How the Heart Works: Four Chambers, Four Valves and One Electrical Spark
The human heart is a muscular pump about the size of a fist that beats, in a resting adult, somewhere between 60 and 100 times a minute. It is really two pumps side by side: the right half sends blood to the lungs to pick up oxygen, and the left half sends that oxygen-rich blood around the rest of the body. This page explains how the parts fit together, where each beat starts, and the numbers that come up most often in quizzes.
Where the heart sits
The heart lies in the middle of the chest, in a space between the lungs called the mediastinum, with roughly two thirds of its mass to the left of the midline. Its pointed lower tip, the apex, aims down and to the left, which is why the beat is easiest to feel just below the left nipple and why many people think the whole heart is on the left. It is protected at the front by the breastbone and the rib cage: as our page on the bones of the body explains, the sternum and the 24 ribs make up a large part of the axial skeleton, and shielding the heart and lungs is one of their main jobs. An adult heart typically weighs around 250 to 350 grams, according to Encyclopaedia Britannica, and it is wrapped in a tough double-layered sac, the pericardium.
Four chambers, two circuits
Each half of the heart has two chambers: a thin-walled upper chamber, the atrium, that receives blood, and a thicker lower chamber, the ventricle, that pumps it out. A wall of muscle, the septum, keeps the two halves apart, so in a healthy adult heart oxygen-poor and oxygen-rich blood do not mix.
- Right atrium. Receives oxygen-poor blood returning from the body through the two largest veins, the superior and inferior venae cavae.
- Right ventricle. Pumps that blood into the pulmonary artery and on to the lungs. This short loop is the pulmonary circulation.
- Left atrium. Receives oxygen-rich blood back from the lungs through the pulmonary veins.
- Left ventricle. Pumps blood into the aorta, the body's largest artery, and from there to every organ. This long loop is the systemic circulation.
Because the left ventricle has to push blood around the whole body rather than just through the lungs, its wall is much thicker than that of the right ventricle. Two quiz traps follow from the circuits. First, the pulmonary artery is the one artery that carries oxygen-poor blood, and the pulmonary veins are the veins that carry oxygen-rich blood: arteries and veins are defined by direction (away from or towards the heart), not by oxygen. Second, the heart muscle does not feed itself from the blood inside its chambers. It has its own supply, the coronary arteries, which branch off the aorta just above the aortic valve; a blockage in one of them is what causes a heart attack.
Four valves keep blood moving one way
Valves made of thin flaps of tissue (leaflets or cusps) open to let blood through and snap shut to stop it flowing backwards. There are four:
| Valve | Between | Leaflets |
|---|---|---|
| Tricuspid | Right atrium and right ventricle | 3 |
| Pulmonary | Right ventricle and pulmonary artery | 3 |
| Mitral (bicuspid) | Left atrium and left ventricle | 2 |
| Aortic | Left ventricle and aorta | 3 |
The familiar "lub-dub" heard through a stethoscope is the sound of valves closing. The first sound comes as the tricuspid and mitral valves shut at the start of ventricular contraction; the second, shorter sound comes as the aortic and pulmonary valves shut when the ventricles relax. A doctor listening for a murmur is listening for turbulent flow, often through a valve that is narrowed or does not close fully.
Where the beat starts
Heart muscle does not wait for orders from the brain to contract. Each beat begins with an electrical impulse from a small cluster of specialised cells in the wall of the right atrium, the sinoatrial (SA) node, often called the heart's natural pacemaker. The impulse spreads across both atria, which contract and top up the ventricles. It then reaches the atrioventricular (AV) node, which holds it back for a fraction of a second so that the ventricles can fill, before passing it down the bundle of His and its branches to the Purkinje fibres, which make the ventricles contract from the bottom up, squeezing blood towards the outlets.
The nervous system adjusts the pace rather than creating it. As our page on the central nervous system describes, the brainstem controls automatic functions such as breathing and heart rate: signals carried by the vagus nerve slow the SA node down, while sympathetic nerves and the hormone adrenaline speed it up and make each contraction stronger. That is why a transplanted heart, whose nerves have been cut, still beats on its own.
The electrical activity can be recorded from the skin as an electrocardiogram (ECG). The Dutch physiologist Willem Einthoven, who developed a practical string galvanometer to record it in the early 1900s, received the 1924 Nobel Prize in Physiology or Medicine for discovering the mechanism of the electrocardiogram.
The numbers worth knowing
- Resting heart rate. The American Heart Association gives 60 to 100 beats per minute as the normal range for adults. Trained endurance athletes often sit below 60, which is usually not a sign of disease.
- Beats per day. At 70 beats a minute, a heart beats 100,800 times in 24 hours, which is why "about 100,000 a day" is the usual round figure.
- Output. Each contraction of the left ventricle ejects roughly 70 millilitres of blood at rest in an adult; multiplied by 70 beats, that is close to 5 litres a minute, which is about the whole volume of blood in an average adult body.
- Pulse. The pulse you feel at the wrist or neck is the pressure wave of each ventricular contraction travelling along the arteries, so counting it for 60 seconds gives the heart rate.
A short history
For many centuries, European medicine followed the Greek physician Galen, who taught that blood was made in the liver and consumed by the body. In 1628 the English physician William Harvey published De Motu Cordis ("On the Motion of the Heart"), which showed by experiment and calculation that the heart pumps the same blood round a closed circuit. He could not see the tiny capillaries that link arteries to veins; Marcello Malpighi observed them under the microscope in the lungs of a frog in 1661. The first human-to-human heart transplant was performed by Christiaan Barnard and his team at Groote Schuur Hospital in Cape Town on 3 December 1967; the patient, Louis Washkansky, lived for 18 days.
Finally, the heart shape that decorates cards has little to do with the organ, whose real outline is closer to a blunt cone. The symbol is one of many signs whose meaning is learned rather than natural, a theme that also runs through our guide to hand gestures around the world.
Quick quiz: the heart
This page explains anatomy for general interest. It is not medical advice: for chest pain, a racing or irregular pulse, or breathlessness, contact a doctor or emergency services.
Sources
- National Heart, Lung, and Blood Institute (NIH), How the Heart Works (consulted October 2026).
- American Heart Association, All About Heart Rate (Pulse).
- Encyclopaedia Britannica, Heart, William Harvey and Christiaan Barnard.
- The Nobel Prize, Willem Einthoven, Nobel Prize in Physiology or Medicine 1924.
- Harvey, W. (1628). Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus. Frankfurt.
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