Measurement of Renal Plasma Flow; Renal Clearance of PAH
Para-aminohippuric acid (PAH) undergoes both glomerular filtration and tubular secretion by a rate limited active transport mechanism. If the plasma PAH concentration (PPAH) is low enough that the peritubular load of PAH is below the PAH Tm, then PAH will be completely cleared from the plasma in a single pass through the kidney. Under these circumstances the renal venous PPAH will be zero and will drop out of the denominator in the Fick equation. The renal clearance of PAH then becomes an estimate of renal plasma flow.
Some PAH, however, does appear in the renal venous blood because 5-10% of the PAH is in blood that perfuses nonfunctional regions of the kidney and PAH has an extraction ratio of about 0.90 to 0.95. This means that approximately 90 to 95 % of the PAH is removed from the renal blood flow in a single passage through the kidneys. The clearance of PAH is therefore called the effective renal plasma flow (ERPF), where effective connotes the fraction of the total renal plasma flow that perfuses functional portions of the nephron where it can be filtered and secreted.
If the PPAH rises to levels at which the peritubular load of PAH exceeds its secretory Tm then the amount secreted becomes constant, PAH appears in the renal venous plasma and PAH excretion increases only in proportion to the increase in filtered PAH. The PAH excreted is less than that contained in the renal plasma flow and CPAH underestimates the ERPF.
Once ERPF is measured by the clearance of PAH, effective renal blood flow (ERBF) can be calculated as shown in the figure from the relationship ERBF = ERPF ¸ (1-Hematocrit). Also shown is that if the PAH extraction ratio is known the actual or total renal plasma flow or blood flow can be calculated.