# The Heart and Circulation Simulation

> A free heart simulation for class: watch the four chambers and valves through the cardiac cycle and follow the blood round the lungs and the body.

Source: https://olyclass.com/simulations/heart-and-circulation/ · Updated 2026-11-03

This is a beating heart the whole class can watch on a projector, TV or interactive whiteboard. Its four chambers fill and squeeze through the cardiac cycle, its four valves snap open and shut at the right moments, and blood cells travel the double circulation: from the body into the right side of the heart, up to the lungs where they turn red, back to the left side, and out to the body where they turn blue again. Drag the heart rate from 40 to 180 beats per minute, switch between Sleep, Rest and Exercise, turn on the "lub-dub", step through the beat one phase at a time, and tap any part to name it. A quiz mode asks the class where the blood goes next.

## How to use it

- **Watch the beat:** the phase is named at the top left (atria contract, ventricles contract, heart relaxes), with what happens in it. The four valve chips on the right turn green when a valve is open.
- **Step through it:** press the step buttons (or the left and right arrow keys) to stop in each phase in turn: Step 1 of 3, 2 of 3, 3 of 3. The heart holds still so you can point at the valves. ¼× plays the beat in slow motion.
- **Sound:** the speaker button (or the S key) plays a soft "lub" when the tricuspid and mitral valves shut and a shorter "dub" when the pulmonary and aortic valves shut. It is off until you turn it on.
- **Stethoscope:** the strip on the right shows the last few beats as sound bursts, over a coloured band of the phases: yellow for the atria, red for the ventricles, green for diastole.
- **Heart rate and activity:** Sleep, Rest and Exercise set a typical heart rate and stroke volume; the slider (or the up and down keys) sets any rate from 40 to 180. The panel shows how long one beat and each phase last, and the cardiac output.
- **Tap a part:** chambers, valves, vessels, the lungs, the body or the septum. The panel names it, says what it does and whether its blood is oxygen-rich or oxygen-poor.
- **The two loops:** press Pulmonary circulation or Systemic circulation to fade everything else.
- **Quiz:** press Quiz. The question names a part, for example "Where does the blood go after the right ventricle?", and four answers appear. Some questions ask whether a vessel's blood is oxygen-rich or oxygen-poor.
- **Projector:** the full screen button in the top right makes everything bigger.

## The heart: four chambers and four valves

The heart is a muscular pump about the size of a fist, divided down the middle by a wall, the **septum**. Each side has two chambers:

- an **atrium** on top, which collects blood coming back in the veins;
- a **ventricle** below, which pumps blood out into an artery.

The right side handles **oxygen-poor** blood from the body; the left side handles **oxygen-rich** blood from the lungs. The septum keeps them apart. The simulation shows the heart from the front, as if you were facing the person, so the right side of the heart is on the left of the screen, as in textbook diagrams.

Four **valves** keep the blood flowing one way:

| Valve | Where | Opens when |
|---|---|---|
| Tricuspid | right atrium → right ventricle | the ventricles relax (diastole) |
| Mitral (bicuspid) | left atrium → left ventricle | the ventricles relax (diastole) |
| Pulmonary | right ventricle → pulmonary artery | the ventricles contract (systole) |
| Aortic | left ventricle → aorta | the ventricles contract (systole) |

The tricuspid and mitral valves are the **atrioventricular (AV) valves**; the pulmonary and aortic valves are the **semilunar valves**, with three half-moon cusps each. The left ventricle has the thickest wall, because it pushes blood all the way round the body.

## The cardiac cycle

One heartbeat is one **cardiac cycle**. At 75 beats per minute it lasts 60 ÷ 75 = 0.8 s, in three phases:

1. **Atrial systole (0.1 s):** the atria contract and push the last of their blood into the ventricles. The AV valves are open, the semilunar valves are shut.
2. **Ventricular systole (0.3 s):** the ventricles contract. The pressure snaps the AV valves shut (the first heart sound, **"lub"**), then pushes the semilunar valves open, and blood rushes into the pulmonary artery and the aorta.
3. **Diastole (0.4 s):** the whole heart relaxes. The semilunar valves shut so blood cannot fall back (the second sound, **"dub"**), the AV valves open, and blood from the veins flows through the atria into the ventricles.

For a short moment at the start of systole and again at the start of diastole all four valves are shut: the ventricle is squeezing or relaxing but its volume does not change yet. In slow motion the four valve chips all turn grey for that moment.

When the heart beats faster, every beat is shorter, but not every phase shrinks the same. Systole shortens a little (the model scales it with the square root of the beat length), and diastole takes most of the cut. At 150 beats per minute a beat lasts 0.4 s, systole about 0.21 s and diastole only about 0.12 s. That is why a very fast heart has less time to fill.

