ultrafiltration The process in which hydrostatic pressure causes water and small dissolved molecules and ions to move across a membrane against a concentration gradient. Ultrafiltration is responsible for the formation of tissue fluid and glomerular filtrate from blood. In both these processes the ultrafiltered fluid has the same composition as the plasma except that it does not contain blood cells or large protein molecules.

ultrafiltration (ultră-fil-tray-shŏn) n. filtration under pressure. In the kidney, blood is subjected to ultrafiltration to remove the waste material that goes to make up urine.