Key Stage 3 science · Biology – Structure and function of living organisms

KS3 breathing, gas exchange and the effects of drugs

In Key Stage 3 biology pupils learn how the lungs get oxygen into the blood and carbon dioxide out, and how the muscles of the chest move air in and out. They find out how exercise, asthma and smoking affect the lungs, how leaves exchange gases through stomata, and what recreational drugs, including alcohol, nicotine and illegal substances, do to behaviour, health and the body’s processes.

Breathing, gas exchange and drugs, explained

Air enters through the nose or mouth and passes down the trachea (windpipe), which is held open by rings of cartilage and lined with mucus and tiny hairs called cilia that trap dust and microbes. The trachea divides into two bronchi, one to each lung, which branch into smaller bronchioles ending in clusters of tiny air sacs called alveoli. The alveoli are where gas exchange happens. They are adapted for it: there are millions of them, which gives a huge surface area, their walls are only one cell thick, they are moist, and they are surrounded by capillaries, so blood brings carbon dioxide and takes oxygen away. Oxygen and carbon dioxide move by diffusion, from high to low concentration.

Breathing is explained using a pressure model. To breathe in, the intercostal muscles between the ribs contract, raising the ribs up and out, and the diaphragm contracts and flattens. This makes the space inside the chest bigger, so the pressure inside falls below the pressure of the air outside, and air flows in. To breathe out, the muscles relax, the ribs fall, the diaphragm rises, the volume of the chest gets smaller, the pressure rises above the outside pressure and air is pushed out. Schools often show this with a model made from a bottle, balloons and a rubber sheet.Pupils also take simple measurements of lung volume, for example by breathing out through a tube into an upside-down bottle full of water standing in a bowl of water, and measuring how much water the breath pushes out, or by using a spirometer if the school has one.

During exercise muscles use more oxygen and produce more carbon dioxide, so breathing gets faster and deeper and the heart beats faster. Over time, regular exercise strengthens the diaphragm and intercostal muscles. Asthma is a condition in which the lining of the airways becomes inflamed, the muscles around them tighten and extra mucus is made, so the airways narrow, often in response to triggers such as pollen, dust, smoke, cold air or exercise. Reliever inhalers contain a medicine that relaxes those muscles and widens the airways. Smoking damages the lungs: tar contains substances that cause cancer, the cilia are damaged so mucus and germs build up, nicotine is addictive and carbon monoxide reduces the oxygen the blood can carry. The walls of the alveoli can break down in a disease called emphysema, which reduces the surface area for gas exchange and leaves people breathless. Many schools also discuss vaping: many vapes contain nicotine, which is addictive, and the long-term effects are still being studied.

In plants, gas exchange happens through tiny holes called stomata, mostly on the underside of leaves. Each is opened and closed by a pair of guard cells. Carbon dioxide diffuses in for photosynthesis, and oxygen and water vapour diffuse out. The programme also includes the effects of recreational drugs, including substance misuse, on behaviour, health and life processes. A drug is a substance that changes the way the body or mind works. Recreational drugs are taken for their effects rather than as medicine, and some, such as alcohol and nicotine, are legal for adults while others are illegal. Teachers cover, for example, how alcohol slows the nervous system, affects judgement and reaction times and can damage the liver over time, why nicotine is addictive, and how other drugs, including solvents, can affect the brain, heart and breathing. The tone is factual and focused on staying safe.

