Which parameters predominantly govern the early decline in plasma concentration during the distribution phase in a two-compartment model?

Which parameters predominantly govern the early decline in plasma concentration during the distribution phase in a two-compartment model?

• A. Elimination rate constant ke
• B. Absorption rate constant ka
• C. Distribution rate constants k12 and k21
• D. Renal clearance

Answer: (C)

In a two-compartment model, the rapid early drop in plasma concentration after an IV dose is driven by the drug moving between the central (plasma) and peripheral (tissues) compartments. The rate at which this inter-compartment transfer happens is described by the distribution rate constants k12 (central to peripheral) and k21 (peripheral back to central). These bidirectional transfers set how quickly the drug leaves the plasma for tissues and then re-equilibrates, producing the distribution phase. Elimination (ke) governs how the drug is removed from the body, shaping the later, slower decline rather than the initial distribution. The absorption rate constant (ka) matters for entry into systemic circulation after non-IV dosing, not for the IV distribution phase. Renal clearance relates to elimination as well, affecting how much and how fast the drug is cleared, not the rapid distribution between compartments.

In a two-compartment model, the rapid early drop in plasma concentration after an IV dose is driven by the drug moving between the central (plasma) and peripheral (tissues) compartments. The rate at which this inter-compartment transfer happens is described by the distribution rate constants k12 (central to peripheral) and k21 (peripheral back to central). These bidirectional transfers set how quickly the drug leaves the plasma for tissues and then re-equilibrates, producing the distribution phase.

Elimination (ke) governs how the drug is removed from the body, shaping the later, slower decline rather than the initial distribution. The absorption rate constant (ka) matters for entry into systemic circulation after non-IV dosing, not for the IV distribution phase. Renal clearance relates to elimination as well, affecting how much and how fast the drug is cleared, not the rapid distribution between compartments.