File:Journal.pone.0158798.g004.png

English: Fig 4. Schematic diagram showing E2 compartmental and GV (gamma variate) variable volume models of drug distribution. The E2 model could also be drawn as a variable volume model in which case a scale factor αexp = VE/Vd(∞) < 1 would define the physical volume at time t to be αexpVd(t). Similarly, for the variable volume adaptively obtained GV model, one can define α = VE/Vd(∞) < 1, and an expanding physical volume αVd(t). Note, both αexp and α are constants at all times for their respective models. The term VSS can be confusing because 1) VSS implies that VE is always a steady state volume, which is not the case as the GV model αVd(t) < VE is concentration depleted at late time, see Eq (30). 2) VSS implies that VE only exists at t = ∞, whereas VE is defined all of the time, i.e., on t = [0,∞) by Eqs (8 & 7). Finally, 3) VSS implies an expected physical volume of distribution for sums of exponential term bolus models, and the apparent volume of distribution for a constant infusion experiment, whereas VE applies to more models as the expected volume of physical distribution of a drug for both the bolus and constant infusion experiments. show less