What is the primary mechanism of water absorption in the large bowel?

• A. Active transport of water via aquaporins
• B. Diffusion of water through tight junctions without solutes
• C. Secretion-driven water loss rather than absorption
• D. Osmosis following absorption of electrolytes and short-chain fatty acids

Answer: (D)

Water movement in the large bowel is a passive process driven by osmotic gradients. As electrolytes, especially sodium, are absorbed by colonocytes (along with chloride via exchangers), solutes leave the lumen and create an osmotic pull. Water then follows through the paracellular path and through cells, moving into the mucosa. Short-chain fatty acids produced by bacterial fermentation are also absorbed by colonocytes and contribute to these osmotic forces, further promoting water reabsorption. This mechanism is about reclaiming water rather than secreting it, unlike conditions that cause watery stools. Aquaporins aren’t the main driver here; the dominant process is osmosis linked to solute (electrolyte and SCFA) absorption.