Extraction ratio

Extraction ratio is a measure in renal physiology, primarily used to calculate renal plasma flow in order to evaluate renal function. It measures the percentage of the compound entering the kidney that was excreted into the final urine.[1]

Parameter Value
renal blood flow RBF=1000 ml/min
hematocrit HCT=40%
glomerular filtration rate GFR=120 ml/min
renal plasma flow RPF=600 ml/min
filtration fraction FF=20%
urine flow rate V=1 mL/min
Sodium Inulin Creatinine PAH
SNa=150 mEq/L SIn=1 mg/mL SCr=0.01 mg/ml SPAH=
UNa=710 mEq/L UIn=150 mg/mL UCr=1.25 mg/mL UPAH=
CNa=5 mL/min CIn=150 ml/min CCr=125 mL/min CPAH=420 ml/min
ER=90%
ERPF=540 ml/min

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

, where Pa is the concentration in renal artery, and Pv 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 (Pv ~0). Therefore, the extraction ratio of PAH ~1. This is why PAH is used in PAH clearance to estimate renal plasma flow.


Note: The "Hepatic Extraction Ratio" is a similar measurement for clearance of a substance (usually a pharmacological drug) by the liver. It is defined as the fraction of drug removed from blood by the liver, and depends on 3 factors— the hepatic blood flow, the uptake into the hepatocytes, and the enzyme metabolic capacity. Examples of drugs with a high hepatic extraction ratio include propranolol, opiates, and lignocaine

References
  1. Renal Mathematics Texas Tech University Health Sciences Center
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