Respiratory organs

The concept of the respiratory system includes:

• upper airways (nasal cavity, nasopharynx, oropharynx, larynx);
• lower airways (trachea and bronchi); lung parenchyma, pleura and its cavity;
• apparatus that ensures respiratory movements (ribs with adjacent bone formations, respiratory muscles).

The trachea begins at the lower border of the larynx at the level of the VI-VII cervical vertebrae and ends at the level of the IV-V thoracic vertebrae, dividing into the right and left main bronchi. It should be remembered that the right main bronchus departs from the trachea at an angle of 15-40°, and its length does not exceed 3 cm. The left main bronchus departs from the trachea at an angle of 50-70° and has a length of 4-5 cm.

The bronchial tree (arbor bronchialis) includes the main bronchi, lobar bronchi, segmental bronchi, numerous branches of the segmental bronchi, lobular bronchi, and terminal bronchioles. The bronchial tree, which constitutes the main part of the airways, has on average 16 dichotomous divisions of the bronchi, the main part of which falls on the branches of the segmental bronchi. The lobular bronchi still contain a cartilaginous framework, but there is no cartilage in the walls of the terminal bronchioles.

Each terminal bronchiole is dichotomously divided into respiratory bronchioles (17th-19th generations of bronchi), on the walls of which the pulmonary alveoli are located. From each respiratory bronchiole, 2-3 alveolar passages (20th-22nd generations) branch off, each of which ends in 3-6 alveolar sacs (23rd generation of airways). The steps of these sacs consist of alveoli.

Each acinus contains about 2000 alveoli, closely adjacent to each other. The alveoli are separated by interalveolar septa, which contain a large number of openings - the pores of Kohn, through which active collateral gas exchange between the alveoli is carried out. In addition, with the help of short channels (Lambert's canals), the alveoli are connected to the bronchioles, which provides another collateral route for air to enter the alveoli.

The inner surface of the alveoli is lined with a layer of surfactant, which reduces the surface tension of the alveoli. Underneath it is a layer of alveolar epithelium, consisting of two types of alveolocytes. Type I cells occupy more than 90% of the inner surface of the alveoli. They mainly perform the function of gas exchange between the alveolar air and blood. About 10% of the inner surface of the alveoli is lined with type II alveolocytes, which mainly provide surfactant secretion. In addition, type II alveolocytes participate in the regeneration of the alveolar epithelium: if necessary, they can differentiate into type I alveolocytes.

Directly adjacent to the alveoli is the capillary network, the total area of which reaches 70 m2 ^. The capillaries are lined with endothelial cells.

The interstitial tissue, consisting of collagen (about 70%), elastin (about 30%), glycosaminoglycans and fibronectin, largely determines the elasticity of the lung tissue. Lymphatic vessels and fibroblasts, alveolar macrophages, mast cells and other types of cells are located in the interstitial tissue.

The mucous membrane of the trachea and bronchi consists of a covering high prismatic ciliated epithelium, a basement membrane, muscular and submucosal layers.

The epithelium is represented mainly by four types of cells. The bulk of them are ciliated cells. They have an irregular prismatic shape. On the free surface of the cell facing the lumen of the bronchus, there are short microvilli and a large number (about 200) of cilia. The cilia rhythmically oscillate in the direction of the nasopharynx, moving a protective layer of mucus from the lungs into it and thereby facilitating the "cleansing" of the airways.

The number of goblet (secretory) cells of the epithelium is 4-5 times lower than that of ciliated cells. The main function of goblet cells is the secretion of mucous secretion. Secretory cells of the epithelium of terminal and respiratory bronchioles (Clara cells) are distinguished by particularly high metabolic activity.

Finally, the basal and intermediate cells are located deep in the epithelium and do not reach the surface. These poorly differentiated cells are responsible for the physiological regeneration of the epithelium. In addition, the bronchial epithelium contains neuroendocrine and chemoreceptor ("brush") cells.

Under the covering epithelium are the basal membrane, the lamina propria, the muscular and submucous layers. The latter contains a large number of bronchial glands that secrete mucous or serous secretion into the lumen of the trachea and bronchi. Some of the bronchial glands are located between the cartilages of the fibrocartilaginous membrane and in the outer membrane.