Define volume of distribution (Vd) and interpret a large Vd.

• A. The actual physical volume in which the drug distributes.
• B. The volume that equals total body water.
• C. The volume of distribution equals the plasma volume.
• D. The theoretical volume that would contain the drug at the same concentration as plasma. A large Vd implies extensive tissue distribution or binding.

Answer: (D)

Volume of distribution is a theoretical amount that links how much drug is in the body to the concentration in plasma. It’s defined as Vd = (amount of drug in the body) / (plasma drug concentration). Because it’s not a real anatomical space, it doesn’t equal a physical volume like the plasma, total body water, or any specific compartment. Instead, it reflects how widely the drug distributes after administration.

A large Vd means the drug leaves the plasma and distributes extensively into tissues or binds strongly to tissue components. In practice, this results in a relatively low plasma concentration for a given dose, since most of the drug resides outside the plasma. This can occur with lipophilic drugs, drugs that bind tissue components, or drugs that sequester in fat, bone, or intracellular spaces.