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Lecture notes, cheat sheets
Histology. Respiratory system
Directory / Lecture notes, cheat sheets Table of contents (expand) Topic 22. RESPIRATORY SYSTEM The respiratory system includes various organs that perform air conduction and respiratory (gas exchange) functions: the nasal cavity, nasopharynx, larynx, trachea, extrapulmonary bronchi and lungs. The main function of the respiratory system is external respiration, i.e., the absorption of oxygen from the inhaled air and the supply of blood to it, as well as the removal of carbon dioxide from the body (gas exchange is carried out by the lungs, their acini). Internal, tissue respiration occurs in the form of oxidative processes in the cells of organs with the participation of blood. Along with this, the respiratory organs perform a number of other important non-gas exchange functions: thermoregulation and humidification of the inhaled air, cleansing it of dust and microorganisms, deposition of blood in a richly developed vascular system, participation in maintaining blood clotting due to the production of thromboplastin and its antagonist (heparin), participation in the synthesis of certain hormones and in water-salt, lipid metabolism, as well as in voice formation, smell and immunological protection. Development On the 22nd - 26th day of intrauterine development, a respiratory diverticulum - the rudiment of the respiratory organs - appears on the ventral wall of the foregut. It is separated from the foregut by two longitudinal esophagotracheal (tracheoesophageal) grooves, which protrude into the lumen of the foregut in the form of ridges. These ridges, coming together, merge, and the esophagotracheal septum is formed. As a result, the foregut is divided into a dorsal part (esophagus) and a ventral part (trachea and pulmonary buds). As it separates from the foregut, the respiratory diverticulum, lengthening in the caudal direction, forms a structure lying in the midline - the future trachea; it ends in two sac-like protrusions. These are pulmonary buds, the most distal parts of which constitute the respiratory rudiment. Thus, the epithelium lining the tracheal primordium and pulmonary buds is of endodermal origin. The mucous glands of the airways, which are derivatives of the epithelium, also develop from the endoderm. Cartilage cells, fibroblasts and SMCs are derived from the splanchic mesoderm surrounding the foregut. The right pulmonary kidney is divided into three, and the left - into two main bronchi, predetermining the presence of three lobes of the lung on the right and two on the left. Under the inductive influence of the surrounding mesoderm, branching continues, eventually forming the bronchial tree of the lungs. By the end of the 6th month there are 17 branches. Later, 6 more additional branchings occur, the branching process ends after birth. At birth, the lungs contain about 60 million primary alveoli, their number increases rapidly in the first 2 years of life. Then the growth rate slows down, and by 8 to 12 years the number of alveoli reaches approximately 375 million, which is equal to the number of alveoli in adults. Stages of development. Differentiation of the lungs goes through the following stages - glandular, tubular and alveolar. The glandular stage (5-15 weeks) is characterized by further branching of the airways (the lungs take on the appearance of a gland), the development of cartilage of the trachea and bronchi, and the appearance of bronchial arteries. The epithelium lining the respiratory bud consists of cylindrical cells. On the 10th week, goblet cells appear from the cells of the cylindrical epithelium of the airways. By the 15th week, the first capillaries of the future respiratory department are formed. The tubular stage (16-25 weeks) is characterized by the appearance of respiratory and terminal bronchioles lined with cubic epithelium, as well as tubules (prototypes of alveolar sacs) and the growth of capillaries to them. The alveolar (or terminal sac stage (26-40 weeks)) is characterized by massive transformation of tubules into sacs (primary alveoli), an increase in the number of alveolar sacs, differentiation of type I and II alveolocytes, and the appearance of surfactant. By the end of the 7th month, a significant part of the cells of the cubic epithelium of the respiratory bronchioles differentiates into flat cells (type I alveolocytes), closely connected by blood and lymphatic capillaries, and gas exchange becomes possible. The rest of the cells remain cuboidal (type II alveolocytes) and begin to produce surfactant. During the last 2 months of prenatal and several years of postnatal life, the number of terminal sacs is constantly increasing. Mature alveoli before birth are absent. lung fluid At birth, the lungs are filled with fluid containing large amounts of chlorides, protein, some mucus from the bronchial glands, and surfactant. After birth, lung fluid is rapidly resorbed by the blood and lymph capillaries, and a small amount is removed through the bronchi and trachea. The surfactant remains as a thin film on the surface of the alveolar