Renal plasma flow is best described as which parameter?

• A. The volume of plasma delivered to the kidneys per unit time.
• B. The volume of urine formed per minute.
• C. The rate of glomerular filtration.
• D. The total renal blood flow.

Answer: (A)

The key idea is distinguishing plasma flow to the kidney from other kidney flow metrics. Renal plasma flow is the volume of plasma that reaches the kidneys per unit time—essentially the fluid portion of blood delivered to the kidneys. It excludes the blood cells, so it isn’t the total blood flow. That’s why the best description is the volume of plasma delivered per minute, usually expressed in mL/min.

To connect the ideas: renal blood flow includes both plasma and cells, so it’s larger than renal plasma flow; they’re related by hematocrit (RBF ≈ RPF/(1 − Hct)). The rate of glomerular filtration measures filtrate formed per minute (GFR), not the plasma arriving, and urine flow is the amount of urine produced per minute, not the plasma entering the kidneys. In practice, PAH clearance is used to estimate RPF, while inulin or creatinine clearance estimates GFR.