2.3 Alveoli + their adaptations

Alveoli role

• Groups of alveoli are found at the ends of bronchioles.

  • Oxygen diffuses from alveoli into the blood.

  • Carbon dioxide diffuses from blood into alveoli.

• Moist lining allows oxygen and carbon dioxide to dissolve before diffusing across the walls.
  
  

Large surface area

• The lungs contain millions of tiny alveoli.

• Each alveolus provides a surface across which gases can diffuse.

• Having many alveoli creates a very large total surface area.

• This allows more oxygen and carbon dioxide molecules to diffuse across the exchange surface at the same time.

Short diffusion distances

• Alveolar epithelium is one cell thick.

• Capillary endothelium is also one cell thick.

• Oxygen only has a short distance to travel from the alveolar air into the blood.

• Carbon dioxide also has a short distance to travel from the blood into the alveoli.

Moist lining

• The inside of the alveoli has a thin layer of moisture.

• Oxygen and carbon dioxide dissolve in this moisture before diffusing across cell membranes.

• This helps the gases cross the exchange surface.

Rich blood supply

• Each alveolus is surrounded by a network of blood capillaries.

• Blood arriving at the lungs has a relatively low oxygen concentration and a relatively high carbon dioxide concentration compared with alveolar air.

• Oxygen diffuses from the alveoli into the blood.

• Blood flow carries oxygen away and brings more blood with a lower oxygen concentration to the lungs.

• This helps maintain a concentration gradient for oxygen. It also helps maintain the gradient for carbon dioxide in the opposite direction.