Trachea

The trachea, commonly known as the windpipe, is a vital component of the respiratory system in vertebrates, serving as the primary airway connecting the larynx to the bronchi of the lungs. It enables the passage of air during breathing and is structurally adapted to remain open while allowing flexibility. Although the term trachea is also used in invertebrate anatomy, in insects it refers to part of an entirely different respiratory system composed of spiracles, tracheae and tracheoles that directly transport gases to tissues.
The vertebrate trachea is a specialised cartilaginous tube that maintains a patent airway, supports mucociliary clearance and provides a conduit for medical procedures such as tracheal intubation and tracheostomy. Its development, microstructure, vascular supply and relationship with surrounding structures are key topics in human anatomy and clinical practice.

Structure

The adult human trachea is typically around 2 centimetres in internal diameter and approximately 10–12 centimetres in length, with males generally having a slightly wider trachea than females. It begins at the lower border of the cricoid cartilage at the level of the sixth cervical vertebra (C6) and terminates at the carina, where it bifurcates into the right and left main bronchi at the level of the fourth thoracic vertebra (T4). Its vertical position varies slightly with breathing movements.
The tracheal wall contains 16 to 20 C-shaped rings of hyaline cartilage, each approximately 4 millimetres high. These incomplete rings are joined by annular ligaments and bridged posteriorly by the trachealis muscle, which adjusts tracheal diameter during activities such as coughing. The outermost connective tissue layer, the adventitia, provides flexibility and helps anchor the trachea to adjacent structures. The trachea, while a midline structure, may be slightly displaced towards the right due to the presence of the aortic arch.

Nearby Structures

The trachea passes through the neck and superior mediastinum and is closely associated with multiple anatomical structures:

• Anteriorly, the upper portion is covered by skin and connective tissue. The jugular venous arch crosses the trachea superiorly, while further down the sternohyoid and sternothyroid muscles lie over it. The thyroid gland spans the upper trachea, with its isthmus typically covering the second to fourth cartilage rings.
• Major vessels, including the superior thyroid arteries and inferior thyroid veins, lie near the gland and trachea. In the lower region, the manubrium of the sternum overlies the trachea, and nearby are the aortic arch, left common carotid artery, brachiocephalic artery and left brachiocephalic vein.
• Posteriorly, the oesophagus runs closely behind the trachea, separated by connective tissue before the vertebral column.
• Laterally, the carotid arteries, inferior thyroid arteries, recurrent laryngeal nerves (superiorly) and vagus nerves (inferiorly) pass alongside the trachea.

This proximity to major vessels, nerves and the oesophagus is clinically significant, particularly in surgical interventions.

Blood and Lymphatic Supply

The trachea’s upper segment receives arterial supply from the inferior thyroid arteries, while the lower trachea is supplied by bronchial arteries, direct branches of the thoracic aorta. These arteries form networks of superior and inferior branches, which further divide to supply anterior and posterior portions of the tracheal wall.
Venous drainage mirrors the arterial pattern, with the upper trachea draining into the inferior thyroid veins and the lower trachea into bronchial veins.Lymphatic drainage is directed to the pretracheal lymph nodes anteriorly and paratracheal lymph nodes laterally.

Development

During the fourth week of embryogenesis, outgrowth of the respiratory bud and formation of tracheoesophageal ridges lead to the separation of the trachea from the foregut via the tracheoesophageal septum. By the fifth week, the primary bronchi begin as outpouchings at the distal end of the developing trachea.
In infancy, the trachea measures only about 4 millimetres in diameter, expanding progressively to its adult size by late childhood. The paediatric trachea is more circular and vertical and its position relative to surrounding structures is more variable than in adults.

Microanatomy

The trachea is lined by pseudostratified ciliated columnar epithelium containing numerous goblet cells. These secrete mucins, which form mucus to trap inhaled particles. The coordinated beating of cilia propels mucus upwards towards the larynx and pharynx, where it may be swallowed or expelled. This system, known as mucociliary clearance, is crucial for airway hygiene and protection.
Beneath the epithelial layer lies the submucosa, composed of connective tissue and containing seromucous glands. The tracheal cartilage rings, situated beneath the submucosa, provide structure and resistance to collapse. The rings are elastic but may undergo calcification with ageing.

Function

The trachea’s principal function is to act as the main airway for transporting air to and from the lungs. Additional roles include:

• Air filtration, via mucus and ciliary action.
• Humidification and warming, ensuring that inspired air reaches the lungs in optimal condition.
• Structural maintenance, achieved by cartilaginous rings preventing collapse under negative pressure during inhalation.

These functions are essential for efficient pulmonary ventilation and airway protection.

Clinical Significance

Inflammation and InfectionTracheitis refers to inflammation of the trachea, most commonly caused by viral infections and frequently associated with inflammation of adjacent structures, particularly the larynx and bronchi, in a condition known as croup. Bacterial infections of the trachea, most often due to Staphylococcus aureus or Streptococcus pneumoniae, occur predominantly in children and may follow an initial viral illness. Additional organisms, such as Haemophilus influenzae and Moraxella catarrhalis, may also play a role.
Symptoms may include cough, fever, stridor and airway narrowing. Severe cases risk obstruction and may require urgent airway management.
Airway Obstruction and InterventionsObstruction of the trachea prevents air from reaching the lungs, necessitating emergency measures such as tracheostomy to bypass the obstruction.During general anaesthesia, a tracheal tube is inserted to maintain airway patency and allows mechanical ventilation. The trachealis muscle may contract during coughing, narrowing the lumen and increasing intrathoracic pressure to expel irritants.