ex·pi·ra·tion / ˌekspəˈrāshən/ • n. 1. the ending of the fixed period for which a contract is valid: the expiration of the lease. ∎ the end of a period of time: the expiration of three years.2. technical exhalation of breath.
expiration (exhalation) The process by which gas is expelled from the lungs (see respiratory movement). In mammals, the volume of the thoracic cavity is reduced by contraction of the internal intercostal muscles and relaxation of the muscles of the diaphragm, assisted by upward pressure of the abdominal organs. As a result, pressure in the lungs exceeds atmospheric pressure and gas flows out of the lungs, allowing the pressures to equalize. Compare inspiration.
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