Trachea

Structure

The trachea is a D-shaped fibrocartilaginous respiratory organ. It consists of 16-20 tracheal cartilages anterolaterally and a fibromuscular wall posteriorly. The tracheal cartilages are composed of hyaline cartilage and interconnected by fibroelastic tissue. They support the trachea and keep it open during pressure changes that accompany air ventilation. The posterior wall of the trachea is formed by the trachealis muscle, making the cartilages appear as incomplete C-shaped rings. The structure of its wall makes the trachea sufficiently flexible and elastic to permit the transient expansion of the esophagus during swallowing.

The trachea consists of four histological layers. The mucosa represents the innermost layer and it is lined with pseudostratified ciliated columnar epithelium. The second histological layer is the submucosa. It consists of connective tissue that contains mucus glands, smooth muscle, vessels, nerves and lymphatics. The third layer is the musculocartilaginous layer which is represented by the cartilaginous rings and intervening smooth muscle. Lastly, the most external layer is provided by the fibroelastic adventitia.

The trachea extends between the larynx and thorax, roughly occupying the midline (slightly tilted to the left). It is divided into two parts:

• The cervical part of trachea, which is located inside the anterior visceral (pretracheal) compartment of the neck. It begins at the inferior border of the larynx (cricoid cartilage) located at the level of vertebra C6. It ends at the level of the jugular notch of sternum, which represents the upper border of the superior mediastinum.
• The thoracic part of trachea, which is located within the superior mediastinum of the thorax. It begins from the superior thoracic aperture and ends at the tracheal bifurcation. The bifurcation can be located anywhere between the levels of the fourth and seventh thoracic vertebrae. Most commonly it is located at the level of the sternal angle and vertebra T5.

The trachea and bronchi collectively form the tracheobronchial tree. The thoracic part divides at the tracheal bifurcation into the right and left main bronchi. The tracheal bifurcation houses a sagittally-oriented cartilaginous ridge called the carina.

The right main bronchus is shorter, wider and courses almost vertically compared to the left one. Therefore, it is more susceptible to foreign body obstructions. Each main bronchus divides into increasingly smaller intrapulmonary bronchi that supply the lungs;

• Lobar bronchi which transport air towards the pulmonary lobes. The right main bronchus branches into three lobar bronchi (superior, middle, inferior) for the three lobes of the right lung. In turn, the left main bronchus divides into two lobar bronchi (superior, inferior) for the two lobes of the left lung.
• Segmental bronchi which aerate the bronchopulmonary segments. On the right side, the three lobar bronchi divide into a total of ten to twelve segmental bronchi, one for each bronchopulmonary segment. Similarly on the left side, the two lobar bronchi branch into a total of eight to ten segmental bronchi.
• Intrasegmental (subsegmental) bronchi which transport air even deeper within the bronchopulmonary segments. Each segmental bronchi provides approximately fifteen intrasegmental bronchi. In turn, the intrasegmental bronchi give off many bronchioles which terminate as the pulmonary lobules and alveoli.

Relations

Blood supply and innervation

The trachea is supplied with arterial blood by tracheal branches of inferior thyroid arteries, that stem from the thyrocervical trunk. Venous blood is drained to the inferior thyroid venous plexus, which empties into the brachiocephalic veins. Lymphatic drainage is to the pretracheal and paratracheal (cervical, thoracic) lymph nodes that empty into the deep cervical lymph nodes.

The trachea receives innervation from the pulmonary plexus. Parasympathetic supply originates from the recurrent laryngeal nerves, branches of the vagus nerve. Sympathetic supply is provided by the sympathetic trunks.

Function

The trachea is part of the conducting zone of the respiratory system. Its main function is to transport oxygenated air from the upper respiratory tract towards the alveoli for gas exchange. The resulting carbon dioxide filled air is transported by the tracheobronchial system towards the oral cavity and nose for excretion.

The trachea is only one part of the respiratory system. Learn the anatomy of all the remaining organs easily and efficiently using Kenhub’s respiratory system quizzes and labeled diagrams!

The trachea also protects the respiratory tract via physical and immunological methods. In addition to oxygen and carbon dioxide, environmental air also contains many potentially harmful agents such as pathogens, debris, gases and chemicals. The goblet cells lining the trachea secrete mucus which traps any unwanted substances. Immune cells and their secretions (lysozyme, IgA antibodies, lymphocytes) prevent harmful microorganisms from reaching the lower airways. The trapped particles are then removed from the trachea via the mucociliary rejection current. This system pushes them upwards back into the pharynx and mouth for excretion. The mucociliary excretion is facilitated by the coughing reflex.

Now that you've learned everything about the trachea, it's time to put that knowledge to the test with the following quiz!