At equilibrium, tissue concentration Ct is related to venous concentration Cv by Ct = Kp * Cv. Which of the following expresses Ct in terms Cv?

• A. Ct = Kp * Cv
• B. Ct = Cv / Kp
• C. Ct = Cv
• D. Ct = Kp + Cv

Answer: (A)

The main idea is tissue-to-plasma partitioning at equilibrium, described by a dimensionless partition coefficient Kp. By definition, Kp equals Ct divided by Cv, so Ct = Kp × Cv. This shows the tissue concentration scales with the venous concentration by the factor Kp. If Kp is greater than 1, the tissue concentrates more drug than plasma; if Kp is less than 1, the tissue concentration is lower than plasma; if Kp equals 1, Ct equals Cv. The other expressions would imply an inverse or additive relationship or no partitioning at all, which doesn’t reflect how Kp governs the distribution between tissue and blood at equilibrium.