Histological types of capillaries table. Histology lectures (cardiovascular system)

Material taken from the site www.hystology.ru

The blood vessels are a closed system of branched tubes of different diameters, which are part of the large and pulmonary circulation. In this system, they are distinguished: arteries through which blood flows from the heart to organs and tissues, veins - through them, blood returns to the heart, and a complex of vessels of the microvasculature, which, along with the transport function, provide the exchange of substances between blood and surrounding tissues.

Blood vessels develop from the mesenchyme. In embryogenesis, the most early period characterized by the appearance of numerous cell clusters of the mesenchyme in the wall of the yolk mark - blood islets. Inside the islet, blood cells are formed and a cavity is formed, and the cells located along the periphery become flat, interconnected with the help of cell contacts and form the endothelial lining of the formed tubule. These primary blood tubes, as they are formed, interconnect and form a capillary network. The surrounding mesenchymal cells are converted into pericytes, smooth muscle cells, and adventitia cells. In the body of the embryo, blood capillaries are laid from the cells of the mesenchyme around the slit-like spaces filled with tissue fluid. When blood flow increases through the vessels, these cells become endothelial, and elements of the middle and outer membranes are formed from the surrounding mesenchyme.

The vascular system is very flexible. First of all, there is a significant variability in the density of the vascular network, since, depending on the needs of the organ for nutrients and oxygen, the amount of blood brought to it varies widely. Change in blood flow rate and blood pressure leads to the formation of new vessels and the restructuring of existing vessels. There is a transformation of a small vessel into a larger one with the characteristic features of the structure of its wall. The greatest changes occur in the vascular system during the development of roundabout, or collateral, blood circulation.

Arteries and veins are built according to a single plan - three shells are distinguished in their walls: inner (tunica intima), middle (tunica media) and outer (tunica adventicia). However, the degree of development of these membranes, their thickness and tissue composition are closely related to the function performed by the vessel and hemodynamic conditions (height of blood pressure and blood flow velocity), which are not the same in different parts of the vascular bed.

Arteries... According to the structure of the walls, arteries of muscular, muscular-elastic and elastic types are distinguished.

TO elastic arteriesinclude the aorta and pulmonary artery. In accordance with the high hydrostatic pressure (up to 200 mm Hg) created by the pumping activity of the ventricles of the heart, and the high blood flow velocity (0.5 - 1 m / s), these vessels have pronounced elastic properties, which ensure the strength of the wall when it is stretched and return to the starting position, and also contribute to the transformation of the pulsating blood flow into a constant continuous one. The wall of the elastic type arteries is distinguished by a significant thickness and the presence of a large number of elastic elements in the composition of all membranes.

The inner shell consists of two layers - endothelial and subendothelial. Endothelial cells, which form a continuous inner lining, have different sizes and shapes, and contain one or more nuclei. Their cytoplasm contains few organelles and many microfilaments. The basement membrane is located under the endothelium. The podendothelial layer consists of loose, fine-fibrous connective tissue, which, along with a network of elastic fibers, contains poorly differentiated stellate cells, macrophages, smooth muscle cells. The amorphous substance of this layer, which is of great importance for the wall nutrition, contains a significant amount of glycosaminoglycans. walls and the development of the pathological process (atherosclerosis) lipids (cholesterol and its esters) accumulate in the subendothelial layer. Cellular elements of the subendothelial layer play an important role in the regeneration of the wall. At the border with the middle membrane is a dense network of elastic fibers.

The middle shell consists of numerous elastic fenestrated membranes, between which are obliquely oriented bundles of smooth muscle cells. Through the windows (fenestra) of the membranes, the intramural transport of substances necessary for the nutrition of the wall cells is carried out. Both the membranes and the cells of smooth muscle tissue are surrounded by a network of elastic fibers, which together with the fibers of the inner and outer shells form a single frame, which ensures high elasticity of the wall.

The outer shell is formed by connective tissue, which is dominated by bundles of collagen fibers oriented longitudinally. In this shell, vessels are located and branched, providing nutrition to both the outer shell and the outer zones of the middle shell.

Muscular arteries... Arteries of this type of different caliber include most of the arteries that deliver and regulate blood flow to various parts and organs of the body (brachial, femoral, splenic, etc.) - When microscopic examination in the wall, the elements of all three membranes are clearly distinguishable (Fig. 202).

The inner membrane consists of three layers: endothelial, subendothelial and internal elastic membrane. The endothelium looks like a thin plate consisting of cells elongated along the vessel with oval nuclei protruding into the lumen. The podendothelial layer is more developed in large diameter arteries and consists of stellate or fusiform cells, thin elastic fibers, and an amorphous substance containing glycosaminoglycans. At the border with the middle membrane lies an internal elastic membrane, clearly visible on the preparations in the form of a shiny wavy strip, stained with eosin in a light pink color.

Figure: 202.

Diagram of the structure of the artery wall (AND)and veins (B)muscle type:
1 - inner shell; 2 - middle shell; 3 - outer shell; and - endothelium; b - inner elastic membrane; in - nuclei of cells of smooth muscle tissue in the middle shell; r - nuclei of cells of connective tissue of adventitia; d - vessels of vessels.

This membrane is permeated with numerous holes that are important for the transport of substances.

The middle shell is built predominantly of smooth muscle tissue, the bundles of cells of which go in a spiral, but when the position of the arterial wall (stretching) changes, the location of the muscle cells can change. The contraction of the muscle tissue of the midline is important in regulating blood flow to organs and tissues in accordance. with their needs and maintaining blood pressure. A network of elastic fibers is located between the bundles of muscle tissue cells, which, together with the elastic fibers of the subendothelial layer and the outer shell, form a single elastic frame, which gives the wall elasticity when it is squeezed. On the border with the outer sheath in large arteries of the muscular type there is an external elastic membrane, consisting of a dense plexus of longitudinally oriented elastic fibers. In smaller arteries, this membrane is not expressed.

The outer sheath consists of connective tissue in which collagen fibers and elastic fiber networks are extended longitudinally. Cells, mainly fibrocytes, are located between the fibers. The outer sheath contains nerve fibers and small blood vessels that feed the outer layers of the artery wall.

Muscular-elastic type arteriesaccording to the structure of the walls, they occupy an intermediate position between the elastic and muscular arteries. In the middle shell, spirally oriented smooth muscle tissue, elastic plates and a network of elastic fibers are developed in equal numbers.

Figure: 203. Scheme of the vessels of the microvasculature:

1 - arteriole; 2 - venula; 3 - capillary network; 4 - arterio-venular anastomosis.

Vessels of the microcirculatory bed... A dense network of small precapillary, capillary and postcapillary vessels is formed at the site of the transition of the arterial bed to the venous one in organs and tissues. This complex of small vessels, which provides blood circulation in organs, transvascular metabolism and tissue homeostasis, is referred to as the microvasculature. It includes various arterioles, capillaries, venules and arterio-venular anastomoses (Fig. 203).

Arterioles... As the diameter decreases in the arteries of the muscle type, all membranes become thinner and they pass into arterioles - vessels with a diameter of less than 100 microns. Their inner shell consists of the endothelium, located on the basement membrane, and individual cells of the subendothelial layer. Some arterioles may have a very thin internal elastic membrane. In the middle shell, one row of spirally located cells of smooth muscle tissue is preserved. In the wall of the terminal arterioles, from which the capillaries branch off, smooth muscle cells do not form a continuous row, but are scattered. These are precapillary arterioles. However, in the place of branching from the arteriole, the capillary is surrounded by a significant number of smooth muscle cells, which form a kind of precapillary sphincter. Due to a change in the tone of such sphincters, blood flow in the capillaries of the corresponding tissue or organ is regulated. There are elastic fibers between the muscle cells. The outer shell contains individual adventitia cells and collagen fibers.

