Alveolar Ducts and Alveoli

• The alveolar ducts are passageways that connect the respiratory bronchioles to the alveoli.
• Alveoli are thin-walled, elastic buds at the end of the respiratory tree.
• There are approximately 300-600 million alveoli.
• Types of cell that can be found inside the alveoli
• 1. Type I Pneumocyte

  • An squamous epithelial cell that creates a very thin diffusion membrane or barrier for gases and facilitates gas exchange.
• 1. Type II Pneumocyte

  • secretes surfactant, which decreases the surface tension or the adhesive force inside the lungs and prevents the alveoli from collapsing during exhalation.
  • Connects the epithelium to other adjacent cells via tight junctions.
  • Plays a role in acting as a progenitor cell to replace injured or damaged type I pneumocytes.
• 1. Type III Pneumocyte

  • Secrete macrophages, a type of white blood cell that protects your body against bacteria through phagocytosis.
  • As a product of phagocytosis, lysozyme is released, which breaks down epithelial cells including the type I pneumocytes.
  • Alpha-1 Antitrypsin prevents the breakdown of type I pneumocytes when lysozyme is released.
• Capillaries inside the alveoli contain erythrocytes that carry oxygen and carbon dioxide. If the erythrocytes came from systemic circulation, it contains more carbon dioxide than oxygen.
• Note that even though it is called deoxygenated blood, partially, the blood still contains oxygen.