Perhaps no act is more associated with life than breathing, yet we take breathing and the air around us very much for granted. Breathing and respiration are often used synonymously, although this is not strictly correct. Respiration has two meanings: internal respiration refers to the consumption of oxygen (O2) by cells and to the production of carbon dioxide (CO2) as a result of metabolism; external respiration refers to the transport of O2 into the lungs and the removal of CO2 from them. In this Chapter, respiration will be used to mean external respiration, breathing. The respiratory system refers to those structures which function in the exchange of gases between the environment and the individual, and which bring air into and out of the body. In the body, external and internal respiration are linked together by the blood in the circulatory system which carries the gases. For air to be moved in and out of the body, muscles must also contract and so the neuromuscular system plays a role in respiration.