The glomerulus is the capillary network in a human nephron that filters blood to begin urine formation.
A glomerulus is a small tuft of blood capillaries enclosed by Bowman's capsule, together forming the renal corpuscle. Blood enters through an afferent arteriole, and pressure across the glomerular filtration barrier pushes water and small dissolved substances into the capsule. The resulting fluid is called glomerular filtrate.
The filtrate then travels through the nephron, where useful substances such as much of the water, glucose, and ions are reabsorbed. The remaining fluid is processed into urine. Normally, blood cells and most large plasma proteins remain in the bloodstream rather than passing through the filtration barrier.
The glomerulus is sometimes confused with the entire nephron or with the kidney itself. It performs the initial filtration step; the renal tubule carries out much of the later adjustment that determines the final composition of urine.