the breathing mechanism involves the action of the respiratory muscles, which causes air to move into and out of the lungs. Moving air into the lungs is known as inhalation, or inspiration, and moving air out is called exhalation, or expiration.
In a healthy adult at rest, the mechanism typically sets up a rate of between 16 and 20 breaths per minute. This is achieved by alternately increasing and decreasing the capacity of the chest cavity. The size of the chest space can be increased in a vertical direction by lowering the floor, which is achieved by contraction of the diaphragm. Contraction causes the diaphragm's domed shape to flatten out, so it effectively descends and increases the vertical dimension of the chest cavity.