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
Hereditary hemorrhagic telangiectasia noted by cyanosis (bluish discoloration of the lips and skin), clubbing of the fingernails (often encountered in chronically low oxygen levels), and a humming noise over the affected part of the lung detectable by stethoscope. The symptoms produced by AVMs in the liver depend on the type of abnormal connection that they form between blood vessels. If the connection is between arteries and veins, a large amount of blood bypasses the body's organs, for which the heart compensates by increasing the cardiac output. Eventually congestive cardiac failure develops ("high-output cardiac failure"), with breathlessness and leg swelling among other problems. If