The Digestion and Absorption of Food
561
defecation has not occurred for several days or even weeks,
depending on the individual. These symptoms of
constipa-
tion
are caused not by toxins, but by distension of the rectum.
The longer that fecal material remains in the large intestine,
the more water is absorbed and the harder and drier the feces
become, making defecation more diffi cult and sometimes
painful.
Decreased motility of the large intestine is the primary
factor causing constipation. This often occurs in the elderly,
or it may result from damage to the colon’s enteric nervous
system or from emotional stress.
One of the factors increasing motility in the large intes-
tine, and thus opposing the development of constipation, is
distension. As noted earlier, dietary fi ber (cellulose and other
complex polysaccharides) is not digested by the enzymes in the
small intestine and is passed on to the large intestine, where its
bulk produces distension and thereby increases motility. Bran,
most fruits, and vegetables are examples of foods that have a
relatively high fi ber content.
Laxatives,
agents that increase the frequency or ease of
defecation, act through a variety of mechanisms. Fiber pro-
vides a natural laxative. Some laxatives, such as mineral oil,
simply lubricate the feces, making defecation easier and less
painful. Others contain magnesium and aluminum salts, which
are poorly absorbed and therefore lead to water retention in
the intestinal tract. Still others, such as castor oil, stimulate
the motility of the colon and inhibit ion transport across the
wall, thus affecting water absorption.
Excessive use of laxatives in an attempt to maintain a
preconceived notion of regularity leads to a decreased respon-
siveness of the large intestine to normal defecation-promoting
signals. In such cases, a long period without defecation may
occur following cessation of laxative intake, appearing to con-
fi rm the necessity of taking laxatives to promote regularity.
Diarrhea
is characterized by large, frequent, watery
stools. Diarrhea can result from decreased fl uid absorption,
increased fl uid secretion, or both. The increased motility that
accompanies diarrhea probably does not cause most cases of
diarrhea (by decreasing the time available for fl uid absorp-
tion), but rather results from the distension produced by
increased luminal fl
uid.
A number of bacterial, protozoan, and viral diseases of
the intestinal tract cause secretory diarrhea.
Cholera,
which
is endemic in many parts of the world, is caused by a bacte-
rium that releases a toxin that stimulates the production of
cyclic AMP in the secretory cells at the base of the intesti-
nal villi. This leads to an increased frequency in the opening
of the chloride channels in the luminal membrane and hence
increased secretion of chloride ions. An accompanying osmotic
fl ow of water into the intestinal lumen occurs, resulting in
massive diarrhea that can be life-threatening due to dehydra-
tion and decreased blood volume that leads to circulatory
shock. The salt and water lost by this severe form of diarrhea
can be balanced by ingesting a simple solution containing salt
and glucose. The active absorption of these solutes is accom-
panied by absorption of water, which replaces the fl uid lost by
diarrhea.
Traveler’s diarrhea,
produced by several species of
bacteria, produces a secretory diarrhea by the same mechanism
as the cholera bacterium, but is less severe.
In addition to decreased blood volume due to salt and
water loss, other consequences of severe diarrhea are potas-
sium depletion and metabolic acidosis (Chapter 14) result-
ing from the excessive fecal loss of potassium and bicarbonate
ions, respectively.
SUMMARY
I. The gastrointestinal system transfers digested organic
nutrients, minerals, and water from the external environment
to the internal environment. The four processes used to
accomplish this function are (a) digestion, (b) secretion,
(c) absorption, and (d) motility.
a. The system is designed to maximize the absorption of most
nutrients, not to regulate the amount absorbed.
b. The system does not play a major role in the removal of
waste products from the internal environment.
Overview: Functions of the Gastrointestinal Organs
I. Figure 15–3 summarizes the names and functions of the
gastrointestinal organs.
II. Each day the gastrointestinal tract secretes about six times
more fl uid into the lumen than is ingested. Only 1 percent of
this fl uid is excreted in the feces.
Structure of the Gastrointestinal Tract Wall
I. Figure 15–6 diagrams the structure of the wall of the
gastrointestinal tract.
a. The area available for absorption in the small intestine is
greatly increased by the folding of the intestinal wall and
by the presence of villi and microvilli on the surface of the
epithelial cells.
b. The epithelial cells lining the intestinal tract are
continuously replaced by new cells arising from cell division
at the base of the villi.
c. The venous blood from the small intestine, containing
absorbed nutrients other than fat, passes to the liver via
the hepatic portal vein before returning to the heart. Fat is
absorbed into the lymphatic vessels (lacteals) in each villus.
Digestion and Absorption
I. Starch is digested by amylases secreted by the salivary glands
and pancreas. The resulting products, as well as ingested
disaccharides, are digested to monosaccharides by enzymes in
the luminal membranes of epithelial cells in the small intestine.
a. Most monosaccharides are then absorbed by secondary
active transport.
b. Some polysaccharides, such as cellulose, cannot be digested
and pass to the large intestine, where bacteria metabolize
them.
II. Proteins are broken down into small peptides and amino acids,
which are absorbed by secondary active transport in the small
intestine.
a. The breakdown of proteins to peptides is catalyzed by
pepsin in the stomach and by the pancreatic enzymes
trypsin and chymotrypsin in the small intestine.
b. Peptides are broken down into amino acids by pancreatic
carboxypeptidase and intestinal aminopeptidase.
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