After iv drug administration, the drug is distributed to all parts of the body via the systemic circulation.
When the drug is rapidly distributed to all tissues and rapid equilibrium is established between the plasma and all tissues, this means that the body behaves as a single vessel or one compartment. This is despite the fact that the drug concentration in different tissues is different.
When the drug is initially distributed in the systemic circulation and surrounding tissues and then slowly distributed to other tissues, this means that the body behaves as two compartments.
