Epithelium lining the alveolar ducts. Changes in the wall of the bronchi as their caliber decreases

Decline height of the epithelial layer mucosa (from multirow cylindrical to double row, and then single row in small caliber bronchi and single row cubic in terminal bronchioles) with a gradual decrease in the number and then disappearance of goblet cells. In the distal portions of the terminal bronchioles, there are no ciliated cells, but there are bronchiolar exocrinocytes.

Decrease mucosal thickness.

Increasing number of elastic fibers.

Increase in the number of mining and metallurgical complexes, so that with a decrease in the caliber of the bronchi, the muscular layer of the mucous membrane becomes more pronounced.

Decrease sizes of plates and islands cartilage tissue followed by its disappearance.

Reduction in the number of mucous glands with their disappearance in the small caliber bronchi and bronchioles.

Respiratory Department

The respiratory section of the respiratory system is formed by parenchymal organs - the lungs. The respiratory section of the lung carries out the function of external respiration - gas exchange between two environments - external and internal. The concept of the respiratory department is associated with the concepts of the acinus and the pulmonary lobule.

Acinus

The respiratory section is a collection of acini. The acini begins with a first-order respiratory bronchiole, which is dichotomously divided into second-order and then third-order respiratory bronchioles. Each third-order respiratory bronchiole, in turn, is divided into alveolar ducts, which pass into the vestibule and then into the alveolar sacs. The alveoli open into the lumen of the respiratory bronchiole and alveolar ducts. The vestibule and alveolar sacs are actually voids formed by the alveoli. The lungs provide the function of external respiration - gas exchange between blood and air. The structural and functional unit of the respiratory department is the acinus, which is the terminal branch of the terminal bronchiole. 12-18 acini make up the lung lobule. The lobules are separated from each other by thin connective tissue layers and have the shape of a pyramid with an apex through which the bronchioles and the blood vessels that accompany them enter. Lymphatic vessels are located along the periphery of the lobules. The base of the lobule faces outward, towards the surface of the lungs, covered with the visceral layer of the pleura. The terminal bronchiole enters the lobule, branches, and gives rise to the lung acini.

Pulmonary acinus. The pulmonary acini make up the respiratory section of the lungs. From the terminal bronchioles, first-order respiratory bronchioles arise, which give rise to the acini. Bronchioles are divided into second and third order respiratory bronchioles. Each of the latter is divided into two alveolar ducts. Each alveolar duct passes through the vestibule into two alveolar sacs. In the walls of the respiratory bronchioles and alveolar ducts there are sac-like protrusions - alveoli. The alveoli form the vestibules and alveolar sacs. Between the acini there are thin layers of connective tissue. The pulmonary lobule includes 12–18 acini.

Pulmonary beforelka

The pulmonary lobule consists of 12–18 acini, separated by thin layers of connective tissue. Incomplete fibrous interlobular septa separate adjacent lobules from each other.

Pulmonary lobule. The lobules of the lung are shaped like pyramids with an apex through which a blood vessel and a terminal bronchiole enter. The base of the lobule faces outward, towards the surface of the lung. The bronchiole, penetrating the lobule, branches and gives rise to respiratory bronchioles, which are part of the pulmonary acini. The latter also have the shape of pyramids, with the base facing outward.

Alveoli

The alveoli are lined with single-layer epithelium located on the basement membrane. The cellular composition of the epithelium is pneumocytes of types I and II. Cells form tight junctions among themselves. The alveolar surface is covered with a thin layer of water and surfactant. Alveoli- bag-like voids separated by thin partitions. On the outside, blood capillaries are closely adjacent to the alveoli, forming a dense network. The capillaries are surrounded by elastic fibers that entwine the alveoli in the form of bundles. The alveolus is lined with single-layer epithelium. The cytoplasm of most epithelial cells is maximally flattened (type I pneumocytes). It contains many pinocytotic vesicles. Pinocytotic vesicles are also abundant in the squamous endothelial cells of capillaries. Between type I pneumocytes are cubic-shaped cells called type II pneumocytes. They are characterized by the presence in the cytoplasm of lamellar bodies containing surfactant. Surfactant is secreted into the alveolar cavity and forms a monomolecular film on the surface of a thin layer of water covering the alveolar epithelium. Macrophages can migrate from the interalveolar septa into the lumen of the alveoli. Moving along the surface of the alveoli, they form numerous cytoplasmic processes, with the help of which they capture foreign particles entering with the air.

