The relationship between plasma concentration of a drug and the concentration of drug in the body is expressed as:

• A. Volume of distribution
• B. Equilibrium constant
• C. Plasma half life
• D. Clearance

Answer: (A)

Volume of distribution describes how a drug distributes between the plasma and the body's tissues, linking the amount of drug in the body to its plasma concentration. It is defined as Vd = amount in body / plasma concentration. A large Vd means the drug extensively leaves the bloodstream and accumulates in tissues, so for a given amount in the body, the plasma concentration is lower. A small Vd means the drug stays mostly in the plasma. This concept specifically describes the relationship between body burden and plasma level, whereas clearance and half-life describe how the drug is eliminated over time and how quickly plasma levels fall. In practice, Vd helps guide loading dose calculations and reflects factors like lipophilicity, tissue binding, and protein binding; in neonates, maturation and body composition can alter Vd and influence dosing. Example: if the body has 100 mg of drug and the plasma concentration is 2 mg/L, the Vd is 50 L.