Which statement best describes the distribution phase in a two-compartment model?

• A. It is the absorption phase
• B. It reflects rapid equilibration between blood and highly perfused tissues
• C. It occurs after complete elimination
• D. It is identical to the elimination phase

Answer: (B)

In a two-compartment model, the distribution phase is the rapid movement of drug from the bloodstream into tissues that receive the most blood flow, causing a quick drop in plasma concentration as the central (blood) compartment exchanges with the peripheral (tissue) compartment. This quick redistribution reflects the drug reaching well-perfused organs such as the liver, kidneys, lungs, brain, and heart, which pulls drug out of the plasma faster than it is eliminated. After this initial rapid equilibration, the concentration then declines more slowly as elimination processes (metabolism and excretion) take over—the elimination phase.

So the distribution phase best describes the rapid equilibration between blood and highly perfused tissues. It’s not about absorption, and it’s not the same as the elimination phase, nor does it occur after complete elimination.