Expiration, also called exhalation, is the process by which air (or water in the case of fish) is expelled from the lungs (or gills). In mammals it involves a reduction in the volume of the chest cavity by relaxation of the diaphragm muscles and contraction of muscles between the ribs. This squeezes the lungs and forces gases out until the pressure in the lungs is the same as atmospheric pressure. Expiration, and its opposite process, inspiration, are fundamental in aiding gas exchange, part of the complete process of respiration.