In fentanyl pharmacokinetics, which statement about the lungs is correct?

In fentanyl pharmacokinetics, which statement about the lungs is correct?

• A. Digestive metabolism
• B. Skeletal muscle uptake
• C. Lungs act as a temporary reservoir influencing kinetics
• D. Kidney excretion

Answer: (C)

The key idea is how fentanyl distributes in the body and how the lungs can influence that distribution. Fentanyl is very lipophilic and is taken up rapidly by highly perfused tissues, with the lungs receiving the entire cardiac output. Because the lungs are lipophilic and richly perfused, they can store a portion of the drug temporarily. This pulmonary reservoir slows the movement of fentanyl from the lung tissue back into the bloodstream, smoothing out the distribution phase and affecting how quickly plasma levels fall and how long the drug acts. This reservoir effect helps explain variations in onset and duration after administration. Digestive metabolism isn’t the main processing route for fentanyl; the liver is responsible, primarily via cytochrome P450 enzymes to inactive metabolites, with renal excretion playing a smaller role. Skeletal muscle uptake occurs but doesn’t drive the overall pharmacokinetic profile as strongly as the lung reservoir does. Kidney excretion is not the primary elimination pathway either.

The key idea is how fentanyl distributes in the body and how the lungs can influence that distribution. Fentanyl is very lipophilic and is taken up rapidly by highly perfused tissues, with the lungs receiving the entire cardiac output. Because the lungs are lipophilic and richly perfused, they can store a portion of the drug temporarily. This pulmonary reservoir slows the movement of fentanyl from the lung tissue back into the bloodstream, smoothing out the distribution phase and affecting how quickly plasma levels fall and how long the drug acts. This reservoir effect helps explain variations in onset and duration after administration.

Digestive metabolism isn’t the main processing route for fentanyl; the liver is responsible, primarily via cytochrome P450 enzymes to inactive metabolites, with renal excretion playing a smaller role. Skeletal muscle uptake occurs but doesn’t drive the overall pharmacokinetic profile as strongly as the lung reservoir does. Kidney excretion is not the primary elimination pathway either.