The aorta is the main blood vessel that carries oxygen-rich blood from your heart to the organs in your body. An aneurysm is a bulge or swelling in a blood vessel. This type of bulge occurs when a blood clot or blood clots develop in the aorta, causing to expand. In your case, your doctor has determined that a portion of the aorta passing through your abdomen - the area between your legs and your chest - has developed a blood clot. In most cases these clots are caused by fatty deposits that build up inside the arteries. Aneurysms are dangerous because the blood clot weakens the blood vessel and can cause it to burst. The surgery your doctor has recommended will remove the blood clot and reinforce the weakened wall of the aorta.