Atherosclerosis is the hardening and narrowing of the arteries. It is caused by the slow buildup of plaque on the inside of artery walls. Arteries are blood vessels that carry oxygen-rich blood from the heart to other parts of the body. Plaque is made up of fat, cholesterol, calcium, and other substances found in the blood. As it grows, the buildup of plaque narrows the inside of the artery and, in time, may restrict blood flow. There are two types of plaque:

• Hard and stable
• Soft and unstable

Hard plaque causes artery walls to thicken and harden. Soft plaque is more likely to break apart from the walls and enter the bloodstream. This can cause a blood clot to partially or totally block the flow of blood in the artery. When this happens, the organ supplied by the blocked artery starves for blood and oxygen. The organ's cells may either die or suffer severe damage. The resulting complications include:

• Coronary artery disease (heart attack and angina) - if the arteries that feed heart are affected.
• Stroke or transient ischemic attack - if the arteries that feed brain are affected.
• Peripheral arterial disease - if the arteries that feed legs, pelvis, or arms are affected.
• Renovascular hypertension - if the arteries that feed kidneys are affected. .

Diseases caused by atherosclerosis are the leading cause of illness and death in the United States.