<P> Exhalation (or expiration) is the flow of the breath out of an organism . In humans it is the movement of air from the lungs out of the airways, to the external environment during breathing . </P> <P> This happens due to elastic properties of the lungs, as well as the internal intercostal muscles which lower the rib cage and decrease thoracic volume . As the thoracic diaphragm relaxes during exhalation it causes the tissue it has depressed to rise superiorly and put pressure on the lungs to expel the air . During forced exhalation, as when blowing out a candle, expiratory muscles including the abdominal muscles and internal intercostal muscles generate abdominal and thoracic pressure, which forces air out of the lungs . </P> <P> Exhaled air is rich in carbon dioxide, a waste product of cellular respiration during the production of energy, which is stored as ATP . Exhalation has a complementary relationship to inhalation which together make up the respiratory cycle of a breath . </P> <P> The main reason for exhalation is to rid the body of carbon dioxide, which is the waste product of gas exchange in humans . Air is brought in the body through inhalation . During this process air is taken in through the lungs . Diffusion in the alveoli allows for the exchange of O into the pulmonary capillaries and the removal of CO and other gases from the pulmonary capillaries to be exhaled . In order for the lungs to expel air the diaphragm relaxes, which pushes up on the lungs . The air then flows through the trachea then through the larynx and pharynx to the nasal cavity and oral cavity where it is expelled out of the body . Exhalation takes longer than inhalation since it is believed to facilitate better exchange of gases . Parts of the nervous system help to regulate respiration in humans . The exhaled air isn't just carbon dioxide; it contains a mixture of other gases . Human breath contains volatile organic compounds (VOCs). These compounds consist of methanol, isoprene, acetone, ethanol and other alcohols . The exhaled mixture also contains ketones, water and other hydrocarbons . </P>

Where does the co2 that we exhale come from