How the human heart works: chambers, valves and the beat you can watch

How to use this simulation

1. Press play and watch one beat: the atria squeeze first (orange on the timeline), then the ventricles (the long middle part), then everything relaxes.
2. Follow one blue dot from the venae cavae into the right atrium and down to the lungs; it comes back red through the pulmonary veins into the left side.
3. Watch the yellow valve flaps: the moment the ventricles start squeezing, the tricuspid and bicuspid valves slam shut and the panel shows "lub"; when the ventricles relax, the semilunar valves shut and you see "dub".
4. Move the heart-rate slider from 75 to 150 bpm: the cycle time, the phase bars and the ECG all speed up, and the cycle duration reading halves.
5. Change the stroke volume and watch cardiac output update, then use the highlight buttons to see the pulmonary and systemic circuits separately.

A pump that never takes a holiday

Put two fingers on the inside of your wrist, just below the thumb, and press gently. That little tap you feel is your heart at work. It started beating weeks before you were born, and it has not stopped since, not during sleep, not during exams, not even while you read this sentence.

At a resting rate of about 72 beats per minute, your heart beats 103,680 times in one day. Over 70 years that is about 2.6 billion beats, every one of them without you having to think about it. No machine made by people runs that long without a service.

Think about a water pump on the roof of a building in Dhaka or Delhi. It pulls water up from the tank and pushes it to every tap. Your heart does the same job with blood: it pulls blood in from the veins and pushes it out through the arteries, so that every cell gets oxygen and food and gets rid of its waste. This page shows exactly how, using a moving diagram you can control.

Where the heart is and how it is protected

The heart sits in the middle of the chest, in the space between the two lungs, behind the breastbone (sternum). It is tilted slightly, so about two-thirds of it lies to the left of the middle line. That is why you feel the beat most strongly on the left side, even though the heart is not fully on the left.

An adult heart is about the size of a closed fist, around 12 cm long, and weighs roughly 250 to 350 grams. It is shaped a bit like an upside-down cone: the broad base is at the top, where the big blood vessels attach, and the pointed tip (the apex) points down and to the left.

The heart is wrapped in a double-layered bag called the pericardium. Between its two layers is a thin film of pericardial fluid. This fluid works like oil in a machine: it lets the heart slide smoothly as it beats thousands of times an hour, reduces friction, and cushions the heart against small shocks. The rib cage and breastbone add a hard, bony shield on the outside.

The wall of the heart and its outside view

The heart wall has three layers. The outer thin layer is the epicardium. The thick middle layer is the myocardium, made of cardiac muscle, the special muscle that contracts rhythmically and never gets tired the way your arm muscles do. The inner smooth lining is the endocardium, which also covers the valves.

From the outside you can see a groove that runs around the heart separating the upper chambers from the lower ones (the coronary sulcus), and grooves on the front and back that mark where the two lower chambers meet. Small blood vessels called coronary arteries run in these grooves. They are the heart's own food supply: the heart muscle does not take oxygen from the blood passing through its chambers, it feeds through these coronary vessels.

At the top you can see the large vessels: the aorta arching over, the pulmonary trunk in front of it, the superior vena cava on the right side and the pulmonary veins at the back.

The four chambers and why the left ventricle is thickest

Inside, the heart is divided into a right half and a left half by a muscular wall called the septum. Blood from the two halves never mixes in a healthy heart. Each half has two rooms: an upper, thin-walled atrium (plural atria, also called auricles) that receives blood, and a lower, thick-walled ventricle that pumps blood out.

Notice the direction in every textbook diagram and in the simulation: it is a front view of someone facing you, so the heart's right side appears on your left. The right atrium and right ventricle are drawn on the left of the picture.

