Human Anatomy, Part 1 - K. A. Dyubenko, A. K. Kolomiysev, Yu. B. Chaykovsky 2002

Special Part
Respiratory System, systema respiratorium [respiratory apparatus, apparatus respiratorius] - General Overview
Lungs - Structure of the Lungs

Each of the main Bronchi divides into lobar bronchi, bronchi lobares, corresponding to the lobular Structure OF THE right and left Lungs. The right superior lobar bronchus, bronchus lobaris superior, passing toward the center of the superior lobe, lies above the pulmonary artery (eparterial bronchus), whereas the other lobar bronchi of the right and left lungs lie below their respective pulmonary Arteries (hyparterial bronchi). The lobar bronchi branch into smaller segmental bronchi, bronchi segmentates, and these, in turn, into even smaller bronchi of the 4th and subsequent orders.

The 8th-to-10th order bronchi (about 1 mm in diameter) enter the apices of the pulmonary lobules, lobulus pulmonis. These are small, pyramid-shaped areas of the pulmonary parenchyma up to 1 cm in diameter, separated from one another by Connective Tissue septa. The total number of lobules in both lungs is approximately 1,000. Within each lobule, the lobular bronchus divides into 12–18 tiny (0.3–0.5 mm in diameter) terminal bronchioles, bronchioli terminales. The entire network of bronchi, starting from the main bronchi and ending with the terminal bronchioles, constitutes the bronchial tree, arbor bronchialis. Its primary function is the conduction of air during inspiration and expiration.

The terminal bronchioles divide into respiratory bronchioles, bronchioli respiratońi, upon the walls of which are located pulmonary alveoli, alveolus pulmonaris. Respiratory bronchioles may form 2nd-to-3rd order branchings. Alveolar ducts, ductuli alveolares, originate from the respiratory bronchioles and terminate in alveolar sacs, sacculi alveolares. The walls of the alveolar ducts and alveolar sacs consist of pulmonary alveoli, alveoli pulmonis. Respiratory bronchioles, alveolar ducts, sacs, and alveoli make up the alveolar tree, arbor alveolaris. Its Structural and functional unit is the pulmonary acinus, acinus pulmonaris (Fig. 292).

When the branches of a terminal bronchiole are reconstructed in space, they resemble a bunch of grapes (Latin: *acinus*, hence the name of this structure). Thus, a pulmonary acinus is a collection of respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli representing the ramifications of a single terminal bronchiole, together with the surrounding connective tissue containing neural elements, Blood Vessels, and Lymphatic vessels.

The main function of the alveolar tree is gas exchange between air and blood. The number of alveoli in both lungs reaches 500 million, and the total respiratory surface area of the lungs ranges from 30 m² during expiration to 100 m² during deep inspiration.

The structure of the intrapulmonary bronchial walls differs slightly from that of the main bronchi. In the lobar bronchi, Cartilage connections appear between the cartilaginous rings, transforming the ring-like structure into a lattice-like one. In the segmental bronchi and their subsequent branches, the cartilages break down into separate plates, the sizes of which decrease as the diameter of the bronchi diminishes. Terminal bronchioles lack cartilage entirely.

The mucous membrane of the lobar and segmental bronchi is lined with ciliated pseudostratified epithelium. In the further Branches of the bronchi, the height of the epithelium decreases, and in the terminal bronchioles, it becomes simple cuboidal ciliated epithelium.

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Fig. 292. Pulmonary acinus, acinus pulmonaris (diagram) (after R. D. Sinelnikov)

In the submucosa of large bronchi lie the bronchial glands, glandulae bronchiales, and lymphoid nodules, noduli lymphatici bronchiales.

The difference between the structure of the main bronchial wall and its subsequent branches is that in the bronchi, starting from the lobar level, the layer of Cytology/cytology/32.html">Smooth Muscle tissue is more developed. Myocytes are characterized by an oblique-circular orientation. Therefore, during expiration, the contraction of muscle bundles leads not only to the narrowing of the bronchus but also to its shortening. This ensures a more efficient clearance of air from the respiratory tract.

In the terminal bronchioles (where cartilage is absent), the muscle layer is particularly well-developed. Contraction of myocytes in pathological states (e.g., Bronchial Asthma) sharply reduces the lumen of the bronchioles and impedes breathing.

In the respiratory bronchioles, the cuboidal epithelial Cells lose their cilia, and the muscle layer thins out and divides into separate, circularly oriented bundles of smooth myocytes. The alveoli are lined with simple squamous epithelium and densely enveloped in blood capillaries. The alveolar epithelium is coated with surfactant, a phospholipid substance. Surfactant reduces surface tension within the alveoli, preventing their walls from collapsing and sticking together during expiration. Surfactant deficiency in newborns can cause severe respiratory disorders.