Atherosclerosis is one of the major causes of abdominal aortic aneurysm. This disease process can be seen in any blood vessel in the body and is the cause of coronary artery disease, stroke, and peripheral arterial disease.

When atherosclerosis develops in any blood vessel of the body, be it a large artery like the aorta or a small one like a coronary artery, a plaque forms inside the blood vessel. This plaque is made up of various cells, including macrophages, foam cells (macrophages with cholesterol in them), collagen, and free cholesterol. The plaque appears like a yellow, firm, shiny layer in the inside of a blood vessel.

The wall of the aorta (and all blood vessels) is a dynamic tissue made up of living cells, requiring nutrients and oxygen. Many of these nutrients seep from the inside of the blood vessel through the walls to nourish the rest of the blood vessel. When the inner lining of the vessel is covered with an atherosclerotic plaque, nutrients can no longer seep through sufficiently. The cells receive no oxygen, and some of them die. As the atherosclerosis progresses and cells continue to die, the walls become weaker and weaker.

At some point, a critical relationship is reached between the pressure experienced in the center of the blood vessel, the wall tension, and the strength of the wall itself. When this point is reached, the wall begins to dilate (grow larger) in the area of the plaque. As the diameter of the vessel grows, the wall tension increases, leading to even more dilation. The end result is an aneurysm.