Cardiovascular complex of organs includes the heart, arteries, microcirculatory vessels, veins, lymphatic vessels. The heart and a closed network of blood vessels ensure blood circulation in the body and transport of lymph to the heart. The activity of the cardiovascular complex is aimed at maintaining metabolism and the constancy of the internal environment of the body - nutrients, oxygen, and biologically flow from the blood to the tissues and cells. active substances regulating their development and functions; Toxins and products of their special activities that cells do not need are removed into the blood and lymph.
Development. Source of development blood vessels is mesenchyme. The first vessels appear outside the body of the embryo - in the wall of the yolk sac and chorion at the beginning of the 3rd week of embryogenesis. Initially, clusters of mesenchymal cells called blood islands are formed. The peripheral cells of the islets flatten and, connecting with each other, form primitive vessels in the form of endothelial tubes. Centrally located mesenchymocytes differentiate into primary blood cells (the initial intravascular stage of hematopoiesis). In the body of the embryo, vessels appear later, also from the mesenchyme through the growth of its cells along the walls of the slit-like spaces of the embryo.
At the end of the 3rd week, communication is established between the primary blood vessels vessels extraembryonic organs and the body of the embryo. After the start of blood circulation, the structure of the vessels becomes noticeably more complex in accordance with regional hemodynamic conditions. In addition to the endothelium, other tissues (also originating from mesenchyme) develop within the walls of blood vessels, which combine to form the inner, middle, and outer membranes of the vessels.
Heart bookmark appears at the beginning of the 3rd week of development in the form of paired mesenchymal tubes. After their fusion, tissue differentiation begins inner shell heart - endocardium. The middle and outer shells of the heart are also formed from paired myoepicardial plates - fragments of the right and left visceral layers of the splanchnotome. Myoepicardial plates approach the endocardium, surround it from the outside, and then, merging, differentiate into the tissue elements of the myo- and epicardium.
Arteries- vessels that ensure the movement of blood from the heart to the microvasculature. Based on their diameter, they are divided into small, medium and large caliber arteries. The wall of all arteries consists of three membranes: internal (tunica intima), middle (tunica media) and external (tunica externa). The tissue composition and degree of development of these membranes in arteries of different calibers are not the same, which is associated with hemodynamic conditions and the characteristics of the functions performed by the vessels of certain parts of the arterial bed. According to the quantitative ratio of elastic and muscular elements in the middle shell of the vessel, arteries of elastic, mixed (muscular-elastic) and muscular types are distinguished.
Arteries elastic type (aorta and pulmonary artery) perform a transport function and the function of maintaining blood pressure in arterial system during cardiac diastole. Their wall experiences rhythmic changes in blood pressure. Blood enters these vessels under high pressure(120-130 mm Hg) and at a speed of about 1 m/s. Under these conditions, the strong development of the elastic frame of the wall is fully justified, which allows the vessels to stretch during systole and take their original position during diastole. Returning to their original position, the elastic wall of such vessels ensures that portions of blood successively ejected from the ventricles of the heart turn into continuous blood flow.
Inner shell vessels elastic type (using the example of the aorta) consists of endothelium, subendothelial layer and a plexus of elastic fibers. In the subendothelial layer, poorly differentiated stellate cells of loose connective tissue, individual smooth muscle cells, large number glycosaminoglycans. With age, there is an accumulation of cholesterol. In the middle shell of the aorta there are up to 50 elastic fenestrated membranes (more precisely, elastic fenestrated cylinders of different diameters inserted into each other), in the openings of which smooth muscle cells and elastic fibers are located. The outer shell consists of loose fibrous connective tissue containing vascular vessels and nerve trunks.
Arteries of the mixed(muscular-elastic) type are characterized by approximately equal numbers of muscle and elastic elements in the middle shell. Between the smooth myocytes lie dense networks of elastic fibrils.
At the border of the inner and middle shells there is a clearly expressed internal elastic membrane. IN outer shell contains bundles of smooth muscle cells, as well as collagen and elastic fibers. Arteries of this type include the carotid, subclavian and others.
