Endothelial cells, glomerular capillary wall

Like the walls of other capillaries, the glomerular capillary wall consists of a single layer of endothelial cells. However, these cells are specialized in that they are fenestrated. The presence of large pores in these capillaries makes them 100 times more permeable than the typical capillary. These pores are too small, however, to permit the passage of blood cells through them. 

The endothelial cell barrier normally restricts the passage of formed elements (e.g., red cells) through the glomerular capillary wall, but it presents no restriction to the passage of different macromolecules. 

Figure 2.16b shows the structure of the glomerular vessel wall. The interior is covered by endothelial cells with multiple holes ( fenestrations ). The podocytes (= foot cells ) form a likewise discontinuous outer layer. Between them is an acellular basal membrane, consisting of proteins and proteoglycans, which has the smallest pore diameter of all three and therefore, as in any the capillaries found elsewhere in the body, represents the effective filter layer. The filter has a cut-off size of very few nanometers, so that most protein molecules will be retained. Salt ions and small molecules - if they are not protein-bound - will be filtrated. The amount of filtrate produced is about 1501 per day in a healthy adult this corresponds to about 1/10 of the blood plasma volume that passes the kidneys. 

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