Blood vessels form a closed circuit that carries blood from the heart to the organs, tissues, and cells throughout the body and then back to the heart. The blood vessels include arteries, arterioles, capillaries, venules, and veins. Arteries carry blood away from the heart under high pressure. The arteries subdivide into smaller, thinner tubes called arterioles. As the arterioles approach capillaries, the walls of the vessels become very thin. Capillaries have the smallest diameter of all the blood vessels. They connect the arterioles with the venules. Venules continue from the capillaries to form the veins.