Water Transport by Epithelia

Abstract Water transport in epithelia can occur down an established concentration gradient, in which case it is indisputably osmotic. In other epithelia, water transport occurs in the absence of or even against differences in the osmolalities of the adjacent solutions. In the latter case, water flow is in the same direction as the net solute flux and in near‐isosmotic proportions. The pathways and mechanisms of water transport in this class of epithelia remain controversial, but the prevailing views are that water flow is largely transcellular, via both water pores ( aquaporins , AQP ) and the phospholipid bilayer, with little contribution of other membrane proteins. In addition, solute–solvent flux coupling is not a molecular, but a thermodynamic phenomenon, i.e. water transport is driven by small differences in osmolality between epithelial fluid compartments created by solute transport. Paracellular water flow is likely to be small relative to transcellular water flow.