The glottis is a specialized structure located centrally within the larynx, commonly known as the voice box. This anatomical region includes both the true vocal folds and the space that separates them, known as the rima glottidis. Positioned between the upper (supraglottis) and lower (subglottis) sections of the larynx, the glottis functions as a valve in the respiratory tract. Its precise movements control the passage of air.
Regulating Airflow
The primary function of the glottis at rest is to allow unimpeded air exchange between the lungs and the outside environment. During quiet respiration, laryngeal muscles cause the vocal folds to move apart (abduction). This creates an open, triangular passage, minimizing resistance to airflow. The vocal folds maintain this relaxed position throughout inhalation and exhalation.
When the body requires a greater volume of air, such as during physical exertion, the glottis widens significantly. Forced inspiration triggers a maximal abduction of the vocal folds to create the largest possible opening. This facilitates a substantial increase in airflow to meet the body’s elevated oxygen demands. The coordinated movement of this passage ensures that breathing is an efficient and continuous process.
Generating Vocal Sound
When sound production begins, the glottis transitions from an open state to a nearly closed one as the vocal folds adduct and approximate each other at the midline. Air pressure builds up beneath the closed vocal folds, creating a subglottic pressure that must overcome the resistance of the folded tissue. Once this pressure reaches a sufficient level, it forces the vocal folds apart, releasing a puff of air into the throat.
The subsequent closure of the vocal folds is not simply due to muscle action but is largely driven by a combination of tissue elasticity and aerodynamics. As the air rushes through the narrow opening, it creates a drop in pressure perpendicular to the flow, according to the Bernoulli effect. This aerodynamic suction helps to pull the vocal folds back toward the midline, initiating the closing phase of the cycle.
This rapid opening and closing creates a wave-like motion along the surface of the vocal folds, called the mucosal wave. The lower edges of the vocal folds open first, followed by the upper edges, and they also close in this bottom-to-top sequence. This vertical phase difference sustains the self-oscillating vibration, converting the continuous stream of air from the lungs into acoustic energy.
The frequency of this vibration determines the perceived pitch of the voice, which is controlled by the tension and length of the vocal folds. Small laryngeal muscles stretch and tense the folds, causing them to vibrate faster for higher notes and slower for lower notes. The loudness, or volume, is regulated by the force of the air pressure from the lungs and how firmly the vocal folds are pressed together. Increasing the subglottic pressure results in a greater amplitude of vibration and a louder sound.
Airway Protection Mechanisms
Beyond voice and breathing, the glottis functions as a sphincter to protect the lower airway from foreign material. During swallowing, the glottis closes, creating a tight seal to prevent food or liquid from entering the trachea. This rapid adduction of the true vocal folds, combined with the folding of the epiglottis over the laryngeal opening, forms a multi-layered barrier against aspiration.
The glottis also plays a mechanical role in generating force throughout the torso, such as in the Valsalva maneuver. This action involves a forceful exhalation against a tightly closed glottis, which increases the pressure within the chest and abdominal cavities. This rise in pressure stabilizes the trunk, assisting in physically demanding actions like lifting heavy objects or in bodily functions like defecation.
A similar protective action occurs during the cough reflex. The glottis begins by closing tightly after a deep inhalation, trapping air in the lungs. Expiratory muscles then contract against this sealed airway, building up high pressure beneath the vocal folds. The sudden opening of the glottis releases this pressurized air in a forceful burst, designed to expel irritants or mucus from the respiratory passages.