Which expression correctly shows the relationship between tissue and venous concentrations at equilibrium?

Which expression correctly shows the relationship between tissue and venous concentrations at equilibrium?

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

Answer: (C)

At equilibrium, how a drug distributes between tissue and blood is captured by a tissue-to-blood partition coefficient, Kp. This coefficient tells you how much drug sits in the tissue relative to the blood. By definition, Kp = Ct / Cv, where Ct is the tissue concentration and Cv is the venous blood concentration. Rearranging gives Ct = Kp × Cv, which is why that expression correctly describes the relationship at equilibrium. This means the tissue concentration scales with the amount in venous blood by the factor Kp. If Kp is greater than one, tissue concentrates more drug than blood; if less than one, it concentrates less. The other forms aren’t consistent with the standard definition. Using Ct = Cv / Kp would imply Kp is defined as Cv/Ct, the reciprocal of the usual tissue-to-blood ratio. Using arterial concentration (Ct = Ca × Kp) would require a different or alternate definition of the partition coefficient. Multiplying by tissue volume (Ct = Vt × Cv) would mix in a volume term that isn’t part of the distribution ratio and would not reflect equilibrium partitioning.

At equilibrium, how a drug distributes between tissue and blood is captured by a tissue-to-blood partition coefficient, Kp. This coefficient tells you how much drug sits in the tissue relative to the blood. By definition, Kp = Ct / Cv, where Ct is the tissue concentration and Cv is the venous blood concentration. Rearranging gives Ct = Kp × Cv, which is why that expression correctly describes the relationship at equilibrium.

This means the tissue concentration scales with the amount in venous blood by the factor Kp. If Kp is greater than one, tissue concentrates more drug than blood; if less than one, it concentrates less.

The other forms aren’t consistent with the standard definition. Using Ct = Cv / Kp would imply Kp is defined as Cv/Ct, the reciprocal of the usual tissue-to-blood ratio. Using arterial concentration (Ct = Ca × Kp) would require a different or alternate definition of the partition coefficient. Multiplying by tissue volume (Ct = Vt × Cv) would mix in a volume term that isn’t part of the distribution ratio and would not reflect equilibrium partitioning.