Capillaries- the most important elements of the microvasculature, in which the exchange of gases and various substances between the blood and surrounding tissues is carried out. In most organs, branching capillary networks are formed between arterioles and venules, located in loose connective tissue. The density of the capillary network in different organs can be different. The more intensive the metabolism in the organ, the denser the network of its capillaries. The most developed network of capillaries in the gray matter of the organs of the nervous system, in the organs of internal secretion, the myocardium of the heart, around the pulmonary alveoli. In skeletal muscles, tendons, nerve trunks, capillary networks are oriented longitudinally.

The capillary network is constantly in a state of restructuring. In organs and tissues, a significant number of capillaries do not function. In their greatly reduced cavity

Figure: 204. Diagram of the ultrastructural organization of the blood capillary wall with continuous endothelial lining:

1 - endothelial cells; 2 - basement membrane; 3 - pericyte; 4 - pinocytic microbubbles; 5 - the zone of contact between endothelial cells (Fig. Kozlov).

only blood plasma (plasma capillaries) circulates. The number of open capillaries increases with the intensification of the organ.

Capillary networks are also found between vessels of the same name, for example, venous capillary networks in the lobules of the liver, adenohypophysis, arterial - in the renal glomeruli. In addition to the formation of branched networks, capillaries can have the form of a capillary loop (in the papillary dermis) or form glomeruli (vascular glomeruli of the kidneys).

Capillaries are the narrowest vascular tubes. Their caliber, on average, corresponds to the diameter of an erythrocyte (7 - 8 microns), however, depending on the functional state and organ specialization, the diameter of the capillaries can be different. Narrow capillaries (4 - 5 microns in diameter) in the myocardium. Special sinusoidal capillaries with a wide lumen (30 microns or more) in the lobules of the liver, spleen, red bone marrow, organs of internal secretion.

The wall of blood capillaries consists of several structural elements... The inner lining is formed by a layer of endothelial cells located on the basement membrane, which contains cells - pericytes. Around the basement membrane are adventitious cells and reticular fibers (Fig. 204).

Squamous endothelial cells are elongated along the length of the capillary and have very thin (less than 0.1 μm) peripheral anucleated areas. Therefore, with light microscopy of the cross section of the vessel, only the region of the location of the nucleus with a thickness of 3 - 5 microns is distinguishable. The nuclei of endothelial cells are often oval in shape, contain condensed chromatin, concentrated near the nuclear envelope, which, as a rule, has irregular contours. In the cytoplasm, the bulk of the organelles are located in the perinuclear region. The inner surface of endothelial cells is uneven, the plasmolemma forms microvilli, protrusions and valve-like structures of various shapes and heights. The latter are especially characteristic of the venous capillaries. Numerous pinocytic vesicles are located along the inner and outer surfaces of endothelial cells, indicating an intensive absorption and transfer of substances through the cytoplasm of these cells. Endothelial cells, due to their ability to quickly swell and then, releasing fluid, decrease in height, can change the size of the capillary lumen, which, in turn, affects the passage of blood cells through it. In addition, electron microscopy revealed microfilaments in the cytoplasm that determine the contractile properties of endothelial cells.

The basement membrane located under the endothelium is revealed by electron microscopy and is a plate 30 - 35 nm thick, consisting of a network of thin fibrils containing type IV collagen and an amorphous component. In the latter, along with proteins, hyaluronic acid is contained, the polymerized or depolymerized state of which determines the selective permeability of the capillaries. The basement membrane also provides elasticity and strength to the capillaries. In the splits of the basement membrane, there are special process cells - pericytes. They cover the capillary with their processes and, penetrating through the basement membrane, form contacts with endothelial cells.

In accordance with the structural features of the endothelial lining and basement membrane, three types of capillaries are distinguished. Most of the capillaries in organs and tissues belong to the first type (general type capillaries). They are characterized by the presence of a continuous endothelial lining and basement membrane. In this continuous layer, the plasmolemmas of neighboring endothelial cells are as close as possible and form compounds according to the type of tight contact, which is impermeable to macromolecules. There are other types of contacts, when the edges of adjacent cells overlap each other like tiles or are connected by jagged surfaces. The narrower (5-7 microns) proximal (arteriolar) and wider (8-10 microns) distal (venular) parts are distinguished along the length of the capillaries. In the cavity of the proximal part, the hydrostatic pressure is greater than the colloid-osmotic pressure created by the proteins in the blood. As a result, the liquid is filtered behind the wall. In the distal part, the hydrostatic pressure becomes less than the colloid-osmotic pressure, which causes the transfer of water and substances dissolved in it from the surrounding tissue fluid into the blood. However, the output fluid flow is greater than the input one, and excess fluid enters the lymphatic system as a component of the connective tissue tissue fluid.

In some organs, in which the processes of absorption and excretion of fluid are intensively occurring, as well as the rapid transport of macromolecular substances into the blood, the endothelium of the capillaries has rounded submicroscopic holes with a diameter of 60 - 80 nm or rounded areas covered by a thin diaphragm (kidneys, organs of internal secretion). These are capillaries with fenestrae (lat. Fenestrae - windows).

Capillaries of the third type are sinusoidal, characterized by a large diameter of their lumen, the presence of wide gaps between the endothelial cells and an intermittent basement membrane. Capillaries of this type are found in the spleen, red bone marrow. Through their walls, not only macromolecules penetrate, but also blood cells.

Venules- the outlet section of the microvasculature and the initial link of the venous section of the vascular system. They collect blood from the capillary bed. The diameter of their lumen is wider than in the capillaries (15 - 50 microns). In the wall of venules, as well as in capillaries, there is a layer of endothelial cells located on the basement membrane, as well as a more pronounced outer connective tissue membrane. In the walls of the chenules, passing into small veins, there are individual smooth muscle cells. In the postcapillary venules of the thymus and lymph nodes, the endothelial lining is represented by high endothelial cells, which contribute to the selective migration of lymphocytes during their recirculation. Due to the thinness of their walls, slow blood flow ai low blood pressure, a significant amount of blood can be deposited in venules.

Arterio-venular anastomoses... In all organs, tubules were found through which blood from the arterioles can be directed directly to the venules, bypassing the capillary network. There are especially many anastomoses in the dermis of the skin, in auricle, the ridge of birds, where they play a role in thermoregulation.

In terms of structure, true arteriolovenular anastomoses (shunts) are characterized by the presence in the wall of a significant number of longitudinally oriented bundles of smooth muscle cells located either in the subendothelial layer of the intima (Fig. 205), or in the inner zone of the middle shell. In some anastomoses, these cells acquire an epithelial-like appearance. Longitudinally located muscle cells are also found in the outer shell. There are not only simple

Figure: 205. Arterio-venular anastomosis:

1 - endothelium; 2 - longitudinally located epithelioid-muscle cells; 3 - circularly located muscle cells of the middle membrane; 4 - outer shell.

anastomoses in the form of single tubes, but also complex, consisting of several branches extending from one arteriole and surrounded by a common connective tissue capsule.

With the help of contractile mechanisms, anastomoses can reduce or completely close their lumen, as a result of which the blood flow through them stops and blood enters the capillary network. Thanks to this, the organs receive blood in. depending on the need associated with their work. In addition, high arterial blood pressure is transmitted through the anastomoses into the venous bed, thus facilitating better blood flow in the veins. The role of anastomoses in the enrichment of venous blood with oxygen is significant, as well as in the regulation of blood circulation during the development of pathological processes in organs.

Veins - blood vessels through which blood from organs and tissues flows to the heart, to the right atrium. The exception is the pulmonary veins, which direct oxygen-rich blood from the lungs to the left atrium.

The wall of the veins, like the wall of the arteries, consists of three sheaths: inner, middle and outer. However, specific histological structure of these membranes in different veins is very diverse, which is due to the difference in their functioning and local (in accordance with the localization of the vein) conditions of blood circulation. Most veins of the same diameter with similar arteries have a thinner wall and a wider lumen.

