Alveolar epithelium and airborne barrier. The mucous membrane is lined with multirow prismatic ciliated epithelium. How gas exchange occurs
1. Concept of the respiratory system Respiratory system consists of two parts :
- airways
- respiratory department.
- nasal cavity;
- nasopharynx;
- trachea;
- bronchial tree (extra- and intrapulmonary bronchi).
- respiratory bronchioles;
- alveolar ducts;
- alveolar sacs.
Source of development The main respiratory organ is the material of the ventral wall of the foregut, called the prechordal plate. At the 3rd week of embryogenesis, it forms a protrusion, which in the lower part is divided into two rudiments of the right and left lungs.
There are 3 stages in lung development:
- glandular stage, begins from the 5th week to the 4th month of embryogenesis. At this stage, the airway system and the bronchial tree are formed. At this time, the lung primordium resembles a tubular gland, since in the section, among the mesenchyme, numerous sections of large bronchi are visible, similar to the excretory ducts of the exocrine glands;
- canalicular stage(4-6 months of embryogenesis) is characterized by the completion of the formation of the bronchial tree and the formation of respiratory bronchioles. At the same time, capillaries are intensively formed, which grow into the mesenchyme surrounding the epithelium of the bronchial tubes;
- alveolar stage and begins from the 6th month of intrauterine development and continues until the birth of the fetus. In this case, alveolar ducts and sacs are formed. Throughout embryogenesis, the alveoli are in a collapsed state.
- conducting air to the respiratory section;
- air conditioning - warming, humidifying and cleaning;
- barrier-protective;
- secretory - the production of mucus, which contains secretory antibodies, lysozyme and other biologically active substances.
Vestibule of the nose lined with a mucous membrane, which contains stratified squamous non-keratinizing epithelium and the lamina propria.
Respiratory part lined with single-layer multirow ciliated epithelium. Its composition includes :
- ciliated cells- have flickering cilia that oscillate against the movement of inhaled air; with the help of these cilia, microorganisms and foreign bodies are removed from the nasal cavity;
- goblet cells secrete mucins - mucus that glues foreign bodies and bacteria together and facilitates their removal;
- microvilli cells are chemoreceptor cells;
- basal cells play the role of cambial elements.
Mucous membrane lining the respiratory part of the nasal cavity has two areas that differ in structure from the rest of the mucosa :
- olfactory part, which is located on most of the roof of each nasal cavity, as well as in the superior turbinate and the upper third of the nasal septum. The mucous membrane lining the olfactory areas forms the olfactory organ;
- mucous membrane in the area of the middle and inferior turbinates differs from the rest of the nasal mucosa in that it contains thin-walled veins, reminiscent of the lacunae of the cavernous bodies of the penis. Under normal conditions, the blood content in the lacunae is small, since they are in a partially collapsed state. When inflamed (rhinitis), the veins become filled with blood and narrow the nasal passages, making nasal breathing difficult.
- olfactory cells have a spindle-shaped shape and two processes. The peripheral process has a thickening (olfactory club) with antennae - olfactory cilia, which run parallel to the surface of the epithelium and are in constant motion. In these processes, upon contact with an odorous substance, a nerve impulse is formed, which is transmitted along the central process to other neurons and further to the cortex. Olfactory cells are the only type of neurons that have a predecessor in the form of cambial cells in an adult individual. Thanks to the division and differentiation of basal cells, olfactory cells are renewed every month;
- supporting cells located in the form of a multirow epithelial layer, on the apical surface they have numerous microvilli;
- basal cells They have a conical shape and lie on the basement membrane at some distance from each other. Basal cells are poorly differentiated and serve as a source for the formation of new olfactory and supporting cells.
The olfactory analyzer is built from 3 neurons.
First The neurons are the olfactory cells, their axons form the olfactory nerves and end in the form of glomeruli in the olfactory bulbs on the dendrites of the so-called mitral cells. This second link olfactory pathway. The axons of mitral cells form the olfactory pathways in the brain. Still others neurons are cells of the olfactory pathways, the processes of which end in the limbic region of the cerebral cortex.
Nasopharynx is a continuation of the respiratory part of the nasal cavity and has a structure similar to it: it is lined with multirow ciliated epithelium lying on the lamina propria. The lamina propria contains secretory sections of small protein-mucosal glands, and on the posterior surface there is an accumulation of lymphoid tissue (pharyngeal tonsil).
