Describe the changes that occur in the thorax during expiration to move air out of the lungs.

Cambridge International IGCSE Biology (0610) — 11.1 Gas exchange in humans · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. The diaphragm relaxes and returns to a dome shape.
2. The intercostal muscles relax, causing the ribcage to move down and inwards.
3. The volume of the thorax decreases.
4. The pressure inside the thorax rises above atmospheric pressure, so air moves out of the lungs.

Examiner tips

Use the exact wording from the mark scheme (e.g., "diaphragm relaxes" and "intercostal muscles relax").
Show the sequence of events in the correct order.
Include the final result – the rise in pressure and movement of air out.
Keep the answer concise – one sentence per point.

Common mistakes

Writing the diaphragm as contracting instead of relaxing.
Omitting the ribcage movement or describing it incorrectly.
Failing to mention the rise in thoracic pressure or the direction of airflow.

Mark scheme (4 marks)

The diaphragm relaxes and returns to a dome shape
The intercostal muscles relax, causing the ribcage to move down and inwards
The volume of the thorax decreases
The pressure inside the thorax rises above atmospheric pressure, so air moves out of the lungs

Key terms in this question

expiration · thorax