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Respiratory system
Last reviewed: 23.04.2024
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The concept of the respiratory system includes:
- upper airways (nasal cavity, nasopharynx, oropharynx, larynx);
- lower airways (trachea and bronchi); parenchyma of the lungs, pleura and its cavity;
- apparatus providing respiratory movements (ribs with adjacent bone formations, respiratory muscles).
The trachea begins from the lower boundary of the larynx at the level of the VI-VII cervical vertebrae and ends at level IV-V of the 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 composition of the bronchial tree (arbore bronchialis) includes the main bronchi, lobar bronchi, segmental bronchi, numerous branchings of segmental bronchi, lobular bronchi, terminal bronchioles. The bronchial tree, which constitutes the main part of the airways, on average, has 16 dichotomous fission of the bronchi, the main part of which is the branching of the segmental bronchi. Lobular bronchi still contain a cartilaginous skeleton, but there are no cartilage in the walls of terminal (terminal) bronchioles.
Each end (terminal) bronchioles is dichotomously divided into respiratory (respiratory) bronchioles (17-19th generation of bronchus), on the walls of which pulmonary alveoli are located. Each respiratory bronchiola recedes but 2-3 alveolar courses (20-22th generation), each of which ends in 3-6 alveolar sacs (23rd generation of airways). Stacks of these pouches consist of alveoli.
Each acinus contains about 2000 alveoli that are closely in contact with each other. The alveoli are separated by interalveolar septa, in which there are a large number of holes - the Kona pores, through which active collateral gas exchange between the alveoli is carried out. In addition, by means of short channels (Lambert channels) the alveoli connect to the bronchioles, which provides one more collateral airway alveoli.
The inner surface of the alveoli is lined with a layer of surfactant, which reduces the surface tension of the alveoli. Under it is located 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 perform mainly the gas exchange between the alveolar air and blood. About 10% of the internal surface of the alveoli is lined with type II alveolocytes, mainly providing the secretion of the surfactant. In addition, type II alveolocytes are involved in the regeneration of the alveolar epithelium: if necessary, they can differentiate into type I alveolocytes.
Directly to the alveoli there is a capillary network, the total area of which reaches 70 m 2. Capillaries are lined with endothelial cells.
Interstitial tissue, consisting of collagen (about 70%), elastin (about 30%), glycosaminoglycans and fibronectin, largely determines the elasticity of the lung tissue. In the interstitial tissue are lymphatic vessels and fibroblasts, alveolar macrophages, obese and other types of cells.
The mucous membrane of the trachea and bronchi consists of a cover high prismatic ciliated epithelium, a basal membrane, a muscular and submucosal layer.
Epithelium is represented mainly by four kinds 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 (about 200) number of cilia. Cilia rhythmically oscillate in the direction of the nasopharynx, moving a protective layer of mucus out of the lungs 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 secretion of mucous secretions. The secretory cells of the terminal and respiratory bronchial epithelium (Clara cells) are characterized by particularly high metabolic activity.
Finally, the basal and intermediate cells are located in the depth of the epithelium and do not reach the surface. Due to these poorly differentiated cells, physiological regeneration of the epithelium occurs. In addition, the epithelium of the bronchi contains neuroendocrine and chemoreceptor ("brush") cells.
Under the cover epithelium, the basal membrane, its own plate, the muscular and submucosal layers are located. The latter contains a large number of bronchial glands secreting a mucous or serous secret in the lumen of the trachea and bronchi. Part of the bronchial glands is located between the cartilages of the fibrous-cartilaginous shell and in the outer shell.
