TRACHEA: CHARACTERISTICS, FUNCTIONS, PARTS, HISTOLOGY, DISEASES - ANATOMY AND PHYSIOLOGY - 2025

The trachea is a flexible cylindrical tube about 12-14 cm long in adult humans, and about 2 cm in diameter. It begins in the cricoid cartilage of the larynx and ends up bifurcating (dividing in two, like a “Y”) and giving rise to the right main bronchus and the left main bronchus.

This structure is part of the air conduction system of the respiratory system of man and many other vertebrate animals. Anatomically, the trachea is located between the larynx and the main bronchus of each lung (right and left).

The trachea begins at the end of the larynx

The air conduction system of the human respiratory system is made up of the nasal passages and nasal cavity, the paranasal sinuses, the pharynx (common passage for food and air), the larynx (which contains the vocal cords), the trachea, the bronchi and the structure of pulmonary tubes and ducts.

The function of the respiratory system is to carry out the gaseous exchange of oxygen and carbon dioxide (CO2) between the air that reaches the lungs and the gases that circulate in the systemic blood. This process is called “external respiration”, to differentiate it from tissue-capillary exchange and from oxygen consumption and cellular production of CO2, which is known as “internal respiration”.

Human respiratory system

The presence of irritants in the trachea or main bronchi triggers the cough reflex, which allows, through an explosive air current, to eliminate the irritant and prevent damage to the lung structures "downstream", such as the pulmonary alveoli.

Tracheal anomalies are very rare, however, there are congenital pathologies such as, for example, tracheoesophageal fistulas, tracheal stenoses, absence of cartilage and abnormal bifurcations, to name a few.

General characteristics

Anatomical diagram of the human larynx, trachea, bronchi and lungs (Source: OpenStax via Wikimedia Commons) The trachea is a tube whose posterior part is flattened and extends from the lower part of the larynx to the level of the fourth dorsal vertebra, where it bifurcates giving rise to the two main bronchi.

Histologically speaking, it is made up of three different tissue layers, known as the mucosa, submucosa, and adventitia.

The cells present in these layers participate in the conduction of air and in the secretion of mucus and the elimination of foreign substances from the airways.

Respiration consists of two phases: inspiration or air entry and expiration or air exit. During inspiration, the trachea widens in diameter and increases in length, while during expiration it returns to its normal condition, that is, it returns to the initial condition prior to inspiration.

Features

Air conduction

The main function of the trachea is to conduct air from the environment, which reaches through the nostrils and larynx, to the main bronchi and subsequently to the lungs.

Defense of the organism

Another important function of this portion of the respiratory tract is to eliminate, through the ciliary sweep of mucus, foreign substances or suspended particles that enter with the air, preventing them from reaching the most sensitive or delicate parts of the lung, that is, to the pulmonary alveoli, which represent the main sites of gas exchange.

The trachea reacts to irritating substances by contracting the smooth muscles associated with it, triggering the cough reflex.

The contraction of these muscles achieves the reduction of the diameter of the trachea and, together with the violent contraction of the expiratory muscles and the abrupt opening of the glottis, helps to increase the speed of air flow and the elimination of irritants.

Thermoregulation

Along with the nasal passages and sinuses, the trachea participates in the heating and humidification (saturation of water vapor) of the air entering the respiratory tract.

Parts and histology

The trachea is made up of three layers of tissue, which are:

- The mucosa

- The submucosa

- The adventitia

Much of the trachea is outside the chest, in the front of the neck, and in front of the esophagus. Then it enters the internal part of the thorax (mediastinum), being located behind the sternum, until it reaches the level of the fourth dorsal vertebra, where it bifurcates.

Adventitial layer of trachea

This is the outermost layer of the trachea, it is formed by fibroelastic connective tissue, hyaline cartilage and fibrous connective tissue. It works in fixing the trachea to adjacent structures such as the esophagus and connective tissues in the neck.

The adventitial layer is where the tracheal rings are located, which are more than a dozen, and which are made up of hyaline cartilage. These rings are shaped like a "C" or horseshoe; the "open" part of the horseshoe is directed towards the back of the trachea, that is, as if looking towards the dorsal part of the body.

Graphic representation of the lungs, trachea and bronchi (Source: Arcadiande translation: Ortisa via Wikimedia Commons) Between each cartilaginous ring is a layer of intermediate fibrous connective tissue. Each ring, at its back, is attached to the next through a layer of smooth muscle known as the tracheal muscle. The contraction of this muscle reduces the diameter of the windpipe and increases the speed of flow and the displacement of foreign substances.

The arrangement of the rings and the tracheal muscle make the posterior part of the trachea flat and the anterior part rounded.

