Breathing and gas exchange · Model
How breathing works
Start here
If you had to guess, how does a healthy lung normally fill with air?
The chest stretches it open. Lungs cannot open themselves.
The chest stretches it open. Lungs cannot open themselves.
Key words
The sheet of muscle under the lungs. It is domed at rest and flattens when it contracts.
The muscles between the ribs, which swing the ribs up and out.
- volume
- pressure
Key knowledge
Breathing in and breathing out
| Row | Breathing in | Breathing out |
|---|---|---|
| Intercostal muscles | ||
| Ribs | ||
| Diaphragm | ||
| Chest volume | ||
| Pressure in the chest | ||
| Air | ||
| Why air moves |
Your lungs cannot move themselves.
The diaphragm and intercostal muscles do the work.
The lungs stretch when the chest gets bigger.
Air particles hit the walls of the chest. This causes pressure.
The same particles in a bigger space hit the walls less often. So the pressure falls.
In a smaller space, they hit the walls more often. So the pressure rises.
A bell jar is a model of the chest.
Pull the sheet down. The volume in the jar increases and the pressure falls.
Air moves in and the balloons inflate.
Push the sheet up. The volume in the jar decreases and the pressure rises.
Air moves out and the balloons deflate.
Take the deepest breath you can.
Then breathe out fully through the tube into the bottle.
The volume of water pushed out equals the volume of air you breathed out.
Wrong: The lungs pull air in.
Right: Muscles make the chest bigger, then air moves in.
Wrong: Air coming in makes the chest bigger.
Right: The chest gets bigger first. Then air moves in.
Wrong: Air gets sucked in.
Right: Air moves in because the pressure outside is higher.
Key fact
Breathing works by changing pressure. Muscles change the volume of the chest, which changes the pressure inside it. Air always moves from high pressure to low pressure.
Practice
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1
Put these in the order they happen when you breathe in.
Rung 2
The balloon starts empty. You seal the tube so no air can get in or out. Then you pull the rubber sheet down. What happens?
Rung 3
A small wound between the ribs lets air into the space around a lung. Explain why this can collapse a lung that is not damaged. Use the words volume, pressure and diaphragm. Say why enlarging the chest no longer helps.
Rung 4
You lie on the bottom of a pool and breathe through a 2 m snorkel. Water pressure increases with depth. Explain, using pressure, why this is far harder than snorkelling at the surface. Say why the problem is the chest, not the tube.
- The jar is rigid. Your ribs can move.
- A balloon is one bag. A lung holds millions of tiny air sacs.
- The rubber sheet is flat. A real diaphragm is a dome.
At rest, you do not have to push air out.
The muscles relax and the stretched lungs spring back to size.
A wound between the ribs lets air into the space around the lung.
The pressure around the lung now matches the air outside.
So a bigger chest no longer stretches that lung.
The lung collapses, even if it is not damaged.
Before this lesson
Connects to
- Gas pressure
The same particles in a bigger space hit the walls less often.
- Testing the model
How to find where a model stops being true.
At GCSE this becomes
- Ventilation, lung volumes and spirometer traces, with pressure and volume treated quantitatively.
Where to next
Ask MrBadmus
Want to check you have the order of events the right way round?
Lesson content © MrBadmus.