Fig. 2. Hydraulic model analogy to three-compartment pharmacokinetic model. The height, h1(5), of the water level in the ith bucket is analogous to the drug concentration in the ith compartment. Each bucket is characterized by a cylindrical area, CA1, and the pipes connecting the buckets with each other or the outside world are characterized by a conductance, G1. Water enters bucket 1 at the rate r(t) and leaks irreversibly through G1. Reprinted with persmission from Hughes MA, Glass PSA, Jacobs JR: Context-sensitive half-time in multicompartment pharmacokinetic models for intravenous anesthetic drugs. Anesthesiology 1992; 76:334–41.