An arteriovenous malformation or AVM is a disorder in which the blood vessels within a specific area of the body are incorrectly formed, resulting in multiple abnormal communications between the arteries and the veins. The circulation in the normal tissue consists of arteries, capillaries, and veins. The capillaries are tiny channels throughout every tissue of the body that carry arterial blood with its oxygen and nutrients to the cells of the body. After passing through capillaries, blood is collected by veins. The blood pressure in the normal veins is much lower than the blood pressure in the arteries. In an arteriovenous malformation, some of the capillaries are replaced by larger channels that connect the arteries directly to the veins. The direct connection is called a shunt because it shunts, or diverts, the blood from the artery directly to the vein, bypassing the capillaries. The channels between the artery and vein are termed the nidus. The nidus can consist of a vast number of tiny shunts, or a smaller number of large arteriovenous shunts. The number of arteries supplying an AVM and the number of veins draining it are quite variable. In a peripheral arteriovenous malformation, the shunt has the effect of reducing the blood supply with its nutrients and oxygen to the tissue around it, raising the blood pressure in the draining veins, and increasing the amount of blood that is pumped through the arteries and veins. In time, the shunting through a peripheral AVM damages the surrounding tissue and increases the work done by the heart.