Mechanisms and consequences of proteinuria.

The glomerulus is a complex structure containing a remarkable capillary bed which is freely permeable to water and solutes up to the size of inulin. Many small proteins are filtered, reabsorbed, and catabolized by the kidney; but most large proteins, such as albumin or immunoglobulins, are almost entirely excluded from the glomerular ultrafiltrate due to the charge-size permselectivity of the glomerular capillary basement membrane. These large proteins appear in the urine when diseases reduce the charge selectivity or result in the development of large pores in this membrane. The reabsorptive capacity of the renal tubules for these proteins is overwhelmed. Hypoalbuminemia results when increased synthetic and decreased catabolic rates of albumin fail to compensate for the urinary loss of the protein. The resulting decrease in serum oncotic pressure increases the flux of fluid out of systemic capillaries into the interstitial space, a process that increases lymphatic flow and returns the relatively protein-poor ultrafiltrate to the plasma compartment. Interstitial proteins are swept into the plasma by the increased lymphatic flow, leading to a depletion of the extravascular pool of albumin even more severe than the depletion of albumin in the plasma compartment. The rate of albumin synthesis is increased but not sufficiently to replace losses and restore the serum concentration to normal. The rate of albumin catabolism is decreased. This decrease from the normal catabolic rate is as important as the increased rate of albumin synthesis in maintenance of albumin homeostasis in nephrosis. Whereas the reduced serum oncotic pressure certainly contributes to edema formation, sodium retention may result from processes intrinsic to the kidney itself; and plasma volume may actually be expanded despite hypoalbuminemia. The hyperlipemia that occurs in nephrosis is due to a combined defect in lipoprotein metabolism: increased hepatic synthesis of VLDL and decreased removal of TG and highly atherogenic remnants of incompletely metabolized CMs. The defects in lipoprotein metabolism may in part be the end result of the urinary loss of highly negative-charged macromolecules of the mucopolysaccharide called orosomucoid, which carries with it heparan sulfate, and important cofactor for LPL.