The trachea, or windpipe, is a 10-11 cm long fibrocartilaginous tube of the lower respiratory tract. It forms the trunk of the tracheobronchial tree, or pulmonary conducting zone. The trachea extends between the larynx and thorax, consisting of two parts; cervical and thoracic. It ends at the level of the sternal angle (T5) where it divides into two main bronchi, one for each lung. Each main bronchus branches out into smaller intrapulmonary bronchi that supply air to the various pulmonary lobes and segments.
The main function of the trachea is to transport air in and out of the lungs during the act of breathing. In addition, it protects the respiratory tract by warming and moistening the air and propelling foreign particles upwards towards the pharynx for expulsion.
This article will describe the anatomy and function of the trachea.
- Blood supply and innervation
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 bronchi branches into three lobar bronchi (superior, middle, inferior) for the three lobes of the right lung. In turn, the left main bronchi 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.
The trachea is related to many anatomical structures as it travels between the neck and mediastinum. The cervical part of trachea is covered anteriorly by several structures, from deep to superficial;
- The visceral cervical fascia
- The isthmus of thyroid gland which crosses the trachea between the second and fourth tracheal cartilages. The inferior thyroid arteries are located superior to the isthmus. The pretracheal fascia, inferior thyroid veins and thymus are located inferior to the isthmus.
- Pretracheal lymph nodes
- The sternohyoid and sternothyroid muscles
- The jugular venous arch
The esophagus is located posterior to the cervical part of trachea. In addition, there are several lateral relations, as follows:
- The lobes of thyroid gland which extend inferiorly until the sixth tracheal cartilage.
- The common carotid artery
- The inferior thyroid arteries and their branches, the inferior laryngeal arteries.
- The recurrent laryngeal nerve, which is located slightly posteriorly compared to the rest.
- Paratracheal cervical lymph nodes
The thoracic part of trachea has several relations as it travels within the superior mediastinum. The anterior relations include, from superior to inferior;
- The inferior thyroid veins, manubrium of sternum and attachments of sternohyoid and sternothyroid muscles. The trachea can be palpated superior to the jugular notch of sternum, all the way to the cricoid cartilage.
- The brachiocephalic trunk and left common carotid artery
- The aortic arch, left brachiocephalic vein, cardiac plexus and paratracheal thoracic lymph nodes
The esophagus remains posteriorly to the thoracic part of trachea. In addition, the lateral relations of the thoracic part are different on the right and left sides;
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.
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.
For more details about the trachea, take a look at the articles, videos, quizzes and labeled diagrams in the following study unit:
Tracheomalacia is a congenital anomaly that affects the development of the tracheal cartilage rings. If the bronchial cartilages are involved the condition is called bronchomalacia. If both the trachea and bronchi are involved, the disorder is called tracheobronchomalacia.
In tracheomalacia, the tracheal cartilages become abnormally soft. As a result, they cannot maintain the trachea open and it continuously collapses during inspiration and expiration. The affected area can span only a few cartilages or affect the entire trachea. Such diffused involvement happens during Williams-Campbell syndrome. Being a congenital anomaly, the tracheomalacia manifests in early infancy. It presents with signs and symptoms of respiratory distress such as dyspnea, cough, stridor, wheezing and tachypnea. If the tracheal collapse is severe, it can even manifest with the cessation of breathing (apnea). In such cases, a tracheal tube or airway stents are used to bypass the obstruction and maintain the trachea open. Tracheostomies and continuous night-time ventilatory support may also be required.