Pneumocytes type I

Type I pneumocytes (respiratory pneumocytes) cover almost 95% of the alveolar surface. These are flat cells with flattened processes; the outgrowths of neighboring cells overlap each other, shifting during inhalation and exhalation. There are many pinocytotic vesicles along the periphery of the cytoplasm. Cells are unable to divide. The function of type I pneumocytes is to participate in gas exchange. These cells are part of the air-blood barrier.

Pneumocytes type II

Type II pneumocytes produce, accumulate and secrete surfactant components. The cells have a cubic shape. They are embedded between type I pneumocytes, rising above the latter; occasionally form groups of 2–3 cells. Type II pneumocytes have microvilli on their apical surface. A peculiarity of these cells is the presence in the cytoplasm of lamellar bodies with a diameter of 0.2–2 µm. The membrane-enclosed bodies consist of concentric layers of lipids and proteins. Lamellar bodies of type II pneumocytes are classified as lysosome-like organelles that accumulate newly synthesized and recycled surfactant components.

Interalveolar partition

The interalveolar septum contains capillaries enclosed in a network of elastic fibers surrounding the alveoli. The endothelium of the alveolar capillary is flattened cells containing pinocytotic vesicles in the cytoplasm. In the interalveolar septa there are small openings - alveolar pores. These pores create the opportunity for air to penetrate from one alveoli to another, which facilitates air exchange. Migration of alveolar macrophages also occurs through the pores in the interalveolar septa.

Lung parenchyma has a spongy appearance due to the presence of many alveoli (1), separated by thin interalveolar septa (2). Hematoxylin and eosin staining.

Aerogematic barrier

Between the cavity of the alveoli and the lumen of the capillary, gas exchange occurs through simple diffusion of gases in accordance with their concentrations in the capillaries and alveoli. Consequently, the fewer structures between the alveolar cavity and the capillary lumen, the more efficient the diffusion. A decrease in the diffusion path is achieved due to the flattening of cells - type I pneumocytes and the capillary endothelium, as well as due to the fusion of the basement membranes of the capillary endothelium and type I pneumocyte and the formation of one common membrane. Thus, the aerohematic barrier is formed by: type I alveolar cells (0.2 µm), common basement membrane (0.1 µm), flattened part of the capillary endothelial cell (0.2 µm). This adds up to about 0.5 microns.

Respiratory exchange CO 2. CO 2 is transported by the blood mainly in the form of the bicarbonate ion HCO 3 - as part of the plasma. In the lungs, where pO 2 = 100 mm Hg, the deoxyhemoglobin–H + complex of red blood cells entering the alveolar capillaries from the tissues dissociates. HCO 3 - is transported from plasma to erythrocytes in exchange for intracellular Cl - using a special anion exchanger (band 3 protein) and combines with H + ions, forming CO 2  H 2 O; Deoxyhemoglobin of the erythrocyte binds O 2, forming oxyhemoglobin. CO 2 is released into the lumen of the alveoli.

Aero-blood barrier- a set of structures through which gases diffuse in the lungs. Gas exchange occurs through the flattened cytoplasm of type I pneumocytes and capillary endothelial cells. The barrier also includes a basement membrane common to the alveolar epithelium and capillary endothelium.

Interstitial space

The thickened section of the alveolar wall, where the fusion of the basement membranes of the capillary endothelium and the alveolar epithelium does not occur (the so-called “thick side” of the alveolar capillary) consists of connective tissue and contains collagen and elastic fibers that create the structural framework of the alveolar wall, proteoglycans, fibroblasts, lipofibroblasts and myofibroblasts , mast cells, macrophages, lymphocytes. Such areas are called interstitial space (interstitium).

Surfactant

The total amount of surfactant in the lungs is extremely small. There is about 50 mm 3 of surfactant per 1 m2 of alveolar surface. The thickness of its film is 3% of the total thickness of the airborne barrier. The main amount of surfactant is produced by the fetus after the 32nd week of pregnancy, reaching its maximum amount by the 35th week. Before birth, excess surfactant is produced. After birth, this excess is removed by alveolar macrophages. 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. After secretion onto a thin layer of water covering the alveolar epithelium, the surfactant undergoes structural rearrangements: in the aqueous layer, the surfactant acquires a mesh-like shape known as tubular myelin, rich in apoproteins; the surfactant then reforms into a continuous monolayer.

