A cough is a sudden, usually involuntary, expulsion of air from the lungs with a characteristic and easily recognizable sound. Although it is known as the most common symptom of respiratory disorders, it serves the functions of defending the respiratory tract against noxious substances and maintaining airway patency by removing excessive secretions from the air passages. Expectoration or sputum production is the act of coughing up and spitting out the material produced in the respiratory tract.
As a cardinal manifestation of respiratory diseases, coughing is one of the most common symptoms encountered in clinical medicine. Being a physiologic reflex, the cough also occurs without any demonstrable evidence of disease when triggered by the stimulation of the irritant receptors. Moreover, it may be a voluntary act or may result from nervous habit. Although the clinical significance of coughing in many instances is trivial, it may be an indication of a serious intrathoracic disease. Pathologic conditions causing the cough are usually the ones that irritate the airways, increase their irritability, result in their deformation, or increase the tracheobronchial secretions. These factors may operate singly or in various combinations. Sputum production with coughing occurs when the respiratory tract secretions are beyond the ability of the mucociliary mechanism to deal with them.