The blood is separated from the glomerular lumen - and hence the external world - by three layers:
- the modified vascular endothelium of the glomerular capillaries
- the glomerular basement membrane which is composed of mainly type IV collagen, laminin and proteoglycans
- the podocytes of the glomerular epithelium
The glomerulus produces a selective ultrafiltrate of the blood. The rate of ultrafiltration is called the glomerular filtration rate (GFR) which is about 120 ml/min or 170 litres per day.
The components of the blood are differentially filtered at the glomerulus:
- the renal filter is too fine to allow any cellular components through, thus red cell, white cells and platelets remain in the blood
- water and electrolytes filter freely due to fenestrations in the vascular endothelium, the slit pores between the foot processes of the podocytes and the high permeability of the glomerular basement membrane
- proteins larger than approximately 3.5 nm diameter (or 65 kDa molecular mass n.b. albumin is 68 kDa) are excluded from the filtrate by the glomerular basement membrane
- negatively charged proteins are preferentially excluded from the filtrate due to the presence of negatively charged proteoglycans in the glomerular basement membrane