A heart attack seems to strike out of nowhere, but it actually involves a sequence of steps that build over years to culminate in a crisis experienced over minutes or hours.

The typical fatality of a heart attack is a man over 65 years old, but heart attacks can occur at any age in men and women.

Women's heart attacks are more likely to involve "silent" symptoms that are harder to recognize than men's. About 1.2 million Americans suffer from heart attacks annually, and about 40 percent die from them.

Heart attacks can be signaled by nausea, dizziness, a shortness of breath, palpitations, unusual sweating and the classic pain in the chest that radiates to the extremities (called angina).

Inside, a catastrophic number of heart cells are starving to death for lack of oxygenated blood and the heart loses its ability to pump.

The pump

The heart is a muscular organ. The average heart beats 100,000 times and pumps about 2,000 gallons of oxygenated and nutrient-rich blood daily.

Blood first travels from the heart to the lungs, where it is re-oxygenated before it returns to the heart to be pumped out to a superhighway of arteries to supply oxygen and nutrients to the brain, digestive tract and the rest of the body's tissues and systems.

Blood from your veins flows into the atrium, then into the lower ventricle. Thick muscles pump blood into the arteries, which are about the diameter of a straw. The average heart beats 60 to 70 times a minute. A blood cell takes about a minute make a round trip through the body.

The first step leading to a heart attack is that the two primary coronary arteries supplying oxygen and nutrients to the heart muscle narrow as a result of fat that packs up over time along the walls of the arteries.

This build-up, called arteriosclerosis, forms a plaque that's hard on the outside and soft on the inside.

Down the line, when one is under physical or other stress, the plaque cracks or tears, and the body's emergency repair system rallies. Blood components called platelets stick to the fat that is now exposed by the crack or tear, forming a clot.

If your arteries are already narrowed, though, the clot can completely block the blood supply downstream to the heart. Within a few minutes, muscle cells in the heart undergo damage and begin to die; this is a myocardial infarction heart attack.

Usually, only part of the heart stops working; it keeps beating, but starts to lose the normal rhythm.

If a larger part of the heart is affected, it can stop beating entirely — cardiac arrest.

Every second counts

If you get to a hospital quickly, doctors can reopen the blood supply to the heart and prevent or even reverse the damage. In heart-attack treatment, every second counts.

Other heart attacks occur as a result of constricting spasms in the coronary arteries. If the arteries are already narrow, these spasms can cut off nutrients to the heart muscle and also result in a heart attack.

Heart attacks affect the region of the heart that was supplied by the blocked artery. If treatment works, in about eight weeks, new heart muscle grows back, but it is scar tissue and will never be never as strong as the original muscle.

Genetics determine who is at high risk for heart attacks. But we can lower the odds by doing things to keep arteries wide open.

Smoking tobacco is one of the worst things you can do, as it narrows the arteries. Exercise helps keep arteries clear and also staves off diabetes and obesity, which also narrow the arteries.

On the bright side, cardiac care has advanced significantly in the past 30 years. While heart attacks still kill, the death rate is much lower today than it once was.

In fact, after decades as the leading killer of Americans under age 85, heart disease gave way to cancer in the top spot, scientists reported in January 2005.

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