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While people can generally cough voluntarily, a
cough is usually a reflex triggered when an irritant stimulates one
or more of the cough receptors found at different points in the
respiratory system. These receptors then send a message to the cough
center in the brain, which in turn tells the body to cough. A cough
begins with a deep breath in, at which point the opening between the
vocal cords at the upper part of the larynx (glottis) shuts,
trapping the air in the lungs. As the diaphragm and other muscles
involved in breathing press against the lungs, the glottis suddenly
opens, producing an explosive outflow of air at speeds greater than
100 mi (160 km) per hour.
In normal situations, most people cough once or
twice an hour during the day to clear the airway of irritants.
However, when the level of irritants in the air is high or when the
respiratory system becomes infected, coughing may become frequent
and prolonged. It may interfere with exercise or sleep, and it may
also cause distress if accompanied by dizziness, chest pain, or
breathlessness. In the majority cases, frequent coughing lasts one
to two weeks and tapers off as the irritant or infection subsides.
If a cough lasts more than three weeks it is considered a chronic
cough, and physicians will try to determine a cause beyond an acute
infection or irritant.
Coughs are generally described as either dry or
productive. A dry cough does not bring up a mixture of mucus,
irritants, and other substances from the lungs (sputum), while a
productive cough does. In the case of a bacterial infection, the
sputum brought up in a productive cough may be greenish, gray, or
brown. In the case of an allergy or viral infection it may be clear
or white. In the most serious conditions, the sputum may contain
blood. |
