Blood vessels. Structure of the vascular wall

The wall of a blood vessel consists of several layers: internal (tunica intima), containing the endothelium, subendothelial layer and internal elastic membrane; middle (tunica media), educated smooth muscle cells and elastic fibers; external (tunica externa), represented by loose connective tissue, which contains nerve plexuses and vasa vasorum. The wall of the blood vessel receives nutrition from branches extending from the main trunk of the same artery or another adjacent artery. These branches penetrate the wall of an artery or vein through the outer membrane, forming a plexus of arteries in it, which is why they are called “vascular vessels” (vasa vasorum).

Blood vessels heading to the heart are usually called veins, and blood vessels leaving the heart are called arteries, regardless of the composition of the blood that flows through them. Arteries and veins differ in the features of their external and internal structure.
1. The following types of artery structure are distinguished: elastic, elastic-muscular and muscular-elastic.

Elastic arteries include the aorta, brachiocephalic trunk, subclavian, common and internal carotid artery, general iliac artery. In the middle layer of the wall, elastic fibers predominate over collagen, lying in the form of a complex network forming membranes. Inner shell The elastic type vessel is thicker than the muscular-elastic type artery. The wall of elastic vessels consists of endothelium, fibroblasts, collagen, elastic, argyrophilic and muscle fibers. The outer shell contains a lot of collagen connective tissue fibers.

Arteries of the elastic-muscular and muscular-elastic types (upper and lower extremities, extraorgan arteries) are characterized by the presence of elastic and muscle fibers in their middle layer. Muscle and elastic fibers are intertwined in the form of spirals along the entire length of the vessel.

2. Intraorgan arteries, arterioles and venules have a muscular type of structure. Their middle shell is formed by muscle fibers (Fig. 362). At the border of each layer of the vascular wall there are elastic membranes. The inner lining in the area where the arteries branch is thickened into pads that resist the vortex impacts of the blood flow. When the muscle layer of blood vessels contracts, blood flow is regulated, which leads to an increase in resistance and an increase in blood pressure. In this case, conditions arise when the blood is directed to another channel, where the pressure is lower due to relaxation of the vascular wall, or the blood flow is discharged through arteriovenular anastomoses into venous system. Blood is constantly redistributed in the body, and first of all it is sent to the organs that need it most. For example, during a layoff, i.e., work, striated muscles their blood supply increases 30 times. But in other organs there is a compensatory slowdown in blood flow and a decrease in blood supply.

362. Histological section of the elastic artery muscular type and veins.
1 - inner layer of the vein; 2 - middle layer of the vein; 3 - outer layer of the vein; 4 - outer (adventitial) layer of the artery; 5 - middle layer of the artery; 6 - inner layer of the artery.

363. Valves in the femoral vein. The arrow shows the direction of blood flow (according to Sthor).
1 - vein wall; 2 - valve leaf; 3 - valve bosom.

3. Veins differ in structure from arteries, which depends on low pressure blood. The wall of the veins (inferior and superior vena cava, all extraorgan veins) consists of three layers (Fig. 362). Inner layer well developed and contains, in addition to the endothelium, muscle and elastic fibers. In many veins there are valves (Fig. 363) that have a connective tissue cusp and at the base of the valve a roller-like thickening of muscle fibers. Middle layer veins are thicker and consist of spiral muscle, elastic and collagen fibers. Veins lack an outer elastic membrane. At the confluence of veins and distal to the valves, which act as sphincters, muscle bundles form circular thickenings. The outer shell consists of loose connective and adipose tissue and contains a denser network of perivascular vessels (vasa vasorum) than the arterial wall. Many veins have a paravenous bed due to the well-developed perivascular plexus (Fig. 364).

364. Schematic illustration vascular bundle representing closed system, Where pulse wave promotes the movement of venous blood.

In the wall of the venules, muscle cells are detected that act as sphincters that function under the control of humoral factors(serotonin, catecholamine, histamine, etc.). Intraorgan veins are surrounded by a connective tissue sheath located between the vein wall and the organ parenchyma. Networks are often located in this connective tissue layer lymphatic capillaries, for example in the liver, kidneys, testicles and other organs. In the abdominal organs (heart, uterus, bladder, stomach, etc.) smooth muscle their walls are woven into the wall of the vein. Veins that are not filled with blood collapse due to the lack of an elastic elastic frame in their wall.

