Explain how the structure of the alveoli makes them well suited for gas exchange.

Eduqas GCSE Biology — 1.2.2 The respiratory system · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Alveoli have a very large surface area because they are many in number and highly folded, which allows a great amount of gas to diffuse at the same time.
The walls of the alveoli are only one cell thick, so the diffusion distance for oxygen and carbon dioxide is very short and the gases move quickly.
They are surrounded by a dense network of capillaries, giving a constant concentration gradient that drives the exchange of gases.
The alveolar lining is moist, which lets the gases dissolve in the fluid before they cross the membrane, further speeding diffusion.

Examiner tips

  • Mention surface area, wall thickness, capillary network and moist lining in separate points
  • Use terms like ‘diffusion distance’, ‘concentration gradient’ and ‘moist lining’
  • Keep each point concise and directly linked to gas exchange

Common mistakes

  • Failing to mention the thinness of the alveolar wall
  • Confusing alveoli with other lung structures
  • Omitting the role of the moist lining

Mark scheme (4 marks)

  1. Large surface area (of alveoli / many alveoli present) allows more gas to diffuse at once
  2. Thin walls (one cell thick) so diffusion distance is short / gases diffuse quickly
  3. Good blood supply (dense capillary network) maintains a concentration gradient for oxygen and carbon dioxide
  4. Moist lining allows gases to dissolve before diffusing across the membrane

Key terms in this question

alveoli

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