Atherosclerosis is the condition in which the inner layers of the artery wall, known as the intima, become thick and irregular due to deposits of fats (mainly in the form of cholesterol and another fat called a phospholipid) and other substances. Atherosclerosis can develop in different parts of the body, but when deposits occur in the arteries that supply blood to the heart, the condition is specifically called coronary atherosclerosis. As the buildup grows, the artery narrows and the flow of blood to the heart muscle is reduced.
Like any other muscle, the heart needs blood to provide it with oxygen. When that blood flow is reduced or completely blocked, some of the cells in the heart muscle can suffocate and die.
Atherosclerosis does not occur overnight. In most cases, the fatty buildups that can narrow the artery develop "silently" over decades. Pathologists divide the telltale signs of atherosclerosis into two major types--the earliest deposits, called fatty streaks, and the advanced deposits, called plaques.
Fatty streaks, which contain about 25 percent fat, can be seen even in the arteries of children. During the development of fatty streaks, the cells that line the arteries are stimulated to take in more cholesterol than they can handle. Although this process is not completely understood, it may begin when damage occurs to the endothelium, the thin layer of cells lining the inside of the artery.
Several factors are suspected of causing this damage, including cigarette smoking and high blood pressure. The key process, however, is a chemical change in LDL whereby LDL is oxidized. LDL becomes oxidized when free radicals, a form of oxygen that has been chemically modified into a highly unstable substance, encounter LDL. This stimulates atherosclerosis in several ways.
First, oxidized LDL is toxic to the walls of the arteries and encourages further injury. Second, it sends out chemical distress signals that cause more blood cells--called monocytes--to flock to the artery. The monocytes then penetrate the endothelium, where they try to "clean up" the problem by swallowing the oxidized LDLs.
Unfortunately, as they swallow the damaged LDLs, the monocytes themselves begin to swell within the endothelium. This sets up a vicious cycle in which the response of the body to this injury both starts and continues the process of building the cholesterol-engorged plaques.
The earliest visible deposits are called fatty streaks, which are present to some extent in all of us. But fatty streaks alone do not obstruct the flow of blood. They become a problem in people who have poor eating habits, which causes the fatty streaks to grow into dangerous and potentially deadly cholesterol-rich plaques.
Plaque is the hallmark of coronary atherosclerosis. It forms as more and more cholesterol is deposited at the site of the fatty streak. A simple plaque can grow into a complicated one when calcium accumulates and hardens the plaque and when blood clots develop.
Clots are a particularly feared complication; if the cap, or covering, of the fibrous plaque tears or ruptures, heavy bleeding occurs. This causes a local clot called a thrombus, which can obstruct the artery and cause a heart attack (also known as a coronary thrombosis). During a heart attack, the blood supply is cut off from a portion of the heart muscle, and that area of the heart dies. If enough of the heart is affected, or if the heart begins to beat rapidly and uncontrollably (called arrhythmia), the victim can die.
When the plaque grows more slowly, it may eventually reduce blood flow through the coronary arteries. If an artery is narrowed to 30 percent or less of its normal diameter, the situation is clinically described as angina, a pain or discomfort in the chest or shortness of breath. Angina usually occurs during a time of exertion, when the heart requires more oxygen than it needs while the body is at rest.
Not all those suffering from threatening heart disease experience the relatively mild warning of angina, however. Sometimes the first sign of heart disease is a full-blown heart attack.
And some consequences of cholesterol are less severe but unhealthy nonetheless. One such condition, metabolic syndrome, is detailed on the next page.
For more information on cholesterol, see:
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