Figure 3. (A) Predicted arterial plasma concentration curves of fentanyl in the “male,”“female,” and “human” models after administration of the indicated dose over 1 min. (B) The amounts of fentanyl, as a percentage of dose, eliminated or present in the large tissue compartments of the “human” model as a function of time. (C, D) The same curves for alfentanil. These plots show the relative importance of the tissue compartments for removing drug from the circulation. Compartments with similar rates of drug uptake have been grouped together:“Visceral”= brain, heart, liver, gut, pancreas, spleen, and kidneys;“peripheral”= muscle, skin, and carcass. “Blood” is great vessel blood only because the tissue compartments include capillary blood. The time scales correspond to approximately 90% elimination of either drug in the “human” model. Note the marked similarities in disposition (except for dose fractions in blood) between the two opioids after shifting of the time scale.