Arterial aneurysm. Coloured angiogram (X-ray) showing an arterial dilation (aneurysm, lower left) in the carotid artery of a 50-year-old man. This artery (black/green) supplies blood to the brain. The front of the skull (pink) is at left. An aneurysm occurs when a weakened part of the arterial wall balloons outwards due to the high pressure of the arterial system. The expansion can damage surrounding tissues and if the aneurysm bursts there can be fatal blood loss.