Surfactant is regularly inactivated and converted into small surface-inactive aggregates. Approximately 70–80% of these aggregates are captured by type II pneumocytes, enclosed in phagolysosomes, and then catabolized or recycled. Alveolar macrophages phagocytose the remaining pool of small surfactant aggregates. As a result, lamellar aggregates of surfactant (“foamy” macrophage) surrounded by a membrane form and accumulate in the macrophage. At the same time, there is a progressive accumulation of extracellular surfactant and cellular debris in the alveolar space, the possibilities for gas exchange decrease, and the clinical syndrome of alveolar proteinosis develops.

Synthesis and secretion of surfactant by type II pneumocytes is an important event in intrauterine lung development. The functions of surfactant are to reduce the surface tension forces of the alveoli and increase the elasticity of the lung tissue. Surfactant prevents collapse of the alveoli at the end of expiration and allows the alveoli to open at reduced intrathoracic pressure. Of the phospholipids that make up surfactant, lecithin is extremely important. The ratio of lecithin content to sphingomyelin content in amniotic fluid indirectly characterizes the amount of intra-alveolar surfactant and the degree of maturity of the lungs. An indicator of 2:1 or higher is a sign of functional maturity of the lungs.

During the last two months of prenatal and several years of postnatal life, the number of terminal sacs constantly increases. Mature alveoli are absent before birth.

Pulmonary surfactant is an emulsion of phospholipids, proteins and carbohydrates; 80% are glycerophospholipids, 10% are cholesterol and 10% are proteins. Approximately half of the surfactant proteins are plasma proteins (mainly albumin) and IgA. The surfactant contains a number of unique proteins that promote the adsorption of dipalmitoylphosphatidylcholine at the interface of two phases. Among the proteins

Respiratory distress syndrome newborns develops in premature infants due to immaturity of type II pneumocytes. Due to the insufficient amount of surfactant secreted by these cells onto the surface of the alveoli, the latter are not straightened (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.

Alveolar macrophage. Bacteria in the alveolar space are covered with a film of surfactant, which activates the macrophage. The cell forms cytoplasmic projections, with the help of which it phagocytizes bacteria opsonized by surfactant.

Antigen-presenting cells

Dendritic cells and intraepithelial dendrocytes belong to the system of mononuclear phagocytes; they are the main Ag-presenting cells of the lung. Dendritic cells and intraepithelial dendrocytes are most abundant in the upper respiratory tract and trachea. As the caliber of the bronchi decreases, the number of these cells decreases. As Ag-presenting, pulmonary intraepithelial dendrocytes and dendritic cells. express MHC I and MHC II molecules.

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 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 cellular organelles are abundant. There are no phagosomes, since dendritic cells have virtually no phagocytic activity.

Antigen presenting cells in the lung. Dendritic cells enter the lung parenchyma with the blood. Some of them migrate to the epithelium of the intrapulmonary airways and differentiate into intraepithelial dendrocytes. The latter capture Ag and transfer it to regional lymphoid tissue. These processes are controlled by cytokines.

Intraepithelial dendrocytes

Intraepithelial dendrocytes are present only in the epithelium of the airways and are absent in the alveolar epithelium. These cells differentiate from dendritic cells, and such differentiation is possible only in the presence of epithelial cells. Connected by cytoplasmic processes penetrating between epithelial cells, intraepithelial dendrocytes form a well-developed intraepithelial network. Intraepithelial dendrocytes are morphologically similar to dendritic cells. A characteristic feature of intraepithelial dendrocytes is the presence in the cytoplasm of specific electron-dense granules in the shape of a tennis racket with a lamellar structure. These granules are involved in the capture of Ag by the cell for its subsequent processing.

Macrophages

Macrophages make up 10–15% of all cells in the alveolar septa. There are many microfolds on the surface of macrophages. The cells form rather long cytoplasmic processes that allow macrophages to migrate through the interalveolar pores. While inside the alveoli, the macrophage, with the help of processes, can attach to the surface of the alveoli and capture particles.

Fill out the table for self-control:

Alveolar macrophages originate from blood monocytes or connective tissue histiocytes and move along the surface of the alveoli, capturing foreign particles that come with the air, destroying epithelial cells. Macrophages, in addition to their protective function, also take part in immune and reparative reactions.