4. Blood capillaries have a diameter of 5-13 microns, but there are also organs with wide capillaries (30-70 microns), for example in the liver, the anterior lobe of the pituitary gland; even wider capillaries in the spleen, clitoris and penis. The capillary wall is thin and consists of a layer of endothelial cells and a basement membrane. WITH outside the blood capillary is surrounded by pericytes (cells connective tissue). There are no muscular and nervous elements in the capillary wall, therefore the regulation of blood flow through the capillaries is completely under the control of the muscular sphincters of arterioles and venules (this distinguishes them from capillaries), and the activity is regulated by the sympathetic nervous system and humoral factors.

In the capillaries, blood flows in a constant stream without pulsating shocks at a speed of 0.04 cm/s under a pressure of 15-30 mm Hg. Art.

Capillaries in organs, anastomosing with each other, form networks. The shape of the networks depends on the design of the organs. IN flat organs- fascia, peritoneum, mucous membranes, conjunctiva of the eye - flat networks are formed (Fig. 365), in three-dimensional - liver and other glands, lungs - there are three-dimensional networks (Fig. 366).

365. Single-layer network of blood capillaries of the mucous membrane of the bladder.

366. Network of blood capillaries of the lung alveoli.

The number of capillaries in the body is enormous and their total lumen exceeds the diameter of the aorta by 600-800 times. 1 ml of blood is distributed over a capillary area of ​​0.5 m2.

The study of the cardiovascular system is called angiocardiology.

For the first time exact description the mechanism of blood circulation and the importance of the heart is given English doctor- V. Harvey. A. Vesalius, the founder of scientific anatomy, described the structure of the heart. The Spanish doctor - M. Servet - correctly described the pulmonary circulation.

Types of blood vessels.

Anatomically blood vessels are divided into arteries, arterioles, precapillaries, capillaries, postcapillaries, venules, veins. Arteries and veins are great vessels, the rest are microvasculature.

Arteries - vessels that carry blood from the heart, regardless of what kind of blood it is.

Structure:

Most arteries have an elastic membrane between the membranes, which gives the wall elasticity and elasticity.

Types of arteries

I. Depending on the diameter:

Large;

Average;

II. Depending on location:

Extraorganic;

Intraorgan.

III. Depending on the structure:

Elastic type - aorta, pulmonary trunk.

Muscular-elastic type - subclavian, general carotid.

Muscular type - smaller arteries contribute to the movement of blood through their contraction. A prolonged increase in the tone of these muscles leads to arterial hypertension.

Capillaries – microscopic vessels that are found in tissues and connect arterioles with venules (through pre- and postcapillaries). Through their walls occur metabolic processes, visible only under a microscope. The wall consists of one layer of cells - endothelium, located on basement membrane formed by loose fibrous connective tissue.

Vienna - vessels that carry blood to the heart, regardless of what kind it is. Consist of three shells:

· Inner lining – consists of endothelium.

· The middle layer is smooth muscle.

· Outer shell – adventitia.

Features of the structure of veins:

The walls are thinner and weaker.

Elastic and muscle fibers are less developed, so their walls may collapse.

The presence of valves (semilunar folds of the mucous membrane) that prevent blood flow. Do not have valves: vena cava, portal vein, pulmonary veins, veins of the head, renal veins.

Anastomoses – branches of arteries and veins; can connect and form an anastomosis.

Collaterals – vessels that provide a roundabout outflow of blood, bypassing the main one.

The following vessels are functionally distinguished:

· The main vessels are the largest - the blood flow resistance is small.

· Resistance vessels (vessels of resistance) are small arteries and arterioles that can change the blood supply to tissues and organs. They have a well-developed muscular layer and can taper.

· True capillaries (exchange vessels) – have high permeability, due to which the exchange of substances occurs between blood and tissues.

· Capacitive vessels – venous vessels (veins, venules), containing 70-80% of the blood.

· Shunt vessels – arteriovenular anastomoses, providing a direct connection between arterioles and venules, bypassing the capillary bed.

Structure of blood vessels

Blood vessels develop from mesenchyme. First, the primary wall is laid, which subsequently turns into the inner lining of the vessels. Mesenchyme cells, connecting, form the cavity of future vessels. The wall of the primary vessel consists of flat mesenchymal cells that form the inner layer of future vessels. This layer of flat cells belongs to the endothelium. Later, the final, more complex vessel wall is formed from the surrounding mesenchyme. It is characteristic that all vessels in embryonic period are laid down and built like capillaries, and only in the process of them further development the simple capillary wall is gradually surrounded by various structural elements, and the capillary vessel becomes either an artery, a vein, or a lymphatic vessel.

