Answer:
D. respiratory membrane
Step-by-step explanation:
The "alveolar walls" and the "pulmonary capillaries" are part of the Respiratory System of the human body. They form the respiratory membrane/respiratory surface (a very thin layer), which makes the exchange of gases possible. It functions by separating the air from the blood. This enables oxygen to enter our body and for carbon dioxide to go out of the body. It makes the gas exchange efficient and fast. This movement is called "diffusion."