Plasma clearance is the volume of plasma that is cleared of a substance by the kidneys per unit of time. It is calculated by multiplying the urine concentration of a substance by the urine flow rate, divided by the plasma concentration of the substance. In this example, the plasma clearance rate of PAH was calculated to be 585 ml/min, with a total blood flow through the kidneys of 1185 ml/min when accounting for the extraction ratio of 90%.