11.1.4Gas Exchange in Humans
Differences in Inspired and Expired Air — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 5 of 8
Section 1 of 6Two samples of air
Inspired air
- Inspired air This is the air breathed in from the atmosphere, so it has the composition of ordinary air.
- It is cooler and its water vapour content depends on the weather.
Expired air
- Expired air This is the air breathed out after it has been in contact with the alveoli.
- Its composition has changed because gases were exchanged with the blood.
Section 2 of 6Typical composition
| Property | Inspired air | Expired air |
|---|---|---|
| Oxygen | About 20 to 21% | About 16% |
| Carbon dioxide | About 0.04% | About 4% |
| Water vapour | Variable, usually low | Higher, about 1% more |
| Nitrogen | About 78% | About 78% (little change) |
| Temperature | Same as surroundings | Warmer, close to body temperature |
Section 3 of 6Reading the numbers
Gas exchange in the alveoli
Oxygen falls
- Oxygen falls Only a small share of the oxygen, around 4 to 5% of the air, is absorbed into the blood, so expired air still holds a lot of oxygen.
Carbon dioxide rises
- Carbon dioxide rises Carbon dioxide diffuses from the blood into the alveoli, so expired air contains about one hundred times as much as inspired air.
Water vapour rises
- Water vapour rises Moisture evaporates from the damp lining of the lungs and airways into the air passing over it.
Temperature rises
- Temperature rises Heat released by respiration warms the body, and the air breathed out is warmed on its way through the airways.
Section 4 of 6How the differences can be shown
1
To detect carbon dioxide, bubble inspired air and expired air through separate tubes of limewater.
2
Limewater turns cloudy faster and more strongly with expired air because it contains more carbon dioxide.
3
To show water vapour, breathe onto a cold mirror or dry cobalt chloride paper; condensation appears and the paper turns from blue to pink.
4
To show the temperature difference, compare readings from a thermometer in the room with breath blown across it.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Inspired air | Air that is breathed in. |
| Expired air | Air that is breathed out. |
| Limewater | A solution that turns cloudy when carbon dioxide is bubbled through it. |
| Water vapour | Water in the form of a gas, found in the atmosphere and in breath. |
| Gas exchange | The diffusion of oxygen into the blood and carbon dioxide out of it at the alveoli. |
Section 6 of 6Exam technique
1
Saying that expired air contains no oxygen — it still holds about 16% oxygen.
2
Writing that expired air is 'pure carbon dioxide' — it contains only about 4%.
3
Reversing the percentages for carbon dioxideinspired air has about 0.04%, expired about 4%.
4
Forgetting that nitrogen does not change much because the body does not use it.
Last updated: