what do the capillaries do
Science Main Index. Capillaries are the smallest blood vessels in the body. They allow water, oxygen, carbon dioxide, and other nutrient and waste substances to exchange between blood and surrounding tissues. Read on to learn more about capillaries, and what they do in your body.cience Main Index. Capillaries are the smallest blood vessels in the body. They allow water, oxygen, carbon dioxide, and other nutrient and waste substances to exchange between blood and surrounding tissues. Read on to learn more about capillaries, and what they do in your body.