Control of Breathing
Breathing is usually automatic, controlled subconsciously by the respiratory center at the base of the brain. Breathing continues during sleep and usually even when the person is unconscious. Small sensory organs in the brain and in the aorta and carotid arteries monitor the blood and sense when oxygen levels are too low or carbon dioxide levels are too high. In healthy people, an increased concentration of carbon dioxide is the strongest stimulus to breathe more deeply and more quickly. Conversely, when the carbon dioxide concentration in the blood is low, breathing is reduced. Then, the brain decreases the frequency and size of breaths. In quiet breathing, the average adult inhales and exhales about 15 times a minute.
 See the figure Diaphragm's Role in Breathing.
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.
The process of breathing out (called exhalation or expiration) is usually passive when a person is not exercising. Energy stored in the elasticity of the lungs and chest wall can be used to expel air out of the lungs. Therefore, when a person is at rest, no effort is needed by the respiratory muscles. During vigorous exercise, however, a number of muscles assist in exhalation. The abdominal muscles are the most important of these. Abdominal muscles contract, raise abdominal pressure, and push a relaxed diaphragm against the lungs, causing air to be expelled.
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