In accordance with the hemodynamic conditions - low blood pressure (15 - 20 mm Hg) and low blood flow velocity (about 10 mm / s) - elastic elements are relatively poorly developed in the vein wall and there is less muscle tissue in the middle membrane. These signs determine the possibility of changing the configuration of the veins: with low blood volume, the walls of the veins become collapsed, and if the outflow of blood is difficult (for example, due to blockage), the wall is easily stretched and the veins expanded.

Essential in the hemodynamics of venous vessels: they have valves located in such a way that, passing blood towards the heart, they block the path of its reverse flow. The number of valves is greater in veins in which blood flows in the opposite direction to gravity (for example, in the veins of the extremities).

According to the degree of development in the wall of the muscular elements, veins of the muscleless and muscular types are distinguished.

Muscular type veins... The characteristic veins of this type are the veins of the bones, central veins hepatic lobules and trabecular veins of the spleen. The wall of these veins consists only of a layer of endothelial cells located on the basement membrane and an outer thin layer of fibrous connective tissue. With the participation of the latter, the wall grows tightly with the surrounding tissues, as a result of which these veins are passive in the movement of blood through them and do not collapse. The muscleless veins of the meninges and retina of the eye, being filled with blood, can easily stretch, but at the same time, the blood, under the influence of its own gravity, easily flows into the larger venous trunks.

Muscle type veins... The wall of these veins, like the wall of the arteries, consists of three membranes, but the boundaries between them are less distinct. The thickness of the muscular membrane in the wall of veins of different localization is not the same, which depends on whether the blood moves in them under the influence of gravity or against it. Based on this, the veins of the muscular type are subdivided into veins with weak, medium and strong muscle development. The veins of the first variety include horizontally located veins of the upper body of the body and veins of the digestive tract. The walls of such veins are thin, in their middle shell, smooth muscle tissue does not form a continuous layer, but is located in bundles, between which there are layers of loose connective tissue.

Veins with a strong development of muscle elements include large veins of the extremities of animals, through which blood flows upward, against the force of gravity (femoral, shoulder, etc.). They are characterized by longitudinally located small bundles of cells of smooth muscle tissue in the subendothelial layer of the intima and well-developed bundles of this tissue in the outer shell. Contraction of the smooth muscle tissue of the outer and inner membranes leads to the formation of transverse folds in the vein wall, which prevents reverse blood flow.

The middle membrane contains circularly located bundles of smooth muscle cells, the contractions of which contribute to the movement of blood to the heart. In the veins of the extremities there are valves, which are thin folds formed by the endothelium and the subendothelial layer. The basis of the valve is fibrous connective tissue, which at the base of the valve leaflets may contain a number of smooth muscle cells. The valves also prevent venous blood from flowing back. For the movement of blood in the veins, the suction effect of the chest during inhalation and the contraction of the skeletal muscle tissue surrounding the venous vessels are essential.

Vascularization and innervation of blood vessels.The wall of large and medium arterial vessels is nourished both from the outside - through the vessels of the vessels (vasa vasorum), and from the inside - due to the blood flowing inside the vessel. Vascular vessels are branches of thin perivascular arteries that run in the surrounding connective tissue. In the outer shell of the vessel wall, arterial branches branch, capillaries penetrate into the middle, the blood from which is collected in the venous vessels of the vessels. The intima and the inner zone of the middle membrane of the arteries do not have capillaries and are fed from the side of the vascular lumen. Due to the significantly lower strength pulse wave, a smaller thickness of the middle shell, the absence of an internal elastic membrane, the mechanism of vein feeding from the side of the cavity is not of particular importance. In the veins, the vessels of the vessels supply arterial blood to all three membranes.

Constriction and expansion of blood vessels, maintenance of vascular tone occur mainly under the influence of impulses coming from the vasomotor center. Impulses from the center are transmitted to the lateral horn cells spinal cord, from where they go to the vessels along the sympathetic nerve fibers. The terminal branches of the sympathetic fibers, which include the axons of the nerve cells of the sympathetic ganglia, form motor nerve endings on the cells of smooth muscle tissue. The efferent sympathetic innervation of the vascular wall is responsible for the main vasoconstrictor effect. The question of the nature of vasodilators has not been finally resolved.

It has been established that parasympathetic nerve fibers are vasodilators in relation to the vessels of the head.

In all three membranes of the vessel wall, the terminal branching of the dendrites of nerve cells, mainly of the spinal ganglia, form numerous sensitive nerve endings. In adventitia and perivascular loose connective tissue, encapsulated bodies are also found among the various free endings. Specialized interoreceptors are of particular physiological importance, which perceive changes in blood pressure and its chemical composition, concentrated in the wall of the aortic arch and in the branching area. carotid artery internal and external - aortic and carotid reflex zones... It has been established that, in addition to these zones, there are a sufficient number of other vascular territories sensitive to changes in blood pressure and chemical composition (baro- and chemoreceptors). From the receptors of all specialized territories, impulses along the centripetal nerves reach the vasomotor center of the medulla oblongata, causing the corresponding compensatory neuroreflex reaction.

The cardiovascular system.

The cardiovascular system includes the heart, blood vessels and lymph vessels. The heart and blood vessels ensure the movement of blood through the body, with which nutrients and biologically active substances, oxygen, heat energy are delivered and metabolic products are excreted.

The heart is the main organ that drives blood. Blood vessels carry out a transport function, regulation of blood supply to organs and metabolism between blood and surrounding tissues.

The vascular system is a complex of tubules of different diameters. The activity of the vascular apparatus is regulated by the nervous system and hormones. The vessels do not form such a dense network in the body that could provide direct communication with each cell. Nutrients and oxygen are brought to the majority of cells with tissue fluid, into which they enter with blood plasma by percolating it through the walls of the capillaries. This fluid carries away metabolic products from the cells and, flowing from the tissues, moves first between the cells and then is absorbed into the lymphatic capillaries. Thus, the vascular system is divided into two parts: circulatory and lymphatic.

In addition, the hematopoietic organs are associated with the cardiovascular system, which simultaneously perform protective functions.

Development of the vascular system.

The first blood vessels appear in the mesenchyme of the walls of the yolk sac at the 2nd - 3rd week of embryogenesis. From the peripheral cells of the blood islets, flat endothelial cells are formed. The surrounding mesenchymal cells are converted into pericytes, smooth muscle cells, and adventitia cells. In the body of the embryo, blood capillaries are laid in the form of an irregularly shaped slit filled with tissue fluid. Their wall is the surrounding mesenchyme. When blood flow increases through the vessels, these cells become endothelial, and elements of the middle and outer membranes are formed from the surrounding mesenchyme. Then the vessels of the embryo begin to communicate with the vessels of the extraembryonic organs. Further development occurs with the beginning of blood circulation under the influence of blood pressure, the speed of blood flow, which are created in different parts of the body.

During the entire postembryonic period of life, the vascular system has great plasticity. There is a significant variability in the density of the vascular network, since, depending on the organ's need for nutrients and oxygen, the amount of blood brought in varies widely.

In connection with a change in the speed of blood movement, blood pressure, the walls of the vessels are rearranged, small vessels can turn into larger ones with characteristic features, or vice versa. At the same time, new vessels can form, and old ones can atrophy.

Especially large changes occur in the vascular system with the development of roundabout or collateral circulation. This is observed when any obstacles are encountered in the path of blood flow. New capillaries and vessels are formed, and the existing ones are transformed into vessels of a larger caliber.

If a section of an artery is cut out in a living animal and a vein is sewn in its place, the latter will be rearranged under conditions of arterial circulation and turn into an artery.

Classification and general characteristics of blood vessels.