Why do the walls differ so much? A chamber's wall is as thick as its job is hard. The atria only push blood a few centimetres down into the ventricles, so they have thin walls. The right ventricle pushes blood to the lungs, which are close by and offer little resistance, so its wall is moderately thick. The left ventricle must push blood to the brain, the toes and every organ in between, against much higher pressure, so its wall is the thickest of all, roughly three times as thick as the right ventricle's. In the simulation the left ventricle is drawn with the thickest outline for exactly this reason.

Valves and the lub–dub heart sounds

Valves are one-way doors. They open when blood pushes from the correct side and slam shut when blood tries to flow backwards. The heart has four of them.

Between the right atrium and right ventricle is the tricuspid valve, with three flaps. Between the left atrium and left ventricle is the bicuspid valve, with two flaps; it is also called the mitral valve because it looks like a bishop's mitre (hat). Together these are the atrioventricular (AV) valves. Thin strings called chordae tendineae, attached to small papillary muscles, hold the flaps so they cannot flip back into the atria when the ventricles squeeze.

At the exits of the ventricles are two semilunar valves, each with three half-moon-shaped pockets: the pulmonary valve at the start of the pulmonary artery and the aortic valve at the start of the aorta. They stop blood from falling back into the ventricles when the ventricles relax.

The sounds a doctor hears through a stethoscope are the sounds of valves closing. The first sound, "lub", is longer and lower: the AV valves shutting as ventricular systole begins. The second sound, "dub", is shorter and sharper: the semilunar valves shutting as the ventricles begin to relax. In the simulation, watch the panel: "lub" flashes exactly when the tricuspid and bicuspid flaps close, and "dub" exactly when the semilunar flaps close.

Arteries, veins and capillaries

The heart pumps blood into a network of tubes. Arteries carry blood away from the heart. Veins carry blood towards the heart. Capillaries are tiny vessels, with walls only one cell thick, that connect the smallest arteries to the smallest veins; this is where oxygen, glucose and waste actually pass between blood and cells.

A common trap: "arteries carry oxygenated blood" is true for most arteries but not all. The pulmonary artery carries deoxygenated blood from the heart to the lungs, and the pulmonary veins carry oxygenated blood from the lungs to the heart. The definition is about direction (away from or towards the heart), not about oxygen.

Double circulation: two loops, one heart

In humans, blood passes through the heart twice for every complete trip around the body. That is called double circulation, and the simulation draws both loops so you can follow them.

The pulmonary circuit is the short loop: right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium. In the lungs, blood drops its carbon dioxide and picks up oxygen, so it turns from blue to red in the diagram.

The systemic circuit is the long loop: left ventricle → aorta → arteries → capillaries in every organ → veins → superior and inferior venae cavae → right atrium. In the body's capillaries, blood gives oxygen to the cells and takes their carbon dioxide, so it turns from red back to blue.

Why is this worth having? Because the two sides never mix, the body always gets fully oxygenated blood at high pressure. Fish have a two-chambered heart and a single circulation, so blood loses pressure passing through the gills. Frogs have three chambers with some mixing. Birds and mammals, which keep a warm body temperature and need a lot of energy, have four chambers and complete double circulation.

• Pulmonary circuit: heart → lungs → heart (short, low pressure).
• Systemic circuit: heart → body → heart (long, high pressure).
• Blood path in one line: venae cavae → RA → RV → pulmonary artery → lungs → pulmonary veins → LA → LV → aorta → body.

The cardiac cycle: one heartbeat, step by step

One cardiac cycle is everything that happens in one heartbeat. Contraction is called systole and relaxation is called diastole. The time for one cycle is simply 60 seconds divided by the heart rate: at 75 bpm, 60 ÷ 75 = 0.8 s.

Textbooks split those 0.8 s into three phases. Atrial systole (about 0.1 s): the atria squeeze and top up the ventricles, which are already mostly full. Ventricular systole (about 0.3 s): the ventricles squeeze, the AV valves shut (lub), the semilunar valves open and blood rushes into the pulmonary artery and aorta. Joint diastole (about 0.4 s): all chambers relax, the semilunar valves shut (dub), and blood flows from the veins into the atria and straight through the open AV valves into the ventricles.

