Aortic aneurysm. Light micrograph showing a critical haematoma (blood-filled swelling, solid wedge at top) splitting the wall of the thoracic aorta. The aorta's inner wall (tunica intima) is at left and the outer wall (tunica externa) is at right and has been pushed outwards by the haematoma.The aorta is the body's main artery, carrying blood from the heart to the rest of the body. An aneurysm is a blood-filled dilation in a blood vessel caused by a weakened vessel wall. If it ruptures, it will be fatal. Treatment of the aneurysm is by repairing or replacing the weakened vessel.