Modeling drug distribution assumes that the drug is moves in and out of one or more "compartments" each of which may show different kinetics for entrance and exit.
In a one compartment model, it is assumed that drug is distributed in one space, so that the concentration in all tissues is the same as within the plasma.
In a 2 compartment model, all of the drug is confined initially to the plasma (intravascular) space, but then it distributes from plasma into one or more extravascular compartments. This results in a biphasic curve when the logarithm of the plasma concentration is plotted over time (semilog plot).
The kinetic modeling of some drugs assumes 3 or more compartments (multi-compartment models).
The elimination or equilibrium phase follows the distribution phase. During this time period there is a slower rate of fall in drug levels as the drug is eliminated from the body.
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