somatic capillaries. The cardiovascular system. Vessels. development of blood vessels

Importance of the cardiovascular system (CCS) in the life of the organism, and consequently the knowledge of all aspects of this area for practical medicine, is so great that cardiology and angiology have separated into the study of this system as two independent areas. The heart and blood vessels are systems that function not periodically, but constantly, therefore, more often than other systems, they are subject to pathological processes. Currently, cardiovascular disease, along with cancer, occupies a leading position in terms of mortality.

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

Classification:

I. The central organ is the heart.

II. Peripheral department:

A. Blood vessels:

1. Arterial link:

a) arteries of the elastic type;

b) muscular arteries;

c) mixed arteries.

2.Microcirculatory bed:

a) arterioles;

b) hemocapillaries;

c) venules;

d) arteriolo-venular anastomoses

3. Venous link:

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

elements;

b) non-muscular type veins.

B. Lymphatic vessels:

1. Lymphatic capillaries.

2. Intraorganic lymphatic vessels.

3. Extraorganic lymphatic vessels.

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

Arterial link - represented by vessels through which blood is delivered from the heart to the organs. The term “artery” is translated as “air-containing”, since at autopsy, researchers often found these vessels empty (not containing blood) and thought that the vital “pneuma” or air was spreading through them throughout the body .. Elastic, muscular and mixed arteries have a common principle of structure: 3 shells are distinguished in the wall - inner, middle and outer adventitia.

The inner shell consists of layers:

1. Endothelium on the basement membrane.

2. Subendothelial layer - snotty fibrous sdt with a high content of poorly differentiated cells.

3. Internal elastic membrane - plexus of elastic fibers.

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

Outer adventitia arteries histologically presented

loose fibrous sdt with vascular vessels and vascular nerves.

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

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

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

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

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

Compound:

1. Arterioles (including precapillary).

2. Hemocapillaries.

3. Venules (including post-capillary).

4. Arteriolo-venular anastomoses.

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

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

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

2. Type II hemocapillaries (fenestrated or visceral type) - the basement membrane is continuous, there are fenestrae in the endothelium - thinned areas in the cytoplasm of endotheliocytes. Diameter 8-12 microns. There are in the capillary glomeruli of the kidney, in the intestine, in the endocrine glands.

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

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

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

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

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

Arteriovenular anastomoses (AVA)- these are shunts (or fistulas) between arterioles and venules, i.e. carry out a direct connection and participate in the regulation of regional peripheral blood flow. They are especially abundant in the skin and kidneys. ABA - short vessels, also have 3 shells; there are myocytes, especially many in the middle shell, acting as a sphincter.

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

Non-muscular type veins are present in the dura mater, bones, retina, placenta, and red bone marrow. The wall of muscleless veins is internally lined with endotheliocytes on the basement membrane, followed by a layer of fibrous sdt; there are no smooth muscle cells.

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

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

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

LK diameter is not constant- there are contractions and expansions. Lymphatic capillaries merge to form intraorganic lymphatic vessels - in structure they are close to veins, because. are in the same hemodynamic conditions. They have 3 shells, the inner shell forms valves; unlike veins, there is no basement membrane under the endothelium. The diameter is not constant throughout - there are expansions at the level of the valves.

Extraorganic lymphatic vessels they are also similar in structure to veins, but the basal membrane of the endothelium is poorly expressed, sometimes absent. In the wall of these vessels, the internal elastic membrane is clearly distinguished. The middle shell receives special development in the lower extremities.

HEART. The heart is laid at the beginning of the 3rd week of embryonic development in the form of a paired rudiment in the cervical region from the mesenchyme under the visceral sheet of splanchnotomes. Paired strands are formed from the mesenchyme, which soon turn into tubules, from which eventually inner lining of the heart - endocardium. Sections of the visceral layer of splanchnotomes, the envelopes of these tubules are called myoepicardial plates, which subsequently differentiate into myocardium and epicardium. As the embryo develops with the appearance of the trunk fold, the flat embryo folds into a tube - the body, while 2 bookmarks of the heart are in the chest cavity, approach and finally merge into one tube. Further, this tube-heart begins to grow rapidly in length and, not fitting in the chest, forms several bends. Neighboring loops of the curving tube grow together and a 4-chambered heart is formed from a simple tube.

