Place your hand on your chest and you can feel it: a steady rhythm that began before you were born and continues every moment of your life. The heart is a muscle about the size of a fist, yet it beats roughly 100,000 times a day and pumps about 2,000 gallons (7,500 liters) of blood through a network of vessels long enough to circle Earth more than twice. It delivers oxygen and nutrients to every cell and carries away waste. This explainer describes how the heart is built, how it keeps its rhythm and how blood travels through the body. It is an explanation of how the body works, not medical advice; if you have concerns about your heart, talk to a doctor.
The heart's job
Every cell in the body needs oxygen and nutrients to produce energy, and every cell produces waste, including carbon dioxide, that must be removed. Blood carries these substances, and the heart is the pump that keeps blood moving. Without circulation, cells begin to suffer within minutes; the brain is especially sensitive to a lack of oxygen.
Two pumps in one
The heart is really two pumps side by side, separated by a muscular wall called the septum.
- The right side receives oxygen-poor blood from the body and pumps it to the lungs, where it picks up oxygen and releases carbon dioxide. This is the pulmonary circulation.
- The left side receives oxygen-rich blood from the lungs and pumps it to the rest of the body. This is the systemic circulation.
Because the left side must push blood all the way around the body, its muscular wall is much thicker than the right side's.
Four chambers
Each side of the heart has two chambers:
- Atria (singular: atrium) are the upper chambers. They receive blood coming into the heart.
- Ventricles are the lower chambers. They pump blood out of the heart.
| Chamber | Receives blood from | Sends blood to |
|---|---|---|
| Right atrium | the body, through the superior and inferior vena cava | right ventricle |
| Right ventricle | right atrium | lungs, through the pulmonary artery |
| Left atrium | lungs, through the pulmonary veins | left ventricle |
| Left ventricle | left atrium | the body, through the aorta |
The aorta, which leaves the left ventricle, is the largest artery in the body, about an inch (2.5 centimeters) across in adults.
Four valves: one-way doors
To keep blood flowing in one direction, the heart has four valves, flaps of tissue that open and close with each beat:
- The tricuspid valve between the right atrium and right ventricle.
- The pulmonary valve between the right ventricle and the pulmonary artery.
- The mitral valve between the left atrium and left ventricle.
- The aortic valve between the left ventricle and the aorta.
The familiar "lub-dub" sound of the heartbeat comes from these valves closing. The "lub" is the sound of the tricuspid and mitral valves closing as the ventricles begin to contract. The "dub" is the sound of the aortic and pulmonary valves closing as the ventricles relax.
The journey of blood
Follow a single red blood cell through the heart:
- Oxygen-poor blood returns from the body into the right atrium.
- It flows through the tricuspid valve into the right ventricle.
- The right ventricle contracts, pushing the blood through the pulmonary valve into the pulmonary artery, which carries it to the lungs.
- In the lungs, blood passes through tiny vessels called capillaries, where it releases carbon dioxide and picks up oxygen.
- Oxygen-rich blood returns through the pulmonary veins to the left atrium.
- It flows through the mitral valve into the left ventricle.
- The left ventricle contracts powerfully, pushing blood through the aortic valve into the aorta.
- The aorta branches into arteries that carry blood to every part of the body. In the capillaries, oxygen and nutrients pass to the cells, and carbon dioxide and wastes pass into the blood.
- Veins carry oxygen-poor blood back to the right atrium, and the cycle starts again.
At rest, a full trip around the body takes about one minute.
Blood vessels
The circulatory system includes three main types of vessels:
- Arteries carry blood away from the heart. Their thick, elastic walls withstand high pressure.
- Capillaries are tiny vessels, some so narrow that red blood cells pass through in single file. Their thin walls allow oxygen, nutrients and wastes to pass between blood and tissues.
- Veins carry blood back to the heart. They have thinner walls and contain one-way valves that prevent blood from flowing backward, especially in the legs. Muscle contractions help squeeze blood upward through the veins.
