Introductory remarks

Abstract

It is a generally accepted viewpoint that the processes underlying active transport in tissues are basically the same as those which are operative in individual cells. However, the organization of cells to form epithelia seems to endow them with new potentialities with respect to apparently active transport of solutes and water not usually recognized in individual cells. The new potentialities probably can be traced back to two main sources: (1) that the cells in an epithelium are polarized so that the ‘inward’ and ‘outward’ facing membranes have different properties, and (2) the presence of a system of interspaces which is usually more or less closed towards one side of the epithelium and more open toward the other. In principle the interspace system allows relatively rapid bulk flow which in turn may give rise to ‘solvent drag’, ‘anomalous solvent drag’, and in specialized structures even to counter-current interactions between different streams.