section_path: Microanatomy/Alveoli
section_name: Alveoli
paragraph_text: Alveoli
  
  Alveoli consist of two types of alveolar cell and an alveolar macrophage. The two types of cell are known as type I and type II cells (also known as pneumocytes). Types I and II make up the walls and alveolar septa. Type I cells provide 95% of the surface area of each alveoli and are flat (" squamous "), and Type II cells generally cluster in the corners of the alveoli and have a cuboidal shape. Despite this, cells occur in a roughly equal ratio of 1:1 or 6:4.
  
  Type I are squamous epithelial cells that make up the alveolar wall structure. They have extremely thin walls that enable an easy gas exchange. These type I cells also make up the alveolar septa which separate each alveolus. The septa consist of an epithelial lining and associated basement membranes. Type I cells are not able to divide, and consequently rely on differentiation from Type II cells.
  
  Type II are larger and they line the alveoli and produce and secrete epithelial lining fluid, and lung surfactant. Type II cells are able to divide and differentiate to Type I cells.
  
  The alveolar macrophages have an important role in the immune system. They remove substances which deposit in the alveoli including loose red blood cells that have been forced out from blood vessels.