sement lamina is a joint product of the endothelium and the podocytes (Ross, Romrell and Kaye, 1995, 566-569). It is the principal filtration barrier and is known as the glomerular basement membrane (GBM). It contains type IV collagen; sialoglycoprotein; other non-collagenous glycoproteins such as laminin; proteoglycans; and glycosaminoglycans, particularly heparan sulfate. The laminae rara interna adjacent to the capillary endothelium is rich in polyanions such as heparan sulfate that specifically impede the passage of positively charged molecules. The sialoglycoproteins are involved in attachment of the endothelial cells to the podocytes and to the GBM.
Type IV collagen is concentrated in the lamina densa, which is a thick physical layer (Ros, Romrell and Kaye, 1995, 566-569). The GBM restricts the movement of particles, particularly proteins, larger than roughly 70,000 Daltons. However, several grams of protein do pass through the barrier each day, but are reabsorbed by endocytosis in the proximal convoluted tubule. If albumin or hemoglobin are present in the urine in significant amounts, this indicates damage to the basement membranes. The polyanionic glycosaminoglycans restrict the movement of cationic particles and molecules across the GBM, even if they are smaller than 70,000 Daltons.
The narrow slit pores form barriers to bulk flow and free diffusion (Ross, Romrell and Kaye, 1995, 567). Glycoproteins of the filtration slit membrane and the glycocalyx of the pedicels also restrict movement of molecules. The fenestrated capillary endothelium restricts the movement of blood cells and other formed blood elements from the capillaries. The flow rate and pressure in the glomerular capillaries also affects the filtration function of the renal corpuscle. The space between the visceral and parietal layers of Bowman's capsule is the urinary space, and is the receptacle for the filtrate.
9.3 Most of the molecu...