The Digestive System
A thick coating of bicarbonate-rich mucus
builds up on the
Te epithelial cells of the mucosa are joined together by tight
that prevent gastric juice from leaking into under-
lying tissue layers.
Damaged epithelial mucosal cells are shed and quickly replaced
by division of
undiﬀerentiated stem cells
that reside where the
(a) Layers of the stomach wall
(b) Enlarged view of
gastric pits and
(c) Location of the HCl-producing parietal cells
and pepsin-secreting chief cells in a gastric
Mucous neck cells
Microscopic anatomy of the stomach.
hormones that diﬀuse into the blood capillaries to inﬂuence sev-
eral digestive system target organs (
, p. 868).
hormone, plays essential roles in regulating stomach secretion and
motility, as we will describe shortly.
The Mucosal Barrier
Te stomach mucosa is exposed to some of the harshest condi-
tions in the entire digestive tract. Gastric juice is corrosively
acidic (the H
concentration in the stomach can be 100,000
times that found in blood), and its protein-digesting enzymes
can digest the stomach itself.
However, the stomach is not a passive victim of its formida-
ble environment. It mounts an aggressive counterattack to pro-
tect itself, producing the
. Tree factors create