3. Structure of the larynx Laryngeal wall
consists of mucous, fibrocartilaginous and adventitial membranes.
Mucous membrane represented by epithelial and lamina propria. The epithelium is multirow ciliated, consists of the same cells as the epithelium of the nasal cavity. Vocal cords covered with stratified squamous non-keratinizing epithelium. The lamina propria is formed by loose fibrous unformed connective tissue and contains many elastic fibers. The fibrocartilaginous membrane plays the role of the frame of the larynx and consists of fibrous and cartilaginous parts. The fibrous part is dense fibrous connective tissue, the cartilaginous part is represented by hyaline and elastic cartilage.
Vocal cords(true and false) are formed by folds of the mucous membrane protruding into the lumen of the larynx. They are based on loose fibrous connective tissue. The true vocal cords contain several striated muscles and a bundle of elastic fibers. Muscle contraction changes the width of the glottis and the timbre of the voice. False vocal cords, lying above the true ones, do not contain skeletal muscles and are formed by loose fibrous connective tissue covered with stratified epithelium. In the mucous membrane of the larynx, in the lamina propria, there are simple mixed protein-mucous glands.
Functions of the larynx:
- air conduction and conditioning;
- participation in speech;
- secretory function;
- barrier-protective function.
- mucous membrane;
- submucosa;
- fibrocartilaginous;
- adventitial.
Submucosa formed by loose fibrous connective tissue in which complex protein-mucosal tracheal glands are located. Their secretion moisturizes the surface of the epithelium and contains secretory antibodies.
Fibrocartilaginous sheath consists of glial cartilaginous tissue, forming 20 half-rings, and dense fibrous connective tissue of the perichondrium. On the posterior surface of the trachea, the ends of cartilaginous half-rings are connected by bundles of smooth myocytes, which facilitates the passage of food through the esophagus lying behind the trachea.
Adventitia formed by loose fibrous connective tissue. The trachea at the lower end is divided into 2 branches, forming the main bronchi, which are part of the roots of the lungs. The bronchial tree begins with the main bronchi. It is divided into extrapulmonary and intrapulmonary parts.
5. Structure of the lungs Basic functions of the lungs:
- gas exchange;
- thermoregulatory function;
- participation in the regulation of acid-base balance;
- regulation of blood coagulation - the lungs form large quantities of thromboplastin and heparin, which participate in the activity of the coagulant-antigoagulant blood system;
- regulation of water-salt metabolism;
- regulation of erythropoiesis by secretion of erythropoietin;
- immunological function;
- participation in lipid metabolism.
- intrapulmonary bronchi (bronchial tree)
- numerous acini forming the lung parenchyma.
6. Structure of the bronchi Bronchial wall consists of of 4 shells :
- mucous membrane;
- submucosa;
- fibrocartilaginous;
- adventitial.
The inner mucous membrane consists of three layers:
- multirow ciliated epithelium;
- own
- muscle plates.
- secretory cells that secrete enzymes that destroy surfactant;
- non-ciliated cells (possibly perform a receptor function);
- border cells, the main function of these cells is chemoreception;
- ciliated;
- goblet;
- endocrine.
Muscular plate of the mucous membrane formed by smooth muscle tissue.
Submucosa represented by loose fibrous connective tissue. It contains the terminal sections of mixed mucous-protein glands. The secretion of the glands moisturizes the mucous membrane .
Fibrocartilaginous sheath formed by cartilaginous and dense fibrous connective tissue. Adventitia represented by loose fibrous connective tissue.
Throughout the bronchial tree, the structure of these membranes changes. The wall of the main bronchus does not contain half rings, but closed cartilaginous rings. In the wall of large bronchi, cartilage forms several plates. Their number and size decrease as the diameter of the bronchus decreases. In the medium-sized bronchi, hyaline cartilaginous tissue is replaced by elastic tissue. In small-caliber bronchi, cartilage is completely absent. The epithelium also changes. In large bronchi it is multirowed, then gradually becomes birowed, and in the terminal bronchioles it turns into a single row cubic. The number of goblet cells in the epithelium decreases. The thickness of the lamina propria decreases, while the thickness of the muscular lamina, on the contrary, increases. In the small-caliber bronchi, the glands disappear in the submucous membrane, otherwise the mucus would close the narrow lumen of the bronchus here. The thickness of the adventitial membrane decreases.
