Bronchi

The right main bronchus is like an extension of the trachea. Its length is from 28 to 32 mm, the diameter of the lumen is 12-16 mm. The left main bronchus 40-50 mm in length has a width from 1 0 to 1 3 mm.

In the direction to the periphery, the main bronchi are dichotomously divided into lobar, segmental, subsegmental and further down to the terminal and respiratory bronchioles. However, there is also a division into 3 branches (trifurcation) and more.

The right main bronchus is divided into the upper lobe and intermediate, and the intermediate bronchus is divided into the mid-lobe and lower-lobe. The left main bronchus is divided into upper lobe and lower lobe. The total number of respiratory tracts is variable. Starting from the main bronchus and ending with alveolar sacs, the maximum number of generations reaches 23-26.

The main bronchi are the first-line bronchuses, the lobar bronchi are of the second order, the segmental bronchi are of the third order, and so on.

Bronchi with 4 th to 13 th generation have a diameter of about 2 mm, the total number of such bronchi 400. In terminal bronchioles, the diameter varies from 0.5 to 0.6 mm. The length of the airways from the larynx to the acini is 23-38 cm.

The right and left main bronchi (bronchi principles dexter and sinister) start from the bifurcation of the trachea at the level of the upper edge of the V thoracic vertebra and are directed to the gates respectively of the right and left lungs. In the area of the lungs' gates, each major bronchus is divided into lobar (second-line bronchi). Above the left main bronchus is the arch of the aorta, above the right is the unpaired vein. The right main bronchus has a more vertical position and a shorter length (about 3 cm) than the left main bronchus (4-5 cm in length). The right main bronchus is wider (diameter 1.6 cm) than the left one (1.3 cm). The walls of the main bronchi have the same structure as the tracheal wall. Inside, the walls of the main bronchi are lined with mucous membranes, and on the outside are covered with adventitia. The basis of the walls are not closed behind the cartilage. In the right main bronchus there are 6-8 cartilaginous semirings, at the left - 9-12 cartilages.

The innervation of the trachea and the main bronchi: branches of the right and left recurrent guttural nerves and sympathetic trunks.

Blood supply: the branches of the lower thyroid, the internal thoracic artery, the thoracic part of the aorta. Venous outflow is carried out in the brachiocephalic veins.

Outflow of lymph: into deep cervical lateral (internal jugular) lymph nodes, pre- and paratracheal, upper and lower tracheobronchial lymph nodes.

[1], [2], [3], [4], [5]

Histological structure of the bronchi

Outside the trachea and large bronchi are covered with a loose connective tissue case - adventitia. The outer cover (adventitia) consists of a loose connective tissue containing fatty cells in large bronchi. In it, there are blood lymph vessels and nerves. Adventism is indistinctly delimited from peribronchial connective tissue and, together with the latter, provides the possibility of some displacement of the bronchi in relation to the surrounding parts of the lungs.

Further inward, go fibro-cartilaginous and partially muscular layers, submucosal layer and mucous membrane. In the fibrous layer, in addition to the cartilaginous semirings, there is a network of elastic fibers. Fibrous-cartilaginous shell of the trachea with the help of loose connective tissue connects with neighboring organs.

The anterior and lateral walls of the trachea and major bronchi are formed by cartilages and annular ligaments located between them. The cartilaginous skeleton of the main bronchi consists of half-rings of hyaline cartilage, which, as the diameter of the bronchi decreases, diminish in size and acquire the character of an elastic cartilage. Thus, only large and medium bronchi consist of hyaline cartilage. Cartilages occupy 2/3 of the circumference, membranous part - 1/3. They form a fibrous-cartilaginous skeleton, which ensures the preservation of the lumen of the trachea and bronchi. 

Muscle bundles are concentrated in the membrane part of the trachea and the main bronchi. There is a surface, or outer layer, consisting of rare longitudinal fibers, and deep, or internal, which is a continuous thin shell formed by transverse fibers. Muscle fibers are located not only between the ends of the cartilage, but also enter the inter-annular intervals of the cartilaginous part of the trachea and, to a greater extent, the main bronchi. Thus, in the trachea, the bundles of smooth muscles with a transverse and oblique arrangement are located only in the membrane part, that is, the muscle layer as such is absent. In the main bronchi rare groups of smooth muscles are present all around the circumference.

With a decrease in the diameter of the bronchi, the muscle layer becomes more developed, and its fibers go in a somewhat oblique direction. The contraction of the muscles causes not only the development of the lumen of the bronchi, but also a certain shortening of the bronchi, so that the bronchi participate in the exhalation due to a reduction in the capacity of the airways. Reduction of muscles allows to narrow the clearance of the bronchi by 1/4. When inhaled, the bronchus lengthens and expands. Muscles reach respiratory bronchioles of the second order.

