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Lecture notes, cheat sheets
Histology. The cardiovascular system
Directory / Lecture notes, cheat sheets Table of contents (expand) Topic 19. CARDIOVASCULAR SYSTEM The heart, blood vessels, and lymphatics together make up the cardiovascular system. Thanks to it, the tissues and organs of the human body are provided with nutrients and biologically active substances, gases, metabolic products and thermal energy. Blood vessels These are tubes of various diameters closed in the form of a ring, which carry out a transport function, as well as establishing blood supply to organs and metabolism between blood and surrounding tissues. In the circulatory system, arteries, arterioles, hemocapillaries, venules, veins and arteriolo-venular anastomoses are isolated. Vessels of small caliber in total make up the microvasculature. Development of blood vessels - angiogenesis Angiogenesis is the process of formation and growth of blood vessels. It occurs both under normal conditions (for example, in the area of the ovarian follicle after ovulation), and in pathological conditions (during wound healing, tumor growth, during immune responses, observed in neovascular glaucoma, rheumatoid arthritis and other pathological conditions). Cells need oxygen and nutrients to survive. The minimum distance for effective gas diffusion from a blood vessel (source of oxygen) to a cell is 100 - 200 µm. If this value is exceeded, new blood vessels are formed. Angiogenesis causes low pO2, decrease in pH, hypoglycemia, mechanical stress in the tissue due to cell proliferation, tissue infiltration with immunocompetent or inflammation-supporting cells, mutations (for example, activation of oncogenes or deletion of tumor suppressor genes that control the formation of angiogenic factors). Angiogenic factors These factors stimulate the formation of blood vessels. These are growth factors produced by tumors, components of the extracellular matrix, angiogenic factors produced by endothelial cells themselves. Angiogenesis is stimulated by vascular endothelial growth factor (VEGF), angiogenin, fibroblast growth factors (aFGF - acidic and bFGF - alkaline), transforming growth factor (TGFa). All angiogenic factors can be divided into two groups: the first - directly acting on endothelial cells and stimulating their mitosis and motility, and the second - factors of indirect influence that act on macrophages, which, in turn, release growth factors and cytokines. The factors of the second group include, in particular, angiogenin. In response to the action of the angiogenic factor, endothelial cells begin to multiply and change their phenotype. The proliferative activity of cells can increase 100 times. Endothelial cells through their own basement membrane penetrate into the adjacent connective tissue, participating in the formation of the capillary bud. After the end of the action of the angiogenic factor, the phenotype of endothelial cells returns to its original calm state. At later stages of angiogenesis, angiopoietin-1 is involved in vessel remodeling, and its action is also associated with a stabilizing effect on the vessel. Inhibition of angiogenesis. This process is important and can be considered as a potentially effective method of combating the development of tumors in the early stages, as well as other diseases associated with the growth of blood vessels (for example, neovascular glaucoma, rheumatoid arthritis). Angiogenesis inhibitors - factors that inhibit the proliferation of the main cell types of the vascular wall: angiostatin, endostatin, matrix metalloproteinase inhibitors - α-IFN, r-IFN, γ-IFN, IL-4, IL-12, IL-18, prolactin, plasma coagulation factor blood IV. A natural source of factors that inhibit angiogenesis are tissues that do not contain blood vessels (epithelium, cartilage). Malignant tumors require an intensive blood supply for growth and reach a noticeable size after the development of a blood supply system in them. Active angiogenesis occurs in tumors associated with the synthesis and secretion of angiogenic factors by tumor cells. Types of blood vessels and their structure Arteries are the vessels that carry blood from the heart to the organs. As a rule, this blood is saturated with oxygen, with the exception of the pulmonary artery systems, which carry venous blood. The venous vessels include the vessels through which blood goes to the heart and contains little oxygen, except for the blood in the pulmonary veins. Through the microcirculation vessels (arterioles, hemocapillaries, venules and arteriolo-venular anastomoses) there is an exchange between tissues and blood. Hemocapillaries connect the arterial link of the circulatory system with the venous one, in addition to networks, the capillaries of which are located either between two arteries (for example, in the glomeruli of the kidney) or between two veins (for example, in the lobules of the liver). The structure of the vessel determines its function, as well as hemodynamic parameters of the blood (blood pressure, blood flow velocity). All arteries are divided into three types: elastic, muscular and mixed (muscular-elastic). The wall of all arteries and veins consists of three shells: internal, middle and external. Their thickness, tissue composition and functional features are not the same in vessels of different types. The elastic type arteries include large-caliber vessels (aorta and pulmonary artery): blood flows into them under high pressure (120 - 130 mm Hg) and at high speed (0,5 - 1,3 m / s) or directly from the heart, or near it from the aortic arch. The main function of these vessels is transport. High pressure and high speed of flowing blood determine the structure of the walls of the vessels of the elastic type. Thus, the inner shell of large arteries includes the endothelium with a basement membrane, followed by the subendothelial layer and the plexus of elastic fibers. The human endothelium consists of cells of various shapes and sizes. Along the entire length of the vessel, the size and shape of the cells are not the same: sometimes the cells can sometimes reach 500 microns in length and 150 microns in width. As a rule, they are single-core, but there are also multi-core ones. The subendothelial layer is represented by loose, thin-fibrillar connective tissue rich in poorly differentiated stellate cells. The thickness of the subendothelial layer is significant. Occasionally, individual longitudinally directed smooth muscle cells may be seen. The intercellular substance of the inner membrane of a large vessel, or less often other membranes, contains a large amount of glycosaminoglycans and phospholipids, which are detected with appropriate processing. At the same time, it is known that cholesterol and fatty acids are found in people older than 40-50 years. Of great importance in the trophism of the vessel wall is an amorphous substance. The middle shell of a large vessel consists of a large number of elastic fenestrated membranes connected by elastic fibers. As a result, together with other shells, they form a single elastic frame. Between the membranes lie smooth muscle cells (SMC), which have an oblique direction with respect to the membranes, and a few fibroblasts. Due to this structure in large vessels, the tremors of blood ejected into the vessel during contraction of the left ventricle of the heart are softened, and the tone of the vascular wall is maintained during diastole. The outer shell consists of loose fibrous connective tissue, which has many elastic and collagen fibers with a longitudinal direction. The structure and functional features of the mixed arteries occupy an intermediate position between the vessels of the muscular and elastic types. These vessels include the carotid and subclavian arteries. Their wall also consists of an inner membrane, a subendothelial layer, and an internal elastic membrane. The middle layer of mixed arteries has the same number of smooth muscle cells, elastic fibers and fenestrated elastic membranes. And in the outer shell of the arteries, two layers are distinguished: the inner, containing separate bundles of smooth muscle cells, and the outer, consisting mainly of longitudinally and obliquely arranged bundles of collagen and elastic fibers and connective tissue cells, vessels and nerve fibers. The arteries of the muscular type mainly include the arteries of the body, limbs and internal organs of medium and small caliber, that is, most of the arteries of the body. Their distinguishing feature is a large number of smooth muscle cells, which provide additional pumping power and regulate blood flow to organs. The inner membrane consists of the endothelium, the subdentelial layer and the internal elastic membrane. From the vessels of the microvasculature, a dense network of anastomoses of precapillary, capillary and postcapillary vessels is formed, and other options are possible with the selection of a preferred channel, for example, precapillary arterioles, etc. Arterioles are small arteries of the muscular type, they gradually pass into capillaries. In the arterioles, three membranes are preserved, which are characteristic of larger arteries, but their degree of severity is small. Under an electron microscope in arterioles, especially in precapillary ones, one can detect perforations in the basement membrane of the endothelium and the internal elastic membrane, due to which there is a direct close contact between endotheliocytes and smooth muscle cells. Blood capillaries are the most numerous and thinnest vessels, but the diameter of their lumen can vary. This is due to both the organ features of the capillaries and the functional state of the vascular system. The cross-sectional area of the cut of the capillary bed in any area is many times greater than the cross-sectional area of the original artery. In the wall of the capillaries, three thin layers are distinguished as the rudiments of the three membranes of the vessels. Slits (or pores) can be found between the cells of the capillary membranes, which are visible even under a light microscope. Fenestra and crevices facilitate the penetration of various macromolecular and corpuscular substances through the capillary wall. The extensibility of the endothelium and the permeability for colloidal particles in the venous part of the capillary is higher than in the arterial part. The capillary wall is a semipermeable membrane, functionally and morphologically closely related to the surrounding connective tissue and actively regulates the metabolism between blood and other tissues. The venous part of the capillaries begins the discharge section of the microvasculature, they are characterized by larger microvilli on the luminal surface of the endothelium and folds resembling valve leaflets, fenestras are more often found in the endothelium. Blood from the capillary bed is collected in postcapillary venules. The structure of these vessels is characterized by shorter sizes of