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U N I T 8
Gastrointestinal and Hepatobiliary Function
substances to the surface of the intestinal villi, where they
are taken into the epithelial cells and used to form new
triglycerides. These new triglycerides are then released
into the lymphatic system as chylomicrons. Small quanti-
ties of short- and medium-chain fatty acids are absorbed
directly into the portal blood rather than being converted
into triglycerides and absorbed by way of the lymphatics.
Fat that is not absorbed in the intestine is excreted in
the stool.
Steatorrhea
is the term used to describe fatty
stools. It usually indicates that there is 20 g or more of
fat in a 24-hour stool sample.
Colonic Absorption and
Intestinal Flora
When the remaining food residue moves into the colon,
fluid not absorbed during movement of the meal through
the small intestine is absorbed and the waste products
are stored in the large intestine until they can be conve-
niently eliminated.
While in the colon, food residues are acted upon
by a large and diverse bacterial community called the
intestinal flora. The stomach and small intestine con-
tain only a few species of bacteria, probably because
the composition of luminal contents (i.e., acids, bile,
pancreatic secretions) kills most ingested microorgan-
isms, and the propulsive movements of these organs
impedes their colonization. The large intestine, on the
other hand, contains a large and complex microbial eco-
system. It has been estimated that each individual has
300 to 500 different species of intestinal bacteria, with
anaerobic bacteria outnumbering aerobic bacteria by a
large percentage.
The major metabolic function of the intestinal
flora is the fermentation of undigestible dietary resi-
due including resistant starches, cellulose, pectins and
Bile salts
synthesized
in liver
Dietary fat
(mainly triglycerides)
Chylomicrons
Lymphatics
Formation of micelles
Bile salts
reabsorbed
Pancreatic
lipase
Mixed micelle
Bile salts
micelle
2-Monoglyceride
Triglyceride
Duodenum:
addition of bile salts
and pancreatic lipase
Stomach:
agitation
FIGURE 28-12.
Mechanisms of dietary fat absorption. During digestion, agitation in the stomach
and bile from the liver break large globules of dietary fat into small particles that facilitate the action
of pancreatic lipase in splitting triglycerides into free fatty acids and monoglycerides (glycerol
with one fatty acid chain attached). Bile salts also facilitate formation of micelles that transport the
monoglycerides and free acids to the intestinal mucosa, where they are absorbed and converted to
chylomicrons for transport in the lymphatic channels.
