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Structure and functions of blood vessels. Types of blood vessels and the structure of their walls Blood vessels and their features

Structure and functions of the vascular wall

Blood in the human body flows through closed system blood vessels. Vessels not only passively limit the volume of circulation and mechanically prevent blood loss, but also have a whole range of active functions in hemostasis. Under physiological conditions, an intact vascular wall helps maintain the fluid state of the blood. Intact endothelium in contact with blood does not have the ability to initiate the coagulation process. In addition, it contains on its surface and releases into the bloodstream substances that prevent clotting. This property prevents the formation of a blood clot on intact endothelium and limits the growth of a blood clot beyond the damage. When damaged or inflamed, the vessel wall takes part in the formation of a blood clot. Firstly, subendothelial structures, which come into contact with blood only when damaged or a pathological process develops, have a powerful thrombogenic potential. Secondly, the endothelium in the damaged area is activated and appears

procoagulant properties. The structure of the vessels is shown in Fig. 2.

The vascular wall of all vessels, except pre-capillaries, capillaries and post-capillaries, consists of three layers: the inner membrane (intima), the middle membrane (media) and the outer membrane (adventitia).

Intimacy. Throughout the bloodstream, under physiological conditions, blood comes into contact with the endothelium, which forms the inner layer of the intima. The endothelium, which consists of a monolayer of endothelial cells, plays the most active role in hemostasis. The properties of the endothelium differ somewhat in different parts of the circulatory system, determining the different hemostatic status of arteries, veins and capillaries. Under the endothelium there is an amorphous intercellular substance with smooth muscle cells, fibroblasts and macrophages. There are also inclusions of lipids in the form of droplets, most often located extracellularly. At the border of the intima and media there is an internal elastic membrane.

Rice. 2. Vascular wall consists of intima, the luminal surface of which is covered with a single-layer endothelium, media ( smooth muscle cells) and adventitia (connective tissue frame): A - large muscular-elastic artery ( schematic illustration), B - arterioles ( histological specimen), IN - coronary artery in cross section

Vascular wall

Media consists of smooth muscle cells and intercellular substance. Its thickness varies significantly depending on various vessels, causing their different contractility, strength and elasticity.

Adventitia consists of connective tissue containing collagen and elastin.

Arterioles ( arterial vessels with a total diameter of less than 100 μm) are transitional vessels from arteries to capillaries. The thickness of the walls of arterioles is slightly less than the width of their lumen. The vascular wall of the largest arterioles consists of three layers. As arterioles branch, their walls become thinner and the lumen narrower, but the ratio of lumen width to wall thickness remains the same. In the smallest arterioles, on a cross section, one or two layers of smooth muscle cells, endothelial cells and a thin outer membrane consisting of collagen fibers are visible.

Capillaries consist of a monolayer of endothelial cells surrounded by a basal plate. In addition, another type of cell is found in the capillaries around endothelial cells - pericytes, the role of which is not well understood.

Capillaries open at their venous end into postcapillary venules (diameter 8-30 µm), which are characterized by an increase in the number of pericytes in the vascular wall. Postcapillary venules, in turn, flow into

collecting venules (diameter 30-50 μm), the wall of which, in addition to pericytes, has an outer shell consisting of fibroblasts and collagen fibers. Collecting venules empty into muscular venules, which have one or two layers of smooth muscle fibers in the tunica media. In general, venules consist of an endothelial lining, a basement membrane directly adjacent to the outside of the endothelial cells, and pericytes, also surrounded by a basement membrane; Outside the basement membrane there is a layer of collagen. Veins are equipped with valves that are oriented to allow blood to flow towards the heart. Most valves are in the veins of the extremities, and in the veins of the chest and organs abdominal cavity they are missing.

Vascular function in hemostasis:

Mechanical restriction of blood flow.

Regulation of blood flow through vessels, including
le spastic reaction damaged from
ships.

