666
UNIT FIVE
Gastric Secretions
The mucous membrane that forms the inner lining of the
stomach is thick. Its surface is studded with many small
openings, called
gastric pits,
located at the ends of tubular
gastric glands
(oxyntic glands)
(f
g. 17.19)
. Although their
structure and the composition of their secretions vary in dif-
ferent parts of the stomach, gastric glands generally contain
three types of secretory cells. One type, the
mucous cell
, is
found in the necks of the glands near the openings of the
gastric pits. The other types,
chief cells
(peptic cells) and
parietal cells
(oxyntic cells), reside in the deeper parts of the
glands (F g. 17.19). The chief cells secrete digestive enzymes,
and the parietal cells release a solution containing hydrochloric
acid. The products of the mucous cells, chief cells, and parietal
cells together form
gastric juice
(gas
trik jo
o
s).
Of the digestive enzymes in gastric juice,
pepsin
is by
far the most important. The chief cells secrete pepsin as an
inactive, nonerosive enzyme precursor called
pepsinogen.
When pepsinogen contacts the hydrochloric acid from the
parietal cells, however, it breaks down rapidly, forming
air. This produces a gastric air bubble, which may be used
as a landmark on a radiograph of the abdomen. The dilated
body,
the main part of the stomach, is between the fundus
and pylorus. The
pyloric antrum
is a funnel-shaped portion
that narrows and becomes the
pyloric canal
as it approaches
the small intestine.
At the end of the pyloric canal, the circular layer of F
bers
in its muscular wall thickens, forming a powerful muscle,
the
pyloric sphincter.
This muscle is a valve that controls
gastric emptying.
Hypertrophic pyloric stenosis
is a birth defect in which muscle over-
growth blocks the pyloric canal. The newborn vomits, with increas-
ing force. To diagnose the condition, a radiograph is taken of the
area after the infant drinks formula containing a radiopaque barium
compound. Surgical splitting of the muscle blocking the passageway
from stomach to small intestine is necessary to enable the infant to
eat normally. Pyloric stenosis can occur later in life as a result of ulcers
or cancer.
Lower esophageal
sphincter
Fundus
Body
Greater
curvature
Lesser
curvature
Gastric
folds
(rugae)
Pyloric
antrum
Pyloric
canal
Pylorus
Pyloric opening
Duodenum
Pyloric
sphincter
Cardia
Esophagus
Esophagus
Longitudinal
fibers
Circular
fibers
Oblique
fibers
Longitudinal
fibers
Esophagus
(b)
(a)
FIGURE 17.17
±Stomach.±(
a
) Some parts of the stomach have
three layers of muscle F
bers. (
b
) Major regions of the stomach and its
associated structures.
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