During inhalation, the muscles increase the volume of the thorax, which lowers the pressure inside the lungs, so air flows in. Exhalation reverses each step.
This lesson follows relating respiratory surfaces to diffusion and belongs to the gas exchange section.
What happens in each phase?
Write the steps in cause-and-effect order. Start with the muscles and end with the air movement.
| Step | Inhalation | Exhalation |
|---|---|---|
| Intercostal muscles | Contract | Relax |
| Ribs | Move upwards and outwards | Move downwards and inwards |
| Diaphragm | Contracts and flattens | Relaxes and curves upwards |
| Volume of thorax | Increases | Decreases |
| Air pressure in lungs | Falls below atmospheric | Rises above atmospheric |
| Air movement | Into the lungs | Out of the lungs |
Worked example: a three-mark answer
Question: explain how air enters the lungs during inhalation.
- The external intercostal muscles contract and the diaphragm contracts and flattens.
- The ribs move up and out, so the volume of the thorax increases.
- The air pressure in the lungs falls below atmospheric pressure, so air flows into the lungs.
Each sentence gives a cause and its effect. The links between the sentences carry the marks.
The mistake that costs marks
The common slip is to say the lungs pull or suck air in. The lungs have no muscles of their own, so they do not expand by themselves.
| Claim | Why it fails |
|---|---|
| “The lungs expand and pull air in” | Lungs expand because the thorax volume increases |
| “The diaphragm relaxes during inhalation” | It contracts and flattens |
| Correct: “Volume increases, so pressure falls, so air flows in” | Uses cause, then effect, then movement |
A useful habit is to write “volume, pressure, flow” in the margin whenever you see a ventilation question.
Check yourself
Describe what happens to the ribs, the diaphragm, the thorax volume and the air pressure in the lungs during exhalation at rest.
Answer
The intercostal muscles relax, so the ribs move downwards and inwards. The diaphragm relaxes and curves upwards.
The volume of the thorax decreases, so the air pressure in the lungs rises above atmospheric pressure. Air flows out of the lungs.
What to study next
Continue with comparing gas exchange in selected organisms, then test the set in the gas exchange practice. The biological data interpretation trainer offers more practice linking cause and effect in short answers.
For a teacher to hear you explain this sequence, see online one-to-one Biology tuition.