Standing gradient osmosis

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Water absorption at the colon typically proceeds against a transmucosal osmotic pressure gradient.

The standing gradient osmosis is a term used to describe the reabsorption of water against the osmotic gradiet at the colon.

Numerous Na⁺/K⁺ pumps are present at the lumenal end of lateral membrane of epithelial cells at the colon. This causes Na+ to be pumped into the lumenal side of the lateral intercellular spaces. This hypertonic fluid creates an osmotic pressure that drives water into the lateral intercellular spaces by osmosis via tight junctions and adjacent cells, which then in turn moves across the basement membrane and into the capillaries.