How the Heart Works: A Guide for Kids
Ask a child what the heart does and you will usually get "it pumps blood". Ask what it pumps blood to and the answer stops. That gap is worth closing, because the heart only makes sense once you know it is doing two different jobs at the same time.
It is two pumps in one case
This is the fact that unlocks everything else, and most diagrams bury it.
The right side of the heart takes blood that has come back from the body with its oxygen used up, and sends it a short distance to the lungs to be refilled. The left side takes that refilled blood and sends it everywhere else - brain, legs, fingers, all of it.
Two pumps, side by side, beating at the same moment. The right one only has to push blood to the lungs and back, which is a few centimetres. The left one has to reach your toes. That is why the left side's muscle wall is visibly thicker, and it is the single best thing to point at on a diagram.
The four chambers follow from this. Each side has a small upper room that collects blood, and a big lower room that does the pushing. Two collectors, two pushers.
What the lub-dub actually is
Not the muscle squeezing. It is the valves slamming shut.
There are valves between the collecting rooms and the pushing rooms, and more at the exits. When the big chambers squeeze, the first set snaps closed so blood cannot go backwards - that is the "lub". When the squeeze ends and the pressure drops, the exit valves snap shut so blood does not fall back in - that is the "dub".
So the sound of a heartbeat is doors closing, not muscle working. Children find this genuinely surprising, and it makes valves memorable in a way that labelling them never does.
Why it speeds up when you run
Muscles that are working need more oxygen. Oxygen arrives in blood. More oxygen therefore means more blood, and there are only two ways to move more blood: send it more often, or send more each time.
Your heart does both, and the numbers are worth showing a child because they are larger than anyone guesses.
| Sitting still | Sprinting | |
|---|---|---|
| Beats per minute | about 70 | about 180 |
| Blood pumped per minute | about 4.9 litres | about 20.7 litres |
| Pushed out per beat | about 70 ml | about 115 ml |
| One full lap of your body | about 61 seconds | about 14 seconds |
Four times the blood, and a lap of the entire body in fourteen seconds. The other thing visible in the simulation above is what happens to the gaps: at rest there is a clear pause between beats, and at 180 there is almost none. The heart rests between beats, not between shifts, and at full effort those rests nearly vanish.
The number that lands hardest
At a resting 70 beats a minute, a heart beats about 100,800 times a day. Roughly 2.5 billion times in an average lifetime. It has done it every second since before the child was born, without ever being told to and without stopping once.
For most children this is the moment the organ stops being a shape in a textbook. It is worth saying out loud.
How to explain it in five minutes
An order that works, if you are doing this at a kitchen table.
Start with the two pumps, not the four chambers. Right side to the lungs, left side to the body. Everything else hangs off that.
Have them feel their own pulse, then do twenty star jumps and feel it again. The change is the lesson, and no explanation substitutes for noticing it in your own wrist.
Then show the numbers. Once they have felt their heart speed up, 4.9 to 20.7 litres means something.
Finish with the lifetime count. End on the fact that stays.
Where 3Eyes Fits
The heart lesson above is one of about sixty interactive science lessons built into 3Eyes, which is a parental controls product for family computers - it filters the machine, approves sites rather than blocking them one at a time, and limits YouTube to channels you choose.
The lessons are there because a computer that only ever says no is a difficult thing to defend to a ten-year-old. When the same laptop has sixty science lessons and a tutor on it, the limits stop being the entire relationship.
About two minutes to set up. See what it includes or create an account.
Frequently asked questions
Why is the left side of the heart bigger? It has much further to pump. The right side pushes blood to the lungs and back, a few centimetres. The left side has to reach your toes, so it needs a thicker muscle wall and higher pressure.
What makes the heartbeat sound? Valves closing, not muscle contracting. The first sound is the valves between the upper and lower chambers snapping shut; the second is the exit valves closing as the squeeze ends.
How fast should a child's heart beat? Faster than an adult's, and the younger the faster - a young child at rest typically runs higher than the adult 60-100 range. Anything you are actually worried about is a question for a doctor rather than an article.
Does the heart ever rest? Between beats, yes - that is what the pause is. It never gets a longer break than that, which is why the gaps shrinking during exercise is worth pointing out.
Why do you breathe harder when you run as well? Same reason the heart speeds up. Muscles want more oxygen, so you both collect it faster (breathing) and deliver it faster (heart rate). The two systems are solving one problem.