<P> The glomerulus receives its blood supply from an afferent arteriole of the renal arterial circulation . Unlike most capillary beds, the glomerular capillaries exit into efferent arterioles rather than venules . The resistance of the efferent arterioles causes sufficient hydrostatic pressure within the glomerulus to provide the force for ultrafiltration . </P> <P> The glomerulus and its surrounding Bowman's capsule constitute a renal corpuscle, the basic filtration unit of the kidney . The rate at which blood is filtered through all of the glomeruli, and thus the measure of the overall renal function, is the glomerular filtration rate (GFR). </P> <P> The glomerulus is a tuft of small blood vessels called capillaries located within Bowman's capsule within the kidney . Glomerular mesangial cells structurally support the tufts . Blood enters the capillaries of the glomerulus by a single arteriole called an afferent arteriole and leaves by an efferent arteriole . The capillaries consist of a tube lined by endothelial cells with a central lumen . The walls have a unique structure: there are pores between the cells that allow water and soluble substances to exit, and after passing through the glomerular basement membrane, and between podocyte foot processes, enter the capsule as ultrafiltrate . </P> <P> Capillaries of the glomerulus are lined by endothelial cells . These contain numerous pores - also called fenestrae - 50--100 nm in diameter . Unlike those of other capillaries with fenestrations, these fenestrations are not spanned by diaphragms . They allow for the filtration of fluid, blood plasma solutes and protein, at the same time preventing the filtration of red blood cells, white blood cells, and platelets . </P>

Where is the glomerulus located in the nephron