Answer:
Type I pneumocytes.
Step-by-step explanation:
Type I pneumocytes are cells that cover more than 90% of the alveolar surface, due to the fact that they are very flattened and extensive, they fundamentally facilitate gas exchange. When the pneumocytes I contact the capillaries of the alveolar wall their basement membrane fuses with that of the endothelium, so that the gases only have to cross the cytoplasm of the pneumocyte, the fused basement membranes and the cytoplasm of the capillary endothelium, a set that measures less than 1 micron thick. In the rest of the alveolar wall, between the pneumocytes and the capillaries, a layer of interstitial tissue is interposed, with amorphous substance and fibrils, which performs a support function and, as it has lymphatic vessels, drains the fluid that permanently ultrafilters from the capillaries, thus preventing it from invading the alveoli.