Found the exact Ganong's section. Let me pull the full passage.

Here's the concept broken down clearly, straight from Ganong's (Chapter 37, "Renal Circulation").

You can't stick a flow meter into a human kidney routinely, so physiologists use the Fick principle indirectly: infuse a substance, measure how much of it leaves the body in urine, and divide by the arteriovenous (A-V) concentration difference across the kidney. That gives you plasma flow because the kidney filters plasma, not whole blood.

Para-aminohippuric acid (PAH) is used because it's both filtered by the glomeruli and secreted by the tubular cells. That combination means almost all the PAH that enters the kidney in one pass gets pulled out of the blood and dumped into the urine. At low infusion doses, about 90% of the PAH in arterial blood is removed in a single trip through the kidney - this is called a high extraction ratio.

If PAH were removed 100% (i.e., renal venous PAH = 0), then measuring urine PAH and plasma PAH alone would give you the true renal plasma flow. Since renal venous blood is hard to sample directly, physiologists cheat: they just use peripheral venous plasma PAH concentration (easy blood draw) as a stand-in for arterial plasma PAH (since PAH concentration is essentially the same in peripheral venous and arterial plasma), and they ignore the renal venous concentration entirely (assuming it's near zero).

$$ERPF = \frac{U_{PAH} \times \dot{V}}{P_{PAH}} = C_{PAH} \text{ (clearance of PAH)}$$

Because this calculation assumes complete (100%) extraction, but extraction is really only ~90%, the value you get is not quite the true renal plasma flow - it's called the "effective" renal plasma flow to flag that assumption. In humans, ERPF averages about 625 mL/min.

Worked example from the text:

ERPF = (14 × 0.9) / 0.02 = 630 mL/min

To correct ERPF into the actual RPF, you divide by the PAH extraction ratio (how much of it really got removed in one pass, average ~0.9):

$$RPF = \frac{ERPF}{Extraction\ ratio} = \frac{630}{0.9} = 700\ mL/min$$

RPF is just the plasma portion. To get whole blood flow, you need to account for the red cells by dividing by (1 - hematocrit):

$$RBF = RPF \times \frac{1}{1-Hct} = 700 \times \frac{1}{0.55} = 1273\ mL/min$$

(using Hct = 45%)

Think of it as three nested corrections: ERPF is your raw, "quick and dirty" estimate assuming perfect extraction. RPF fixes that by scaling up for the ~10% of PAH that didn't get extracted. RBF then scales up again to include the red blood cells that don't carry PAH at all.

In a resting adult, total renal blood flow is about 1.2-1.3 L/min, which is just under 25% of cardiac output - a disproportionately large share for organs that make up a small fraction of body weight, reflecting the kidney's role in filtration rather than just its own metabolic needs.