Figure 1.

Diagram showing the movement of a drug between the central compartment (systemic circulation) and peripheral compartment(s).

A 3-compartment model will include the peripheral compartment 2 with dashed lines. k₁₂ and k₂₁; k₁₃ and k₃₁ are rate constants associated with the to and fro transfer of drug between the compartments; k_e is the rate constant for elimination from the central compartment.