Final answer:
The alveoli where gas exchange occurs are made up of Type I pneumocytes, which are the primary surface cells, and Type II pneumocytes, which secrete surfactant.
Step-by-step explanation:
The type of cells that make up the alveoli where gas exchange occurs are the Type I pneumocytes and Type II pneumocytes. Type I alveolar cells are squamous epithelial cells and are the major cell type covering about 97 percent of the alveolar surface area. They are extremely thin, about 25 nm thick and highly permeable to gases, which facilitates the gas exchange process. Type II alveolar cells are cuboidal epithelial cells, which are fewer in number but very important as they secrete pulmonary surfactant to reduce the surface tension of the alveoli, preventing the collapse of alveoli during exhalation.