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

WJEC GCSE Biology (Wales) — 1.2.2 The respiratory system · Describe · 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 branched, giving more area for gas exchange.
The alveolar walls are only one cell thick, so the diffusion distance for oxygen and carbon dioxide is very short.
Each alveolus is surrounded by a dense network of capillaries, which keeps a concentration gradient for gases and allows rapid transfer.
The alveolar lining is moist, providing a medium in which gases can dissolve and diffuse efficiently.

Examiner tips

  • Mention surface area, wall thickness, capillary network and moisture in separate points.
  • Use the exact terms ‘large surface area’, ‘one cell thick’, ‘dense network of capillaries’, ‘moist lining’.
  • Keep each point brief and to the point to fit the 4‑mark limit.

Common mistakes

  • Failing to state that the walls are only one cell thick.
  • Omitting the moist lining or the capillary network.
  • Using vague language such as ‘thin walls’ instead of ‘one cell thick’.

Mark scheme (4 marks)

  1. Large surface area (of alveoli / many alveoli)
  2. Walls are one cell thick / thin walls to give a short diffusion distance
  3. Rich / dense network of capillaries surrounding the alveoli (maintains a concentration gradient)
  4. Moist / damp lining (allows gases to dissolve for diffusion)

Key terms in this question

alveoli

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