The body's ability to breathe involves the nose, mouth, chest muscles and diaphragm. Breathing is usually automatic and controlled by the respiratory center at the base of the brain. We breathe during sleep and usually even during unconsciousness. Small sensors in the brain and aorta and carotid arteries monitor the blood. If there is too little oxygen in the blood these sensors trigger faster or deeper breathing. (In quiet breathing, the average adult inhales and exhales about 15 times a minute.)
The work of breathing is done by the diaphragm and the muscles between the ribs, in the neck and in the abdomen. The diaphragm, a bell-shaped sheet of muscle that separates the lungs from the abdomen, is the most important muscle used for breathing. The diaphragm is attached to the base of the breastbone, the lower parts of the ribcage and the spine.
A breath starts when the ribs and the chest wall expand and the diaphragm tightens and flattens, which causes the lungs to fill with air. All the muscles used in breathing contract only if the nerves connecting them to the brain are healthy. In some neck and back injuries, the spinal cord can be severed, in which case, a person will die unless he or she has a machine to help with breathing.
As the air enters our mouth and nose, the mucus membranes lining the mouth and nose make the air moist and warm, and they trap any particles. The air then passes down the throat into the trachea (or windpipe), the bronchi, the bronchioles and then the alveoli.
When the air rushes into the lungs, it fills the alveoli like balloons. Each alveolus is surrounded by tiny blood vessels. The oxygen that moves across the walls of the air sacs is picked up by the blood and carried to the rest of the body. The carbon dioxide and waste gases that the blood carried to the lungs pass into the air sacs and are exhaled.
Once the blood has picked up fresh oxygen and released carbon dioxide into the alveoli, the diaphragm and chest muscles relax. This relaxation pushes the air out of the alveoli, through the bronchioles and the bronchi, up through the windpipe and out through the nose or mouth.
When we are at rest, the process of breathing out requires no effort from the respiratory muscles. During vigorous exercise, however, many 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.