Arteries muscular type perform not only transport, but also distribution functions, regulating blood flow to organs under conditions of various physiological loads (these are the so-called organ arteries). Muscular arteries contain smooth myocytes in the tunica media. This allows the arteries to regulate blood flow to the organs and maintain blood pumping, which is important for the blood supply to the organs located on the great distance from the heart. Arteries of the muscular type can be large, medium and small in caliber. The inner lining of the wall of these arteries is formed by the endothelium lying on the basement membrane, the subendothelial layer and the internal elastic membrane, but in small arteries the internal elastic membrane is poorly expressed.
The middle shell is formed by smooth muscle tissue with a small amount of fibroblasts, collagen and elastic fibers. Smooth myocytes are located in the tunica media in a gentle spiral. Together with radially and arcuately located elastic fibers, myocytes create a single springy frame that prevents the collapse of the arteries, ensuring their gaping and continuity of blood flow. At the border between the middle and outer shells there is an outer elastic membrane. The latter refers to the outer shell, consisting of loose connective tissue. Collagen fibers have an oblique and longitudinal direction. The outer shell of muscular arteries contains blood vessels and nerves that feed them.
Scanning electron microscopy has shown that the inner surface of the endothelium arteries has numerous folds and depressions, microscopic outgrowths of various shapes. This creates an uneven and complex microrelief of the internal (luminal) surface of the vessels. This microrelief increases the free surface of contact between the endothelium and the blood, which has trophic significance and creates favorable conditions for hemodynamics.
Both elastic fibers and the outer one, consisting of fibrous connective tissue containing collagen fibers. The inner lining is formed by endothelium, which lines the lumen of the vessel, a subendothelial layer and an internal elastic membrane. The middle layer of the artery consists of spirally arranged smooth myocytes, between which a small amount of collagen and elastic fibers pass, and an outer elastic membrane formed by longitudinal thick intertwining fibers. The outer shell is formed by loose fibrous connective tissue containing elastic and collagen fibers; blood vessels and nerves pass through it (Fig. 204).
Depending on the development of the various layers of the artery wall, they are divided into vessels of the muscular (predominant), mixed (muscular-elastic) and elastic types. In the wall of muscular arteries, the tunica media is well developed. Myocytes and elastic fibers are located in it like a spring. Myocytes of the middle "shell of the wall of muscular arteries regulate blood flow to organs and tissues by their contractions. As the diameter of the arteries decreases, all the membranes of the artery walls become thinner. The thinnest arteries of the muscular type, arterioles with a diameter of less than 100 microns, pass into capillaries. To the arteries mixed type These include arteries such as the carotid and subclavian. In the middle shell of their wall there is approximately an equal number of elastic fibers and myocytes, fenestrated elastic membranes appear. Elastic arteries include the aorta and pulmonary trunk, into which blood flows under high pressure and at high speed from the heart.
The tunica media is formed by concentric elastic fenestrated membranes, between which myocytes lie.
The large arteries located near the heart (aorta, subclavian arteries and carotid arteries) have to withstand high pressure from the blood pushed out by the left ventricle of the heart. These vessels have thick walls, the middle layer of which consists mainly of elastic fibers. Therefore, during systole they can stretch without rupturing. After the end of systole, the artery walls contract, which ensures continuous blood flow throughout the arteries.
Arteries located further from the heart have a similar structure, but contain more smooth muscle fibers in the middle layer. They are innervated by fibers of the sympathetic nervous system, and impulses arriving through these fibers regulate their diameter.
From the arteries, blood flows into smaller vessels called
Everyone knows that in the human body the function of transmitting blood to all tissues from the heart muscle is performed by vessels. The peculiarity of the structure of the circulatory system makes it possible to provide permanent job all systems. Length of all vessels human body is thousands of meters, or more precisely, about one hundred thousand. This bed is represented by capillaries, veins, aorta, arteries, venules and arterioles. What are arteries and what is their structure? What function do they perform? What types of human arteries are there?