endothelial cells, roundness of the nuclei, and a pronounced outer connective tissue membrane. The venous section of the microvasculature performs a drainage function, regulating the balance between blood and extravascular fluid, removing metabolic products of tissues. Leukocytes often migrate through the walls of venules. Slow blood flow and low blood pressure, as well as the distensibility of these vessels, create conditions for the deposition of blood. Arteriovenular anastomoses are connections of vessels carrying arterial and venous blood bypassing the capillary bed. They are present in almost all organs. There are two groups of anastomoses: 1) true arteriovenular anastomoses (shunts), through which pure arterial blood is discharged; 2) atypical arteriovenular fistulas (semi-shunts), through which mixed blood flows. The external form of the first group of anastomoses can be different - in the form of straight short anastomoses, loop-like, sometimes in the form of branching connections. Histostructurally, they are divided into two subgroups: 1) vessels that do not have special locking devices; 2) vessels equipped with special contractile structures. In the second subgroup, anastomoses may have special contractile sphincters in the form of longitudinal ridges or pillows in the subendothelial layer (arteriovenular anastomoses of the trailing arteries type). The contraction of the muscle pads protruding into the lumen of the anastomosis leads to the cessation of blood flow. Simple anastomoses of the epithelioid type (second subgroup) are characterized by the presence in the middle shell of the inner longitudinal and outer circular layers of smooth muscle cells, which, as they approach the venous end, are replaced by short oval light cells, similar to epithelial cells, capable of swelling and swelling, due to which change in the lumen of the anastomosis. In the venous segment of the arteriovenular anastomosis, its wall sharply becomes thinner. The middle shell here contains only a small number of bands of circularly arranged smooth muscle cells. The outer shell consists of dense connective tissue. Arteriovenular anastomoses, especially of the glomerular type, are richly innervated, and they can periodically contract. Arteriovenular anastomoses play an important role in compensatory reactions of the body in case of circulatory disorders. The venous system is the outlet link of the blood. It begins with postcapillary venules in the vessels of the microvasculature. The structure of veins is closely related to the hemodynamic conditions of their functioning. The number of smooth muscle cells in the wall of the veins is not the same and depends on whether the blood moves in them to the heart under the influence of gravity or against it. Due to the fact that in the lower extremities the blood must be lifted against gravity, there is a strong development of smooth muscle elements in the veins of the lower extremities, in contrast to the veins of the upper extremities, head and neck. Veins, especially subcutaneous veins, have valves. The exception is the veins of the brain and its membranes, veins of internal organs, hypogastric, iliac, hollow and nameless. According to the degree of development of muscle elements in the wall of the veins, they can be divided into two groups: veins of the non-muscular type and veins of the muscular type. Muscular veins, in turn, are divided into veins with weak development of muscle elements and veins with medium and strong development of muscle elements. In veins, as well as in arteries, three membranes are distinguished: internal, middle and external. At the same time, the degree of expression of these membranes in the veins differs significantly. Veins of the non-muscular type are veins of the dura mater, pia mater, veins of the retina, bones, spleen, and placenta. Under the action of blood, these veins are capable of stretching, but the blood accumulated in them flows relatively easily under the influence of its own gravity into larger venous trunks. Veins of the muscular type are distinguished by the development of muscle elements in them. These veins include the veins of the lower body. Also, in some types of veins there are a large number of valves, which prevents the reverse flow of blood, under the force of its own gravity. In addition, the rhythmic contractions of the circularly arranged muscle bundles also contribute to the movement of blood towards the heart. In addition, a significant role in the promotion of blood towards the heart belongs to the contractions of the skeletal muscles of the lower extremities. Lymphatic vessels The lymphatic vessels drain lymph into the veins. Lymphatic vessels include lymphatic capillaries, intra- and extraorganic lymphatic vessels that drain lymph from organs, and lymphatic trunks of the body, which include the thoracic duct and the right lymphatic duct, which flow into the large veins of the neck. Lymphatic capillaries are the beginning of the lymphatic system of vessels, into which metabolic products come from tissues, and in pathological cases - foreign particles and microorganisms. It has also long been proven that cells of malignant tumors can also spread through the lymphatic vessels. Lymphatic capillaries are a system of closed and anastomosing with each other and penetrating the entire body. The diameter of the lymphatic capillaries may be larger than the blood capillaries. The wall of the lymphatic capillaries is represented by endothelial cells, which, unlike