688
U N I T 8
Gastrointestinal and Hepatobiliary Function
and water. The second type of secretion consists of surface
enzymes that aid absorption. These enzymes include pep-
tidases, which separate amino acids, and disaccharidases,
which split sugars. Also present in intestinal secretions
are mucins secreted by goblet cells. The mucins, which
are glycoproteins high in carbohydrates, form a gel that
serves to lubricate the mucosal surface and protect it from
mechanical injury from solid food particles.
The crypts of Lieberkühn also function to replace epi-
thelial cells. The epithelial cells lining the small intestine
are constantly being sloughed off into the lumen and
replaced as a result of the high mitotic index of the epi-
thelial stem cells in the crypts. The epithelium is entirely
replaced approximately every 4 to 5 days.
Paneth cells
,
which are present in the bottom of the crypts, secrete
antibacterial substances and are involved in the host
defense system of the small intestine.
The large intestine secretes mainly mucus and bicar-
bonate. Mucus not only protects the mucosa of the
colon, but also facilitates compaction of the feces.
Bicarbonate adheres to the mucus and acts as a buffer,
protecting the mucosa from acid by-products of bacte-
rial metabolism within the feces.
ANS activity strongly influences mucus production in
the bowel, as in other parts of the digestive tract. During
intense parasympathetic stimulation, mucus secretion
may increase to the point that the stool contains large
amounts of obvious mucus.
Gastrointestinal Hormones
The GI tract is the largest endocrine organ in the body.
It produces hormones that act locally, pass into the gen-
eral circulation for distribution to more distant sites,
and interact with the CNS through the enteric and auto-
nomic nervous systems. These hormones influence appe-
tite, GI motility, enzyme activity, electrolyte levels, and
the secretion and actions of hormones such as growth
hormone, insulin, and glucagon. The actions of many of
these hormones overlap: two or more GI hormones may
affect the same process in the same direction, or they
may inhibit each other. The GI tract hormones and their
functions are summarized in Table 28-2.
The stomach is the source of two important GI hor-
mones: gastrin and ghrelin. Gastrin is produced by
G cells, located predominantly in the antrum of the
Enterocyte being extruded
from a villus
Enterocyte
Vein
Artery
Lacteal
Crypt of
Lieberkühn
Paneth cells
FIGURE 28-9.
A single villus from the small intestine.
TABLE 28-2
Selected Gastrointestinal Hormones andTheir Actions
Hormone
Site of Secretion Stimulus for Secretion
Action
Cholecystokinin
Duodenum, jejunum Products of protein digestion and
long-chain fatty acids
Stimulates contraction of gallbladder and
secretion of pancreatic enzymes; slows
gastric emptying; inhibits food intake
Gastrin
Antrum of stomach,
duodenum
Vagal stimulation; epinephrine;
neutral amino acids; calcium-
containing foods such as milk;
alcohol
Secretion inhibited by acid content
of stomach antrum (pH < 2.5)
Stimulates secretion of gastric acid and
pepsinogen; increases gastric blood
flow; stimulates gastric smooth muscle
contractions; stimulates growth of
gastric and intestinal mucosal cells
Ghrelin
Fundus of stomach Nutritional (fasting) and hormonal
(decreased levels of growth
hormone)
Stimulates secretion of growth hormone;
acts as an appetite-stimulating signal
from stomach when an increase in
metabolic efficiency is necessary
Glucagon-like
peptide-1 (GLP-1)
Distal small intestine High-carbohydrate meal
Augments insulin release; suppresses
glucagon release; slows gastric
emptying; decreases appetite and
body weight
Glucose-dependent
insulinotropic
polypeptide (GIP)
Small intestine,
mainly jejunum
High-carbohydrate meal
Augments insulin release
Secretin
Duodenum
Acid pH or chyme entering
duodenum (pH < 3.0)
Stimulates secretion of bicarbonate-
containing fluids by pancreas and liver
