Blood vessels are tubes that start out big at the heart, and then branch out smaller and smaller and smaller and smaller as they reach through the body . . . just like the branches of a tree. Arteries are larger than arterioles, and veins are larger than venules. Capillaries are the smallest of all.

These tubes are where all the transport and exchanges of oxygen and carbon dioxide take place so your body can keep working. The exchange of oxygen and carbon dioxide happens within the capillaries.