The final formed walls of the vessels of both arteries and veins are not the same along their entire length, but both of them consist of three main layers (Fig. 231). Common to all vessels is a thin inner membrane, or intima (tunica intima), lined on the side of the vascular cavity with the thinnest, very elastic and flat polygonal endothelial cells. The intima is a direct continuation of the endothelium and endocardium. This inner lining with a smooth and even surface protects the blood from clotting. If the endothelium of a vessel is damaged by injury, infection, inflammatory or dystrophic process, etc., then small blood clots (clotting - thrombi) form at the site of damage, which can increase in size and cause blockage of the vessel. Sometimes they break away from the site of formation, are carried away by the blood stream and, as so-called emboli, clog a vessel in some other place. The effect of such a thrombus or embolus depends on where the vessel is blocked. Thus, blockage of a vessel in the brain can cause paralysis; blockage coronary artery heart disease deprives the heart muscle of blood flow, which results in a severe heart attack and often leads to death. Blockage of a vessel leading to any part of the body or internal organ, deprives it of nutrition and can lead to necrosis (gangrene) of the supplied part of the organ.

Outside the inner layer is the middle shell (media), consisting of circular smooth muscle fibers with an admixture of elastic connective tissue.

The outer shell of the vessels (adventitia) covers the middle one. In all vessels it is built of fibrous fibrous connective tissue, containing predominantly longitudinally located elastic fibers and connective tissue cells.

At the border of the middle and inner, middle and outer shells of blood vessels, elastic fibers form a kind of thin plate (membrana elastica interna, membrana elastica externa).

In the outer and middle membranes of blood vessels, the vessels that feed their wall (vasa vasorum) branch.

Walls capillary vessels extremely thin (about 2 μ) and consist mainly of a layer of endothelial cells that form the capillary tube. This endothelial tube is braided on the outside with a thin network of fibers on which it is suspended, thanks to which it moves very easily and without damage. The fibers extend from a thin, main film, with which they are also connected special cells- pericytes covering capillaries. The capillary wall is easily permeable to leukocytes and blood; It is at the level of capillaries through their wall that exchange takes place between blood and tissue fluids, as well as between blood and external environment(in the excretory organs).

Arteries and veins are usually divided into large, medium and small. The smallest arteries and veins that turn into capillaries are called arterioles and venules. The arteriole wall consists of all three membranes. The innermost is endothelial, and the next middle one is built from circularly arranged smooth muscle cells. When an arteriole passes into a capillary, only single smooth muscle cells are observed in its wall. With the enlargement of the arteries, the number of muscle cells gradually increases to a continuous annular layer - a muscle-type artery.

The structure of small and medium arteries differs in some other feature. Under the inner endothelial membrane there is a layer of elongated and stellate cells, which in larger arteries form a layer that plays the role of cambium (germ layer) for the vessels. This layer is involved in the processes of regeneration of the vessel wall, i.e. it has the property of restoring the muscular and endothelial layers of the vessel. In medium-sized arteries or mixed type the cambial (germ) layer is more developed.

Large-caliber arteries (aorta and its large branches) are called elastic arteries. Elastic elements predominate in their walls; in the middle shell, strong elastic membranes are concentrically laid, between which lies a significantly smaller number of smooth muscle cells. The cambial layer of cells, well defined in small and medium-sized arteries, in large arteries turns into a layer of subendothelial loose connective tissue rich in cells.

Due to the elasticity of the walls of the arteries, like rubber tubes, they can easily stretch under the pressure of blood and do not collapse, even if the blood is released from them. All the elastic elements of the vessels together form a single elastic frame, which works like a spring, each time returning the vessel wall to its original state as soon as the smooth muscle fibers relax. Since arteries, especially large ones, have to withstand quite high blood pressure, then their walls are very strong. Observations and experiments show that arterial walls can withstand even such strong pressure as in the steam boiler of a conventional locomotive (15 atm.).

The walls of veins are usually thinner than the walls of arteries, especially their tunica media. There is also significantly less elastic tissue in the venous wall, so the veins collapse very easily. The outer shell is made of fibrous connective tissue, which is dominated by collagen fibers.

