BRONCHIAL TREE: PARTS, HISTOLOGY AND FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2024

The bronchial tree is the set of tubes and tubes that connect the lower part of the trachea with the pulmonary alveoli. It is one of the fundamental structures of the lung.

Its main function is to distribute the air that enters through the upper respiratory tract towards structures contained in the lung that, due to their conformation, house a huge contact surface (80 square meters), which facilitates the diffusion of gases.

Schematic of the trachea and bronchial tree (Source: DataBase Center for Life Science (DBCLS) via Wikimedia Commons)

Most of the bronchial tubing system has unique air conduction functions, but some of the final portions of this system have both conduction and diffusion functions.

As the bronchial tree penetrates into the lung, it subdivides and each division receives the name "bronchial generation."

Likewise, the structure of the bronchial wall, as the bronchi penetrate the depth of the lung, is modified; the diameter or cross-section of these structures decreases and the wall of the bronchus becomes increasingly thin, losing some structures, such as cartilage.

In this way, the bronchial tree, according to its structure, is composed of main bronchi, medium and small bronchi, bronchioles and respiratory bronchioles, ending in the alveolar sacs.

Parts of the bronchial tree

The bronchial tree begins in the main bronchi, one right and one left, each directed towards the respective lung. These bronchi originate from the terminal bifurcation of the trachea, both lungs "hang" from them in the central portion of the thorax.

From each bronchus to the alveolar sacs the bronchi divide and each division forms a “bronchial generation”. There are 23 of these generations from the bronchi to the alveoli.

The first 16 bronchial generations form what is known as the "exclusive conduction zone" and include medium and small bronchi, bronchioles and terminal bronchioles. From generation 17 to generation 23 there is the so-called “transition and breathing zone”.

The bronchial tree and the lungs (Source: Internet Archive Book Images via Wikimedia Commons)

The latter is composed of respiratory bronchioles, alveolar ducts, and alveolar sacs. Air is conducted in this area, but gaseous diffusion also occurs between the air contained in the bronchial tree and the capillary blood that surrounds it.

The nomenclature of bronchi and bronchioles depends on the distribution of cartilage in the wall of the airways. The bronchioles do not have cartilage and, within the bronchial tree, they are located away from the bronchi and closer to the alveoli.

Pulmonary acinus

The final portion of the bronchial tree corresponds to a terminal bronchiole. A set of 3 to 5 terminal bronchioles forms a lobule.

An "acinus" or "pulmonary respiratory unit" is the area of ​​the lung that receives air through a single terminal bronchiole and can contain 3 to 5 generations of respiratory bronchioles.

Histology

The bronchial tree, as discussed, begins at the bifurcation of the trachea and begins with the left and right main bronchi. These bronchi are also known as "extrapulmonary bronchi" and once inside the lung they divide and become intrapulmonary bronchial passages.

Primary bronchi

The histological structure of the primary or main bronchi is identical to that of the trachea, with the exception that they are smaller in diameter than the latter and their walls are thinner.

Each main bronchus, along with the pulmonary arteries, veins, and lymphatic vessels, enters the lung through the pulmonary hile. The right bronchus is subdivided into three branches and the left into two; each branch goes to a lung lobe, which is why they are called “lobar bronchi”.

Histologically, then, the main bronchi, like the trachea, are made up of three layers of tissue: a mucosa, a submucosa, and an adventitia.

- The mucosa is composed of a ciliated, pseudostratified respiratory epithelium, and a lamina propria of subepithelial connective tissue. This layer covers the inner layer of the bronchi.

- The submucosa is the layer that contains mucous and seromucosal glands, which are embedded in fibroelastic tissue. This layer is between the mucosa and the adventitia and is rich in blood and lymphatic vessels.

- The adventitia contains hyaline cartilage and fibroelastic connective tissue, it is the outermost layer of the bronchi.

Intrapulmonary bronchi

Each intrapulmonary or lobar bronchus is directed toward a lung lobe. Its structure is similar to that of the primary or main bronchi, except for the cartilage, which no longer forms rings (as in the trachea) but rather irregular plates that completely surround the perimeter of the bronchus.

These structures are associated with smooth muscle, located between the lamina propria and the submucosa, distributed in two layers arranged in a spiral and in opposite directions.

As the intrapulmonary bronchi are subdivided, their individual diameter decreases, although the total cross-sectional area of ​​the same subdivision or "bronchial generation" increases.

Little by little, the cartilage plates decrease in size, until they only appear at the subdivision sites. Finally, the cartilage disappears completely, originating the bronchioles.

Bronchioles

The bronchioles are found between the 10th and 15th bronchial generation; these generally have a diameter of less than 1 mm.

The epithelial covering of these varies from a simple ciliated to simple cuboid columnar epithelium, with eventual Clara cells (columnar cells with dome-shaped apices and short microvilli) and no goblet cells in the smaller bronchioles.

The terminal bronchioles form the smallest and most distal region of the conductive portion of the respiratory system. Its epithelium is made up of Clara and cuboid cells, with very few cilia.

Respiratory bronchioles and alveoli

In this region of the bronchial tree, diffusion or gas exchange occurs for the first time. Respiratory bronchioles have a structure very similar to that of terminal bronchioles, but their structure is occasionally interrupted by the presence of alveolar sacs.

The alveolar sacs have much thinner walls than the terminal bronchioles (with a diameter of approximately 200 micrometers). The epithelium of these sacs is composed of two types of cells: type I pneumocytes and type II pneumocytes.

Pneumocytes are very thin flattened cells that form tight junctions. Type II pneumocytes have lamellar bodies in their cytosol and function in the production of the surfactant substance.

In a human lung there are approximately 300 million alveoli, which add up to an approximate area that ranges between 80 and 140 square meters of exchange surface.

Features

The functions can be divided into the functions exercised by the air conduction zone and those of the transition and breathing zone.

The air conduction zone has, as its name implies, the main function of conducting air from the upper respiratory tract to the terminal bronchioles.

However, due to its ciliated epithelium, this area also contributes to the filtration process of the incoming air, as well as the heating and humidification of the incoming air. Although these last two functions are typical of the upper respiratory tract, these areas participate to a lesser extent.

The transition and respiration zone, from the respiratory bronchioles, involves conduction and gas exchange and, upon reaching the alveolar sacs, this zone only fulfills a function of gas exchange between alveolar air and capillary blood, in both directions.

References

1. Ganong, WF, & Barrett, KE (2012). Ganong's review of medical physiology. McGraw-Hill Medical.
2. Gartner, LP, & Hiatt, JL (2006). Color textbook of histology ebook. Elsevier Health Sciences.
3. Hall, JE (2015). Guyton and Hall textbook of medical physiology e-Book. Elsevier Health Sciences.
4. Netter, FH, & Colacino, S. (1989). Atlas of human anatomy. Ciba-Geigy Corporation.
5. West, JB (2012). Respiratory physiology: the essentials. Lippincott Williams & Wilkins.