Capillary: A tiny blood vessel that connects an arteriole (the smallest division of an artery) with a venule (the smallest division of a vein). Although tiny, the capillary plays an imortant role in the circulatory system. The walls of capillaries act as semipermeable membranes that permit the exchange of various substances, including fluids and the gases oxygen and carbon dioxide, between the blood stream and the tissues of the body.
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