Regulation of hemostatic reactions by
synthesis and representation on the surface
dothelium and in the subendothelial layer of proteins,
peptides and non-protein substances, directly
directly involved in hemostasis.

Presentation on cell surface recipe
tors for enzymatic complexes,
treated with coagulation and fibrinolysis.

Endothelium

Characteristics of the enlothelial cover

The vascular wall has an active surface, with inside lined with endothelial cells. The integrity of the endothelial lining is the basis for the normal functioning of blood vessels. The surface area of the endothelial lining in the vessels of an adult is comparable to the area of a football field. The cell membrane of endothelial cells has high turnover, which is an important condition for the antithrombogenic properties of the vascular wall. High fluidity ensures a smooth inner surface of the endothelium (Fig. 3), which functions as an integral layer and excludes contact of blood plasma procoagulants with subendothelial structures.

Endotheliocytes synthesize, present on their surface and release into the blood and subendothelial space a whole range of biologically active substances. These are proteins, peptides and non-protein substances that regulate hemostasis. In table Table 1 lists the main products of endothelial cells involved in hemostasis.

Vascular wall

– the most important physiological mechanism responsible for feeding body cells and excreting them from the body harmful substances. The main structural component is blood vessels. There are several types of vessels, differing in structure and function. Vascular diseases lead to serious consequences, negatively affecting the entire body.

General information

A blood vessel is hollow formations in the form of a tube that penetrates the tissues of the body. Blood is transported through vessels. In humans, the circulatory system is closed, due to which the movement of blood in the vessels occurs at high temperatures. Transportation through the vessels is carried out due to the work of the heart, which performs a pumping function.

Blood vessels capable of changing under the influence of certain factors. Depending on external influence, they expand or contract. The process is regulated by the nervous system. The ability to expand and contract provides specific structure human blood vessels.

The vessels consist of three layers:

External. Outer surface the vessel is covered with connective tissue. Its function is to protect against mechanical stress. Also, the task of the outer layer is to separate the vessel from nearby tissues.
Average. Contains muscle fibers characterized by mobility and elasticity. They provide the ability of the vessel to expand or contract. In addition, the function of the muscle fibers of the middle layer is to maintain the shape of the vessel, due to which full, unimpeded blood flow occurs.
Interior. The layer is represented by flat single-layer cells - endothelium. The fabric makes the vessels smooth inside, thereby reducing resistance to blood movement.

It should be noted that the walls of venous vessels are much thinner than arteries. This is due to the small number of muscle fibers. The movement of venous blood occurs under the influence of skeletal blood, while arterial blood moves due to the work of the heart.

In general, the blood vessel is the main structural component the cardiovascular system, through which blood moves to tissues and organs.

Types of vessels

Previously, the classification of human blood vessels included only 2 types - arteries and veins. Currently, there are 5 types of vessels, differing in structure, size, and functional tasks.

Types of blood vessels:

. Vessels ensure the movement of blood from the heart to the tissues. They are distinguished by thick walls with high content muscle fibers. Arteries constantly narrow and dilate depending on the level of pressure, preventing excess blood flow to some organs and deficiency in others.
Arterioles. Small vessels that are the terminal branches of arteries. Consist primarily of muscle tissue. They are a transitional link between arteries and capillaries.
Capillaries. The smallest vessels that penetrate organs and tissues. A special feature is the very thin walls through which blood is able to penetrate outside the vessels. Capillaries supply cells with oxygen. At the same time, the blood is saturated with carbon dioxide, which is subsequently removed from the body through the venous routes.

Venules. They are small vessels connecting capillaries and veins. They transport oxygen spent by cells, residual waste products, and dying blood particles.
Vienna. Provide blood movement from organs to the heart. Contains fewer muscle fibers, which is associated with low resistance. Because of this, the veins are less thick and are more likely to be damaged.

Thus, several types of vessels are distinguished, the totality of which forms the circulatory system.

Functional groups

Depending on their location, vessels perform different functions. The structure of blood vessels differs depending on the functional load. Currently there are 6 main functional groups.

