Differentiate between one-compartment and two-compartment models in terms of their distribution phases.

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In pharmacokinetics, how the drug distributes after dosing distinguishes one- and two-compartment models. A one-compartment model treats the body as a single, well-mixed space, so after administration the drug equilibrates almost instantly and the plasma concentration declines in a single exponential due to elimination—there’s no separate distribution phase to observe. In a two-compartment model, the body has a central compartment (plasma and highly perfused tissues) and a peripheral compartment (less perfused tissues). After a dose, the drug rapidly moves between these compartments, producing a quick initial fall in plasma concentration (the alpha distribution phase) as it distributes to the peripheral tissues, followed by a slower phase (the beta distribution phase) as redistribution between compartments continues. Elimination occurs from the central compartment, shaping the later part of the curve. This results in a biexponential plasma concentration-time profile with two distinct distribution phases, unlike the single-phase decline seen in the one-compartment model.