THE COUGH MECHANISM AND THE FUNCTION OF THE COUGH REFLEX
Coughing is an airway reflex mediated by airway receptors that react to either
pressure or chemical stimuli. In several cardiopulmonary diseases, such as left
atrial enlargement, tracheal collapse and primary neoplasms, coughing is primarily
associated with mechanical compression of the airway. In other conditions a combination
of inflammatory mediators stimulating irritant receptors, and airway exudates
stimulating mechanoreceptors, results in coughing.
Large numbers of cough receptors are located in the larynx, at the thoracic inlet
and tracheal bifurcation. The numbers of receptors decrease further down the
respiratory tract and there are no receptors present in the peripheral airways
The cough reflex therefore functions mainly in the larger airways, although material
from the lower airways and alveoli can move to the level of the larger airways
where coughing will help to remove it.
The function of coughing is to assist the removal of material from the airways.
This material may have been inhaled or produced in the airways. Coughing also
prevents additional inhalation of material, or movement of inhaled material into
the peripheral airways.
With respiratory diseases, airway defence mechanisms result in increased production
of mucus from goblet cells and mucous glands in the airway. Respiratory diseases
will also result in inflammatory exudates entering the respiratory tract. This
material can accumulate in the lower airways and alveoli.
Alveolar and lower airway material is usually removed through phagocytosis by
alveolar macrophages, but because of the presence of the surface tension reducing
agent surfactant in the alveoli, this material can also move cranially by a capillary
Once material comes in contact with ciliated epithelium, it is transported by
ciliary beating towards the trachea where there is the highest density of cough
Coughing then propels the material into the oropharynx where it is swallowed.
Considering these factors it can be appreciated that coughing is an extremely
important protective mechanism for the respiratory system.
However, coughing caused by mechanical compression of the airways has no protective
function, may cause airway epithelial damage and should be controlled, particularly
if it is causing exhaustion.