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A glomerulus is a capillary tuft surrounded by Bowman's capsule in nephrons of the vertebrate kidney. It receives its blood supply from an afferent arteriole of the renal circulation, and empties into an efferent arteriole. The resistance of the arterioles results in high pressure in the glomerulus aiding the process of ultrafiltration where fluids and soluble materials in the blood are forced out of the capillaries and into Bowman's capsule. The rate at which blood is filtered through the glomeruli is the glomerular filtration rate (GFR), measurements of which are often used to determine renal function.
Blood supply
The afferent arteriole that supplies the glomerulus is a branch off of an interlobular artery in the cortex. The efferent arteriole, into which the glomerulus delivers blood, empties into an interlobular vein.
Unique features
The glomerulus has several characteristics that deviate from the features of most other capillaries of the body. Firstly, the endothelial cells of the glomerulus contain numerous pores (fenestrae) that, unlike those of other fenestrated capillaries, are not spanned by diaphragms. Consequently, the endothelial cells lining the glomerulus are not considered part of the renal filtration barrier.
Secondly, the glomerular endothelium sits on a very thick basement membrane. It is not only uncharacteristically thick compared to most other basement membranes, but it is also rich in negatively charged glycosaminoglycans such as heparan sulfate. The negatively-charged basement membrane repels negatively-charged ions from the blood, helping to prevent their passage into Bowman's space.
Thirdly, blood is carried out of the glomerulus by an efferent arteriole instead of a venule, as is observed in most other capillary systems. This provides tighter control over the bloodflow through the glomerulus, since arterioles can be dilated and constricted more readily than venules, owing to arterioles' larger smooth muscle layer (tunica media).
Vasa recta
Efferent arterioles of juxtamedullary nephrons (ie, the 15% of nephrons closest to the medulla) send straight capillary branches that deliver isotonic blood to the renal medulla. Along with the loop of Henle, these vasa recta play a crucial role in the establishment of the nephron's countercurrent exchange system.
Juxtaglomerular cells
The walls of the afferent arteriole contain specialized smooth muscle cells that synthesize renin. These juxtaglomerular cells play a major role in the renin-angiotensin system, which helps regulate blood volume and pressure.
Mesangial cells
Mesangial cells are found both within and outside (near the macula densa) the glomerulus. They are specialized pericytes that participate indirectly in filtration.
External links
"Glomerulus"