Define renal clearance.

• A. The rate of urine formation.
• B. The amount of plasma filtered per minute.
• C. The actual amount of substance excreted per minute.
• D. The imaginary volume of plasma from which a substance is completely cleared per unit time.

Answer: (D)

Renal clearance is the imaginary volume of plasma from which a substance is completely removed by the kidneys each minute. It’s calculated as clearance = (urine concentration of the substance × urine flow rate) / plasma concentration of the substance, giving units of mL per minute. This concept ties together how much of the substance is excreted and how concentrated it is in the plasma, to express the rate at which plasma would have to be cleared to account for the observed excretion.

It differs from the rate of urine formation (that’s urine flow), and from the actual amount excreted per minute (which is the excretion rate, not a plasma-based volume). It also isn’t simply the amount filtered per minute, which relates to the glomerular filtration rate and plasma concentration but does not directly define the clearance concept.