A common misconception is that blood in veins is blue. Blood is always red: bright red when rich in oxygen and darker red when it carries less. Veins look bluish through the skin because of the way light is absorbed and scattered by skin and tissue.
The heart's electrical system
Unlike most muscles, which need a signal from the brain to contract, the heart has its own built-in electrical system. It can keep beating even when nerves to the heart are cut, as in heart transplants.
- The sinoatrial (SA) node, a small cluster of specialized cells in the wall of the right atrium, acts as the heart's natural pacemaker. It produces electrical impulses at regular intervals.
- Each impulse spreads across the atria, causing them to contract and push blood into the ventricles.
- The impulse reaches the atrioventricular (AV) node, which delays it briefly, about a tenth of a second, giving the ventricles time to fill with blood.
- The impulse then travels down specialized fibers, the bundle of His and the Purkinje fibers, spreading through the ventricles and causing them to contract from the bottom up, squeezing blood out efficiently.
An electrocardiogram (ECG or EKG) records this electrical activity through electrodes on the skin. The characteristic waves on an ECG show the spread of electrical signals through the atria and ventricles.
Heart rate
A typical resting heart rate for adults is between about 60 and 100 beats per minute. Well-trained athletes often have lower resting rates, sometimes in the 40s or 50s, because their hearts pump more blood with each beat.
Heart rate changes constantly to meet the body's needs. The autonomic nervous system adjusts it:
- The sympathetic nervous system, part of the "fight or flight" response, speeds up the heart and makes it contract more forcefully. Adrenaline has a similar effect.
- The parasympathetic nervous system, mainly through the vagus nerve, slows the heart during rest.
During exercise, the heart can pump four to six times as much blood per minute as at rest.
Blood pressure
Blood pressure is the force of blood pushing against the walls of arteries. It is measured in millimeters of mercury (mmHg) and written as two numbers, such as 120/80:
- Systolic pressure, the higher number, is the pressure when the ventricles contract.
- Diastolic pressure, the lower number, is the pressure when the heart relaxes between beats.
Blood pressure varies throughout the day and with activity. Persistently high blood pressure, called hypertension, can strain the heart and damage blood vessels over time, which is why doctors measure it regularly. Guidelines on what counts as normal or high vary slightly between countries; a doctor can interpret readings for an individual.
The heart's own blood supply
The heart muscle itself needs a constant supply of oxygen. It receives blood through the coronary arteries, which branch off the aorta and wrap around the surface of the heart. If a coronary artery becomes blocked, for example by a buildup of fatty deposits and a blood clot, the part of the heart muscle it supplies can be damaged. This is what happens in a heart attack, a medical emergency that requires immediate help.
How the heart adapts
The heart is remarkably adaptable. Regular aerobic exercise strengthens the heart muscle, allowing it to pump more blood with each beat. Over time, this can lower resting heart rate and blood pressure. The heart also responds to growth, pregnancy and aging. Health organizations recommend regular physical activity, a balanced diet, not smoking, managing stress and regular health checkups to support heart health.
Questions readers ask
How many times does the heart beat in a lifetime? At an average of about 70 beats per minute, the heart beats more than 2.5 billion times in an 80-year life.
Why does the heart make a "lub-dub" sound? The sounds come from heart valves closing: the first sound as the ventricles begin to contract, the second as they relax.
Is blood in veins blue? No. Blood is always red. Veins appear blue because of the way skin absorbs and reflects light.
What controls the heartbeat? The sinoatrial node, the heart's natural pacemaker, generates electrical impulses. The nervous system and hormones speed it up or slow it down.
What does blood pressure measure? The force of blood against artery walls, during contraction (systolic) and relaxation (diastolic) of the heart.
In short
The heart is a double pump with four chambers and four valves. The right side sends oxygen-poor blood to the lungs, and the left side sends oxygen-rich blood to the body. Its own electrical system, led by the sinoatrial node, sets the rhythm, while the nervous system adjusts the rate to meet the body's needs. Arteries, capillaries and veins carry blood to and from every cell, and blood pressure reflects the force of each beat. Day and night, the heart quietly keeps us alive, adapting to every activity of life.