The alveoli are lined with two types of epithelial cells: Type I alveolar cells (simple squamous epithelial cells) which are the main sites of gas exchange and Type II alveolar cells (septal cells) which secrete alveolar fluid.

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Type I alveolar cells are indeed simple squamous epithelial cells that facilitate gas exchange, while Type II alveolar cells (septal cells) secrete alveolar fluid containing surfactant, which reduces surface tension within the alveoli to prevent their collapse.

The Alveoli Are Lined with Two Types of Epithelial Cells