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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. The main muscles involved in the movements of breathing are the diaphragm, the large muscle which forms the floor of the chest cavity, and the intercostal muscles, which are attached to the ribs. When inhaling, the muscles work together to increase the size of the chest space, lowering the pressure inside the lungs and drawing air in from outside. Exhaling involves relaxation of the diaphragm and intercostal muscles, reducing the volume of the chest cavity and this, combined with the lungs' natural elasticity, serves to increase the pressure inside the lungs and move air out.
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.
The amount of room in the chest from front to back is increased by the action of the intercostal muscles. These work to elevate the ribs, drawing them together and lifting them toward the uppermost rib, which is fixed by the neck muscles. This movement of the upper ribs resembles that of a pump handle being raised, pushing the bottom of the breastbone forward and increasing the diameter of the chest from back to front. The ribs are shaped so they curve around at the sides of the chest, and the lifting movement also raises these curved ends up and out at the sides in an action similar to raising a bucket handle. This aspect of the breathing mechanism increases the size of the chest space as measured from side to side.
During exhalation, as the muscles relax, all of the actions associated with inhalation are reversed, and the dimensions of the chest space reduce. Normal, quiet breathing is an involuntary process, in that a person does not have to think about it for it to happen. Sometimes, what is known as forced respiration occurs, where a person takes voluntary control of breathing, for example when playing a wind instrument or taking vigorous exercise. Here, the breathing mechanism is slightly different and extra muscles are brought into action, including some from the neck and abdomen.