Inside the muscular layer is a submucosal layer consisting of loose connective tissue. It contains vascular and neural formations, submucous lymphatic network, lymphoid tissue and a significant part of the bronchial glands, which belong to the tubular-acinous type with mixed mucus-serous secretion. They consist of end sections and excretory ducts, which are opened by the bulbous extensions on the surface of the mucous membrane. The relatively long length of the ducts contributes to a prolonged course of bronchitis in inflammatory processes in the glands. Atrophy of the glands can lead to the drying of the mucous membrane and inflammatory changes.

The largest number of large glands is found over the bifurcation of the trachea and in the division of the main bronchi into the lobar bronchi. A healthy person secrets up to 100 ml of secret per day. On 95% it consists of water, and on 5% it is necessary equal quantity of proteins, salts, lipids and inorganic substances. Secret is dominated by mucins (high molecular weight glycoproteins). To date, there are 14 types of glycoproteins, 8 of which are contained in the respiratory system.

Bronchial mucosa

The mucosa consists of a covering epithelium, a basal membrane, a propria of the mucous membrane and a muscular plate of the mucosa.

The bronchial epithelium contains high and low basal cells, each of which is attached to the basal membrane. The thickness of the basal membrane varies from 3.7 to 10.6 microns. The epithelium of the trachea and major bronchi is multi-row, cylindrical, ciliary. The thickness of the epithelium at the level of segmental bronchi is 37 to 47 microns. It consists of 4 main types of curd: ciliate, goblet, intermediate and basal. In addition, there are serous, brush, Clara and Kulchitsky cells.

The resected cells predominate on the free surface of the epithelial layer (Romanova LK, 1984). They have an irregular prismatic shape and an oval vesicle-shaped nucleus located in the middle part of the cell. Electron-optical density of the cytoplasm is low. Mitochondria are few, the endoplasmic granular reticulum is poorly developed. Each cell carries on its surface short microvilli and about 200 ciliated cilia with a thickness of 0.3 μm and a length of about 6 μm. In humans, the density of the cilia is 6 μm ².

Between the adjacent cells, spaces are formed; among themselves the cells are connected with the help of finger-like outgrowths of the cytoplasm and desmosomes.

The population of ciliate cells according to the degree of differentiation of their apical surface is subdivided into the following groups:

1. Cells that are in the phase of basal body formation and axonem. Cilia at this time on the apical surface absent. During this period, there is accumulation of centrioles, which move to the apical surface of cells, and the formation of basal bodies, from which axons of cilia begin to form.
2. Cells in the phase of moderately expressed ciliogenesis and growth of cilia. On the apical surface of such cells appears a small number of cilia, the length of which is 1 / 2-2 / 3 of the length of the cilia of differentiated cells. In this phase, microvilli predominate on the apical surface.
3. Cells in the phase of active ciliogenesis and growth of cilia. The apical surface of such cells is almost completely covered with cilia, the size of which corresponds to the size of the cilia of cells in the previous phase of ciliogenesis.
4. Cells in the phase of complete ciliogenesis and growth of cilia. The apical surface of such cells is entirely covered with densely located long cilia. On the electron diffraction patterns it is seen that the cilia of the adjacent cells are oriented in one direction and are bent. This is an expression of mucociliary transport.

All these groups of cells are clearly distinguishable in photos obtained with the help of light electron microscopy (SEM).

Cilia are attached to basal bodies located in the apical part of the cell. The ciliary axoneme is formed by microtubules, of which 9 pairs (duplexes) are located at the periphery, and 2 single (singlet) - in the center. Duplets and singlets are connected by non-new fibrils. On each of the doublets there are 2 short "handles" on the one hand, which contain ATP-ase, which participates in the release of ATP energy. Due to this structure, the cilia rhythmically oscillate with a frequency of 16-17 in the direction of the nasopharynx.

They move the mucosal film covering the epithelium at a rate of about 6 mm / min, thereby ensuring a continuous drainage function of the bronchus.

Resynate epitheliocytes, according to most researchers, are at the stage of terminal differentiation and are not capable of dividing by mitosis. According to the modern concept, basal cells are precursors of intermediate cells, which can differentiate into ciliate cells.

Goblet cells, like ciliated cells, reach the free surface of the epithelial layer. In the membrane part of the trachea and large bronchi, the share of ciliated cells accounts for 70-80%, and for the goblet cells - no more than 20-30%. In those places where there are cartilaginous semicircles along the perimeter of the trachea and bronchi, zones with different ratios of ciliated and goblet cells are found:

1. with a predominance of ciliate cells;
2. with an almost equal ratio of ciliated and secretory cells;
3. with a predominance of secretory cells;
4. with complete or almost complete absence of ciliated cells ("biorescent").

