Lungs

The right and left lungs are located in the chest cavity, each in its own half, in the pleural sacs. Between the lungs are the organs of the mediastinum: the heart with the pericardium, the aorta and the superior vena cava, the trachea with the main bronchi, the esophagus, the thymus, the lymph nodes, etc.

The shape and structure of the lungs. The shape of the lung resembles a cone with a flattened medial side and a rounded apex. The right lung is about 25-27 cm long and 12-14 cm wide. It is shorter than the left lung by about 2-3 cm and narrower by 3-4 cm, which is due to the higher location of the right dome of the diaphragm compared to the left.

The lung (pulmo) has an apex (apex pulmonis), a base (basis pulmonis) and 3 surfaces: diaphragmatic, costal and mediastinal. The diaphragmatic surface (facies diaphragmatiса) corresponds to the base of the lung, it is concave, facing the diaphragm. The costal surface (facies costalis) is convex, adjacent to the inner surface of the chest wall - to the ribs and intercostal spaces. The vertebral (back) part (pars vertebralis) of this surface is rounded and borders the spine. The mediastinal (mediastinal) part (pars mediastinalis) of the lung faces the mediastinum. The surfaces of the lung are separated by edges. The anterior edge of the lung (margo anterior) separates the costal and medial surfaces, the lower edge (margo inferior) separates the costal and medial surfaces from the diaphragmatic. On the anterior edge of the left lung there is a depression - the cardiac notch (incisura cardiaca), limited below by the tongue of the left lung (lingula pulmonis sinistri).

Each lung is divided into large sections, called lobes, by means of deep fissures. The right lung has 3 lobes: the upper (lobus superior), the middle (lobus medius), and the lower (lobus inferior). The left lung has 2 lobes: the upper and the lower. Both lungs have an oblique fissure (fissura obliqua). This fissure begins at the posterior edge of the lung, 6-7 cm below its apex (the level of the spinous process of the third thoracic vertebra), and goes forward and down to the anterior edge of the organ at the level of the transition of the bony part of the sixth rib into its cartilage. Then the oblique fissure passes to the medial surface and is directed toward the gates of the lung. The oblique fissure in both lungs separates the upper lobe from the lower. The right lung has a horizontal fissure (fissura horizontalis pulmonis dextri). It begins on the costal surface approximately in the middle of the oblique fissure, where it intersects the mid-axillary line. Next, the horizontal fissure first goes transversely to the anterior edge, then turns to the gates of the right lung (along the medial surface). The horizontal fissure separates the middle lobe from the upper. The middle lobe of the right lung is visible only from the front and from the medial side. Between the lobes of each lung are their interlobar surfaces (facies interlobares)

The medial surface of each lung has a depression - the hilum of the lung (hillum pulmonis), through which the vessels, nerves and main bronchus pass, forming the root of the lung (radix pulmonis). In the hilum of the right lung, in the direction from top to bottom, is the main bronchus, below is the pulmonary artery, under which lie two pulmonary veins. In the hilum of the left lung, at the top is the pulmonary artery, below it is the main bronchus, and even lower are two pulmonary veins. The hilum of the right lung is somewhat shorter and wider than that of the left.

In the area of the gates, the right main bronchus (bronchus principalis dexter) divides into 3 lobar bronchi: the right superior lobar bronchus (bronchus lobaris superior dexter), the middle lobar bronchus (bronchus lobaris medius dexter), and the inferior lobar bronchus (bronchus lobaris inferior dexter). When entering the upper lobe of the right lung, the superior lobar bronchus is located above the lobar artery (a branch of the pulmonary artery), i.e. it is located epiarterially, and in the other lobes of the right and left lungs, the lobar bronchus passes under the lobar artery (hypoarterially).

The left main bronchus (bronchus principalis sinister) at the hilum of the lung divides into two lobar bronchi: the left superior lobar bronchus (bronchus lobaris superior sinister) and the left inferior lobar bronchus (bronchus lobaris inferior sinister). The lobar bronchi give rise to smaller segmental (tertiary) bronchi, which further divide dichotomously.