In the blood vessel system, there are:

1) Arteries, through which blood flows to organs and tissues (rich in O 2, except for the pulmonary artery);

2) Veinsthrough which blood returns to the heart (little O 2, except for the pulmonary vein);

3) Microcirculatory bed providing, along with the transport function, the exchange of substances between blood and tissues. This channel includes not only hemocapillaries, but also the smallest arteries (arterioles), veins (venules), as well as arterio-venular anastomoses.

Hemocapillaries connect the arterial link of the circulatory system with the venous one, except for the "miraculous systems" in which capillaries are located between two vessels of the same name - arterial (in the kidneys), or venous (in the liver and pituitary gland).

Arterio-venular anastomoses provide a very rapid transition of blood from the artery to the veins. They are short vessels connecting small arteries with small veins and are capable of quickly closing their lumen. Therefore, anastomoses play an important role in regulating the amount of blood brought to the organs.

Arteries and veins are built according to a single plan. Their walls consist of three shells: 1) the inner one, built of endothelium and the elements of connective tissue located above it; 2) middle - muscular or muscular-elastic and 3) external - adventitia formed from loose connective tissue.

Arteries.

According to the structural features of the artery, there are 3 types: elastic, muscular and mixed (muscular-elastic). The classification is based on the ratio of the number of muscle cells and elastic fibers in the middle lining of the arteries.

TO elastic arteries include large-caliber vessels, such as the aorta and pulmonary artery, into which blood is infused under high pressure (120 - 130 mm Hg) and at high speed (0.5 - 1.3 m / s). These vessels perform mainly a transport function.

High pressure and high speed of flowing blood determine the structure of the elastic-type vessel wall; in particular, the presence of a large number of elastic elements (fibers, membranes) allows these vessels to stretch during heart systole and return to their original position during diastole, and also contributes to the conversion of pulsating blood flow into a constant, continuous one.

Inner shell includes the endothelium and the subendothelial layer. The aortic endothelium consists of cells of various shapes and sizes. Sometimes cells reach 500 microns in length and 150 microns in width, more often they are mononuclear, but there are also multinucleated ones (from 2 - 4 to 15 - 30 nuclei). The endothelium secretes anti-clotting substances in the blood and coagulants, participates in metabolism, secretes substances that affect blood formation.

In their cytoplasm, the endoplasmic reticulum is poorly developed, but there are a lot of microfilaments. The basement membrane is located under the endothelium.

Subendothelial layerconsists of loose fine fibrillar connective tissue, rich in poorly differentiated stellate cells, macrophages, smooth myocytes. The amorphous substance of this layer contains many glucosaminoglycans. In case of wall damage or pathology (atherosclerosis), lipids (cholesterol and ethers) accumulate in this layer.

Deeper than the subendothelial layer, in the inner shell, there is a dense plexus of thin elastic fibers.

Middle shell The aorta consists of a large number (40-50) of elastic fenestrated membranes, interconnected by elastic fibers. Smooth muscle cells with an oblique direction to them lie between the membranes. This structure of the middle membrane creates high elasticity of the aorta.

Outer sheaththe aorta is built of loose connective tissue with a large number of thick elastic and collagen fibers, which have a mainly longitudinal direction.

In the middle and outer membranes of the aorta, as well as in large vessels in general, feed vessels and nerve trunks pass.

The outer shell protects the vessel from overstretching and rupture.

To muscle-type arteriesincludes most of the arteries of the body, that is, of medium and small caliber: the arteries of the body, limbs and internal organs.

The walls of these arteries contain a relatively large number of smooth myocytes, which provides additional pumping force and regulates blood flow to the organs.

Part inner shell includes the endothelium, the podendothelial layer and the inner elastic membrane.

Endothelial cells are elongated along the axis of the vessel and have convoluted borders. The endothelial cover is followed by the basement membrane and subendothelial layer, consisting of thin elastic and collagen fibers, mainly longitudinally directed, as well as poorly differentiated connective tissue cells and an amorphous substance containing glycosaminoglycans. On the border with the middle shell lies internal elastic membrane... IN

27. Cardiovascular system

Arteriovenular anastomoses are connections of vessels carrying arterial and venous blood bypassing the capillary bed. Their presence is noted 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 (half shunts) through which mixed blood flows.

The external form of the first group of anastomoses can be different: in the form of straight short fistulas, loop-like, sometimes in the form of branching joints.

In histostructural terms, they are divided into two subgroups:

a) vessels that do not have special locking devices;

b) vessels equipped with special contractile structures.

In the second subgroup, anastomoses have special contractile sphincters in the form of longitudinal ridges or pillows in the subendothelial layer. The contraction of the muscle cushions protruding into the anastomotic lumen leads to the cessation of blood flow. Simple anastomoses of the epithelioid type 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 ones, capable of swelling and swelling, due to which the lumen of the anastomosis changes. In the venous segment of the arteriovenular anastomosis, its wall sharply becomes thinner. The outer shell consists of dense connective tissue. Arteriovenular anastomoses, especially of the glomerular type, are richly innervated.

The structure of the veins is closely related to the hemodynamic conditions of their functioning. The number of smooth muscle cells in the vein wall is not the same and depends on whether the blood in them moves to the heart under the influence of gravity or against it. According to the degree of development of muscle elements in the vein wall, they can be divided into two groups: non-muscle type veins and muscle type veins. Veins of the muscular type, in turn, are subdivided into veins with a weak development of muscle elements and veins with medium and strong development of muscle elements. In the veins (as well as in the arteries), there are three shells: internal, middle and external, while the severity of these membranes in the veins is significantly different. Veins of the non-muscular type are veins of the hard and pia mater, veins of the retina of the eye, bones, spleen and placenta. Under the influence 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 the larger venous trunks. Veins of the muscle type are distinguished by the development of muscle elements in them. These veins include the veins in the lower torso. Also, some types of veins have a large number of valves, which prevents blood from flowing back under its own gravity.

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The importance of the cardiovascular system (CCC) in the vital activity of the organism, and consequently the knowledge of all aspects of this area for practical medicine, is so great that cardiology and angiology have isolated themselves as two independent directions in the study of this system. The heart and blood vessels belong to systems that do not function periodically, but constantly, therefore, more often than other systems are prone to pathological processes. Currently, CVD diseases, along with oncological diseases, takes the leading place in mortality.

The cardiovascular system ensures the movement of blood through the body, regulates the supply of nutrients and oxygen to tissues and the removal of metabolic products, blood deposition.

Classification:

I. The central organ is the heart.

II. Peripheral department:

A. Blood vessels:

1. Arterial link:

a) elastic type arteries;

b) arteries of the muscular type;

c) mixed type arteries.

2.Microcirculatory bed:

a) arterioles;

b) hemocapillaries;

c) venules;

d) arterio-venular anastomoses

3. Venous link:

a) veins of the muscle type (with weak, medium, strong development of muscle

elements;

b) veins of the muscleless type.

B. Lymphatic vessels:

1. Lymphatic capillaries.

2. Intraorganic lymphatic vessels.

3. Extraorganic lymphatic vessels.

In the embryonic period, the first blood vessels are laid at the 2nd week in the wall of the yolk sac from the mesenchyme (see the stage of megaloblastic hematopoiesis on the topic "Hematopoiesis") - blood islets appear, the peripheral cells of the islet flatten and differentiate into the endothelial lining, and form the surrounding mesenchyme connective tissue and smooth muscle elements of the vascular wall. Soon, blood vessels are formed from the mesenchyme in the body of the embryo, which are connected to the vessels of the yolk sac.

Arterial link - represented by vessels through which blood is delivered from the heart to the organs. The term "artery" is translated as "air-containing", since upon opening, researchers often found these vessels empty (not containing blood) and thought that vital "pneuma" or air was spreading through them through the body. Arteries of elastic, muscular and mixed types have a common structural principle: 3 shells are distinguished in the wall - inner, middle and outer adventitious.