Blood vessels are a kind of tubes of different sizes and different structures through which blood circulates. These organs are very durable and can withstand significant chemical exposure. High strength is ensured by the special structure of the vessels, consisting of an internal layer, middle and outer layers. Inside, the vessels consist of the thinnest epithelium, which ensures the smoothness of the vascular walls. The middle layer is somewhat thicker than the inner layer and consists of muscle, collagen and elastic tissues. The outside of the vessels is covered with fibrous fabric, which protects the loose texture from damage.
Medicine divides vessels according to type of structure, functions and some other characteristics into veins, arteries and capillaries. The most major artery called the aorta, and the largest veins are the pulmonary veins. What are arteries and what types are they? In anatomy, there are three types of arteries: elastic, muscular-elastic and muscular. Their walls consist of three shells: outer, middle and inner.
Elastic vessels emerge from the ventricles of the heart. These include: aorta, pulmonary trunk, carotid and pulmonary artery. The walls of these channels contain many elastic cells, due to which they have elasticity and are able to stretch when blood leaves the heart under pressure and at tremendous speed. When the ventricles are at rest, the stretched walls of the vessels contract. This principle of operation helps to maintain normal vascular pressure until the ventricle is filled with blood from the arteries.
What is an artery, what is its structure? As you know, vessels consist of three shells. Inner layer called intima. In the elastic type of vessels it occupies about twenty percent of their walls. This membrane is lined with endothelium located on the basement membrane. Under this layer there is connective tissue, which contains macrophages, muscle cells, fibroblasts, and intercellular substance. There are special valves where the arteries leave the heart. These types of formations are also observed along the aorta.
The middle layer of the artery is formed of elastic tissue with a large number of membranes. With age, their number increases, and the middle layer itself thickens. Between the adjacent membranes are smooth muscle cells that are capable of producing collagen, elastin and some other substances.
The outer lining of the arteries is very thin and formed by fibrous connective tissue. It protects the vessel from rupture and overstretching. In this place there are multiple nerve endings, small vessels that nourish the outer and middle linings of the arteries.
The pulmonary column and aorta are divided into numerous branches that deliver blood to different areas body: to skin, internal organs. Arteries also arise from these branches lower limbs. Parts of the body experience different stress, which is why they need different amounts of blood. Arteries must have the ability to change their lumen in order to deliver the required volume of blood at different times. Because of this feature, the arteries must have a well-developed layer smooth muscles, capable of contracting and reducing the lumen.
These types of vessels belong to the muscular type. Their diameter is controlled by the sympathetic nervous system. This type includes the arteries of the neck, brachial, radial, vessels and some others.
The walls of muscular-type vessels consist of endothelium lining the lumen of the channel, and there is also connective tissue and an elastic internal membrane. Elastic and collagen cells, an amorphous substance, are well developed in connective tissue. This layer is best developed in large and medium-sized vessels. Outside the connective tissue is an internal elastic membrane, which is clearly visible in large arteries.
The middle layer of the vessel is formed by smooth muscle cells arranged in a spiral. When they contract, the volume of the lumen decreases, and blood begins to push through the channel into all parts of the body. Muscle cells are connected to each other by an intercellular substance containing elastic fibers. They are located between muscle fibers and are connected to the outer and inner membranes. This system forms an elastic frame that gives elasticity to the walls of the arteries.
On the outside, the shell is formed by loose connective tissue, which contains many collagen fibers. Here are the nerve endings, lymphatic and blood vessels that supply the walls of the arteries.
What are mixed type arteries? These are vessels that, in function and structure, occupy an intermediate position between muscular and elastic types. These include the femoral, iliac vessels, as well as the celiac trunk and some other vessels.
The middle layer of mixed arteries consists of elastic fibers and fenestrated membranes. In the deepest places of the outer shell there are bundles of muscle cells. On the outside, they are covered with connective tissue and well-developed collagen fibers. These types of arteries differ from others in their high elasticity and ability to contract strongly.
As the arteries approach the site of division into arterioles, the lumen decreases and the walls become thinner. There is a decrease in the thickness of the connective tissue, internal elastic membrane, muscle cells, the elastic membrane gradually disappears, and the thickness of the outer membrane is disrupted.
