glomerular filtration rate (GFR) The volume of fluid (see glomerular filtrate) that is filtered from the capillaries of the glomeruli into the kidney tubules per unit time. It is influenced chiefly by blood pressure and by changes in the diameter of arterioles in the glomeruli. In humans GFR is typically about 125 ml/min, although some 99% of this filtered volume is reabsorbed as it passes along the kidney tubules to form urine. The GFR is usually estimated by injecting the polysaccharide inulin into the bloodstream, measuring the rate at which it appears in the urine, and dividing this by the inulin concentration in plasma. Inulin is neither reabsorbed nor secreted by the kidneys, so its appearance in urine is directly related to the GFR.

glomerular filtration rate (GFR) (glom-e-roo-ler) n. the rate at which substances are filtered from the blood of the glomeruli into the Bowman's capsules of the nephrons. It is calculated by measuring the clearance of specific substances (e.g. creatinine) and is an index of renal function.