A mathematical model describing the displayed selectivity of the kidney nephron to macromolecules of varying particle size is presented. Diffusion and electrical forces are considered to be the sole mechanisms of transfer. The mass transfer principles responsible for selectivity are considered to occur within the capillary structure, and not within the membrane wall. A charge is assumed to be characteristic of the macromolecule and the membrane wall. Models describing both charged and uncharged solutes are developed.

A non-linear least squares technique, developed by Marquardt, is used to curve fit the derived functions to data relating fractional clearances of dextran and dextran sulfate to varying particle radius. It is shown, that the charged case model gives reasonable results in the simulation of nephron function whrn dextran is the considered solute. Also, curve fittings obtained by the charged case model in the dextran sulfate simulation indicate the realization of charge as a determining variable in the prediction of fractional clearance.