During contraction, the heart pushes blood with great force into the aorta, and from there it enters the arteries, spreading throughout the body. As the vessels fill with blood, the elastic walls contract along with the heart, pushing blood through the vascular bed. Pulse wave is formed during periods of blood expulsion from the left ventricle. At this time, the pressure in the aorta rises sharply, and the walls begin to stretch. The wave then propagates from the aorta to the capillaries, passes through the vertebral artery and other vessels.
Initially, the blood is ejected by the heart into the aorta, the walls of which are stretched, and it passes further. With each contraction, the ventricle ejects a certain amount of blood: the aorta stretches, then narrows. Thus, the blood passes further along the channel, to other vessels of smaller diameter. When the heart relaxes, the blood tries to return back through the aorta, but this process is prevented by special valves located in large vessels. They close the lumen from the reverse flow of blood, and the narrowing of the lumen of the bed promotes further movement.
There are certain fluctuations in the cardiac cycle that cause blood pressure not always the same. Based on this, two parameters are distinguished: diastole and systole. The first represents the moment of relaxation of the ventricle and its filling with blood, and systole is the contraction of the heart. You can determine the strength of blood flow through the arteries by placing your hand on the places where the pulse is palpated: at the base thumb hands, on the carotid or popliteal artery.
In the human body there are coronary arteries that nourish the heart. They begin the third circle of blood circulation - coronary. Unlike small and large, it feeds only the heart.
As you approach the arterioles, the lumen of the vessels decreases, their walls become thinner, and the outer membrane disappears. After the arteries, arterioles begin - these are small vessels that are considered a continuation of the arteries. Gradually they turn into capillaries.
The walls of arterioles have three layers: inner, middle and outer, but they are very weakly expressed. Then the arterioles are divided into even smaller vessels - capillaries. They fill all space and penetrate all cells of the body. This is where they come from metabolic processes, helping to maintain the vital functions of the body. Then the capillaries increase in volume and form venules, then veins.
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Large vessels - aorta, pulmonary trunk, hollow and pulmonary veins– serve primarily as pathways for blood movement. All other arteries and veins, even small ones, can, in addition, regulate the flow of blood to the organs and its outflow, since they are capable of changing their lumen under the influence of neurohumoral factors.
Distinguish arteries three types:
The wall of all types of arteries, as well as veins, consists of three layers (shells):
The relative thickness of these layers and the nature of the tissues that form them depend on the type of artery.
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Arteries elastic type exit directly from the ventricles of the heart - these are the aorta, pulmonary trunk, pulmonary and common carotid arteries. Their walls contain a large number of elastic fibers, due to which they have the properties of elongation and elasticity. When blood under pressure (120–130 mm Hg) and at high speed (0.5–1.3 m/s) is pushed out of the ventricles during heart contraction, the elastic fibers in the walls of the arteries are stretched. After the end of ventricular contraction, the distended walls of the arteries contract and thus maintain pressure in the vascular system until the ventricle is filled with blood again and its contraction occurs.
Inner lining (intima) of arteries elastic type is approximately 20% of their wall thickness. It is lined with endothelium, the cells of which lie on the basement membrane. Underneath it is a layer of loose connective tissue containing fibroblasts, smooth muscle cells and macrophages, as well as a large amount of intercellular substance. The physicochemical state of the latter determines the permeability of the vessel wall and its trophism. In older people, cholesterol deposits can be seen in this layer ( atherosclerotic plaques). Externally, the intima is limited by an internal elastic membrane.
At the point where it leaves the heart, the inner membrane forms pocket-like folds - valves. Intimal folding is also observed along the aorta. The folds are oriented longitudinally and have a spiral course. The presence of folding is also characteristic of other types of vessels. This increases the area of the inner surface of the vessel. The thickness of the intima should not exceed a certain value (for the aorta - 0.15 mm) so as not to interfere with the nutrition of the middle layer of the arteries.
