* The thin squamous epithelial cells of the alveoli (Type I pneumocytes): These cells form the inner lining of the alveoli, where gas exchange occurs.

* The thin squamous epithelial cells of the capillaries: These cells line the blood vessels that surround the alveoli.

* The fused basement membranes of the alveolar and capillary cells: These membranes provide structural support and act as a barrier between the air and blood.

In total, the respiratory membrane is incredibly thin, often less than 0.5 micrometers thick, which allows for efficient gas exchange by diffusion. This delicate structure is also permeable to gases, allowing oxygen to pass from the alveoli into the capillaries and carbon dioxide to move from the capillaries into the alveoli.