Pharmaceutics Distribution of Drugs Practice Exam

In a perfusion-limited distribution, the speed at which drug gets into tissue is controlled by how much blood is flowing to that tissue. The assumption is that the drug crosses tissue membranes quickly enough, so transfer into the tissue isn’t the bottleneck; instead, delivery is limited by the rate of perfusion. That means tissue drug levels rise and track the amount of blood flowing through the tissue and carrying drug with it.

Membrane permeability would matter if distribution were diffusion-limited, where the barrier to entry is the membrane itself. Plasma protein binding affects how much drug is free to distribute, but in this perfusion-limited scenario the rate-limiting step is blood flow, not the availability of unbound drug at the membrane. The rate of hepatic metabolism influences clearance from the body over time, altering overall exposure, but it doesn’t determine the immediate tissue levels in the perfusion-limited distribution phase the way tissue perfusion does.