A feature of the veins is the presence of valves in them in the form of semilunar pockets (Fig. 232), formed from doubling the inner membrane (intima). However, not all veins in our body have valves; The veins of the brain and its membranes, the veins of the bones, as well as a significant part of the veins of the viscera, lack them. Valves are more often found in the veins of the limbs and neck; they are open towards the heart, i.e. in the direction of blood flow. By blocking the reverse outflow that may occur due to low blood pressure and due to the law of gravity ( hydrostatic pressure), valves facilitate blood flow.

If there were no valves in the veins, the entire weight of a column of blood more than 1 m high would put pressure on the blood entering the lower limb and thereby greatly impede blood circulation. Further, if the veins were inflexible tubes, the valves alone could not ensure blood circulation, since the entire column of liquid would still press on the underlying sections. Veins are located among large skeletal muscles, which, contracting and relaxing, periodically compress the venous vessels. When a contracting muscle compresses a vein, the valves located below the clamping point close, and those located above open; when the muscle relaxes and the vein is again free from compression, the upper valves in it close and retain the above-located column of blood, while the lower ones open and allow the vessel to refill with blood coming from below. This pumping action of the muscles (or "muscle pump") greatly aids blood circulation; standing for many hours in one place, in which the muscles help little to move the blood, is more tiring than walking.

AFO of the cardiovascular system.

Anatomy and physiology of the heart.

The structure of the circulatory system. Structural features in different age periods. The essence of the blood circulation process. Structures that carry out the blood circulation process. Basic indicators of blood circulation (number of heartbeats, blood pressure, electrocardiogram indicators). Factors affecting blood circulation (physical and nutritional stress, stress, lifestyle, bad habits etc.). Circulation circles. Vessels, types. The structure of the walls of blood vessels. Heart - location, external structure, anatomical axis, projection onto the surface of the chest in different age periods. Heart chambers, orifices and heart valves. Principles of operation of heart valves. The structure of the heart wall - endocardium, myocardium, epicardium, location, physiological properties. Conduction system of the heart. Physiological properties. The structure of the pericardium. Vessels and nerves of the heart. Phases and duration cardiac cycle. Physiological properties of the heart muscle.

Circulatory system

The functions of the blood are performed due to the continuous operation of the circulatory system. Blood circulation - This is the movement of blood through the vessels, ensuring the exchange of substances between all tissues of the body and the external environment. The circulatory system includes the heart and blood vessels. Blood circulation in the human body through a closed cardiovascular system is ensured by rhythmic contractions hearts- her central authority. The vessels through which blood from the heart is carried to tissues and organs are called arteries, and those through which blood is delivered to the heart - veins. In tissues and organs, thin arteries (arterioles) and veins (venules) are interconnected by a dense network blood capillaries.

Structural features at different age periods.

A newborn's heart has rounded shape. Its transverse diameter is 2.7-3.9 cm, the length of the heart is on average 3.0-3.5 cm. The anteroposterior size is 1.7-2.6 cm. The atria are large compared to the ventricles, and the right Of these, the left one is significantly larger. The heart grows especially quickly during a child's year of life, and its length increases more than its width. Individual parts of the heart change differently at different ages: during the 1st year of life, the atria grow more than the ventricles. At the age of 2 to 6 years, the growth of the atria and ventricles occurs equally rapidly. After 10 years, the ventricles enlarge faster than the atria. The total weight of the heart in a newborn is 24 g, at the end of the 1st year of life it increases approximately 2 times, by 4-5 years - by 3 times, at 9-10 years - by 5 times and by 15-16 years - by 10 once. Up to 5-6 years of age, the heart weight is greater in boys than in girls; at 9-13 years of age, on the contrary, it is greater in girls, and at 15 years of age, the heart weight is again greater in boys than in girls. In newborns and infants, the heart is located high and lies transversely. The transition of the heart from a transverse to an oblique position begins at the end of the 1st year of a child’s life.

Factors affecting blood circulation (physical and nutritional stress, stress, lifestyle, bad habits, etc.).

Circulation circles.

Large and small circles of blood circulation. IN In the human body, blood moves through two circles of blood circulation - large (trunk) and small (pulmonary).