epithelium. Malformations Tracheoesophageal fistula occurs as a result of incomplete splitting of the primary intestine into the esophagus and trachea. Principles of organization of the respiratory system The lumen of the airways and alveoli of the lung is the external environment. In the airways and on the surface of the alveoli - there is a layer of epithelium. The epithelium of the airways performs a protective function, which is performed, on the one hand, by the very fact of the presence of the layer, and on the other hand, due to the secretion of a protective material - mucus. It is produced by the goblet cells present in the epithelium. In addition, under the epithelium there are glands that also secrete mucus, the excretory ducts of these glands open to the surface of the epithelium. The airways function as an air junction unit. The characteristics of the external air (temperature, humidity, contamination with different types of particles, the presence of microorganisms) vary quite significantly. But the air that meets certain requirements must enter the respiratory department. The function of bringing air to the required conditions is played by the airways. Foreign particles are deposited in the mucosal film located on the surface of the epithelium. Further, contaminated mucus is removed from the airways with its constant movement towards the exit from the respiratory system, followed by coughing. Such a constant movement of the mucous film is ensured by synchronous and undulating oscillations of the cilia located on the surface of the epithelial cells directed towards the exit from the airways. In addition, by moving the mucus to the exit, it is prevented from reaching the surface of the alveolar cells, through which diffusion of gases occurs. Conditioning of the temperature and humidity of the inhaled air is carried out with the help of blood located in the vascular bed of the airway wall. This process occurs mainly in the initial sections, namely in the nasal passages. The mucous membrane of the airways is involved in protective reactions. The epithelium of the mucous membrane contains Langerhans cells, while its own layer contains a significant number of various immunocompetent cells (T- and B-lymphocytes, plasma cells synthesizing and secreting IgG, IgA, IgE, macrophages, dendritic cells). Mast cells are very numerous in their own mucosal layer. Mast cell histamine causes bronchospasm, vasodilation, hypersecretion of mucus from the glands, and mucosal edema (as a result of vasodilation and increased permeability of the wall of postcapillary venules). In addition to histamine, mast cells, along with eosinophils and other cells, secrete a number of mediators, the action of which leads to inflammation of the mucous membrane, damage to the epithelium, reduction of SMC and narrowing of the airway lumen. All of the above effects are characteristic of bronchial asthma. The airways do not collapse. The clearance is constantly changing and adjusting in connection with the situation. The collapse of the lumen of the airways prevents the presence in their wall of dense structures formed in the initial sections by bone, and then by cartilage tissue. The change in the size of the lumen of the airways is provided by the folds of the mucous membrane, the activity of smooth muscle cells and the structure of the wall. Regulation of MMC tone. The tone of the SMC of the airways is regulated by neurotransmitters, hormones, metabolites of arachidonic acid. The effect depends on the presence of the corresponding receptors in the SMC. SMC walls of the airways have M-cholinergic receptors, histamine receptors. Neurotransmitters are secreted from the terminals of the nerve endings of the autonomic nervous system (for the vagus nerve - acetylcholine, for neurons of the sympathetic trunk - norepinephrine). Bronchoconstriction is caused by choline, substance P, neurokinin A, histamine, thromboxane TXA2, leukotrienes LTC4, LTD4, LTE4. Bronchodilation is caused by VIP, epinephrine, bradykinin, prostaglandin PGE2. The reduction of MMC (vasoconstriction) is caused by adrenaline, leukotrienes, angiotensin-II. Histamine, bradykinin, VIP, prostaglandin PG have a relaxing effect on the SMC of blood vessels. The air entering the respiratory tract is subjected to chemical examination. It is carried out by the olfactory epithelium and chemoreceptors in the wall of the airways. Such chemoreceptors include sensitive endings and specialized chemosensitive cells of the mucous membrane. airways The airways of the respiratory system include the nasal cavity, nasopharynx, larynx, trachea, and bronchi. When the air moves, it is purified, moistened, the temperature of the inhaled air approaches the body temperature, the reception of gas, temperature and mechanical stimuli, as well as the regulation of the volume of inhaled air. In addition, the larynx is involved in sound production. Nasal cavity It is divided into the vestibule and the nasal cavity itself, consisting of the respiratory and olfactory regions. The vestibule is formed by a cavity, located under the cartilaginous part of the nose, covered with stratified squamous