Physiology, Glomerular Filtration Rate

Glomerular filtration rate (GFR) represents the flow of plasma from the glomerulus into Bowman’s space over a specified period and is the chief measure of kidney function. The kidneys receive 20% to 25% of the cardiac output (about 1.0 to 1.1 liters per minute) with the blood entering individual glomerular tufts via the afferent arteriole and exiting through the efferent arteriole. Of this renal blood flow (RBF), only the plasma can cross the structures comprising the glomerulus. Thus, the renal plasma flow (RPF) is a more accurate expression and is calculated as follows: RBF*(1-Hct) The RPF is approximately 600 to 720 ml per minute. Within the plasma, organic and inorganic solutes are freely filtered- meaning that they can be found in the ultrafiltrate (the fluid in Bowman’s space) and plasma at the same concentrations. GFR is approximately 120 ml per min (180 L per day). Average urine output, on the other hand, averages only about 1.5 L daily. The reabsorption of 178.5 L requires a sophisticated tubular network.