Volume of distribution describes the relationship between the total amount of drug in the body and the plasma concentration.

• A. Plasma concentration and total amount of drug in the body
• B. Dose and plasma concentration
• C. Clearance and half-life
• D. Absorption rate and bioavailability

Answer: (A)

The main idea is how much drug is in the body compared with how much is found in the plasma. Volume of distribution is defined as the amount of drug in the body divided by the plasma concentration (Vd = A/C). This makes Vd the bridge between total body drug amount and what you measure in plasma. A large Vd means the drug spreads out into tissues and compartments beyond the plasma, often giving a lower plasma concentration for a given amount. A small Vd means the drug largely stays in the blood.

The other aspects describe different pharmacokinetic properties: dose with plasma concentration relates to how much you give versus what appears in plasma, but not the distribution ratio; clearance and half-life pertain to elimination; and absorption rate with bioavailability relate to how the drug enters circulation.