The segmental bronchus (bronchus segmentalis) is part of a segment that is a section of the lung with its base facing its surface and its apex facing the root. In the center of the segment are the segmental bronchus and segmental artery. At the border between adjacent segments, in the connective tissue, there is a segmental vein. Segmental bronchi are divided into subsegmental, then lobular.

The lobular bronchus (bronchus lobularis) enters the lobule of the lung, the number of which in one lung is approximately 80 or more. Each lobule is shaped like a pyramid with a polygonal base measuring 5-15 mm. The length of the lobule reaches 20-25 mm. The apex of each lobule faces the inside of the lung, and the base faces its surface covered with pleura. The lobular bronchus, entering the lobule from the side of its apex, divides into 12-20 terminal bronchioles (bronchioli terminates), the number of which in both lungs reaches 20,000. The terminal bronchioles and the respiratory bronchioles (bronchioli respiratorii) formed by their branching no longer have cartilage in their walls.

The structure of the bronchi has common features throughout the bronchial tree (to the terminal bronchioles). The walls of the bronchi are formed by a mucous membrane with a submucosa, outside of which there are fibrocartilaginous and adventitial membranes.

The mucous membrane of the bronchi is lined with ciliated epithelium. The thickness of the epithelial cover decreases as the caliber of the bronchi decreases as a result of a change in the shape of the cells from high prismatic to low cubic. In the walls of small-caliber bronchi, the epithelium is bilayered, then single-row. Among the epithelial cells (in addition to ciliated), there are goblet cells, endocrinocytes, basal cells (similar to the cells of the trachea walls). In the distal parts of the bronchial tree, among the epithelial cells, there are secretory Clara cells that produce enzymes that break down surfactant. The proper plate of the mucous membrane contains a significant number of longitudinal elastic fibers. These fibers help stretch the bronchi during inhalation and return to their original position during exhalation. In the thickness of the proper plate of the mucous membrane, there is lymphoid tissue (lymphoid cells), vessels and nerves. The relative thickness of the muscular plate of the mucous membrane (in relation to the bronchial wall) increases from large to small bronchi. The presence of oblique and circular bundles of smooth muscle cells of the muscular plate contributes to the formation of longitudinal folds of the bronchial mucosa. These folds are present only in large bronchi (5-15 mm in diameter). In the submucosa of the bronchi, in addition to vessels, nerves, and lymphoid tissue, there are secretory sections of numerous mucous-protein glands. Glands are absent only in small-caliber bronchi (diameter less than 2 mm).

The fibrocartilaginous membrane changes its character as the diameter of the bronchi decreases. The main bronchi contain unclosed cartilaginous rings. The walls of the lobar, segmental, and subsegmental bronchi contain cartilaginous plates. A lobular bronchus with a diameter of 1 mm contains only individual small plates of cartilaginous tissue. Bronchi of a smaller caliber (bronchioles) do not have cartilaginous elements in their walls. The outer adventitial membrane of the bronchi is constructed of fibrous connective tissue, which passes into the interlobular connective tissue of the lung parenchyma.

In addition to the bronchial tree (bronchi of different diameters), the lungs also include the alveolar tree, which has not only air-conducting functions, but also respiratory functions.

The alveolar tree, or pulmonary acinus, is the structural and functional unit of the lung. Each lung contains up to 150,000 acini. The acinus is a branching system of one terminal bronchiole. The terminal bronchiole is subdivided into 11-16 respiratory bronchioles of the first order, which are dichotomously divided into respiratory bronchioles of the second order, and the latter are also dichotomously divided into respiratory bronchioles of the third order.

