Aneurysm. Human spleen with aneurysm of the splenic artery. The aneurysm has been cut open to expose the fatty atherosclerotic plaques on the distended artery walls (bottom of picture). An aneurysm is an abnormal dilation (ballooning) of the blood vessel occurring where the vessel wall is weakened. Pressure of blood in this region causes the wall to distend. Weakening of the blood vessel wall is often caused by atherosclerosis in which fatty plaques develop and coat the inner walls of arteries. Atherosclerosis can be a result of smoking. An aneurysm may exert pressure on surrounding organs, or may rupture, sometimes causing fatal blood loss.