When the diaphragm contracts, the chest cavity enlarges, reducing the pressure inside. To equalize the pressure, air rushes into the lungs. When the diaphragm relaxes, the elasticity of the lungs and chest wall pushes air out of the lungs.
Because the lungs have no skeletal muscles of their own, the work of breathing is done by the diaphragm, the muscles between the ribs (intercostal muscles), the muscles in the neck, and the abdominal muscles. The diaphragm, a bell-shaped sheet of muscle that separates the lungs from the abdomen, is the most important muscle used for breathing in (called inhalation or inspiration). The diaphragm is attached to the base of the sternum, the lower parts of the rib cage, and the spine. When the diaphragm contracts, it increases the length and diameter of the chest cavity and thus expands the lungs. The intercostal muscles help move the rib cage and thus assist in breathing. All the muscles used in breathing contract only if the nerves connecting them to the brain are intact. In some neck and back injuries, the spinal cord can be severed, and the person will die unless he is artificially ventilated.
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