All living things need oxygen. Oxygen enables them to metabolize, or burn, nutrients, releasing the energy required to grow, reproduce, and maintain life. In the metabolic process, oxygen combines with the carbon in food and forms carbon dioxide as a waste product. The respiratory system is an organism’s apparatus for taking in oxygen and expelling carbon dioxide and water vapor.
"Respiratory system." Britannica School, Encyclopædia Britannica, 13 Aug. 2017. school.eb.com/levels/middle/article/respiratory-system/276699. Accessed 4 Sep. 2017.
Breathing. Image. Britannica School, Encyclopædia Britannica, 13 Aug. 2017. school.eb.com/levels/middle/assembly/view/87779. Accessed 4 Sep. 2017.