The inner shell consists of layers:

1. Endothelium on the basement membrane.

2. The podendothelial layer is a snout fibrous SDM with a high content of poorly differentiated cells.

3. Internal elastic membrane - a plexus of elastic fibers.

Middle shell contains smooth muscle cells, fibroblasts, elastic and collagen fibers. At the border of the middle and outer adventitia membrane, there is an external elastic membrane - a plexus of elastic fibers.

Outer adventitia arteries histologically presented

loose fibrous SDT with vascular vessels and vascular nerves.

Features in the structure of the varieties of arteries are due to differences in the hemadynamic conditions of their functioning. Differences in structure mainly relate to the middle shell (different ratios of the constituent elements of the shell):

1. Arteries of the elastic type - these include the aortic arch, pulmonary trunk, thoracic and abdominal aorta. Blood enters these vessels in jerks under high pressure and moves at high speed; there is a large pressure drop during the transition from systole to diastole. The main difference from other types of arteries is in the structure of the middle shell: in the middle shell of the above components (myocytes, fibroblasts, collagen and elastic fibers), elastic fibers predominate. Elastic fibers are located not only in the form of individual fibers and plexuses, but form elastic fenestrated membranes (in adults, the number of elastic membranes reaches 50-70 words). Due to the increased elasticity, the wall of these arteries not only withstands high pressure, but also smoothes out large drops (surges) in pressure during systole-diastole transitions.

2. Arteries of muscle type - these include all arteries of medium and small caliber. A feature of the hemodynamic conditions in these vessels is a drop in pressure and a decrease in blood flow velocity. Muscular arteries differ from other types of arteries by the predominance of myocytes in the middle membrane over other structural components; the inner and outer elastic membrane is clearly expressed. Myocytes in relation to the lumen of the vessel are oriented spirally and are found even in the outer shell of these arteries. Due to the powerful muscular component of the middle membrane, these arteries control the intensity of blood flow of individual organs, maintain the falling pressure and push the blood further, therefore, muscle-type arteries are also called "peripheral heart".

3. Arteries of mixed type - these include large arteries extending from the aorta (carotid and subclavian arteries). In terms of structure and function, they occupy an intermediate position. The main feature in the structure: in the middle membrane, myocytes and elastic fibers are represented approximately the same (1: 1), there is a small amount of collagen fibers and fibroblasts.

Microcirculatory bed- a link located between the arterial and venous links; provides regulation of blood circulation in the organ, metabolism between blood and tissues, blood deposition in organs.

Composition:

1. Arterioles (including precapillary).

2. Hemocapillaries.

3. Venules (including postcapillary).

4. Arterio-venular anastomoses.

Arterioles- vessels connecting arteries with hemocapillaries. They preserve the principle of the structure of the arteries: they have 3 membranes, but the membranes are weakly expressed - the subendothelial layer of the inner membrane is very thin; the middle membrane is represented by one layer of myocytes, and closer to the capillaries - by single myocytes. As the diameter in the middle membrane increases, the number of myocytes increases, first one, then two or more layers of myocytes are formed. Due to the presence of myocytes in the wall (in the precapillary arterioles in the form of a sphincter), arterioles regulate the blood supply of the hemocapillaries, thereby the exchange rate between the blood and the tissues of the organ.

Hemocapillaries... The wall of hemocapillaries has the smallest thickness and consists of 3 components - endotheliocytes, basement membrane, pericytes in the thickness of the basement membrane. There are no muscle elements in the capillary wall, however, the diameter of the inner lumen may change somewhat as a result of changes in blood pressure, the ability of the nuclei of pericytes and endothelial cells to swell and contract. The following types of capillaries are distinguished:

1. Type I hemocapillaries (somatic type) - capillaries with continuous endothelium and continuous basement membrane, diameter 4-7 microns. Found in skeletal muscles, skin and mucous membranes ..

2. Hemocapillaries of type II (fenestrated or visceral type) - the basement membrane is solid, in the endothelium there are fenestra - thinned areas in the cytoplasm of endothelial cells. Diameter 8-12 microns. They are present in the capillary glomeruli of the kidney, in the intestines, in the endocrine glands.

3. Type III hemocapillaries (sinusoidal type) - the basement membrane is not continuous, in places it is absent, and there are gaps between the endotheliocytes; diameter 20-30 microns and more, not constant throughout - there are widened and narrowed areas. The blood flow in these capillaries is slowed down. Available in the liver, hematopoietic organs, endocrine glands.

Around the hemocapillaries there is a thin layer of loose fibrous SDM with a high content of poorly differentiated cells, the state of which determines the intensity of exchange between blood and working tissues of the organ. The barrier between the blood in the hemocapillaries and the surrounding working tissue of the organ is called the histohematogenous barrier, which consists of endothelial cells and the basement membrane.

Capillaries can change their structure, rearrange themselves into vessels of a different type and caliber; new branches can form from existing hemocapillaries.

Precapillaries are different from hemocapillaries the fact that in the wall, in addition to endotheliocytes, basement membrane, pericytes, there are single or groups of myocytes.

Venules begin with postcapillary venules, which differ from capillaries by a high content of pericytes in the wall and by the presence of valve-like folds of endothelial cells. As the diameter of the venules in the wall increases, the content of myocytes increases - first single cells, then groups, and finally solid layers.

Arterio-venular anastomoses (AVA)- these are shunts (or fistulas) between arterioles and venules, i.e. carry out direct communication and participate in the regulation of regional peripheral blood flow. They are especially abundant in the skin and kidneys. AVA - short vessels, also have 3 membranes;there are myocytes, especially in the middle membrane, which play the role of a sphincter.

VIENNA.A feature of hemodynamic conditions in the veins is low pressure (15-20 mm Hg) and low blood flow rate, which leads to a lower content of elastic fibers in these vessels. There are valves in the veins - duplication of the inner shell. The number of muscle elements in the wall of these vessels depends on whether the blood moves under the influence of gravity or against it.

Muscular type veins are found in the dura mater, bones, retina, placenta, in the red bone marrow. The wall of non-muscular veins is lined with endotheliocytes on the basement membrane from the inside, followed by an interlayer of fibrous SDT; there are no smooth muscle cells.

Veins of the muscular type with weakly expressed muscular elements are located in the upper half of the body - in the superior vena cava system. These veins are usually collapsed. In the middle membrane, they have a small number of myocytes.

Veins with highly developed muscle elements make up the vein system of the lower half of the body. A feature of these veins is well-defined valves and the presence of myocytes in all three membranes - in the outer and inner membranes in the longitudinal direction, in the middle - in the circular direction.

LYMPHATIC VESSELS begin with lymphatic capillaries (LC). LK, unlike hemocapillaries, start blindly and have a larger diameter. The inner surface is lined with endothelium, the basement membrane is absent. Under the endothelium, there is a loose fibrous SDM with a high content of reticular fibers.

The diameter of the LC is not constant - there are contractions and expansions. Lymphatic capillaries merge to form intraorgan lymphatic vessels - in structure they are similar to veins, because are in the same hemodynamic conditions. They have 3 shells, the inner shell forms the valves; unlike the veins under the endothelium, the basement membrane is absent. The diameter is not constant throughout - there are expansions at the level of the valves.

Extraorganic lymphatic vesselsare also structurally similar to veins, but the basal endothelial membrane is poorly expressed, in places absent. The inner elastic membrane is clearly visible in the wall of these vessels. The middle shell receives a special development in the lower extremities.