The renewal of the epithelial lining of the alveoli is carried out by type II alveolocytes.

While studying the pleura, find out that the visceral pleura is tightly fused with the lungs and differs from the parietal pleura in the quantitative content of elastic fibers and smooth myocytes.

The respiratory system of organs, in connection with the performance of basic functions, is divided into two sections: airways (nasal cavity, nasopharynx, larynx, trachea, extra- and pulmonary bronchi), which perform the functions of conducting, purifying, warming air, sound production; and respiratory sections - acini - systems of pulmonary vesicles located in the lungs and providing gas exchange between air and blood.

Sources of development. The rudiments of the larynx, trachea and bronchi arise as protrusions of the ventral wall of the foregut, formed at 3-4 weeks of embryonic development. Smooth muscle tissue of the bronchi, as well as cartilaginous, fibrous connective tissue, and a network of blood vessels are differentiated from the mesenchyme. From the visceral and parietal layers of the splanchnotome, the visceral and parietal layers of the pleura are formed.

Airways They are a system of interconnected tubes conducting air. They are lined with a mucous membrane of the respiratory type with multirow ciliated epithelium. The exception is the vestibule of the nasal cavity, vocal cords and epiglottis, where the epithelium is stratified squamous. The wall of most organs of the airways of the respiratory system has a layered structure and consists of 4 membranes: mucous membrane, submucosa with glands, fibrocartilaginous with the inclusion of hyaline or elastic cartilaginous tissue and adventitia. The degree of expression of the membranes in different organs varies depending on the location and functional characteristics of the organ. Thus, in the small and terminal bronchi there is no submucosa and fibrocartilaginous membrane.

Mucous membrane usually includes three plates that have their own organ characteristics: 1. epithelial, represented by multirow prismatic ciliated epithelium, characteristic of the mucous membrane of the respiratory type;

2. the lamina propria of the mucous membrane, in the loose connective tissue of which there are many elastic fibers; 3. The muscular plate of the mucous membrane (absent in the nasal cavity, larynx, trachea), represented by smooth myocytes.

Trachea- a hollow tube consisting of all 4 membranes: the inner mucous membrane with two plates; submucosa with complex protein-mucosal glands, the secretion of which moisturizes the surface of the mucous membrane; fibrocartilaginous and outer adventitia. In the ciliated multirow epithelium of the mucous membrane there are ciliated, goblet cells that produce mucus, basal cambial cells and endocrine cells that produce norepinephrine, serotonin, dopamine, regulating the contraction of smooth myocytes of the airways. Failures in their activities can lead to serious disturbances in the functioning of the respiratory system. The fibrocartilaginous membrane of the trachea consists of 16-20 hyaline rings, not closed on the posterior wall of the organ. The ends of the open rings are connected by bundles of smooth muscles, which makes the wall of the trachea pliable and which is of great importance during swallowing, pushing the bolus of food through the esophagus.

Lung consists of a system of airways - bronchi, which make up the bronchial tree, and of respiratory sections - acini - a system of pulmonary vesicles, which form the alveolar tree.