epithelium. Under the epithelium in the connective tissue layer there are sebaceous glands and bristle hair roots. Bristle hairs perform a very important function: they retain dust particles from the inhaled air in the nasal cavity. The inner surface of the nasal cavity proper in the respiratory part is lined with a mucous membrane consisting of a multi-row prismatic ciliated epithelium and a connective tissue proper plate. The epithelium consists of several types of cells: ciliated, microvillous, basal and goblet. Intercalated cells are located between the ciliated cells. Goblet cells are unicellular mucous glands that secrete their secret on the surface of the ciliated epithelium. The lamina propria is formed by a loose, fibrous, unformed connective tissue containing a large number of elastic fibers. It contains the terminal sections of the mucous glands, the excretory ducts of which open on the surface of the epithelium. The secret of these glands, like the secret of goblet cells, moisturizes the mucous membrane. The mucous membrane of the nasal cavity is very well supplied with blood, which contributes to the warming of the inhaled air in the cold season. Lymphatic vessels form a dense network. They are associated with the subarachnoid space and perivascular sheaths of various parts of the brain, as well as with the lymphatic vessels of the major salivary glands. The mucous membrane of the nasal cavity has abundant innervation, numerous free and encapsulated nerve endings (mechano-, thermo- and angioreceptors). Sensitive nerve fibers originate from the semilunar ganglion of the trigeminal nerve. In the region of the superior nasal concha, the mucous membrane is covered with a special olfactory epithelium containing receptor (olfactory) cells. The mucous membrane of the paranasal sinuses, including the frontal and maxillary sinuses, has the same structure as the mucous membrane of the respiratory part of the nasal cavity, with the only difference that their own connective tissue plate is much thinner. Larynx The organ of the air-bearing section of the respiratory system, complex in structure, is involved not only in air conduction, but also in sound production. The larynx in its structure has three membranes - mucous, fibrocartilaginous and adventitial. The mucous membrane of the human larynx, in addition to the vocal cords, is lined with multi-row ciliated epithelium. The mucosal lamina propria, formed by loose fibrous unformed connective tissue, contains numerous elastic fibers that do not have a specific orientation. In the deep layers of the mucous membrane, elastic fibers gradually pass into the perichondrium, and in the middle part of the larynx they penetrate between the striated muscles of the vocal cords. In the middle part of the larynx there are folds of the mucous membrane, forming the so-called true and false vocal cords. The folds are covered by stratified squamous epithelium. Mixed glands lie in the mucous membrane. Due to the contraction of the striated muscles embedded in the thickness of the vocal folds, the size of the gap between them changes, which affects the pitch of the sound produced by the air passing through the larynx. The fibrocartilaginous membrane consists of hyaline and elastic cartilages surrounded by dense fibrous connective tissue. This shell is a kind of skeleton of the larynx. The adventitia is composed of fibrous connective tissue. The larynx is separated from the pharynx by the epiglottis, which is based on elastic cartilage. In the region of the epiglottis, there is a transition of the mucous membrane of the pharynx into the mucous membrane of the larynx. On both surfaces of the epiglottis, the mucous membrane is covered with stratified squamous epithelium. Trachea This is an air-conducting organ of the respiratory system, which is a hollow tube consisting of a mucous membrane, submucosa, fibrocartilaginous and adventitious membranes. The mucous membrane, with the help of a thin submucosa, is connected with the underlying dense parts of the trachea and, due to this, does not form folds. It is lined with multi-row prismatic ciliated epithelium, in which ciliated, goblet, endocrine and basal cells are distinguished. Ciliated prismatic cells flicker in the direction opposite to the inhaled air, most intensively at the optimum temperature (18 - 33 ° C) and in a slightly alkaline environment. Goblet cells - unicellular endoepithelial glands, secrete a mucous secretion that moisturizes the epithelium and creates conditions for adherence of dust particles that enter with air and are removed when coughing. The mucus contains immunoglobulins secreted by immunocompetent cells of the mucous membrane, which neutralize many microorganisms that enter with the air. Endocrine cells have a pyramidal shape, a rounded nucleus and secretory granules. They are found both in the trachea and in the bronchi. These cells secrete peptide hormones and biogenic amines (norepinephrine, serotonin, dopamine) and regulate the contraction of airway muscle cells. Basal cells are cambial cells that are oval or triangular in shape. The submucosa of the trachea consists of loose