# Explain how the alveoli in the lungs are adapted for efficient gas exchange.

> WJEC A-Level Biology (Wales) — 2.2 Adaptations for gas exchange · Explain · 5 marks

> The lungs contain millions of tiny air sacs called alveoli. Oxygen must pass from the air in the alveoli into the blood, while carbon dioxide passes in the opposite direction.

## Mark scheme (5 marks)

1. Large surface area increases the rate of diffusion / allows more molecules to diffuse across at once
2. Thin walls / short diffusion distance so gases diffuse across the membrane more quickly
3. Moist lining / surfaces are moist so gases can dissolve before diffusing across the membrane
4. Good blood supply / capillaries carry oxygen away from the alveoli maintaining a large concentration gradient
5. Large concentration gradient between alveolar air and blood means diffusion occurs faster / down a steep concentration gradient

## Related

- [Revision notes for WJEC A-Level Biology (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-alveoli-in-the-66926eab) · Published by Druglandscape Ltd.