Arteries are blood vessels that deliver oxygen-rich blood (except the pulmonary artery) from the heart to the tissues of the body, which have thick muscular walls and a narrow lumen. They are like balloons or pressure reservoirs, able to expand and recoil with every heartbeat. If they were rigid like pipes, they’d eventually leak or burst after being battered by so many waves of pressure.

Veins carry blood away from the tissues and towards the heart and have thin walls. Their internal lumen is larger than that of the arteries. This is due to the fact that they contain blood under low pressure. They also have valves that prevent the blood from flowing backwards.

Capillaries serve as a bridge between your arterial and venous systems. They may be small, but these are where the vital exchange of materials actually happens. Capillary walls are made of just a single layer of epithelial tissue, which form only the tunica intima, so they’re able to deliver the oxygen and other nutrients in your blood to their cellular destinations through diffusion.