fibrous unformed connective tissue, without a sharp border passing into dense fibrous connective tissue of the perichondrium of open cartilaginous semirings. In the submucosa there are mixed protein-mucous glands, the excretory ducts of which, forming flask-shaped extensions on their way, open on the surface of the mucous membrane. The fibrocartilaginous membrane of the trachea consists of 16-20 hyaline cartilaginous rings, not closed on the posterior wall of the trachea. The free ends of these cartilages are connected by bundles of smooth muscle cells attached to the outer surface of the cartilage. Due to this structure, the posterior surface of the trachea is soft, pliable. This property of the posterior wall of the trachea is of great importance: when swallowing, food boluses passing through the esophagus, located directly behind the trachea, do not encounter obstacles from its cartilaginous skeleton. The adventitial membrane of the trachea consists of loose, fibrous, irregular connective tissue that connects this organ to the adjacent parts of the mediastinum. The blood vessels of the trachea, just as in the larynx, form several parallel plexuses in its mucous membrane, and under the epithelium - a dense capillary network. Lymphatic vessels also form plexuses, of which the superficial is directly below the network of blood capillaries. The nerves approaching the trachea contain spinal (cerebrospinal) and autonomic fibers and form two plexuses, the branches of which end in its mucous membrane with nerve endings. The muscles of the posterior wall of the trachea are innervated from the ganglia of the autonomic nervous system. Lungs The lungs are paired organs that occupy most of the chest and constantly change their shape depending on the phase of breathing. The surface of the lung is covered with a serous membrane (visceral pleura). Structure. The lung consists of branches of the bronchi, which are part of the airways (bronchial tree), and a system of pulmonary vesicles (alveoli), which act as the respiratory sections of the respiratory system. The composition of the bronchial tree of the lung includes the main bronchi (right and left), which are divided into extrapulmonary lobar bronchi (large bronchi of the first order), and then into large zonal extrapulmonary (4 in each lung) bronchi (bronchi of the second order). Intrapulmonary segmental bronchi (10 in each lung) are subdivided into bronchi of III-V orders (subsegmental), which are medium in diameter (2-5 mm). The middle bronchi are subdivided into small (1-2 mm in diameter) bronchi and terminal bronchioles. Behind them, the respiratory sections of the lung begin, performing a gas exchange function. The structure of the bronchi (although not the same throughout the bronchial tree) has common features. The inner shell of the bronchi - the mucous membrane - is lined like the trachea with ciliated epithelium, the thickness of which gradually decreases due to a change in the shape of the cells from high prismatic to low cubic. Among epithelial cells, in addition to ciliated, goblet, endocrine and basal, in the distal sections of the bronchial tree, secretory cells (Clara cells), bordered (brush), and non-ciliated cells are found in humans and animals. Secretory cells are characterized by a dome-shaped top, devoid of cilia and microvilli and filled with secretory granules. They contain a rounded nucleus, a well-developed endoplasmic reticulum of an agranular type, and a lamellar complex. These cells produce enzymes that break down the surfactant that coats the respiratory compartments. Ciliated cells are found in bronchioles. They are prismatic in shape. Their apical end rises somewhat above the level of adjacent ciliated cells. The apical part contains accumulations of glycogen granules, mitochondria, and secretion-like granules. Their function is not clear. Border cells are distinguished by their ovoid shape and the presence of short and blunt microvilli on the apical surface. These cells are rare. They are believed to function as chemoreceptors. The lamina propria of the bronchial mucosa is rich in longitudinally directed elastic fibers, which provide stretching of the bronchi during inhalation and their return to their original position during exhalation. The mucous membrane of the bronchi has longitudinal folds due to the contraction of oblique bundles of smooth muscle cells that separate the mucous membrane from the submucosal connective tissue base. The smaller the diameter of the bronchus, the relatively thicker is the muscular plate of the mucosa. In the mucous membrane of the bronchi, especially large ones, there are lymphatic follicles. In the submucosal connective base, the terminal sections of the mixed mucosal-protein glands lie. They are located in groups, especially in places that are devoid of cartilage, and the excretory ducts penetrate the mucous membrane and open on the surface of the epithelium. Their secret moisturizes the mucous membrane and promotes adhesion, enveloping of dust and other particles, which are subsequently released to the outside. Mucus has bacteriostatic and bactericidal properties. In the bronchi of