Generally speaking, the walls of large vessels are thicker than those of small vessels. Large blood vessels is the collective name of large arteries and large veins, in general, the aorta, the main trunk of the pulmonary artery and the largest branch called the large arteries, including the aortic arch, the thoracic aorta, the abdominal aorta, the common iliac artery, the subclavian artery, the axillary artery, the brachial artery and so on, corresponding to the veins for the large veins. From the appearance, large blood vessels have a large caliber, thick diameter, and the walls are rich in elastic fibers. As the branches of the vessels become thinner, the walls gradually become thinner and the elastic fibers are relatively reduced. Blood vessels are divided into arteries, veins, capillaries, capillaries for the smallest blood vessels, mainly composed of endothelial cells and basement membranes, the thinnest wall, conducive to the exchange of substances between blood and tissue fluids in the capillaries. That is why the walls of large blood vessels are thicker than those of small ones.