The airways end terminal bronchioles, having a diameter of up to 0.5 mm. Their wall is formed by the mucous membrane. The epithelium is single-layered cubic ciliated. It consists of ciliated, brush, borderless cells and Clara secretory cells. The lamina propria is formed by loose fibrous connective tissue, which passes into the interlobular loose fibrous connective tissue of the lung. The lamina propria contains bundles of smooth myocytes and longitudinal bundles of elastic fibers.
7. Respiratory section of the lungs The structural and functional unit of the respiratory department is acini. Acinus is a system of hollow structures with alveoli in which gas exchange occurs.
The acinus begins with a respiratory or alveolar bronchiole of the 1st order, which is dichotomously sequentially divided into respiratory bronchioles of the 2nd and 3rd orders. Respiratory bronchioles contain a small number of alveoli; the rest of their wall is formed by a mucous membrane with cuboidal epithelium, thin submucosa and adventitia. Respiratory bronchioles of the 3rd order are dichotomously divided and form alveolar ducts with a large number of alveoli and correspondingly smaller areas lined with cuboidal epithelium. The alveolar ducts pass into the alveolar sacs, the walls of which are completely formed by alveoli in contact with each other, and there are no areas lined with cuboidal epithelium.
Alveolus
- structural and functional unit of the acinus. It has the appearance of an open vesicle, lined from the inside with single-layer squamous epithelium. The number of alveoli is about 300 million, and their surface area is about 80 square meters. m. The alveoli are adjacent to each other, between them there are interalveolar walls, which contain thin layers of loose fibrous connective tissue with hemocapillaries, elastic, collagen and reticular fibers. Pores connecting them were found between the alveoli. These pores allow air to penetrate from one alveoli to another, and also ensure gas exchange in the alveolar sacs, whose own airways are closed as a result of the pathological process.
The alveolar epithelium consists of 3 types of alveolocytes:
- alveolocytes Type I or respiratory alveolocytes, gas exchange occurs through them, and they also participate in the formation of the aerohematic barrier, which includes the following structures - the endothelium of the hemocapillary, the basement membrane of the continuous type endothelium, the basement membrane of the alveolar epithelium (the two basement membranes are tightly adjacent to each other and are perceived as one); alveolocyte type I; surfactant layer lining the surface of the alveolar epithelium;
- alveolocytes Type II or large secretory alveolocytes, these cells produce surfactant- a substance of glycolipid-protein nature. Surfactant consists of two parts (phases) - the lower (hypophase). The hypophase smoothes out the surface irregularities of the alveolar epithelium; it is formed by tubules that form a lattice structure on the surface (apophase). Apophase forms a phospholipid monolayer with the orientation of the hydrophobic parts of the molecules towards the alveolar cavity.
- reduces the surface tension of the alveoli and prevents their collapse;
- prevents fluid from leaking from the vessels into the cavity of the alveoli and the development of pulmonary edema;
- has bactericidal properties, as it contains secretory antibodies and lysozyme;
- participates in the regulation of the functions of immunocompetent cells and alveolar macrophages.
- alveolocytes III type or alveolar macrophages, which adhere to other cells. They come from blood monocytes. The function of alveolar macrophages is to participate in immune reactions and in the work of the surfactant-antisurfactant system (splitting of surfactant).
8. Blood supply to the lungs Blood supply to the lungs coming by 2 vascular systems:
- the pulmonary artery brings venous blood to the lungs. Its branches are divided into capillaries, which surround the alveoli and participate in gas exchange. The capillaries collect in the system of pulmonary veins, which carry oxygenated arterial blood;
- bronchial arteries depart from the aorta and carry out trophism of the lung. Their branches go along the bronchial tree up to the alveolar ducts. Here, capillaries that anastomose with each other extend from the arterioles to the alveoli. At the top of the alveoli, capillaries become venules. There are anastomoses between the vessels of the two arterial systems.
Alveoli are the smallest structures of the lungs, but thanks to them the process of breathing and ensuring all vital functions is possible. These microscopic vesicles that end the bronchioles are responsible for gas exchange in the body. Both lungs contain about 700 million alveoli, the size of each of them does not exceed 0.15 microns. Thanks to them, the tissues of all organs and systems without exception receive the amount of oxygen necessary for normal functioning. The structure of the alveoli is complex.
Anatomy
The alveoli have the form of sacs, located in clusters at the end of the terminal bronchioles, connecting with them by the alveolar ducts. Outside they are entwined with a network of small capillary vessels. The main structures through which gas exchange occurs are:
- One layer of epithelial cells located on the basement membrane. These are pneumocytes of orders 1–3.
