Coughing is an involuntary protective reflex designed to clear the respiratory passages of irritants, excess mucus, or foreign material. This action is a highly coordinated mechanical event involving a precise sequence of muscle contractions. It is a rapid, three-stage process that recruits muscle groups from the neck, chest, abdomen, and larynx to generate the necessary force for airway clearance. This process involves a complex interplay between muscles of inhalation and forced exhalation.
Phase One: Deep Inhalation
The first step in generating an effective cough is a preparatory deep breath, which is necessary to maximize the volume of air available for expulsion. This initial inspiration is powered primarily by the diaphragm, a dome-shaped sheet of muscle situated beneath the lungs. When the diaphragm contracts, it flattens and moves downward into the abdominal cavity, creating more space within the chest.
This increase in thoracic volume lowers the internal pressure in the lungs, drawing a large volume of air inward. The External Intercostal muscles, located between the ribs, also contract during this phase, pulling the rib cage upward and outward. This substantial intake of air provides the fuel for the high-velocity, forceful expulsion that will follow.
Phase Two: Compression and Pressure Build-up
Following the deep breath, the system seals itself to build tremendous internal pressure against the trapped air. This compression phase begins with the closure of the glottis (the opening between the vocal cords), as intrinsic laryngeal muscles contract to rapidly adduct the vocal cords, sealing the airway.
While the glottis remains closed, the muscles of forceful expiration contract forcefully against the sealed airway. The Internal Intercostal muscles pull the rib cage downward and inward, actively decreasing the chest cavity volume. Simultaneously, powerful abdominal wall muscles contract to press against the organs beneath the diaphragm. The collective action of these muscles rapidly increases the pressure inside the chest, which can reach levels as high as 300 millimeters of mercury (mm Hg).
Phase Three: Forceful Expulsion
The final stage is the explosive release of air that defines the cough, driven by the continued, intense contraction of the expiratory musculature. The abdominal wall muscles—including the Rectus Abdominis, External Obliques, and Transversus Abdominis—are the primary force generators in this phase. Their coordinated contraction dramatically increases intra-abdominal pressure, pushing the relaxed diaphragm upward with immense force.
This upward thrust rapidly compresses the lungs, causing an instantaneous and massive spike in intrathoracic pressure. When the glottis abruptly opens, the high-pressure air is suddenly released, generating an explosive airflow that can reach velocities exceeding 500 miles per hour. The sudden, explosive opening of the glottis is what produces the characteristic sound of a cough.