Breathing is usually automatic, controlled subconsciously by the respiratory center at the base of the brain. Breathing continues during sleep and usually even when a person is unconscious. People can also control their breathing when they wish, for example during speech, singing, or voluntary breath holding. Sensory organs in the brain and in the aorta and carotid arteries monitor the blood and sense oxygen and carbon dioxide levels. Normally, an increased concentration of carbon dioxide is the strongest stimulus to breathe more deeply and more frequently. Conversely, when the carbon dioxide concentration in the blood is low, the brain decreases the frequency and depth of breaths. During breathing at rest, the average adult inhales and exhales about 15 times a minute.

You are watching: The large band of muscle that controls the size of the chest cavity

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 dome-shaped sheet of muscle that separates the chest cavity 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. As 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.

The process of breathing out (called exhalation or expiration) is usually passive when a person is not exercising. The elasticity of the lungs and chest wall, which are actively stretched during inhalation, causes them to return to their resting shape and to expel air out of the lungs when inspiratory muscles are relaxed. Therefore, when a person is at rest, no effort is needed to breathe out. During vigorous exercise, however, a number of muscles participate 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 pushed out.

The muscles used in breathing can contract only if the nerves connecting them to the brain are intact. In some neck and back injuries, the spinal cord can be severed, which breaks the nervous system connection between the brain and the muscles, and the person will die unless artificially ventilated.

When the diaphragm contracts and moves lower, the chest cavity enlarges, reducing the pressure inside the lungs. To equalize the pressure, air enters the lungs. When the diaphragm relaxes and moves back up, the elasticity of the lungs and chest wall pushes air out of the lungs.

See more: How Many Calories In 1/4 Cup Almonds In A Serving? Dry Roasted Almonds (1/4 Cup = 170 Calories)



Shortness of Breath
Breath-Holding Spells
Riot-Control Chemical Agents
Illnesses linked to air pollution are categorized as environmental lung diseases. Exposure to air pollution for an extended period may increase respiratory infections and symptoms of respiratory disorders, such as coughing. In which of the following groups is long-term exposure to air pollution especially likely to increase respiratory infections and symptoms of respiratory disorders?
Children Young adults ages 20 to 30 years Adults ages 40 to 60 years Older adults (ages 65 years and older)

MSD and the MSD Manuals

Merck and Co., Inc., Kenilworth, NJ, USA (known as MSD outside of the US and Canada) is a global healthcare leader working to help the world be well. From developing new therapies that treat and prevent disease to helping people in need, we are committed to improving health and well-being around the world. The Manual was first published as the Merck Manual in 1899 as a service to the community. The legacy of this great resource continues as the MSD Manual outside of North America. Learn more about our commitment to Global Medical Knowledge.