- A layer of stroma represented by interstitial tissue.
- Endothelium of small capillary vessels immediately adjacent to the alveoli; the wall of one capillary is in contact with several alveoli.
- A layer of surfactant is a special substance that lines the alveoli from the inside. It is formed by cells from blood plasma, helps maintain a constant volume of the respiratory sacs, and prevents them from sticking together. Thanks to this special substance, the main function of the alveoli is ensured - gas exchange.
The surfactant is fully “ripened” by the time the baby is born, allowing the newborn to breathe independently. That is why premature babies have a high risk of developing respiratory distress syndrome due to the inability to breathe independently.
All of these structures form a so-called aerohematic barrier, through which oxygen enters and carbon dioxide is removed. In addition to the indicated structural elements, there are special ones necessary to maintain homeostasis:
- Chemoreceptors that detect fluctuations in changes in gas exchange or surfactant production by cells. Having received a signal about the slightest deviations, they contribute to the production of special active peptides involved in the restoration of altered functions.
- Macrophages – have an antimicrobial effect, protect the alveoli from damage by pathogenic microorganisms.
Thanks to collagen and elastic fibers, the shape is maintained and the volume of the alveolar sacs changes during breathing.
Functions
The most important task performed by the alveolar epithelium is the exchange of gases between the capillaries and the lungs. Its implementation is possible due to the large area of the respiratory surface of the alveoli, amounting to more than 90 square meters, and the same size as the area of the capillary network that forms the pulmonary circulation.
In addition, the alveolar part of the lungs, as the most important structural unit, is involved in performing the following functions:
- Excretory. Through the lungs, gaseous substances formed in the body are removed from the bloodstream and enter from the environment: carbon dioxide, oxygen, methane, ethanol, drugs, nicotine and others.
- Regulation of water-salt balance. Water evaporates from the surface of the alveoli, reaching up to 500 ml/day.
- Heat transfer. Up to 15% of the thermal energy generated by the body is released using the alveolar apparatus of the lung tissue. Before entering the bloodstream, the incoming air is warmed by the alveoli to approximately 37 degrees.
- Protective. Viruses and pathogenic microbes penetrate from the surrounding space through the inhaled air. The coordinated work of macrophages and chemoreceptors, thanks to the production of lysozyme and immunoglobulins, foreign aggressive agents are neutralized and removed from the body.
- Filtration and hemostasis. Small blood clots or emboli from the pulmonary circulation are destroyed with the help of fibrinolytic enzymes produced by the alveolar epithelium.
- Depositing blood. Up to 15% of the volume of circulating blood can remain and fill the capillary network of the pulmonary circulation, while being saturated with oxygen, providing the body with reserve capabilities during critical situations.
- Metabolic. They take part in the formation and destruction of biologically active compounds: heparin, polysaccharides, surfactant. The alveolar epithelium carries out the processes of synthesis of protein molecules, collagen, and elastin fibers.
The lungs are the site of deposition of serotonin, histamine, norepinephrine, insulin and other active substances, which ensures their rapid entry into the blood when acute stressful situations occur. It is this mechanism that is the basis for the development of shock reactions.
How does gas exchange occur?
Inhaled oxygen, passing through a thin layer of alveolar epithelium and the capillary wall, enters the bloodstream. Blood saturation occurs due to low blood flow velocity. In addition, the size of the red blood cell significantly exceeds the diameter of the capillary. Under pressure, the shaped element undergoes deformation, squeezing into the lumen of the vessel, which increases the area of contact with the alveolar wall. This mechanism promotes maximum saturation of hemoglobin with oxygen.
Carbon dioxide diffusion occurs in the opposite direction. The process is carried out due to the difference in pressure on both sides of the air-hematic barrier.
Age, lifestyle, diseases lead to the fact that lung tissue undergoes changes. By the time of adulthood, the number of alveoli increases by more than 10 times compared to their number in a newborn. Playing sports helps increase the respiratory surface.
With age and with certain lung diseases, due to tobacco smoking and inhalation of toxic substances, a gradual proliferation of connective tissue fibers occurs, reducing the respiratory surface of the alveolar structures. Such conditions are the cause of respiratory failure.
Decline height of the epithelial layer mucosa (from multirow cylindrical to double row, and then single row in small 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 tolka
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, toward 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 feature 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 bicarbonate ion HCO 3 - in 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 area of the alveolar wall, where the basement membranes of the capillary endothelium and the alveolar epithelium do not merge (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 used again. 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 saccules 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.
