In pharmacokinetic modeling, there are compartments with instant distribution and slower distribution. Which statement is true?

• A. Compartment 1 equilibrates slowly; compartment 2 instantly
• B. Compartment 1 equilibrates instantly; compartment 2 has slower distribution
• C. Both compartments equilibrate instantly
• D. Neither compartment equilibrates to blood

Answer: (B)

In a two-compartment pharmacokinetic model, one part of the system is treated as in near-instantaneous equilibrium with the blood, while the other part takes longer to reach distribution equilibrium. This means the central, blood-like compartment equilibrates very quickly, and the peripheral compartment distributes more slowly as drug moves into deeper tissues. Saying that compartment 1 equilibrates instantly while compartment 2 has slower distribution fits this picture: the first compartment represents the rapid, immediate exchange with blood, while the second reflects the slower distribution into tissues. The other possibilities would imply no distinct distribution phases or no rapid exchange with blood, which doesn’t match the common two-compartment framework.