filtration unit of vertebrate nephrons, functional units of the kidney. It consists of a knot of capillaries (glomerulus) surrounded by a double-walled capsule (Bowman's capsule) that opens into a tubule. Blood pressure forces plasma minus its macromolecules (e.g., proteins) from the glomerular capillaries into the Bowman's capsule. This filtrate, called capsular urine, then passes into the tubule for further processing.