similar cells in the blood capillaries, do not have a basement membrane. Cell borders are tortuous. The endothelial tube of the lymphatic capillary is closely connected with the surrounding connective tissue. In the lymphatic vessels that bring the lymphatic fluid to the heart, a distinctive feature of the structure is the presence of valves in them and a well-developed outer shell. This can be explained by the similarity of the lympho- and hemodynamic conditions for the functioning of these vessels: the presence of low pressure and the direction of the fluid flow from the organs to the heart. According to the size of the diameter, all lymphatic vessels are divided into small, medium and large. Like veins, these vessels can be non-muscular and muscular in their structure. Small vessels are mainly intraorganic lymphatic vessels, there are no muscle elements in them, and their endothelial tube is surrounded only by a connective tissue membrane. Medium and large lymphatic vessels have three well-developed membranes - internal, middle and external. In the inner shell, covered with endothelium, there are longitudinally and obliquely directed bundles of collagen and elastic fibers. There are valves on the inner lining of the vessels. They consist of a central connective tissue plate covered with endothelium on the inner and outer surfaces. The boundary between the inner and middle membranes of the lymphatic vessel is not always clearly defined internal elastic membrane. The middle membrane of the lymphatic vessels is poorly developed in the vessels of the head, upper body and upper limbs. In the lymphatic vessels of the lower extremities, on the contrary, it is expressed very clearly. In the wall of these vessels there are bundles of smooth muscle cells that have a circular and oblique direction. The muscular layer of the wall of the lymphatic vessel reaches good development in the collectors of the iliac lymphatic plexus, near the aortic lymphatic vessels and the cervical lymphatic trunks accompanying the jugular veins. The outer shell of the lymphatic vessels is formed by loose, fibrous, unformed connective tissue, which, without sharp boundaries, passes into the surrounding connective tissue. Vascularization. All large and medium-sized blood vessels have their own system for their nutrition, which is called "vascular vessels". These vessels are necessary to feed the very wall of a large vessel. In the arteries, the vessels of the vessels penetrate to the deep layers of the middle shell. The inner lining of the arteries receives nutrients directly from the blood flowing in this artery. Protein-mucopolysaccharide complexes, which are part of the main substance of the walls of these vessels, play an important role in the diffusion of nutrients through the inner lining of the arteries. The innervation of the vessels is obtained from the autonomic nervous system. The nerve fibers of this part of the nervous system, as a rule, accompany the vessels and end in their wall. By structure, vascular nerves are either myelinated or unmyelinated. Sensory nerve endings in capillaries are diverse in shape. Arteriovenular anastomoses have complex receptors located simultaneously on the anastomosis, arteriole and venule. The terminal branches of nerve fibers end on smooth muscle cells with small thickenings - neuromuscular synapses. Effectors on arteries and veins are of the same type. Along the vessels, especially large ones, there are individual nerve cells and small ganglia of a sympathetic nature. Regeneration. Blood and lymphatic vessels have a high ability to recover both after injuries and after various pathological processes occurring in the body. Recovery of defects in the vascular wall after its damage begins with the regeneration and growth of its endothelium. Already after 1-2 days, a massive amitotic division of endothelial cells is observed at the site of the former injury, and on the 3rd-4th day, a mitotic type of reproduction of endothelial cells appears. The muscle bundles of the damaged vessel, as a rule, recover more slowly and incompletely compared to other tissue elements of the vessel. In terms of the rate of recovery, the lymphatic vessels are somewhat inferior to the blood vessels. Vascular afferents pO changes2, рСО2 blood, the concentration of H+, lactic acid, pyruvate and a number of other metabolites have both a local effect on the vascular wall and are recorded by chemoreceptors embedded in the vascular wall, as well as baroreceptors that respond to pressure in the lumen of the vessels. These signals reach the centers of regulation of blood circulation and respiration. The responses of the central nervous system are realized by motor autonomic innervation of the smooth muscle cells of the vascular wall and myocardium. In addition, there is a powerful system of humoral regulators of smooth muscle cells of the vascular wall (vasoconstrictors and vasodilators) and endothelial permeability. Baroreceptors are especially numerous in the aortic arch and in the wall of large veins close to the heart. These nerve endings are formed by the terminals of the fibers passing through the vagus nerve. The reflex regulation of blood circulation involves the carotid sinus and carotid body, as well as similar formations of the aortic arch, pulmonary trunk, and right subclavian artery. Structure and function of the carotid sinus. The carotid sinus is located near the bifurcation of the common carotid artery. This is an expansion of the