Atherosclerosis, or hardening of the arteries, is a condition in which plaque builds up inside the arteries. Plaque is made of cholesterol, fatty substances, cellular waste products, calcium and fibrin (a clotting material in the blood). 

Atherosclerosis is a type of arteriosclerosis. Arteriosclerosis is a general term for the thickening and hardening of arteries.  

What damage does atherosclerosis cause?
Plaque may partially or totally block the blood's flow through an artery in the heart, brain, pelvis, legs, arms or kidneys. Some of the diseases that may develop as a result of atherosclerosis include coronary heart disease, angina (chest pain), carotid artery disease, peripheral artery disease (PAD) and chronic kidney disease.

Two things that can happen where plaque occurs are:

• A piece of the plaque may break off.
• A blood clot (thrombus) may form on the plaque's surface.

If either of these occurs and blocks the artery, a heart attack or stroke may result.

Atherosclerosis affects large and medium-sized arteries. The type of artery affected and where the plaque develops varies with each person.

Atherosclerosis is a slow, progressive disease that may start in childhood. In some people the disease progresses rapidly in their 30s. In others it doesn't become dangerous until they reach their 50s or 60s. However, it is normal to have some hardening of the arteries as you get older.

How does atherosclerosis start and progress?

It's a complex process. Exactly how atherosclerosis begins or what causes it isn't known, but some theories have been proposed. Many scientists believe plaque begins to form because the inner lining of the artery, called the endothelium, becomes damaged. Three possible causes of damage to the arterial wall are:

• Elevated levels of cholesterol and triglycerides in the blood
• High blood pressure
• Cigarette smoking

Smoking greatly aggravates and speeds up the growth of atherosclerosis in the coronary arteries, the aorta and the arteries of the legs.

Because of the damage, fats, cholesterol, platelets, cellular debris and calcium accumulate over time in the artery wall. These substances may stimulate the cells of the artery wall to produce other substances, resulting in the accumulation of more cells in the innermost layer of the artery wall where the atherosclerotic lesions form. These cells accumulate, and many divide. At the same time, fat builds up within and around these cells. They also form connective tissue.

The arterial wall becomes markedly thickened by these accumulating cells and surrounding material. The artery narrows and blood flow is reduced, thus decreasing the oxygen supply.

Often a blood clot forms and blocks the artery, stopping the flow of blood. If the oxygen supply to the heart muscle is reduced, a heart attack can occur. If the oxygen supply to the brain is cut off, a stroke can occur. And if the oxygen supply to the extremities is reduced, gangrene can result.

Source:http://www.heart.org/HEARTORG/Conditions/Cholesterol/WhyCholesterolMatters/Atherosclerosis_UCM_305564_Article.jsp#.WGxRdPmLSbg