668
UNIT FIVE
secrete intrinsic factor, required for vitamin B
12
absorption
from the small intestine.
Table 17.5
summarizes the compo-
nents of gastric juice.
PRACTICE
20
Where is the stomach located?
21
What are the secretions of the chief cells and parietal cells?
22
Which is the most important digestive enzyme in gastric juice?
23
Why doesn’t the stomach digest itself?
Regulation of Gastric Secretions
Gastric juice is produced continuously, but the rate varies
considerably and is controlled both neurally and hormon-
ally. In the gastric glands, specialized cells closely associated
with the parietal cells secrete the hormone
somatostatin,
which inhibits acid secretion. However, acetylcholine (ACh)
released from nerve endings in response to parasympathetic
impulses arriving on the vagus nerves suppresses the secre-
tion of somatostatin and stimulates the gastric glands to
secrete abundant gastric juice, which is rich in hydrochlo-
ric acid and pepsinogen. These parasympathetic impulses
also stimulate certain stomach cells, mainly in the pyloric
region, to release a peptide hormone called
gastrin,
which
increases the secretory activity of gastric glands
(f
g. 17.20)
.
Furthermore, parasympathetic impulses and gastrin promote
release of
histamine
from gastric mucosal cells, which, in
turn, stimulates additional gastric secretion.
Gastrin stimulates cell growth in the mucosa of the stomach and
intestines, except where gastrin is produced. This eF
ect helps replace
mucosal cells damaged by normal stomach function, disease, or
medical treatments.
Gastric secretion occurs in three stages—the cephalic, gas-
tric, and intestinal phases. The
cephalic phase
begins before
food reaches the stomach and possibly even before eating.
In this stage, parasympathetic refl exes operating through the
vagus nerves stimulate gastric secretion at the taste, smell,
sight, or thought of food. The greater the hunger, the greater
the gastric secretion. The cephalic phase is responsible for
30% to 50% of the secretory response to a meal.
pepsin. Pepsin, in turn, can also break down pepsinogen to
release more pepsin.
Pepsin begins the digestion of nearly all types of dietary
protein into polypeptides. This enzyme is most active in an
acidic environment, which is provided by the hydrochloric
acid in gastric juice.
Gastric juice contains small quantities of a fat-splitting
enzyme,
gastric lipase.
However, its action is weak due in
part to the low pH of gastric juice. Gastric lipase acts mainly
on butterfat.
Much of what we know about the stomach’s functioning comes from
a ±rench-Canadian explorer, Alexis St. Martin, who in 1822 acciden-
tally shot himself in the abdomen. His extensive injuries eventually
healed, but a hole, called a ²
stula, was left, allowing observers to look
at his stomach in action. A U.S. Army surgeon, William Beaumont,
spent eight years watching food digesting in the stomach, noting
how the stomach lining changed in response to stress.
In 1984, our knowledge of digestive function expanded when
medical resident Barry Marshall at Royal Perth Hospital in western
Australia performed a daring experiment. His mentor, J. Robin Warren,
had hypothesized that a bacterial infection causes gastritis (in³
am-
mation of the stomach lining) and peptic ulcers (sores in the lining of
the esophagus, stomach, or small intestine). At the time, these condi-
tions were attributed to poor diet and stress. Marshall drank “swamp
water”—billions of bacteria. He developed gastritis, which, fortu-
nately, cleared up. A colleague who repeated the experiment devel-
oped an ulcer and required antibiotics. After a decade of debate, the
medical community ²
nally concurred that the bacterium
Helicobacter
pylori,
which thrives under acidic conditions, causes many cases of gas-
tritis and peptic ulcers. A short course of antibiotics and acid-lowering
drugs has replaced lifelong treatments. Marshall and Warren were
awarded a Nobel Prize in 2005 for their discovery.
The mucous cells of the gastric glands (
mucous neck
cells
) and the mucous cells, associated with the stomach’s
inner surface, release a viscous, alkaline secretion that coats
the inside of the stomach wall. This coating is especially
important because pepsin can digest the proteins of stomach
tissues, as well as those in foods. The coating normally pre-
vents the stomach from digesting itself.
Another component of gastric juice is
intrinsic factor
(in-trin
sik fak
tor). The parietal cells of the gastric glands
TABLE
17.5
|
Major Components of Gastric Juice
Component
Source
Function
Pepsinogen
Chief cells of the gastric glands
Inactive form of pepsin
Pepsin
±ormed from pepsinogen in the presence of hydrochloric acid
A protein-splitting enzyme that digests nearly all types of dietary protein
Hydrochloric acid
Parietal cells of the gastric glands
Provides the acid environment needed for production and action of pepsin
Mucus
Mucous cells
Provides a viscous, alkaline protective layer on the stomach’s inner surface
Intrinsic factor
Parietal cells of the gastric glands
Aids in vitamin B
12
absorption
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