An aortic aneurysm is a swelling of the wall of the aorta. The aorta is the main artery of the body that carries oxygenated blood out of and away from the heart. It is thus also the largest artery of the body which divides into smaller vessels that carry blood throughout all the tissues of the body. The aorta extends down through the thorax and into the abdomen, and thus an aneurysm can develop anywhere in these regions. A thoracic aneurysm naturally occurs where the aorta is much closer to the heart, while an abdominal aneurysm is found lower down where the aorta passes through the abdomen.