3.14 : Compartment Models: Two-Compartment Model

The two-compartment model assumes that drugs are not uniformly distributed throughout the body because the blood perfusion rate varies among different organs and tissues. So, this model divides the body into central and peripheral compartments.

The central compartment comprises blood and highly perfused tissues where the drug is rapidly distributed.

The peripheral compartment consists of tissues where the drug distribution is slow.

Following a single IV bolus dose, the drug concentration is high in the plasma and low in the tissues.

The distribution of the drug between the compartments is a first-order process defined by rate constants, termed as transfer constant or micro constant.

Due to distribution, drug concentration rapidly declines in the plasma and increases in the tissues until an equilibrium is reached.

After this, the drug concentration slowly declines in both compartments due to elimination.

In short, the plasma drug concentration declines bi-exponentially, where the rapid initial decline is the distribution or ɑ phase, and the subsequent decline is the elimination or β phase.