small caliber (diameter 1 - 2 mm) glands are absent. The fibrocartilaginous membrane, as the caliber of the bronchus decreases, is characterized by a gradual change of open cartilage rings in the main bronchi by cartilaginous plates (lobar, zonal, segmental, subsegmental bronchi) and islets of cartilaginous tissue (in medium-sized bronchi). In medium-sized bronchi, hyaline cartilage tissue is replaced by elastic cartilage tissue. In the bronchi of small caliber, the fibrocartilaginous membrane is absent. The outer adventitial membrane is built of fibrous connective tissue, passing into the interlobar and interlobular connective tissue of the lung parenchyma. Among the connective tissue cells, tissue basophils are found, which are involved in the regulation of the composition of the intercellular substance and blood coagulation. The terminal (terminal) bronchioles are about 0,5 mm in diameter. Their mucous membrane is lined with a single layer of cubic ciliated epithelium, in which brush cells and secretory Clara cells occur. In the lamina propria of the mucous membrane of these bronchioles, longitudinally extending elastic fibers are located, between which individual bundles of smooth muscle cells lie. As a result, the bronchioles are easily distensible during inhalation and return to their original position during exhalation. Respiratory department. The structural and functional unit of the respiratory section of the lung is the acinus. It is a system of alveoli located in the wall of the respiratory bronchiole, alveolar ducts and sacs that carry out gas exchange between the blood and air of the alveoli. The acinus begins with a respiratory bronchiole of the 12st order, which is dichotomously divided into respiratory bronchioles of the 18nd, and then of the XNUMXrd order. In the lumen of the bronchioles, the alveoli open, which in this regard are called alveolar. Each respiratory bronchiole III order, in turn, is divided into alveolar passages, and each alveolar passage ends with two alveolar sacs. At the mouth of the alveoli of the alveolar ducts there are small bundles of smooth muscle cells, which are visible in transverse sections in the form of button-like thickenings. The acini are separated from each other by thin connective tissue layers, XNUMX-XNUMX acini form the lung lobule. Respiratory bronchioles are lined with a single layer of cuboidal epithelium. The muscular plate becomes thinner and breaks up into separate, circularly directed bundles of smooth muscle cells. On the walls of the alveolar passages and alveolar sacs there are several dozen alveoli. Their total number in adults reaches an average of 300 - 400 million. The surface of all alveoli with a maximum breath in an adult can reach 100 m2, and when exhaling, it decreases by 2 - 2,5 times. Between the alveoli are thin connective tissue septa, through which the blood capillaries pass. Between the alveoli there are messages in the form of holes with a diameter of about 10 - 15 microns (alveolar pores). The alveoli look like an open vesicle. The inner surface is lined by two main types of cells: respiratory alveolar cells (type I alveolocytes) and large alveolar cells (type II alveolocytes). In addition, in animals, type III cells exist in the alveoli - kamchatye. Type I alveolocytes have an irregular, flattened, elongated shape. On the free surface of the cytoplasm of these cells, there are very short cytoplasmic outgrowths facing the cavity of the alveoli, which significantly increases the total area of air contact with the surface of the epithelium. Their cytoplasm contains small mitochondria and pinocytic vesicles. An important component of the air-blood barrier is the surfactant alveolar complex. It plays an important role in preventing the collapse of the alveoli on exhalation, as well as in preventing them from penetrating through the wall of the alveoli of microorganisms from the inhaled air and transuding fluid from the capillaries of the interalveolar septa into the alveoli. Surfactant consists of two phases: membrane and liquid (hypophase). Biochemical analysis of the surfactant showed that it contains phospholipids, proteins and glycoproteins. Type II alveolocytes are somewhat larger in height than type I cells, but their cytoplasmic processes, on the contrary, are short. In the cytoplasm, larger mitochondria, a lamellar complex, osmiophilic bodies, and an endoplasmic reticulum are revealed. These cells are also called secretory because of their ability to secrete lipoprotein substances. In the wall of the alveoli, brush cells and macrophages containing trapped foreign particles and an excess of surfactant are also found. The cytoplasm of macrophages always contains a significant amount of lipid droplets and lysosomes. The oxidation of lipids in macrophages is accompanied by the release of heat, which warms the inhaled air. Surfactant The total amount of surfactant in the lungs is extremely small. 1 m2 alveolar surface accounts for about 50 mm3 surfactant. The thickness of its film is 3% of the total thickness of the air-blood barrier. The components of the surfactant enter the type II alveolocytes from the blood. Their synthesis and storage in lamellar bodies of these cells is also possible. 