Cough is the most common symptom of lower respiratory tract disease. It is caused by the mechanical or chemical stimulation of the cough receptors in the epithelium of the pharynx, larynx, trachea and bronchi. Afferents receptors go to the cough centre in the medulla where efferent signals are generated to the expiratory musculature. (Parveen Kumar, Michael Clark, Clinical Medicine 6th Edition, Elsevier Saunders, London, UK, 2005, page 882)

Coughing occurs when the airways are irritated. Respiratory infections usually bacterial or viral, irritate the airways and are a common cause of coughing. Allergies can irritate the airways as well. People who smoke often cough. Smoke not only irritates the airways but also damages the cells that line the airways, including the hairlike projections that normally cleanse the airways of debris (cilia). Coughing may also result from postnasal drip, in which nasal secretions drain down the back of the nose into the throat and sometimes into the trachea and other airways, where they produce irritation. Coughing may result from gastroesophageal reflux, in which stomach or esophageal contents flow backward from the esophagus into the trachea and airways, producing irritation. Another cause of cough can be drugs, for example, angiotensin-converting enzyme (ACE) inhibitors. Narrowing of the airways below the windpipe (bronchoconstriction), foreign bodies, or tumors in the airway can cause cough, wheezing, or both. Bronchoconstriction occurs in asthma, in chronic obstructive pulmonary disease, and heart failure (when fluid accumulates in the lungs). Coughs vary considerably. A cough may be distressing, especially if coughing episodes are accompanied by chest pain, shortness of breath, blood, or unusually large amounts of or very sticky sputum. However, if coughing increases slowly over decades, as it may in a smoker, the person may hardly be aware of it.

Mechanism of cough

The bronchi and trachea are so sensitive to light touch that very slight amounts of foreign matter or other causes of irritation initiate the cough reflex.

Afferent nerve impulses pass from the respiratory passages mainly through the vagus nerves to the medulla of the brain

Deep and rapidly inspired

Epiglottis closes and the vocal cords shut tightly to entrap the air within the lungs.

The abdominal muscles contract forcefully, pushing against the diaphragm while other expiratory muscles such as the internal intercostals, also contract forcefully.

The pressure in the lungs rises rapidly to as much as 100mmHg or more.

The vocal cords and the epiglottis suddenly open widely.

Air under this high pressure in the lungs explodes outward.

The rapidly moving air usually carries with it any foreign matter that is present in the bronchi or trachea.