Trachea

The trachea, commonly known as the windpipe, is a cylindrical tube that serves as a vital part of the respiratory system. It is approximately 10-12 centimeters in length and 2-2.5 centimeters in diameter in adults. The trachea is made of cartilage rings, smooth muscle, and connective tissue, which help maintain its structure and keep the airway open for the passage of air.

Location

The trachea is located in the neck and upper part of the chest. It begins just below the larynx (voice box) and extends downward through the midline of the neck into the thoracic cavity, where it ends at the carina, where it bifurcates into the left and right main bronchi. It is positioned anterior to the esophagus and runs along the midline of the body.

Structure and Anatomy

The trachea is an essential component of the respiratory system that connects the larynx to the bronchi. Its structure is designed to maintain a clear airway while allowing flexibility and movement. Below is a detailed description of the trachea’s anatomy.

Size

In adults, the trachea is approximately 10 to 12 centimeters long and about 2 to 2.5 centimeters in diameter. It runs from the level of the C6 vertebra to the T4/T5 vertebrae, where it ends at the carina, the point where the trachea divides into the left and right main bronchi.

Cartilaginous Structure

• The trachea is supported by C-shaped rings of hyaline cartilage, which help maintain its shape and keep the airway open. These rings are incomplete posteriorly, creating an open side toward the esophagus, allowing flexibility during swallowing.
• The trachea typically has 16 to 20 cartilage rings, spaced evenly along its length. Each ring is about 4 millimeters in height, and the rings are interconnected by fibrous connective tissue known as annular ligaments.

Trachealis Muscle

• The posterior side of the trachea, where the cartilage rings are incomplete, is bridged by the trachealis muscle, a band of smooth muscle. This muscle allows the trachea to contract or relax, adjusting the diameter of the airway.
• The trachealis muscle also provides flexibility to the trachea, enabling it to move slightly during breathing, swallowing, and coughing. It is located adjacent to the esophagus, allowing the esophagus to expand when food passes through it.

Lining of the Trachea

• The trachea’s inner lining is composed of pseudostratified ciliated columnar epithelium, a type of mucosal tissue designed to trap and remove foreign particles from the airways. This epithelium is composed of ciliated cells and goblet cells.
• Goblet cells secrete mucus, which traps dust, pathogens, and other particles from inhaled air. The cilia on the epithelial cells move the mucus upward toward the larynx and pharynx, where it can be swallowed or expelled through coughing.

Submucosa

• Beneath the epithelial lining is the submucosa, a layer of connective tissue that contains seromucous glands. These glands produce additional mucus, which helps to moisten and protect the airway.
• The submucosa also contains blood vessels, nerves, and lymphatic vessels, which nourish and support the tracheal tissue and play a role in immune responses.

Adventitia

• The outermost layer of the trachea is called the adventitia, a layer of connective tissue that surrounds the trachea and anchors it to surrounding structures within the neck and thoracic cavity.
• The adventitia contains fat, blood vessels, and lymph nodes, providing structural support and a pathway for the trachea’s vascular and lymphatic supply.

Blood Supply

The trachea receives its blood supply from various arteries:

• Inferior Thyroid Arteries: These arteries, branches of the thyrocervical trunk of the subclavian artery, supply the upper portion of the trachea.
• Bronchial Arteries: The lower part of the trachea is supplied by small branches from the bronchial arteries, which originate from the thoracic aorta.

Venous drainage from the trachea is provided by the inferior thyroid veins and bronchial veins, which drain into larger veins like the brachiocephalic veins and azygos vein.

Nervous Innervation

The trachea is innervated by both the autonomic nervous system and sensory nerves.

• Parasympathetic Innervation: The trachea receives parasympathetic fibers from the vagus nerve (cranial nerve X). These fibers regulate the tone of the trachealis muscle and control secretions of mucus by the goblet cells.
• Sympathetic Innervation: Sympathetic nerve fibers from the thoracic sympathetic ganglia innervate the trachea, playing a role in regulating the diameter of the airway by relaxing the smooth muscles.
• Sensory Innervation: Sensory fibers from the vagus nerve detect irritants in the trachea and help trigger the cough reflex, which clears the airway.

Lymphatic Drainage

The lymphatic drainage of the trachea is essential for maintaining immune defense and fluid balance.

• Pretracheal and paratracheal lymph nodes are located near the trachea and drain lymph from the tracheal tissue.
• Lymph from these nodes eventually drains into the bronchomediastinal lymphatic trunks, which lead to the thoracic duct or right lymphatic duct, depending on the side of the body.

Division at the Carina

• The trachea ends at the carina, a cartilaginous ridge at the level of the T4/T5 vertebrae, where the trachea bifurcates into the left and right main bronchi.
• The carina is a highly sensitive structure and is involved in triggering the cough reflex when irritated by foreign particles or substances. It is positioned in the midline of the body, and the division of airflow at the carina directs air into each lung.

Function

The trachea, commonly known as the windpipe, is an essential part of the respiratory system, playing multiple roles that ensure efficient breathing and airway protection. Below is a detailed explanation of the trachea’s functions.

