So if we zoom in on a single alveolis, we basically get the following diagram. So, inside this region, we have air, we have carbon dioxide and we have oxygen. Now, this purple section is the wall of the alveola of the alveola. So it's the alveolar wall. And now within the wall, within the inside portion of the wall, between the wall and the air, we have this layer of fluid we call the alveolar fluid. And this is a polar fluid.