The length of one respiratory bronchiole is 0.5-1 mm, the diameter is 0.15-0.5 mm. Respiratory bronchioles received their name due to the fact that on their thin walls (25-45 μm) there are single alveoli. Respiratory bronchioles are divided into alveolar ducts (ductuli alveolares), ending in alveolar sacs (sacculi alveolares). The diameter of the alveolar ducts and alveolar sacs in an adult is 200-600 μm (in children - 150-400 μm). The length of the alveolar ducts and sacs is 0.7-1 m. The alveolar ducts and sacs have protrusions in their walls - bubbles - alveoli of the lung (alveoli pulmonis). There are approximately 20 alveoli per alveolar duct. The diameter of one alveolus is 200-300 µm, and its surface area is on average 1 mm ². The total number of alveoli in both lungs reaches 600-700 million. The total surface area of the alveoli varies from 40 m ² during exhalation to 120 m ² during inhalation.

The acinus has a complex structure. The respiratory bronchioles are lined with cuboidal epithelium, which contains non-ciliated epithelial cells. The underlying layer of smooth myocytes is very thin and discontinuous. The alveolar ducts are lined with squamous epithelium. The entrance to each alveolus from the alveolar duct is surrounded by thin bundles of smooth myocytes. The alveoli are lined with two types of cells: respiratory (squamous) and large (granular) alveolocytes, located on a continuous basal membrane. Macrophages are also found in the alveolar epithelial lining. Respiratory alveolocytes are the main part of the alveolar wall structure. These cells are 0.1-0.2 μm thick and have a slightly convex nucleus, as well as numerous micropinocytic vesicles, ribosomes, and other poorly developed organelles. Gas exchange occurs through respiratory alveolocytes. Large alveolocytes are located in groups of 2-3 cells. These are large cells with a large round nucleus and well-developed organelles. The apical surface of large alveolocytes contains microvilli. Large alveolocytes are the source of restoration of the cellular lining of the alveoli; they actively participate in the formation of surfactant.

Surfactant is a complex of substances of protein-carbohydrate-lipid nature. Surfactant is located on the inner surface of the alveoli and prevents the collapse and adhesion of the alveoli during exhalation, maintains the surface tension of the alveoli. Surfactant has bactericidal properties.

The air-blood (aerohematic) barrier formed by thin (90-95 nm) respiratory alveolocytes, the basement membrane of the alveolocytes merging with the basement membrane of the blood capillaries, a thin (20-30 nm) layer of endothelial cells through which gas exchange occurs, is very thin (0.2-0.5 μm). The thickness of the total basement membrane is 90-100 nm. The capillaries form a dense hemocapillary network around the alveoli. Each capillary borders on one or more alveoli. Oxygen passes from the lumen of the alveolus through the air-blood barrier into the lumen of the blood capillary during diffusion, and CO2 _passes in the opposite direction. In addition to gas exchange, the lungs perform other functions. This is the regulation of acid-base balance, the production of immunoglobulins by plasma cells, the release of immunoglobulins into the lumen of the airways, etc.

Topography of the lungs (projection onto the chest wall). The right and left lungs are each located in their own half of the chest cavity, and their topography is largely the same. However, there are differences in the location of the anterior edge of the lungs and their lower border due to the presence of nearby organs (the heart turned to the left, a higher right dome of the diaphragm). In this regard, the skeletotopy of the right and left lungs is not the same. The apex of the right lung in front is 2 cm above the clavicle, 3-4 cm above the 1st rib. Behind, the apex of the right lung is projected at the level of the spinous process of the 7th cervical vertebra. The anterior border of the right lung from the apex goes to the right sternoclavicular joint, then passes through the middle of the junction of the manubrium and the body of the sternum. The anterior edge of the right lung goes down behind the sternum (slightly to the left of the midline) to the level of the cartilage of the 4th rib, passing into the lower border of the lung. The lower border of the right lung along the midclavicular line is at the level of the 6th rib, along the anterior axillary line - at the level of the 7th rib, along the middle axillary line - the 8th, along the posterior axillary line - the 9th rib, along the scapular line - the 10th rib, along the paravertebral line - at the level of the neck of the 11th rib. At the level of the 11th rib, the lower border of the right lung turns upward and passes into the posterior border, which rises to the head of the 2nd rib.

