Fig. 2.

A simple two-membrane model of epithelial transport. (A) Secretion of fluid without boundary conditions on the secretion. The basolateral membranes are washed with a well-stirred solution of osmolarity c_o; the intracellular osmolarity C_i(X) is allowed to vary across the epithelium, but the calculations suggest it is essentially constant because difiusion is very effective over short distances; the secreted solution has osmolarity c_s, which is determined by the membrane water permeability and the rate of salt transport, j. Since the flux of salt in the secretion is carried entirely by convection, it is given by j = uc_s. Hence the boundary condition on the osmolarity of the secretion is c_s = j/u for model A. (B) The same simple model of an epithelium, but now each side is washed with a well-stirred solution whose osmolarity is c_o. Thus the difference in the two models is the boundary condition on the secretion, which is c_s = c_o for model B.