What it does
Your heart is two pumps side by side. The right side collects blood low in oxygen from the body and pumps it to the lungs; the left side collects oxygen-rich blood from the lungs and pumps it to the body. Each beat follows the same pattern, the cardiac cycle: the atria contract, then the ventricles, then the whole heart relaxes and fills. Valves stop blood flowing backwards.
This page animates a simple four-chamber heart (blue for blood low in oxygen, red for oxygen-rich blood) together with an ECG-like trace and the volume of the left ventricle:
- AV valves (tricuspid on the right, bicuspid or mitral on the left) open while the heart fills and shut as the ventricles contract: the “lub” sound.
- Semilunar valves (pulmonary and aortic) open only while the ventricles push blood out, and shut as they relax: the “dub”.
- The P wave comes as the atria are told to contract, QRS as the ventricles are, and T as the ventricles recover.
It fits Class 10 Science: life processes (the human heart, double circulation) and Class 11 Biology: body fluids and circulation (cardiac cycle, ECG, cardiac output). It is a teaching model, not a medical tool. Photosynthesis, from the same Class 10 chapter, is in the photosynthesis lab; work out cardiac output from oxygen use with fick cardiac output; and see training heart rates in heart rate zones.
How to use it
- Pick the level. Class 10 shows the chambers, valves, heart rate and output, and explains double circulation. Class 11 adds the end-systolic volume, ejection fraction and a table of each phase with its time, valves and ECG wave.
- Slide the heart rate (40 to 180 beats a minute) and the filling of the ventricle.
- Slow motion shows the beat at a quarter of real speed; Pause stops it so you can study one moment. The line under the picture says what is happening and which valves are open.
- Read the cardiac output with the sum filled in.
- Download the picture and numbers, or (Class 11) one beat’s phases as a CSV file. Check yourself gives fresh questions every time.
Worked example
At the start: 75 beats a minute, 120 mL in the ventricle when full and 50 mL left after each beat. One beat takes 60 ÷ 75 = 0.80 s: 0.1 s atrial systole, 0.3 s ventricular systole, 0.4 s joint diastole. Stroke volume = 120 − 50 = 70 mL, so cardiac output = 70 mL × 75 = 5250 mL/min = 5.25 L/min. At 150 beats a minute a beat takes only 0.40 s: systole shrinks to 0.21 s but diastole to just 0.12 s, so there is less time to fill.
Tips
- Pause just after the tall R spike: both sets of valves are shut and the ventricle volume is flat. That is isovolumetric contraction.
- The left ventricle wall is thickest because it pushes blood all round the body.
- Real ECGs come from electrodes on the skin; doctors read them carefully. Do not use this trace to judge a real one.
FAQ
Why does the volume not fall to zero? The ventricle never empties completely; about 50 mL stays behind at rest.
Why do the atria only add a little blood? Most filling happens on its own while the heart relaxes; the atria top up the last part.
Does the right ventricle pump the same amount? Yes, on average both sides pump the same volume each beat, or blood would pile up on one side.
Where the numbers come from
- Timing at 75 beats a minute (cycle 0.8 s: atrial systole 0.1 s, ventricular systole 0.3 s, joint diastole 0.4 s), cardiac output = stroke volume × heart rate, and the order of valve events are standard physiology; written by fibub.
- Systole is scaled with the square root of the cycle length (as the QT interval roughly is) so that diastole shortens most at fast rates; this is a simplification for teaching.
- The ECG-like trace and the volume curve are smooth made-up shapes drawn by fibub, not recordings, and must not be used to read a real ECG. Volumes are typical round resting values. Nothing you enter is stored or sent anywhere.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.