Which of the following best describes the respiratory membrane?

The respiratory membrane is best described as the site where oxygen and carbon dioxide are exchanged. It is specifically formed by the thin walls of the alveoli (tiny air sacs in the lungs) and the surrounding capillaries (small blood vessels). The primary function of this membrane is to facilitate the gas exchange process during respiration, allowing oxygen to move from the alveoli into the bloodstream and carbon dioxide to move from the blood into the alveoli to be exhaled. This efficient exchange occurs due to the thinness of the respiratory membrane, which minimizes the distance across which gases must diffuse.

In contrast, the other options refer to structures or functions that do not accurately describe the specific role of the respiratory membrane. The layer of tissue surrounding the lungs pertains more to the pleura, while a barrier preventing pathogen entry describes a protective feature of the immune system rather than gas exchange. The muscles involved in lung expansion are primarily the diaphragm and intercostal muscles and are crucial for the mechanics of breathing, but they don't directly pertain to the function of the respiratory membrane itself.