A cough occurs as an involuntary reflex triggered when the nerves in the respiratory system become stimulated. The Merck Manual reports that a cough is one of the most common reasons for visiting the doctor. Although everyone coughs as a normal response to keep the throat and airways free of irritating substances, many medical conditions and triggers can cause a cough.
Inhaled Irritants
As the lungs expand during breathing, air enters the mouth and nose and travels through the trachea. The trachea branches into two bronchi, the tubes that lead to each lung. In the lungs the bronchi branch into smaller tubes known as the bronchial tubes that branch further into bronchioles, the smallest tubes that lead to the tiny air sacs. Tiny hairs known as cilia line the tubes of the respiratory tract, also known as the airways. Inhaled irritants describe substances that affect the cilia and stimulate the nerve endings, therefore causing a cough.
Although not everyone is sensitive to the same irritants, some of the most common inhaled irritants include cigarette smoke, air pollution, paint fumes and chemicals. Some scented products such as perfumes and air fresheners can also trigger a cough.
Stomach Acid
Stomach acid, the liquid secreted by the parietal cells in the lining of the stomach, functions to help break down food particles during digestion. As food leaves the mouth, it travels through the esophagus and enters the stomach through the lower esophageal sphincter, or LES, which is a ring of muscles that relaxes to allow food into the stomach. When the LES malfunctions, it may fail to close completely or may relax sporadically, allowing stomach acid to rise into the esophagus; this is known as acid reflux or heartburn, and causes a burning sensation in the chest or throat.
Persistent acid reflux that occurs more than twice a week causes a condition known as gastroesophageal reflux disease, or GERD, according to the National Digestive Diseases Information Clearinghouse. Acid in the esophagus can rise into the larynx and throat, causing irritation that leads to a cough. Acid can even move through the throat into the respiratory tract to the lungs, causing a dry, persistent cough.
Excess Mucus
Mucus consists of water, carbohydrates, proteins and lipids. Mucus lines the airways and functions to protect the lungs by trapping foreign particles such as bacteria, viruses and allergens. Too much mucus, however, interferes with the flow of air into the lungs and triggers the cough reflex as a way to remove the mucus.
Illnesses such as the common cold and flu trigger the respiratory tract to produce excess mucus. More serious conditions such as chronic bronchitis, or inflammation of the bronchial tubes, and pneumonia, or inflammation of the lungs, also cause an increase in the production of mucus. The presence of allergens such as dust, mold and pollens, stimulate the immune system to react, also causing an increase in mucus production.
