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

OCR A-Level Biology B — Advancing Biology (H422) — 2.4 Gas exchange surfaces · Explain · 5 marks · View as Markdown

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 inside the alveoli into the blood, whilst carbon dioxide passes in the opposite direction.

Model answer (5 marks)

Alveoli provide a very large surface area for gas exchange.

The large surface area increases the rate of diffusion of O₂ and CO₂.

Alveolar walls are only one cell thick, so the diffusion distance is very short.

The short diffusion distance allows gases to cross the alveolar–capillary barrier quickly.

A rich capillary network surrounds each alveolus, maintaining a steep concentration gradient for O₂ and CO₂.

Examiner tips

Mention all five points in the order given; use the exact terms "large surface area", "thin walls", "steep concentration gradient".
Explain how each feature improves diffusion – surface area → rate, thin walls → distance, capillaries → gradient.
Keep the answer concise – 5–6 sentences is enough for full marks.
Use UK spelling (e.g., "capillaries", "diffusion").

Common mistakes

Failing to mention the capillary network or the steep concentration gradient.
Using vague phrases like "big area" instead of "large surface area".
Omitting the fact that the alveolar wall is only one cell thick.

Mark scheme (5 marks)

Alveoli provide a large surface area
Large surface area increases the rate of diffusion (of gases)
Alveoli walls are very thin / one cell thick
Thin walls reduce the diffusion distance so gases can pass across quickly
Alveoli have a good blood supply / rich capillary network, which maintains a steep concentration gradient

Key terms in this question

alveoli