Measuring Renal Blood Flow: Fick Principle
The amount of a substance taken up, utilized, or excreted unchanged by the kidney or any other organ, (or the whole body for that matter), per unit time is equal to the difference between the arterial and venous levels of that substance multiplied by the organ blood flow. Stated differently, it is possible to calculate renal plasma flow by measuring the amount of a given substance excreted (used) by the kidney, and dividing this value by the plasma arteriovenous concentration difference of this substance.
For this method to provide accurate measurements certain conditions must be satisfied: (1) the concentration of the substance in the blood plasma must be maintained constant; (2) the measured substance must not have an affect on blood flow; (3) it must not be sequestered, manufactured, or metabolized by the kidney. Any substance that satisfies these conditions can be used to measure renal blood flow by the Fick method. Under these conditions the urinary excretion rate of the substance may be used to measure renal uptake by collecting urine over a measured time period and obtaining an arterial blood sample during that period. Both of these procedures are easily accomplished. The difficulty with the method, particularly in its clinical application, is catheterization of the renal vein to obtain a sample of renal venous blood.