A researcher needs to deliver a drug directly to the cells of an organ for rapid effect. Which blood vessel should the delivery target for optimal diffusion to surrounding cells?
Capillaries because of their one-cell thick walls
Exchange between blood and tissue happens only where the vessel wall is thin enough for substances to cross it. Capillaries have a wall just one cell thick and form a dense network threading between the cells of every organ, which is why oxygen, nutrients and drugs diffuse out there.
A drug released into the capillary bed therefore reaches the surrounding cells over the shortest possible diffusion path and acts fastest.
Arteries carry thick muscular and elastic walls built to withstand pressure, and veins are collecting vessels with valves; neither wall permits appreciable diffusion into tissue.
Hence, the answer is Capillaries because of their one-cell thick walls.
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