The process of breathing is called external respiration. Under normal, resting conditions, the respiratory rate in an adult is12-14 times per minute. Breathing is involuntary that is you are not aware of it until you consciously think about it. Yet, you can change your rate of breathing therefore overriding the involuntary mechanism regulated by the autonomic nervous system and the respiratory centre in the brain. Some people can hold their breath for several minutes
The reason that you breathe is so the cells in the body receive oxygen gas (O2) required to convert glucose, fats or proteins into ATP in the mitochondria. In this process, carbon dioxide (CO2) is released as a waste product. This process within the cells is internal respiration.
During respiration oxygen enters the lungs and diffuses into the blood. Carbon dioxide is exhaled from the lungs (after diffusing out of the blood into the lungs). In addition, respiration, together with the kidneys, regulate acid-base balance in the body.