Words your child will hear

Alveoli: Tiny air sacs in the lungs where oxygen passes into the blood and carbon dioxide passes out. One is an alveolus.
Diaphragm: A sheet of muscle under the lungs that flattens as you breathe in.
Intercostal muscles: Muscles between the ribs that move the ribcage up and out when you breathe in.
Gas exchange: Oxygen moving into the blood and carbon dioxide moving out, by diffusion.
Asthma: A condition in which the airways become inflamed and narrow, making breathing harder.
Stomata: Small pores, mainly under leaves, through which gases enter and leave. One is a stoma.
Substance misuse: Using drugs, alcohol or other substances in a way that harms health.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about the structure and functions of the gas exchange system in humans, including adaptations to function
Pupils should be taught about the mechanism of breathing to move air in and out of the lungs, using a pressure model to explain the movement of gases, including simple measurements of lung volume
Pupils should be taught about the impact of exercise, asthma and smoking on the human gas exchange system
Pupils should be taught about the role of leaf stomata in gas exchange in plants
Pupils should be taught about the effects of recreational drugs (including substance misuse) on behaviour, health and life processes

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: a model answer on how we breathe in

Explain, using the idea of pressure, how air moves into the lungs when you breathe in.

Start with the muscles. The intercostal muscles contract and move the ribs up and out. The diaphragm contracts and flattens, moving down.
Link to volume. Both movements make the space inside the chest, the thorax, bigger. The same amount of air now has more room.
Link to pressure. When a gas has a bigger space, its pressure falls, so the pressure inside the lungs falls below the pressure of the air outside.
Explain the movement. Air moves from a region of higher pressure (outside) to one of lower pressure (inside the lungs), so air flows in through the nose and trachea to the alveoli.
Add the model. In a bottle model, pulling the rubber sheet at the bottom downwards (the diaphragm) makes the space inside bigger, and the balloons (the lungs) fill up. Say too where the model is weak: it has no ribs, and the rubber sheet is not a real muscle.

Contracting muscles increase the volume of the chest, which lowers the pressure inside the lungs below atmospheric pressure, so air flows in until the pressures are equal.

Common mistakes and how to help

Thinking the lungs pull air in by themselves

We feel our chest swell, and it seems as if the lungs are muscles that expand.

What helps: The lungs cannot expand on their own. The intercostal muscles and diaphragm change the volume of the chest, and air follows because of the change in pressure.

Believing we breathe in only oxygen and breathe out only carbon dioxide

Textbook diagrams show oxygen going in and carbon dioxide coming out, with nothing else.

What helps: Air is mostly nitrogen. Breathed-in air is about 21% oxygen and 0.04% carbon dioxide, while breathed-out air is roughly 16% oxygen and 4% carbon dioxide, so we use only some of the oxygen.

Confusing breathing, gas exchange and respiration

The words are used loosely, and “respiratory system” and “respiration” sound as if they are the same process.

What helps: Breathing moves air in and out. Gas exchange swaps oxygen and carbon dioxide in the alveoli. Respiration is the chemical reaction in cells that releases energy from food.

Thinking plants take in only carbon dioxide and give out only oxygen

Photosynthesis is taught first, so plants are seen as the opposite of animals.

What helps: Plants respire all the time, taking in oxygen and giving out carbon dioxide. In bright light photosynthesis is faster, so there is a net gain of oxygen.

How to help at home

Measure breathing rate. Count the breaths in 30 seconds at rest and double it. Then do a minute of star jumps or brisk marching in the living room and count again. Ask why it rose. If your child has asthma, keep their inhaler close and let them stop whenever they want.
Build a bottle lung. An adult cuts the bottom off a plastic bottle (and tapes over the sharp edge) and fixes a balloon inside the neck. Cover the cut end with a stretched balloon or rubber sheet and pull it down to make the inside balloon inflate. Ask your child to explain it using pressure.
Compare balloon breaths. Take one deep breath, blow it all into a balloon, tie it and measure round the widest part with a piece of string. Each person should use their own balloon. Compare a few attempts or different people, and discuss why this is only a rough measure of lung volume and how it could be made fairer.
Have a calm talk about drugs. Ask what they have learned in school about alcohol, vaping and other drugs, and listen first. Share facts, not scare stories, and agree who they could talk to if a friend or situation worried them.

Builds on

Year 6: Classifying living things

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Updated 20 October 2026.