In a two-compartment model, which of the following best describes the central and peripheral compartments?

• A. Central compartments include blood/plasma; peripheral compartments include tissues.
• B. Central includes tissues; peripheral is bloodstream.
• C. Central is brain only.
• D. Central is elimination site.

Answer: (A)

In a two-compartment PK model, the central compartment represents the blood/plasma and other well-perfused areas where the drug is initially distributed and from which elimination is often described. The peripheral compartment represents tissues where the drug distributes more slowly, leading to a two-phase distribution with exchange between these compartments and slower overall elimination. This distinction—central = blood/plasma, peripheral = tissues—best captures how the model describes drug movement and clearance.

The other descriptions misplace the compartments: the central space is not limited to the brain, it is not defined as the tissues themselves, and while elimination is modeled as occurring from the central compartment, that doesn’t define what the central compartment is.