During a heart attack, a blood clot blocks one of the coronary arteries that feeds the heart muscle, cutting off oxygen to part of the heart. Within minutes, the starved muscle cells begin to die. The longer the blockage lasts, the more tissue is permanently lost, which is why speed of treatment determines how much of the heart survives.
How the Blockage Forms
A heart attack almost always starts with a buildup of fatty plaque inside a coronary artery, a process that develops over years or decades. The plaque sits beneath the artery’s inner lining, covered by a fibrous cap that keeps it sealed off from the bloodstream. The trouble begins when that cap becomes extremely thin and tears open.
Once the cap ruptures, the material inside the plaque is exposed to flowing blood. This core material is highly reactive, and the body treats it the way it would any wound: it sends clotting factors to seal the breach. A blood clot (thrombus) rapidly forms at the rupture site. In a full-blown heart attack, the clot grows large enough to completely block the artery, halting blood flow to everything downstream. Not all heart attacks start with a rupture. Some begin with plaque erosion, where the surface wears away without tearing, but the end result is the same: a clot that chokes off blood supply.
What Happens to Starving Heart Cells
Heart muscle cells are energy-hungry. They rely on a constant supply of oxygenated blood to produce the fuel they need to contract. When that supply stops, a predictable cascade unfolds.
Within seconds, the affected cells switch from their normal oxygen-based energy production to a much less efficient backup system. This generates lactic acid and other waste products that accumulate quickly. The cells lose their energy reserves, and the chemical balance inside them starts to collapse. Potassium leaks out, calcium floods in, and the internal environment becomes increasingly acidic.
Animal studies show that irreversible cell damage begins after roughly 30 minutes of complete blockage. By the first few hours, large numbers of heart muscle cells are dying through multiple pathways, including a process where cells swell until their outer membranes rupture. This is why the phrase “time is muscle” is central to heart attack treatment. Every additional minute without blood flow means more cells cross the point of no return.
Why a Heart Attack Causes Chest Pain
The chest pain, pressure, or tightness you feel during a heart attack comes from sensory nerve endings embedded in the heart muscle. When oxygen is cut off, the buildup of lactic acid and inflammatory chemicals activates specific receptors on these nerve fibers, particularly a type of receptor that functions as an ischemia sensor, detecting when tissue is being starved. These nerves relay pain signals up through the spinal cord to the brain.
The reason heart attack pain often radiates to the left arm, jaw, or neck is that the heart’s sensory nerves share spinal cord pathways with nerves from those body regions. The brain has difficulty distinguishing the source, so it interprets the signals as coming from multiple locations at once. Some people, particularly women and those with diabetes, experience minimal or atypical pain because their cardiac nerve signaling is blunted.
How the Heart’s Electrical System Breaks Down
The heart beats because of a coordinated electrical wave that travels from cell to cell through specialized connections called gap junctions. During a heart attack, oxygen deprivation disrupts this system in a dangerous way.
In the first 10 minutes of ischemia, cells lose their normal electrical excitability as potassium leaks out and membranes become depolarized. Initially, this accounts for about 30% of the slowdown in electrical conduction. As minutes pass, rising calcium levels and falling energy reserves cause the gap junctions between cells to shut down, a process called cellular uncoupling. This is when things become most dangerous.
As cells uncouple from each other, the affected region of the heart becomes a patchwork: some areas can still conduct electrical signals, others are completely unresponsive, and others fall somewhere in between. This creates the perfect conditions for chaotic, disorganized electrical activity. The highest risk of a life-threatening rhythm called ventricular fibrillation occurs when cell-to-cell uncoupling has reached roughly the halfway point, because the mix of responsive and unresponsive tissue creates multiple competing electrical circuits. In ventricular fibrillation, the heart quivers instead of pumping, which is why a heart attack can trigger sudden cardiac arrest.
Heart Attack vs. Cardiac Arrest
These two events are related but fundamentally different. A heart attack is a circulation problem: a blocked artery prevents blood from reaching part of the heart. The heart typically continues beating, though it may beat abnormally. Cardiac arrest is an electrical problem: the heart abruptly stops beating because its rhythm collapses into something incompatible with pumping.
A heart attack can cause cardiac arrest (through the electrical disruption described above), but cardiac arrest can also happen independently, from inherited rhythm disorders, drug reactions, or other causes. During a heart attack, the person is usually conscious and experiencing symptoms. During cardiac arrest, the person loses consciousness within seconds and stops breathing. This distinction matters because the immediate response is different: a heart attack requires restoring blood flow, while cardiac arrest requires CPR and defibrillation to restart the heart’s rhythm.
How the Heart Heals After the Damage
Once blood flow is restored, the heart begins a weeks-long repair process. Unlike skin or liver, heart muscle cannot regenerate. The dead tissue is replaced with scar tissue, and the process follows a roughly predictable timeline.
Within the first 72 hours, immune cells flood the damaged area. Macrophages, monocytes, and neutrophils arrive to clear dead cells and debris. During this early phase, enzymes released by these immune cells break down the structural scaffolding between muscle fibers, causing the damaged wall to thin and stretch outward. This stretching, called infarct expansion, increases mechanical stress on the heart.
By about day seven, new collagen fibers become visible in the damaged zone. Over the next three weeks, collagen production ramps up dramatically. By day 28, the dead muscle is entirely replaced by a dense fibrous scar. This scar is structurally stable but cannot contract. It is stiff filler where flexible, pumping muscle used to be.
Long-Term Changes to Heart Function
The scar itself is only part of the story. After a heart attack, the entire heart undergoes a process called ventricular remodeling, where the surviving muscle tries to compensate for the lost tissue. Remaining muscle cells enlarge, and the heart’s overall shape and architecture shift to redistribute the increased wall stress.
When this process goes well, the heart stabilizes at a somewhat reduced but functional level. When it goes poorly, the heart progressively dilates, pulling border-zone tissue (the muscle surrounding the scar) into the damaged area. This worsens pumping efficiency and can lead to heart failure over months or years. A healthy heart pumps out about 55% to 70% of its blood volume with each beat (ejection fraction). After a large heart attack, that number can drop significantly, and whether it recovers depends on how much muscle was lost, how quickly blood flow was restored, and how effectively remodeling is managed with medication.
The size and location of the scar determine the practical impact. A small scar on the heart’s side wall may barely affect daily life. A large scar on the front wall, where the heart’s main pumping chamber does most of its work, can permanently reduce exercise capacity and increase the risk of dangerous rhythms for years afterward.