Expansion of the vascular compartment would have what effect on drug concentrations?

• A. It increases the concentration gradient and enhances absorption.
• B. It leads to relative dilution and lower concentrations, reducing absorption.
• C. It causes immediate saturation of tissues.
• D. It has no effect on drug distribution.

Answer: (B)

Expansion of the vascular compartment increases the volume into which the drug can distribute. The same amount of drug is spread over a larger blood volume, so plasma concentration falls—this is relative dilution. Because distribution and absorption depend on concentration gradients, the lower plasma concentration reduces the driving force for the drug to move into tissues or into systemic circulation, leading to lower overall concentrations. It doesn’t raise the gradient, cause immediate tissue saturation, or leave distribution unchanged. So the effect is relative dilution with lower concentrations, reducing absorption.