Perfusion-limited distribution is primarily determined by which factor?

• A. Membrane permeability
• B. Blood flow
• C. Plasma protein binding
• D. Drug metabolism

Answer: (B)

Perfusion-limited distribution occurs when a drug can cross capillary walls quickly, so the speed and extent of its tissue uptake are governed mainly by how much blood reaches the tissue each minute. In this scenario, tissue blood flow becomes the rate-limiting factor: tissues with high perfusion receive drug faster and accumulate it more quickly, while poorly perfused tissues take up drug more slowly.

Membrane permeability would only control the rate if the barrier to entry were itself slow; when permeability isn’t the bottleneck, distribution is not limited by how easily the drug moves across membranes. Plasma protein binding affects how much drug is free to diffuse, which can influence overall extent, but it doesn’t determine the rate when perfusion is the controlling factor. Drug metabolism affects clearance and duration, not the immediate distribution rate.

So the factor chiefly determining perfusion-limited distribution is the rate of blood flow to the tissue.