C h a p t e r 2 8
Structure and Function of the Gastrointestinal System
689
stomach. The primary function of
gastrin
is the stimula-
tion of gastric acid secretion. Gastrin also has a trophic,
or growth-producing, effect on the mucosa of the small
intestine, colon, and acid-secreting area of the stomach.
Removal of the tissue that produces gastrin results in
atrophy of these structures.
Ghrelin
is a recently discov-
ered peptide hormone produced by endocrine cells in the
mucosal layer of the fundus of the stomach. It displays
potent growth hormone–releasing activity and has a
stimulatory effect on food intake and digestive function,
while reducing energy expenditure. The isolation of this
hormone has led to new insights into the gut–brain regu-
lation of growth hormone secretion and energy balance.
The intestine is the source of secretin, cholecystoki-
nin, and incretin hormones.
Secretin
, which is secreted
by S cells in the mucosa of the duodenum and jeju-
num, inhibits gastric acid secretion. The entry of an
acid chyme into the intestine stimulates the release of
secretin, which inhibits the release of gastrin. Secretin
also stimulates the pancreas to secrete large quantities
of fluid with a high bicarbonate concentration and low
chloride concentration. The primary function of
cho-
lecystokinin
(CCK), secreted by I cells in the intestinal
mucosa, is the stimulation of pancreatic enzyme secre-
tion. It also potentiates the action of secretin, increasing
the pancreatic bicarbonate response to low circulating
levels of secretin; and regulates gallbladder contrac-
tion and gastric emptying. CCK has also been shown to
inhibit food intake and to be an important mediator for
appetite and amount of food consumed during a meal.
Several gut-derived hormones have been identified as
having what is termed an
incretin
effect, meaning that they
increase insulin release after an oral glucose load. This sug-
gests that gut-derived factors can stimulate insulin secre-
tion after a high-carbohydrate meal. The two hormones
that account for about 90% of the incretin effect are
glucagon-like peptide 1 (GLP-1), which is released from
L cells in the distal small intestine, and glucose-dependent
insulinotropic peptide (GIP), which is released by K cells
in the upper small intestine (mainly the jejunum). Because
increased levels of GLP-1 and GIP can lower blood glucose
levels by augmenting insulin release in a glucose-depen-
dent manner (i.e., at low blood glucose levels no further
insulin is secreted, minimizing the risk of hypoglycemia),
these hormones have been targeted as possible antidia-
betic drugs. Moreover, GLP-1 can exert other metaboli-
cally beneficial effects, including suppression of glucagon
release, slowing of gastric emptying, augmenting glucose
clearance, and decreasing appetite and body weight.
Digestion and Absorption in the
Gastrointestinal Tract
Digestion
involves the dismantling of foods into their
constituent parts, a process that requires hydrolysis,
enzyme cleavage, and fat emulsification.
Hydrolysis
is
the breakdown of a compound that involves a chemi-
cal reaction with water. The importance of hydrolysis
to digestion is evidenced by the amount of water (7
to 8 L) that is secreted into the GI tract daily.
Enzyme
cleavage
requires the use of enzymes to cut substances
into smaller components.
Emulsification
involves the
breakdown of large globules of dietary fat into smaller
particles.
Absorption
is the process of moving nutrients and
other materials from the external environment in the
lumen of the GI tract into the blood or lymph of the
internal environment. Absorption is accomplished by
active transport and diffusion. A number of substances
require a specific carrier or transport system. For exam-
ple, vitamin B
12
is not absorbed in the absence of intrin-
sic factor, which is secreted by the parietal cells of the
stomach. Transport of amino acids and glucose occurs
mainly in the presence of sodium. Water is absorbed
passively along an osmotic gradient.
SUMMARY CONCEPTS
■■
Secretory glands throughout the gastrointestinal
tract serve two basic purposes: production of
mucus to lubricate and protect its mucosal layer
and secretion of fluids and enzymes to aid in the
digestion and absorption of nutrients.
■■
The stomach, in addition to producing mucus,
secretes hydrochloric acid (HCl); pepsinogen,
which is converted to pepsin, which is
important in protein metabolism; and intrinsic
factor, which is involved in the absorption of
vitamin B
12
.
■■
The cells of the small intestine, where most
digestion and absorption of foods take place,
produces a watery alkaline fluid that aids in
the digestive process and surface enzymes
that aid in the digestion and absorption of
carbohydrates and proteins. Digestive and
absorptive processes in the small intestine are
aided by bile from the liver and enzymes from
the pancreas.
■■
In addition to secreting fluids containing
digestive enzymes, the GI tract produces and
secretes hormones that act locally, then pass
into the general circulation for distribution
to more distant sites. Among the hormones
produced by the GI tract are gastrin, ghrelin,
secretin, cholecystokinin, and incretin hormones
(glucagon-like peptide-1 [GLP-1] and glucose-
dependent insulinotropic polypeptide [GIP]).
These hormones influence appetite, GI motility,
enzyme activity, electrolyte levels, and the
secretion and action of hormones such as growth
hormone, insulin, and glucagon.
