HINDERED TRANSPORT OF MACROMOLECULES THROUGH A SINGLE ROW OF CYLINDERS - APPLICATION TO GLOMERULAR-FILTRATION

The slit diaphragms of renal glomerular capillaries form an ultrafiltr ation barrier which may be approximated as a row of cylindrical fibers of macromolecular dimensions. To describe the hindered transport of p lasma proteins and other macromolecules through this barrier, we devel oped an approximate hydrodynamic model for spherical, Brownian particl es passing through a row of infinitely long cylinders. The selectivity of the slit diaphragm was assessed by computing concentration profile s for a wide range of molecular sizes for Pe less than or equal to 1, where Pe is a Peclet number based on the cylinder radius. The sieving coefficient the slit diaphragm was computed as theta(SD) = C-B/C-0, wh ere C-0 was the average concentration at a specified distance upstream from the cylinders (corresponding to the location of the basement mem brane), and C-B was the concentration far downstream (corresponding to Bowman's space). The results of previous experimental sieving studies using rats could be accounted for approximately by postulating a wide distribution of spacings between the fibers of the slit diaphragm. Co mparing the results for theta(SD) with calculations for a model of the glomerular basement membrane suggests that the slit diaphragm is by f ar the more size-restrictive part of the overall barrier.