85% of the surfactant components are recycled and only a small amount is resynthesized. Removal of surfactant from the alveoli occurs in several ways: through the bronchial system, through the lymphatic system and with the help of alveolar macrophages. The main amount of surfactant is produced after the 32nd week of pregnancy, reaching a maximum amount by the 35th week. Before birth, an excess of surfactant is formed. After birth, this excess is removed by alveolar macrophages. Respiratory distress syndrome of the newborn develops in preterm infants due to the immaturity of type II alveolocytes. Due to the insufficient amount of surfactant secreted by these cells to the surface of the alveoli, the latter are unexpanded (atelectasis). As a result, respiratory failure develops. Due to alveolar atelectasis, gas exchange occurs through the epithelium of the alveolar ducts and respiratory bronchioles, which leads to their damage. Compound. Pulmonary surfactant is an emulsion of phospholipids, proteins and carbohydrates, 80% glycerophospholipids, 10% cholesterol and 10% proteins. The emulsion forms a monomolecular layer on the surface of the alveoli. The main surface active component is dipalmitoylphosphatidylcholine, an unsaturated phospholipid that makes up more than 50% of the surfactant's phospholipids. The surfactant contains a number of unique proteins that promote the adsorption of dipalmitoylphosphatidylcholine at the interface between two phases. Among the surfactant proteins, SP-A, SP-D are isolated. Proteins SP-B, SP-C and surfactant glycerophospholipids are responsible for reducing surface tension at the air-liquid interface, while SP-A and SP-D proteins are involved in local immune responses by mediating phagocytosis. SP-A receptors are present in type II alveolocytes and in macrophages. Production regulation. The formation of surfactant components in the fetus is facilitated by glucocorticosteroids, prolactin, thyroid hormones, estrogens, androgens, growth factors, insulin, cAMP. Glucocorticoids enhance the synthesis of SP-A, SP-B and SP-C in the lungs of the fetus. In adults, surfactant production is regulated by acetylcholine and prostaglandins. Surfactant is a component of the lung defense system. Surfactant prevents direct contact of alveolocytes with harmful particles and infectious agents that enter the alveoli with inhaled air. The cyclic changes in surface tension that occur during inhalation and exhalation provide a breath-dependent cleaning mechanism. Enveloped by the surfactant, dust particles are transported from the alveoli to the bronchial system, from which they are removed with mucus. Surfactant regulates the number of macrophages migrating into the alveoli from the interalveolar septa, stimulating the activity of these cells. Bacteria entering the alveoli with air are opsonized by surfactant, which facilitates their phagocytosis by alveolar macrophages. The surfactant is present in bronchial secretions, coating the ciliated cells, and has the same chemical composition as lung surfactant. Obviously, surfactant is needed to stabilize the distal airways. immune protection macrophages Macrophages make up 10-15% of all cells in the alveolar septa. Many microfolds are present on the surface of macrophages. The cells form rather long cytoplasmic processes that allow macrophages to migrate through the interalveolar pores. Being inside the alveolus, the macrophage can attach itself to the surface of the alveolus with the help of processes and capture particles. Alveolar macrophages secrete α1-antitrypsin, a glycoprotein from the family of serine proteases that protects alveolar elastin from: splitting of leukocytes by elastase. Mutation of the α1-antitrypsin gene leads to congenital emphysema (damage to the elastic framework of the alveoli). Migration paths. Cells loaded with phagocytosed material can migrate in various directions: up the acinus and into the bronchioles, where macrophages enter the mucous membrane, which is constantly moving along the surface of the epithelium towards the exit from the airways; inside - into the internal environment of the body, i.e., into the interalveolar septa. Function. Macrophages phagocytize microorganisms and dust particles that enter with the inhaled air, have antimicrobial and anti-inflammatory activity mediated by oxygen radicals, proteases and cytokines. In lung macrophages, the antigen presenting function is poorly expressed. Moreover, these cells produce factors that inhibit the function of T-lymphocytes, which reduces the immune response. Antigen presenting cells Dendritic cells and Langerhans cells belong to the system of mononuclear phagocytes, they are the main antigen-presenting cells of the lung. Dendritic cells and Langerhans cells are numerous in the upper respiratory tract and trachea. With a decrease in the caliber of the bronchi, the number of these cells decreases. As antigen-presenting pulmonary Langerhans cells and dendritic cells express MHC class 1 molecules. These cells have receptors for the Fc fragment of IgG, the fragment of