Compartment(s)

The space or spaces in the body which a drug appears to occupy after it has been absorbed. Pharmacokinetic compartments are mathematical constructs and need not correspond to the fluid volumes of the body which are defined physiologically and anatomically, i.e., the intravascular, extracellular and intracellular volumes.
Some drugs make the body "behave" as if it consisted of only a single pharmacokinetic compartment. Tissue and plasma concentrations of the drug rapidly and simultaneously reach equilibrium in all the tissues to which the drug is distributed. A plot of plasma concentration against time after intravenous administration can be rectified into only a single straight line of negative slope, which can intersect the ordinate at only one point; only one volume of distribution can be calculated. Hence, the existence of only one compartment or volume of distribution can be inferred.
Some drugs make the body appear to consist of two or more pharmacokinetic compartments, since tissue/plasma equilibrium is achieved at different times in different tissues or groups of tissues. A plot of plasma concentration against time after intravenous administration can, at best, be resolved into a series of connected straight-line segments with progressively decreasing slopes. Each of these segments may be extrapolated to intersect the ordinate, and one may infer the existence of as many pharmacokinetic compartments, of volumes of distribution, for the the drug as there are intersections or segments.
Compartments in which equilibrium is achieved relatively late are referred to as "deep" compartments; compartments in which equilibrium is achieved early - and from which drug is redistributed to other sites - are referred to as "shallow" or "superficial" compartments.
Cf. a , b , Volume of Distribution , Vd (Vd)