Which respiratory surface allows oxygen to diffuse directly into capillaries?

The correct answer is external skin, as it serves as a respiratory surface that allows for the direct diffusion of oxygen into the capillaries. In many organisms, particularly amphibians and some other vertebrates, the skin is highly vascularized, meaning it is supplied with numerous blood vessels. This extensive capillary network is located just beneath the skin surface, facilitating the direct exchange of gases.

When oxygen-rich air or water comes into contact with the moist surface of the skin, oxygen molecules can easily diffuse through the skin and enter the capillaries. This process is efficient because it relies on the concentration gradient; oxygen moves from an area of higher concentration (outside) to an area of lower concentration (inside the capillaries). The thinness of the skin also aids in this process, reducing the distance that gases need to travel.

While gills, lungs, and tracheal systems also facilitate gas exchange, they typically involve specialized structures and mechanisms that are more complex than simple diffusion through skin. Gills extract oxygen from water, lungs are tailored for air breathing and require ventilation mechanisms, and the tracheal system in insects delivers oxygen directly to tissues without using blood. In contrast, utilizing external skin for respiration allows for an efficient and straightforward diffusion of