capillaries- these are the terminal branches of blood vessels in the form of endothelial tubules with a very simply arranged membrane. So, the inner shell consists only of the endothelium and the basement membrane; the middle shell is virtually absent, and the outer shell is represented by a thin pericapillary layer of loose fibrous connective tissue. Capillaries 3-10 µm in diameter and 200-1000 µm long form a highly branched network between metarterioles and post-capillary venules.

capillaries- these are places of active and passive transport of various substances, including oxygen and carbon dioxide. This transport depends on various factors, among which the selective permeability of endothelial cells for certain specific molecules plays an important role.

Depending on the structure of the walls, capillaries can be divided into continuous, fenestrated and sinusoidal.

Endothelial cells have flat nuclei that protrude slightly into the capillary lumen; cell organelles are well developed.

In the cytoplasm of endotheliocytes, several actin microfilaments and numerous microvesicles (MB) with a diameter of 50-70 nm are found, which sometimes merge and form transendothelial channels (TCs). The transendothelial transport function in two directions with the help of microvesicles is greatly facilitated by the presence of microfilaments and the formation of channels. Openings (Ov) of microvesicles and transendothelial channels on the inner and outer surfaces of the endothelium are clearly visible.

Rough, 20-50 nm thick basement membrane (BM) is located under the endothelial cells; on the border with pericytes (Pe), it often splits into two sheets (see arrows), which surround these cells with their processes (O). Outside of the basement membrane there are isolated reticular and collagen microfibrils (CM), as well as autonomic nerve endings (NO), corresponding to the outer shell.

continuous capillaries found in brown adipose tissue (see figure), muscle tissue, testicles, ovaries, lungs, central nervous system (CNS), thymus, lymph nodes, bones, and bone marrow.

Endothelial cells have flattened, elongated perinuclear cytoplasmic zones that protrude slightly into the capillary lumen. The internal structure of endothelial cells is identical to the internal structure of the same cells in continuous capillaries. Due to the presence of actin microfilaments in the cytoplasm, endothelial cells can shrink.

The basement membrane (BM) has the same thickness as in continuous capillaries and surrounds the outer surface of the endothelium. Around fenestrated capillaries, pericytes (Pe) are less common than in continuous capillaries, but they are also located between two layers of the basement membrane (see arrows).

Reticular and collagen fibers (KB) and autonomic nerve fibers (not shown) run along the outside of the fenestrated capillaries.

Fenestrated capillaries found mainly in the kidneys, choroid plexuses of the ventricles of the brain, synovial membranes, endocrine glands. The exchange of substances between blood and tissue fluid is greatly facilitated by the presence of such intraendothelial fenestrations.

Endothelial cells are connected through nexuses and locking zones, zonulae occludentes, as well as using overlapping zones (indicated by an arrow).

The nuclei of endothelial cells are flattened; the cytoplasm contains well-developed organelles, few microfilaments, and in some organs a noticeable amount of lysosomes (L) and microvesicles (Mv).

The basement membrane in this type of capillaries is almost completely absent, thus allowing the blood plasma and intercellular fluid to mix freely, there is no permeability barrier.

In rare cases, pericytes occur; delicate collagen and reticular fibers (RV) form a loose network around sinusoidal capillaries.

This type of capillaries is found in the liver, spleen, pituitary gland, adrenal cortex. It is believed that endothelial cells sinusoidal capillaries liver and bone marrow show phagocytic activity.

The vital cardiovascular system consists of the heart, blood and lymph vessels. Vessels are present in almost all organs. Blood vessels play an important role in the transport of blood to organs and tissues, regulate their blood supply. Through the wall of blood capillaries there is an intensive exchange between blood and tissues. Violation of the histophysiology of the heart and blood vessels, which are present in almost all organs, leads to the pathology of the cardiovascular system, which makes it necessary to study this section by doctors of all specialties.

Blood vessels are divided into arteries of various types, veins and vessels of the microvasculature:

arterioles, venules, capillaries and AVA, connecting the arterial and venous bed. There may also be “miraculous networks” - capillaries connecting two vessels of the same name, for example, in the glomeruli of the kidneys. AVA connect arteries and veins, bypassing the capillary bed. All vessels are of mesenchymal origin. The structure of the vessel wall, the degree of development of the membranes and belonging to one or another type depends on the conditions of hemodynamics and the function of the vessel.

