Figure 12-5 Three-compartment model (including the biophase) schematizing the basic pharmacokinetic processes that occur after intravenous drug administration. I, dosing scheme as a function of time; k₁₀ , rate constant reflecting all processes acting to irreversibly remove drug from the central compartment; k_ij intercompartmental rate constants; V₁ , central compartment volume, usually expressed in liters or liters per kilogram.