Vascular structure The cardiovascular system (CVS) consists of the heart, blood and lymph vessels. Vessels in embryogenesis are formed from the mesenchyme. They are formed from the mesenchyme of the marginal zones of the vascular strip of the yolk sac or the mesenchyme of the embryo. In late embryonic development and after birth, vessels are formed by budding from capillaries and postcapillary structures (venules and veins). Blood vessels are subdivided into great vessels (arteries, veins) and vessels of the microvasculature (arterioles, precapillaries, capillaries, postcapillaries, and venules). In the main vessels, blood flows at a high speed and there is no exchange of blood with tissues; in the vessels of the microvasculature, blood flows slowly for better exchange of blood with tissues. All organs of the cardiovascular system are hollow and, in addition to the vessels of the microvasculature system, contain three membranes: 1. The inner membrane (intima) is represented by the inner endothelial layer. Behind it is the subendothelial layer (PBST). The subendothelial layer contains a large number of poorly differentiated cells that migrate into the middle membrane, and delicate reticular and elastic fibers. In muscular-type arteries, the inner membrane is separated from the middle membrane by an inner elastic membrane, which is an accumulation of elastic fibers. 2. The middle sheath (media) in the arteries consists of smooth myocytes arranged in a gentle spiral (almost circular), elastic fibers or elastic membranes (in elastic-type arteries); In the veins in it there can be smooth myocytes (in the veins of the muscle type) or connective tissue (veins of the nonmuscular type) predominate. In veins, unlike arteries, the middle sheath (media) is much thinner than the outer sheath (adventitia).

3. The outer shell (adventitia) is formed by the RVST. In the arteries of the muscular type, there is a thinner than the inner - outer elastic membrane.

Arteries Arteries have 3 membranes in the wall structure: intima, media, adventitia. Arteries are classified according to the predominance of elastic or muscular elements on the artery: 1) elastic, 2) muscular and 3) mixed.

In arteries of elastic and mixed types, in comparison with arteries of muscle type, the subendothelial layer is much thicker. The middle membrane in elastic-type arteries is formed by fenestrated elastic membranes - an accumulation of elastic fibers with areas of their rare distribution ("windows"). Between them there are interlayers of PBST with single smooth myocytes and fibroblastic cells. Muscle-type arteries contain many smooth muscle cells. The farther from the heart, the arteries with a predominance of the muscular component are located: the aorta is of the elastic type, the subclavian artery is mixed, the brachial artery is muscular. An example of a muscle type is also the femoral artery.

Veins Veins have 3 shells in the structure: intima, media, adventitia. Veins are subdivided into 1) non-muscular and 2) muscular (with weak, medium or strong development of the muscular elements of the middle shell). Veins of the non-muscular type are located at the level of the head, and vice versa - veins with a strong development of the muscular membrane on the lower extremities. Veins with a well-developed muscular membrane have valves. The valves are formed by the inner lining of the veins. Such a distribution of muscle elements is associated with the action of gravity: it is more difficult to raise blood from the legs to the heart than from the head, therefore, in the head it is a muscleless type, in the legs it has a highly developed muscle layer (for example, the femoral vein). The blood supply to the vessels is limited to the outer layers of the middle membrane and adventitia, while in the veins the capillaries reach the inner membrane. The innervation of the vessels is provided by autonomic afferent and efferent nerve fibers. They form the adventitious plexus. Efferent nerve endings reach mainly the outer regions of the meninges and are predominantly adrenergic. Afferent nerve endings of baroreceptors that respond to pressure form local subendothelial accumulations in the great vessels.

An important role in the regulation of vascular muscle tone, along with the autonomic nervous system, is played by biologically active substances, including hormones (adrenaline, norepinephrine, acetylcholine, etc.).

Blood capillaries Blood capillaries contain endothelial cells that lie on the basement membrane. The endothelium has an apparatus for metabolism, is capable of producing a large number of biologically active factors, including endothelin, nitric oxide, anticoagulant factors, etc., which control vascular tone, vascular permeability. Adventitial cells are closely adjacent to the vessels. In the formation of the basement membranes of the capillaries, pericytes take part, which can be in the cleavage of the membrane. There are capillaries: 1. Somatic type. The diameter of the lumen is 4-8 microns. The endothelium is continuous, not fenestrated (i.e., not thinned, fenestra is a window in translation). The basement membrane is continuous, well defined. The layer of pericytes is well developed. There are adventitious cells. Such capillaries are located in the skin, muscles, bones (what is referred to as soma), as well as in organs where cells need to be protected - as part of histohematogenous barriers (brain, gonads, etc.) 2. Visceral type. Clearance up to 8-12 microns. The endothelium is continuous, fenestrated (in the area of \u200b\u200bthe windows, the cytoplasm of the endotheliocyte is practically absent and its membrane is adjacent directly to the basement membrane). All types of contacts prevail between endothelial cells. The basement membrane is thinned. There are fewer pericytes and adventitia cells. Such capillaries are found in internal organsfor example, in the kidneys, where urine must be filtered.

3. Sinusoidal type. The diameter of the lumen is more than 12 microns. The endothelial layer is discontinuous. Endothelial cells form pores, hatches, fenestra. The basement membrane is intermittent or absent. There are no pericytes. Such capillaries are necessary, where not only the exchange of substances between blood and tissues takes place, but also the "exchange of cells", i.e. in some organs of blood formation (red bone marrow, spleen), or large substances in the liver.

Arterioles and precapillaries. Arterioles have a lumen diameter of up to 50 microns. Their wall contains 1-2 layers of smooth myocytes. The endothelium is elongated along the course of the vessel. Its surface is flat. The cells are characterized by a well-developed cytoskeleton, an abundance of desmosomal, locking, and tiled contacts. In front of the capillaries, the arteriole narrows and passes into the precapillary. Precapillaries have a thinner wall. The muscular membrane is represented by separate smooth myocytes. Postcapillaries and venules. Postcapillaries have a smaller lumen than venules. The structure of the wall is similar to the structure of the venule. Venules are up to 100 microns in diameter. The inner surface is uneven. The cytoskeleton is less developed. Contacts are mostly simple, in a "joint". Often, the endothelium is higher than in other vessels of the microvasculature. Cells of the leukocytic series penetrate through the venule wall, mainly in the areas of intercellular contacts. The outer layers are similar in structure to capillaries. Arterio-venular anastomoses.

Blood can flow from the arterial systems to the venous system, bypassing the capillaries, through arterio-venular anastomoses (AVA). There are true AVA (shunts) and atypical AVA (half shunts). In semi-shunts, the inflowing and outflowing vessels are connected through a short, wide capillary. As a result, mixed blood enters the venule. In true shunts, the exchange between the vessel and the organ does not occur and arterial blood enters the vein. True shunts are divided into simple (one anastomosis) and complex (multiple anastomoses). It is possible to distinguish shunts without special locking devices (smooth myocytes play the role of a sphincter) and with a special contractile apparatus (epithelioid cells, which, when swollen, squeeze the anastomosis, closing the shunt).

Lymphatic vessels. Lymphatic vessels are represented by microvessels lymphatic system (capillaries and postcapillaries), intraorgan and extraorganic lymphatic vessels. Lymphatic capillaries begin blindly in the tissues, contain a thin endothelium and a thinned basement membrane.

In the wall of medium and large lymphatic vessels there is an endothelium, a podendothelial layer, a muscular membrane and an adventitia. According to the structure of the membranes, the lymphatic vessel resembles a muscle-type vein. The inner lining of the lymphatic vessels forms valves, which are an integral attribute of all lymphatic vessels after the capillary section.

Clinical significance. 1. In the body, arteries are most sensitive to atherosclerosis, especially of the elastic and muscular-elastic types. This is due to hemodynamics and the diffuse nature of the trophic supply of the inner membrane, its significant development in these arteries. 2. In the veins, the valve apparatus is most developed in the lower extremities. This greatly facilitates the movement of blood against the hydrostatic pressure gradient. Violation of the structure of the valve apparatus leads to a gross violation of hemodynamics, edema and varicose veins of the lower extremities. 3. Hypoxia and low molecular weight products of cell destruction and anaerobic glycolysis are among the most powerful factors in stimulating the formation of new blood vessels. Thus, areas of inflammation, hypoxia, etc., are characterized by the subsequent rapid growth of microvessels (angiogenesis), which ensures the restoration of the trophic supply of the damaged organ and its regeneration.

