Part of the cardiovascular system, the vasa vasorum is a network of small blood vessels that help supply larger vessels with blood. This phrase literally means "vessels of the vessels" in Latin, describing their function of providing blood and oxygen to arteries and veins that supply blood and oxygen to the rest of the body. The largest blood vessels in the body, such as the aorta, depend on this support network to maintain healthy function. Both oxygenated and deoxygenated blood are carried to and from larger vessels by these tiny blood vessels.
The vasa vasorum is needed to supply large arteries and veins because of their size. In order to effectively receive oxygen from the bloodstream, cells must be very close to a blood vessel or capillary so the oxygen can pass into each individual cell. Most blood vessels and veins absorb oxygen from the blood flowing inside them. However, because the large veins and arteries are by necessity so thick, their outer and middle cell layers cannot be adequately nourished without this additional network of blood vessels to support them by providing oxygenated blood and carrying away deoxygenated blood.
There are three major types of vasa vasorum, classified by where they originate and where they lead. The vasa vasorum internae originate from inside the main artery or vein and pass into the vessel's walls. Vasa vasorum externae originate in the main artery's branches, then return to the main artery or vein to nourish the cells farther away from the vessel's interior. Venous vasa vasorae have their origins in the main artery, then drain into the artery's concomitant vein, or "partner" vein. The exact structure and function of these blood vessels varies depending on which of these types it is and where it is located.
The function of vasa vasorum in supporting the aorta has been the subject of much study. In some areas, the human aorta does not have vasa vasorum, and in these areas, the aorta's walls are much thinner, making anyeurism more likely to occur in those locations. By contrast, dogs and some other mammals do have these vessels in these areas of the aorta, allowing the vessel walls to be thicker and less susceptible to anyeurism. This complex network of vessels is more commonly found in arteries than in veins, possibly because arterial walls tend to be thicker and more muscular than the walls of even the largest veins.