lumen of the internal carotid artery immediately at the place of its branch from the common carotid artery. In the area of expansion, the middle shell is thinned, while the outer one, on the contrary, is thickened. Here, in the outer shell, there are numerous baroreceptors. Considering that the median sheath of the vessel within the carotid sinus is relatively thin, it is easy to imagine that the nerve endings in the outer sheath are highly sensitive to any changes in blood pressure. From here, information enters the centers that regulate the activity of the cardiovascular system. The nerve endings of the baroreceptors of the carotid sinus are the terminals of the fibers passing through the sinus nerve, a branch of the glossopharyngeal nerve. carotid body. The carotid body responds to changes in the chemical composition of the blood. The body is located in the wall of the internal carotid artery and consists of cell clusters immersed in a dense network of wide sinusoid-like capillaries. Each glomerulus of the carotid body (glomus) contains 2–3 glomus cells (or type I cells), and 1–3 type II cells are located on the periphery of the glomerulus. Afferent fibers for the carotid body contain substance P and peptides related to the calcitonin gene. Type I cells form synaptic contacts with afferent fiber terminals. Type I cells are characterized by an abundance of mitochondria, light, and electron-dense synaptic vesicles. Type I cells synthesize acetylcholine, contain an enzyme for the synthesis of this neurotransmitter (choline acetyltransferase), as well as an efficient choline uptake system. The physiological role of acetylcholine remains unclear. Type I cells have H- and M-cholinergic receptors. Activation of any of these types of cholinergic receptors causes or facilitates the release of another neurotransmitter, dopamine, from type I cells. With a decrease in pO2 secretion of dopamine from type I cells increases. Type I cells can form synapse-like contacts with each other. Efferent innervation On the glomus cells, the fibers that pass as part of the sinus nerve (Hering) and the postganglionic fibers from the superior cervical sympathetic ganglion end. The terminals of these fibers contain light (acetylcholine) or granular (catecholamines) synaptic vesicles. Function The carotid body registers changes in pCO2 and ro2, as well as shifts in blood pH. Excitation is transmitted through synapses to afferent nerve fibers, through which impulses enter the centers that regulate the activity of the heart and blood vessels. Afferent fibers from the carotid body pass through the vagus and sinus nerves (Hering). The main cell types of the vascular wall Smooth muscle cell. The lumen of the blood vessels decreases with contraction of the smooth muscle cells of the middle membrane or increases with their relaxation, which changes the blood supply to the organs and the magnitude of blood pressure. Vascular smooth muscle cells have processes that form numerous gap junctions with neighboring SMCs. Such cells are electrically coupled, through the contacts, excitation (ionic current) is transmitted from cell to cell. This circumstance is important, since only MMCs located in the outer layers of t are in contact with motor terminals. media. SMC walls of blood vessels (especially arterioles) have receptors for various humoral factors. Vasoconstrictors and vasodilators. The effect of vasoconstriction is realized by the interaction of agonists with α-adrenergic receptors, serotonin receptors, angiotensin II, vasopressin, thromboxane. Stimulation of α-adrenergic receptors leads to contraction of vascular smooth muscle cells. Norepinephrine is predominantly an α-adrenergic receptor antagonist. Adrenaline is an antagonist of α- and β-adrenergic receptors. If the vessel has smooth muscle cells with a predominance of α-adrenergic receptors, then adrenaline causes a narrowing of the lumen of such vessels. Vasodilators. If α-adrenergic receptors predominate in the SMC, then adrenaline causes the expansion of the lumen of the vessel. Antagonists that in most cases cause relaxation of the SMC: atriopeptin, bradykinin, VIP, histamine, peptides related to the calcitonin gene, prostaglandins, nitric oxide NO. Motor autonomic innervation. The autonomic nervous system regulates the size of the lumen of the vessels. Adrenergic innervation is regarded as predominantly vasoconstrictor. Vasoconstrictive sympathetic fibers abundantly innervate small arteries and arterioles of the skin, skeletal muscles, kidneys and celiac region. The density of innervation of the veins of the same name is much less. The vasoconstrictor effect is realized with the help of norepinephrine, an antagonist of α-adrenergic receptors. cholinergic innervation. Parasympathetic cholinergic fibers innervate the vessels of the external genitalia. With sexual arousal, due to the activation of parasympathetic cholinergic innervation, there is a pronounced expansion of the vessels of the genital organs and an increase in blood flow in them. The cholinergic vasodilating effect has also been observed in relation to the small arteries of the pia mater. Proliferation The size of the SMC population of the vascular wall is controlled by growth factors and cytokines. Thus, cytokines of macrophages and B-lymphocytes (transforming growth factor IL-1) inhibit the proliferation of SMCs. This problem is important in atherosclerosis, when SMC proliferation is enhanced by growth factors produced in the vascular wall (platelet