Investigating the Differences in Inspired & Expired Air
- Air that is breathed in and air that is breathed out has a different composition of gases in it due to exchanges that take place in the alveoli
- Atmospheric air contains around 20 – 21% oxygen, of which we only absorb around 4 – 5%, breathing out air containing around 16% oxygen
- The normal carbon dioxide content of air is around 0.04% and, as carbon dioxide diffuses into the alveoli from the blood, we breathe out air containing around 4% carbon dioxide
- The air we breathe out contains more water vapour than when we breathe it in, and the temperature of exhaled air is usually higher than inhaled air
Composition of Air Table
| Gas | Inspired air | Expired air |
| Oxygen (O2) | 21% | 16% |
| Carbon dioxide (CO2) | 0.04% | 4% |
| Nitrogen (N2) | 78% | 78% |
| Other gases (water vapour, argon, trace gases) | 0.96% | 2% |
- A simple experimental setup using limewater (calcium hydroxide solution, Ca(OH)2) can be used to investigate the differences between inspired and expired air
Limewater Analysis of Air Diagram
The limewater test
- When we breathe in, the air is drawn through boiling tube A
- When we breathe out, the air is blown into boiling tube B
- Limewater is clear but becomes cloudy (or milky) when carbon dioxide is bubbled through it
- The lime water in boiling tube A will remain clear, but the limewater in boiling tube B will become cloudy
- This shows us that the percentage of carbon dioxide in exhaled air is higher than in inhaled air
Reason for Differences Between Inspired & Expired Air Table
| Gas | Inspired air | Expired air | Reason for Difference |
| Oxygen | 21% | 16% | Oxygen is removed from the 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 | 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 | 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 | 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 |
