How are blood vessels fit for purpose?

Arteries - carry blood at high pressure away from the heart. Their narrow lumen and thick, muscular wall allows them to do this effectively. The elastic, muscular wall allows for expansion and relaxation of the artery as the blood is pumped around the body by the heart. It also allows for contraction of the muscle, to maintain the blood pressure, or divert blood towards particular areas of the body.
Veins - carry low pressure blood towards the heart. Their large lumen, and thin walls allows them to do this. Furthermore, veins contain semi-lunar valves to help prevent backflow of blood.
Capillaries - these are usually in exchange surfaces. They have walls 1 cell thick to allow for effective exchange due to small diffusion distances. Blood flow through capillaries is slow, due to their very narrow lumen (just about wide enough for a RBC to squeeze through), which also allows for effective exchange.