Bronchi by location they are divided into extrapulmonary: main, lobar, zonal and pulmonary, starting with segmental and subsegmental, and ending with terminal bronchioles. By caliber, large, medium, small bronchi and terminal bronchioles are distinguished. All bronchi have a general structural plan. In their wall there are 4 membranes: the inner mucosa, the submucosa, the fibrocartilaginous membrane and the outer adventitial membrane. The degree of expression of the membrane component structures depends on the diameter of the bronchus. So, if in the main, large and middle bronchi there are all four membranes, then in the small bronchi there are only two: the mucous membrane and the adventitia. The bronchial mucosa has three plates: the epithelial plate, the lamina propria and the muscular plate of the mucosa. The epithelial plate of the mucous membrane, facing the lumen of the bronchus, is represented by multirow ciliated prismatic epithelium. As the caliber of the bronchi decreases, the multilayered epithelium decreases. The cells become lower - to low cubic ones in the small bronchi, the number of goblet cells decreases. In addition to ciliated, goblet, endocrine and basal cells, secretory cells that break down surfactant, border cells - chemoreceptors and non-ciliated cells, found in bronchioles, are found in the distal parts of the bronchial tree. The epithelial lamina is followed by the lamina propria of the mucous membrane, which is represented by loose connective tissue with elastic fibers. With a decrease in the caliber of the bronchi, the number of elastic fibers in it increases. The mucous membrane of the bronchi is closed by its third plate - the muscular plate of the mucous membrane. It appears in the main and reaches a maximum in the small bronchus. In bronchial asthma, contraction of muscle elements in the small and smallest bronchi sharply reduces their lumen. In the submucosa of the bronchi, the terminal sections of mixed protein-mucosal glands are located in groups. Their secretion has bacteriostatic and bactericidal properties; the secretion envelops dust particles and moisturizes the mucous membrane. There are no glands in the small bronchi, and there is no submucosa. The fibrocartilaginous membrane also undergoes changes as the caliber of the bronchi decreases; open cartilaginous rings in the main bronchi are replaced by cartilaginous plates in the large lobar bronchi. In the small bronchi there is no cartilaginous tissue, there is no fibrocartilaginous membrane. The outer adventitia of the bronchi consists of fibrous connective tissue with vessels and nerves; it passes into the connective tissue septa of the lung parenchyma.

Terminal, terminal bronchioles (D - 0.5 mm) are lined with single-layer cuboidal epithelium. The lamina propria of the mucous membrane contains longitudinally running elastic fibers, with individual bundles of smooth myocytes lying between them. Terminal bronchioles end the airways.

Respiratory tree. Respiratory department. Its structural and functional unit is the acinus. Acinus is a system of pulmonary vesicles that provide gas exchange. The acini are attached to the terminal bronchioles. Composition of the acini: respiratory bronchioles of the 1st, 2nd, 3rd order, alveolar ducts and alveolar sacs. All these formations have alveoli, which means gas exchange is possible. In the respiratory bronchioles, areas of single-layer cuboidal non-ciliated epithelium alternate with alveoli lined with single-layer squamous epithelium. There are already many alveoli in the alveolar ducts; club-shaped thickenings (muscle brushes) containing smooth myocytes are visible in the interalveolar septa. The alveolar sacs are formed by many alveoli; they lack muscle elements. In the interalveolar septa, in addition to the blood capillaries adjacent to the basement membrane of the alveolar epithelium, there is a network of elastic fibers intertwining the alveoli. The alveoli are closely adjacent to each other, so one capillary borders on its sides two alveoli, which provides maximum conditions for gas exchange. Alveolus has the appearance of a vesicle, lined from the inside with single-layer squamous epithelium with two types of cells: respiratory and large granular epithelial cells. Respiratory epithelial cells are type 1 cells with small mitochondria and pinocytotic vesicles. Gas exchange occurs through these cells. Adjacent to the nuclear-free areas of type 1 epithelial cells are the nuclear-free areas of the endothelium of the blood capillary. Separating respiratory epithelial cells and capillary endothelial cells, their basement membranes are tightly adjacent to each other. The listed structures (respiratory alveolocytes, basement membranes and capillary endothelium) constitute an aerohematic barrier between the air of the alveoli and the blood of the blood capillaries. It is very thin - 0.5 microns. The barrier also includes a surfactant alveolar complex, which lines the alveoli from the inside and makes up 2 phases: a membrane phase, similar to a biological membrane, with proteins and phospholipids, and a liquid hypophase, located deeper and containing glycoproteins. Surfactant prevents the alveoli from collapsing during exhalation, protects against the penetration of microbes from the air and from the transudation of fluid from the capillaries into the alveoli. Surfactant is produced by large granular epithelial cells - type 2 cells. They contain large mitochondria, the Golgi complex, the endoplasmic reticulum and surfactant granules. Macrophages are also found in the alveolar wall;

they contain a lot of lysosomes and lipids, due to the oxidation of which heat is released to warm the air in the alveoli.

Function respiratory section of the lungs - gas exchange.

Structural and functional unit of the respiratory department – acini. The acini is a system of hollow structures with alveoli in which gas exchange occurs.

Acinus is formed:

• respiratory bronchioles of the 1st, 2nd and 3rd orders , which are consistently dichotomously divided;
• alveolar ducts
• alveolar sacs .

12-18 acini form the pulmonary lobule.

