Breathing, also known as ventilation, is the process of moving air or water in order to facilitate the gas exchange between the external and internal (body) environment, mostly to eliminate carbon dioxide (CO2) and bring in oxygen (O2). For example, fish are aquatic animals that need to run water over their gills to get the O2 that they need to survive (gills are organs that create a large surface area for ventilation). Moreover, insects ventilate their tracheal system to get O2. Finally, terrestrial vertebrates (i.e., amphibians, reptiles, birds, and mammals) use organs called lungs to exchange O2 and CO2 between their cells and the external environment.
