If a drug has a volume of distribution of about 5 L, where is it mainly distributed?

If a drug has a volume of distribution of about 5 L, where is it mainly distributed?

• A. Plasma (intravascular)
• B. Interstital fluid only
• C. Intracellular tissues
• D. Bone and hair

Answer: (A)

The key idea is that volume of distribution (Vd) tells you how far a drug spreads from the blood into body compartments. A small Vd around the plasma volume (about 4–5 L in an average adult) means most of the drug stays in the intravascular space. So a Vd of roughly 5 L indicates the drug is largely confined to the bloodstream, possibly due to limited membrane permeability or binding to plasma proteins, keeping it in the central blood compartment rather than leaking into interstitial fluid or entering cells. If the drug mainly distributed into interstitial fluid, the Vd would be closer to the extracellular fluid volume (about 12 L). If it distributed extensively into intracellular tissues, bone, or other tissues, the Vd would be much larger, reflecting widespread distribution beyond the plasma. Therefore, a Vd around 5 L best corresponds to distribution primarily within the plasma.

The key idea is that volume of distribution (Vd) tells you how far a drug spreads from the blood into body compartments. A small Vd around the plasma volume (about 4–5 L in an average adult) means most of the drug stays in the intravascular space. So a Vd of roughly 5 L indicates the drug is largely confined to the bloodstream, possibly due to limited membrane permeability or binding to plasma proteins, keeping it in the central blood compartment rather than leaking into interstitial fluid or entering cells.

If the drug mainly distributed into interstitial fluid, the Vd would be closer to the extracellular fluid volume (about 12 L). If it distributed extensively into intracellular tissues, bone, or other tissues, the Vd would be much larger, reflecting widespread distribution beyond the plasma. Therefore, a Vd around 5 L best corresponds to distribution primarily within the plasma.