678
UNIT FIVE
them into the bile ducts. Liver cells synthesize bile salts,
which replace the small amounts lost in the feces.
PRACTICE
40
Explain how bile forms.
41
Describe the function of the gallbladder.
42
How is secretion of bile regulated?
43
How do bile salts function in digestion?
17.9
SMALL INTESTINE
The
small intestine
is a tubular organ that extends from the
pyloric sphincter to the beginning of the large intestine. With
its many loops and coils, it F
lls much of the abdominal cav-
ity (see F
g. 17.1 and reference plates 4 and 5). The small
intestine is 5.5–6.0 meters (18–20 feet) long in a cadaver
when the muscular wall lacks tone, but it may be only half
this long in a living person.
The small intestine receives secretions from the pan-
creas and liver. It also completes digestion of the nutrients in
chyme, absorbs the products of digestion, and transports the
remaining residue to the large intestine.
solid crystals. If cholesterol continues to come out of solution,
the crystals enlarge, forming
gallstones
(f g. 17.29)
. This may
happen if the bile is too concentrated, hepatic cells secrete too
much cholesterol, or the gallbladder is infl amed (cholecysti-
tis). Gallstones in the bile duct may block the fl ow of bile,
causing obstructive jaundice and considerable pain. Clinical
Application 17.4 discusses disorders of the gallbladder.
Regulation of Bile Release
Normally, bile does not enter the duodenum until chole-
cystokinin stimulates the gallbladder to contract. The
intestinal mucosa releases this hormone in response to
proteins and fats in the small intestine. (Recall from earlier
in this chapter, page 672, that cholecystokinin also stimu-
lates pancreatic enzyme secretion.) The hepatopancreatic
sphincter usually remains contracted until a peristaltic
wave in the duodenal wall approaches. Just before the
wave hits, the sphincter relaxes, and bile squirts into the
duodenum
(f
g. 17.30)
.
Table 17.8
summarizes the hor-
mones that control digestion.
Functions of Bile Salts
Bile salts aid digestive enzymes. Molecules of fats clump into
fat globules.
Bile salts reduce surface tension and break fat
globules into droplets, much like the action of soap or deter-
gent. This process is called
emulsi±
cation.
Monoglycerides
that form from the action of pancreatic lipase on triglycer-
ide molecules aid emulsification. Overall, emulsification
greatly increases the total surface area of the fatty substance,
and the resulting droplets mix with water. Lipases can then
digest the fat molecules more effectively.
Bile salts enhance absorption of fatty acids and choles-
terol by forming complexes (micelles) that are very soluble
in chyme and that epithelial cells can more easily absorb.
The fat-soluble vitamins A, D, E, and K are also absorbed in
the presence of bile salts. Lack of bile salts results in poor
lipid absorption and vitamin deF
ciencies.
The mucous membrane of the small intestine reabsorbs
nearly all of the bile salts, along with fatty acids. The blood
carries bile salts to the liver, where hepatic cells resecrete
FIGURE 17.29
Falsely colored radiograph of a gallbladder that
contains gallstones (arrow).
TABLE
17.8
|
Hormones of the Digestive Tract
Hormone
Source
Function
Gastrin
Gastric cells, in response to food
Increases secretory activity of gastric glands
Intestinal gastrin
Cells of small intestine, in response to
chyme
Increases secretory activity of gastric glands
Somatostatin
Gastric cells
Inhibits secretion of acid by parietal cells
Intestinal somatostatin
Intestinal wall cells, in response to fats
Inhibits secretion of acid by parietal cells
Cholecystokinin
Intestinal wall cells, in response to
proteins and fats in the small intestine
Decreases secretory activity of gastric glands and inhibits gastric motility; stimulates
pancreas to secrete ±
uid with a high digestive enzyme concentration; stimulates gallbladder
to contract and release bile
Secretin
Cells in the duodenal wall, in response to
acidic chyme entering the small intestine
Stimulates pancreas to secrete ±
uid with a high bicarbonate ion concentration
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