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What is the middle layer of the vessel wall called? The structure of the walls of blood vessels

Functional classification blood vessels.

Main vessels.

Resistive vessels.

Exchange vessels.

Capacitive vessels.

Shunt vessels.

The main vessels are the aorta, large arteries. The wall of these vessels contains many elastic elements and many smooth muscle fibers. Meaning: convert the pulsating ejection of blood from the heart into a continuous flow of blood.

Resistive vessels - pre- and post-capillary. Precapillary vessels - small arteries and arterioles, capillary sphincters - vessels have several layers smooth muscle cells. Postcapillary vessels - small veins, venules - are also present smooth muscle. Meaning: provide the greatest resistance to blood flow. Precapillary vessels regulate blood flow in the microvasculature and maintain a certain blood pressure in large arteries. Post-capillary vessels - maintain a certain level of blood flow and pressure in the capillaries.

Exchange vessels - 1 layer of endothelial cells in the wall - high permeability. They carry out transcapillary exchange.

Capacitive vessels are all venous. They contain 2/3 of all blood. They have the least resistance to blood flow, their wall is easily stretched. Meaning: due to expansion, they deposit blood.

Shunt vessels - connect arteries with veins bypassing capillaries. Meaning: provide unloading of the capillary bed.

The number of anastomoses is not a constant value. They occur when there is poor circulation or lack of blood supply.

Sensitivity - there are many receptors in all layers of the vascular wall. When the pressure, volume, or chemical composition of the blood changes, the receptors are excited. Nerve impulses go to the central nervous system and reflexively affect the heart, blood vessels, and internal organs. Due to the presence of receptors, the vascular system is connected to other organs and tissues of the body.

Motility is the ability of blood vessels to change the lumen in accordance with the needs of the body. The change in the lumen occurs due to the smooth muscles of the vascular wall.

Vascular smooth muscles have the ability to spontaneously generate nerve impulses. Even at rest, there is moderate tension in the vascular wall - basal tone. Under the influence of factors, smooth muscles either contract or relax, changing blood supply.

Meaning:

regulation of a certain level of blood flow,

ensuring constant pressure, blood redistribution;

the capacity of the vessels is adjusted to the volume of blood

Blood circulation time is the time during which a cow passes both circles of blood circulation. At a heart rate of 70 per minute, the time is 20 - 23 s, of which 1/5 of the time is for the small circle; 4/5 of the time - for a large circle. Time is determined using control substances and isotopes. - they are injected intravenously into the v.venaris of the right hand and it is determined after how many seconds, this substance will appear in the v.venaris of the left hand. Time is affected by volumetric and linear speeds.

Volume velocity is the volume of blood that flows through the vessels per unit time. Vlin. - the speed of movement of any blood particle in the vessels. The highest linear velocity is in the aorta, the lowest is in the capillaries (0.5 m/s and 0.5 mm/s, respectively). Linear speed depends on total area sections of blood vessels. Due to low linear speed in capillaries there are conditions for transcapillary exchange. This speed in the center of the vessel is greater than at the periphery.

Blood movement is subject to physical and physiological laws. Physical: - laws of hydrodynamics.

1st law: the amount of blood flowing through the vessels and the speed of its movement depends on the pressure difference at the beginning and end of the vessel. The greater this difference, the better the blood supply.

2nd law: blood flow is prevented by peripheral resistance.

Physiological patterns of blood movement through vessels:

heart function;

closedness of the heart vascular system;

suction effect of the chest;

elasticity of blood vessels.

During the systole phase, blood enters the vessels. The wall of blood vessels stretches. During diastole there is no ejection of blood, the elastic vascular wall returns to its original state, and energy accumulates in the wall. When the elasticity of blood vessels decreases, pulsating blood flow appears (normally in the vessels of the pulmonary circulation). In pathological sclerotic vessels - Musset's symptom - head movements in accordance with pulsation.

Blood vessels in vertebrates form a dense closed network. The wall of the vessel consists of three layers:

The inner layer is very thin, it is formed by one row of endothelial cells, which give the smoothness of the inner surface of the vessels.
The middle layer is the thickest, containing many muscle, elastic and collagen fibers. This layer ensures the strength of the blood vessels.
The outer layer is connective tissue; it separates the vessels from the surrounding tissues.

