Illustration of an aneurysm of the aortic arch that has ruptured externally. The aorta, the body's main artery, carries oxygenated blood from the heart to the body. An aneurysm is a blood-filled dilation in a blood vessel wall. It is caused by a weakening of the vessel wall, which may be caused by hardening of the vessel (atherosclerosis), high blood pressure or injury. If left untreated an aneurysm can rupture, which is often fatal. Illustration by Thomas Godart.