Adaptations of alveoli for gas exchange

OCR J257/01 — Extended response

Explain how the structure of the alveoli is adapted for efficient gas exchange. [6 marks]

Mark scheme — level response:

Level 3 (5–6 marks): Detailed account of adaptations with clear mechanistic explanation of how each adaptation increases exchange efficiency. Four or more features discussed.

Level 2 (3–4 marks): Two or three adaptations described with some explanation of mechanism. Some logical development.

Level 1 (1–2 marks): One or two features stated with little/no explanation.

Indicative content:

• Large surface area: ~300 million alveoli / total SA ~70 m² → more molecules can cross simultaneously
• Thin walls: alveolus wall + capillary wall each one cell thick → short diffusion path → faster diffusion by Fick's Law
• Rich blood supply: capillaries continually remove O₂ and deliver CO₂ → steep concentration gradient maintained
• Ventilation: breathing movements constantly renew air → high O₂, low CO₂ in alveolus → steep gradient maintained
• Moist lining: gases dissolve in water film → can cross the membrane

Total: 6 marks