Aortic aneurysm in lung tissue. Three sections of a gross clinical specimen showing an aortic aneurysm (swelling) which has protruded into lung tissue. The aorta is the main artery that takes blood from the heart to the body. An aneurysm is caused by a weakness in the wall of the aorta which then swells under the pressure of arterial blood. The most common cause of the weakening of the wall is atherosclerosis, the deposition of fatty plaques of atheroma on the inside walls of arteries. In this case, pressure from the swelling has compressed and damaged the lung. The aneurysm may rupture, leading to heavy internal bleeding. An aneurysm can often be surgically repaired by grafting a reinforcing implant to replace the weakened part of the artery, but the