5.1 - Lungs & Gas Exchange
How the alveolus is adapted for gas exchange
- The alveolus is specialised so oxygen and carbon dioxide diffuse across it as fast as possible.
- Several features work together to make it an efficient exchange surface.
The adaptations
- Large surface area - There are so many alveoli (a total of roughly 75 square metres in an adult) that a large amount of gas can diffuse across at the same time.
- Thin walls - Each alveolus wall is only one cell thick, so gases have only a tiny distance to travel between the air and the blood. A short diffusion path means exchange is fast.
- Moist lining - Gases dissolve in the moisture so they can cross the surface more easily.
- Permeable wall - The wall lets the gases pass through.
- Good blood supply - Capillaries wrapped around each alveolus constantly bring fresh blood that is low in oxygen and high in carbon dioxide, and carry away the exchanged gases, keeping the concentration gradient across the wall steep.
- Good ventilation - Breathing keeps moving fresh air in and stale air out, which also helps keep the concentration gradients steep.