The functional groups of blood vessels include:

Shock-absorbing. Vessels belonging to this group have greatest number muscle fibers. They are the largest in the human body and are located in close proximity to the heart (aorta, pulmonary artery). These vessels are the most elastic and resilient, which is necessary to smooth out the systolic waves formed during heart rate. The amount of muscle tissue in the walls of blood vessels decreases depending on the degree of distance from the heart.
Resistive. These include the terminal, thinnest blood vessels. Due to the smallest lumen, these vessels offer the greatest resistance to blood flow. Resistive vessels contain many muscle fibers that control the lumen. Due to this, the volume of blood entering the organ is regulated.
Capacitive. They perform a reservoir function, storing large volumes of blood. IN this group includes large venous vessels that can hold up to 1 liter of blood. Capacitance vessels regulate the movement of blood to, controlling its volume to reduce the load on the hearts.
Sphincters. Found in the terminal branches of small capillaries. Due to narrowing and expansion, sphincter vessels control the amount of incoming blood. When the sphincters narrow, blood does not flow, as a result of which the trophic process is disrupted.
Exchange. Represented by the terminal branches of capillaries. Metabolism occurs in the vessels, providing nutrition to tissues and removing harmful substances. Venules perform similar functional tasks.
Shunting. Vessels provide communication between veins and arteries. In this case, the capillaries are not affected. These include atrial, great and organ vessels.

In general, there are several functional groups of vessels that provide adequate blood flow and nutrition to all cells of the body.

Regulation of vascular activity

The cardiovascular system responds instantly to external changes or impact negative factors inside the body. For example, when stressful situations occur, a rapid heartbeat is noted. The vessels narrow, due to which it increases, and the muscle tissue is supplied a large number blood. While at rest, more blood flows to the brain tissues and digestive organs.

Nerve centers located in the cerebral cortex and hypothalamus are responsible for the regulation of the cardiovascular system. The signal arising as a result of the reaction to the stimulus affects the center that controls vascular tone. In the future, through nerve fibers the impulse moves into the vascular walls.

In the walls of blood vessels there are receptors that perceive pressure surges or changes in the composition of the blood. The vessels are also capable of transmitting nerve signals to the appropriate centers, notifying of possible danger. This makes it possible to adapt to changing environmental conditions, such as temperature changes.

The functioning of the heart and blood vessels is affected. This process is called humoral regulation. Adrenaline, vasopressin, and acetylcholine have the greatest effect on blood vessels.

Thus, the activity of the cardiovascular system is regulated nerve centers brain and endocrine glands responsible for the production of hormones.

Diseases

Like any organ, the vessel can be affected by diseases. Reasons for development vascular pathologies often associated with a person’s unhealthy lifestyle. Less commonly, diseases develop as a result of congenital abnormalities, acquired infections, or against the background of concomitant pathologies.

Common vascular diseases:

. Considered one of the most dangerous pathologies cardiovascular system. With this pathology, blood flow through the vessels feeding the myocardium - the heart muscle - is disrupted. Gradually, due to atrophy, the muscle weakens. Complications include heart attack, as well as heart failure, which can cause sudden stop hearts.
Neurocirculatory dystonia. A disease in which the arteries are affected due to malfunctions of the nerve centers. In vessels due to excess sympathetic influence on muscle fibers, a spasm develops. The pathology often manifests itself in the vessels of the brain and also affects arteries located in other organs. The patient develops intense pain, interruptions in heart function, dizziness, changes in pressure.
Atherosclerosis. A disease in which the walls of blood vessels narrow. This leads to a number of negative consequences, including atrophy of nutritional tissues, as well as a decrease in the elasticity and strength of the vessels located behind the narrowing. is a provoking factor in many cardiovascular diseases, and leads to the formation of blood clots, heart attack, and stroke.
Aortic aneurysm. With this pathology, sac-like bulges form on the walls of the aorta. Subsequently, scar tissue forms and the tissue gradually atrophies. As a rule, the pathology develops against the background of a chronic form of hypertension, infectious lesions, including syphilis, as well as with abnormalities in the development of the vessel. If left untreated, the disease provokes rupture of the vessel and death of the patient.
. Pathology in which the veins of the lower extremities are affected. They expand greatly due to increased load, and the flow of blood to the heart slows down greatly. This leads to swelling and pain. Pathological changes in the affected veins of the legs are irreversible; the disease in the later stages can only be treated surgically.

