A drug would leave slowly from tissues with a ____ Kp and a ____Q.

• A. High Kp and high Q
• B. Low Kp and low Q
• C. High Kp and low Q
• D. Low Kp and high Q

Answer: (C)

The key idea is how fast a drug can move out of tissue depends on two factors: how much the drug likes to stay in the tissue (the tissue/plasma partition coefficient, Kp) and how much blood is delivering and carrying it away (tissue blood flow, Q). If a drug has a high Kp, it tends to accumulate in tissue relative to plasma, so it is more “retained” in the tissue. If the blood flow to that tissue is low (low Q), the rate at which drug can be carried away from the tissue into the blood is reduced. Put together, a high Kp means strong tissue retention, and a low Q means slow removal by circulation. This combination leads to slow leaving from tissues.

If Q were high, the circulating blood would move drug away more quickly, offsetting the retention, and if Kp were low, there would be less tendency for the drug to stay in tissue in the first place.