The apex of the left lung also protrudes above the clavicle by 2 cm. From the apex, the anterior border (edge) of the left lung goes to the left sternoclavicular joint, then behind the body of the sternum to the level of the cartilage of the 4th rib. Then the anterior border of the left lung deviates to the left, goes along the lower edge of the cartilage of the 4th rib to another line near the chest, turns sharply down to the cartilage of the 6th rib, where it sharply passes to the left into the lower border of the lung. The lower border of the left lung passes approximately half a rib lower than that of the right lung. Along the paravertebral line, the lower border of the left lung passes into its posterior border, which passes upward along the spine. The posterior borders of the left and right lungs coincide.

Blood supply to the lungs

The blood vessels of the lungs are classified as the small and large circles of blood circulation.

The pulmonary vessels (a. et v. pulmonales) make up the pulmonary circulation and perform mainly the function of gas exchange between blood and air, while the system of bronchial vessels (a. et v. bronchiales) provides nutrition to the lungs and belongs to the systemic circulation.

The pulmonary arteries, branching off from the pulmonary trunk, carry venous blood to the lungs. The pulmonary trunk is located entirely intrapericardial. Its length is 4-6 cm, diameter - 3.5 cm. The right pulmonary artery in its direction and size is like a continuation of the pulmonary trunk, which is of practical importance in selective angiopulmonography, as well as in the case of emboli carried into it.

The division point of the pulmonary trunk is located below the bifurcation of the trachea by 1.5-2 cm. Having entered the lungs through the root, the pulmonary arteries are divided into lobar and segmental branches and repeat the branches of the bronchi, located next to them. Respiratory bronchioles are accompanied by arterioles. Precapillary arterioles are wider than those of the systemic circle and create little resistance to blood flow.

From the capillaries, blood is collected in postcapillaries, venules and veins, which, unlike arteries, are located between the lobules. Intrasegmental branches of the pulmonary veins, which are not constant in caliber and length, flow into intersegmental veins, each of which collects blood from two adjacent segments. The veins unite into large trunks (two from each lung), flowing into the left atrium.

Bronchial arteries, 2 to 4 in number, originate from the thoracic aorta, go to the roots of the lungs and, giving branches to the pleura, branch together with the bronchi, reaching the level of the bronchioles. The branches of the bronchial arteries are located in the peribronchial connective tissue and the adventitia of the bronchi. Smaller branches, forming a capillary network, reach the proper plate of the mucous membrane of the bronchial wall. From the capillaries, the blood passes into small veins, some of which flow into the pulmonary venous system, the other part (from the large bronchi) - into the bronchial veins, draining into the azygos (hemizygos) vein. Between the branches of the pulmonary and bronchial arteries and veins there are anastomoses, the function of which is regulated by the occlusive arteries.

Innervation of the lungs and bronchi

According to modern concepts, the innervation of the lungs is carried out by nerve branches separating from the vagus nerve, nodes of the sympathetic trunk, bronchial and pulmonary branches, and the phrenic nerve, which form the pulmonary plexus in the gates of the lungs, which is divided into anterior and posterior. The branches of the anterior and posterior plexuses form peribronchial and perivasal plexuses in the lungs, which enter the lung segments, implementing afferent (sensory) and efferent (motor) innervation, with the effect of parasympathetic innervation on the bronchi being more pronounced than sympathetic. Between the aortic arch, the bifurcation of the pulmonary trunk and the trachea there is a reflexogenic zone - the deep extracardiac nerve plexus. Here, in the adventitia of the bifurcation of the pulmonary trunk, there is a permanent nerve ganglion, and in front - the superficial extracardiac nerve plexus.

The nerves form plexuses at the hilum of the lungs, anastomosing with the plexuses of the trachea and heart. The presence of connections between the nerves of the lungs and heart partly explains the reflex cardiac arrest during manipulations in the area of the root of the lung.