. A disease in which varicose veins develops in the area of hemorrhoidal veins that supply lower sections intestines. Late stages of the disease are accompanied by loss hemorrhoids, heavy bleeding, stool disorder. Infectious lesions, including blood poisoning, are complications.
Thrombophlebitis. The pathology affects the venous vessels. The danger of the disease is explained by the potential for a blood clot to break off, which blocks the lumen of the pulmonary arteries. However, large veins are extremely rarely affected. Thrombophlebitis affects small veins, the defeat of which does not pose a significant threat to life.

Exists wide range vascular pathologies that have a negative impact on the functioning of the entire body.

While watching the video you will learn about the cardiovascular system.

Blood vessels - important element human body, responsible for blood movement. There are several types of vessels, differing in structure, functional purpose, size, location.

Human arteries and veins perform various jobs in the body. In this regard, significant differences can be observed in the morphology and conditions of blood flow, although general structure, with rare exceptions, all vessels have the same. Their walls have three layers: inner, middle, outer.

The inner shell, called intima, necessarily has 2 layers:

the endothelium lining the inner surface is a layer of cells squamous epithelium;
subendothelium - located under the endothelium, consists of connective tissue with a loose structure.

The middle shell consists of myocytes, elastic and collagen fibers.

The outer shell, called “adventitia,” is a fibrous connective tissue with a loose structure, equipped with vascular vessels, nerves, lymphatic vessels.

Arteries

These are blood vessels that carry blood from the heart to all organs and tissues. There are arterioles and arteries (small, medium, large). Their walls have three layers: intima, media and adventitia. Arteries are classified according to several criteria.

Based on the structure of the middle layer, three types of arteries are distinguished:

Elastic. Their middle layer of the wall consists of elastic fibers that can withstand high blood pressure blood, developing during its release. This type includes the pulmonary trunk and aorta.
Mixed (muscular-elastic). The middle layer consists of varying numbers of myocytes and elastic fibers. These include the carotid, subclavian, and iliac.
Muscular. Their middle layer is represented by individual myocytes arranged in a circular pattern.

According to their location relative to the organs, arteries are divided into three types:

Trunk – supply parts of the body with blood.
Organ - carry blood to the organs.
Intraorgan - have branches inside organs.

Vienna

They are non-muscular and muscular.

The walls of muscleless veins consist of endothelium and connective tissue of a loose structure. Such vessels are located in bone tissue, placenta, brain, retina, spleen.

Muscular veins, in turn, are divided into three types depending on how the myocytes are developed:

poorly developed (neck, face, upper part body);
medium (brachial and small veins);
strongly ( bottom part body and legs).

Veins, except the umbilical and pulmonary veins, carry blood that gives oxygen and nutrients and takes carbon dioxide and decomposition products as a result of metabolic processes. It moves from the organs to the heart. Most often, she has to overcome gravity and her speed is lower, which is due to the peculiarities of hemodynamics (lower pressure in the vessels, lack of it sharp drop, low amount of oxygen in the blood).

Structure and its features:

Larger in diameter compared to arteries.
The subendothelial layer and elastic component are poorly developed.
The walls are thin and fall off easily.
The smooth muscle elements of the middle layer are rather poorly developed.
Pronounced outer layer.
The presence of a valve apparatus, which is formed by the inner layer of the vein wall. The base of the valves consists of smooth myocytes, inside the valves there is fibrous connective tissue, and on the outside they are covered by a layer of endothelium.
All wall membranes are endowed with vascular vessels.

