The two cell layers being referred from the question are a layer of alveolar cells and layer of endothelial cells. Gases must diffuse to these layers to proceed to the plasma. The layer of alveolar cells make up the alveolar wall while the layer endothelial cells consist the capillary wall.
Alveoli are tiny air sacs found in the lungs. Gases such as Oxygen and carbon dioxide move from alveoli to plasma through the alveolar and capillary walls.
These layers are referred to as the alveolar-capillary (respiratory) membrane.
