The heart is a carefully timed, self-regulating engine that keeps blood moving in the right direction, at the right pressure, to the right places. To understand how it works, it helps to look at its four chambers, its one-way valves, the pattern of its contractions, and the wiring that keeps everything coordinated.

Chambers

The right atrium is the first stop for blood returning from the body. This blood is low in oxygen and full of carbon dioxide and other waste, having just completed a loop through your organs and tissues. It enters through two large veins, the superior and inferior vena cava. This chamber also houses the sinoatrial (SA) node, the heart’s natural pacemaker. Every heartbeat starts here, triggered by a small burst of electrical activity.

From there, blood moves into the right ventricle, which pushes it toward the lungs through the pulmonary artery. This trip doesn’t require much force since the lungs are nearby and the resistance is low, so the right ventricle has relatively thin muscular walls. But if pressure builds up in the pulmonary circuit (as in chronic pulmonary hypertension), the right ventricle can start to thicken in response. This thickening is called right ventricular hypertrophy, and while it may help at first, it can eventually lead to problems.

Next is the left atrium, which receives freshly oxygenated blood from the lungs through the pulmonary veins. It’s usually a low-pressure environment, but if the mitral valve (which connects the left atrium to the left ventricle) becomes narrowed (a condition called mitral stenosis), the atrium has to work harder to push blood through. Over time, it can stretch and enlarge.

The left ventricle is the most powerful chamber. It pumps blood out to the entire body through the aorta, which means it needs to generate enough force to overcome systemic resistance, especially in high-pressure arteries like those supplying your brain or legs. That’s why the left ventricle has the thickest walls of all. If the aortic valve becomes narrowed (aortic stenosis), the ventricle has to push even harder, which can lead to concentric hypertrophy, a thickening of the muscle that, while adaptive at first, can eventually impair function and lead to heart failure.

Valves

The heart’s four valves are like turnstiles that let blood through in one direction only. Two sit between the atria and ventricles (the atrioventricular valves), and two sit at the exits of the ventricles (the semilunar valves).

On the right side, the tricuspid valve separates the right atrium from the right ventricle. It has three flaps, or cusps, tethered by thin cords called chordae tendineae, which prevent them from flipping backward under pressure. If the tricuspid valve becomes leaky (a condition called tricuspid regurgitation), blood can flow the wrong way and cause congestion in the veins, eventually leading to signs of right-sided heart failure like leg swelling or liver enlargement.

On the left, the mitral valve (also known as the bicuspid valve) separates the left atrium and ventricle. If it’s too floppy (mitral valve prolapse) or too narrow (mitral stenosis), blood flow gets disrupted, which can reduce cardiac output or lead to backup into the lungs.

At the exits of the heart are the pulmonary valve (right ventricle to pulmonary artery) and the aortic valve (left ventricle to aorta). These semilunar valves open during contraction and slam shut during relaxation, which creates the familiar “lub-dub” sound of a heartbeat. Their job is to make sure blood doesn’t come rushing back into the ventricles after it’s been pumped out.

The Cardiac Cycle

Each heartbeat has two main phases: systole and diastole.

During systole, the ventricles contract to push blood out. At this moment, the AV valves snap shut (producing the “lub” sound) and the semilunar valves open, which allows blood to leave the heart.

During diastole, the heart muscle relaxes and the ventricles fill back up. The semilunar valves close (producing the “dub” sound), and the AV valves open to let in new blood from the atria. This cycle repeats roughly once every second in a resting adult, with all the valves and chambers working in tightly choreographed sequence.

The Conduction Pathway

Behind the mechanical heartbeat is an electrical sequence that keeps everything in rhythm.

It starts in the SA node, which sends out a wave of depolarization across the atria, which triggers their contraction. That signal then hits the AV node, which introduces a slight delay just long enough for the ventricles to fill with blood. From there, the impulse travels down the Bundle of His, splits into the right and left bundle branches, and spreads through the Purkinje fibers, which causes the ventricles to contract together in a powerful squeeze.

If any part of this conduction pathway is damaged, the heart’s rhythm can become erratic. That’s how you get arrhythmias, which range from benign irregular beats to life-threatening conditions like ventricular fibrillation.

Coronary Circulation

While the heart pumps blood for the whole body, it also needs to feed itself. The first branches off the ascending aorta are the right and left coronary arteries, which wrap around the surface of the heart and supply the muscle with oxygen-rich blood.

If a coronary artery starts to narrow (usually because of a plaque building up along the vessel wall) blood flow to part of the heart muscle gets restricted. That area still gets some oxygen, but not quite enough to meet its needs, especially if the heart is working harder than usual. This kind of oxygen shortage is called myocardial ischemia, and while the muscle is under stress, the cells are still alive at this point.

Things change quickly if that plaque breaks open. When it does, a clot can form right on top of it, and if the clot grows large enough, it can block the artery completely. Now the oxygen supply to that region of the heart drops off entirely, and without oxygen, the tissue starts to die. That’s what we call a myocardial infarction, or heart attack: a section of heart muscle that’s been permanently damaged because blood couldn’t reach it in time.

Cardiac Output

How much blood the heart pumps each minute is called cardiac output. It’s calculated by multiplying heart rate (beats per minute) by stroke volume (the amount of blood pumped with each beat).

CO = HR × SV

Autonomic Control

The heart is constantly tuned by signals from the autonomic nervous system, which controls involuntary functions like heart rate, digestion, and pupil size.

When you’re stressed or exercising, the sympathetic system kicks in, where it releases norepinephrine and epinephrine, which speed up the heart and increase contractility to deliver more blood to your muscles.

When you’re resting, the parasympathetic system (especially the vagus nerve) slows the heart rate to conserve energy.

If this balance gets disrupted (for example, if the sympathetic system stays overactive for too long) it can contribute to problems like persistent tachycardia, palpitations, or in some cases, stress-related cardiac conditions such as arrhythmias or even cardiomyopathy.