Capillary << KAP uh lehr ee >> is the smallest blood vessel in the body. It can be seen only under the microscope. Most capillaries are so small that only one blood cell can pass through them at a time. Capillaries connect the smallest arteries with the smallest veins. They make up a network of blood vessels throughout the body. The largest capillaries are in the bone marrow and skin. The smallest are in the brain and lining of the intestine.
Capillaries have thin walls. Nutrients that are carried in the bloodstream are able to pass through their walls into the tissues. Waste materials from the tissues also pass into the bloodstream through capillary walls. In the capillaries of the lungs, oxygen goes through the walls into the blood, and carbon dioxide goes out. Other waste products pass through tiny capillary walls into the kidneys, intestine, and skin.
See also Blood; Circulatory system; Vein.
