. Angiotensin I is converted to angiotensin II by an
2. Secretion of erythropoietin—This hormone is enzyme found in lung tissue and vascular endothe-
secreted whenever the blood oxygen level decreases lium.
(a state of hypoxia). Erythropoietin stimulates 4. Angiotensin II causes vasoconstriction and stimulates
the adrenal cortex to secrete aldosterone.
the red bone marrow to increase the rate of RBC
production. With more RBCs in circulation, the
oxygen-carrying capacity of the blood is greater,
and the hypoxic state may be corrected (see also citriol (D ) by the kidneys. Calcitriol is the most
2
Box 18–3: Erythropoietin). active form of vitamin D, which increases the
3. Activation of vitamin D—This vitamin exists in absorption of calcium and phosphate in the small
several structural forms that are converted to cal- intestine.
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Copyright © 2007 by F. A. Davis.
The Urinary System 431
angular area called the trigone, which has no rugae
BOX 18–3 ERYTHROPOIETIN
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- The smooth muscle layer in the wall of the bladder
ing erythropoietin, a natural stimulus for RBC is called the detrusor muscle. It is a muscle in the
production. Erythropoietin can be produced by form of a sphere; when it contracts it becomes a
genetic engineering and is available for hemo- smaller sphere, and its volume diminishes. Around the
dialysis patients. In the past, their anemia could opening of the urethra the muscle fibers of the detru-
only be treated with transfusions, which exposed sor form the internal urethral sphincter (or sphinc-
these patients to possible immunologic compli- ter of the bladder), which is involuntary.
cations of repeated exposure to donated blood
or to viral diseases. The synthetic erythropoietin
eliminates such risks. Others who benefit from URETHRA
this medication are cancer patients and AIDS The urethra (see Fig. 18–7) carries urine from the
patients with severe anemia.
bladder to the exterior. The external urethral
sphincter is made of the surrounding skeletal muscle
of the pelvic floor, and is under voluntary control.
In women, the urethra is 1 to 1.5 inches (2.5 to 4
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
The ureters, urinary bladder, and urethra do not part just outside the bladder is called the prostatic ure-
change the composition or amount of urine, but are thra because it is surrounded by the prostate gland.
responsible for the periodic elimination of urine. The next inch is the membranous urethra, around
which is the external urethral sphincter. The longest
URETERS portion is the cavernous urethra (or spongy or penile
urethra), which passes through the cavernous (or erec-
Each ureter extends from the hilus of a kidney to the tile) tissue of the penis. The male urethra carries
lower, posterior side of the urinary bladder (see Fig. semen as well as urine.
18–1). Like the kidneys, the ureters are retroperi-
toneal, that is, behind the peritoneum of the dorsal THE URINATION REFLEX
abdominal cavity.
The smooth muscle in the wall of the ureter con- Urination may also be called micturition or voiding.
tracts in peristaltic waves to propel urine toward the This reflex is a spinal cord reflex over which voluntary
urinary bladder. As the bladder fills, it expands and control may be exerted. The stimulus for the reflex is
compresses the lower ends of the ureters to prevent stretching of the detrusor muscle of the bladder. The
backflow of urine. bladder can hold as much as 800 mL of urine, or even
more, but the reflex is activated long before the maxi-
mum is reached.
URINARY BLADDER
When urine volume reaches 200 to 400 mL, the
The urinary bladder is a muscular sac below the peri- stretching is sufficient to generate sensory impulses
toneum and behind the pubic bones. In women, the that travel to the sacral spinal cord. Motor impulses
bladder is inferior to the uterus; in men, the bladder is return along parasympathetic nerves to the detrusor
superior to the prostate gland. The bladder is a reser- muscle, causing contraction. At the same time, the
voir for accumulating urine, and it contracts to elimi- internal urethral sphincter relaxes. If the external ure-
nate urine. thral sphincter is voluntarily relaxed, urine flows into
The mucosa of the bladder is transitional epithe- the urethra, and the bladder is emptied.
lium, which permits expansion without tearing the Urination can be prevented by voluntary contrac-
lining. When the bladder is empty, the mucosa tion of the external urethral sphincter. However, if the
appears wrinkled; these folds are rugae, which also bladder continues to fill and be stretched, voluntary
permit expansion. On the floor of the bladder is a tri- control is eventually no longer possible.
18Scanlon(p3)-ch18 8/17/06 11:01 AM Page 432
Copyright © 2007 by F. A. Davis.
432 The Urinary System
Parietal peritoneum Ureter
Detrusor muscle
Openings of
ureters
Rugae
Ureter
Trigone B
Prostate gland
Prostatic urethra
Trigone
Internal Membranous
urethral sphincter urethra
A
External
urethral sphincter
Cavernous (spongy)
urethra
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.
and others are included in Appendix D: Normal
CHARACTERISTICS OF URINE
Values for Some Commonly Used Urine Tests.
The characteristics of urine include the physical and Amount—normal urinary output per 24 hours is 1
chemical aspects that are often evaluated as part of a to 2 liters. Many factors can significantly change
urinalysis. Some of these are described in this section, output. Excessive sweating or loss of fluid through
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