Mechanism of Proteinuria



The serum proteins, which carry a net negative charge, are excluded from the urine on the basis of both their size and their net charge. The latter causes them to be repelled by the fixed negative charge of the glomerular capillary wall. Albumin, with a molecular radius of about 3.6 nm, is almost totally excluded from passage into the urine, but a neutral molecule of equal size would appear in urine at a concentration of approximately 20 per cent that in the blood.

Proteinuria in glomerular disease most likely represents a loss of the functional, negatively charged barrier of the glomerulus rather than a disruption in the structural integrity of the cap­illary wall by “holes.” Proteinuria of glomerular disease does not represent either a failure by the tubules to reabsorb normally filtered proteins or an increase in the tubular secretion of serum pro­teins.