What are capillaries?

Capillaries are the smallest blood vessels in the body. They form a network throughout the body for the exchange of oxygen, metabolic waste products, and carbon dioxide between blood and tissue cells. This exchange is possible due to the thin walls of the capillaries, which are only one cell thick. Blood flows from the heart to the arteries, which branch into smaller and smaller vessels, eventually becoming so small they are called capillaries. After the body has used the oxygen and nutrients in the blood, the capillaries then carry waste and carbon dioxide away from the cells and back to larger vessels that return to the heart.