Vein cells called tunica intima. The precise location of veins varies much more from person to person than that of arteries. Veins serve to return blood from organs to the heart. Veins are also called "capacitance vessels" because most of the blood volume (60%) is contained within veins. In systemic circulation oxygenated blood is pumped by the left ventricle through the arteries to the muscles and organs of the body, where its nutrients and gases are exchanged at capillaries. After taking up cellular waste and carbon dioxide in capillaries, blood is channeled through vessels that converge with one another to form
Great arteries of the aortic arch. The 5th pair of vessels only form in some cases without any known contribution to the final structure of the great arteries. The right 6th almost completely regresses with only the proximal part contributing to the base of the pulmonary arteries, while the left 6th forms the ductus arteriosus, which disappears after birth. Abnormalities in the development of the great arteries during embryonic development may lead to congenital cardiovascular defects, such as: Some of these defects can be asymptomatic and cause no complications in patients, with others being more severe and require immediate surgery right after