Your heart is a four-chambered muscular pump that weighs about 11 oz and is about the size of your fist. Throughout your life, it delivers blood rich with oxygen and nutrients to all the body's tissues and organs, including the heart muscle itself. The demands on your heart occur nonstop. 
With every heartbeat, your heart pumps oxygenated blood through the aorta—the body's largest artery—to smaller arteries throughout your body. These arteries branch into even smaller blood vessels, called arterioles, and eventually into a microscopic network of capillaries that deliver oxygen and nutrients to every cell.
Of course, like every other organ, your heart needs its own supply of oxygenated blood. It receives this supply through the coronary arteries, which emerge from the base of the aorta. The two primary coronary arteries—the left main coronary artery and the right coronary artery—lie on the surface of the heart and branch off into a series of smaller blood vessels. The smallest of these vessels extend deep into the heart muscle.
In a healthy heart, blood flow through the coronary arteries increases to meet the heart's demand for oxygen. For example, the heart needs more oxygen during exercise than at rest. But as we age, the coronary arteries become thickened and narrowed by atherosclerosis and are less able to respond to increased oxygen demands. During the development of atherosclerosis, fatty deposits called plaques build up within the inner walls of the coronary arteries. When atherosclerosis impairs blood flow to the heart, the result is a condition called coronary heart disease (also known as coronary artery disease).
Heart attacks, which doctors call myocardial infarctions, typically occur in people with coronary heart disease when a coronary artery becomes completely blocked by a blood clot. This blockage cuts off the blood supply to the muscular layer of the heart's wall (myocardium), which plays a crucial role in the heart's pumping action. Such a blockage can produce crushing pain or pressure in the middle of the chest as well as tissue death (infarction) in the portions of the heart muscle normally supplied by that coronary artery.
Many people who suffer a heart attack have angina (episodes of pain, pressure, or squeezing in the chest caused by the buildup of plaques in the coronary arteries) and may experience more frequent or severe pain in the days leading up to a heart attack. Other people, particularly women, may experience less obvious heart attack warning signs such as shortness of breath, dizziness, or nausea. In many cases, however, a heart attack strikes suddenly and without warning in individuals who think they are healthy. But many of these people haven't been to a doctor in years and don't realize that their arteries are sustaining damage from high blood pressure or high cholesterol levels.
Once the heart muscle is deprived of oxygen, irreparable damage can develop within 20 to 30 minutes. In fact, half of the deaths from heart attacks occur within the first hour. That's why you need to get immediate medical help when symptoms suggest a heart attack. Doctors who treat heart attacks have coined a phrase to describe the importance of early treatment: "time is muscle."
Like the rest of the muscles in your body, the heart requires steady supply of oxygenated blood in order to function. But heart muscle cells cannot extract oxygen directly from the blood that passes through the heart’s chambers. Instead, the heart must pump blood to itself via the right coronary artery and left main coronary artery and their branches, which nourish every portion of the heart with oxygen and nutrients.
If blood flow through one of these vital arteries is obstructed by atherosclerotic plaque, the heart muscle cells supplied by that artery may not receive enough blood during periods of exertion, resulting in ischemia (oxygen deprivation to cells). In most cases, rest or medications such as nitrates can promptly restore sufficient blood flow to the area.
On the other hand, if a coronary artery becomes completely blocked by the formation of a blood clot at the site of a plaque, a heart attack occurs (see the illustration above). Unless the blood supply is restored within minutes, heart muscle cells will die from lack of oxygen.
The extent of permanent damage depends on the location of the blockage (the larger the affected artery, the more heart muscle will die), the availability of alternative (collateral) arteries to supply blood to the deprived area, and the promptness of treatment.