According to the circles of blood circulation, blood vessels can be divided into:

Arteries great circle blood circulation [show]
- The largest arterial vessel in the human body is the aorta, which emerges from the left ventricle and gives rise to all the arteries that form the systemic circulation. The aorta is divided into the ascending aorta, aortic arch and descending aorta. The aortic arch in turn is divided into thoracic aorta and abdominal aorta.
- Arteries of the neck and head
  General carotid artery(right and left), which at the level of the upper edge of the thyroid cartilage is divided into the external carotid artery and the internal carotid artery.
  - The external carotid artery gives off a number of branches, which, according to their topographical characteristics, are divided into four groups - anterior, posterior, medial and a group of terminal branches supplying blood thyroid gland, muscles of the hyoid bone, sternocleidomastoid muscle, muscles of the mucous membrane of the larynx, epiglottis, tongue, palate, tonsils, face, lips, ear (external and internal), nose, back of the head, dura mater.
  - The internal carotid artery in its course is a continuation of both carotid arteries. It distinguishes between the cervical and intracranial (head) parts. In the cervical part, the internal carotid artery usually does not give branches. In the cranial cavity, branches extend from the internal carotid artery to big brain and the orbital artery, which supply blood to the brain and eye.
  Subclavian artery - steam, begin in anterior mediastinum: right - from the brachiocephalic trunk, left - directly from the aortic arch (therefore the left artery is longer than the right). IN subclavian artery Topographically, three divisions are distinguished, each of which gives its branches:
  - Branches of the first department - vertebral artery, internal thoracic artery, thyroid-cervical trunk - each of which gives off its own branches that supply blood to the brain, cerebellum, neck muscles, thyroid gland, etc.
  - Branches of the second section - here only one branch departs from the subclavian artery - the costocervical trunk, which gives rise to arteries supplying blood to the deep muscles of the back of the head, spinal cord, back muscles, intercostal spaces
  - Branches of the third section - one branch also departs here - the transverse artery of the neck, which supplies blood to the back muscles
- Arteries upper limb, forearms and hands
- Arteries of the trunk
- Pelvic arteries
- Arteries of the lower limb
Veins of the systemic circulation [show]
- Superior vena cava system
  - Veins of the trunk
  - Veins of the head and neck
  - Veins of the upper limb
- Inferior vena cava system
  - Veins of the trunk
- Veins of the pelvis
  - Veins of the lower extremities
Vessels of the pulmonary circulation [show]
The vessels of the pulmonary, pulmonary, circulation include:
- pulmonary trunk
- pulmonary veins in two pairs, right and left
Pulmonary trunk divided into two branches: right pulmonary artery and the left pulmonary artery, each of which is directed to the gate of the corresponding lung, bringing venous blood from the right ventricle to it.
The right artery is slightly longer and wider than the left. Entering lung root it is divided into three main branches, each of which enters the gate of the corresponding lobe of the right lung.
The left artery at the root of the lung is divided into two main branches that enter the portal of the corresponding lobe of the left lung.
A fibromuscular cord (arterial ligament) runs from the pulmonary trunk to the aortic arch. During the period intrauterine development this ligament is the ductus arteriosus through which most of the blood from the pulmonary trunk of the fetus passes into the aorta. After birth, this duct is obliterated and turns into the indicated ligament.
Pulmonary veins, right and left, - remove arterial blood from the lungs. They leave the hilum of the lungs, usually two from each lung (although the number of pulmonary veins can reach 3-5 or even more), the right veins are longer than the left ones, and flow into the left atrium.