growth factor [PDGF], alkaline fibroblast growth factor, insulin-like growth factor 1 [IGF-1], and tumor necrosis factor). Phenotypes of MMC There are two variants of SMC of the vascular wall: contractile and synthetic. Contractile phenotype. SMCs have numerous myofilaments and respond to vasoconstrictors and vasodilators. The granular endoplasmic reticulum in them is expressed moderately. Such SMCs are not capable of migration and do not enter mitoses, since they are insensitive to the effects of growth factors. synthetic phenotype. SMCs have a well-developed granular endoplasmic reticulum and the Golgi complex, cells synthesize components of the intercellular substance (collagen, elastin, proteoglycan), cytokines and factors. SMCs in the area of atherosclerotic lesions of the vascular wall are reprogrammed from a contractile to a synthetic phenotype. In atherosclerosis, SMCs produce growth factors (eg, platelet-derived factor PDGF), alkaline fibroblast growth factor [bFGF], which enhance the proliferation of neighboring SMCs. Regulation of the SMC phenotype. The endothelium produces and secretes heparin-like substances that maintain the contractile phenotype of SMC. Paracrine regulatory factors produced by endothelial cells control vascular tone. Among them are derivatives of arachidonic acid (prostaglandins, leukotrienes and thromboxanes), endothelin-1, nitric oxide NO, etc. Some of them cause vasodilation (for example, prostacyclin, nitric oxide NO), others cause vasoconstriction (for example, endothelin-1, angiotensin -II). Insufficiency of NO causes an increase in blood pressure, the formation of atherosclerotic plaques, an excess of NO can lead to collapse. endothelial cell The wall of a blood vessel reacts very subtly to changes in hemodynamics and chemical composition of the blood. A peculiar sensitive element that captures these changes is the endothelial cell, which on the one hand is washed by the blood, and on the other hand is turned to the structures of the vascular wall. Restoration of blood flow in thrombosis. The effect of ligands (ADP and serotonin, thrombin thrombin) on the endothelial cell stimulates the secretion of NO. His targets are located nearby the MMC. As a result of relaxation of the smooth muscle cell, the lumen of the vessel in the area of the thrombus increases, and blood flow can be restored. Activation of other endothelial cell receptors leads to a similar effect: histamine, M-cholinergic receptors, α2-adrenergic receptors. Blood clotting. The endothelial cell is an important component of the hemocoagulation process. On the surface of endothelial cells, prothrombin can be activated by clotting factors. On the other hand, the endothelial cell exhibits anticoagulant properties. The direct participation of the endothelium in blood coagulation is the secretion of certain plasma coagulation factors (for example, von Willebrand factor) by endothelial cells. Under normal conditions, the endothelium weakly interacts with blood cells, as well as with blood coagulation factors. The endothelial cell produces prostacyclin PGI2, which inhibits platelet adhesion. growth factors and cytokines. Endothelial cells synthesize and secrete growth factors and cytokines that influence the behavior of other cells in the vascular wall. This aspect is important in the mechanism of atherosclerosis development, when, in response to the pathological effects of platelets, macrophages, and SMCs, endothelial cells produce platelet-derived growth factor (PDGF), alkaline fibroblast growth factor (bFGF), and insulin-like growth factor-1 (IGF-1). ), IL-1, transforming growth factor. On the other hand, endothelial cells are targets for growth factors and cytokines. For example, endothelial cell mitosis is induced by alkaline fibroblast growth factor (bFGF), while endothelial cell proliferation is stimulated by platelet-derived endothelial cell growth factor. Cytokines from macrophages and B-lymphocytes - transforming growth factor (TGFp), IL-1 and α-IFN - inhibit the proliferation of endothelial cells. hormone processing. The endothelium is involved in the modification of hormones and other biologically active substances circulating in the blood. So, in the endothelium of the vessels of the lungs, angiotensin-I is converted to angiotensin-II. Inactivation of biologically active substances. Endothelial cells metabolize norepinephrine, serotonin, bradykinin, prostaglandins. Breakdown of lipoproteins. In endothelial cells, lipoproteins are broken down to form triglycerides and cholesterol. Homing of lymphocytes. Venules in the paracortical zone of the lymph nodes, tonsils, Peyer's patches of the ileum, containing an accumulation of lymphocytes, have a high endothelium that expresses on its surface a vascular addressin, recognizable by the CD44 molecule of lymphocytes circulating in the blood. In these areas, lymphocytes attach to the endothelium and are removed from the bloodstream (homing). barrier function. The endothelium controls the permeability of the vascular wall. This function is most clearly manifested in the blood-brain and hematothymic barriers. Heart Development The heart is laid on the 3rd week of intrauterine development. In the mesenchyme, between the endoderm and the visceral layer of the splanchiotoma, two endocardial tubes lined with endothelium are formed. These tubes are the rudiment of the endocardium. The tubes grow and are surrounded by a visceral splanchiotome. These areas of the splanchiotome thicken and give