Explain why the lungs contain a large number of alveoli rather than a single large air sac.

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)

A large number of alveoli gives a much greater total surface area.
This larger surface area allows more gas exchange to occur simultaneously.
A single large air sac would have a much smaller surface‑area‑to‑volume ratio.
Thus the rate of diffusion of O₂ into the blood (and CO₂ out) would be too slow to meet the body’s needs.

Examiner tips

  • Use the term ‘surface area’ and ‘surface‑area‑to‑volume ratio’. Explain the effect on diffusion rate. Show the logical sequence: many alveoli → large area → fast diffusion.
  • common_mistakes
  • :
  • Saying ‘more alveoli means more air’ instead of surface area. Confusing ‘volume’ with ‘surface area’. Omitting the link between surface area and diffusion rate.

Mark scheme (4 marks)

  1. A large number of alveoli provides a much greater total surface area
  2. A greater surface area allows more gas exchange to occur at the same time
  3. A single large air sac would have a much smaller surface area to volume ratio
  4. This means the rate of diffusion of oxygen into the blood (and carbon dioxide out) would be too slow to meet the body's needs

Key terms in this question

alveoli

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