Arteriovenous malformation or AVM is an abnormal connection between arteries and veins, bypassing the capillary system. Arteries and veins are part of the human cardiovascular system. Normally, the arteries in the vascular system carry oxygen-rich blood, except in the case of the pulmonary artery. Structurally, arteries divide and sub-divide repeatedly, eventually forming a sponge-like capillary bed. Blood moves through the capillaries, giving up oxygen and taking up waste products, including CO2, from the surrounding cells. Capillaries in turn successively join together to form veins that carry blood away. The heart acts to pump blood through arteries and uptake the venous blood.