The middle layer of the membrane of elastic arteries is formed by a large number of fenestrated elastic membranes located concentrically. Their number changes with age. A newborn has about 40 of them, an adult has up to 70. These membranes thicken with age. Between adjacent membranes lie poorly differentiated smooth muscle cells capable of producing elastin and collagen, as well as an amorphous intercellular substance. With atherosclerosis, deposits can form in the middle layer of the wall of such arteries cartilage tissue in the form of rings. This is also observed with significant dietary violations.
Elastic membranes in the walls of arteries are formed due to the secretion of amorphous elastin by smooth muscle cells. In the areas lying between these cells, the thickness of the elastic membranes is much less. Here are formed fenestrae(windows) through which nutrients pass to the structures of the vascular wall. As the vessel grows, the elastic membranes stretch, the fenestrae expand, and newly synthesized elastin is deposited at their edges.
The outer shell of elastic-type arteries is thin, formed by loose fibrous connective tissue with a large number of collagen and elastic fibers, located mainly longitudinally. This membrane protects the vessel from overstretching and rupture. Nerve trunks and small blood vessels (vasa vascularis) pass here, feeding the outer tunic and part of the middle tunic of the main vessel. The number of these vessels is directly dependent on the wall thickness of the main vessel.
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Numerous branches depart from the aorta and pulmonary trunk, which deliver blood to various parts of the body: to the limbs, internal organs, and integument. Since individual areas of the body have different functional loads, they need different amounts of blood. The arteries that supply them with blood must have the ability to change their lumen in order to deliver the currently required amount of blood to the organ. In the walls of such arteries there is a well-developed layer of smooth muscle cells that can contract and reduce the lumen of the vessel or relax, increasing it. These arteries are called arteries muscular type, or distribution. Their diameter is controlled by the sympathetic nervous system. These arteries include the vertebral, brachial, radial, popliteal, cerebral arteries and others. Their wall also consists of three layers. The inner layer includes endothelium lining the lumen of the artery, subendothelial loose connective tissue and an internal elastic membrane. The connective tissue has well-developed collagen and elastic fibers located longitudinally and an amorphous substance. The cells are poorly differentiated. The layer of connective tissue is better developed in large and medium-sized arteries and weaker in small ones. Outside the loose connective tissue there is an internal elastic membrane closely associated with it. It is more pronounced in large arteries.
The middle lining of the muscular artery is formed by spirally arranged smooth muscle cells. The contraction of these cells leads to a decrease in the volume of the vessel and pushes blood into more distal sections. Muscle cells are connected by an intercellular substance with a large number of elastic fibers. The outer boundary of the middle shell is the outer elastic membrane. Elastic fibers located between muscle cells are connected to the inner and outer membranes. They form a kind of elastic frame that gives elasticity to the artery wall and prevents its collapse. Smooth muscle cells of the tunica media, when contracting and relaxing, regulate the lumen of the vessel, and therefore the flow of blood into the vessels of the microvasculature of the organ.
The outer shell is formed by loose connective tissue with a large number of elastic and collagen fibers located obliquely or longitudinally. This layer contains nerves and blood and lymphatic vessels that supply the artery wall.
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Mixed arteries, or muscular-elastic type in structure and functional characteristics occupy an intermediate position between elastic and muscular arteries. These include, for example, subclavian, external and internal iliac, femoral, mesenteric arteries, celiac trunk. In the middle layer of their wall, along with smooth muscle cells, there is a significant amount of elastic fibers and fenestrated membranes. In the deep part of the outer shell of such arteries there are bundles of smooth muscle cells. On the outside, they are covered with connective tissue with well-developed bundles of collagen fibers lying obliquely and longitudinally. These arteries are highly elastic and can contract strongly.
As you approach the arterioles, the lumen of the arteries decreases and their wall becomes thinner. In the inner shell, the thickness of the connective tissue and internal elastic membrane decreases, in the middle layer the number of smooth muscle cells decreases, and the outer elastic membrane disappears. The thickness of the outer shell decreases.
Arterioles, capillaries and venules, as well as arteriole-venular anastomoses form microvasculature. Functionally, there are afferent microvessels (arterioles), exchange microvessels (capillaries) and efferent microvessels (venules). It was found that the microcirculation systems of various organs differ significantly from each other: their organization is closely related to the functional characteristics of organs and tissues.
