Explain how the alveoli in the lungs are adapted for efficient gas exchange.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
1. The alveoli have a very large surface area, which allows many molecules of O₂ and CO₂ to diffuse at the same time.
2. Their walls are extremely thin, giving a short diffusion distance for gases.
3. The alveolar lining is moist, so gases dissolve in the liquid before diffusing across the membrane.
4. A dense capillary network surrounds each alveolus, carrying O₂ away and maintaining a high concentration gradient.
5. The large concentration gradient between alveolar air and blood drives rapid diffusion of O₂ into the blood and CO₂ out of it.
2. Their walls are extremely thin, giving a short diffusion distance for gases.
3. The alveolar lining is moist, so gases dissolve in the liquid before diffusing across the membrane.
4. A dense capillary network surrounds each alveolus, carrying O₂ away and maintaining a high concentration gradient.
5. The large concentration gradient between alveolar air and blood drives rapid diffusion of O₂ into the blood and CO₂ out of it.
Examiner tips
- Use the exact terms: surface area, thin walls, moist lining, capillaries, concentration gradient.
- Show the causal link: each adaptation improves diffusion rate or capacity.
Common mistakes
- Confusing ‘surface area’ with ‘volume’ of alveoli.
- Forgetting that the moist lining is for dissolving gases, not just for wetness.
Mark scheme (5 marks)
- Large surface area increases the rate of diffusion / allows more molecules to diffuse across at once
- Thin walls / short diffusion distance so gases diffuse across the membrane more quickly
- Moist lining / surfaces are moist so gases can dissolve before diffusing across the membrane
- Good blood supply / capillaries carry oxygen away from the alveoli maintaining a large concentration gradient
- Large concentration gradient between alveolar air and blood means diffusion occurs faster / down a steep concentration gradient
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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