According to their structural features and functions, blood vessels can be divided into:

Groups of vessels according to the structural features of the wall

Arteries

Blood vessels going from the heart to the organs and carrying blood to them are called arteries (aer - air, tereo - contain; on corpses the arteries are empty, which is why in the old days they were considered air tubes). 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. 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. The elastic elements of the arterial wall form a single elastic frame that works like a spring and determines the elasticity of the arteries.
Elastic fibers give arteries elastic properties, which ensure continuous blood flow throughout the vascular system. The left ventricle pushes out during contraction high pressure more blood than flows out of the aorta into the arteries. In this case, the walls of the aorta stretch, and it accommodates all the blood ejected by the ventricle. When the ventricle relaxes, the pressure in the aorta drops, and its walls, due to their elastic properties, collapse slightly. Excess blood contained in the distended aorta is pushed out of the aorta into the arteries, although no blood flows from the heart at this time. Thus, the periodic expulsion of blood by the ventricle, due to the elasticity of the arteries, turns into a continuous movement of blood through the vessels.
The elasticity of the arteries provides another physiological phenomenon. It is known that in any elastic system a mechanical shock causes vibrations that propagate throughout the system. IN circulatory system This impulse is the impact of the blood ejected by the heart against the walls of the aorta. The resulting vibrations propagate along the walls of the aorta and arteries at a speed of 5-10 m/s, which significantly exceeds the speed of blood movement in the vessels. In areas of the body where large arteries come close to the skin - on the wrist, temples, neck - you can feel the vibrations of the artery walls with your fingers. This is the arterial pulse.
Arteries muscular type- medium and small arteries, in which the inertia of the cardiac impulse weakens and its own contraction of the vascular wall is required for further movement of blood, which is ensured by the relatively greater development of smooth muscle tissue in the vascular wall. Smooth muscle fibers, contracting and relaxing, narrow and dilate arteries and thus regulate blood flow in them.

Individual arteries supply blood to entire organs or parts thereof. 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 arteries that form anastomoses are called anastomosing (they are the majority). Arteries that do not have anastomoses with neighboring trunks before they pass into 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 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 of 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; their diameter is 5-10 microns. 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). On an area of 1 mm 2 of cross-section of skeletal muscle at rest, there are 100-300 open capillaries. In a working muscle, where the need for oxygen and nutrients increases, the number of open capillaries reaches 2 thousand per 1 mm 2.

Widely anastomosing among themselves, the capillaries form networks (capillary networks), which include 5 links:

arterioles as the most distal parts of the arterial system;
precapillaries, which are an intermediate link between arterioles and true capillaries;
capillaries;
postcapillaries
venules, which are the roots of veins and pass into veins

All these links are equipped with mechanisms that ensure the permeability of the vascular wall and the regulation of blood flow at the microscopic level. Blood microcirculation is regulated by the work of the muscles of the arteries and arterioles, as well as special muscle sphincters, which are located in the pre- and post-capillaries. Some vessels of the microvasculature (arterioles) perform primarily a distributive function, while others (precapillaries, capillaries, postcapillaries and venules) perform a predominantly trophic (metabolic) function.

Vienna

Unlike arteries, veins (Latin vena, Greek phlebs; hence phlebitis - inflammation of the veins) do not carry, but collect blood from the organs and carry it in the opposite direction to the arteries: from the organs to the heart. The walls of the veins have the same structure as the walls of the arteries, but the blood pressure in the veins is very low, so the vein walls are thin and have less elastic and muscle tissue, causing the empty veins to collapse. The veins widely anastomose with each other, forming venous plexuses. Merging with each other, small veins form large venous trunks - veins that flow into the heart.

The movement of blood through the veins is due to the 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, the contraction of striated and smooth muscles of the organs and other factors. The contraction of the muscular lining of the veins is also important, which in the veins of the lower half of the body, where conditions for venous outflow are more difficult, is 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, which make up the 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. From this rule, in addition to some deep veins, the exception is mainly the superficial veins going into subcutaneous tissue and almost never accompanying arteries.

The walls of blood vessels have their own thin arteries and veins, vasa vasorum, serving them. 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 adventitia; this layer is called the 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.

Functional groups of blood vessels

All vessels, depending on the function they perform, can be divided into six groups:

shock-absorbing vessels (elastic type vessels)
resistance vessels
sphincter vessels
exchange vessels
capacitive vessels
shunt vessels

Shock-absorbing vessels. These vessels include elastic-type arteries with a relatively high content of elastic fibers, such as the aorta, pulmonary artery and adjacent sections of large arteries. The pronounced elastic properties of such vessels, in particular the aorta, cause a shock-absorbing effect, or the so-called Windkessel effect (Windkessel in German means “compression chamber”). This effect is to dampen (smooth) the periodic systolic waves of blood flow.

