The air exhaled from the lungs has a lower oxygen content and higher carbon dioxide content than normal air, leading to an increase in the respiratory rate. Receptors in the arteries detect the partial pressure of oxygen and carbon dioxide in the bloodstream and increase or decrease the respiratory rate to establish balance, according to an article in The Medical Journal of Australia.
Reference.com notes that the initial effects of hypercapnia, or the increase in carbon dioxide in the lungs, are minor and include muscle twitches, flushed skin or slight increases in blood pressure. However, as the levels of carbon dioxide increase, the symptoms become more serious.
Normal air contains 21 percent oxygen and 0.04 percent carbon dioxide, according to HowStuffWorks. However, air expelled from the lungs contains approximately 4.5 percent carbon dioxide. According to The University of Chicago Medicine, breathing reduces the oxygen content to 17 percent. These differences in air from the atmosphere and air expelled from the lungs have caused some researchers to question the effectiveness of rescue breathing in the early stages of CPR for cardiac patients. The differences necessitate special measures in enclosed areas, such as submarines or airplanes, where oxygen generators supply replacement oxygen and soda lime traps carbon dioxide from the air. These steps allow humans inside to maintain a normal breathing rate without an increase in breathing rate.