Explain how the structure of the alveoli is adapted to allow efficient gas exchange in the lungs.

Pearson Edexcel International GCSE Biology (4BI1) — 2.9 Gas exchange: humans · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Large surface area of the alveoli allows many gas molecules to diffuse at once.
Thin walls (one cell thick) give a short diffusion distance for gases.
A rich blood supply and dense capillary network maintain a steep concentration gradient for O₂ and CO₂.
The moist lining lets gases dissolve before diffusing across the membrane.

Examiner tips

Use the exact terms: surface area, thin walls, capillary network, steep concentration gradient, moist lining.
Show the link between structure and function – e.g. short diffusion distance = faster exchange.
Include all four points to reach full marks.

Common mistakes

Omitting the moist lining or the capillary network.
Using vague phrases like "good structure" instead of specific features.
Not linking structure to the efficiency of gas exchange.

Mark scheme (4 marks)

Large surface area (of alveoli) allows more gas molecules to diffuse simultaneously
Thin walls (one cell thick) give a short diffusion distance / pathway for gases
Rich blood supply / dense capillary network maintains a steep concentration gradient for both oxygen and carbon dioxide
Moist lining allows gases to dissolve before diffusing across the membrane

Key terms in this question

alveoli

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