Fenestral diaphragms

Plasma is filtered through a sieve consisting of three main layers (1) large endothelial fenestrations, (2) the dense network of the glomerular basement membrane, and (3) the slit diaphragm between podocyte processes. 

Figure 5.1 Schematic illustration of the structure of the wall of different classes of blood capillaries. (1) Continuous capillary (as found in the general circulation). The endothelium is continuous with tight junctions between adjacent endothelial cells. The subendothehal basement membrane is also continuous. (2) Fenestrated capillary (as found in exocrine glands and the pancreas). The endothelium exhibits a series of fenestrae which are sealed by a membranous diaphragm. The subendothehal basement membrane is continuous. (3) Discontinuous (sinusoidal) capillary (as found in the liver, spleen and bone marrow). The overlying endothelium contains numerous gaps of varying size. The subendothehal basement is either absent (hver) or present as a fragmented interrupted structure (spleen, bone marrow)...

Drugs exchange freely between blood and interstitium in the liver, where endothelial cells exhibit large fenestrations (100 nm in diameter) facing Disse s spaces (D) and where neither diaphragms nor basement membranes impede drug movement. 

In the glomerular capillaries, a portion of the plasma water is forced through a filter that has three basic components fenestrated capillary endothelial cells, a basement membrane lying just beneath the endothelial cells, and the filtration slit diaphragms formed by the epithelial cells that cover the basement membrane on its urinary space side. Solutes of small size flow with filtered water (solvent drag) into the urinary (Bowman s) space, whereas formed elements and macromolecules are retained by the filtration barrier. 

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