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Arterioles They are small, up to 100 microns in diameter, blood vessels that are a continuation of the arteries. They gradually turn into capillaries. The wall of the arterioles is formed by the same three layers as the wall of the arteries, but they are very weakly expressed. The inner lining consists of endothelium lying on the basement membrane, a thin layer of loose connective tissue and a thin internal elastic membrane. The middle shell is formed by 1–2 layers of smooth muscle cells arranged in a spiral. In terminal precapillary arterioles, smooth muscle cells lie singly; they are necessarily present at the sites where arterioles divide into capillaries. These cells surround the arteriole in a ring and perform the function precapillary sphincter(from Greek sphincter hoop). In addition, terminal arterioles are characterized by the presence of holes in the basement membrane of the endothelium. Due to this, endothelial cells come into contact with smooth muscle cells, which are able to respond to substances that enter the blood. For example, when adrenaline is released into the blood from the adrenal medulla, it reaches the muscle cells in the walls of the arterioles and causes them to contract. The lumen of the arterioles sharply decreases, and blood flow in the capillaries stops.
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Capillaries – these are the thinnest blood vessels that make up the longest part of the circulatory system and connect the arterial and venous beds. Are formed true capillaries as a result of branching of precapillary arterioles. They are usually located in the form of networks, loops (in the skin, synovial bursae) or vascular glomeruli (in the kidneys). The size of the lumen of the capillaries, the shape of their networks and the speed of blood flow in them are determined by the organ characteristics and functional state vascular system. The narrowest capillaries are found in skeletal muscles (4–6 µm), nerve sheaths, and lungs. Here they form flat networks. In the skin and mucous membranes, the lumens of the capillaries are wider (up to 11 microns), they form a three-dimensional network. Thus, in soft tissues The diameter of the capillaries is larger than in dense ones. In the liver, glands internal secretion and hematopoietic organs, the lumens of the capillaries are very wide (20–30 µm or more). Such capillaries are called sinusoidal or sinusoids.
The density of capillaries varies in different organs. The largest number of them per 1 mm 3 is found in the brain and myocardium (up to 2500–3000), in skeletal muscle – 300–1000, and in bone tissue even less. Under normal physiological conditions, approximately 50% of capillaries are in an active state in tissues. The lumen of the remaining capillaries decreases significantly, they become impassable for blood cells, but plasma continues to circulate through them.
The capillary wall is formed by endothelial cells covered on the outside with a basement membrane (Fig. 2.9).
Rice. 2.9. Structure and types of capillaries:
A – capillary with continuous endothelium; B – capillary with fenestrated endothelium; B – sinusoidal type capillary; 1 – pericyte; 2 – fenestrae; 3 – basement membrane; 4 – endothelial cells; 5 – pores
In its cleavage lie pericytes – branched cells surrounding the capillary. Efferent nerve endings are found on these cells in some capillaries. Outside, the capillary is surrounded by poorly differentiated adventitial cells and connective tissue. There are three main types of capillaries: with continuous endothelium (in the brain, muscles, lungs), with fenestrated endothelium (in the kidneys, endocrine organs, intestinal villi) and with discontinuous endothelium (sinusoids of the spleen, liver, hematopoietic organs). Capillaries with continuous endothelium are the most common. The endothelial cells in them are connected by tight intercellular junctions. Transport of substances between blood and tissue fluid occurs through the cytoplasm of endothelial cells. In the capillaries of the second type, along the endothelial cells, there are thinned areas - fenestrae, which facilitate the transport of substances. In the wall of the third type of capillaries - sinusoids - the spaces between the endothelial cells coincide with the holes in the basement membrane. Not only macromolecules dissolved in the blood or tissue fluid, but also the blood cells themselves easily pass through such a wall.
Capillary permeability is determined by a number of factors: the condition of surrounding tissues, pressure and chemical composition blood and tissue fluid, the effect of hormones, etc.
There are arterial and venous ends of the capillary. The diameter of the arterial end of the capillary is approximately the size of a red blood cell, and the venous end is slightly larger.
