Figure 3-19 Hydraulic model of fentanyl pharmacokinetics. Drug is administered into the central tank, from which it can distribute into two peripheral tanks, or it may be eliminated. The volume of the tanks is proportional to the volumes of distribution. The cross-sectional area of the pipes is proportional to the clearance. (Adapted from Youngs EJ, Shafer SL: Basic pharmacokinetic and pharmacodynamic principles. In White PF [ed]: Textbook of Intravenous Anesthesia. Baltimore, Williams & Wilkins, 1997, p 10.)