What is the relationship between total systemic clearance and its component clearances?

• A. CL_total = CL_renal + CL_hepatic + CL_other
• B. CL_total = CL_renal − CL_hepatic
• C. CL_total equals the hepatic clearance only
• D. CL_total is independent of organ clearances

Answer: (A)

Total systemic clearance is the sum of the clearances through all elimination pathways. Clearance is the volume of plasma from which the drug is completely removed per unit time, so when multiple organs can eliminate the drug in parallel, their elimination rates add up. At a given plasma concentration, the kidney clears at a rate C × CL_R, the liver at C × CL_H, and other routes at C × CL_O. The total rate of elimination is therefore C × (CL_R + CL_H + CL_O), which means CL_total = CL_R + CL_H + CL_O. This holds under linear (dose-independent) kinetics.

That’s why the best answer is the sum of the individual clearances. The other options imply subtraction, dependence on only one pathway, or independence from organ clearances, which aren’t consistent with how parallel elimination pathways contribute to overall clearance.