Final answer:
The respiratory membrane is coated by pulmonary surfactant, (option 1) which is secreted by type II alveolar cells and helps reduce alveolar surface tension. The respiratory membrane is formed by a simple squamous epithelium, enabling efficient gas exchange through simple diffusion.
Step-by-step explanation:
The surface of the respiratory membrane is coated by a substance called pulmonary surfactant, which is secreted by type II alveolar cells. Pulmonary surfactant is composed of phospholipids and proteins that significantly reduce the surface tension inside the alveoli. This reduction in surface tension is crucial because it helps prevent the collapse of alveoli during exhalation, thereby ensuring efficient gas exchange between the air in the alveoli and the blood in the surrounding capillaries.
The respiratory membrane itself is formed by the simple squamous epithelium of the alveoli and the endothelial membrane of capillaries. This membrane is approximately 0.5 μm thick and allows for the diffusion of gases, such as oxygen entering the blood and carbon dioxide being expelled from the blood into the air of the alveoli. Gases cross this barrier by simple diffusion due to the partial pressure gradients between the blood and the alveolar air.
In addition to type II alveolar cells and the simple squamous epithelium, the alveolar wall also consists of type I alveolar cells and alveolar macrophages. Type I cells make up the majority of the alveolar surface and are involved in the gas exchange process. Alveolar macrophages roam the alveolar wall, clearing away debris and pathogens that have reached the alveoli.