compartment model

A compartment model is a simplified picture that treats the body not as a tangle of organs but as one or a few imaginary tanks of fluid into which a drug mixes. Each compartment is assumed to be well stirred, so the drug spreads evenly within it instantly; the model then tracks how the drug moves between tanks and out of the body. It trades anatomical realism for mathematical tractability.

In a one-compartment model the body behaves as a single tank — the drug equilibrates everywhere at once and the concentration falls as one smooth exponential. A two-compartment model adds a central compartment (blood and well-perfused organs) and a peripheral one (slower tissues like fat and muscle), producing a two-phase decline: a fast distribution phase as drug spreads into tissues, then a slower elimination phase.

These compartments are mathematical conveniences, not real boxes you could dissect out. Their value is practical: they let clinicians estimate parameters like volume of distribution, clearance, and half-life, and predict drug levels for any dosing schedule. Physiologically based models go further by using real organ blood flows and volumes, at the cost of far more complexity.