Cambridge O Level Biology · Syllabus 5090 · Human Gas Exchange
Expired Air
What is Expired Air?
The air breathed out of the lungs. Compared with inspired air it contains less oxygen, approximately 16 per cent rather than approximately 21 per cent, because oxygen has diffused from the alveolar air into the blood and been used in respiration in the body's cells. It contains much more carbon dioxide, approximately 4 per cent rather than approximately 0.04 per cent, because carbon dioxide produced by respiration has diffused from the blood into the alveolar air. Its nitrogen content is approximately unchanged at about 78 per cent, because nitrogen is not exchanged in the lungs. It carries more water vapour, because the air has passed over the moist linings of the airways and alveoli, and it is close to body temperature, because it has been warmed on the way through.
This definition is part of the Human Gas Exchange chapter in Cambridge O Level Biology.
Common mistakes with Expired Air
- 2. Saying expired air has no oxygen. It has about 16 %. Only about a fifth of the oxygen you breathe in is absorbed. This is why rescue breathing works at all.
- 6. Reporting a conclusion as an observation. “Limewater went cloudy faster” is what you saw. “Expired air contains a higher concentration of carbon dioxide” is what it means. Practical questions frequently award those two separately, and answers that only give one of them get half the marks.
- Nitrogen is not “used up”, and it is not “produced”. Its percentage looks unchanged because nothing exchanges it. Strictly, because the total volume of expired air is slightly different from the volume inspired, the nitrogen percentage can shift by a fraction of a point — but at this level it is treated as unchanged, and you should write “approximately the same” rather than “exactly the same”.
Questions students ask about Expired Air
How does expired air differ from inspired air?
It contains less oxygen, about 16 % rather than about 21 %, because oxygen has diffused into the blood and been used in respiration. It contains much more carbon dioxide, about 4 % rather than about 0.04 %, because carbon dioxide produced by respiration has diffused out of the blood into the alveolar air. It carries more water vapour, because the air has passed over moist linings, and it is close to body temperature, because it has been warmed on the way through. The percentage of nitrogen is approximately unchanged.
Does expired air contain any oxygen?
Yes, about 16 %. Only about a fifth of the oxygen breathed in is actually absorbed: a large fraction of each breath never reaches an alveolus at all, and even in the alveoli the blood does not take up all the oxygen available in a single pass. This is why one person’s expired air can still supply another person during rescue breathing.
Why does limewater go cloudy, and what does it prove?
Limewater turns cloudy when carbon dioxide is bubbled through it, and it turns cloudy faster when the concentration of carbon dioxide is higher. Comparing two identical tubes shows that expired air contains a higher concentration of carbon dioxide than inspired air. It does not measure how much: the test is qualitative, and the time taken cannot be converted into a percentage.