Sieving equations and effective glomerular filtration pressure

Sieving equations and effective glomerular filtration pressure. The curvilinear relationship between the sieving coefficients for 125I-PVP (fractional clearances/GFR) and the radii of hydrodynamically equivalent spheres has been studied in 15 normal mongrel dogs. Starting from the formula used to describe membrane permeability in terms of irreversible thermodynamics (Kedem and Katchalsky), new equations have been developed to account for the glomerular sieving of these molecules. The new equations differ from the older equations based merely on kinetics (Pappenheimer and Renkin, Landis and Pappenheimer) by the values given to the concentration term and the restriction factors used in calculating the contributions of bulk flow and diffusion to solute flow across the membrane. The equations allow the derivation of two parameters characterizing the porosity of an isoporous membrane equivalent to the glomerular sieve: r, the radius of cylindrical pores and A p/Δx, the total area of the pores per unit of path length. From these, effective glomerular filtration pressure (GFPe) has been calculated applying Poiseuille's law. A mean value of 14.5± 1.4 or 17.8 ± 2.1 mm Hg has been derived, depending on the limits for molecular radii within which the mean pore radius is calculated. Sieving equations assuming a log normal distribution model are less satisfactory for calculating GFPe than those based on the isoporous model, although they provide an excellent alignment of the calculated and the experimental sieving curves over a wide range of molecular sizes