Explain how the ventilation of the lungs maintains the concentration gradients necessary for gas exchange.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. Ventilation brings fresh air into the alveoli, which has a high partial pressure of O₂ and a low partial pressure of CO₂.
2. This keeps the alveolar pO₂ higher than the pO₂ in the pulmonary capillary blood.
3. Expired air removes CO₂ from the alveoli, so alveolar pCO₂ remains lower than the pCO₂ in the blood coming from tissues.
4. Because gases diffuse down their concentration gradients, O₂ moves from alveolar air into blood and CO₂ moves from blood into alveolar air, sustaining efficient gas exchange.
2. This keeps the alveolar pO₂ higher than the pO₂ in the pulmonary capillary blood.
3. Expired air removes CO₂ from the alveoli, so alveolar pCO₂ remains lower than the pCO₂ in the blood coming from tissues.
4. Because gases diffuse down their concentration gradients, O₂ moves from alveolar air into blood and CO₂ moves from blood into alveolar air, sustaining efficient gas exchange.
Examiner tips
- Use the exact terms ‘partial pressure’, ‘alveoli’, ‘pulmonary capillaries’, and ‘concentration gradients’.
- Show the cause–effect link: ventilation → fresh air → high pO₂, low pCO₂ → gradients → diffusion.
- Mention both gases in the same sentence to cover all points.
Mark scheme (4 marks)
- Ventilation brings fresh air (high oxygen / low carbon dioxide) into the alveoli, maintaining a higher partial pressure of oxygen in the alveoli than in the blood of the pulmonary capillaries.
- Expired air removes carbon dioxide from the alveoli, keeping the partial pressure of carbon dioxide in the alveoli lower than in the blood arriving from the body tissues.
- Because gases diffuse down their concentration gradients, oxygen diffuses from the alveolar air into the blood and carbon dioxide diffuses from the blood into the alveolar air.
- Without continuous ventilation the gradients would diminish as oxygen is consumed and carbon dioxide accumulates, reducing the rate of gas exchange; ventilation therefore sustains efficient exchange.
Key terms in this question
ventilation · concentration gradient
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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