BOOK 2. THE BATH SCHOOL. N. LEVASHOV'S «SVETL BROOM» IN A. KHATYBOV'S «BATH SCHOOL» AND A LABOUR SPADE

F. Shkrudnev N. Levashov's "SvetL Broom" in A. Khatybov's "Bath School" and A Labour Spade 209 The walls of the trachea and bronchi consist of an internal epithelial layer, an external connective tissue layer and a middle layer containing cartilaginous rings and smooth muscle fibers. In a person suffering from asthma, these muscle fibers are greatly reduced, which causes a narrowing of the lumen of the small bronchi and makes breathing difficult. The epithelium contains ciliated cells. Beating of cilia occurs continuously in one direction, and when solid particles, for example, dust particles enter the moist surface of the epithelium, they are delayed by the mucus secreted by the epithelium, and the ciliary beat carries them back to the pharynx. This is an important mechanism for protecting the body against inhaled bacteria (some bacteria also have cilia). As the bronchioles and their branchings narrow, their walls become thinner, the cartilage layer disappears, and the ciliary cells are replaced by a flat epithelium. The walls of the alveoli consist of only one layer of flat epithelial cells. It was assumed that the alveolar epithelium was also single-layered, but studies using an electron microscope showed that it consists of two layers – the alveolar epithelium and the endothelium of the capillaries, which separates the air in the alveoli from the blood. Between the alveoli there are supporting strands of elastic connective tissue. This gives the lungs such elasticity that if you immediately puff them out of the animal's body through the trachea like a balloon, and then open the trachea hole, then they, due to their elasticity, compress and push the air out. The lung is provided with both motor nerves going to the smooth musculature of the bronchi and bronchioles, and sensitive nerves branched throughout (it is known that if this nerve is dipped in curare (a strong poison), the rest may not be considered). Each lung, as well as the inner surface of the chest cavity, in which the lungs are, is covered with a thin layer of smooth epithelium called pleura. Both pleura leafs are always moist, which reduces friction when the lungs move in the chest cavity when breathing. The pressure in the pleural cavity (between two pleura leafs) is usually less than atmospheric (in some cases, the lungs can be swapped).