There are four basic mechanisms of diarrhea: increased intestinal secretion, decreased intestinal absorption, increased osmotic load, and abnormal intestinal motility. Knowledge of normal intestinal function assists in understanding these various mechanisms. Normally, the jejunum receives between 6 and 8 L/day of fluid in the form of oral intake and gastric, pancreatic, and biliary secretions. Dietary intake actually constitutes a small portion of the jejunal load (1.5 L). A healthy small intestine absorbs nearly 75 percent of the fluid to which it is exposed. The 2 L of fluid not absorbed by the small intestine then enter the colon, where fluid is absorbed at an even higher rate. The absorptive power of the colon approaches 90 percent efficiency and far exceeds that of the small intestine. In fact, the colon can make up for a decrease in small-intestinal absorption. Under normal conditions, very little fluid (<100 mL) is lost in the stool each day. 8
In diarrheal states, normal intestinal physiology is disrupted. At a cellular level, intestinal absorption occurs through the villi, while secretion occurs through the crypts. Often, in diarrheal states, enterotoxins, inflammation, or ischemia damage the intestinal villi preferentially. As a result, diarrhea occurs because of diminished intestinal villi absorption and unopposed crypt secretion (the crypts are more resilient after injury).9 Diarrhea also accompanies the delivery of an osmotic load to the intestine. For example, administration of a laxative results in the collection of an osmotically active, nondigestible agent within the intestinal lumen. Osmosis occurs, drawing fluid into the intestinal lumen, and results in diarrhea. Finally, increased intestinal motility causes diarrhea, as in patients with irritable bowel syndrome, neuropathies, or a shortened intestine secondary to surgery.
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