The Windkessel effect for smoothing the movement of liquid can be explained by the following experiment: water is released from the tank in an intermittent stream simultaneously through two tubes - rubber and glass, which end in thin capillaries. At the same time, from glass tube water flows out in spurts, while from rubber it flows evenly and in greater quantities than from glass. The ability of an elastic tube to equalize and increase the flow of liquid depends on the fact that at the moment when its walls are stretched by a portion of liquid, elastic tension energy of the tube arises, i.e., a portion of the kinetic energy of liquid pressure is converted into potential energy of elastic tension.

In the cardiovascular system, part of the kinetic energy developed by the heart during systole is spent on stretching the aorta and the large arteries extending from it. The latter form an elastic, or compression, chamber into which a significant volume of blood enters, stretching it; in this case, the kinetic energy developed by the heart transforms into the energy of elastic tension arterial walls. When systole ends, this elastic tension of the vascular walls created by the heart maintains blood flow during diastole.

More distally located arteries have more smooth muscle fibers, so they are classified as muscular-type arteries. Arteries of one type smoothly pass into vessels of another type. Obviously, in large arteries, smooth muscles influence mainly the elastic properties of the vessel, without actually changing its lumen and, consequently, hydrodynamic resistance.

Resistive vessels. Resistive vessels include terminal arteries, arterioles and, to a lesser extent, capillaries and venules. It is the terminal arteries and arterioles, i.e. precapillary vessels that have a relatively small lumen and thick walls with developed smooth muscles, have the greatest resistance to blood flow. Changes in the degree of contraction of the muscle fibers of these vessels lead to distinct changes in their diameter and, therefore, in the total cross-sectional area (especially when we're talking about about numerous arterioles). Considering that hydrodynamic resistance largely depends on the cross-sectional area, it is not surprising that it is the contractions of the smooth muscles of the precapillary vessels that serve as the main mechanism for regulating the volumetric velocity of blood flow in various vascular areas, as well as the distribution of cardiac output (systemic blood flow) among different organs .

The resistance of the postcapillary bed depends on the condition of the venules and veins. The relationship between precapillary and postcapillary resistance has great value for hydrostatic pressure in the capillaries and therefore for filtration and reabsorption.

Sphincter vessels. The number of functioning capillaries, i.e., the exchange surface area of the capillaries (see Fig.), depends on the narrowing or expansion of the sphincters - the last sections of the precapillary arterioles.

Exchange vessels. These vessels include capillaries. It is in them that such things happen critical processes like diffusion and filtration. Capillaries are not capable of contraction; their diameter changes passively following pressure fluctuations in pre- and post-capillary resistive vessels and sphincter vessels. Diffusion and filtration also occur in venules, which should therefore be classified as exchange vessels.

Capacitive vessels. Capacitive vessels are mainly veins. Due to their high distensibility, veins are able to accommodate or eject large volumes of blood without significantly affecting other parameters of blood flow. In this regard, they can play the role of blood reservoirs.

Some veins at low intravascular pressure are flattened (i.e., have an oval lumen) and therefore can accommodate some additional volume without stretching, but only acquiring a more cylindrical shape.

Some veins have a particularly high capacity as blood reservoirs, which is due to their anatomical structure. These veins include primarily 1) the veins of the liver; 2) large veins celiac region; 3) veins of the subpapillary plexus of the skin. Together, these veins can hold more than 1000 ml of blood, which is released when needed. Short-term deposition and release of sufficiently large quantities of blood can also be carried out by the pulmonary veins connected to the systemic circulation in parallel. This changes the venous return to the right heart and/or the output of the left heart [show]

Intrathoracic vessels as a blood depot

Due to high stretchability pulmonary vessels the volume of blood circulating in them can temporarily increase or decrease, and these fluctuations can reach 50% of the average total volume of 440 ml (arteries - 130 ml, veins - 200 ml, capillaries - 110 ml). Transmural pressure in the vessels of the lungs and their distensibility change slightly.

The volume of blood in the pulmonary circulation, together with the end-diastolic volume of the left ventricle of the heart, constitutes the so-called central blood reserve (600-650 ml) - a quickly mobilized depot.