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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.
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 figure shows a segment of the alveolar septum (AS) under high magnification; we will look at the structure of the alveolar epithelium and the air-hematic barrier. Unfortunately, the figure does not show all the listed structures, which will be discussed further.
Alveolar epithelium formed by alveolar cells of types I and II.
Alveolar cells type I (AC I) are highly flattened epithelial cells in contact with air. In addition to the flattened nucleus (R), the perikaryon (P) contains a small Golgi complex, several small mitochondria, a small number of granular endoplasmic reticulum cisterns, many microvesicles (MV) and free ribosomes. The remaining cytoplasm forms an extremely thin continuous layer 70 nm thick with a cell surface area of about 4000 μm2. Alveolar cells of type I, connecting with each other, form a continuous alveolar lining lying on the basement membrane (BM). Type I alveolar cells are capable of transporting small amounts of inhaled material in microvesicles to the underlying interstitial connective tissue space.
Alveolar cells type II (AC II)- round or cuboidal secretory alveolar cells with a diameter of 10-15 microns, located in small recesses of the alveolar wall. The round nucleus (R) occupies a central position, all cellular organelles, especially the Golgi complex and the granular endoplasmic reticulum (RER), are well developed. Numerous mitochondria (M) are also located here. The apical cytoplasm contains varying numbers of multivesicular bodies (MVBs), which gradually transform into multilamellar bodies (MLBs). The latter are secreted by cells, and their lamellar components spread over the entire epithelial surface, turning into surfactant. On the sides, type II alveolar cells are in contact with the cytoplasmic processes of type I alveolar cells. The free surface of type II alveolar cells is dotted with prominent multilamellar bodies and laterally with microvilli (MV).
Lung surfactant, or anti-atelectatic factor, is a three-layer film about 30 nm thick covering the alveolar epithelium. Biochemically lung surfactant- a complex mixture of phospholipids (there are most of them), proteins and glycoproteins. Surfactant not only reduces surface tension at the air-liquid interface, thus preventing collapse (atelectasis) of the alveoli, but also fixes inhaled dust particles, which are then processed by alveolar macrophages.
This substance performs three main functions:
1. “Lubricating” the alveoli from the inside, lung surfactant reliably protects lung tissue from penetration of microorganisms, dust particles, etc.
2. The barrier is very thin. So why can air from the alveoli transfer oxygen to the capillary, but the capillary cannot, in the opposite direction, along with carbon dioxide, transfer some liquid - plasma? This is the second merit lung surfactant: It prevents fluid from the blood from leaking into the lumen of the alveoli.
3. Phospholipids surfactant are able to withstand enormous force - the desire of the elastic interalveolar walls to shrink. Each time you exhale, collapse of the alveoli could occur if the surfactant did not overcome the physical factors contributing to this. That is why the production of this secretion begins already at the 24th week of intrauterine development, so that by the time of birth and the first human breath, the lungs immediately expand and cannot collapse.
Air-blood barrier (ABB) is a very thin multilayer biological membrane between air and blood capillaries (Cap). In humans, its thickness is about 2.2 ± 0.2 µm.
To more clearly depict the air-blood barrier, the type I alveolar cell segment and the epithelial and capillary basement membranes are shown in the figure to extend to the outer surface of the capillary endothelial cell. Aero-blood barrier formed by a very thin layer of cytoplasm of alveolar type I cells (AC I), the epithelial basement membrane (BM), the capillary basement membrane (BCM), and the very flattened cytoplasm of the endothelial cells of the non-fenestrated capillary. The two basement membranes almost fuse where the alveolar and endothelial cells are located opposite each other. The exchange of gases between the air of the alveoli and the capillaries occurs by passive diffusion.
In order not to interfere with the free exchange of gases, the nuclei (N) of endothelial cells (EC) are almost always located on the periphery of the cells closer to the capillary wall.
The interstitial space of connective tissue also contains fibroblasts (F), collagen microfibrils (CMf) and fibrils (Fr), as well as elastic fibers (EF).
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: internal - mucous membrane, submucosa, fibrocartilaginous and outer adventitial membranes. 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 of the mucosa, 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 externally adjacent to the basement membrane of the alveolar epithelium, there is a network of elastic fibers entwining 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 anucleate areas of type 1 epithelial cells are the anucleate 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 of the alveoli.
