It's a very small type of pulmonary vein. Now let's take a look at the actual membrane within which we have this diffusion of oxygen and carbon dioxide taking place. So notice we have two important types of cells within the alveolis. We have the cell labeled as number four. That is our alveolar cell type number two. And what this cell does is it produces and releases the pulmonary surfactant that is necessary to prevent the alveolis from actually collapsing when we exhale and to decrease the surface tension and therefore the pressure that is needed to actually inflate our alveolis.