Glomerular filtration rate

Glomerular filtration rate, or GFR, is the volume of filtrate that the glomeruli of both kidneys produce each minute. The glomeruli are tufts of capillaries inside the nephrons, where blood pressure pushes water and small dissolved substances out of the blood and into the kidney tubules. In healthy young adults GFR is roughly 100 to 125 milliliters per minute, which adds up to around 150 to 180 liters of filtrate a day. Nearly all is reabsorbed, leaving one or two liters of urine.

GFR depends on net filtration pressure: the blood pressure inside the glomerular capillaries minus the opposing pressure of fluid already in the capsule and the osmotic pull of blood proteins. Because this net pressure is small, the kidneys guard it. The afferent arteriole narrows or widens as blood pressure changes, and signals from the tubule adjust the flow, so GFR stays fairly steady across a wide range of blood pressures.

Measuring GFR directly needs a marker such as inulin, which is filtered but neither reabsorbed nor secreted. Clinicians usually estimate it as eGFR from the blood level of creatinine, a muscle waste product, along with factors such as age and sex. A low GFR is used to judge how advanced kidney disease is, and it also guides the dosing of drugs that the kidneys clear.