Just above where the bifurcation of the trachea occurs, cartilaginous rings come together to completely surround the airway. The muscular covering of the muscular part of the trachea is reorganized forming a separate layer of interlocking fascicles within said cartilages.

Submucosal layer of the trachea

The submucosal layer contains mucous and seromucosal glands embedded in a dense and irregular fibroelastic tissue. It is located spatially between the mucous layer and the adventitia and is rich in blood and lymphatic vessels.

The ducts of the glands in this layer are short and pierce the lamina propria of the epithelium, draining its products towards the inner surface of the trachea.

Mucous layer of the trachea

It is the layer that covers the inner portion of the trachea (covering layer) and is separated from the submucosa by a bundle of fairly thick elastic fibers. It is composed of a respiratory epithelium (ciliated pseudostratified epithelium) and a lamina propria of subepithelial connective tissue.

The respiratory epithelium

It is made up of various types of cells, including goblet cells, ciliated cylindrical cells, other brush cells, basal cells, serous cells, and cells of the diffuse neuroendocrine system.

All of these cells reach the basement membrane, but not all reach the lumen of the trachea (the inner space of the duct). The most abundant are ciliated columnar cells, goblet cells, and basal cells.

- Cylindrical ciliated cells, as their name indicates, are cells with an apical plasma membrane differentiated into cilia and microvilli, whose movement is upward, that is, from bottom to top or towards the nasopharyngeal route.

The main function of these cells is to "facilitate" the movement of the mucus and the particles contained in it out of the trachea.

- Goblet cells are responsible for the production of mucinogen, which is one of the main components of mucus and, in the respiratory system, these are cells with a narrow base and an expanded upper part, rich in secretory granules loaded with mucin.

- The basal cells are short in length and are attached to the basement membrane, but do not reach the luminal surface of the mucosa. Stem cells are considered for the regeneration of goblet cells, hair cells, and brush cells.

- Serous cells are the least abundant in the tracheal mucosa. They are cylindrical cells that have microvilli and apical granules loaded with the electrodense serous fluid that they secrete.

- Brush cells, as well as serous cells, are found in a very low proportion. They also have high microvilli and some researchers suggest that they may have sensory functions, as they have been linked to nerve endings.

- Cells of the diffuse neuroendocrine system, also known as “small granule cells”, are scarce in the mucosa. These contain granules that are apparently released into the connective tissue spaces of the lamina propria, secretions that appear to control the functions of other cells of the respiratory epithelium.

The lamina propria

This layer is made up of loose fibroelastic connective tissue and contains lymphoid tissues such as lymph nodes, lymphocytes, and also neutrophils. Some seromucosal glands and mucus are also found in the lamina propria.

Diseases

Like all organic tissue, the trachea is susceptible to certain conformational alterations caused by congenital problems, by infections and benign or malignant tumors, and by alterations in its structure due to the continuous inhalation of irritants.

Tracheal metaplasia

One of the most frequent alterations of the trachea is tracheal metaplasia, which consists of a reduction in the number of hair cells in the mucosa and an increase in mucus-producing goblet cells, typical of chronic cigarette smoking or recurrent exposure to coal dust.

The increase in the number of goblet cells increases the thickness of the mucus layer, but the decrease in the number of hair cells reduces their elimination from the tracheal duct, resulting in chronic congestion of the airways and lungs.

Tracheoesophageal fistulas

Among the congenital alterations of the trachea, it is worth mentioning the tracheoesophageal fistulas, which are abnormal conduits that connect the trachea with the esophagus; tracheal stenosis (a congenital reduction in the diameter of the trachea); cartilage agenesis (absence of tracheal cartilage that causes the collapse and closure of the trachea), among others.

Infections or tumors

Other tracheal pathologies have to do with infections of viral or bacterial origin, or the development of benign or carcinomatous tumors.

Other

Finally, there are alterations associated with retractable scars that occur in the trachea due to penetrating injuries or tracheostomy, an intervention in which a tube is placed inside the trachea to connect patients who require assisted respiration for very long periods of time.

These scars produce local narrowing of the trachea that can cause difficulty in breathing and that must be treated surgically.

References

1. Ganong, WF (1980). Manual of medical physiology.
2. Gartner, LP, & Hiatt, JL (2006). Color textbook of histology ebook. Elsevier Health Sciences.
3. Johnson, K. (1991). Histology and Cell Biology (2nd ed.). Baltimore, Maryland: The National medical series for independent study.
4. Netter, FH, & Colacino, S. (1989). Atlas of human anatomy. Ciba-Geigy Corporation.
5. Ross, M., & Pawlina, W. (2006). Histology. A Text and Atlas with correlated cell and molecular biology (5th ed.). Lippincott Williams & Wilkins.