The alveolar tree is the terminal part of the respiratory system, forming the site of major gas exchange.

Figure 1. Histology of the alveolar tree

Structure

The respiratory part of the lungs includes the respiratory bronchioles, which give rise to the alveolar ducts, which in turn give rise to the alveolar sacs that open up to a cluster of alveoli.

  • Alveolar ducts resemble "hallways" bordered almost exclusively by alveoli, and surrounded by smooth-muscle cells and connective tissue.
  • Alveolar sacs are the distal ends of the alveolar ducts, opening up to clusters of alveoli.
  • Alveoli are air spaces surrounded alveolar cells and capillaries, which allow the exchange of gases between the blood and air.
  • An interalveolar septa separates one alveolus from its neighbor, with interalveolar pores (of Kohn) allowing collateral air circulation between adjacent alveoli.

Cells

The alveolar epithelium consists of alveolar cells, with the occasional brush cells, alveolar macrophages, and fibroblasts.

  • Alveolar cells (pneumocytes) are divided into two types of cells:
    • Type I alveolar cells (type I

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