However, carbon dioxide levels can build up in closed spaces like breathing in and out of a bag, living in a fish tank, living in the space shuttle, etc. Therefore, it is advantageous for diffusion to be continuously exposed to fresh air with little carbon dioxide. Respiratory organs in most organisms have evolved to have very large surface areas over which gases can diffuse. External gills increase the surface area for gas exchange with water in larval amphibians and many insect species. Many mollusks, arthropods, and fishes have evolved internal gills to better protect the thin and vulnerable gas exchange surface.
Lungs also have large surface areas for gas exchange.
In addition ventilation and perfusion are maximized for effective gas exchange. The gills and lung membranes are very thin . Breathing ventilates the gas exchange areas to provide fresh air to give oxygen continuously and take away carbon dioxide. Circulation provides blood flowing through capillaries very close to the air and removes gases that have already diffused across. Insects breathe through a special system of air tubes called tracheae that open to the air through spiracles.