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processes of digestion and absorption by maintaining the liquidity of the small intestine chyme. These small intestinal secretions, which may be as much as 1500 mL/day, are normally reabsorbed by the large intestine. Thus, under physiologic conditions there is a careful balance between absorption by the small intestinal villus cells and secretion by their crypt cells. This concerted effort results in the efficient digestion and absorption of nutrients without compromising the liquidity of the chyme.

BALANCE BETWEEN ABSORPTION AND SECRETION: ORAL REHYDRATION THERAPY

The amount of fluid that flows per unit of time from the small intestine through the ileocecal valve is determined by the balance between absorptive processes carried out by cells located on the upper half of the villus and by secretory processes apparently mediated by cells located in the crypts (Fig. 9). Under physiologic conditions, these oppositely directed fluid movements are balanced to achieve the highly efficient digestion and absorption of nutrients and at the same time maintain the liquidity of the luminal contents. The fluid that enters the colon is relatively devoid of nutrients, and its volume is normally well below the maximum absorptive capacity of that organ. Secretory diarrheas result when secretion by the crypt cells so markedly exceeds the villus absorptive processes that the volume of fluid entering the colon exceeds the maximum absorptive capacity of that organ.

The presence of sugars or amino acids in the lumen greatly increases the rate of total solute absorption by the small intestinal villous cells via the Na + -coupled entry mechanisms illustrated in Fig. 3B. This finding suggested that fluid loss from the small intestine because of secretion might be diminished by stimulating fluid absorption by that organ. Clearly, because the absorption of cfrl

FIGURE 9 Illustration of the relation between absorptive and secretory flows that, in the final analysis, determine net fluid movments. S, solute.

sugars and amino acids is coupled to Na + , every molecule of sugar or amino acid absorbed will be accompanied by one Na+ ion and one anion, primarily CP, a total of three osmotically active solutes. This reasoning led to the use of glucose- and amino acid-containing saline solutions that, when administered orally, markedly reduce salt and water losses resulting from secretory diarrheas. These oral replacement (rehydration) therapies have markedly reduced deaths and disabilities attributable to secretory diarrheas worldwide.

Suggested Readings

Schultz SG. Cellular models of epithelial ion transport. In Andreoli TE, Huffman JF, Fanestil DD, Schultz SG, eds. Physiology of membrane disorders, 2nd ed. New York: Plenum Press, 1986, pp 519-534.

Sellin JH. Intestinal electrolyte absorption and secretion. In Feldman M, Friedman LS, Sleisenger H, eds. Sleisenger Fordtran's gastrointestinal and liver disease, 7th ed. Philadelphia: WB Saunders, 2002, pp 1693-1714.

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FIGURE 9 Illustration of the relation between absorptive and secretory flows that, in the final analysis, determine net fluid movments. S, solute.

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