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CHAPTER SEVENTEEN
Digestive System
PRACTICE
24
What controls gastric juice secretion?
25
Distinguish among the cephalic, gastric, and intestinal phases of
gastric secretion.
26
What is the function of cholecystokinin?
Gastric Absorption
Gastric enzymes begin breaking down proteins, but the
stomach wall is not well-adapted to absorb digestive prod-
ucts. The stomach absorbs only some water and certain
salts, as well as certain lipid-soluble drugs. Most nutrients
are absorbed in the small intestine. Alcohol, which is not a
nutrient, is absorbed both in the small intestine and stom-
ach. This is why the intoxicating effects of alcohol are felt
soon after consuming alcoholic beverages.
Mixing and Emptying Actions
Food stretches the smooth muscles of the stomach wall. The
stomach may enlarge, but its muscles maintain their tone,
and internal pressure of the stomach normally is unchanged.
When a person eats more than the stomach can comfortably
hold, the internal pressure may rise enough to stimulate pain
receptors. The result is a stomachache. Clinical Application
17.2 discusses this common problem along with its associ-
ated indigestion.
Following a meal, the mixing movements of the stom-
ach wall aid in producing a semifl uid paste of food particles
and gastric juice called
chyme
(kı¯m). Peristaltic waves push
the chyme toward the pylorus of the stomach, and as chyme
accumulates near the pyloric sphincter, this muscle begins
to relax. Stomach contractions push chyme a little (5–15
milliliters) at a time into the small intestine. These stomach
contraction waves push most of the chyme backward into
the stomach, mixing it further. The lower esophageal sphinc-
ter prevents refl ux of stomach contents into the esophagus.
Figure 17.21
illustrates this process.
The rate at which the stomach empties depends on the
uidity of the chyme and the type of food. Liquids usually
pass through the stomach rapidly, but solids remain until
The
gastric phase
of gastric secretion, which accounts
for 40% to 50% of the secretory activity, starts when food
enters the stomach. The presence of food and the distension
of the stomach wall trigger the stomach to release gastrin,
which stimulates production of more gastric juice.
As food enters the stomach and mixes with gastric juice,
the pH of the contents rises, which enhances gastrin secre-
tion. Consequently, the pH of the stomach contents drops.
As the pH approaches 3.0, secretion of gastrin is inhibited.
When the pH reaches 1.5, gastrin secretion ceases.
For the stomach to secrete hydrochloric acid, hydrogen
ions are removed from the blood, and an equivalent num-
ber of alkaline bicarbonate ions are released into the blood.
Following a meal, the blood concentration of bicarbonate
ions increases, and the urine excretes excess bicarbonate
ions. This phenomenon is called the
alkaline tide.
The
intestinal phase
of gastric secretion, which accounts
for about 5% of the total secretory response to a meal, begins
when food leaves the stomach and enters the small intestine.
When food first contacts the intestinal wall, it stimulates
intestinal cells to release a hormone,
intestinal gastrin,
that
again enhances gastric gland secretion.
As more food moves into the small intestine, a sympa-
thetic refl ex triggered by acid in the upper part of the small
intestine inhibits secretion of gastric juice from the stomach
wall. At the same time, proteins and fats in this region of
the intestine stimulate release of the peptide hormone
chole-
cystokinin
(ko
le-sis
to-ki
nin) from the intestinal wall,
which decreases gastric motility. Similarly, fats in the small
intestine stimulate intestinal cells to release
intestinal soma-
tostatin,
which inhibits release of gastric juice. Overall, these
actions decrease gastric secretion and motility as the small
intestine ±
lls with food.
Table 17.6
summarizes the phases
of gastric secretion.
1
2
3
4
Release into
bloodstream
Stimulation
Parasympathetic
preganglionic
nerve fiber (in
vagus nerve)
Parasympathetic
postganglionic
impulses stimulate
the release of
gastric juice from
gastric glands
Impulses
stimulate
the release
of gastrin
Gastrin stimulates
gastric glands to
release more gastric
juice
Bloodstream
FIGURE 17.20
The secretion of gastric juice is regulated in part by
parasympathetic nerve impulses that stimulate the release of gastric
juice and gastrin.
TABLE
17.6
|
Phases of Gastric Secretion
Phase
Action
Cephalic phase
The sight, taste, smell, or thought of food triggers
parasympathetic reF
exes. Gastric juice is secreted in
response.
Gastric phase
±ood in stomach chemically and mechanically
stimulates release of gastrin, which, in turn, stimulates
secretion of gastric juice; reF
ex responses also
stimulate gastric juice secretion.
Intestinal phase
As food enters the small intestine, it stimulates
intestinal cells to release intestinal gastrin, which, in
turn, promotes the secretion of gastric juice from the
stomach wall.
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