Respiratory bronchioles contain a small number of alveoli, the rest of their wall is similar to the wall of the terminal bronchioles: mucous membrane with cuboidal epithelium, a thin lamina propria with smooth myocytes and elastic fibers and a thin adventitia. In the distal direction (from the 1st order bronchioles to the 3rd order bronchioles), the number of alveoli increases, the spaces between them decrease.

Alveolar ducts are formed during the dichotomous division of respiratory bronchioles of the 3rd order; their the wall is formed by alveoli, between which, at the mouth of the alveoli, bundles of smooth myocytes are located in a ring shape, protruding into the lumen (in the form of “buttons”); areas lined with cuboidal epithelium are absent.

Alveolar ducts pass into alveolar sacs– clusters of alveoli at the distal edge of the alveolar duct.

Alveoli- rounded formations with a diameter of 200-300 microns; lined with single-layer squamous epithelium and surrounded by a dense capillary network. The number of alveoli is about 300 million, and their surface area is about 80 km.

In the epithelium of the alveoli there are 2 types of cells - alveolocytes (pneumocytes):

• type I alveolocytes or respiratory alveolocytes;
• type II alveolocytes or large secretory alveolocytes .

Alveolocytes type I occupy 95-97% of the surface area of ​​the alveoli; consist of a thicker part containing the nucleus and a very thin non-nuclear part (about 0.2 µm thick); organelles are poorly developed, there are poorly developed organelles, a large number of pinocytotic vesicles. Type I alveolocytes are components air-blood barrier , and are connected to type 2 cells by tight junctions.

Alveolocytes of the 2nd type are larger cells, cubic shape;

have well-developed organelles of the synthetic apparatus and special lamellar osmiophilic granules – lamellar bodies; the contents of the granules are released into the lumen of the alveoli, forming surfactant.

Functions of type 2 alveolocytes:

Production and renewal of surfactant;

Secretion of lysozyme and interferon;

Neutralization of oxidizers;

Cambial elements of the alveolar epithelium (renewal rate - 1% per day)

Participation in regeneration (for example, during lung resection), since these cells are capable of mitotic divisions.

Surfactant– a layer of surfactant of glycolipid-protein nature; consists of two phases (parts):

hypophase – lower, “tubular myelin”; has a lattice appearance; smoothes out uneven surfaces of the epithelium;

apophase - surface monomolecular film of phospholipids.

Surfactant functions:

Reducing the surface tension of the tissue fluid film → promotes the straightening of the alveoli and prevents their walls from sticking together; if the production of surfactant is impaired, the lung collapses (atelectasis);

Anti-edematous barrier → prevents the release of fluid into the lumen of the alveoli;

Protective (bactericidal, immunomodulatory, stimulation of alveolar macrophage activity).

Surfactant is constantly renewed; type 2 alveolocytes, alveolar macrophages and bronchiolar exocrinocytes (Clara cells) participate in the renewal of surfactant.

Surfactant is produced at the end of fetal development. In its absence or deficiency (in premature babies), respiratory failure syndrome develops because the alveoli do not straighten. Surfactant secretion can be stimulated by corticosteroids.

Aero-blood barrier– this is a barrier of minimal thickness (0.2-0.5 microns) between the lumen of the alveoli and the capillary, which ensures gas exchange (through passive diffusion)

The air-blood barrier includes the following structures:

A layer of surfactant lining the surface of the alveolar epithelium;

Thinned area of ​​the cytoplasm of type 1 alveolocyte;

Common fused basement membrane of type 1 alvolocyte and endotheliocyte;

A thinned section of the cytoplasm of the capillary endothelial cell (somatic type capillary).

Material taken from the site www.hystology.ru

Respiratory section of the lung. The functional unit of the lung is the acinus. It consists of respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli in combination with associated blood and lymphatic vessels, connective tissue and nerves. The diameter of the respiratory bronchiole is about 0.5 mm. In the initial section, it is lined with a single-layer prismatic ciliated epithelium, which in its final section turns into a cubic single-layer without cilia.

Under the epithelium in the wall of the bronchiole lies a thin layer of connective tissue, including elastic fibers and smooth muscle cells. The wall of the respiratory bronchiole contains separate alveoli. Respiratory bronchioles break up into alveolar ducts, which, branching, end in alveolar sacs, consisting of a set of respiratory alveoli: The alveoli are lined with respiratory epithelium located on the basement membrane.

