A glomerulus is a network (as a tuft) of capillaries located at the beginning of a nephron in the kidney. It serves as the first stage in the filtering process of the blood carried out by the nephron in its formation of urine.

The glomerulus is surrounded by a cup-like sac known as Bowman's capsule. The blood plasma is filtered through the capillaries of the glomerulus into the capsule. The Bowman's capsule empties the filtrate into the proximal tubule that is also part of the duct system of the nephron.

A glomerulus receives its blood supply from an afferent arteriole of the renal circulation. Unlike most other capillary beds, the glomerulus drains into an efferent arteriole rather than a venule. The resistance of these arterioles results in high pressure within 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.

A 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).