There are two short moments when all four valves are closed: right after "lub" the ventricles are building up pressure but have not yet pushed the semilunar valves open (isovolumetric contraction), and right after "dub" they are relaxing but not yet low enough for the AV valves to open (isovolumetric relaxation). In the simulation's "Valves open" reading you will see "All closed" flash during these moments.

A note on the numbers: many textbooks (NCERT and NCTB included) use 72 beats per minute and a 0.8 s cycle. Strictly, 60 ÷ 72 = 0.83 s; 0.8 s is exact at 75 bpm, which is why the simulation starts there. Also, the simulation shrinks all three phases equally when you raise the rate. A real heart mostly shortens diastole, which is why a very fast heart has less time to fill.

The pacemaker: who tells the heart to beat?

Your heart beats even if the nerves to it are cut; a heart removed for a transplant can keep beating for a while. That is because the signal for each beat is made inside the heart itself. Such a heart is called myogenic.

The natural pacemaker is the sinoatrial node (SA node), a small patch of special muscle in the wall of the right atrium, near where the superior vena cava enters. It fires an electrical impulse about 70 to 75 times a minute at rest. The impulse spreads across both atria, making them contract.

It then reaches the atrioventricular node (AV node), near the bottom of the wall between the atria. The AV node holds the signal back for about a tenth of a second. That little delay gives the atria time to finish emptying into the ventricles.

From the AV node the signal runs down the bundle of His inside the septum, splits into right and left bundle branches, and spreads through the Purkinje fibres in the ventricle walls. The ventricles then contract from the tip upwards, squeezing blood up and out like toothpaste from a tube.

Nerves and hormones do not start the beat, but they change its speed. When you run or feel scared, sympathetic nerves and the hormone adrenaline speed the SA node up; when you rest, the vagus nerve slows it down.

• SA node (pacemaker) → atria contract
• AV node (short delay)
• Bundle of His → right and left bundle branches
• Purkinje fibres → ventricles contract

ECG basics: reading P, QRS and T

An electrocardiogram (ECG or EKG) records the heart's electrical signals from electrodes stuck on the skin. It does not measure blood flow; it measures the electrical waves that trigger each squeeze. The simulation draws a simple schematic trace that moves in time with the phases.

The P wave is small and rounded: the atria being electrically activated (depolarised) just before they contract. The QRS complex is the tall, sharp spike: the ventricles being activated, just before ventricular systole. The T wave is a gentle hump: the ventricles recovering (repolarising) as they relax. The atria recover too, but that small signal is hidden inside the QRS.

Doctors read the spacing as well as the shapes. The distance from one R peak to the next is one full cycle, so heart rate = 60 ÷ (R–R interval in seconds). Only a trained doctor can interpret a real ECG; this page is about understanding what the waves mean, not about reading your own report.

Pulse, blood pressure and cardiac output

Each time the left ventricle pushes blood into the aorta, a pressure wave travels along the arteries. Where an artery runs close to the skin over a bone, you can feel that wave as the pulse: at the wrist (radial artery) and at the side of the neck (carotid artery). Counting it for 15 seconds and multiplying by 4 gives your heart rate. A normal resting adult rate is roughly 60 to 100 beats per minute; children and babies have faster rates, and trained athletes often slower ones.

Blood pressure is the push of blood on artery walls, measured in millimetres of mercury (mm Hg) with a sphygmomanometer. It is written as two numbers, such as 120/80. The top number (systolic) is the highest pressure, during ventricular systole; the bottom number (diastolic) is the lowest, while the heart relaxes. 120/80 mm Hg is the typical healthy adult value used in textbooks. Readings that stay high on repeated checks are called hypertension, and only a doctor should decide that.

Cardiac output is how much blood one ventricle pumps per minute: cardiac output = stroke volume × heart rate. With the simulation's defaults, 70 mL × 75 = 5,250 mL, or 5.25 litres every minute. An adult has only about 5 litres of blood, so the whole amount goes round roughly once a minute.

