Measurement of the glomerular filtration pressure from sieving data for macromolecules

Equations describing restricted filtration and diffusion of molecules in solution through porous membranes, macromolecular gels and capillary walls have been formulated by Pappenheimer; Renkin; and Landis and Pappenheimer. These formulations allow to calculate the radius of pores, r, (supposed cylindrical) and their total area per unit of path length: {Mathematical expression}. In this study, they have been applied to characterize the glomerular sieve, starting from sieving data for PVP125I. From the mean values for r and {Mathematical expression}, supposing the membrane isoporous, the effective glomerular filtration pressure (GFP) was derived, applying Poiseuille's law. {Mathematical expression} was 30(±2.2) mm Hg in dogs (n=20), and 19.6(±1.9) in humans (n=11). The isoporous model however does not account for the passage in the urine of comparatively large amounts of molecules with a radius higher than 41 Å. Assuming a lognormal distribution of pore radii allows a quite satisfactory adjustment of calculated and experimental values for sieving coefficients in a larger range of molecular radii (23 to 79 Å). This assumption necessitates to compute the value for GFP simultaneously to those characterizing the distribution (median and standard deviation). The values for GFP obtained by this method are slightly lower than [27.1(±2.8) mm Hg], but closely correlate with, those derived supposing the membrane isoporous in dogs. In humans {Mathematical expression} was 11.7(±1.7) mm Hg. However the sieving coefficients higher than 0.65 have to be discarded from the calculations on which rests the choice of the final value for {Mathematical expression}. The reasons for the discrepancies between experimental and calculated values for sieving coefficients close to 1 are discussed by light of recent studies on sieving processes and membrane permeability. © 1971 Springer-Verlag.SCOPUS: ar.j