Renal Plasma Flow (2)

The Renal Plasma Flow calculator computes the renal plasma flow based on the urine flow rate and the para-aminohuippurate concentration of urine, arterial concentration and venal concentration.

INSTRUCTIONS:  Choose units and enter the following:

• (V_u) Volumetric Flow rate of Urine
• (U_PAH)  Urine concentration of para-aminohuippurate in the urine sample
• (RA_PAH) Arterial concentration of para-aminohuippurate (PAH)
• (RV_PAH) Venal concentration of para-aminohuippurate (PAH)

Renal Plasma Flow (RPF): The flow is returned in milliliters per second.  However, this can be automatically converted to compatible units via the pull-down menu.

The Math / Science

The Renal Plasma Flow equation computes the true renal plasma flow from the measurements taken of urine flow rate and the concentration of a specific substance, para-aminohippurate, introduced into the bloodstream.

          RPF = (U_PAH • V) / (RA_PAH - RV_PAH) 

where:

• RPF = Renal Plasma Flow
• V_u = volumetric flow rate of urine
• U_PAH - the urine concentration of para-aminohuippurate in the urine sample
• RA_PAH - the arterial concentration of para-aminohuippurate (PAH)
• RV_PAH - the venal concentration of para-aminohuippurate (PAH)

DERIVING THE GFR EQUATION

The Fick principle implies that the amount of PAH entering the kidney through the renal artery equals the amount of PAH leaving the kidney through the renal vein plus the amount of PAH leaving the kidney through the ureter. Therefore, the equation for “true” renal plasma flow is as follows:

This is derived from the Fick Principle, which can be applied more generically to any organ in the body.  The Fick principle states that blood flow to an organ can be calculated by introducing a substance into the bloodstream and then measuring the following:

• the amount of the substance removed from the blood flow by the organ per unit time
• the concentration of the substance in arterial blood supplying the organ
• the concentration of  substance in venous blood leaving the organ

RPF =