Final answer:
The alveolus wall is primarily made of thin, gas-permeable type I alveolar cells or squamous epithelial cells, with interspersed type II alveolar cells producing pulmonary surfactant. Alveolar macrophages also reside in the alveolus, removing debris and pathogens.
Step-by-step explanation:
The wall of the alveolus consists of a single layer of type I alveolar cells, which are also known as squamous epithelial cells. These cells constitute up to 97 percent of the alveolar surface area and are only about 25 nm thick. This allows them to be highly permeable to gases, enabling efficient gas exchange. In the alveolar wall, the type I cells are joined by type II alveolar cells that secrete pulmonary surfactant, a critical substance made of phospholipids and proteins that reduces surface tension within the alveoli. Additionally, alveolar macrophages patrol the alveoli, ingesting foreign particles and pathogens to protect the respiratory system.
The close association between the alveoli and the capillaries forms a respiratory membrane that is approximately 0.5 μm thick. This membrane supports gas diffusion; oxygen passes into the blood while CO₂ is expelled into the alveoli's air space. The extensive surface area of the alveoli, roughly 75 m² in total, along with their thin walls, ensures that gas exchange is both rapid and efficient.