Comparing Inspired & Expired Air
- Inspired and expired air has different amounts of gases in due to exchanges that take place in the alveoli and respiring cells of the body
Composition of air table
Gas | Inspired air | Expired air | Reason for the difference |
Oxygen | 21% | 16% | Oxygen is removed from the blood during cellular respiration so blood returning to the lungs to be expired has a lower oxygen concentration |
Carbon dioxide | 0.04% | 4% | Carbon dioxide is diffused into the blood during cellular respiration so blood returning to the lungs to be expired has a higher carbon dioxide concentration |
Nitrogen | 78% | 78% | Nitrogen is an inert gas and is not used by the body so the same concentration is inspired and expired |
Water vapour | Varies | Saturated with water vapour | Water evaporates from the moist alveolar lining into expired air as a result of the warmth from the body |
The 'huff and puff' test
- A simple test using lime water can detect the presence of carbon dioxide
- It is used to compare the carbon dioxide content of inspired and expired air
Carbon dioxide test diagram
The limewater test for carbon dioxide
- When we breathe in, the air is drawn through boiling tube A
- When we breathe out, the air is blown into boiling tube B
- Lime water is colourless 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
The alveoli and gas exchange
- Gas exchange occurs by the process of diffusion
- The air entering the alveoli has a high concentration of oxygen
- The surrounding capillaries contain blood with a low concentration of oxygen: deoxygenated blood is brought to the lungs
- The oxygen diffuses from a region of high concentration (within the alveoli) across the walls of the alveoli and capillaries and into the red blood cells where there is a low concentration of oxygen; this oxygenated blood is then taken to the heart to be pumped all around the body
- The opposite can be said of carbon dioxide: deoxygenated blood is brought to the lungs which contains a high concentration of carbon dioxide
- The alveoli contain a low concentration of carbon dioxide
- Carbon dioxide diffuses from a region of high concentration (the blood) into the alveoli where the is a low concentration of carbon dioxide
- The alveoli are highly specialised for gas exchange
- Alveoli (and the capillaries around them) have thin, single layers of cells to minimise diffusion distance
- Ventilation maintains high levels of oxygen and low levels of carbon dioxide in the alveolar air space
- A good blood supply ensures a constant supply of blood high in carbon dioxide and low in oxygen
- A layer of moisture on the surface of the alveoli helps diffusion as gases dissolve
- Additionally, there are many rounded alveolar sacs within the lungs which give a very large surface area to volume ratio
- All of these adaptations maximise the rate of diffusion of oxygen and carbon dioxide
Adaptations of alveoli diagram
Alveoli are specifically adapted to maximise gas exchange