capillary

A capillary is the smallest kind of blood vessel and the place the circulatory system does its work. Arteries carry blood away from the heart under high pressure, veins carry it back at low pressure with valves to stop backflow, and between them the vessels narrow into capillaries, whose walls are thin enough for exchange: oxygen and nutrients leave the blood for the tissues, and carbon dioxide and wastes enter it. At the alveoli the capillary network wraps the air sacs, which is why the lung is the body's gas-exchange surface, and in the tissues the same networks deliver what the cells need. Blood flows fastest in the arteries and slowest in the capillaries, where the total cross-section is widest. A capillary is the one vessel with no job of its own besides exchange: direction, wall and valves separate artery from vein.