Extraction ratio is a measure in renal physiology, primarily used to calculate renal plasma flow in order to evaluate renal function. It is the amount of compound entering the kidney that got excreted into the final urine. .

Measured in concentration in blood plasma, it may thus be expressed as:

$Extraction\; ratio\; =\; frac\{P\_a\; -\; P\_v\}\{P\_a\}$

, where P_a is the concentration in renal artery, and P_v is the concentration in the renal vein.

For instance, para aminohippuric acid (PAH) is almost completely excreted in the final urine, and thus almost none is found in the venous return (P_v ~0). Therefore, the extraction ration of PAH ~1. This is why PAH is used in PAH clearance to estimate renal plasma flow.

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