In a two-compartment model, what do alpha and beta phases represent?

• A. Alpha is the rapid distribution phase; beta is the slower elimination phase.
• B. Alpha is the elimination phase; beta is the distribution phase.
• C. Alpha and beta are both distribution phases.
• D. Alpha is the slow elimination phase; beta is the rapid distribution phase.

Answer: (A)

In a two-compartment model the drug dose creates a pattern with two distinct phases in the plasma concentration over time. The rapid, early phase is governed by alpha and represents distribution: drug quickly moves from the central (blood) compartment into the peripheral tissues. This quick movement makes the concentration fall steeply at early times. The slower, later phase is governed by beta and represents elimination: the drug is cleared from the body (primarily from the central compartment) while some exchange with the peripheral compartment continues, but at a slower rate. On a log concentration vs time plot, alpha controls the steep initial slope, while beta controls the more gradual terminal slope. Because alpha corresponds to distribution (fast) and beta to elimination (slower), this pairing is the best description.