A fist-sized, four-chambered pump seated between the lungs, the heart beats roughly 100,000 times a day without pause, moving the body's entire blood volume every minute. This guide maps every key structure and what it does.
| ~100,000 | Beats per day |
| ~5L | Blood pumped per minute at rest |
| ~7,500L | Blood pumped daily |
| ~300g | Average adult heart weight |
The heart is divided into four hollow chambers, functionally split into two separate pumps working in series — right heart and left heart — separated by the septum.
The upper-right chamber receives deoxygenated blood returning from the body via the superior and inferior vena cavae, plus drainage from the heart muscle itself via the coronary sinus. Its wall is thin, since it only needs to push blood a short distance into the right ventricle below.
A crescent-shaped chamber that receives blood from the right atrium and pumps it through the pulmonary valve into the pulmonary artery, toward the lungs for oxygenation. Its wall is roughly a third as thick as the left ventricle's, since it only has to generate enough pressure to perfuse the nearby lungs, not the entire body.
The upper-left chamber receives freshly oxygenated blood from the lungs via the four pulmonary veins. Like the right atrium, its walls are relatively thin, acting mainly as a reservoir and priming pump for the left ventricle.
The most muscular chamber of the heart, with walls up to 15mm thick. It receives oxygenated blood from the left atrium and ejects it through the aortic valve into the aorta, generating enough pressure to drive blood through the entire systemic circulation — from the brain to the toes. Left ventricular function is the single most common measure of overall cardiac health.
A thick muscular wall separating the left and right ventricles, preventing oxygenated and deoxygenated blood from mixing. Its lower, muscular portion is far thicker than the small membranous portion near the valves — a common site for congenital septal defects.
A double-layered fibroserous sac enclosing the entire heart. The outer fibrous layer anchors the heart within the chest and resists overstretching; the inner serous layer produces a thin film of lubricating fluid that lets the beating heart move freely against surrounding structures with minimal friction.
The thick, muscular middle layer of the heart wall, made of cardiac muscle cells (cardiomyocytes) that contract in a coordinated wave to generate each heartbeat. Unlike skeletal muscle, cardiomyocytes are interconnected by intercalated discs, allowing electrical signals to spread rapidly from cell to cell.
One-way valves ensure blood flows in a single direction through the heart, opening and closing purely in response to pressure differences on either side.
Situated between the right atrium and right ventricle, this three-leaflet (cuspid) valve prevents blood from flowing backward into the atrium during ventricular contraction. Its leaflets are anchored by chordae tendineae ("heart strings") to papillary muscles in the ventricle wall, which tension the valve to stop it inverting under pressure.
A three-cusped semilunar valve at the exit of the right ventricle, opening to let blood into the pulmonary artery during contraction and snapping shut during relaxation to prevent backflow into the ventricle.
Also called the bicuspid valve, it sits between the left atrium and left ventricle and has only two leaflets, unlike its three-leaflet counterpart on the right. It must withstand far higher pressures than the tricuspid valve, making it a common site for valve disease such as mitral regurgitation and prolapse.
A three-cusped semilunar valve guarding the exit from the left ventricle into the aorta. It endures the highest pressures of any heart valve and is the most common valve to calcify and narrow (aortic stenosis) with age, often eventually requiring surgical or transcatheter replacement.
The heart generates its own electrical rhythm — it does not need a signal from the brain to beat, which is why a transplanted heart continues beating in a new body.
A small cluster of specialised cells in the wall of the right atrium that spontaneously depolarises roughly 60–100 times per minute, initiating each heartbeat. Its rate is continuously adjusted by the autonomic nervous system — the vagus nerve slows it, sympathetic input speeds it up.
Located near the border between the atria and ventricles, the AV node receives the SA node's signal and deliberately delays it by about 0.1 seconds. This delay allows the atria to finish contracting and fill the ventricles completely before the ventricles themselves contract.
The signal leaving the AV node travels down the Bundle of His, which splits into left and right bundle branches running down either side of the interventricular septum, carrying the impulse rapidly toward the base of the ventricles.
A fine network of fibres spreading from the bundle branches throughout the ventricular walls, conducting the electrical impulse extremely quickly — up to 4 m/s — so that both ventricles contract in a near-simultaneous, coordinated squeeze rather than a slow, uneven ripple.
Despite pumping blood constantly, the heart muscle cannot feed itself from the blood passing through its chambers — it needs its own dedicated circulation.
The left and right coronary arteries branch from the aorta immediately above the aortic valve, the only vessels to originate there. The left coronary artery quickly splits into the left anterior descending (LAD) artery, supplying the front of the heart and much of the septum, and the circumflex artery, supplying the left side and back. The right coronary artery (RCA) supplies the right side and, in most people, the SA and AV nodes.
Most deoxygenated blood from the heart muscle drains through the cardiac veins into the coronary sinus, a short vessel that empties directly into the right atrium. A smaller amount drains via tiny thebesian veins directly into the heart chambers.
Because the coronary arteries are narrow and supply muscle that cannot tolerate interruption for more than a few minutes, they are a common site of atherosclerotic plaque build-up. A sudden blockage — a heart attack (myocardial infarction) — starves a region of muscle of oxygen, causing tissue death within 20–40 minutes if blood flow isn't restored.
Each heartbeat is a tightly choreographed cycle of electrical activation, muscular contraction, and valve movement, repeating roughly once per second at rest.
Over an average lifetime, the human heart beats roughly 2.5 to 3 billion times and pumps enough blood to fill more than three Olympic swimming pools. It performs this work continuously from around three weeks after conception until death, without ever fully resting.
Unlike liver tissue, mature heart muscle has very limited capacity to regenerate after injury. Cardiomyocytes lost during a heart attack are typically replaced by non-contractile scar tissue rather than new muscle, which is why the damage from a major infarction is largely permanent — a key reason cardiovascular disease remains a leading cause of death worldwide.
The electrocardiogram (ECG/EKG), developed by Willem Einthoven in the early 1900s, records the heart's electrical activity from electrodes on the skin. Its distinctive P wave, QRS complex, and T wave correspond respectively to atrial depolarisation, ventricular depolarisation, and ventricular repolarisation — making it possible to diagnose arrhythmias, heart attacks, and conduction disorders non-invasively.
Pain signals from the heart travel into the spinal cord at the same level as signals from the left arm, shoulder, and jaw, so the brain can struggle to distinguish their source. This is why the pain of a heart attack is so often felt radiating into the left arm or jaw rather than localised to the chest.
Because the heart's own SA node generates its rhythm independently of the nervous system, a transplanted heart — surgically disconnected from the recipient's nerves — will still beat on its own once reconnected to the circulation. It simply loses the fine, moment-to-moment autonomic adjustment a native heart has, responding to exertion more slowly via circulating hormones like adrenaline instead.
Human Heart Anatomy — A Field Guide — Educational reference · Cardiology & Human Anatomy