Para-aminohippurate blood flow

A substance that fulfills these criteria is para-aminohippuric acid (PAH). All of the PAH not filtered at the glomerulus is secreted by the proximal tubule. The net effect is that all of the plasma flowing through the nephrons is completely cleared of PAH. It is important to note that about 10 to 15% of the total renal plasma flow supplies regions of the kidneys that are not involved with filtration or secretion. Consequently, this plasma cannot be cleared of PAH. Therefore, the plasma clearance of PAH provides a measurement of the effective renal plasma flow, that is, the volume of plasma that actually flows through the nephrons. The ERPF is normally about 625 ml/ min. (This value is based on a renal blood flow of about 1.1 1/min and a hematocrit of about 42.)... 

Para-aminohippurate (PAH)—A small molecule which is completely secreted from the tubules into urine, so that blood leaving the kidney is virtually free of PAH a marker that is often used to measure renal plasma flow (RPF). 

Cassin S, Vogh B. Effect of hydrochlorothiazide on renal blood flow and clearance of para-aminohippurate and creatinine. Proc Soc Exp Biol Med 1966 122 970-973. 

An indirect measurement of RBF can be made using para-aminohippuric acid (PAH) clearance. This molecule is an ideal marker of the effective renal plasma flow (eRPF), as it freely filters through the glomerulus, and any amotmt remaining in the peritubular capillary plasma is secreted into the proximal tubule. Therefore, essentially aU PAH passing through the kidneys appears in the urine. For this reason, the PAH clearance is directly proportional to the rate of plasma flow through the kidneys. If the hematocrit is known, the total renal blood flow can be easily calculated from the eRPF value. 

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