Clearance studies studies using various probe molecules established that the passage of molecules/proteins across the glomerular capillary wall of mammalian kidneys is increasingly restricted as their size and net negative charge increase. An extended mathematical model, based on the Fiber Matrix theory, was developed to describe the dynamics of the size- and charge-selective functions of the glomerular capillary barrier using mainly its hemodynamic, morphometric, and electrostatic variables. The glomerular basement membrane was represented as a homogeneous three-dimensional network of fibers of uniform length (L-f), radius (R-f), and packing density (N-fv) and characteristic Darcy permeability. The model was appropriate for simulating fractional clearance data of neutral and charged solutes from an experimental modeling exercise. We believe that the L-f and R-f best-fit numerical values may signify new insights for the diagnosis of some human nephropathies. (C) 2010 Elsevier Inc. All rights reserved.