Explain how the alveoli are adapted to allow efficient gas exchange in the lungs.

Edexcel GCSE Biology (1BI0) — 8.1 Gas exchange · Explain · 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 alveolar walls are extremely thin, only one cell thick, reducing the diffusion distance for O₂ and CO₂.
2. The alveoli are highly branched and numerous, giving a very large total surface area for gas exchange.
3. The alveolar lining is moist, allowing gases to dissolve into the fluid before diffusing across the membrane.
4. A rich capillary network surrounds each alveolus, maintaining a steep concentration gradient and ensuring rapid transfer of gases.

Examiner tips

  • Use the exact terms: ‘thin walls’, ‘large surface area’, ‘moist lining’, ‘rich blood supply’.
  • Show the causal link: each adaptation reduces resistance to diffusion or increases capacity.
  • Keep each point short and focused on one feature.

Common mistakes

  • Confusing alveoli with bronchioles; forgetting to mention the capillary network; using vague phrases like ‘many alveoli’ without quantifying surface area.

Mark scheme (4 marks)

  1. Large surface area increases the rate of diffusion / more molecules can diffuse at once
  2. Thin walls reduce the diffusion distance so gases diffuse more quickly
  3. Good/rich blood supply maintains a steep concentration gradient for oxygen and carbon dioxide
  4. Moist lining allows gases to dissolve so they can diffuse across the exchange surface

Key terms in this question

alveoli

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