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 move from the air inside the alveoli into the blood, and carbon dioxide must move from the blood into the alveoli.
Model answer (5 marks)
Alveoli are highly specialised for gas exchange because they provide a very large surface area, allowing many molecules to diffuse at once. Their walls are extremely thin, giving a short diffusion distance so gases move quickly. The alveolar lining is moist, which lets oxygen and carbon dioxide dissolve before crossing the membrane. A dense network of capillaries surrounds each alveolus, carrying oxygen away and keeping the concentration gradient high. Finally, ventilation continually replaces the air in the alveoli, maintaining a low CO₂ concentration and a large gradient for diffusion.
Examiner tips
- Mention all five adaptations in a concise way; use the exact terms from the mark scheme.
- Show the link between each adaptation and the diffusion process (surface area, thickness, moisture, blood flow, ventilation).
Common mistakes
- Omitting the role of ventilation or blood flow; confusing diffusion with convection.
- Using vague phrases like "many alveoli" instead of "large surface area"; not specifying that the lining is moist for dissolution.
Mark scheme (5 marks)
- Large surface area (of the alveoli) allows more molecules to diffuse across at once
- Thin walls / short diffusion distance so gases diffuse across quickly
- Moist lining so gases can dissolve before diffusing across the membrane
- Good blood supply / capillaries carry oxygen away, maintaining a large concentration gradient
- Ventilation maintains a large concentration gradient by replacing air in the alveoli / keeping carbon dioxide concentration low in the alveoli
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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