The Bowman's capsule gathers filtrate from the tight knot of capillaries (the glomerulus) fed by the renal artery. The blood goes through the glomerulus at high pressure, forcing small molecules out through the leaky capillary wall and into the capsule. Blood cells and larger molecules, such as proteins, remain in the capillary as it flows through the capsule and so the process separates large from small blood constituents. The process is called 'ultrafiltration'.
