Which statement best defines volume of distribution and what a large Vd implies about tissue distribution?

• A. Vd relates the amount of drug in the body to plasma concentration; large Vd implies extensive tissue distribution.
• B. Vd is the physical volume of the body.
• C. Vd measures the rate of distribution.
• D. Vd equals clearance divided by rate constant.

Answer: (A)

Volume of distribution tells you how a drug spreads between the blood and the tissues. It's defined as the amount of drug in the body divided by the plasma concentration. When this value is large, it means most of the drug is distributing into tissues rather than staying in the plasma, indicating extensive tissue distribution. This often happens with lipophilic drugs or those that bind strongly to tissue components, so a given dose yields a relatively low plasma concentration compared with the total amount in the body. Remember, Vd is a theoretical construct, not a physical organ or space, and a large Vd can even exceed the volume of body water, reflecting widespread tissue uptake. In practice, a large Vd also relates to longer elimination in many cases (t1/2 = 0.693 × Vd / Cl), since more drug resides in tissue away from plasma while clearance acts.