Reflection coefficient and permeability of urea in the proximal convolution of the rat kidney

The transport theory of Kedem and Katchalsky which was derived for passive transport in a two-compartment system is generalized for a multicomponent system with active transport, so that it can be applied to more complicated biological membranes. Equations have been derived to describe the transport of urea through the proximal convolution of the rat kidney and the permeability and the reflection coefficient have been determined. The permeability coefficient $(\tilde P_u )$ measured with the microperfusion and stop flow microperfusion methods, was found to be 6.0 and 5.2×10⁻⁵ mm²/sec, respectively. The reflection coefficient (σ) was determined in a stationary state situation and found to be 0.68. Earlier free flow micropuncture results together with theP _u andσ _u of this study indicate that 50% of the filtered urea is reabsorbed proximally and that approximately half of this amount is reabsorbed by solvent drag and the rest by diffusion. In the Appendix, a theoretical treatment of nonelectrolyte transport in a multicomponent system with active transport is given.