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. About 2 L/d is delivered to the colon; about 0.2 L/d is excreted in the stool. 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.