So, if it is necessary to increase the output of the left ventricle within a short time, then about 300 ml of blood can come from this depot. As a result, the balance between the output of the left and right ventricles will be maintained until another mechanism for maintaining this balance is activated - an increase in venous return.

Humans, unlike animals, do not have a true depot in which blood could be retained in special formations and released as necessary (an example of such a depot is the spleen of a dog).

In a closed vascular system, changes in the capacity of any department are necessarily accompanied by a redistribution of blood volume. Therefore, changes in the capacity of the veins that occur during contractions of smooth muscles affect the distribution of blood throughout the entire circulatory system and thereby directly or indirectly on general function blood circulation

Shunt vessels - These are arteriovenous anastomoses present in some tissues. When these vessels are open, blood flow through the capillaries is either reduced or stopped completely (see figure above).

According to function and structure various departments and the characteristics of innervation, all blood vessels have recently begun to be divided into 3 groups:

pericardial vessels that begin and end both circles of blood circulation - the aorta and pulmonary trunk (i.e., elastic arteries), hollow and pulmonary veins;
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;
organ vessels that provide exchange reactions between blood and organ parenchyma. These are intraorgan arteries and veins, as well as capillaries

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Large vessels- aorta, pulmonary trunk, vena cava and pulmonary veins - serve primarily as routes 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:

elastic,
muscular and
muscular-elastic.

The wall of all types of arteries, as well as veins, consists of three layers (shells):

internal,
middle and
outdoor

The relative thickness of these layers and the nature of the tissues that form them depend on the type of artery.

Elastic arteries

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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. In their walls is large number 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 stretched 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 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 (atherosclerotic plaques) can be seen in this layer. 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 a large number fenestrated elastic membranes located concentrically. Their number changes with age. A newborn has about 40 of them, and 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 of cartilage tissue in the form of rings can form in the middle layer of the wall of such arteries. 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, supplying 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.

Muscular arteries

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Numerous branches depart from the aorta and pulmonary trunk, which deliver blood to various areas body: to the limbs, internal organs, integument. Since individual areas of the body bear different functional load, 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 the endothelium lining the lumen of the artery, subendothelial loose connective tissue and 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 vessels and lymphatic vessels, feeding the wall of the arteries.

Arteries of mixed, or muscular-elastic type

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Mixed arteries, or muscular-elastic type in structure and functional features occupy intermediate position between elastic and muscular arteries. These include, for example, the subclavian, external and internal iliac, femoral, mesenteric arteries, and 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 functional features organs and tissues.

Arterioles

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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.

Capillaries

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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 of the 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, endocrine glands 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 dense intercellular contacts. 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.

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. IN distal end metarterioles can flow into true capillaries. 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 becomes special meaning during thermoregulation (for example, in subcutaneous tissue).

Venules

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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 venous section create the prerequisites for the deposition and redistribution of blood in various organs and tissues. Calculations show that the blood in the circulatory system is distributed in such a way that arterial system it is contained up to 15%, in the capillaries – 5–12%, and in 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 is 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.

Vienna

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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. The middle, muscular layer is 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 inner shell veins due to low blood pressure. In the outer shell there are also developed lymphatic capillaries, through which excess tissue fluid flows.

According to the degree of development of 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 thoracic vein) 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 small quantity 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 action 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 due to the pressure of the surrounding muscles, but the superficial veins do not experience such pressure. In this regard, the wall of the latter is much thicker, it has a well-developed muscle layer the middle shell, 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 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 the 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 veins depends on physiological conditions blood flow in them.

Blood not only flows in the veins, but is also reserved in certain sections of the riverbed. 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 the control of the nervous system.

Innervation of blood vessels

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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.

Blood vessels - elastic tubes through which blood is transported to all organs and tissues and then collected again to the heart. The study of blood vessels, along with lymphatic vessels, is a branch of medicine - angiology. Blood vessels form: a) the macrocirculatory bed - these are arteries and veins through which blood moves from the heart to the organs and returns to the heart; b) microcirculatory bed - includes capillaries, arterioles and venules located in organs that ensure the exchange of substances between blood and tissues.

