Human Body Organs for Kids: What Each One Does
Your liver is doing about five hundred different jobs right now, and most adults cannot name one of them. That is roughly the state of organ knowledge in general: everyone knows the heart pumps and the lungs breathe, and it stops there. The organs children remember are the ones they have seen in three dimensions with a single startling number attached, so this is a guide built that way - one job, one number, one thing to point at.
Everything below comes from the 3Eyes Human Anatomy lesson - a 3D interactive where a child rotates photoreal organs, opens the kidney and stomach in cutaway and reads each one's numbers. The recordings below are taken straight from it; the lesson itself is part of 3Eyes.
Start with the four that do the work
A body has dozens of organs but four carry the story. The heart is a fist-sized muscle that beats about 100,000 times a day and moves roughly 7,500 litres of blood in that time - enough to fill a small swimming pool. The lungs are two spongy bags that pull oxygen from about 20,000 breaths a day; unfolded, the 500 million tiny air sacs inside them would cover a tennis court. The brain weighs about 1.3 kg, holds around 86 billion nerve cells, and burns a fifth of all the energy you eat while sitting still. The liver is the largest internal organ, filters about 1.4 litres of blood a minute, and is the only human organ that can regrow itself from a piece.
Learn those four with their numbers and the rest of the body starts to make sense as a set of systems that feed them, clean up after them, and carry their orders.
The heart is two pumps, not one
The single most useful fact about the heart is that it is two pumps built into one case. The right side sends used-up blood a few centimetres to the lungs to collect oxygen. The left side sends that refreshed blood everywhere else, all the way to the toes. That is why the left wall is visibly thicker on any model - it has further to push.
Blood leaves through the aorta at roughly 1 metre per second, which is faster than most children guess. It goes round the whole body about once a minute at rest and about four times a minute during a sprint, because the heart both beats faster and pushes more each beat. There is a longer walk through this in How the Heart Works, including what the lub-dub sound actually is.
The lungs are a tennis court folded into your chest
Air comes down the windpipe, splits into two bronchi, then keeps splitting - about twenty-three times - until it reaches half a billion alveoli, the tiny sacs where oxygen crosses into blood. Folding all that surface into a space the size of two footballs is the engineering trick that keeps a child alive.
The right lung has three lobes and the left has two, and the reason is worth pointing at on a model: the left lung is smaller to make room for the heart, which sits slightly to the left. The lungs themselves have no muscle. The diaphragm below them and the muscles between the ribs do the pulling, which is why a stitch or a broken rib makes breathing hurt even though the lungs feel nothing.
The liver is the body's chemical plant
Everything absorbed from food goes through the liver before it reaches the rest of the body. It stores sugar and releases it between meals, makes the bile that digests fat, builds the proteins that let blood clot, and breaks down poisons - including medicines and alcohol - into things the kidneys can remove.
Five hundred jobs is the usual count. The one children like best is that it regrows: a liver can lose two-thirds of itself and rebuild to full size within months, which is why a living person can donate part of theirs.
The kidneys and stomach have layers worth opening
Two organs reward a cutaway. The kidney is not a bag of filter; it is about a million microscopic tubes called nephrons packed into an outer cortex and an inner medulla. Blood comes in, water and waste are squeezed out into the tubes, the useful parts are pulled back, and what is left runs down the ureter as urine. The two kidneys filter your entire blood supply around forty times a day.
The stomach wall has layers too - a mucous lining that protects it from its own acid, muscle that churns food into paste, and a ring of muscle at the bottom called the pylorus that decides when to let food through to the intestine. The acid is strong enough to dissolve metal; the reason it does not dissolve the stomach is that the lining replaces itself every few days.
The numbers, in one place
| Organ | Weight | Every day it… | The fact that sticks |
|---|---|---|---|
| Heart | 250-350 g | Beats ~100,000 times, pumps ~7,500 litres | Two pumps in one case |
| Lungs | ~1 kg the pair | Takes ~20,000 breaths | Alveoli would cover a tennis court |
| Brain | 1.3-1.4 kg | Uses ~20% of your energy | 86 billion neurons |
| Liver | 1.4-1.6 kg | Filters ~2,000 litres of blood | Regrows from a piece |
| Kidneys | ~150 g each | Filters all your blood ~40 times | A million nephrons each |
| Stomach | ~1 litre capacity | Replaces its own lining every few days | Acid that could dissolve metal |
How to teach it at a kitchen table
One organ at a time, one number each. "Your heart will beat about a hundred thousand times today" is a better opening than a labelled diagram. Ask the child to guess first - guesses are always low, and the gap is the lesson.
Point at the asymmetry. Why is the left lung smaller? Why is the left side of the heart thicker? Questions with a visible answer teach better than facts with none.
Turn it. Organs are three-dimensional and paper is not. Rotating a model of the liver reveals the gall bladder tucked underneath it, which no flat diagram shows well. The explorer above is built for exactly this: drag to turn, scroll to zoom, click a structure to read about it.
Where 3Eyes Fits
The anatomy explorer 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 says no is a hard 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 whole relationship.
About two minutes to set up. See what it includes or create an account.
Frequently asked questions
What are the five most important organs in the body? The usual list is the brain, heart, lungs, liver and kidneys - the ones you cannot live without for more than minutes to days. Everything else can be removed, replaced or worked around; these five cannot.
Which is the largest organ in the human body? The skin, by a wide margin - about 2 square metres and 4 kg in an adult. The largest internal organ is the liver, at around 1.5 kg, followed by the brain.
How many organs does a human have? Around 78 by the common count, grouped into eleven systems. The exact number depends on what you count - whether each tooth is an organ, for example - which is a good thing to tell a child who asks.
Why does the brain use so much energy? Because it never switches off. Eighty-six billion neurons are firing signals continuously, even in sleep, and each signal costs energy to reset. Two per cent of your weight uses twenty per cent of your fuel.
Can you live with one kidney? Yes, comfortably. One healthy kidney can do the filtering work of two, which is why living kidney donation is possible and why many people are born with one and never know.