Explore the heart

Click any part of the heart to find out what it does.

Deoxygenated blood (low in oxygen) Oxygenated blood (rich in oxygen) Heart muscle (carries neither)

Remember: the heart is drawn as if it is facing you, so the right side of the heart — and the right lung — appear on the left of the screen.

Follow the blood

Blood makes a figure-of-eight through the heart. It passes through the heart twice on every complete lap. This is called double circulation.

Speed Normal Show flow arrows on the vessels

Step-by-step journey

Work through one complete lap, one stage at a time.

Stage – of 12

Press “Next” to begin

The diagram will highlight each structure as you go.

Pump it yourself

You are in charge of this heart. Nothing moves until you press the button. The two sides work together, exactly as they do in your chest: both atria squeeze at the same moment, then both ventricles.

The heart is caught at the end of a rest: both atria are already full. Squeeze them to begin.

Stage 1 of 4 · both sides at once

Both atria contract — the AV valves open

The two atria contract together, forcing the tricuspid and bicuspid valves open, and blood is pushed down into the ventricle below on each side.

What is happening right now

Speed Normal

Laps completed: 0

The order to remember

One heartbeat, in slow motion

A single beat is called the cardiac cycle. The muscle squeezes (systole) and relaxes (diastole) in a fixed order, and the valves snap shut to make the “lub dub” sound. This starts in slow motion so you can follow it.

Heart rate 72 bpm Show the electrical wiring (SA node → AV node → Purkinje fibres)

The five stages of one beat

The heart works through these in order, about once every second, far too fast to read as it goes. Click any stage to freeze the heart there and study it. The stage running right now is highlighted.

Live ECG trace

Patria contractQRSventricles contractTventricles relax

Fitness lab

Your heart changes how hard it works to match what your body needs. Drag the slider and watch what happens.

Activity Sitting still

Heart rate70beats per minute

Stroke volume70ml per beat

Cardiac output4.9litres per minute

cardiac output = heart rate × stroke volume

At rest your whole blood supply (roughly 5 litres) travels round your body about once a minute.

Try it with your own pulse

Find your pulse at your wrist or neck, count the beats for 15 seconds and type the number in.

Beats in 15 seconds

Did you know?

A trained athlete’s heart pumps more blood per beat, so it can beat far more slowly at rest — some rest at only 40 beats per minute.