Final answer:
The alveolar sacs of the lungs are lined with type I alveolar cells, which are squamous epithelial cells facilitating gas exchange. Type II alveolar cells, which are cuboidal, also contribute by secreting surfactant that lowers alveolar surface tension.
Step-by-step explanation:
The alveolar sacs of the lungs are lined with type I alveolar cells, which are squamous epithelial cells. These cells are extremely thin (about 25 nm thick) and make up to 97 percent of the alveolar surface area, making them highly permeable to gases. This characteristic facilitates efficient gas exchange, allowing oxygen to enter the blood and carbon dioxide to be expelled from the body.
Type II alveolar cells are also present in the alveolar walls but in a smaller proportion. These cuboidal epithelial cells are crucial for secreting pulmonary surfactant, thus reducing surface tension within the alveoli and preventing their collapse.
The combined action of these cell types and the presence of a thin respiratory membrane ensure an optimal gas exchange environment in the alveolar sacs.