Explain why the concentration of carbon dioxide in expired air is higher than in inspired air.

Cambridge International IGCSE Biology (0610) — 11.1 Gas exchange in humans · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Oxygen is taken up by cells during cellular respiration.
Carbon dioxide is produced as a metabolic waste product.
The CO₂ diffuses from the blood into the alveolar air because its concentration is higher in the blood.
The CO₂ is then exhaled during expiration, so expired air contains more CO₂ than inspired air.

Examiner tips

Use the exact terms "cellular respiration", "metabolic waste", "diffuses", "exhaled" to match the mark scheme.
Show the cause–effect chain: oxygen uptake → CO₂ production → diffusion into alveoli → exhalation.
Keep the answer concise and to the point – 4 marks only.

Common mistakes

Confusing inspired and expired air concentrations.
Omitting the diffusion step or the role of cellular respiration.
Using vague language such as "more gas" instead of specifying CO₂.

Mark scheme (4 marks)

Oxygen is used in respiration by cells / tissues
Carbon dioxide is produced as a waste product of respiration
Carbon dioxide diffuses from the blood into the alveoli
This carbon dioxide is then breathed out / expelled during expiration

Key terms in this question

expired air · inspired air · carbon dioxide