rise to myoepicardial plates. As the intestinal tube closes, both anlages approach and grow together. Now the common bookmark of the heart (heart tube) looks like a two-layer tube. The endocardium develops from its endocardial part, and the myocardium and epicardium develop from the myoepicardial plate. Cells migrating from the neural crest are involved in the formation of the efferent vessels and valves of the heart (neural crest defects are the cause of 10% of congenital heart defects, such as transposition of the aorta and pulmonary trunk). Within 24 - 26 days, the primary heart tube quickly lengthens and acquires an s-shape. This is possible due to local changes in the shape of the cells of the heart tube. At this stage, the following sections of the heart are distinguished: the venous sinus is a chamber at the caudal end of the heart, large veins flow into it. Cranial to the venous sinus is an expanded part of the heart tube, which forms the region of the atrium. From the middle curved part of the heart tube develops the ventricle of the heart. The ventricular loop bends caudally, which moves the future ventricle, which was cranial to the atrium, to the definitive position. The area of narrowing of the ventricle and its transition to the arterial trunk is a cone. An opening is visible between the atrium and the ventricle - the atrioventricular canal. Division into right and left heart. Immediately after the formation of the atrium and ventricle, there are signs of the division of the heart into the right and left halves, which occurs at the 5th and 6th weeks. At this stage, the interventricular septum, interatrial septum and endocardial cushions are formed. The interventricular septum grows from the wall of the primary ventricle in the direction from the apex to the atrium. Simultaneously with the formation of the interventricular septum in the narrowed part of the heart tube between the atrium and the ventricle, two large masses of loosely organized tissue are formed - endocardial pads. Endocardial cushions, consisting of dense connective tissue, are involved in the formation of the right and left atrioventricular canals. At the end of the 4th week of intrauterine development, a median septum in the form of a semicircular fold appears on the cranial wall of the atrium - the primary interatrial septum. One arc of the fold runs along the ventral wall of the atria, and the other along the dorsal. The arcs merge near the atrioventricular canal, but the primary interatrial opening remains between them. Simultaneously with these changes, the venous sinus moves to the right and opens into the atrium to the right of the atrial septum. In this place, venous valves are formed. Complete division of the heart. Complete separation of the heart occurs after the development of the lungs and their vasculature. When the primary septum fuses with the endocardial cushions of the atrioventricular valve, the primary atrial opening closes. Mass death of cells in the cranial part of the primary septum leads to the formation of many small holes that form the secondary interatrial opening. It controls the even flow of blood to both halves of the heart. Soon, a secondary atrial septum forms between the venous valves and the primary atrial septum in the right atrium. Its concave edge is directed upwards to the confluence of the sinus, and later - the inferior vena cava. A secondary opening is formed - an oval window. The remnants of the primary atrial septum, which close the foramen ovale in the secondary atrial septum, form a valve that distributes blood between the atria. Direction of blood flow Since the outlet of the inferior vena cava lies near the foramen ovale, blood from the inferior vena cava enters the left atrium. When the left atrium contracts, blood presses the cusp of the primary septum against the foramen ovale. As a result, blood does not flow from the right atrium to the left, but moves from the left atrium to the left ventricle. The primary septum functions as a one-way valve in the foramen ovale of the secondary septum. Blood enters from the inferior vena cava through the foramen ovale into the left atrium. Blood from the inferior vena cava mixes with blood entering the right atrium from the superior vena cava. Fetal blood supply. Oxygenated placental blood with a relatively low concentration of CO2 enters the liver through the umbilical vein, and from the liver into the inferior vena cava. Part of the blood from the umbilical vein through the venous duct, bypassing the liver, immediately enters the system of the inferior vena cava. In the inferior vena cava, the blood is mixed. CO high blood2 enters the right atrium from the superior vena cava, which collects blood from the upper body. Through the foramen ovale, part of the blood flows from the right atrium to the left. With atrial contraction, the valve closes the foramen ovale, and blood from the left atrium enters the left ventricle and then into the aorta, that is, into the systemic circulation. From the right ventricle, blood is directed to the pulmonary trunk, which is connected with the aorta by an arterial or botallic duct. Consequently, small and large circles of blood circulation are communicated through the ductus arteriosus. In the early stages of fetal development, the need for blood in the immature lungs is still small, blood from the right ventricle enters the pool of the pulmonary artery. Therefore, the level of development of the right ventricle will be determined by