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.