Breathing and gas exchange
How you breathe in (Supplement)
- Breathing changes the volume and pressure inside the chest (the thorax).
- Breathing in (inhaling):
- external intercostal muscles contract → ribs move up and out,
- the diaphragm contracts and flattens (moves down),
- the thorax gets bigger, so pressure inside drops,
- air is pushed in.
- Breathing out is the opposite: muscles relax, thorax smaller, pressure rises, air out.
Practice
When you breathe IN, the diaphragm:
Breathing in: the diaphragm contracts/flattens and ribs move up and out, enlarging the thorax so pressure drops and air enters.
Practice
Air is pushed INTO the lungs because the pressure inside the thorax:
A bigger thorax lowers the inside pressure below atmospheric, so air flows in.
Inspired vs expired air
| Gas | Breathed in | Breathed out |
|---|---|---|
| oxygen | ~21% | ~16% (less) |
| carbon dioxide | ~0.04% | ~4% (more) |
| water vapour | a little | a lot (more) |
- Limewater tests for carbon dioxide — it turns cloudy faster with expired air.
Practice
Compared with inspired air, expired air has:
Expired air has about 16% oxygen (less) and about 4% carbon dioxide (more), plus more water vapour.
Exercise and clean airways
- During exercise muscles respire faster → more carbon dioxide in the blood.
- (Supplement) The brain detects this and makes breathing faster and deeper.
- (Supplement) Goblet cells make mucus that traps dirt; ciliated cells sweep it up to the throat to be swallowed.
Practice
During exercise, more carbon dioxide in the blood makes the brain increase the rate and depth of breathing.
The brain detects the rise in carbon dioxide and speeds and deepens breathing to remove it and bring in oxygen.
You've got it
Key idea
- breathing in: intercostals + diaphragm contract → thorax bigger → pressure lower → air in (out is the opposite)
- expired air = less oxygen, more carbon dioxide and water vapour; limewater tests for CO₂
- exercise → more CO₂ → brain → faster, deeper breathing; goblet + ciliated cells clean the airways