Which statement describes the relationship between plasma concentration changes and tissue distribution?

• A. There is no relationship between plasma and tissue concentrations.
• B. Plasma changes cause only tissue changes but not redistribution.
• C. Plasma concentration changes lead to redistribution between plasma and tissue.
• D. Tissue concentration determines plasma concentration with no effect back.

Answer: (C)

Distribution between plasma and tissues is a dynamic, bidirectional exchange driven by concentration gradients, tissue perfusion, lipophilicity, and binding. When plasma levels rise, more drug diffuses into tissues; as plasma levels fall (due to elimination or dosing changes), drug leaves tissues and returns to plasma. This back-and-forth movement, or redistribution, continues until a new equilibrium is reached. That’s why the statement describing redistribution as arising from changes in plasma concentration best captures the relationship.

In short, plasma concentrations don’t just affect tissues in one direction; the relationship is two-way, with movement between compartments responding to and reshaping both plasma and tissue levels.