Porth's Essentials of Pathophysiology, 4e

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Gastrointestinal and Hepatobiliary Function

U N I T 8

Dietary fat (mainly triglycerides)

Stomach: agitation

Bile salts synthesized in liver

Duodenum: addition of bile salts and pancreatic lipase

Formation of micelles

Triglyceride

Chylomicrons

Pancreatic lipase

2-Monoglyceride

Lymphatics

Bile salts micelle

Bile salts reabsorbed

Mixed micelle

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.

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.

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

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