The nerve trunks that form a plexus at the gates of the lungs send out small branches that form a finely looped nerve plexus on the walls of the large bronchi and pulmonary vessels, continuing along the walls of the bronchi to the smallest sections of the bronchial tree. The connections formed between the nerve branches form a peribronchial nerve plexus, individual branches of which penetrate into the thickness of the bronchial wall, forming an intrabronchial plexus. Along their course, small clusters of nerve cells are encountered.

The walls of the pulmonary vessels are the site of origin of afferent impulses that have a regulatory effect on respiration and circulation.

Afferent fibers originate from "irritation receptors" in the mucous membrane of the larynx, trachea, and bronchi, and from stretch receptors in the alveolar walls. "Irritation receptors" involved in the cough reflex are found between cells in the integumentary epithelium of the respiratory tract. A significant portion of the afferent fibers in the vagus nerve are directed to the sensory cells of the nodose ganglion, another portion to the stellate ganglion, the lower cervical and upper thoracic ganglia, and sometimes to the caudally located spinal ganglia.

Efferent vagal fibers originate mainly from the cells of the dorsal nuclei in the medulla oblongata. In the bronchial plexuses, they are replaced by short postganglionic fibers that carry impulses to the muscles and glands of the trachea, bronchi and bronchioles, as well as to the vessels. Vagal innervation is related to cholinergic innervation and causes contraction of the smooth muscles of the respiratory tract, secretion of glands and dilation of blood vessels.

Efferent sympathetic fibers originate in the spinal cord at the level of the I-II to V-VI thoracic segment. Fibers innervating the larynx and upper trachea switch to postganglionic fibers in the superior cervical sympathetic ganglion. Fibers carrying impulses to the caudal trachea, bronchi, and bronchioles switch to the superior thoracic ganglia of the border sympathetic trunk. They are directed to the pulmonary plexuses and are adrenergic. Stimulation of the sympathetic nerve causes relaxation of the bronchial and bronchiole muscles, inhibition of gland secretion, and vasoconstriction.

The innervation of the lungs is under the control of the hypothalamus and the cerebral cortex, which ensures the integration of respiration and the functions of other organs, as well as dual (automatic and voluntary) regulation of respiration.

Lymphatic vascular network of the lungs

The lymphatic vessels of the lungs are divided into superficial and deep. The superficial ones form a large- and small-mesh network in the thickness of the pleura, anastomosing with the deep vessels located in the connective tissue layers between the lobules, subsegments, segments, and in the walls of the bronchi. The deep lymphatic network of the lung consists of capillaries, the finest vessels located around the alveoli, respiratory and terminal bronchi, as well as lymphatic vessels accompanying the bronchi and large blood vessels. The alveoli are devoid of lymphatic capillaries. The beginning of the lymphatic system are the lymphatic capillaries in the interalveolar spaces. From the intraorgan networks, outflow lymphatic collectors are formed, which accompany the bronchi and go to the gates of the lung.

There are several groups of bronchopulmonary lymph nodes on the path of lymph outflow to the roots of the lungs. They are located along the course and mainly in the places of branching of the bronchi. Near the main bronchi and trachea, there are lower tracheobronchial, upper right and left tracheobronchial, right and left tracheal (paratracheal) lymph nodes.

According to modern concepts, bifurcation lymph nodes are the main regional nodes for the lower lobes of both lungs. The bulk of bifurcation nodes (in 52.8% of cases) are located under the right main bronchus. In this regard, it is advisable to puncture the bifurcation nodes through the inner wall of the right main bronchus, retreating 5-6 mm from the carina, since almost always the bifurcation lymph node is located 2/3 of its size under the right bronchus, and 1/3 - directly under the carina.

The outflow of lymph into the left tracheobronchial lymph nodes is carried out from the left bronchopulmonary (root) and bifurcation nodes, from the left lung and trachea, esophagus. In most cases, the outflow of lymph from these nodes is directed directly into the thoracic duct, in 1/3 of cases - to the upper right tracheobronchial lymph nodes, and then - into the thoracic duct.

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