4. Antiangiogenic factors that prevent the growth of new vessels, according to a number of modern authors, could become one of the effective antitumor drug groups. By blocking the growth of blood vessels in rapidly growing tumors, doctors, thereby, could cause hypoxia and death of cancer cells.

cytohistology.ru

Private histology of the cardiovascular system

Vascular development.

The first vessels appear in the second - third week of embryogenesis in the yolk sac and chorion. From the mesenchyme, an accumulation is formed - blood islands. The central islet cells are rounded and converted into blood stem cells. Peripheral islet cells differentiate into the vascular endothelium. The vessels in the body of the embryo are laid a little later; blood stem cells do not differentiate in these vessels. Primary vessels are similar to capillaries, their further differentiation is determined by hemodynamic factors - pressure and blood flow velocity. Initially, a very large part is laid in the vessels, which is reduced.

The structure of blood vessels.

In the wall of all vessels, 3 shells can be distinguished:

1.internal

2.medium

3.Outdoor

Arteries

Depending on the ratio of muscle elastic components, arteries of the following types are distinguished:

Elastic

The large main vessels are the aorta. Pressure - 120-130 mm / Hg / st, blood flow velocity - 0.5 1.3 m / sec. The function is transport.

Inner sheath:

A) endothelium

flattened polygonal cells

B) subendothelial layer (subendothelial)

It is represented by loose connective tissue, contains stellate cells that perform combined functions.

Middle shell:

It is represented by fenestrated elastic membranes. There are a small number of muscle cells between them.

Outer sheath:

It is represented by loose connective tissue, contains blood vessels and nerve trunks.

Muscular

Arteries of small and medium-sized hummingbirds.

Inner sheath:

A) endothelium

B) subendothelial layer

B) inner elastic membrane

Middle shell:

Smooth muscle cells located in a gentle spiral prevail. Between the middle and outer shell is an outer elastic membrane.

Outer sheath:

Presented by loose connective tissue

Mixed

Arterioles

Similar to arteries. Function - regulation of blood flow. Sechenov called these vessels - taps of the vascular system.

The middle shell is represented by 1-2 layers of smooth muscle cells.

Capillaries

Classification:

Depending on the diameter:

narrow 4.5-7 microns - muscles, nerves, musculoskeletal tissue

medium 8-11 microns - skin, mucous membranes

sinusoidal up to 20-30 microns - endocrine glands, kidneys

lacunae up to 100 microns - found in cavernous bodies

Depending on the structure:

Somatic - continuous endothelium and continuous basement membrane - muscles, lungs, central nervous system

Capillary structure:

3 layers, which are analogous to 3 shells:

A) endothelium

B) pericytes, enclosed in the basement membrane

C) adventitious cells

2. Finished - have thinning or windows in the endothelium - endocrine organs, kidneys, intestines.

3. perforated - there are through holes in the endothelium and in the basement membrane - hematopoietic organs.

similar to capillaries, but have more pericytes

Classification:

● fibrous (non-muscular) type

They are located in the spleen, placenta, liver, bones, meninges. In these veins, the podendothelial layer passes into the surrounding connective tissue

● muscle type

There are three subtypes:

● Depending on the muscle component

A) veins with poor development of muscle elements are located above the level of the heart, blood flows passively due to its severity.

B) veins with an average development of muscle elements - brachial vein

C) veins with a strong development of muscle elements, large veins lying below the level of the heart.

Muscle elements are found in all three membranes

Structure

Inner sheath:

Endothelium

Subendothelial layer - longitudinally directed bundles of muscle cells. A valve is formed behind the inner shell.

Middle shell:

Circularly arranged bundles of muscle cells.

Outer sheath:

Loose connective tissue, and longitudinally located muscle cells.

DEVELOPMENT

The heart is laid at the end of the 3rd week of embryogenesis. Under the visceral leaf of the splanchnotome, an accumulation of mesenchymal cells forms, which turn into elongated tubules. These mesenchymal accumulations protrude into the cilomic cavity, bending the visceral sheets of the splanchnotome. And the sites are myoepicardial plates. Subsequently, the endocardium, myoepicardial plates, myocardium and epicardium are formed from the mesenchyme. The valves develop as a duplicate of the endocardium.

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BSMU

Discipline: Histology | Comment on

The importance of the cardiovascular system (CVS) in the vital activity of the organism, and hence the knowledge of all aspects of this area for practical medicine, is so great that cardiology and angiology have been isolated as two independent directions in the study of this system. The heart and blood vessels belong to systems that do not function periodically, but constantly, therefore, more often than other systems, they are susceptible to pathological processes. At present, CVS diseases, along with cancer, occupy a leading place in mortality. The cardiovascular system ensures the movement of blood through the body, regulates the supply of nutrients and oxygen to tissues and the removal of metabolic products, blood deposition.

Classification: I. The central organ is the heart. II. Peripheral section: A. Blood vessels: 1. Arterial link: a) elastic type arteries; b) arteries of the muscular type; c) mixed type arteries. 2. Microcirculatory bed: a) arterioles; b) hemocapillaries; c) venules; d) arterio-venular anastomoses 3. Venous link: a) veins of the muscular type (with weak, medium, strong development of muscle elements; b) veins of the non-muscular type. B. Lymphatic vessels: 1. Lymphatic capillaries. 2. Intraorganic lymphatic vessels. 3. Extraorganic lymphatic vessels. In the embryonic period, the first blood vessels are laid at the 2nd week in the wall of the yolk sac from the mesenchyme (see the stage of megaloblastic hematopoiesis on the topic "Hematopoiesis") - blood islets appear, the peripheral cells of the islet flatten and differentiate into the endothelial lining, and form the surrounding mesenchyme connective tissue and smooth muscle elements of the vascular wall. Soon, blood vessels are formed from the mesenchyme in the body of the embryo, which are connected to the vessels of the yolk sac. Arterial link - represented by vessels through which blood is delivered from the heart to the organs. The term "artery" is translated as "air-containing", since upon opening, researchers often found these vessels empty (not containing blood) and thought that vital "pneuma" or air was spreading through them through the body. Arteries of elastic, muscular and mixed types have a common structural principle: 3 shells are distinguished in the wall - inner, middle and outer adventitious. The inner shell consists of layers: 1. Endothelium on the basement membrane. 2. The podendothelial layer is a snout fibrous SDM with a high content of poorly differentiated cells. 3. Internal elastic membrane - a plexus of elastic fibers. The middle shell contains smooth muscle cells, fibroblasts, elastic and collagen fibers. At the border of the middle and outer adventitia membrane, there is an external elastic membrane - a plexus of elastic fibers. The outer adventitia of the arteries is histologically represented by a loose fibrous SDM with blood vessels and vascular nerves. Features in the structure of the varieties of arteries are due to differences in the hemadynamic conditions of their functioning. Differences in structure mainly relate to the middle shell (different ratios of the constituent elements of the shell): 1. Elastic arteries - these include the aortic arch, pulmonary trunk, thoracic and abdominal aorta. Blood enters these vessels in jerks under high pressure and moves at high speed; there is a large pressure drop during the transition from systole to diastole. The main difference from other types of arteries is in the structure of the middle shell: in the middle shell of the above components (myocytes, fibroblasts, collagen and elastic fibers), elastic fibers predominate. Elastic fibers are located not only in the form of individual fibers and plexuses, but form elastic fenestrated membranes (in adults, the number of elastic membranes reaches 50-70 words). Due to the increased elasticity, the wall of these arteries not only withstands high pressure, but also smoothes out large drops (surges) in pressure during systole-diastole transitions. 2. Muscular arteries - these include all arteries of medium and small caliber. A feature of the hemodynamic conditions in these vessels is a drop in pressure and a decrease in blood flow velocity. Muscular arteries differ from other types of arteries by the predominance of myocytes in the middle membrane over other structural components; the inner and outer elastic membrane is clearly expressed. Myocytes in relation to the lumen of the vessel are oriented spirally and are found even in the outer shell of these arteries. Due to the powerful muscular component of the middle membrane, these arteries control the intensity of blood flow of individual organs, maintain the falling pressure and push the blood further, therefore, muscle-type arteries are also called "peripheral heart".

