The filtration apparatus is a very complex semipermeable barrier, with properties that allow for high filtration rate of water, nonrestricted passage of small and middle-sized molecules, and almost total exclusion of serum albumins and other larger proteins. The filtration apparatus may thus be described as a barrier having two discontinuous cellular layers, the endothelium of glomerular capillaries and visceral layer of Bowman’s capsule applied to either side of a continuous extracellular layer of the glomerular basement membrane. These three layers have traditionally been considered as the glomerular filtration barrier. However, recently two additional physiologically important layers, the endothelial surface layer of glomerular capillaries and subpodocyte space are included as part of the filtration apparatus.
 Endothelial surface layer of the glomerular capillaries consists of a thick carbohydrate–rich meshwork (200–400 nm) attached to the luminal surface of glomerular endothelial cells. It contains glycocalyx, which refers to plasma membrane-bound negatively charged proteoglycans (such as perlecan, syndecan, and versican) associated with glycosaminoglycan side-chains (such as heparan sulphate and chondroitin sulphate) and peripheral membrane proteins. Plasma proteins (e.g., albumins) adsorbed from the blood coat the luminal surface of glycocalyx.
 Subpodocyte space represents a narrow space between the foot processes with their filtration slit diaphragms on one side and a cell body of the podocyte on the other side (see Fig. 20.13). Recent three-dimensional reconstruction of these spaces revealed their interconnected but structurally restrictive character. They cover approximately 60% of the entire surface area of the glomerular filtration barrier and may function in regulating glomerular fluid flux across the filtration apparatus.
The glomerular basement membrane (GBM) acts as a physical barrier and an ion-selective filter.
As discussed earlier, the GBM contains type IV and XVIII collagens, sialoglycoproteins, and other noncollagenous glycoproteins (e.g., laminin, fibronectin, entactin), as well as proteoglycans (e.g., perlecan, agrin) and glycosaminoglycans, particularly heparan sulfate (Fig. 20.15). These components are localized in particular portions of the GBM:  The lamina rara externa, adjacent to the podocyte processes. It is particularly rich in polyanions, such as heparan sulfate, that specifically impede the passage of negatively charged molecules.
 The lamina rara interna, adjacent to the capillary endothelium. Its molecular features are similar to those of the lamina rara externa.
 The lamina densa, the overlapping portion of the two basal laminae, sandwiched between the laminae rarae. It contains type IV collagen, which is organized into a network that acts as a physical filter. Type XVIII collagen, perlecan, and agrin are responsible for the bulk of anionic charges found in glomerular basement membrane. The laminin and other proteins present in the laminae rara interna and externa are involved in the attachment of the endothelial cells and podocytes to the GBM.
The GBM restricts the movement of particles, usually proteins, larger than approximately 70,000 daltons or 3.6 nm radius (e.g., albumin or hemoglobin). Although albumin is not a usual constituent, it may sometimes be found in urine, indicating that the size of albumin is close to the effective pore size of the filtration barrier. The polyanionic glycosaminoglycans of the laminae rarae have strong negative charges and restrict the movement of anionic particles and molecules across the GBM, even those smaller than 70,000 daltons. Despite the ability of the filtration barrier to restrict protein, several grams of protein do pass through the barrier each day. This