How Does the Heart Work? Chambers, Valves and the Electricity Behind Every Beat

Put your hand on your chest and you can feel it: a steady rhythm that has not stopped once since before you were born. Your heart beats roughly 100,000 times a day and more than 2.5 billion times in an average lifetime, without a single conscious command. It is a pump, an electrical system and a set of precision valves, all packed into an organ about the size of your fist.

A double pump

The heart sits in the middle of the chest, behind the breastbone and between the lungs, tilted slightly to the left. It is really two pumps side by side, separated by a muscular wall called the septum:

• The right side pumps blood to the lungs.
• The left side pumps blood to the rest of the body.

Each side has two chambers. The upper chamber, the atrium, collects blood coming in. The lower chamber, the ventricle, is thicker and more muscular and pushes blood out. The left ventricle has the thickest wall of all, because it must push blood all the way to your toes and brain.

The journey of a drop of blood

According to the US National Heart, Lung, and Blood Institute (NHLBI), blood travels through the heart in a fixed order:

1. Oxygen-poor blood returns from the body through two large veins into the right atrium.
2. It passes through the tricuspid valve into the right ventricle.
3. The right ventricle pumps it through the pulmonary valve to the lungs, where it releases carbon dioxide and picks up oxygen.
4. Oxygen-rich blood returns to the left atrium.
5. It passes through the mitral valve into the left ventricle.
6. The left ventricle pumps it through the aortic valve into the aorta, the body’s largest artery, and from there to every organ.

This creates two circuits: a short pulmonary circulation through the lungs, and a long systemic circulation through the body. At rest, the heart moves around 5 litres of blood a minute, roughly the whole volume of blood in an adult body.

Valves: one-way doors

The four valves are thin, strong flaps of tissue that open in one direction only. They open to let blood through and snap shut to stop it flowing backwards. That snapping is what a doctor hears through a stethoscope. According to the NHLBI, the familiar “lub-DUB” comes from two sets of valves closing: “lub” when the valves between atria and ventricles close, and “dub” when the aortic and pulmonary valves close.

When a valve does not close properly, some blood leaks backwards, creating an extra sound called a murmur. Many murmurs are harmless, but some point to valve problems that need treatment.

The electricity behind every beat

Unlike the muscles in your arms, the heart does not wait for instructions from the brain. It has its own electrical system:

• A small cluster of cells in the right atrium, the sinoatrial (SA) node, fires an electrical signal. It is the heart’s natural pacemaker.
• The signal spreads across both atria, which contract and push blood down into the ventricles.
• At the atrioventricular (AV) node, the signal pauses for a fraction of a second, giving the ventricles time to fill.
• The signal then races down special fibres into the ventricles, which contract powerfully from the bottom up, squeezing blood out like toothpaste from a tube.

The brain does influence the heart: nerves and hormones such as adrenaline speed it up during exercise or fear and slow it down during rest. But the basic rhythm comes from the heart itself. An electrocardiogram (ECG) records these electrical signals from the skin, and doctors use it to spot rhythm problems.

The heart in numbers

A normal resting heart rate for adults is generally 60 to 100 beats per minute. During hard exercise it can more than double, and the amount of blood pumped each minute can rise several times over. Trained athletes often have resting rates below 60, because their stronger hearts pump more blood with each beat.

Blood pressure measures the force of blood against artery walls. The first number (systolic) is the pressure when the ventricles contract; the second (diastolic) is the pressure between beats.

Feeding the pump

Although the heart is full of blood, its muscle cannot absorb oxygen from the blood inside its chambers. It has its own supply: the coronary arteries, which branch off the aorta and wrap around the heart. When one of these arteries becomes blocked, part of the heart muscle is starved of oxygen; this is a heart attack. Chest pain or pressure, shortness of breath and pain spreading to the arm or jaw are warning signs that need emergency help immediately.

Keeping your heart healthy

Heart disease is one of the leading causes of death worldwide, but much of the risk can be reduced. Regular physical activity, not smoking, a balanced diet with plenty of vegetables and whole grains, a healthy weight, enough sleep and keeping blood pressure, cholesterol and blood sugar under control all help the heart do its lifelong job.

In short

• The heart is a double pump: the right side sends blood to the lungs, the left side to the body.
• It has four chambers (two atria, two ventricles) and four valves that keep blood flowing one way.
• The “lub-dub” sound is the valves closing.
• Each beat starts in the sinoatrial node, the heart’s built-in pacemaker.
• At rest the heart beats about 60–100 times a minute and pumps about 5 litres of blood a minute.
• The heart muscle gets its own blood through the coronary arteries.