Larger vessels can also arise from the terminal arteriole - metarteriols(main channels). They cross the capillary bed and flow into the venule. In their wall, especially in the initial part, there are smooth muscle cells. Numerous true capillaries extend from their proximal end and there are precapillary sphincters. True capillaries may flow into the distal end of the metarteriole. These vessels play the role of local regulation of blood flow. They may also serve as channels to enhance the flow of blood from arterioles into venules. This process takes on particular importance during thermoregulation (for example, in subcutaneous tissue).
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There are three varieties venulus: postcapillary, collecting and muscular. The venous parts of the capillaries are collected in postcapillary venules, the diameter of which reaches 8–30 µm. At the junction, the endothelium forms folds similar to the valves of veins, and the number of pericytes increases in the walls. Plasma and blood cells can pass through the wall of such venules. These venules empty into collecting venules with a diameter of 30–50 microns. Individual smooth muscle cells appear in their walls, often not completely surrounding the lumen of the vessel. The outer shell is clearly defined. muscle venules, 50–100 μm in diameter, contain 1–2 layers of smooth muscle cells in the middle shell and a pronounced outer shell.
The number of vessels draining blood from the capillary bed is usually twice the number of bringing vessels. Numerous anastomoses are formed between individual venules; along the course of the venules, expansions, lacunae and sinusoids can be observed. These morphological features of the venous section create the prerequisites for the deposition and redistribution of blood in various organs and tissues. Calculations show that located in circulatory system the blood is distributed in such a way that the arterial system contains up to 15%, the capillaries – 5–12%, and the venous system – 70–80%.
Blood from arterioles can enter venules bypassing the capillary bed - through arteriolo-venular anastomoses (shunts). They are present in almost all organs, their diameter ranges from 30 to 500 microns. The walls of anastomoses contain smooth muscle cells, due to which their diameter can change. Through typical anastomoses arterial blood discharged into the venous bed. Atypical anastomoses are the metarterioles described above, through which mixed blood flows. Anastomoses are richly innervated, the width of their lumen is regulated by the tone of smooth muscle cells. Anastomoses control blood flow through the organ and blood pressure, stimulate venous outflow, participate in the mobilization of deposited blood and regulate the transition of tissue fluid into the venous bed.
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As the venules merge into small veins, the pericytes in their wall are completely replaced by smooth muscle cells. The structure of the veins varies greatly depending on the diameter and location. The number of muscle cells in the walls of the veins depends on whether the blood in them moves towards the heart under the influence of gravity (veins of the head and neck) or against it (veins of the lower extremities). Medium-sized veins have significantly thinner walls than the corresponding arteries, but they are made up of the same three layers. The inner lining consists of endothelium, the internal elastic membrane and subendothelial connective tissue are poorly developed. Average, muscularis propria usually poorly developed, and elastic fibers are almost absent, so a vein cut across, unlike an artery, always collapses. There are almost no muscle cells in the walls of the veins of the brain and its membranes. The outer lining of the veins is the thickest of the three. It consists predominantly of connective tissue with a large number of collagen fibers. Many veins, especially those in the lower half of the body, such as the inferior vena cava, contain large numbers of smooth muscle cells, the contraction of which prevents blood from flowing back and pushes it towards the heart. Since the blood flowing in the veins is significantly depleted of oxygen and nutrients, in the outer shell there are more feeding vessels than in the arteries of the same name. These vascular vessels can reach the inner lining of the vein due to the slight blood pressure. Lymphatic capillaries are also developed in the outer shell, through which excess tissue fluid flows.
By degree of development muscle tissue in the wall of the veins they are divided into veins fibrous type - in them the muscular layer is not developed (veins of hard and soft meninges, retina, bones, spleen, placenta, jugular and internal mammary veins) and veins muscular type. In the veins of the upper body, neck and face, and the superior vena cava, blood moves passively due to its gravity. Their middle shell contains a small number of muscle elements. In the veins digestive tract the muscular layer is unevenly developed. Thanks to this, the veins can expand and perform the function of depositing blood. Among the veins of large caliber, in which the muscular elements are poorly developed, the upper one is most typical vena cava. The movement of blood to the heart through this vein occurs due to gravity, as well as the suction effect of the chest cavity during inhalation. A factor stimulating venous flow to the heart is also the negative pressure in the atrial cavity during diastole.
