NORMAL GASTROINTESTINAL FUNCTION
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ABSORPTION OF FLUID AND ELECTROLYTES
Fluid delivery to the GI tract is 8–10 L/d, including 2 L/d ingested; most is absorbed in small bowel. Colonic absorption is normally 0.05–2 L/d, with capacity for 6 L/d if required. Intestinal water absorption passively follows active transport of Na+, Cl–, glucose, and bile salts. Additional transport mechanisms include Cl–/HCO3– exchange, Na+/H+ exchange, H+, K+, Cl–, and HCO3– secretion, Na+-glucose cotransport, and active Na+ transport across the basolateral membrane by Na+,K+-ATPase.