How does the heart work?

Your heart is a muscle about the size of your fist. It sits in the middle of your chest, tipped so its point, the apex, leans to your left. We are looking at it from the front, so the patient's right side is on your left, just like a doctor facing you.

It is really two pumps joined together. The right side takes oxygen-poor blood (blue here) from the body and sends it to the lungs. The left side takes oxygen-rich blood (red) from the lungs and sends it to the whole body. A wall of muscle, the septum, keeps the two apart.

Each side has a small upper room, an atrium, that collects blood, and a big lower room, a ventricle, that pumps it out. Four valves act as one-way doors: the tricuspid and mitral valves let blood into the ventricles, and the pulmonary and aortic valves let it out. The left ventricle's wall is about three times thicker than the right's, because it has to push blood all the way to your toes.

The heart feeds itself too: the coronary arteries on its surface are the first branches off the aorta. A thin bag, the pericardium, wraps the whole heart so it can beat without rubbing.

Each heartbeat has two halves. In diastole the muscle relaxes and the ventricles fill. Blood flows in from the atria through the open tricuspid and mitral valves. Right at the end, the atria give a small squeeze, the atrial kick, to top the ventricles up.

In systole the ventricles squeeze. For a split second all four valves are shut and the pressure shoots up with no blood moving: isovolumetric contraction. When the pressure inside beats the pressure in the arteries, the aortic and pulmonary valves pop open and blood rushes out: ejection.

The sounds a doctor hears are valves snapping shut. “Lub” is the tricuspid and mitral valves closing as systole starts. “Dub” is the aortic and pulmonary valves closing as it ends.

Each beat pushes out about 70 mL, the stroke volume. That is roughly 55–70% of the blood in the full ventricle, the ejection fraction. Multiply by the heart rate and you get the cardiac output: about 5 litres a minute at rest. A squeezing chamber is a pump just like a syringe: see Pascal's principle for how pressure pushes a fluid, and Newton's laws for why pushing blood forward pushes back on the heart.

Nobody has to tell your heart to beat. A tiny patch of special cells in the right atrium, the sinoatrial (SA) node, fires an electrical spark by itself, about 60 to 100 times a minute at rest. It is the heart’s natural pacemaker.

The spark spreads across both atria like a ripple, and they squeeze. Then it reaches the atrioventricular (AV) node, which holds it back for about a tenth of a second. That pause gives the atria time to empty into the ventricles.

Next the signal races down the bundle of His in the septum, splits into the left and right bundle branches, and spreads through the fast Purkinje fibres. The ventricles squeeze from the bottom up, like squeezing toothpaste towards the cap, pushing blood up and out.

Electrodes on the skin pick up this wave as an ECG. The P wave is the atria firing, the tall QRS is the ventricles firing, and the T wave is the ventricles recharging. When the rhythm goes wrong, too fast, too slow or irregular, it is called an arrhythmia. Doctors read ECGs to spot them, and some people get an artificial pacemaker, a small device that sends the spark when the heart's own system can't. For the electricity itself, see energy.

Your blood travels round two loops, and it passes through the heart once for each.

In the pulmonary circulation, the right ventricle pumps oxygen-poor blood to the lungs. There it squeezes through capillaries thinner than a hair, wrapped round tiny air sacs, dumps carbon dioxide and picks up oxygen. It returns to the left atrium bright red, about 98% full of oxygen.

In the systemic circulation, the left ventricle pumps it through the aorta to every organ. In the body's capillaries it hands over oxygen and comes back through the veins to the right atrium, still about 75% full at rest. That spare oxygen is a reserve for when you run.

An adult has about 5 litres of blood, and at rest the heart pumps about 5 litres a minute. So a drop of blood makes the whole trip in about a minute. The heart muscle feeds itself through the coronary arteries, which take about 4–5% of each beat. William Harvey worked out this circulation in 1628, by showing that the heart pumps far more blood than the body could possibly make.

Each squeeze of the left ventricle pushes blood into arteries that are already full, so the pressure inside them rises and falls with every beat. The peak is the systolic pressure; the low point just before the next beat is the diastolic. A healthy adult reading is around 120/80 mmHg: “120 over 80”, in millimetres of mercury. (For how pressure pushes on everything around it, see Pascal's principle.)

To measure it, a cuff round the upper arm is pumped up until it squashes the artery flat. Then it is let down slowly, 2–3 mmHg a second. When the cuff drops just below the systolic peak, blood spurts through at the top of each beat and makes a tapping sound, a Korotkoff sound. When the cuff drops below diastolic, the artery stays open, the flow goes smooth and the tapping stops.

Stretchy arteries are the secret to a steady flow. The aorta balloons out with each beat and springs back between beats, like an air tank on a pump, the Windkessel effect. By the time blood reaches the capillaries the pressure has dropped to about 35 mmHg and barely pulses; in the veins it is under 10. Why fast narrow jets have lower pressure is Bernoulli's principle.

A reading of 140/90 or more, again and again, is called high blood pressure (hypertension). The WHO estimates 1.4 billion adults aged 30–79 had it in 2024; its 2023 profile for India counted about 188 million. Nearly half of the people who have it don't know, because it usually causes no symptoms. Only a proper check can tell, so it is worth having your blood pressure measured by a health worker, and a doctor can explain what your numbers mean.

Heart and blood-vessel diseases are the world’s leading cause of death, so they are worth understanding. This chapter shares general facts only. For anything about your own heart, talk to a doctor.

Narrowed arteries. Over many years, fatty deposits called plaque can build up inside the coronary arteries: atherosclerosis. Poiseuille's law says the flow through a tube, for the same push, goes with its radius to the fourth power, so halving the radius cuts the flow to one-sixteenth. Arteries cope with mild narrowing, but a severe one starves the muscle beyond it, which can cause chest pain on effort. If a plaque cracks, a clot can block the artery completely: a heart attack. Doctors can open arteries with a tiny balloon and a mesh tube, a stent.

Warning signs of a heart attack include pain or pressure in the chest that may spread to the arm, jaw or back, breathlessness, sweating and feeling sick. It is an emergency: call for help straight away (112 or 108 in India). Other problems: valves can become stiff or leaky, and in heart failure the heart can't pump as well as the body needs. Both are treatable, and a doctor can explain the options.

CPR. If someone collapses and isn’t breathing normally, call the emergency number and push hard and fast in the centre of the chest, 100–120 times a minute, 5–6 cm deep. Your hands become the pump, keeping some blood flowing to the brain until help or a defibrillator (AED) arrives. A certified course teaches it properly.

What protects the heart. The WHO lists the main risks: tobacco, unhealthy diet, too little physical activity, harmful alcohol, and the high blood pressure, blood sugar and cholesterol they can lead to; air pollution adds to them. Regular exercise makes the heart pump more per beat, which is why fit people often have a lower resting heart rate.