the level of development of the lung. As the lungs develop and their volume increases, more and more blood is sent to them and less passes through the ductus arteriosus. The ductus arteriosus closes shortly after birth as the lungs take all the blood from the right heart. After birth, they cease to function and are reduced, turning into connective tissue cords and other vessels - the umbilical cord, the venous duct. The foramen ovale also closes shortly after birth. The heart is the main organ that moves blood through the blood vessels, a kind of "pump". The heart is a hollow organ consisting of two atria and two ventricles. Its wall consists of three membranes: internal (endocardium), middle, or muscular (myocardium) and external, or serous (epicardium). The inner shell of the heart - the endocardium - from the inside covers all the chambers of the heart, as well as the valves of the heart. In different areas, its thickness is different. It reaches its largest size in the left chambers of the heart, especially on the interventricular septum and at the mouth of large arterial trunks - the aorta and pulmonary artery. While on tendon threads it is much thinner. The endocardium is made up of several types of cells. So, on the side facing the cavity of the heart, the endocardium is lined with endothelium, consisting of polygonal cells. Next comes the subendothelial layer, formed by a connective tissue rich in poorly differentiated cells. Muscles are located deeper. The deepest layer of the endocardium, lying on the border with the myocardium, is called the outer connective tissue layer. It consists of connective tissue containing thick elastic fibers. In addition to elastic fibers, the endocardium contains long, tortuous collagen and reticular fibers. The nutrition of the endocardium is carried out mainly diffusely due to the blood in the chambers of the heart. Next comes the muscle layer of cells - the myocardium (its properties were described in the chapter on muscle tissue). Myocardial muscle fibers are attached to the supporting skeleton of the heart, which is formed by fibrous rings between the atria and ventricles and dense connective tissue at the mouths of large vessels. The outer shell of the heart, or epicardium, is a visceral sheet of the pericardium, similar in structure to the serous membranes. Between the pericardium and the epicardium there is a slit-like cavity, in which there is a small amount of fluid, due to which, when the heart contracts, the friction force decreases. Valves are located between the atria and ventricles of the heart, as well as the ventricles and large vessels. However, they have specific names. So, the atrioventricular (atrioventricular) valve in the left half of the heart is bicuspid (mitral), in the right - tricuspid. They are thin plates of dense fibrous connective tissue covered with endothelium with a small number of cells. In the subendothelial layer of the valves, thin collagen fibrils were found, which gradually pass into the fibrous plate of the valve leaflet, and at the site of attachment of the bi- and tricuspid valves - into the fibrous rings. A large amount of glycosaminoglycans was found in the ground substance of the valve leaflets. In this case, you need to know that the structure of the atrial and ventricular sides of the valve leaflets is not the same. So, the atrial side of the valve, smooth from the surface, has a dense plexus of elastic fibers and bundles of smooth muscle cells in the subendothelial layer. The number of muscle bundles markedly increases at the base of the valve. The ventricular side is uneven, equipped with outgrowths from which tendon filaments begin. Elastic fibers in a small amount are located on the ventricular side only directly under the endothelium. There are also valves on the border between the ascending aortic arch and the left ventricle of the heart (aortic valves), between the right ventricle and the pulmonary trunk there are semilunar valves (so named because of the specific structure). On a vertical section in the leaflet of the valve, three layers can be distinguished - inner, middle and outer. The inner layer, facing the ventricle of the heart, is a continuation of the endocardium. In it, under the endothelium, elastic fibers run longitudinally and transversely, followed by a mixed elastic-collagen layer. The middle layer is thin, consists of loose fibrous connective tissue rich in cellular elements. The outer layer, facing the aorta, contains collagen fibers that originate from the annulus fibrosus around the aorta. The heart receives nutrients from the system of coronary arteries. Blood from the capillaries is collected in the coronary veins, which flow into the right atrium, or venous sinus. Lymphatic vessels in the epicardium accompany the blood vessels. Innervation. Several nerve plexuses and small nerve ganglia are found in the membranes of the heart. Among the receptors, there are both free and encapsulated endings located in the connective tissue, on muscle cells and in the wall of the coronary vessels. The bodies of sensory neurons lie in the spinal nodes (C7 - Th6), and their axons, covered with a myelin sheath, enter the medulla oblongata. There is also an intracardiac conduction system - the so-called autonomous conduction system, which generates impulses to contract the heart. Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu. << Back: Nervous system >> Forward: Endocrine system
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