A heart attack happens when blood flow to part of the heart muscle gets blocked, usually by a blood clot that forms inside a coronary artery. In the United States, someone has a heart attack every 40 seconds, adding up to roughly 805,000 cases per year. About 605,000 of those are a first heart attack.
The blockage starves heart muscle cells of oxygen. Within the first six hours, cells begin dying through a process called apoptosis (programmed cell death). After that, the damage shifts to outright tissue destruction. The longer the artery stays blocked, the more muscle is permanently lost, which is why speed of treatment matters so much.
How Plaque Builds Up in Your Arteries
The vast majority of heart attacks trace back to atherosclerosis, a slow buildup of fatty deposits (plaques) inside the walls of the coronary arteries. This process is driven by cholesterol-carrying particles in the blood that burrow into the artery wall and trigger inflammation. Over years or decades, the body responds with a cycle of inflammation, cell death, scar tissue formation, and calcium deposits that gradually thicken and stiffen the artery wall.
Not all plaques are equally dangerous. The ones most likely to cause a heart attack tend to have a large, soft core of dead cells and fat covered by a thin, fragile cap. These “thin-cap” plaques can look modest on an imaging scan but are primed to rupture. When that cap tears open, the contents of the plaque spill into the bloodstream. Your body treats this the way it treats any wound: it sends platelets and clotting proteins to the site, forming a blood clot. If that clot grows large enough to seal off the artery, blood flow stops and a heart attack begins.
A clot can also form without a full rupture. In a process called plaque erosion, the inner lining of the artery wears away over the surface of a plaque, exposing the tissue underneath. Platelets stick to the raw surface and a clot builds. Plaque erosion accounts for a meaningful share of heart attacks, particularly in younger patients and women.
The Risk Factors That Set the Stage
Heart attacks rarely come out of nowhere. They’re typically the end result of risk factors that have been damaging arteries for years.
High blood pressure. Blood pressure consistently at or above 130/80 mmHg is classified as hypertension. The higher your numbers climb, the more force your blood exerts against artery walls with every heartbeat. Over time, that pressure damages the inner lining of arteries, making it easier for cholesterol to infiltrate and plaques to form.
High cholesterol. LDL cholesterol (often called “bad” cholesterol) is the primary fuel for plaque growth. Current guidelines recommend keeping LDL below 100 mg/dL for people at moderate risk of heart disease, and below 70 mg/dL for those at high risk. People who already have heart disease and are at very high risk are advised to get their LDL below 55 mg/dL. The lower the LDL, the slower plaques grow and the less likely they are to rupture.
Diabetes. Chronically elevated blood sugar accelerates artery damage and plaque formation. Diabetes also carries a hidden danger: nerve damage. The same process that causes numbness in the feet can affect the nerves around the heart, a condition called autonomic neuropathy. When those nerves are dulled, you may not feel the typical chest pain of a heart attack. About 1 in 5 heart attacks are “silent,” meaning the damage happens without the person knowing it, and diabetes is a major reason why.
Smoking. Tobacco smoke damages the endothelium (the thin inner lining of arteries), promotes inflammation, raises LDL cholesterol, and makes blood more likely to clot. It attacks nearly every step in the process that leads to a heart attack.
Chronic inflammation. Inflammation isn’t just a consequence of plaque buildup; it actively destabilizes plaques and makes them more likely to rupture. A blood test called high-sensitivity CRP can measure general inflammation in the body. A level below 2.0 mg/L is considered lower risk, while levels at or above 2.0 mg/L are associated with higher heart attack risk. Levels above 8 or 10 mg/L are considered high and can signal significant inflammatory activity, though many conditions besides heart disease can raise CRP.
What Can Trigger a Heart Attack
Risk factors build vulnerable plaques over years. Triggers are the events that push a vulnerable plaque past its breaking point in a matter of minutes.
Intense physical exertion and strong emotional stress, especially anger, are two of the best-documented triggers. Both activate the sympathetic nervous system, the body’s “fight or flight” response. This floods the bloodstream with stress hormones that constrict blood vessels, spike heart rate and blood pressure, and make platelets stickier. All of those changes increase oxygen demand on the heart while simultaneously making a clot more likely to form on a plaque that was already fragile. A large international study called INTERHEART confirmed that both physical exertion and emotional upset can independently precipitate a heart attack through these mechanisms.
This doesn’t mean exercise is dangerous. Regular physical activity actually strengthens the cardiovascular system and reduces long-term risk. The danger comes from sudden, intense bursts of effort in people who are otherwise sedentary and already have significant plaque buildup.
Heart Attacks Without Plaque Buildup
While atherosclerosis is behind most heart attacks, it’s not the only cause. Two other mechanisms can cut off blood flow to the heart even in people with clean-looking arteries.
Coronary Artery Spasm
In some people, a segment of a coronary artery is abnormally prone to clamping down in response to stimuli that wouldn’t bother a normal artery. When such a spasm is severe enough, it can temporarily shut off blood flow entirely. The underlying problem is hyperreactivity in the smooth muscle cells of the artery wall. The inner lining of the artery may also be damaged, reducing its ability to produce nitric oxide, a natural chemical that keeps arteries relaxed and open.
Known triggers for coronary spasm include cold exposure, cigarette smoking, cocaine or stimulant use, and even hyperventilation (which changes blood chemistry in a way that promotes calcium influx into muscle cells). Platelets can also play a role: when activated, they release substances like serotonin and thromboxane that cause blood vessels to constrict. Spasm-related heart attacks can strike people who appear healthy and have no traditional risk factors, making them particularly alarming.
Spontaneous Coronary Artery Dissection (SCAD)
SCAD occurs when the inner layers of a coronary artery wall tear away from the outer layers. Blood seeps into the gap and forms a pocket (called a false lumen) that presses inward, narrowing or blocking the artery. Unlike a typical heart attack, SCAD most commonly affects young and middle-aged women, often those with few or no traditional cardiovascular risk factors.
Conditions that weaken blood vessel walls raise the risk: fibromuscular dysplasia (an abnormality in artery structure), connective tissue disorders like Ehlers-Danlos and Marfan syndrome, and hormonal factors including pregnancy, oral contraceptives, and fertility treatments. Systemic inflammatory diseases and recreational drug use are also linked to SCAD. Because it strikes people who don’t fit the typical heart attack profile, it can be missed or diagnosed late.
Why Speed of Treatment Matters
Once a coronary artery is blocked, the clock starts. Within one to three hours, heart muscle fibers in the affected area begin to warp and distort, though inflammation hasn’t fully set in yet. By four to twelve hours, cells are dying in large numbers, with visible tissue destruction, swelling, and bleeding into the damaged area. The wave of damage starts in the innermost layer of the heart wall and spreads outward toward the surface the longer blood flow is cut off.
This timeline is why the phrase “time is muscle” exists in cardiology. Every minute of blocked blood flow means more heart tissue lost permanently. Restoring flow within the first few hours can save a significant portion of the muscle at risk, preserving the heart’s ability to pump effectively for the rest of your life.