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Renal Clearance:
Renal Clearance:
Renal plasma (or blood) clearance ClR is the volume of plasma (or blood) freed of a substance by only renal mechanisms, per unit time. The amount of drug (AU) excreted in the urine during the time interval t - t' is determined; the plasma (or blood) concentration at the mid-point of the interval (Cp) is found by interpolation on the line relating log C and t. The urinary excretion rate of the drug,
AU/(t - t'), divided by Cp is the renal clearance.
Renal plasma clearance will vary with such factors as age, weight, and sex of subject, the state of cardiovascular and renal function, the nature of the material being excreted, species, etc. Renal clearance by only glomerular filtration is defined and measured as the clearance of the sugar inulin, which is eliminated from the body by no route other than glomerular filtration. Total renal clearance is defined and measured by clearance of para-amino-hippurate (PAH), a substance that is eliminated by both glomerular filtration and tubular excretion (at the maximum rate of which the tubular mass is capable). Neither inulin nor PAH undergoes reabsorption by the tubules as some materials do. (N.B.: Blood and plasma are completely cleared of PAH by a single "pass" through the kidney; PAH clearance is therefore, the standard measure of renal plasma, or blood, flow).
In normal adult human males, plasma clearance of inulin is about 130 ml plasma/min; of PAH, about 700 ml plasma/min. In normal adult human females, clearance of inulin is about 115 ml plasma/min; of PAH, about 600 ml plasma/min. The relationship between clearance of blood and clearance of plasma is given by the relationship ClR (blood) = ClR (plasma)/(1-Hct), where "Hct" is the hematocrit, the proportion, as a fraction - of the blood which consists of cells, not plasma; on the average, normal adult human subjects can be assumed to have a hematocrit of about 0.45.
Like
many other physiological "constants," renal plasma clearance varies regularly and exponentially with body weight, across mammalian species ( Science 109: 757, 1949). Renal plasma clearances, in normal animals, can be predicted using the following relationships, where Cl R is in ml/hr, and body weight (B) is in grams:
ClR (inulin) = 1.74B0.77
ClR (PAH) = 5.40B0.80
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