Blood vessels are used to transport oxygen and nutrients, as well as to remove carbon dioxide and metabolic products. All tissues and organs of the body need nutrients and oxygen supply, and also need to transport metabolic products away, and blood vessels are used to accomplish this task. The blood vessels are divided into arteries and veins. In the body circulation, the arteries deliver oxygen and nutrients to all tissues and organs of the body and exchange them in the capillaries, while the products of the exchange gradually converge through the veins and return to the heart, forming a complete closed loop. And in the pulmonary circulation, the arteries are to bring the lack of itchy blood to the lungs to complete the blood exchange, and through the pulmonary veins back to the heart, these two systems will complete the confluence as well as exchange in the heart, the above is the role of blood vessels.