C h a p t e r 2 8
Structure and Function of the Gastrointestinal System
687
from vagal nerves innervating the stomach and binds
to acetylcholine receptors on the parietal cells. Gastrin
is secreted by G cells in the antrum of the stomach and
reaches the parietal cells through the circulation. It binds
to as-yet uncharacterized receptors on the parietal cells.
Histamine is released from special endocrine cells in
the gastric mucosa and diffuses to nearby parietal cells,
where it binds to histamine-2 (H
2
) receptors.
One of the important characteristics of the gastric
mucosa is resistance to the highly acid secretions that it
produces. The mucosal surface is protected by prosta-
glandin E
2,
which inhibits acid secretion and stimulates
mucus production. Hence, it is an important factor in the
maintenance of the gastric mucosal barrier. Aspirin and
nonsteroidal anti-inflammatory drugs (NSAIDs) decrease
prostaglandin synthesis, which can impair the integrity of
the mucosal surface. Ethyl alcohol, or refluxed bile salts
from the intestine, can also disrupt this barrier. When this
occurs, hydrogen ions move into the tissue. Hydrogen
ions accumulate in the mucosal cells, intracellular pH
decreases, enzymatic reactions become impaired, and cel-
lular structures are disrupted. The result is local ischemia,
vascular stasis, hypoxia, and tissue necrosis.
Intestinal Secretions
The small intestine, which is where most of the digestion
and absorption of food takes place, secretes digestive
juices and receives secretions from the liver and pancreas
(see Chapter 30). An extensive array of mucus-produc-
ing glands, called
Brunner glands
, is concentrated at the
site where the contents from the stomach and secretions
from the liver and pancreas enter the duodenum. These
glands secrete large amounts of alkaline mucus that pro-
tect the duodenum from the acid content in the gastric
chyme and from the action of the digestive enzymes.
In addition to mucus, the intestinal mucosa produces
two other types of secretions. The first is an isotonic alka-
line fluid (pH 6.5 to 7.5) secreted by specialized cells in the
crypts of Lieberkühn
, which are tubelike glands that dip
down into the mucosal surface between the intestinal villi
(Fig. 28-9). Unlike the gastric pits, the intestinal crypts do
not secrete enzymes, but do secrete mucus, electrolytes,
Chief cell
Gastric
glands
Mucous
cell
Gastric pits
Mucosa
Submucosa
Parietal or
oxyntic cell
FIGURE 28-7.
Gastric pit from
body of the stomach.
Histamine
Gastrin
Acetylcholine
HCO
3
–
H
+
H
+
ATPase
Gastric
(HCl)
acid
Blood
Stomach
Mucous cell
Parietal cell
H
2
CO
3
CO
2
+
H
2
O
Cl
–
Cl
–
Basolateral
membrane
Luminal
membrane
K
+
H
+
Proton
pump
FIGURE 28-8.
Mechanism of gastric acid secretion by the
parietal cells in the stomach.
