Ileal Function: Water, Electrolyte, Bile Salt, and Vitamin B12 Absorption
The ileum, the terminal small‑bowel segment, performs critical absorptive and immune functions:
- Water & electrolyte absorption: Active Na⁺ transport (Na⁺/K⁺‑ATPase) creates osmotic gradient for water reabsorption; Cl⁻, K⁺, HCO₃⁻ follow.
- Bile salt reclamation: Apical sodium‑dependent bile acid transporter (ASBT) reabsorbs ~95% of bile acids for enterohepatic circulation.
- Vitamin B12 uptake: Intrinsic factor–B12 complex binds cubilin/amnionless receptors on distal ileal enterocytes.
- Mucosal immunity: Peyer’s patches and gut‑associated lymphoid tissue (GALT) sample luminal antigens, induce IgA, maintain tolerance.
Ileal resection or disease (Crohn’s) leads to bile acid diarrhea, B12 deficiency, and electrolyte depletion.
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