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 average GFR values vary with age and range from about 116 ml/min/1.73 m² in people aged 20–29 to 60 in those aged 70+. Some 99% of this filtered volume is reabsorbed as it passes along the kidney tubules to form urine. The GFR is usually estimated by measuring the creatine concentration in a blood sample. This is then combined with other factors such as age, gender, height, weight, and ethnicity to give an estimate of GFR.