the C3b complement component, IL-2, they synthesize a number of cytokines, including IL-1, IL-6, tumor necrosis factor, stimulate T-lymphocytes, showing increased activity against the antigen that first appeared in the body. Dendritic cells Dendritic cells are found in the pleura, interalveolar septa, peribronchial connective tissue, and in the lymphoid tissue of the bronchi. Dendritic cells, differentiating from monocytes, are quite mobile and can migrate in the intercellular substance of the connective tissue. They appear in the lungs before birth. An important property of dendritic cells is their ability to stimulate the proliferation of lymphocytes. Dendritic cells have an elongated shape and numerous long processes, an irregularly shaped nucleus, and typical cell organelles in abundance. There are no phagosomes, since the cells practically do not have phagocytic activity. Langerhans cells Langerhans cells are present only in the epithelium of the airways and absent in the alveolar epithelium. Langerhans cells differentiate from dendritic cells, and such differentiation is possible only in the presence of epithelial cells. Connecting with cytoplasmic processes penetrating between epitheliocytes, Langerhans cells form a developed intraepithelial network. Langerhans cells are morphologically similar to dendritic cells. A characteristic feature of Langerhans cells is the presence in the cytoplasm of specific electron-dense granules with a lamellar structure. Metabolic lung function In the lungs, it metabolizes a number of biologically active substances. Angiotensins. Activation is only known for angiotensin I, which is converted to angiotensin II. The conversion is catalyzed by an angiotensin-converting enzyme localized in the endothelial cells of the alveolar capillaries. Inactivation. Many biologically active substances are partially or completely inactivated in the lungs. So, bradykinin is inactivated by 80% (with the help of angiotensin-converting enzyme). In the lungs, serotonin is inactivated, but not with the participation of enzymes, but by excretion from the blood, part of the serotonin enters the platelets. Prostaglandins PGE, PGE2, PGE2a and norepinephrine are inactivated in the lungs with the help of appropriate enzymes. Pleura The lungs are covered on the outside with a pleura called the pulmonary (or visceral). The visceral pleura fuses tightly with the lungs, its elastic and collagen fibers pass into the interstitial tissue, so it is difficult to isolate the pleura without injuring the lungs. The visceral pleura contains smooth muscle cells. In the parietal pleura, which lines the outer wall of the pleural cavity, there are fewer elastic elements, and smooth muscle cells are rare. Blood supply in the lung is carried out through two vascular systems. On the one hand, the lungs receive arterial blood from the systemic circulation through the bronchial arteries, and on the other hand, they receive venous blood for gas exchange from the pulmonary arteries, that is, from the pulmonary circulation. The branches of the pulmonary artery, accompanying the bronchial tree, reach the base of the alveoli, where they form a capillary network of the alveoli. Through the alveolar capillaries, the diameter of which varies between 5 - 7 microns, erythrocytes pass in 1 row, which creates an optimal condition for the implementation of gas exchange between erythrocyte hemoglobin and alveolar air. The alveolar capillaries gather into postcapillary venules, which merge to form the pulmonary veins. Bronchial arteries depart directly from the aorta, nourish the bronchi and lung parenchyma with arterial blood. Penetrating into the wall of the bronchi, they branch out and form arterial plexuses in their submucosa and mucous membrane. In the mucous membrane of the bronchi, the vessels of the large and small circles communicate by anastomosis of the branches of the bronchial and pulmonary arteries. The lymphatic system of the lung consists of superficial and deep networks of lymphatic capillaries and vessels. The superficial network is located in the visceral pleura. The deep network is located inside the pulmonary lobules, in the interlobular septa, lying around the blood vessels and bronchi of the lung. Innervation is carried out by sympathetic and parasympathetic nerves and a small number of fibers coming from the spinal nerves. Sympathetic nerves conduct impulses that cause bronchial dilatation and constriction of blood vessels, parasympathetic - impulses that, on the contrary, cause bronchial constriction and dilation of blood vessels. The ramifications of these nerves form a nerve plexus in the connective tissue layers of the lung, located along the bronchial tree and blood vessels. In the nerve plexuses of the lung, large and small ganglia are found, from which nerve branches depart, innervating, in all likelihood, the smooth muscle tissue of the bronchi. Nerve endings were identified along the alveolar ducts and alveoli. Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu. << Back: Digestive system >> Forward: Leather and its derivatives
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