General plan of the structure of the vessel wall

The wall of the vessel consists of three shells: inner, middle and outer. The inner shell is represented by the endothelium, the subendothelial layer is loose, fibrous unformed connective tissue, the internal elastic membrane (in the arteries of the muscular type). The middle shell consists of smooth myocytes and between them located elastic and collagen fibers, as well as elastic fenestrated membranes (in the arteries of the elastic type). In muscular-type arteries, the middle membrane is separated from the outer elastic membrane. The outer shell is formed by loose fibrous irregular connective tissue. In the middle (near large vessels) and outer shells of veins and arteries, there are small vessels that supply blood to the vascular wall, vascular vessels and nerve trunks. According to the diameter, the vessels are divided into vessels of large, medium and small caliber.

Muscular type artery consists of three shells. The inner shell is represented by the endothelium, subendothelial layer and the inner elastic membrane. The latter separates the inner shell from the middle one. The middle shell is most developed in the arteries. It consists of smooth myocytes arranged in a spiral, which, during their contraction, reduce the lumen of the vessel, maintain blood pressure and push blood into the distal sections. Between myocytes in a small amount there are mainly elastic fibers. On the border between the outer and middle shell is the outer elastic membrane. The outer shell consists of loose connective tissue with nerve fibers and blood vessels. The elastic framework, elastic fibers and elastic boundary membranes prevent the arteries from collapsing, which ensures the continuity of blood flow in them.

Artery elastic type. Aorta. There are three shells in its powerful wall. The inner layer consists of endothelium and subendothelial layer with fine fibrillar connective tissue. It contains a lot of glycosaminoglycans and phospholipids. The subendothelial layer has a considerable thickness, it contains many stellate poorly differentiated cells. On the border with the middle shell is a dense plexus of elastic fibers. The middle shell is very wide, represented by a large number of elastic fenestrated membranes and elastic fibers connected to them and to each other, which, together with the elastic fibers of the inner and outer shells, form a pronounced elastic frame that softens blood tremors during systole and maintains tone during diastole. Between the membranes there are smooth myocytes. The outer elastic membrane is absent. In the loose fibrous connective tissue of the outer shell, there are elastic and collagen fibers, vascular vessels and nerve trunks.

Muscular vein. Its wall is represented by three shells. The inner layer consists of the endothelium and the subendothelial layer. In the middle shell - bundles of smooth myocytes, between which predominantly collagen fibers. In the outer, widest shell, in its loose fibrous connective tissue, there are vessels and there may be transversely cut smooth myocytes. The lumen of the vessel is irregular in shape, erythrocytes are visible in the lumen.

Differences between a muscular artery and a muscular vein. The wall of the arteries is thicker than the walls of the corresponding veins; there are no internal and external elastic membranes in the veins; the widest shell in the atreria is the middle one, and in the veins it is the outer one. The veins are equipped with valves; in the veins, muscle cells in the middle membrane are less developed than in the arteries, and are located in bundles separated by connective tissue layers, in which collagen fibers predominate over elastic ones. The lumen of the vein is often collapsed and blood cells are visible in the lumen. In the arteries, the lumen gapes and blood cells are usually absent.

blood capillaries. The thinnest and most numerous vessels. Their lumen can vary from 4.5 µm in somatic capillaries to 20–30 µm in sinusoidal capillaries. This is due to both the organ features of the capillaries and the functional state. There are even wider capillaries - capillary receptacles - gaps in the cavernous bodies of the penis. The walls of the capillaries are sharply thinned to three thinnest layers, which is necessary for metabolic processes. In the capillary wall, there are: the inner layers, represented by endotheliocytes lining the vessel from the inside and located on the basement membrane; the middle one is from the process cells-pericytes located in the crevices of the basement membrane and participating in the regulation of the lumen of the vessel. The outer layer is represented by thin collagen and argyrophilic fibers and adventitial cells that accompany the wall of capillaries, arterioles, and venules from the outside. Capillaries connect arteries and veins.

