<prompt> PAH is used to measure-Extracellular fluidGlomerular filtration rateRenal plasma flowPlasma value  <response>: Ans. is c i.e., Renal plasma flow Imagine a substance that not only passes freely across the glomerular membrane but is also secreted by the tubules to such an extent that the plasma leaving the kidney does not contain that substance any more. In that case the entire plasma flowing through the kidney would be cleared of the subsance. Or, in other words, the clearance of the substance would give us the renal plasma flow RPF. A substance which closely satisfies this description is para-aminohipporic acid PAH. Therefore, PAH is used t o measure renal plasma flow. The PAH clearance in a normal adult is about 625 mlminute. This is a measure of effective renal plasma flow ERPF. PAH is only 90 excreted in the urine in a single passage through the kidney, i.e., its extraction ratio is 0.9 90. Therefore, PAH understimates RPF by about 10 because it does not measure the plasma flowing through those portions of the kidney which do not secrete PAH, the renal medulla, perirenal fat, renal capsule, pelvis and calyces. Thus actual RPF is 10 more than ERPF i.e., 690 mlmin.Knowing RPF and hematocrit RBCs, renal blood flow can be calculated. In simple words, clearance of PAH is a measure of effective renal plasma flow and actual renal plasma flow is 10 more than the PAH renal clearance or ERPF. 