Glomerulus

The glomerulus is a tiny capillary network (twisted mass of microscopic tubes) which is located in the Bowman's capsule. Both the glomerulus and the Bowman's capsule make up the renal corpuscle of a nephron. The glomerulus is semipermeable; this means that it allows water and soluble wastes to seep through and be excreted out of the Bowman's capsule as urine. The filtered blood flows out of the glomerulus into the efferent arteriole to be returned through the medullary plexus to the intralobular vein.

The glomerulus is the main filter of the nephron, and along with the Bowman's capsule, the basic filtration unit of the kidney. An afferent arteriole of the renal circulation drains into the glomerulus. In contrast with other capillary beds, the glomerulus drains into an efferent arteriole rather than a venule. The rate at which blood is filtered through all of the glomeruli in the kidney, and thus the measure of the overall renal function, is the glomerular filtration rate (GFR).

There are numerous pores, called fenestrae, in endothelial cells of the glomerulus. Unlike those of other fenestrated capillaries, are not spanned by diaphragms. These cells have openings which are so large that nearly anything smaller than a red blood cell passes through that layer. Because of this, the endothelial cells lining the glomerulus are not usually considered part of the renal filtration barrier. The glomerular endothelium sits on a very thick (250-350 nm) glomerular basement membrane. It is not only uncharacteristically thick compared to most other basement membranes (40-60 nm), but it is also rich in negatively charged glycosaminoglycans such as heparan sulfate.

Glomerulus and Bowman`s capsule