There are three types of capillaries: 1. somatic type capillaries(in the skin, in the muscles), their endothelium is not fenestrated, the basement membrane is continuous; 2. visceral type capillaries(kidneys, intestines), their endothelium is fenestrated, but the basement membrane is continuous; 3. sinusoidal capillaries(liver, hematopoietic organs), with a large diameter (20-30 microns), there are gaps between endotheliocytes, the basement membrane is discontinuous or may be completely absent, there are also no structures of the outer layer.

In addition to capillaries, the microcirculatory bed includes arterioles, venules, and arteriolo-venular anastomoses.

Arterioles are the smallest arterial vessels. Shells in arterioles and venules are thinned. Arterioles contain components of all three membranes. The inner one is represented by the endothelium lying on the basement membrane, the middle one is represented by one layer of smooth muscle cells with a spiral direction. The outer shell is formed by adventitial cells of loose connective tissue and connective tissue fibers. Venules (postcapillary) have only two membranes: internal with endothelium and external with adventitial cells. There are no smooth muscle cells in the vessel wall.

Arterio-venular anastomoses (AVA). There are true AVA - shunts, through which arterial blood is discharged, and atypical AVA - half-shunts, through which mixed blood flows. True anastomoses are divided into those that do not have special devices and anastomoses equipped with special locking devices. The latter include arteriolo-venular anastomoses of the epithelioid type, containing cells with light cytoplasm in the middle membrane. There are many unequal endings on their surface. These cells secrete acetylcholine. These epithelioid cells are able to swell and, according to other authors, shrink. As a result, the lumen of the vessel is closed. Anastomoses of the epithelial type can be complex (glomerular) and simple. Complex AVAs of the epithelioid type differ from simple ones in that the afferent afferent arteriole divides into 2-4 branches that pass into the venous segment. These branches are surrounded by one common connective tissue sheath (for example, in the skin dermis and hypodermis). There are also anastomoses of the closing type, in which in the subendothelial layer in the form of rollers there are smooth myocytes protruding into the lumen and closing it during their contraction. An important role belongs to ABA in the body's compensatory reactions in case of circulatory disorders and the development of pathological processes.

Lymphatic vessels subdivided into lymphatic capillaries, intra- and extraorganic lymphatic vessels and main lymphatic trunks: the thoracic duct and the right lymphatic duct. Lymphatic capillaries begin in tissues blindly. Their wall consists of large endotheliocytes. The basement membrane and pericytes are absent. The endothelium is connected with the surrounding tissue by fixing filaments that are woven into the surrounding connective tissue. Larger lymphatic vessels resemble veins in structure. They are characterized by the presence of valves and a well-developed outer shell. Among the lymphatic vessels, vessels of the muscular type and lymphatic vessels of the non-muscular fibrous type are distinguished.

Heart. Wall of the heart consists of three membranes: endocardium, myocardium and epicardium. The endocardium lines the inside of the chamber of the heart and is similar in structure to the wall of an artery. Develops from mesenchyme. It distinguishes the following layers: 1. endothelium, which lies below the thick basement membrane, 2. subendothelial layer, represented by loose fibrous connective tissue, 3. muscular-elastic layer with smooth myocytes and elastic fibers, 4. outer connective tissue layer, consisting of connective tissue with thick collagen, elastic and reticulin fibers.

Valves are located in the heart between the atria and ventricles, as well as on the border of the ventricle with the aortic arch and pulmonary artery. These are thin connective tissue plates covered with endothelium. On the atrial side of the atrioventricular (atrioventricular) valve, many elastic fibers are located under the endothelium, and collagen fibers predominate on the ventricular side. The latter continue into tendon threads.

