Water and electrolyte transport

Balance. About 2 L is ingested. Guyton's typical secretion figures are saliva ≈1 L, gastric juice ≈1.5 L, bile ≈1 L, pancreatic juice ≈1 L and intestinal secretions ≈2 L, giving about 9 L in all. The small intestine reabsorbs about 7.5 L, the colon about 1.3–1.4 L of the roughly 1.5 L delivered to it, and 100–200 mL is lost in the faeces. The colon has substantial reserve absorptive capacity.

Water follows solute, through aquaporins and through the paracellular route. The jejunum is "leaky" and absorbs iso-osmotically. The distal colon is "tight" and can absorb against an osmotic gradient.

Mechanisms of Na⁺ absorption, all driven by the basolateral Na⁺/K⁺-ATPase, differ by segment. Nutrient-coupled transport (SGLT1, Na⁺–amino acid carriers) predominates in the jejunum after meals. Electroneutral NaCl absorption by NHE3 together with Cl⁻/HCO₃⁻ exchange (DRA) occurs in the ileum and proximal colon between meals. Electrogenic absorption through ENaC occurs in the distal colon and rectum; it is stimulated by aldosterone, which also increases K⁺ secretion. SCFA-coupled Na⁺ absorption occurs in the colon.

Secretion. Crypt cells take up Cl⁻ basolaterally through NKCC1 and secrete it apically through CFTR. CFTR is activated by cAMP (VIP, PGE₂, cholera toxin), cGMP (guanylin, heat-stable E. coli toxin) and Ca²⁺ (ACh). Na⁺ and water follow paracellularly. K⁺ is absorbed passively in the small intestine and secreted in the colon. HCO₃⁻ is secreted in the ileum and colon.