QS Study

Alveoli of Lung is the functional unit of lungs where the gaseous exchange between blood and the atmospheric air takes place. It is small air spaces in the lungs where carbon dioxide leaves the blood and oxygen enters. Alveoli are peculiar to mammalian lungs; different structures are involved in gas exchange in other vertebrates. These are found in the respiratory zone of the lungs.


  • In new-born: 10 million in each lung.
  • In adult: 150 million in each lung.


Each alveolus is about 200 µm – 300 µm in diameter.

Surface area – 70-100 square meters.

Connection – Adjacent alveoli intercommunicate by alveolar pores.

Lining epithelium By simple squamous epithelium.

Alveoli of Lung 1

Fig: Alveoli of Lung

Types of cells lining the alveoli

  1. Type-I cells: Through these cells, a gaseous exchange takes place. Type I cells are involved in the process of gas exchange between the alveoli and blood.
  2. Type-II cells: They secrete surfactant, which prevents the collapse of alveoli during expiration by reducing surface tension. Type II alveolar cells cover a small fraction of the alveolar surface area.
  3. Type-III cells: They phagocytes bacteria, dust particles or other debris.
Related Study: