lumen
Renal Corpuscle As discussed earlier, the renal corpuscle consists of a tuft of capillaries called the glomerulus that is largely surrounded by Bowmans glomerular capsule. The glomerulus is a high-pressure capillary bed between afferent and efferent arterioles. Bowmans capsule surrounds the glomerulus to form a lumen, and captures and directs this filtrate to the PCT. The outermost part of Bowmans capsule, the parietal layer, is a simple squamous epithelium. It transitions onto the glomerular capillaries in an intimate embrace to form the visceral layer of the capsule. Here, the cells are not squamous, but uniquely shaped cells podocytes extending finger-like arms pedicels to cover the glomerular capillaries Figure 25.11 . These projections interdigitate to form filtration slits, leaving small gaps between the digits to form a sieve. As blood passes through the glomerulus, 10 to 20 percent of the plasma filters between these sieve-like fingers to be captured by Bowmans capsule and funneled to the PCT. Where the fenestrae windows in the glomerular capillaries match the spaces between the podocyte fingers, the only thing separating the capillary lumen and the lumen of Bowmans capsule is their shared basement membrane Figure 25.12 . These three features comprise what is known as the filtration membrane. This membrane permits very rapid movement of filtrate from capillary to capsule though pores that are only 70 nm in diameter.