3. Arteries of mixed type - these include large arteries extending from the aorta (carotid and subclavian arteries). In terms of structure and function, they occupy an intermediate position. The main feature in the structure: in the middle membrane, myocytes and elastic fibers are represented approximately the same (1: 1), there is a small amount of collagen fibers and fibroblasts.

Microcirculatory bed - a link located between the arterial and venous links; provides regulation of blood circulation in the organ, metabolism between blood and tissues, blood deposition in organs. Composition: 1. Arterioles (including precapillary). 2. Hemocapillaries. 3. Venules (including postcapillary). 4. Arterio-venular anastomoses. Arterioles are vessels that connect arteries to hemocapillaries. They preserve the principle of the structure of the arteries: they have 3 membranes, but the membranes are weakly expressed - the subendothelial layer of the inner membrane is very thin; the middle membrane is represented by one layer of myocytes, and closer to the capillaries - by single myocytes. As the diameter in the middle membrane increases, the number of myocytes increases, first one, then two or more layers of myocytes are formed. Due to the presence of myocytes in the wall (in the precapillary arterioles in the form of a sphincter), arterioles regulate the blood supply of the hemocapillaries, thereby the exchange rate between the blood and the tissues of the organ. Hemocapillaries. The wall of hemocapillaries has the smallest thickness and consists of 3 components - endotheliocytes, basement membrane, pericytes in the thickness of the basement membrane. There are no muscle elements in the capillary wall, however, the diameter of the inner lumen may change somewhat as a result of changes in blood pressure, the ability of the nuclei of pericytes and endothelial cells to swell and contract. There are the following types of capillaries: 1. Hemocapillaries of type I (somatic type) - capillaries with continuous endothelium and continuous basement membrane, diameter 4-7 microns. They are present in skeletal muscles, in the skin and mucous membranes .. 2. Hemocapillaries of type II (fenestrated or visceral type) - the basement membrane is continuous, in the endothelium there are fenestra - thinned areas in the cytoplasm of endothelial cells. Diameter 8-12 microns. They are present in the capillary glomeruli of the kidney, in the intestines, in the endocrine glands. 3. Hemocapillaries of type III (sinusoidal type) - the basement membrane is not continuous, in places it is absent, and there are gaps between the endothelial cells; diameter 20-30 microns and more, not constant throughout - there are widened and narrowed areas. The blood flow in these capillaries is slowed down. Available in the liver, hematopoietic organs, endocrine glands. Around the hemocapillaries there is a thin layer of loose fibrous SDM with a high content of poorly differentiated cells, the state of which determines the intensity of exchange between blood and working tissues of the organ. The barrier between the blood in the hemocapillaries and the surrounding working tissue of the organ is called the histohematogenous barrier, which consists of endothelial cells and the basement membrane. Capillaries can change their structure, rearrange themselves into vessels of a different type and caliber; new branches can form from existing hemocapillaries. Precapillaries differ from hemocapillaries in that in addition to endotheliocytes, basement membrane, pericytes, there are single or groups of myocytes in the wall.

Arterio-venular anastomoses (AVA) are shunts (or fistulas) between arterioles and venules, i.e. carry out direct communication and participate in the regulation of regional peripheral blood flow. They are especially abundant in the skin and kidneys. AVA - short vessels, also have 3 membranes; there are myocytes, especially in the middle membrane, which play the role of a sphincter.

VIENNA. A feature of hemodynamic conditions in the veins is low pressure (15-20 mm Hg) and low blood flow rate, which leads to a lower content of elastic fibers in these vessels. There are valves in the veins - a duplication of the inner membrane. The number of muscle elements in the wall of these vessels depends on whether the blood moves under the influence of gravity or against it. Veins of the non-muscular type are found in the dura mater, bones, retina, placenta, and in the red bone marrow. The wall of the veins of the non-muscular type is lined with endothelial cells on the basement membrane from the inside, followed by an interlayer of fibrous SDT; there are no smooth muscle cells. Veins of the muscle type with weakly expressed muscle elements are located in the upper half of the body - in the system of the superior vena cava. These veins are usually collapsed. In the middle membrane, they have a small number of myocytes.

Veins with highly developed muscle elements make up the vein system of the lower half of the body. A feature of these veins is well-defined valves and the presence of myocytes in all three membranes - in the outer and inner membranes in the longitudinal direction, in the middle - in the circular direction.

LYMPH VESSELS begin with lymphatic capillaries (LC). LK, unlike hemocapillaries, start blindly and have a larger diameter. The inner surface is lined with endothelium, the basement membrane is absent. Under the endothelium is a loose fibrous SDM with a high content of reticular fibers. The diameter of the LK is not constant - there are contractions and expansions. Lymphatic capillaries merge to form intraorgan lymphatic vessels - in structure they are close to veins, because are in the same hemodynamic conditions. They have 3 shells, the inner shell forms the valves; unlike the veins under the endothelium, the basement membrane is absent. The diameter is not constant throughout - there are expansions at the level of the valves. Extraorganic lymphatic vessels are also structurally similar to veins, but the basal endothelial membrane is poorly expressed, in places absent. The inner elastic membrane is clearly visible in the wall of these vessels. The middle shell receives a special development in the lower extremities.

HEART. The heart is laid at the beginning of the 3rd week of embryonic development in the form of a paired anlage in the cervical region from the mesenchyme under the visceral leaf of splanchnotomes. Paired strands are formed from the mesenchyme, which soon turn into tubes, from which the inner shell of the heart - the endocardium - is ultimately formed. Areas of the visceral sheet of splanchnotomes that bend around these tubules are called myoepicardial plates, which subsequently differentiate into the myocardium and epicardium. As the embryo develops with the appearance of the trunk fold, the flat embryo folds into a tube - the body, while 2 heart anlages appear in the chest cavity, come closer and finally merge into one tube. Further, this tube-heart begins to grow rapidly in length and does not fit in the chest wall forms several bends. The adjacent loops of the bending tube grow together and a 4-chambered heart is formed from a simple tube. The HEART is the central organ of the CVS, it has 3 shells: the inner one is the endocardium, the middle (muscular) one is the myocardium, the outer (serous) one is the epicardium. The endocardium consists of 5 layers: 1. Endothelium on the basement membrane. 2. The subendothelial layer is made of loose fibrous SDM with a large number of poorly differentiated cells. 3. Muscular-elastic layer (myocytes are elastic fibers). 4. Elastic-muscular layer (myocyte-elastic fibers). 5. Outer SDT-th layer (loose fibrous SDT). In general, the structure of the endocardium resembles the structure of the wall blood vessel... The muscular layer (myocardium) consists of 3 types of cardiomyocytes: contractile, conducting and secretory (for structural and functional features, see the topic " Muscle tissue"). The endocardium is a typical serous membrane and consists of layers: 1. Mesothelium on the basement membrane. 2. Superficial collagen layer. 3. A layer of elastic fibers. 4. Deep collagen layer. 5. Deep collagen-elastic layer (50% of the entire thickness of the epicardium). Under the mesothelium, there are fibroblasts in all layers between the fibers. ССС regeneration. Vessels, endocardium and epicardium regenerate well. Reparative regeneration of the heart is poor, the defect is replaced by a SDT scar; physiological regeneration - well expressed, due to intracellular regeneration (renewal of worn out organelles). Age-related changes in CVS. In the vessels in old and senile age, a thickening of the inner membrane is observed, deposits of cholesterol and calcium salts (atherosclerotic plaques) are possible. In the middle vascular membrane, the content of myocytes and elastic fibers decreases, the amount of collagen fibers and acidic mucopolysaccharides increases.