The veins of the lower extremities are arranged in a special way. The wall of these veins, especially the superficial ones, must resist the hydrostatic pressure created by the column of fluid (blood). Deep veins maintain their structure thanks to the pressure of surrounding muscles, but superficial veins They don't feel that kind of pressure. In this regard, the wall of the latter is much thicker; the muscular layer of the middle shell is well developed in it, containing longitudinally and circularly located smooth muscle cells and elastic fibers. The movement of blood through the veins can also occur due to contraction of the walls of adjacent arteries.
A characteristic feature of these veins is the presence valves. These are semilunar folds of the inner membrane (intima), usually located in pairs at the confluence of two veins. The valves are shaped like pockets open towards the heart, which prevents blood from flowing back due to gravity. A cross section of the valve shows that the outside of the leaflet is covered with endothelium, and the base is a thin plate of connective tissue. At the base of the valve leaflets there are a small number of smooth muscle cells. Typically, the vein dilates slightly proximal to the valve insertion. In the veins of the lower half of the body, where blood moves against gravity, the muscular layer is better developed and valves are more common. There are no valves in the vena cava (hence their name), in the veins of almost all the insides, the brain, head, neck and small veins.
The direction of the veins is not as straight as the arteries - they are characterized by a tortuous course. Another feature of the venous system is that many small and medium-sized arteries are accompanied by two veins. Often the veins branch and reconnect with each other, forming numerous anastomoses. There are well-developed venous plexuses in many places: in the pelvis, in spinal canal, around bladder. The significance of these plexuses can be seen in the example of the intravertebral plexus. When filled with blood, it occupies those free spaces that are formed when cerebrospinal fluid is displaced when changing body position or during movements. Thus, the structure and location of the veins depends on the physiological conditions of blood flow in them.
Blood not only flows in the veins, but is also reserved in separate areas beds. Approximately 70 ml of blood per 1 kg of body weight is involved in blood circulation and another 20–30 ml per 1 kg are in venous depots: in the veins of the spleen (approximately 200 ml of blood), in the veins gate system liver (about 500 ml), in the venous plexuses gastrointestinal tract and skin. If during hard work it is necessary to increase the volume of circulating blood, it leaves the depot and enters the general circulation. Blood depots are under control nervous system.
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The walls of blood vessels are richly supplied with motor and sensory nerve fibers. Afferent endings perceive information about blood pressure on the walls of blood vessels (baroreceptors) and the content of substances such as oxygen, carbon dioxide and others in the blood (chemoreceptors). Baroreceptor nerve endings, most numerous in the aortic arch and in the walls of large veins and arteries, are formed by the terminals of fibers passing through the vagus nerve. Numerous baroreceptors are concentrated in the carotid sinus, located near the bifurcation (bifurcation) of the common carotid artery. In the wall of the internal carotid artery there is carotid body. Its cells are sensitive to changes in the concentration of oxygen and carbon dioxide in the blood, as well as its pH. The fibers of the glossopharyngeal, vagus and sinus nerves form afferent nerve endings on the cells. Through them, information flows to the centers of the brain stem that regulate the activity of the heart and blood vessels. Efferent innervation is carried out by fibers of the superior sympathetic ganglion.
The blood vessels of the torso and limbs are innervated by fibers of the autonomic nervous system, mainly sympathetic, passing through the spinal nerves. Approaching the vessels, the nerves branch and form a plexus in the superficial layers of the vessel wall. The nerve fibers extending from it form the second, supramuscular or border, plexus at the border of the outer and middle membranes. From the latter, the fibers go to the middle layer of the wall and form the intermuscular plexus, which is especially pronounced in the wall of the arteries. Individual nerve fibers penetrate the inner layer of the wall. The plexuses include both motor and sensory fibers.
