What are the special functions of the alveoli?
To provide a large surface area for gas exchange.
The alveoli are very small air sacs in the lungs. Oxygen rich air travels along the airways, into the lungs (via the primary, secondary and tertiary bronchi) then along tiny bronchioles where it reaches the alveoli. Covered in tiny capillaries, they provide a huge surface area for oxygen to diffuse across into the bloodstream (ready to be distributed out to the body) and carbon dioxide to diffuse out of the blood to be exhaled (and removed from the body).
The alveoli are specially adapted for this purpose:
Thin membrane Only one cell thick, there is only a short distance for gases to travel across, making diffusion easier.
They are very small and folded This maximises the surface area that gas can diffuse across.
They are covered in capillaries By removing oxygen rich blood instantaneously, it keeps the concentration gradient steep, so diffusion is quicker and easier.
Hope this helps; let me know if I can do anything else:)