An aneurysm or aneurism (from - aneurusma "dilation", from ἀνευρύνειν - aneurunein "to dilate") is a localized, blood-filled balloon-like bulge
Bulge may refer to:*Slang metonym for penis, referring to a visible protrusion through clothing*Hernia, sometimes referred to as a bulge or bulge out-Military:*A localized discontinuity in an extended military line...

 in the wall of a blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

. Aneurysms can commonly occur in arteries
Arteries are blood vessels that carry blood away from the heart. This blood is normally oxygenated, exceptions made for the pulmonary and umbilical arteries....

 at the base of the brain
Cerebral aneurysm
A cerebral or brain aneurysm is a cerebrovascular disorder in which weakness in the wall of a cerebral artery or vein causes a localized dilation or ballooning of the blood vessel.- Signs and symptoms :...

 (the circle of Willis
Circle of Willis
The Circle of Willis is a circle of arteries that supply blood to the brain...

) and an aortic aneurysm
Aortic aneurysm
An aortic aneurysm is a general term for any swelling of the aorta to greater than 1.5 times normal, usually representing an underlying weakness in the wall of the aorta at that location...

 occurs in the main artery carrying blood from the left ventricle
Ventricle may refer to:* Ventricle , the pumping chambers of the heart* Ventricular system in the brain* Ventricle of the larynx, a structure in the larynx* Stomach of the gastrointestinal tract...

 of the heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

. When the size of an aneurysm increases, there is a significant risk of rupture, resulting in severe hemorrhage, other complications or death.