Big circle blood circulation begins in the left ventricle, from which arterial blood is ejected into the largest artery in diameter - aorta. The aorta arches to the left and then runs along the spine, branching into smaller arteries that carry blood to the organs. In organs, arteries branch into smaller vessels - arterioles, that go online capillaries, penetrating tissues and delivering oxygen and nutrients. Venous blood through the veins collects in two large vessels - top And inferior vena cava, which pour it into the right atrium.

Pulmonary circulation begins in the right ventricle, from where the arterial pulmonary trunk emerges, which divides into colorpulmonary arteries, carrying blood to the lungs. In the lungs, large arteries branch into smaller arterioles, which pass into a network of capillaries that densely entwine the walls of the alveoli, where the exchange of gases occurs. Oxygenated arterial blood flows through the pulmonary veins into the left atrium. Thus, venous blood flows in the arteries of the pulmonary circulation, and arterial blood flows in the veins.

Not all the blood volume in the body circulates evenly. A significant portion of the blood is in blood depots- liver, spleen, lungs, subcutaneous choroid plexuses. The importance of blood depots lies in the ability to quickly provide oxygen to tissues and organs in emergency situations.

Vessels, types. The structure of the walls of blood vessels.

The wall of the vessel consists of three layers:

1. The inner layer is very thin, it is formed by a single row of endothelial cells, which give smoothness inner surface vessels.

2. The middle layer is the thickest, it contains a lot of muscle, elastic and collagen fibers. This layer ensures the strength of the blood vessels.

3. Outer layer connective tissue, it separates the vessels from the surrounding tissues.

Arteries The blood vessels that go from the heart to the organs and carry blood to them are called arteries. Blood from the heart flows through the arteries under high pressure, which is why the arteries have thick elastic walls.

According to the structure of the walls, arteries are divided into two groups:

· Elastic arteries - the arteries closest to the heart (aorta and its large branches) primarily perform the function of conducting blood.

· Arteries of the muscular type - medium and small arteries in which the inertia of the cardiac impulse weakens and the own contraction of the vascular wall is required for further movement of blood

In relation to an organ, there are arteries that go outside the organ before entering it - extraorgan arteries - and their continuations that branch inside it - intraorgan or intraorgan arteries. Lateral branches of the same trunk or branches of different trunks can connect to each other. This connection of vessels before they break up into capillaries is called anastomosis or anastomosis (the majority of them). Arteries that do not have anastomoses with neighboring trunks before they become capillaries are called terminal arteries (for example, in the spleen). Terminal, or terminal, arteries are more easily blocked by a blood plug (thrombus) and predispose to the formation of a heart attack (local death of an organ).

The last branches of the arteries become thin and small and are therefore called arterioles. They directly pass into the capillaries, and due to the presence of contractile elements in them, they perform a regulatory function.

An arteriole differs from an artery in that its wall has only one layer smooth muscle, thanks to which it carries out a regulatory function. The arteriole continues directly into the precapillary, in which the muscle cells are scattered and do not form a continuous layer. The precapillary differs from the arteriole in that it is not accompanied by a venule, as is observed with the arteriole. Numerous capillaries extend from the precapillary.

Capillaries- the smallest blood vessels located in all tissues between arteries and veins. The main function of capillaries is to ensure the exchange of gases and nutrients between blood and tissues. In this regard, the capillary wall is formed by only one layer of flat endothelial cells, permeable to substances and gases dissolved in the liquid. Through it, oxygen and nutrients easily penetrate from the blood to the tissues, and carbon dioxide and waste products in the opposite direction.

In every at the moment Only part of the capillaries functions (open capillaries), while the other remains in reserve (closed capillaries).

