There are 2 sorts of blood vessels in the body - Arteries and Veins.

Arteries take blood from the heart to the tissues to supply the tissue with oxygen and nutrients (food). The blood in the arteries is under high pressure and so the walls of the artery are thick.

Veins return blood back to the heart from the tissues, bringing back the waste products - carbon dioxide, water and waste metabolites (such as urea). Blood in the veins is under low pressure and the walls are thin. The role of veins in the body is explained in The Veins Website.

Aneurysm is the name given to the "swelling" or "ballooning" of an Artery that can occur. The blood in an artery is under high pressure from the beating of the heart and so both the flow of blood and the pressure pulses along the blood vessel. Therefore the wall of an artery has to be both strong enough to resist the pressure of the blood (so the artery doesn't split) and be elastic enough to return to it's normal shape when a pulse of blood passes through it.

An aneurysm occurs when the wall weakens and starts ballooning outwards, and it doesn't have the elastic properties anymore to return to it's origins state.

This website is concerned with the 2 commonest sorts of aneurysms that cause vascular surgical problems:

1) Abdominal Aortic Aneurysm (AAA)

AAA accounts for 10,000 deaths per year in the UK. Therefore the treatment of AAA is aimed to try to prevent death from this cause.

2) Popliteal Aneurysm

50% of people who thrombose (block off) their popliteal aneurysms need an amputation. Therefore the treatment of popliteal aneurysm is to try to save legs from amputation.