Try these in the simulation

Each experiment takes less than a minute and makes one idea stick.

Solved problems

Every answer below is worked from the simulation's own model, so you can check it on the readings panel.

Common mistakes

Most lost marks in heart questions come from these few slips. Check each one before you hand in.

• Drawing the right atrium on the right of the page. In a front view the heart's right side is on your left.
• Saying all arteries carry oxygenated blood. The pulmonary artery carries deoxygenated blood.
• Mixing up the valves: tricuspid is on the right, bicuspid (mitral) on the left.
• Saying "lub" is the semilunar valves. "lub" is the AV valves closing; "dub" is the semilunar valves closing.
• Thinking the heart sound is the muscle squeezing. The sounds come from valves closing.
• Saying the right ventricle is thickest. The left ventricle is thickest because it pumps to the whole body.
• Thinking the brain starts each beat. The SA node inside the heart starts it; nerves only change the speed.
• Writing cardiac output in mL without converting when the question asks for litres per minute.

Looking after your heart

Heart disease is one of the leading causes of death in both Bangladesh and India, but many of its causes are habits we can change, and those habits start in school.

Move every day. Health bodies such as the WHO suggest at least an hour of activity a day for children and teenagers: playing cricket or football, cycling to school, skipping rope, even brisk walking with friends. Exercise makes the heart muscle stronger, so it pumps more blood per beat and can beat more slowly at rest.

Go easy on salt. South Asian food is often salty, from pickles (achar) and chanachur to chips, instant noodles and extra salt sprinkled on rice or fruit. Too much salt raises blood pressure over the years. The WHO recommends less than 5 g of salt a day for adults, about one level teaspoon in total.

Never start smoking, and avoid jarda, gul, gutkha and other tobacco. Tobacco narrows and damages blood vessels. Secondhand smoke harms the heart too. Sleep well, drink water instead of sugary soft drinks, eat plenty of vegetables, lentils and fruit, and choose fish or nuts over deep-fried snacks most of the time.

Common heart problems in brief

Knowing the names helps you follow health news and talk to a doctor; it does not replace a check-up.

Exam corner

Diagram questions are almost guaranteed. Practise drawing a labelled front view with the four chambers, the septum, all four valves, the venae cavae, pulmonary artery, pulmonary veins and aorta, and use arrows to show the direction of blood. Colour or mark deoxygenated and oxygenated sides if allowed.

Short answers that come up again and again: why the left ventricle wall is thickest; why valves are needed; the difference between arteries and veins; what double circulation means and why it is useful to mammals and birds; the role of the SA node; what the P, QRS and T waves represent; and the meaning of 120/80.

For numerical questions, write the formula first, substitute with units, and give the answer with units: cycle time = 60 ÷ rate, cardiac output = stroke volume × rate. Convert mL to L when asked.

One-screen revision

Read this list the night before the exam; every line is a likely question.

• Heart: fist-sized, in the chest between the lungs, tilted left, wrapped in the pericardium.
• Four chambers: two thin atria receive, two thick ventricles pump; the left ventricle is thickest.
• Septum keeps oxygenated and deoxygenated blood apart.
• Valves: tricuspid (right), bicuspid/mitral (left), pulmonary and aortic semilunar.
• Sounds: lub = AV valves close; dub = semilunar valves close.
• Double circulation: pulmonary (heart–lungs–heart) + systemic (heart–body–heart).
• Cardiac cycle at 75 bpm: 0.8 s = 0.1 s atrial systole + 0.3 s ventricular systole + 0.4 s diastole.
• Conduction: SA node → AV node → bundle of His → Purkinje fibres.
• ECG: P = atria, QRS = ventricles activate, T = ventricles recover.
• Cardiac output = stroke volume × rate = 70 × 75 = 5.25 L/min; typical blood pressure 120/80 mm Hg.

Frequently asked questions