The lung alveoli contain the air-blood barrier or interalveolar septum. The wall of the alveolus is especially thin and consists of a layer of surfactant, the flattened type I pneumocyte, a shared basement membrane, and the endothelial cell of a capillary.

• Carefully examine walls of alveoli in this section of lung and the images to the right. In this specimen of lung the macrophages are easy to see because they are filled with black material that they have ingested. In these slides identify:
  • The flattened type I pneumocytes
  • The rounded type II pneumocytes
  • Capillaries
  • Macrophages (dust cells)
• Finally, locate an intact edge of the specimen and look at the simple squamous epithelium or mesothelium of the visceral pleura.

Now try this short quiz to practice what you’ve learned.

Now comes immune and lymphatic system.