Goblet cells are single-cell glands of a mercrinic type that secrete a mucous secret. The shape of the cell and the location of the nucleus depend on the phase of secretion and filling of the supernuclear part with mucus granules, which merge into larger granules and are characterized by a low electron density. The goblet cells have an elongated shape that during the accumulation of the secret takes the form of a glass with a base located on the basement membrane and intimately connected to it. The wide end of the cell domes on the free surface and is equipped with microvilli. The cytoplasm is electronically dense, the core is round, the endoplasmic reticulum is of a rough type, well developed.

Goblet cells are distributed unevenly. Scanning electron microscopy revealed that the different zones of the epithelial layer contain heterogeneous regions consisting either of ciliated epithelial cells or only of secretory cells. However, solid accumulations of goblet cells are relatively few. Along the perimeter of the segmental bronchus of a healthy person, there are areas where the ratio of ciliated epithelial cells to goblet cells is 4: 1-7: 1, and in other regions this ratio is 1: 1.

The number of goblet cells decreases distally in the bronchi. In bronchioles, goblet cells are replaced by Clara cells involved in the production of serous components of mucus and alveolar hypophase.

In small bronchi and bronchioles, goblet cells are normally absent, but can appear in pathology.

In 1986, Czech scientists studied the reaction of the epithelium of the airways of rabbits to the oral administration of various mucolytic substances. It turned out that the target cells of mucolytics are goblet cells. After excretion of mucus, the goblet cells, as a rule, degenerate and are gradually removed from the epithelium. The degree of damage to goblet cells depends on the substance administered: the most irritating effect is produced by lasolvan. After the administration of broncholysin and bromhexine, a massive differentiation of new goblet cells occurs in the epithelium of the airways, resulting in hyperplasia of goblet cells.

Basal and intermediate cells are located in the depth of the epithelial layer and do not reach the free surface. These are the least differentiated cellular forms, due to which physiological regeneration is carried out mainly. The shape of the intermediate cells is elongated, basal cells are irregularly cubic. Both have a rounded, rich DNA core and a small amount of cytoplasm, which has a high density in the basal cells.

Basal cells are able to give rise to both ciliate and goblet cells.

Secretory and ciliary cells are united under the name "mucociliary apparatus".

The process of movement of mucus in the airways of the lungs is called mucociliary clearance. The functional efficiency of MZ depends on the frequency and synchronism of ciliary ciliary epithelium movement, and, what is very important, on the characteristics and rheological properties of mucus, that is, on the normal secretory capacity of goblet cells.

Serous cells are few, reach the free surface of the epithelium and are distinguished by small electron-dense granules of protein secretion. The cytoplasm is also electron-dense. Mitochondria and a rough reticulum are well developed. The nucleus is round, usually located in the middle part of the cell.

Secretory cells, or Clara cells, are most numerous in small bronchi and bronchioles. They, like serous, contain small electron-dense granules, but they have a low electron density of the cytoplasm and a predominance of a smooth, endoplasmic reticulum. The rounded nucleus is in the middle part of the cell. Clara cells participate in the formation of phospholipids and, possibly, in the production of surfactant. Under conditions of increased irritation, they apparently can transform into goblet cells.

The brush cells are carried on the free surface of the microvilli, but are devoid of cilia. Cytoplasm of their small electron density, the nucleus is oval, bubble-shaped. In the manual of Ham A. And Cormack D. (1982) they are considered as goblet cells that have identified their secret. Many functions are attributed to them: absorption, contractile, secretory, chemoreceptor. However, in the airways of man they are practically not investigated.

Kulchitsky cells are found all over the bronchial tree at the base of the epithelial layer, differing from the basal ones by the low electron density of the cytoplasm and the presence of small granules that are detected under the electron microscope and under the light microscope when impregnated with silver. They are referred to the neurosecretory cells of the APUD system.

Under the epithelium is the basal membrane, which consists of collagen and non-collagen glycoproteins; it provides support and attachment of the epithelium, participates in metabolism and immunological reactions. The condition of the basal membrane and the underlying connective tissue determines the structure and function of the epithelium. A layer of loose connective tissue between the basal membrane and the muscle layer is called the proprietary plate. It contains fibroblasts, collagen and elastic fibers. In its own plate there are blood and lymphatic vessels. Capillaries reach the basal membrane, but do not penetrate into it.

In the mucosa of the trachea and bronchi, mainly in the own plate and near the glands, free cells are constantly present in the submucosa, which can penetrate through the epithelium into the lumen. Among them, lymphocytes predominate, plasma cells, histiocytes, mast cells (labrocytes), neutrophilic and eosinophilic leukocytes are less common. The permanent presence of lymphoid cells in the bronchial mucosa is denoted by the special term "broncho-associated lymphoid tissue" (BALT) and is considered as an immunological defense response to antigens penetrating with air into the respiratory tract. 

[6], [7], [8], [9], [10], [11]