198
chapter
12
Gastrointestinal Digestion and Absorption
and in adrenocortical hyperfunction (e.g.,
aldosteronism),
this ratio is low. The chief salivary organic constituent
is
mucin,
which is a mixture of glycoproteins containing
60-85% oligosaccharide by weight. The oligosaccharide
units are linked by O-glycosidic linkage to either serine
or threonine residues (Chapter 9). Mucin-rich saliva is se-
creted by submaxillary and sublingual glands, and because
of the high carbohydrate content, mucins form viscous so-
lutions responsible for much of the lubricating action of
saliva that aids swallowing.
Amylase
is a salivary enzyme
that catalyzes the hydrolysis of
a{\ —>■
4) glucosidic link-
ages of starch and glycogen. Its role is minimal because of
the limited time spent by the food in the mouth and the in-
activation of amylase in the stomach by the acid pH. Other
enzymes detected in saliva include carbonic anhydrase, a
lipase (lingual lipase), a phosphatase, and a protease called
kallikrein. Whether these enzymes function in saliva is not
known. Lingual lipase initiates hydrolysis of dietary fat in
the stomach and facilitates the duodenal-jejunal hydro-
lysis of triacylglycerols.
Even when no food is present in the mouth, saliva is
secreted to keep the mouth moist and facilitates speech.
Saliva also has some antibacterial action. Patients with
xerostomia
(dry mouth) exhibit higher than normal inci-
dence of dental caries. Saliva maintains the oral cavity at
a pH of about 7.0. The teeth do not lose calcium to oral
fluids because of the high concentration of calcium in the
saliva at pH 7.0.
Like the thyroid gland (Chapter 33), salivary glands
(and the gastric mucosa) can take up iodide ions against a
concentration gradient.
The mouth is the normal point of entry of food and
drink. There, solid food is reduced in size by mastication,
blended with saliva, and temperature-moderated before
being swallowed. Conditions that interfere with any of
these processes (e.g., tooth loss) can affect food choice
and hence the nutritional status and health of the indi-
vidual. The esophagus is a muscular tube through which
masticated food is transported from the mouth to the stom-
ach. It consists of both striated muscle (upper one third)
and smooth muscle (lower two thirds). These muscles un-
dergo periodic contractions in the form of peristaltic waves
that push the swallowed boluses toward the stomach. The
esophagus has no digestive function but secretes mucus to
protect the esophageal mucosa from excoriation.
Stomach
The stomach stores food temporarily, retards its entry into
the small intestine, and secretes pepsin to begin the di-
gestion of protein. Hydrogen ions in the stomach acti-
vate pepsinogen to form pepsin and aid in maintaining
the sterility of the upper GI tract. The stomach also se-
cretes intrinsic factor (a glycoprotein), which is required
for vitamin B
12
absorption in the ileum (Chapter 38), helps
prepare some of the essential minerals (e.g., iron) for ab-
sorption by the small intestine (Chapter 29), and provides
mucus for protective and lubricative functions.
The stomach wall is made up of four concentric lay-
ers: the mucosa, the submucosa, the muscularis external,
and the serosa. Functionally, the gastric mucosa is divided
into the oxyntic gland area (which includes the body and
fundus of the stomach) and the pyloric gland area. The
oxyntic gland area contains tightly packed parallel glands
and is covered with a layer of tall, columnar surface epithe-
lial cells that secrete mucous and line the gastric pits. Un-
differentiated cells in the region joining the gastric gland
to the pit (the neck region) undergo division with some
daughter cells migrating upward and differentiating into
surface epithelial cells, with other daughter cells migrating
downward and differentiating into neck mucus cells, and
with
chief
and
parietal
cells. Thus, the mucosa renews
itself rapidly, approximately every 3-5 days in humans.
Replacement of the mucosa after injury occurs even more
rapidly. Pepsinogen is synthesized and secreted by chief
cells and HC1 by parietal cells.
A proteolytic enzyme secreted by gastric mucosa of in-
fants is
chymosin
(rennin), which functions to clot milk
and promote its digestion by preventing rapid passage
from the stomach. Chymosin hydrolyzes casein, a mix-
ture of several related milk proteins, to paracasein, which
reacts with Ca2+ to yield the insoluble curd. Pepsin per-
forms the same functions as chymosin. Chymosin is found
in the fourth stomach of ruminants. Calf stomach is a
source of this enzyme, which is used in the manufacture of
cheese. Chymosin has been synthesized by recombinant
DNA techniques and successfully used in the production
of cheese.
The pyloric mucosa contains surface epithelial cells
that secrete mucus, which increases during digestion of
food. Inadequate secretion of mucus can lead to stomach
ulcers. The cells of the pyloric glands secrete a fluid simi-
lar in composition to an ultrafiltrate of plasma. These cells
also secrete a pepsinogen that differs structurally from the
pepsinogen secreted by the oxyntic gland mucosa. Secre-
tion from all gastric glands amounts to 2-2.3 L/day and
is regulated by neural and humoral systems. Neural con-
trol is mediated by the vagus nerves. The sight, smell, and
thought of pleasant-tasting food can elicit gastric secre-
tion during the
cephalic phase.
Neural control is also ex-
erted by local autonomic reflexes mediated by cholinergic
neurons. Anger and hostility can cause gastric secretion
through nerve impulses down the vagi. Secretory signals
can arises from the cerebral cortex.
