Atherosclerosis is a descriptive term to denote the appearance of plaques on the inner lining of our arteries namely "Athero" (meaning porridge or gruel) and "sclerosis" (meaning hardness). These plaques occur on the inner lining of an artery and are due to deposits of fatty substances like cholesterol, cells such as smooth muscle cells and macrophages as well as calcium and blood clots. These plaques develop in infancy and build-up over years. The rate at which plaques build up depends in part on one’s genes, cholesterol levels, smoking habits, blood pressure and presence or absence of diabetes. Usually, most people do not present with the problems related to plaque build up until they are in their fifties.

Atherosclerosis can affect large and medium sized arteries. Some arteries such as the arteries in the forearm or those on the inner lining of our chest wall called the internal mammary arteries, are less likely to develop atherosclerosis. When the plaques grow to a large size, they push into the inner channel in our arteries and thus limit the amount of blood the artery can transport. In addition to the build up of plaques, the plaques may also tear and cause a blood clot to develop at the site of the tear that can block the blood flowing down the artery. Sometimes bleeding can occur within a plaque that causes it to swell causing the plaque to push into the inner channel of the artery. This a major problem if it occurs in the coronary arteries which supply blood to the heart muscle as a heart attack can occur. If the blood supply to the brain is cut off, a stroke can occur and if the supply to the limbs is blocked, gangrene can result. (see angina, heart attack and stroke).

It is not exactly known what starts the atherosclerotic process but many researchers agree that it starts with damage to the inner lining of arteries which is called the endothelium. In addition, there is increasing evidence that certain spots on the inner lining of arteries become more sticky such that the circulating cholesterol particles can become trapped at specific sites.

As the saying goes, if you throw enough dirt at a wall some of it sticks. The same is true for cholesterol and fat. The more elevated the cholesterol levels the more fat that is available to stick to the wall. To continue with this analogy, certain parts of the wall may be more sticky and thus hold onto more of the fat thrown at it. It is likely that high blood pressure, smoking and diabetes all enhance the stickiness of the artery wall and thus the interaction of all of these risk factors would lead to a greater deposition of plaques. When these lipid or fat particles get stuck in the inner lining of the artery, they stimulate cells in the artery wall to produce substances which result in further accumulation of cells in the innermost layer of the artery. Macrophages are among the cells that reside within the inner lining of the arteries. These cells accumulate cholesterol particles and grow to form large fat filled cells called "foam cells". In addition to the formation of foam cells, connective tissue (or packaging) builds up inside the artery wall. As a result the innermost layer of the artery thickens and if this is severe, the artery becomes narrowed at specific sites. Your doctor may refer to this site as a "stenosis", "lesion" or "narrowing". They may also mention a figure to denote the degree of narrowing, e.g. a 50 % stenosis or a 75% stenosis.