Vienna- blood vessels that carry venous blood from organs and tissues to the heart. The exception is the pulmonary veins, which carry from the lungs to the left atrium arterial blood. The collection of veins forms the venous system, which is part cardiovascular system. The network of capillaries in organs turns into small postcapillaries, or venules. At a considerable distance they still retain a structure similar to the structure of capillaries, but have a wider lumen. Venules merge into more large veins, connected by anastomosis, and form venous plexuses in or near organs. Veins are collected from the plexuses, carrying blood out of the organ. There are superficial and deep veins. Superficial veins located in the subcutaneous fatty tissue, starting from the superficial venous networks; their number, size and position vary greatly. Deep veins , starting on the periphery from small deep veins, accompany the arteries; Often one artery is accompanied by two veins (“companion veins”). As a result of the fusion of the superficial and deep veins, two large venous trunks are formed - the superior and inferior vena cava, which flow into the right atrium, where the common drainage of the cardiac veins - the coronary sinus - also flows. Portal vein carries blood from unpaired organs abdominal cavity.
Low pressure and low blood flow speed cause poor development of elastic fibers and membranes in the venous wall. The need to overcome the gravity of blood in the veins lower limb led to the development of muscular elements in their wall, in contrast to veins upper limbs and the upper half of the body. On the inner lining of the vein there are valves that open along the blood flow and promote the movement of blood in the veins towards the heart. A feature of venous vessels is the presence of valves in them, which are necessary to ensure unidirectional blood flow. The walls of the veins are arranged according to the same plan as the walls of the arteries, however, the blood pressure in the veins is very low, so the walls of the veins are thin, they have less elastic and muscle tissue, causing empty veins to collapse.

Heart- a hollow fibromuscular organ that, functioning as a pump, ensures the movement of blood in the circulatory system. The heart is in anterior mediastinum in the pericardium between the layers of the mediastinal pleura. It has the shape of an irregular cone with the base at the top and the apex facing downwards, to the left and anteriorly. The sizes of S. are individually different. The length of the S. of an adult varies from 10 to 15 cm (usually 12-13 cm), the width at the base is 8-11 cm (usually 9-10 cm) and the anteroposterior size is 6-8.5 cm (usually 6.5-7 cm ). The average weight of S. in men is 332 g (from 274 to 385 g), in women - 253 g (from 203 to 302 g).
In relation to midline the body of the heart is located asymmetrically - about 2/3 to the left of it and about 1/3 to the right. Depending on the direction of the projection of the longitudinal axis (from the middle of its base to the apex) on the anterior chest wall distinguish between transverse, oblique and vertical position of the heart. Vertical position more common in people with narrow and long hair chest, transverse - in persons with a wide and short chest.

The heart consists of four chambers: two (right and left) atria and two (right and left) ventricles. The atria are at the base of the heart. The aorta and pulmonary trunk emerge from the heart in front, and the upper one enters it on the right side. vena cava, in the posteroinferior - the inferior vena cava, behind and to the left - the left pulmonary veins, and somewhat to the right - the right pulmonary veins.

The function of the heart is to rhythmically pump blood into the arteries, which comes to it through the veins. The heart beats about 70-75 times per minute when the body is at rest (1 time per 0.8 s). More than half of this time it rests - relaxes. The continuous activity of the heart consists of cycles, each of which consists of contraction (systole) and relaxation (diastole).

There are three phases of cardiac activity:

· contraction of the atria - atrial systole - takes 0.1 s

· contraction of the ventricles - ventricular systole - takes 0.3 s

general pause - diastole (simultaneous relaxation of the atria and ventricles) - takes 0.4 s

Thus, during the entire cycle, the atria work for 0.1 s and rest for 0.7 s, the ventricles work for 0.3 s and rest for 0.5 s. This explains the ability of the heart muscle to work without getting tired throughout life. The high performance of the heart muscle is due to increased blood supply to the heart. Approximately 10% of the blood ejected by the left ventricle into the aorta enters the arteries that branch from it, which supply the heart.

Circulatory system consists of a central organ - the heart - and closed tubes of various calibers connected to it, called blood vessels(Latin vas, Greek angeion - vessel; hence - angiology). The heart, with its rhythmic contractions, sets in motion the entire mass of blood contained in the vessels.

Arteries. Blood vessels that go from the heart to the organs and carry blood to them, called arteries(aeg - air, tereo - contain; on corpses the arteries are empty, which is why in the old days they were considered air tubes).

The wall of the arteries consists of three membranes.Inner shell, tunica intima. lined on the side of the lumen of the vessel with endothelium, under which lie the subendothelium and internal elastic membrane; middle, tunica media, built from fibers of non-striated muscle tissue, myocytes, alternating with elastic fibers; outer shell, tunica externa, contains connective tissue fibers. Elastic elements arterial wall form a single elastic frame that works like a spring and determines the elasticity of the arteries.

