3D coloured computed tomography (CT) scan of the abdomen showing an aneurysm (swelling, lower centre) of the infrarenal aorta. The aorta is the body's main artery that distributes blood from the heart to the rest of 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.