Blood vessels are part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels.
Arteries carry blood away from the heart. Both arteries and veins have the same structure with three layers: tunica intima, tunica media, and tunica adventitia. The largest artery is the aorta, which carries blood out of the heart.
Capillaries enable the exchange of water and chemicals between the blood and tissues. Capillaries consist of a layer of endothelium and occasional connective tissue. Capillaries are the smallest blood vessels.
Veins carry blood from the capillaries back toward the heart. Veins are either arterial or venous, depending on whether the blood in it is flowing away from (arterial) or toward (venous) the heart. Most veins carry deoxygenated blood from the tissues back to the heart except for the pulmonary and umbilical veins. Veins are classified in a number of ways. Superficial veins are those that are close to the surface of the body and have no corresponding arteries. Deep veins are deeper in the body and have corresponding arteries. Pulmonary veins are a set of veins that deliver oxygenated blood from the lungs to the heart. Systemic veins drain the tissues of the body and deliver deoxygenated blood to the heart.