1Breathing in
To breathe in, your intercostal muscles contract and pull the ribs up and out, and your diaphragm contracts and flattens. Your chest gets bigger, so the pressure inside falls and air is pushed in.
intercostal musclesdiaphragmpressure
Lesson 7.2 notes
Breathing in and out
See how muscles change the size of your chest, why that makes air move, how a bell jar models it, and how to measure the air you breathe out.
2Breathing out
To breathe out, the intercostal muscles and diaphragm relax. The ribs move down and in, the diaphragm moves back up, the chest gets smaller, the pressure inside rises and air is pushed out.
relax
3Why air moves
Air particles push on everything they hit. Spread the same air through a bigger space and it pushes less: the pressure falls. Air always moves from higher pressure to lower pressure.
pressure
4A model of the chest
In the bell jar model, pulling the rubber sheet down makes the space inside bigger. The pressure falls, so air is pushed into the balloons, just as it is into your lungs.
bell jar model
5How good is the model?
The model shows how a bigger space makes air flow in. Its limitations: the glass jar can't move like ribs, and balloons aren't spongy like real lungs.
limitations
6Measuring lung volume
Fill a bottle with water and turn it upside down in a trough. Take a deep breath, then breathe out through a tube into the bottle. Read the volume of air on the scale.
volume
7Normal and deep breaths
At rest each breath is only about 0.5 litres. The most air you can breathe out after your deepest breath in is your vital capacity: 3.5 litres in this trace.
vital capacity
Explore: Breathing model
Breathe in and out with a real chest or the bell jar model. Watch the space inside, the pressure and which way the air moves.
Common mistake: Your lungs don't suck air in
Your lungs can't make themselves bigger. The intercostal muscles and diaphragm make your chest bigger, the pressure inside falls, and the higher pressure outside pushes air in.
Remember
- Breathing in: the intercostal muscles and diaphragm contract, the chest gets bigger, the pressure inside falls and air is pushed in.
- Breathing out: the muscles relax, the chest gets smaller, the pressure inside rises and air is pushed out.
- In the bell jar model the jar is the chest, the balloons are the lungs and the rubber sheet is the diaphragm.
Going further at GCSE