Learn more about the diffusion of gases in Chemical World 9.2
Alveoli and the Exchange of Gases The lungs contain millions of tiny air sacs called alveoli. The alveoli are in close and extensive contact with blood in capillaries. This is where gases are exchanged between the air in lungs and dissolved gases in the blood. z z
zz
Oxygen moves from the alveoli to capillaries. Carbon dioxide moves from capillaries to the alveoli.
Alveoli have many adaptations that make them very efficient at exchanging gases:
z z Air out
Blood containing carbon dioxide
Air in zz
The walls of alveoli and capillaries are very thin, which makes them extremely permeable to gases.
There are approximately 300,000,000 alveoli in each lung. This provides a large surface area for gaseous exchange.
Carbon dioxide Capillary Oxygen Alveolus zz
Each alveolus is covered in capillaries. This allows for the fast diffusion of gases between the air in alveoli and the dissolved gases in the blood flowing through capillaries.
zz
Blood containing oxygen
z Fig. 8.4.3 The exchange of gases at an alveolus
Continuous breathing maintains a difference in the concentration of oxygen and carbon dioxide in alveoli and capillaries. This helps diffusion because gases will always move from an area of high concentration to one of low concentration.
Comparing the composition of inhaled and exhaled air
Consider the table below, which shows the composition of inhaled and exhaled air. Inhaled air (%)
Oxygen
Carbon dioxide Water vapour Nitrogen
Noble gases
20.9 0.03
Variable 78.1 0.94
Exhaled air (%) 16
4.0
Variable but more than inhaled air 78.1 0.94
1. Why is there a significant difference in the composition of gases in inhaled and exhaled air? (Hint: consider respiration.)
2. Why does the concentration of nitrogen and noble gases not change?
Now plan and carry out an investigation into the composition of inhaled and exhaled air. 3. Record your observations.
4. Predict any differences in the composition of exhaled air between a smoker and a non-smoker. 140
