Explain why the same drug can have different Vd in different species?

Explain why the same drug can have different Vd in different species?

• A. Species differences in enzyme activity only.
• B. Differences in diet alone.
• C. Species differences in body composition, organ perfusion, protein binding, and barrier permeability influence distribution and partitioning, altering Vd.
• D. Vd is the same across species.

Answer: (C)

Volume of distribution reflects how a drug partitions between plasma and tissues. It is shaped by how extensively the drug leaves the blood and moves into body tissues, which varies across species. Differences in body composition (more or less body water vs fat), organ perfusion (rate at which tissues are supplied with blood), protein binding (only free drug can distribute), and barrier permeability (like the blood–brain barrier) all influence where the drug goes and how much accumulates in different compartments. Because these factors differ among species, the same drug can show a larger or smaller Vd in different animals. Enzyme activity mainly affects metabolism and clearance, not distribution; diet can influence some factors but doesn’t determine tissue partitioning as broadly; and Vd is not identical across species.

Volume of distribution reflects how a drug partitions between plasma and tissues. It is shaped by how extensively the drug leaves the blood and moves into body tissues, which varies across species. Differences in body composition (more or less body water vs fat), organ perfusion (rate at which tissues are supplied with blood), protein binding (only free drug can distribute), and barrier permeability (like the blood–brain barrier) all influence where the drug goes and how much accumulates in different compartments. Because these factors differ among species, the same drug can show a larger or smaller Vd in different animals. Enzyme activity mainly affects metabolism and clearance, not distribution; diet can influence some factors but doesn’t determine tissue partitioning as broadly; and Vd is not identical across species.