B3.1B The mammalian lung: alveoli, ventilation and lung volumes

Mammalian Lungs Pack a Huge, Thin, Well-Supplied Surface Into a Small Space

1. Mammalian lungs are built around millions of tiny air sacs called alveoli (singular: alveolus), the sites of gas exchange.
2. Air reaches them through branching airways: the trachea, bronchi, and bronchioles.
3. Together these features give a large surface area and a short diffusion distance, the two things gas exchange depends on most.

A Huge Alveolar Surface Speeds Up Gas Exchange

1. Each lung holds around 300 million alveoli, giving a total surface area of about 70 m², roughly the size of a tennis court.
2. A large surface area lets more oxygen enter the blood and more carbon dioxide leave it at the same time.

The bigger the surface, the faster gases can diffuse across it.

Thin Type I Pneumocytes Give the Shortest Possible Diffusion Path

1. The alveolar wall is made mainly of type I pneumocytes, extremely thin cells specialised for gas exchange.
2. The alveolar wall and the wall of the surrounding capillary are each only one cell thick.
3. This thin barrier lets oxygen and carbon dioxide cross quickly.