As they move away from the heart, the arteries divide into branches and become smaller and smaller. The arteries closest to the heart (aorta and its large branches) primarily perform the function of conducting blood. In them, counteraction to stretching by the mass of blood, which is ejected by the heart impulse, comes to the fore. Therefore, structures in their wall are relatively more developed mechanical in nature, i.e. elastic fibers and membranes. Such arteries are called elastic arteries. In middle and small arteries, in which the inertia of the cardiac impulse weakens and the own contraction of the vascular wall is required for further movement of blood, prevails contractile function. It is ensured by a relatively large development of muscle tissue in the vascular wall. Such arteries are called muscular arteries. Individual arteries supply blood to entire organs or parts thereof.

In relation to the organ distinguish between arteries, going outside the organ, before entering it - extraorgan arteries, and their continuations, branching inside it - intraorgan, or infraorgan, arteries. Lateral branches of the same trunk or branches of different trunks can connect to each other. This connection of vessels before they break up into capillaries is called anastomosis, or anastomosis (stoma - mouth). The arteries that form anastomoses are called anastomosing (they are the majority). Arteries that do not have anastomoses with neighboring trunks before they become capillaries (see below) are called terminal arteries (for example, in the spleen). Terminal, or terminal, arteries are more easily blocked by a blood plug (thrombus) and predispose to the formation of a heart attack (local death of the organ).

The last branches of the arteries become thin and small and therefore stand out under name of arterioles.

Arteriole differs from an artery in that its wall has only one layer of muscle cells, thanks to which it carries out a regulatory function. The arteriole continues directly into the precapillary, in which the muscle cells are scattered and do not form a continuous layer. The precapillary also differs from the arteriole in that it is not accompanied by a venule.

From precapillary Numerous capillaries emerge.

Capillaries represent the finest vessels, performing an exchange function. In this regard, their wall consists of one layer of flat endothelial cells, permeable to substances and gases dissolved in liquid. Widely anastomosing with each other, the capillaries form networks (capillary networks), turning into postcapillaries, constructed similarly to the precapillary. The postcapillary continues into the venule accompanying the arteriole. Venules form thin initial segments of the venous bed, which make up the roots of the veins and pass into the veins.

Veins (Latin vena, Greek phlebs; hence phlebitis - inflammation of the veins) carry blood in the opposite direction to the arteries, from the organs to the heart. Walls they are arranged according to the same plan as the walls of the arteries, but they are much thinner and have less elastic and muscle tissue, due to which the empty veins collapse, while the lumen of the arteries gapes in the cross section; veins, merging with each other, form large venous trunks - veins flowing into the heart.

The veins widely anastomose with each other, forming venous plexuses.

Movement of blood through veins carried out due to the activity and suction action of the heart and chest cavity, in which during inhalation it is created negative pressure due to the difference in pressure in the cavities, as well as due to the contraction of the skeletal and visceral muscles of the organs and other factors.

Abbreviations also matter muscularis propria vein, which is in the veins of the lower half of the body, where the conditions for venous outflow more complex, more developed than in the veins of the upper body. The reverse flow of venous blood is prevented by special devices of the veins - valves, components features of the venous wall. Venous valves consist of a fold of endothelium containing a layer of connective tissue. They face the free edge towards the heart and therefore do not interfere with the flow of blood in this direction, but keep it from returning back. Arteries and veins usually run together, with small and medium-sized arteries accompanied by two veins, and large ones by one. The exception to this rule, apart from some deep veins, is mainly superficial veins, going to subcutaneous tissue and almost never accompanying arteries. The walls of blood vessels have their own services thin arteries and veins, vasa vasorum. They arise either from the same trunk, the wall of which is supplied with blood, or from a neighboring one and pass in the connective tissue layer surrounding the blood vessels and more or less closely connected with their outer membrane; this layer is called vascular vagina, vagina vasorum. The walls of arteries and veins contain numerous nerve endings (receptors and effectors) associated with the central nervous system, due to which the mechanism of reflexes carries out neural regulation blood circulation Blood vessels represent extensive reflexogenic zones playing big role in the neurohumoral regulation of metabolism.

According to function and structure various departments and the characteristics of the innervation of all blood vessels in lately sent to share into 3 groups: 1) pericardial vessels that begin and end both circles of blood circulation - the aorta and pulmonary trunk (i.e., elastic arteries), vena cava and pulmonary veins; 2) main vessels that serve to distribute blood throughout the body. These are large and medium-sized extraorgan arteries of the muscular type and extraorgan veins; 3) organ vessels that provide exchange reactions between blood and organ parenchyma. These are intraorgan arteries and veins, as well as parts of the microcirculatory bed.