Compare one-compartment and two-compartment pharmacokinetic models.

• A. One-compartment models treat body as multiple distinct tissues; two-compartment models treat body as a single uniform space.
• B. One-compartment models treat body as a single, uniform space; two-compartment models separate a central (blood/plasma) and a peripheral (tissues) compartment.
• C. One-compartment models include distribution and elimination phases; two-compartment do not.
• D. Both models assume instantaneous distribution.

Answer: (B)

In pharmacokinetics, the number of compartments used to describe the body changes how we view distribution after a dose. A one-compartment model treats the body as a single, uniform space where the drug mixes instantly, so the concentration declines in a single exponential manner and there’s no separate distribution phase. A two-compartment model recognizes a central compartment (the blood/plasma) and a peripheral compartment (tissues), with slower exchange between them. After an IV dose you see a rapid distribution phase as the drug moves between central and peripheral compartments, followed by a slower elimination phase from the central compartment, giving a biexponential plasma concentration-time profile. This distinction lines up with viewing the body as one uniform space in the simple model and as two distinct compartments in the more detailed model.