11.15: Measurement of Renal Plasma Flow

Renal plasma flow could be determined by introducing a substance and its clearance from the plasma. A frequently used agent for this purpose is para-aminohippuric acid (PAH). Approximately 90% of a small dose of PAH is cleared (filtered and secreted) within a single circulation of the kidney. As the extraction ratio is known, adequate renal plasma flow can be calculated once arterial and urinary PAH levels are determined, alongside the measurement of urinary volume.

\[\text { Renal Plasma Flow }=\frac{\text { Urinary PAH Level }(\mathrm{mg} / \mathrm{ml}) \times \text { Urine Volume }(\mathrm{ml} / \mathrm{min})}{\text { Plasma PAH Level }(\mathrm{mg} / \mathrm{ml})} \nonumber \]

Knowledge of the hematocrit makes it possible to estimate the total renal blood flow based on the renal plasma flow calculation. In a normal physiological state, the total renal blood flow is estimated to be approximately 1.2 liters of blood per minute.²⁷

 

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