Air Conduction

The primary function of the trachea is to serve as the main conduit for air between the upper respiratory system and the lungs.

• Passage of Air: The trachea provides a direct path for air to move from the larynx to the bronchi and then into the lungs. During inhalation, air flows through the trachea into the left and right main bronchi, which then distribute it throughout the lungs. During exhalation, the air is expelled back through the trachea.
• Maintaining Open Airway: The trachea is supported by C-shaped cartilaginous rings, which ensure that the airway remains open at all times, preventing collapse during the pressure changes that occur during breathing. These rings provide structural integrity to the airway, allowing a smooth and unobstructed passage of air.

Protection of the Lower Airways

The trachea plays a crucial protective role in safeguarding the lower respiratory tract, including the bronchi and lungs.

• Filtering Air: The inner lining of the trachea is composed of pseudostratified ciliated columnar epithelium, which contains goblet cells. These goblet cells secrete mucus that traps dust, bacteria, and other foreign particles present in the inhaled air, preventing them from reaching the lungs.
• Mucociliary Clearance: The trachea is equipped with tiny hair-like structures called cilia, which constantly beat in a coordinated manner. These cilia move mucus, along with trapped particles, upward toward the throat, where it can be swallowed or coughed out. This process, known as mucociliary clearance, protects the lungs by keeping them free from harmful substances.

Cough Reflex

The trachea is highly sensitive to irritants and foreign objects, and it plays a central role in initiating the cough reflex.

• Sensory Detection: The trachea is innervated by sensory nerve fibers, primarily from the vagus nerve, which detect the presence of irritants, such as dust, smoke, or foreign objects. When these irritants stimulate the nerve endings, signals are sent to the brain to trigger the cough reflex.
• Coughing Action: The cough reflex causes a rapid and forceful expulsion of air from the lungs, helping to remove the irritants from the airway. This protective mechanism ensures that foreign particles are cleared before they can enter the bronchi and lungs, thereby preventing infection or damage.

Humidification and Warming of Air

The trachea plays a key role in conditioning the air that enters the respiratory system, ensuring that it is warm and humid before reaching the delicate lung tissue.

• Humidification: The mucus produced by goblet cells in the tracheal lining helps to humidify inhaled air. This moisture prevents the drying out of the respiratory tract and alveolar surfaces, ensuring that gas exchange in the lungs occurs efficiently.
• Warming of Air: Blood vessels located in the trachea’s submucosa help to warm the air as it passes through. By the time air reaches the bronchi and lungs, it is close to body temperature, protecting the lung tissue from the potentially harmful effects of cold air.

Regulation of Airflow

The trachea contributes to the regulation of airflow into and out of the lungs, adjusting the airway diameter as needed.

• Trachealis Muscle Contraction: The trachealis muscle, located along the posterior part of the trachea, can contract to slightly reduce the diameter of the trachea. This action increases the speed and force of air expelled during coughing, making the cough more effective at clearing the airway.
• Relaxation for Increased Airflow: During periods of increased oxygen demand, such as exercise, the trachealis muscle relaxes to allow the trachea to expand slightly. This increases the airway’s diameter, allowing a greater volume of air to pass through and reach the lungs.

Flexibility for Swallowing

The design of the trachea, with its incomplete posterior cartilaginous rings and trachealis muscle, allows it to be flexible and adjust its shape during swallowing.

• Expansion of the Esophagus: The esophagus, which lies directly behind the trachea, needs to expand during swallowing to accommodate food and liquids. The open posterior part of the trachea, formed by the trachealis muscle, allows the esophagus to bulge into the tracheal space when food passes through, preventing obstruction in both the esophagus and the trachea.
• Maintaining Airway Patency During Swallowing: Even while swallowing occurs, the trachea maintains its patency (openness) due to its supportive cartilaginous rings, ensuring that breathing is not interrupted.

Anchoring for the Bronchi

The trachea provides structural support and acts as an anchor for the left and right main bronchi.

• Division at the Carina: At its inferior end, the trachea bifurcates into the left and right bronchi at the carina, a cartilaginous ridge. This bifurcation allows the trachea to serve as the central pathway, evenly distributing air into both lungs.
• Structural Continuity: The cartilaginous rings of the trachea continue into the bronchi, ensuring that the airway remains structurally sound throughout its entire length, from the larynx to the lungs. This continuity prevents airway collapse during breathing and helps regulate airflow into the lungs.

Clinical Significance

The trachea plays a vital role in maintaining an open airway and facilitating the flow of air to the lungs, making it central to respiratory health. Tracheal disorders can have serious consequences. Conditions like tracheal stenosis (narrowing of the trachea), often caused by prolonged intubation or trauma, can lead to breathing difficulties. Tracheomalacia, a condition where the tracheal walls are weakened and prone to collapse, can also result in airway obstruction and impaired breathing.

Tracheal infections, such as tracheitis, and conditions like tracheoesophageal fistula, where an abnormal connection forms between the trachea and esophagus, can lead to complications, including severe respiratory distress. In emergency medical care, the trachea is often a site for tracheostomy or intubation procedures, allowing for direct access to the airway in cases of obstruction or respiratory failure.