How Cardiovascular disease Works

Atherosclerosis begins when the delicate inner lining of arteries sustains damage from high blood pressure, smoking, or elevated cholesterol. In response, immune cells migrate to these injury sites and begin consuming the excess fats circulating in the bloodstream. These engorged cells, along with cholesterol crystals, smooth muscle cells, and calcium deposits, gradually build up into structures called plaques that bulge into the artery's channel.

As plaques grow over years or decades, they progressively narrow the space available for blood flow, much like mineral deposits constricting a water pipe. When coronary arteries—the vessels feeding the heart muscle itself—become 70% or more blocked, the reduced blood flow can't meet the heart's oxygen demands during exertion. This mismatch produces the characteristic chest pain of angina, a warning sign that tissues are being starved.

The location of blockages determines which organs suffer. Plaques in the carotid arteries limit blood to the brain, while those in peripheral arteries of the legs cause cramping during walking. The gradual nature of plaque buildup sometimes allows the body to develop collateral vessels—small detours around blockages—though these rarely provide adequate compensation for severely narrowed arteries.

The most dangerous plaques aren't necessarily the largest, but those with thin fibrous caps covering a lipid-rich core. When physical stress, blood pressure surges, or inflammatory processes cause these caps to rupture, the plaque's interior contents spill into the bloodstream. The body's clotting system, evolved to seal wounds, interprets this rupture as an emergency requiring immediate repair.

Platelets rushing to the scene stick to the exposed plaque material and to each other, while clotting proteins weave a fibrin mesh to create a solid plug. This clot formation happens within minutes, and unlike the gradual narrowing from plaque buildup, can completely obstruct an artery almost instantly. When this acute blockage occurs in a coronary artery, it causes a heart attack—myocardial infarction—as heart muscle downstream from the clot begins dying from oxygen starvation.

The clot can also break free and travel through the bloodstream as an embolus, lodging in smaller vessels downstream and blocking them instead. Whether the clot stays put or travels, the result is the same: tissues suddenly lose their blood supply. Time becomes critical—"time is muscle" in cardiology—because each minute of blockage means more irreversible tissue death.

Heart failure develops when the heart muscle becomes too weak or stiff to efficiently pump blood through the body. Previous heart attacks leave behind scar tissue that can't contract, reducing the heart's pumping capacity. Chronic high blood pressure forces the heart to work harder for years, causing the muscle to thicken abnormally—a process called hypertrophy—which makes the walls stiff and the chambers smaller, limiting how much blood they can hold and eject.

As pumping efficiency declines, blood backs up in the lungs and body tissues because the heart can't move it forward quickly enough. This congestion causes the hallmark symptoms of heart failure: shortness of breath as fluid accumulates in the lungs, and swelling in the legs and abdomen as fluid leaks into tissues. The kidneys respond to the reduced blood flow by retaining salt and water, paradoxically worsening the fluid overload.

The weakened heart tries to compensate through several mechanisms: it stretches to hold more blood, beats faster, and triggers hormonal systems that temporarily boost blood pressure and cardiac output. While helpful initially, these adaptations eventually become harmful, further straining the struggling heart. The progressive nature of heart failure means that without treatment, the condition typically worsens over time, with the heart becoming increasingly unable to meet the body's circulatory demands.

The heart's rhythmic contractions depend on precisely timed electrical signals that originate in the sinoatrial node—the heart's natural pacemaker—and spread through specialized conducting pathways. When cardiovascular disease damages these pathways or the heart muscle they traverse, the electrical system can misfire. Scar tissue from heart attacks blocks signal transmission, forcing electricity to take chaotic detours. Stretched heart chambers from heart failure disrupt the normally coordinated wave of activation.

Atrial fibrillation, the most common serious arrhythmia, occurs when the upper heart chambers quiver chaotically at rates exceeding 300 beats per minute rather than contracting uniformly. This happens when multiple rogue electrical signals fire simultaneously from various locations, overriding the natural pacemaker's authority. Blood swirls sluggishly in the fibrillating atria rather than being pumped forward efficiently, allowing clots to form that can travel to the brain and cause strokes.

Other arrhythmias involve dangerously slow heart rates when conducting pathways fail to transmit signals between chambers, or life-threatening rapid rhythms when damaged heart muscle generates repetitive electrical loops. Ventricular fibrillation represents complete electrical chaos in the main pumping chambers, producing no effective heartbeat at all—the heart quivers uselessly, and death follows within minutes unless normal rhythm is restored by defibrillation.

Stroke occurs when part of the brain suddenly loses its blood supply, causing neurons to die within minutes of oxygen deprivation. Ischemic strokes—accounting for 87% of cases—happen when a blood clot blocks an artery feeding the brain. These clots either form locally on atherosclerotic plaques in brain arteries, or more commonly, travel from elsewhere: the heart during atrial fibrillation, or plaques in the carotid arteries of the neck. Hemorrhagic strokes occur when weakened blood vessels rupture, often due to chronic high blood pressure that damages vessel walls over time.

Unlike other organs that can tolerate brief interruptions in blood flow, the brain has no energy reserves and extraordinarily high metabolic demands. Within seconds of a blocked artery, affected neurons stop functioning. After three to four minutes without oxygen, cells begin dying in an irreversible cascade. The stroke's core—the territory directly fed by the blocked vessel—suffers the worst damage, but surrounding areas in the "penumbra" remain viable for several hours, representing salvageable tissue if blood flow can be restored quickly.

The specific disabilities a stroke causes depend entirely on which brain region loses blood supply. A clot blocking the middle cerebral artery typically causes paralysis and numbness on the body's opposite side, along with language problems if the left hemisphere is affected. Posterior circulation strokes disrupt vision, balance, and coordination. Because different cardiovascular diseases create clots through different mechanisms—atrial fibrillation, carotid atherosclerosis, or heart valve problems—preventing stroke requires identifying and treating its cardiovascular source.