The myocardium (together with the epicardium) develops from the myoepicardial plate, and consists of striated cardiac muscle tissue. It is represented by typical contractile cardiomyocytes that make up the contractile myocardium, and atypical conductive cardiac myocytes that form the conduction system of the heart. Contractile cardiomyocytes have 1-2 nuclei in the center and longitudinally located myofibrils along the periphery. Through intercalated discs (desmosomes, gap-like junctions), cardiomyocytes are combined into cardiac muscle fibers that anastomose with each other. Longitudinal and lateral connections of cardiomyocytes provide contraction of the myocardium as a whole. Contractile cardiomyocytes contain many mitochondria located both in the center, near the cell nucleus, and in chains between myofibrils. The lamellar Golgi complex is well developed, the endoplasmic reticulum does not form terminal cisterns, but instead forms terminal extensions of the tubules of the endoplasmic reticulum that are adjacent to the T-tubule membranes. The heart muscle is rich in enzymes involved in redox processes. These are mainly aerobic type enzymes. In the connective tissue of the myocardium, among the reticular, and to a lesser extent, collagen and elastic fibers, there are many blood and lymphatic vessels.

The conduction system of the heart consists of the sinus-atrial, atrioventricular nodes, the atrioventricular bundle-trunk, the right and left legs and their branches. These formations consist of conductive cardiac myocytes, well innervated. Among these cardiac myocytes, P-cells are distinguished - pacemakers in the sinus node, transitional cells of the atrioventricular node and cells of the bundle of the conducting system and its legs. The latter transmit excitation from transitional cells to the contractile myocardium. Conductive cardiac myocytes often form clusters under the endocardium. They are larger and lighter in color (richer in sarcaplasm) compared to contractile cardiac myocytes. Their nuclei are larger and eccentrically located. There are fewer myofibrils in conducting cardiac myocytes and they are located on the periphery. There are few mitochondria in conducting cardiac myocytes, a lot of glycogen, but less ribonucleoproteins and lipids. Enzymes involved in anaerobic glycolysis predominate.

The epicardium is a visceral sheet of the pericardium, represented by a thin connective tissue plate. It contains collagen and elastic fibers, vessels, nerve trunks. The free surface of the epicardium is covered with mesothelium.

The microcirculatory bed includes the following components:

arterioles;

precapillaries;

capillaries;

postcapillaries;

• arteriolo-venular anastomoses.

The functions of the microcirculatory bed are as follows:

trophic and respiratory functions, since the exchange surface of capillaries and venules is 1000 m 2, or 1.5 m 2 per 100 g of tissue;

depositing function, since a significant part of the blood is deposited in the vessels of the microvasculature at rest, which is included in the bloodstream during physical work;

drainage function, since the microcirculatory bed collects blood from the supplying arteries and distributes it throughout the organ;

regulation of blood flow in the organ, this function is performed by arterioles due to the presence of sphincters in them;

transport function, i.e. blood transport.

Three links are distinguished in the microcirculatory bed:

arterial (arterioles of precapillaries);

capillary;

venous (postcapillaries, collecting and muscle venules).

Arterioles have a diameter of 50-100 microns. In their structure, three shells are preserved, but they are less pronounced than in the arteries. In the area of ​​​​discharge from the arteriole of the capillary there is a smooth muscle sphincter that regulates blood flow. This area is called the precapillary.

capillaries- these are the smallest vessels, they differ in size by:

narrow type 4-7 microns;

normal or somatic type 7-11 microns;

sinusoidal type 20-30 µm;

lacunar type 50-70 microns.

In their structure, a layered principle can be traced. The inner layer is formed by the endothelium. The endothelial layer of the capillary is an analogue of the inner shell. It lies on the basement membrane, which first splits into two sheets, and then connects. As a result, a cavity is formed in which pericyte cells lie. On these cells, on these cells, vegetative nerve endings end, under the regulatory action of which the cells can accumulate water, increase in size and close the lumen of the capillary. When water is removed from the cells, they decrease in size, and the lumen of the capillaries opens. Functions of pericytes:

change in the lumen of capillaries;

source of smooth muscle cells;

control of endothelial cell proliferation during capillary regeneration;

synthesis of basement membrane components;

phagocytic function.

Basement membrane with pericytes- analogue of the middle shell. Outside of it is a thin layer of the ground substance with adventitial cells that play the role of cambium for loose fibrous irregular connective tissue.

Capillaries are characterized by organ specificity, and therefore there are three types of capillaries:

capillaries of the somatic type or continuous, they are in the skin, muscles, brain, spinal cord. They are characterized by a continuous endothelium and a continuous basement membrane;

capillaries of fenestrated or visceral type (localization - internal organs and endocrine glands). They are characterized by the presence of constrictions in the endothelium - fenestra and a continuous basement membrane;

intermittent or sinusoidal capillaries (red bone marrow, spleen, liver). In the endothelium of these capillaries there are true holes, they are also in the basement membrane, which may be absent altogether. Sometimes capillaries include lacunae - large vessels with a wall structure as in a capillary (cavernous bodies of the penis).

