Capillary action is what happens when a liquid, such as water, rises into a very small diameter tube, against the force of gravity. The tube may be made of glass, or a small tube like a reed. This happens due to the intermolecular forces between the water molecules and the molecules of the container. Capillary tubes have long been employed in the medical industry, used extensively in lab work. Capillary action works effectively in the body, as well. Capillaries are the smallest blood vessels in the body, bridging the gap between arteries, which carry oxygenated blood away from the heart, and veins, which carry deoxygenated blood back to the heart. Capillaries are so small, they supply blood to the cells themselves, which is the smallest division in the body, as far as cellular structures go. Capillary action is instrumental in getting glucose and oxygen into the cells, where energy may be manufactured.