Respiratory Function
The larynx plays an essential role as part of the respiratory passage. The glottis is the narrowest part of the respiratory tract. Under normal conditions, the central nervous system controls the movement of the vocal cords via neural signals, regulating the size of the glottis. During physical activity, the vocal cords abduct, widening the glottis to allow for increased airflow and oxygen intake. Conversely, during rest, the glottis narrows, reducing the amount of air intake.
Phonation Function
The larynx functions as the primary organ of phonation, with the vocal cords serving as the main site for sound production. The mechanism of phonation is complex, and most scholars believe that during phonation, the central nervous system causes the vocal cords to adduct via laryngeal innervation. Air expelled from the lungs induces vibration of the vocal cords, generating a fundamental frequency. This sound is then modified by resonation within the pharynx, oral cavity, and nasal cavity, and further shaped by the articulatory movements of the tongue, soft palate, teeth, cheeks, and lips, resulting in different sounds and speech. The "body-cover theory" of mucosal wave vibration is currently widely accepted as an explanation of vocal cord mechanics. According to this theory, the epithelial layer and the superficial layer of the vocal cord lamina propria form the cover, while the vocal ligament and the underlying muscle layer constitute the body. The body stabilizes the vocal cords, maintains their tension, and provides the resistance necessary for phonation. During phonation, the vocal cords adduct and the glottis closes. Subglottic airflow pressure causes the glottis to open, while the Bernoulli effect draws the vocal cords back together, resulting in repeated cycles of opening and closing. The mucosal wave generated during this process can be visualized using dynamic laryngoscopy. Regular closure of the vocal cords produces periodic vibrations in the airflow, creating alternating compressions and rarefactions that form glottic waves, which determine the fundamental frequency of human voice production.
Protective Function for the Lower Respiratory Tract
The larynx plays a protective role for the lower respiratory tract. During swallowing, the larynx is elevated, and the epiglottis tilts backward to cover the laryngeal inlet, acting as the first line of defense. Simultaneously, the vestibular folds on both sides adduct toward the midline, forming a second line of defense. The vocal cords also adduct and close the glottis, constituting the third line of defense. During food intake, these three lines of defense close simultaneously, the esophageal opening relaxes, and food passes through the piriform recess into the esophagus. However, if food or secretions inadvertently enter the laryngeal cavity or the lower respiratory tract, a strong reflexive cough is typically triggered to expel the material.
Valsalva Function (Breath-Holding)
The larynx also contributes to the generation of increased intrathoracic and intra-abdominal pressure during certain physiological activities, such as coughing, defecation, childbirth, or heavy lifting. During these activities, the vocal cords adduct and the glottis closes tightly, enabling the breath-holding maneuver. This process typically follows inhalation, during which breathing pauses, the thorax becomes stabilized, the diaphragm descends, and muscles of the chest wall and abdomen contract. The duration of glottic closure varies depending on the activity. For example, during coughing, the closure is brief, whereas during defecation, childbirth, or lifting heavy objects, it remains closed for a longer period.