Venules are divided into:

post-capillary;

collective;

muscular.

Postcapillary venules are formed as a result of the fusion of several capillaries, they have the same structure as the capillary, but a larger diameter (12–30 μm) and a large number of pericytes. Collective venules (diameter 30–50 μm), which are formed by the fusion of several postcapillary venules, already have two distinct membranes: the inner (endothelial and subendothelial layers) and the outer, loose, fibrous, unformed connective tissue. Smooth myocytes appear only in large venules, reaching a diameter of 50 µm. These venules are called muscular and have a diameter of up to 100 microns. Smooth myocytes in them, however, do not have a strict orientation and form a single layer.

Arteriovenular anastomoses or shunts- this is a type of vessels of the microcirculatory bed, through which blood from arterioles enters venules, bypassing the capillaries. It is necessary, for example, in the skin for thermoregulation. All arteriolo-venular anastomoses are divided into two types:

true - simple and complex;

atypical anastomoses or half shunts.

In simple anastomoses, there are no contractile elements, and the blood flow in them is regulated by a sphincter located in the arterioles at the site of the anastomosis. In complex anastomoses, there are elements in the wall that regulate their lumen and the intensity of blood flow through the anastomosis. Complex anastomoses are divided into glomus type anastomoses and trailing artery type anastomoses. In anastomoses of the trailing arteries type, there are accumulations of longitudinally smooth myocytes in the inner shell. Their contraction leads to protrusion of the wall in the form of a pillow into the lumen of the anastomosis and its closure. In anastomoses such as glomus (glomerulus) in the wall there is an accumulation of epithelioid E-cells (they look like epithelium) that can suck up water, increase in size and close the lumen of the anastomosis. When water is released, the cells decrease in size, and the lumen opens.

In half shunts there are no contractile elements in the wall, the width of their lumen is not adjustable. Venous blood from venules can be thrown into them, therefore, in half-shunts, unlike shunts, mixed blood flows. Anastomoses perform the function of blood redistribution, regulation of blood pressure.

Vascular development.

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

The structure of the vessels.

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

1. internal

2. middle

3. outer

arteries

Depending on the ratio of muscular elastic components, arteries of the type are distinguished:

elastic

Large main vessels - aorta. Pressure - 120-130 mm / hg / st, blood flow velocity - 0.5 1.3 m / s. The function is transport.

Inner shell:

A) endothelium

flattened polygonal cells

B) subendothelial layer (subendothelial)

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

Middle shell:

Represented by fenestrated elastic membranes. Between them a small number of muscle cells.

Outer shell:

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

muscular

Arteries of small and medium caliber.

Inner shell:

A) endothelium

B) subendothelial layer

B) internal elastic membrane

Middle shell:

Smooth muscle cells predominate, arranged in a gentle spiral. Between the middle and outer shell is the outer elastic membrane.

Outer shell:

Represented by loose connective tissue

Mixed

Arterioles

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

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

capillaries

Classification:

Depending on diameter:

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

medium 8-11 microns - skin, mucous membranes

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

gaps up to 100 microns - found in the cavernous bodies

Depending on the structure:

Somatic - continuous endothelium and continuous basement membrane - muscles, lungs, CNS

The structure of the capillary:

3 layers, which are analogues of 3 shells:

A) endothelium

B) pericytes enclosed in a basement membrane

B) adventitial cells

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

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

Venules

postcapillary venules

similar to capillaries but have more pericytes

collecting venules

muscle venules

Vienna

Classification:

● fibrous (muscleless) type

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

● muscular type

There are three subtypes:

● Depending on the muscle component

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

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

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

Muscular elements are found in all three sheaths

Structure

Inner shell:

Endothelium

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

Middle shell:

Circularly arranged bundles of muscle cells.

Outer shell:

Loose connective tissue, and longitudinally arranged muscle cells.

HEART

DEVELOPMENT

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