## Cardiac output

How much blood the heart pumps each minute is the **cardiac output**:

cardiac output = heart rate × stroke volume

The **stroke volume** is the blood one ventricle pumps in one beat: the volume when it is full (about 120 mL) minus what is left after it squeezes (about 50 mL), so about 70 mL. At rest, 75 beats × 70 mL ≈ 5.3 L per minute: roughly all the blood in an adult's body, every minute.

| Activity | Heart rate | Stroke volume | Cardiac output |
|---|---|---|---|
| Sleep | 55 /min | 65 mL | 3.6 L/min |
| Rest | 75 /min | 70 mL | 5.3 L/min |
| Exercise | 150 /min | 100 mL | 15 L/min |

In exercise the heart beats faster and squeezes harder, so the muscles get about three times as much blood. The slider changes only the heart rate, so the class can see each factor on its own.

## The double circulation

The blood passes through the heart twice on every lap round the body:

- **Pulmonary circulation** (heart → lungs → heart): right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium. In the capillaries of the lungs the blood gives up carbon dioxide and takes up oxygen, so it turns bright red.
- **Systemic circulation** (heart → body → heart): left ventricle → aorta → body → venae cavae → right atrium. In the capillaries of the organs and muscles it gives up oxygen and nutrients and picks up carbon dioxide, so it turns dark (blue in diagrams).

Two surprises worth stopping on: the **pulmonary artery** is an artery that carries oxygen-poor blood, and the **pulmonary veins** are veins that carry oxygen-rich blood. Arteries and veins are named by the way they go (away from or back to the heart), not by the oxygen they carry. Real oxygen-poor blood is dark red, not blue; blue is the colour textbooks use for it.

## Teaching ideas

The simulation fits the human body systems units: the circulatory system in KS3 biology and GCSE (the heart's structure, the double circulation, cardiac output), and NGSS MS-LS1-3 (body systems made of interacting subsystems).

1. **Hands on hearts (3 minutes):** ask the class to find their own pulse and count for 15 seconds. Multiply by 4 and set the slider to the class average.
2. **Name the parts (5 minutes):** pause, then tap each chamber and vessel with the class. Ask: "Why is the left ventricle's wall thicker than the right's?"
3. **Follow one cell (5 minutes):** in slow motion, follow a blue cell from the venae cavae. "Where does it turn red? Where does it go next?" Let the class call out each stop.
4. **Step the beat (8 minutes):** step through the three phases. In each one ask: "Which valves are open? Which way is the blood going?" Turn the sound on and match lub and dub to the valves that shut.
5. **Exercise (5 minutes):** switch from Rest to Exercise. "What happens to the heart rate, the stroke volume and the cardiac output? Which phase got shortest?"
6. **Quiz (5 minutes):** switch to Quiz and let teams take turns answering. Then make a review game: the whole class plays on the same screen in teams, with your phone as the remote.

## Questions teachers ask

### Why is the right side of the heart on the left of the screen?

The heart is drawn from the front, as if you were looking at a person facing you, which is how textbooks and doctors draw it. The person's right is then on your left. The words Right and Left under the heart are the person's sides.

### What makes the "lub-dub" sound?

Valves shutting. "Lub" is the tricuspid and mitral valves closing as the ventricles start to contract; "dub" is the pulmonary and aortic valves closing as the ventricles relax. The simulation plays both at those moments and draws them on the stethoscope strip.

### Is oxygen-poor blood really blue?

No. Blood is always red: bright red when it carries a lot of oxygen and darker red when it carries less. Veins look bluish through the skin because of how skin scatters light. Diagrams use blue for oxygen-poor blood to make the two kinds easy to tell apart, and so does the simulation.

### Why does the pulmonary artery carry oxygen-poor blood?

Arteries carry blood away from the heart and veins carry it back. The pulmonary artery takes oxygen-poor blood from the right ventricle to the lungs to pick up oxygen; the pulmonary veins bring that oxygen-rich blood back to the left atrium.

### Are the times and volumes real?

They are typical values for a healthy adult: 0.8 s per beat at 75 beats per minute (0.1 s + 0.3 s + 0.4 s), about 70 mL per beat and about 5 L per minute at rest. Real people vary, and athletes have a bigger stroke volume and a slower resting rate. The blood cells move much more slowly than real blood, so the class can follow them.

### Do I need to install anything, and does it work on a touch board?

No installing and no sign-up: it runs in the browser and it's free. On an interactive whiteboard students tap the parts and the buttons; on a phone the panel sits below the heart.

[Make a review game](https://olyclass.com/create?subject=science&grade=8&topic=The%20heart%20and%20circulation:%20chambers,%20valves,%20cardiac%20cycle,%20pulmonary%20and%20systemic%20circulation)

## Credits

Model and drawings: Oly. The cardiac cycle uses the textbook beat at 75 beats per minute (0.8 s: atrial systole 0.1 s, ventricular systole 0.3 s, diastole 0.4 s); at other rates systole scales with the square root of the cycle length, after Bazett (1920), and diastole takes the rest. Volumes and cardiac output are typical adult values (end-diastolic volume about 120 mL, end-systolic about 50 mL, stroke volume about 70 mL, about 5 L/min at rest), as in [OpenStax Anatomy and Physiology 2e](https://openstax.org/books/anatomy-and-physiology-2e/pages/19-3-cardiac-cycle) (CC BY 4.0). The drawing is a simplified front view; the sounds are made in the browser.