In the normal adult, 7-8 litres of of water and electrolytes are secreted daily into the gastrointestinal tract. This, together with dietary fluid, is absorbed by epithelial cells in the small and large bowel. Water follows the osmotic gradients which result from shifts of electrolytes across the intestinal epithelium, and sodium and chloride transport mechanisms are central to the causation and management of diarrhoea, especially that caused by bacteria and viruses. The energy for the process is provided by the activity of Na+/K+ ATPase.
Absorption of sodium into the epithelium is effected by:
• Sodium-glucose-coupled entry. Glucose stimulates the absorption of sodium and the resulting water flow also sweeps additional sodium and chloride along with it (solvent drag). This important mechanism remains active in diarrhoea of various aetiologies and improvement of sodium and water absorption by glucose (and amino acids) is the basis of oral rehydration regimens (see below). Absorption of sodium and water in the colon is stimulated by short-chain fatty acids (see below, cereal-based ORT).
• Sodium-ion-coupled entry. Na+ and Cl~ enter the epithelial cell, either as a pair or, as seems more likely, there is a double exchange: Na+ (extracellular) with H+ (intracellular) and CI' (extracellular) with 20H" or 2HC03" (intracellular). Oral rehydration solutions (see below) contain sodium, chloride and bicarbonate.
Secretion is the opposite process to that of absorption. In response to various stimuli, crypt cells actively transport chloride into the gut lumen and sodium and water follow. This stimulus-secretion coupling is modulated by cyclic AMP and GMP, calcium, prostaglandins and leukotrienes.
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