Explain why the concentration of carbon dioxide is higher in the air inside the alveoli than in the blood arriving at the alveoli from the pulmonary artery.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During gas exchange in the lungs, carbon dioxide moves from the blood into the air inside the alveoli. The direction of this movement depends on concentration gradients.
Model answer (4 marks)
Carbon dioxide is produced in body cells during aerobic respiration.
Blood carrying the CO₂ from the tissues enters the pulmonary artery.
Thus the blood arriving at the alveoli has a higher CO₂ concentration than the alveolar air.
CO₂ therefore diffuses down its concentration gradient from the blood into the alveolar air.
Blood carrying the CO₂ from the tissues enters the pulmonary artery.
Thus the blood arriving at the alveoli has a higher CO₂ concentration than the alveolar air.
CO₂ therefore diffuses down its concentration gradient from the blood into the alveolar air.
Examiner tips
- Use the word ‘diffuses’ and mention a concentration gradient. Show the sequence: production → transport → higher concentration in blood → movement into alveoli. Keep answer to 4 short sentences to match the 4 marks.
Common mistakes
- Confusing the direction of diffusion (writing that CO₂ moves from alveoli to blood). Omitting that CO₂ is produced in tissues. Using vague terms like ‘moves’ instead of ‘diffuses’ and not mentioning concentration gradients.
Mark scheme (4 marks)
- Carbon dioxide is produced during (aerobic) respiration in body cells / tissues
- Carbon dioxide diffuses from body cells into the blood / blood picks up / carries carbon dioxide from respiring tissues
- So the blood arriving at the alveoli has a high concentration of carbon dioxide / the concentration of carbon dioxide in the blood is higher than in alveolar air
- Therefore carbon dioxide diffuses down the concentration gradient from the blood into the alveolar air / from higher to lower concentration
Key terms in this question
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Gas exchange: humans questions
- Explain how the structure of the alveoli is adapted to allow efficient gas excha…
- Explain why oxygen moves from the air in the alveoli into the blood in the surro…
- Describe the role of cilia and mucus in protecting the gas exchange system from …
- Describe the roles of the intercostal muscles and diaphragm during inhalation in…