Explain how ventilation of the lungs maintains the concentration gradients necessary for gas exchange in the alveoli.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Oxygen is continually removed from alveolar air by diffusion into the blood, so fresh air must be delivered to keep the alveolar O₂ partial pressure high.
Carbon dioxide diffuses from the blood into the alveoli, and expiration removes this CO₂‑rich air, keeping the alveolar CO₂ partial pressure low.
These actions maintain steep concentration gradients across the alveolar membrane, which, by Fick’s law, increases the net rate of gas diffusion.
Blood flow through the alveolar capillaries works with ventilation: it carries O₂ away and delivers CO₂, preventing equilibration of gases on the blood side of the membrane.
Carbon dioxide diffuses from the blood into the alveoli, and expiration removes this CO₂‑rich air, keeping the alveolar CO₂ partial pressure low.
These actions maintain steep concentration gradients across the alveolar membrane, which, by Fick’s law, increases the net rate of gas diffusion.
Blood flow through the alveolar capillaries works with ventilation: it carries O₂ away and delivers CO₂, preventing equilibration of gases on the blood side of the membrane.
Examiner tips
- Use the word ‘ventilation’ to link air flow with concentration gradients.
- Show the two gases separately (O₂ and CO₂) and explain how each is affected.
- Mention Fick’s law to justify the effect of gradient steepness.
- Include the role of perfusion to show the system works together.
Common mistakes
- Confusing ventilation with perfusion or omitting one of the gases.
- Failing to state that ventilation keeps the alveolar O₂ high and CO₂ low.
- Not mentioning Fick’s law or the role of blood flow in maintaining gradients.
Mark scheme (4 marks)
- Oxygen is continuously removed from alveolar air by diffusion into the blood, so fresh air must be delivered to maintain a high alveolar oxygen partial pressure / concentration.
- Carbon dioxide diffuses from the blood into the alveoli, and expiration removes this CO₂-rich air, maintaining a low partial pressure / concentration of carbon dioxide in the alveoli.
- The steeper / maintained concentration gradient increases the net rate of diffusion of gases across the alveolar membrane, according to Fick's law / principle.
- Continuous blood flow (perfusion) through the alveolar capillaries works in concert with ventilation: blood constantly carries oxygen away and delivers CO₂, preventing equilibration of gases on the blood side of the membrane.
Key terms in this question
ventilation · concentration gradient
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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