Differences in Inspired & Expired Air (Cambridge International IGCSE Biology)
Exam board Cambridge International · IGCSEUpdated 2026-08-28
Differences in Inspired & Expired Air: Extended
Spec point —
11.1.9· Explain the differences in composition between inspired and expired air
Extended Tier Only
Reason for differences between inspired and expired air table
| Gas | Inspired air (%) | Expired air (%) | Reason for difference |
|---|---|---|---|
| Oxygen (O2) | 21 | 16 | Oxygen is removed from blood by respiring cells, so blood returning to the lungs has a lower oxygen concentration than the air in the alveoli, which means oxygen diffuses into the blood in the lungs |
| Carbon dioxide (CO2) | 0.04 | 4 | Carbon dioxide is produced by respiration and diffuses into blood from respiring cells; the blood transports the carbon dioxide to the lungs, where it diffuses into the alveoli as it is in a higher concentration in the blood than in the air in the alveoli |
| Water vapour (H2O) | Lower | Higher | Water evaporates from the moist lining of the alveoli into the expired air as a result of the warmth of the body |
| Nitrogen (N2) | 78 | 78 | Nitrogen gas is very stable and so cannot be used by the body; for this reason, its concentration does not change in inspired or expired air |
Exam Hint Make sure you can explain the reasons behind the differences between inspired and expired air, not just recall the differences themselves, this is a common Extended Tier requirement.
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