Solute-solvent coupling in epithelia: a critical examination of the standing-gradient osmotic flow theory

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Abstract
An examination of the standing-gradient osmotic flow theory by numerical solution of the differential equation describing the velocity profile in a local osmotic space, and by use of an analytical approximation to the emergent osmolarity, indicates that abnormally high values for the osmotic permeability of the cell membranes are required if the secretion is to be isotonic, as observed in many systems. As these permeabilities are not observed experimentally and cannot be predicted on general theoretical grounds, it seems virtually impossible that the intracellular and lateral spaces of fluid-transporting epithelia are in fact local osmotic coupling spaces.