The trachea, commonly known as the windpipe, is the primary tube that carries air from the upper respiratory tract into the lungs. While often viewed as a rigid tube due to its cartilage support, it possesses a dynamic component. The trachea does contain smooth muscle, a specialized band of tissue called the trachealis muscle. This muscle is fundamental to the respiratory system, serving regulatory and protective roles. Its presence transforms the windpipe into a responsive, adjustable airway.
Cartilage Rings and the Posterior Gap
The structural integrity of the trachea is maintained by 16 to 20 horseshoe-shaped rings made of hyaline cartilage. These rings are incomplete, forming a C-shape that opens toward the posterior side, resting directly against the esophagus. This anatomical arrangement ensures the trachea remains consistently open, preventing collapse during inhalation, while allowing for flexibility.
The trachealis muscle spans this posterior gap, connecting the free ends of the C-rings. This band of smooth muscle fibers completes the ring structure, creating the back wall of the trachea. Its placement is functional, maintaining the airway’s shape while accommodating the adjacent esophagus. When swallowing a large bolus of food, the soft, muscular posterior wall can flatten or bulge inward, allowing the esophagus to expand without obstruction.
Regulating Airflow and Diameter
The contraction and relaxation of the trachealis muscle modulate the flow of air into the lungs. This smooth muscle is under involuntary control, constantly making small adjustments to the diameter of the tracheal lumen. During increased respiratory demand, the muscle relaxes slightly to widen the airway, minimizing resistance and allowing a greater volume of air to pass through.
Conversely, a slight, tonic contraction can reduce the airway diameter. This mild narrowing helps reduce the volume of anatomical “dead space”—air that fills the conducting airways but does not participate in gas exchange. By decreasing this volume, contraction marginally increases the efficiency of ventilation and helps regulate the velocity of the airflow. This diameter adjustment is a continuous function.
The Trachealis Muscle and Protective Reflexes
The primary protective function of the trachealis muscle occurs during the reflex known as coughing. When irritants, such as dust, mucus, or foreign particles, stimulate the sensory receptors in the respiratory lining, a rapid and forceful reflex contraction is initiated. The trachealis muscle contracts violently, pulling the ends of the C-rings inward toward the center of the trachea. This action significantly narrows the diameter of the tracheal lumen, sometimes by 10 to 30 percent.
This sudden reduction in the airway’s cross-sectional area dramatically increases the velocity of the air being forcefully expelled from the lungs. According to the principles of fluid dynamics, decreasing the tube diameter while maintaining a constant flow rate results in a massive increase in speed. The resultant high-speed air current, which can exceed 100 miles per hour, generates the necessary shear force to dislodge and propel foreign matter and mucus out of the lower respiratory tract. This forceful contraction is a defense mechanism, ensuring the airways remain clear.