Arteries - blood vessels through which blood moves from the heart to organs and tissues. The walls of the arteries have three layers:

outer layer built of loose connective tissue, it contains nerves that regulate the expansion and contraction of blood vessels;

middle layer consists of smooth muscle membrane And elastic fibers(due to muscle contraction or relaxation, the lumen of blood vessels can change, regulating the flow of blood, and elastic fibers give elasticity to the vessels)

inner layer - formed by a special connective tissue, the cells of which have very smooth membranes and do not interfere with the movement of blood.

Depending on the diameter of the arteries, the structure of the wall in them also changes, therefore three types of arteries are distinguished: elastic (for example, the aorta, pulmonary trunk), muscular (arteries of organs) and mixed, or muscle-elastic (for example, carotid artery) type.

Capillaries- the smallest blood vessels that connect arteries and veins and ensure the exchange of substances between blood and tissue fluid. Their diameter is about 1 micron, the total surface of all capillaries of the body is 6300 m2. The walls consist of one layer of flat epithelial cells- endothelium. The endothelium is the inner layer of flat, elongated cells with uneven wavy edges, which line the capillaries, as well as all other vessels and the heart. Endotheliocytes produce a number of physiologically active substances. Among them, nitric oxide causes relaxation of smooth muscle cells, thereby causing vasodilation. In organs, capillaries provide microcirculation of blood and form a mesh, but they can also form loops (for example, in the papillae of the skin), as well as glomeruli (for example, in the nephrons of the kidneys). Various organs have different levels development capillary mesh. For example, in the skin there are 40 capillaries per 1 mm2, and in muscles there are about 1000. The significant development of the capillary network has gray matter CNS organs, endocrine glands, skeletal muscles, heart, adipose tissue.

Vienna- blood vessels through which blood moves from organs and tissues to the heart. They have the same wall structure as arteries, but thin and less elastic. Medium and some large veins have semilunar valves that allow blood to flow in only one direction. The veins are muscular (hollow) and non-muscular (retina, bones). The movement of blood through the veins to the heart is facilitated by the suction action of the heart, the stretching of the vena cava in the chest cavity when air is inhaled, and the presence of a valve apparatus.

Comparative characteristics of vessels

signs	arteries	capillaries	veins
structure	Thick walls made of 3 layers. lack of valves	Walls of one layer of flat cells	Thin walls made of 3 layers Availability of valves
	Movement of blood away from the heart	Metabolism between blood and tissues	Movement of blood to the heart
blood speed	About 0.5 m/s	About 0.5mm/s	About 0.2 m/s
blood pressure	Up to 120 mm Hg. Art.	Up to 20 mm Hg. Art.	From 3-8 mm Hg. Art. and below

Large vessels consist of three layers:

the inner layer is the endothelium, it reduces friction;
the middle layer contains smooth muscles that regulate the lumen of the vessel, and elastic fibers that impart elasticity;
outer layer consists of loose fibrous connective tissue, provides protection, strengthening, blood supply and innervation of the vessel.

3 types of vessels:
Arteries- large three-layer vessels through which blood flows from the heart. They contain a well-developed middle layer, which allows them to withstand high pressure.
Capillaries- microscopic single-layer vessels consisting only of endothelium. In capillaries, the exchange of substances between blood and intercellular fluid occurs.
Vienna- large three-layer vessels through which blood flows to the heart. Contain semilunar valves that prevent backflow of blood. They have a poorly developed middle layer, which is why they easily stretch (to deposit blood) and contract (therefore, contraction of skeletal muscles increases venous blood flow).

Tests

1. Which blood vessel has a wall consisting of a single layer of cells?
A) intestinal artery
B) superior vena cava
B) portal vein of the liver
D) capillary of the nephron glomerulus

2. What factor ensures the movement of blood in the veins?
A) work of the leaflet valves of the heart
B) large branching of blood vessels
B) contraction of nearby skeletal muscles
D) different speeds of blood movement through the vessels

3. Valves located in the veins provide
A) regulation of blood pressure
B) redistribution of blood in the body
B) better blood clotting
D) blood movement in one direction

4. The thickest muscle layer of the vessel wall is characteristic of
A) blood capillaries
B) lymphatic vessels
B) arteries
D) veins

5. Strengthening and blood supply of the blood vessel provides(s)
A) smooth muscles
B) connective tissue
B) elastic fibers
D) endothelium