A simple method for determining the homoporous solute-membrane permeability from plasma-to-lymph measurements.

The exact equation for ultrafiltration of a neutral solute across a homoporous membrane contains only two adjustable parameters, the reflection coefficient (sigma s) and the permeability coefficient (Ps)-surface area (S) product, Ps X S. Specifying sigma s and Ps X S therefore defines completely the dependence of the protein ratio (R) on the volume flow (Jv) for a homoporous membrane. The reflection coefficient is determined from the high Jv limit of R: lim Jv----infinity [R] = (1 - sigma s). The purpose of this paper is to present a simple procedure for estimating Ps X S from the slope of R as Jv approaches zero: lim Jv----0 [dR/dJv] = -sigma s/(Ps X S). Both relations are exact limits of the nonlinear neutral solute/homoporous membrane transport equation and thus provide a simple, yet rigorous, method for estimating sigma s and Ps X S.