The balance between venous and arterial blood is ensured by several factors:

a large number of veins;
their larger caliber;
density of the vein network;
formation of venous plexuses.

Differences

How are arteries different from veins? These blood vessels differ significantly in many ways.

Arteries and veins, first of all, differ in the structure of the wall

According to the structure of the wall

Arteries have thick walls, they have a lot of elastic fibers, smooth muscles are well developed, they do not fall off unless they are filled with blood. Due to contractility tissues that make up their walls is carried out fast delivery oxygenated blood to all organs. The cells that make up the layers of the walls ensure the unhindered passage of blood through the arteries. Inner surface theirs is corrugated. The arteries must withstand the high pressure that is created by powerful surges of blood.

The pressure in the veins is low, so the walls are thinner. They fall off when there is no blood in them. Their muscle layer unable to contract like arteries. The surface inside the vessel is smooth. Blood moves through them slowly.

In veins, the thickest membrane is considered to be the outer one, in arteries it is the middle one. Veins do not have elastic membranes, arteries have an internal and an external one.

According to form

The arteries have a fairly regular cylindrical shape, they are round in cross section.

Due to the pressure of other organs, the veins are flattened, their shape is tortuous, they either narrow or expand, which is due to the location of the valves.

By quantity

In the human body there are more veins and fewer arteries. Most middle arteries are accompanied by a pair of veins.

According to the presence of valves

Most veins have valves that prevent blood from flowing backwards. They are located in pairs opposite each other throughout the entire length of the vessel. They are not in the portal hollow, brachiocephalic, iliac veins, as well as in the veins of the heart, head and red bone marrow.

In arteries, valves are located as vessels exit the heart.

By blood volume

Veins circulate approximately twice as much blood as arteries.

By location

The arteries lie deep in the tissues and approach the skin only in a few places, where the pulse is heard: on the temples, neck, wrist, and instep of the feet. Their location is approximately the same for all people.

Veins are mostly located close to the surface of the skin

The location of the veins may vary from person to person.

To ensure blood movement

In the arteries, blood flows under the pressure of the force of the heart, which pushes it out. At first the speed is about 40 m/s, then gradually decreases.

Blood flow in the veins occurs due to several factors:

pressure forces depending on the push of blood from the heart muscle and arteries;
suction force of the heart when relaxing between contractions, that is, the creation in the veins negative pressure due to enlargement of the atria;
suction effect on the veins of the chest breathing movements;
contractions of the muscles of the legs and arms.

In addition, approximately a third of the blood is in the venous depots (in portal vein, spleen, skin, walls of the stomach and intestines). It is pushed out from there if it is necessary to increase the volume of circulating blood, for example, with massive bleeding, with high physical activity.

By color and composition of blood

Arteries carry blood from the heart to the organs. It is enriched with oxygen and has a scarlet color.

Veins provide blood flow from tissues to the heart. Venous blood, which contains carbon dioxide and breakdown products formed during metabolic processes, differs more dark color.

Arterial and venous bleeding have different symptoms. In the first case, the blood is ejected in a fountain, in the second it flows in a stream. Arterial – more intense and dangerous for humans.

Thus, the main differences can be identified:

Arteries transport blood from the heart to the organs, veins transport blood back to the heart. Arterial blood carries oxygen, venous blood returns carbon dioxide.
The walls of the arteries are more elastic and thicker than the walls of the veins. In arteries, blood is pushed out with force and moves under pressure, in veins it flows calmly, while valves prevent it from moving in the opposite direction.
There are twice as many arteries as veins, and they are located deep. The veins are located in most cases superficially, their network is wider.

Veins, unlike arteries, are used in medicine to obtain material for analysis and to introduce medications and other fluids directly into the bloodstream.

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 special cells are also associated - pericytes, covering the 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 the blood and the external environment (in the excretory organs).

Arteries and veins are usually divided into large, medium and small. The most small arteries and the veins turning 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 - the artery muscular type.

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 upper 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.