Measuring
Values of Pv are difficult to obtain in patients. In practice, PAH clearance is used instead to calculate the effective renal plasma flow (eRPF). PAH (para-aminohippurate) is freely filtered and it is not reabsorbed within the nephron. Although freely filtered not all PAH crosses into the primary urine within Bowman's capsule. PAH remaining in the vasa recta or peritubular capillaries is taken up actively by epithelial cells of the proximal convoluted tubule and secreted into the tubular lumen. In this way PAH, at low doses, is completely cleared from the blood during a single pass through the kidney. Accordingly, the venous plasma concentration of PAH is approximately zero. Setting Pv to zero in the equation for RPF yields
which is the equation for renal clearance. For PAH, this is commonly represented as
Since the venous plasma concentration of PAH is not exactly zero (in fact, it is usually 10% of the PAH arterial plasma concentration), eRPF usually underestimates RPF by approximately 10%. This margin of error is generally acceptable considering the ease with which PAH infusion allows eRPF to be measured.
Read more about this topic: Renal Blood Flow
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