At the mouth of the alveoli there are groups of smooth muscle cells. The interalveolar connective tissue contains blood vessels

Rice. 290. Walls of the alveoli and blood capillary of the lung (diagram):

1 - alveolar cavity; 2 - alveolar epithelial cell; 3 - endothelial cell of the blood capillary; 4 - capillary lumen; 5 - basement membranes; 6 - erythrocyte.

capillaries, thin bundles of collagen fibers, fragments of the elastic network and single connective tissue cells. Between adjacent alveoli, openings 10 - 20 µm in diameter were identified - alveolar pores.

The alveoli of the lung are lined by two types of cells: type I pneumocytes (respiratory alveolocytes) and type II pneumocytes (large alveolocytes).

Respiratory alveolocytes cover most of the inner surface of the alveoli. They have the form of extensive thin plates, the height of which ranges from 0.2 to 0.3 microns. The nuclear part of the cells protrudes into the cavity of the alveoli, reaching a height of 5 - 6 microns (Fig. 290). These cells contain numerous organelles: mitochondria, ribosomes, endoplasmic reticulum, etc. There is a significant number of pinocytotic vesicles in the cytoplasm. The free surface of the cells is covered with a layer of surfactant, consisting of phospholipids, proteins and glycoproteins, which protects the alveoli from collapse and the penetration of microorganisms into the underlying tissues.

Respiratory alveolocytes, the basement membrane of the alveolar epithelium, the interalveolar line, the basement membrane of blood vessels and their endothelium together form an air-blood barrier with a thickness of 0.1 to 0.5 microns (Fig. 291).

Large alveolocytes are located in the alveolar wall singly or in groups between respiratory alveolocytes. These are large cells with a large nucleus. On their free surface they have short microvilli. In their cytoplasm, the Golgi complex, vesicles and cisterns of the granular endoplasmic reticulum and free ribosomes are well developed. The cytoplasm of these cells is characterized by numerous dense

Rice. 291. Respiratory alveolocytes (electron micrograph):

1 - basement membrane of the epithelium; 2 - basement membrane of the capillary endothelium; 3 - respiratory alveolocyte; 4 - endothelial cell cytoplasm; 5 - erythrocyte.

Rice. 292. Large alveolocyte (electron micrograph):

1 - core; 2 - cytoplasm; 3 - lamellar bodies; 4 - mitochondria; 5 - microvilli; 6 - contact with respiratory alveolocyte.

osmophilic bodies (cytosomes), rich in phospholipids. They consist of parallel plates with a diameter of 0.2 to 1.0 microns. They secrete surfactant onto the surface of the alveoli, which stabilizes their size (Fig. 292). The interalveolar septa contain fixed and free macrophages.

Interstitial tissue of the lung accompanies blood vessels and airways. It delimits the lobes and lobules of the organ parenchyma and forms its subpleural layer. Its elements are detected in the lobules of the organ, in the walls of the alveolar ducts and alveoli.

The connective tissue accompanying the bronchi is characterized by accumulations of lymphoid tissue that form lymphoid nodules along the bronchial tree. The interstitial connective tissue of the lungs is rich in elastic elements. The latter entwine the alveoli, condensing at their mouth in the form of a ring. The lungs of horses and cattle are richest in elastic tissue.

Lung vascularization. The lungs receive blood through the vessels of two systems: the pulmonary artery and the bronchial artery. Most of the blood enters through the pulmonary arteries, which carry venous blood from the right ventricle of the heart. These are elastic arteries. They accompany the bronchi to the bronchioles and break up into a capillary network surrounding the alveoli; the small diameter of the capillaries and their intimate adherence to the wall of the alveoli provide conditions for gas exchange between red blood cells and alveolar air. Blood entering through the bronchial arteries is carried out through the bronchial veins.

Lymphatic vessels The lungs are represented by a superficial network - the visceral pleura and a deep network - pulmonary tissue. The pleural vessels, connecting, form several large trunks that carry lymph to the lymph nodes of the hilum of the lungs. The lymphatic vessels of the lungs accompany the bronchial vessels, pulmonary arteries and pulmonary veins.

Pleura- serous membrane covering the lung and chest cavity. It consists of a thin layer of loose connective tissue and an overlying layer of flat mesothelial cells. The connective tissue of the pleura, especially its visceral layer, is rich in elastic fibers.