The first process by which the kidneys produce urine is called glomerular filtration. Blood enters the glomerulus under high pressure, forcing substances across the leaky endothelial-capsular membrane into the nephron.

This membrane acts like a sieve allowing small substances to be filtered into the nephron, whilst large molecules such as plasma proteins remain within the blood. The filtered fluid is called ultrafiltrate and passes from the Bowman's Capsule into the proximal convoluted tubule.