The Alveolus and the Blood Air Barrier

• The alveolar cell layer consists mainly of simple squamous epithelium
• The squamous epithelial cells, called type I alveolar cells, are unusually thin and are the sites of gas diffusion
• Roaming alveolar macrophages patrol the epithelial surface
• Large type II alveolar cells are scattered among the squamous cells
• Type II alveolar cells produce surfactant, an oily secretion containing phospholipids and proteins
• Gas exchange occurs across the three layered blood air barrier of the alveoli
• Blood air barrier made up of

  • alveolar cell layer
  • capillary endothelial layer
  • fused basement membrane between