Why do we breathe?
You breathe about 20 000 times a day without thinking about it. Every breath brings in oxygen, a gas your cells need to release energy from food. Every breath out removes carbon dioxide, a waste gas your cells produce. Without oxygen, brain cells begin to die within a few minutes.
Breathing is the job of the respiratory system.
The path of air
| Part | What it does |
|---|---|
| Nose and mouth | Air enters. Nose hairs and mucus trap dust and germs; the nose warms and moistens the air. |
| Pharynx (throat) | Carries air (and food) down. |
| Larynx (voice box) | Holds the vocal cords. A flap called the epiglottis covers it when you swallow so food doesn’t go down the wrong way. |
| Trachea (windpipe) | A tube held open by rings of cartilage. You can feel the rings at the front of your neck. |
| Bronchi | Two branches, one to each lung. |
| Bronchioles | Thousands of smaller branches, like the twigs of an upside-down tree. |
| Alveoli | Millions of tiny air sacs at the ends of the bronchioles. |
The lungs
Your lungs are two spongy organs in your chest, protected by your ribs. The left lung is a little smaller than the right to make room for your heart.
Each lung contains hundreds of millions of alveoli. If you spread them all out flat, they would cover about the area of a tennis court! This huge surface lets lots of oxygen enter the blood quickly.
Gas exchange
Each alveolus is wrapped in tiny blood vessels called capillaries. Their walls are only one cell thick.
- Oxygen moves from the air in the alveoli into the blood.
- Carbon dioxide moves from the blood into the alveoli, to be breathed out.
Gases move from where there is more of them to where there is less. This movement is called diffusion. Red blood cells then carry oxygen around the body.
| Air breathed in | Air breathed out | |
|---|---|---|
| Oxygen | about 21% | about 16% |
| Carbon dioxide | about 0.04% | about 4% |
| Water vapour | Varies | More (that’s why breath fogs a mirror) |
How breathing works
Lungs have no muscles of their own. The diaphragm and the muscles between your ribs do the work.
| Breathing in (inhaling) | Breathing out (exhaling) |
|---|---|
| Diaphragm contracts and moves down | Diaphragm relaxes and moves up |
| Ribs move up and out | Ribs move down and in |
| Chest gets bigger; air pressure inside drops | Chest gets smaller; air pressure inside rises |
| Air rushes in | Air is pushed out |
A hiccup is a sudden twitch of the diaphragm!
Controlling your breathing
Your brain controls breathing automatically. It watches the amount of carbon dioxide in your blood. When you exercise, your muscles make more carbon dioxide, so your brain makes you breathe faster and deeper. You can also control your breathing on purpose for a short time, like when you hold your breath underwater.
Keeping lungs healthy
| Problem | What happens |
|---|---|
| Asthma | Airways swell and narrow, making breathing hard. Inhalers open them up. |
| Smoking and vaping | Damage airways and alveoli; smoking causes lung cancer and emphysema |
| Pneumonia | An infection fills alveoli with fluid |
| Air pollution | Irritates airways and can worsen asthma |
Exercise, not smoking or vaping, and staying away from polluted air all help keep lungs healthy.
Breathing in other animals
- Fish use gills to take oxygen from water.
- Insects have tiny tubes called spiracles along their bodies.
- Frogs can breathe through their moist skin as well as their lungs.
- Birds have air sacs that keep air flowing through their lungs in one direction, which helps them fly high where air is thin.
Common mistakes
- Thinking we breathe out only carbon dioxide. Breathed-out air is still mostly nitrogen and has some oxygen. That’s why rescue breathing works.
- Thinking lungs pull air in on their own. The diaphragm does the work.
Try it yourself
- Count your breaths for one minute while sitting. Then do 30 jumping jacks and count again. Explain the difference.
- Make a model lung with a plastic bottle, a balloon inside and a stretched balloon "diaphragm" over the bottom. Pull the bottom balloon down and watch the inside balloon inflate.
- Measure your lung capacity by breathing out through a tube into an upside-down bottle of water.
Quick recap
- Breathing brings in oxygen and removes carbon dioxide.
- Air travels through the nose, trachea and bronchi to the alveoli.
- Gas exchange happens in the alveoli by diffusion.
- The diaphragm and rib muscles make breathing happen.
- The brain speeds up breathing when carbon dioxide rises.