Cardiovascular system. Particular histology of the cardiovascular system Cardiovascular system interesting facts histology
According to their structural and functional characteristics, three types of capillaries are distinguished: somatic, fenestrated and sinusoidal, or perforated.
The most common type of capillaries is somatic. Such capillaries have a continuous endothelial lining and a continuous basement membrane. Capillaries of the somatic type are found in muscles, organs of the nervous system, connective tissue, and exocrine glands.
Second type - fenestrated capillaries. They are characterized by thin endothelium with pores in the endothelial cells. The pores are covered by the diaphragm, the basement membrane is continuous. Fenestrated capillaries are found in endocrine organs, in the intestinal mucosa, in brown adipose tissue, in the renal corpuscle, and in the choroid plexus of the brain.
The third type is capillaries perforated type, or sinusoids. These are capillaries of large diameter, with large intercellular and transcellular pores (perforations). The basement membrane is discontinuous. Sinusoidal capillaries are characteristic of the hematopoietic organs, in particular the bone marrow, spleen, and also the liver.
Venous part of the microvasculature: postcapillaries, collecting venules and muscle venules
Postcapillaries(or postcapillary venules) are formed as a result of the fusion of several capillaries; their structure resembles the venous section of a capillary, but more pericytes are noted in the wall of these venules. In the organs of the immune system there are post-capillaries with a special high endothelium, which serve as a place for lymphocytes to exit the vascular bed. Together with capillaries, postcapillaries are the most permeable areas of the vascular bed, reacting to substances such as histamine, serotonin, prostaglandins and bradykinin, which cause disruption of the integrity of intercellular connections in the endothelium.
Collecting venules are formed as a result of the fusion of postcapillary venules. Individual smooth muscle cells appear in them and the outer membrane is more clearly defined.
Muscular venules have one or two layers of smooth muscle cells in the middle shell and a relatively well-developed outer shell.
The venous section of the microcirculatory bed, together with the lymphatic capillaries, performs a drainage function, regulating the hematolymphatic balance between blood and extravascular fluid, removing tissue metabolic products. Leukocytes migrate through the walls of venules, as well as through capillaries. Slow blood flow and low blood pressure, as well as the distensibility of these vessels, create conditions for blood deposition.
Arteriolo-venular anastomoses
Arteriovenular anastomoses (ABA) are connections between vessels that carry arterial blood into veins, bypassing the capillary bed. They are found in almost all organs. The volume of blood flow in anastomoses is many times greater than in capillaries, and the speed of blood flow is significantly increased. ABAs are highly responsive and capable of rhythmic contraction.
Classification. There are two groups of anastomoses: true ABA (or shunts), and atypical ABA (or half-shunts). IN true anastomoses Purely arterial blood is discharged into the venous bed. IN atypical anastomoses mixed blood flows, because gas exchange takes place in them. Atypical anastomoses (half-shunts) are a short but wide capillary. Therefore, the blood discharged into the venous bed is not entirely arterial.
The first group - true anastomoses - can have a different external shape - straight short anastomoses, loops, branching connections. True ABA is divided into two subgroups: simple and complex. Complex AVAs are equipped with special contractile structures that regulate blood flow. This includes anastomoses with muscle regulation, as well as so-called anastomoses. glomus, or glomerular, type - with special epithelioid cells.
ABA, especially the glomus type, is richly interned. ABAs take part in the regulation of blood supply to organs, redistribution of arterial blood, regulation of local and general blood pressure, as well as in the mobilization of blood deposited in venules.
capillaries with a continuous endothelial layer - somatic type, localized in the brain, muscles, skin;
fenestrated capillaries - visceral type, with exudations of endothelial cytoplasm - (capillaries of the glomeruli of the kidney, intestinal villi);
capillaries with slit-like openings in the endothelium and basement membrane - sinusoidal type capillaries (in the spleen, liver and other organs).
Arteriolovenular anastomoses (ABA). This part of the microvasculature ensures direct passage of arterial blood into the veins, bypassing the capillaries. ABAs are localized in almost all organs.
There are two groups of anastomoses:
true ABA (shunts) through which pure arterial blood is discharged. They, in turn, are divided into two groups according to their structure:
simple ABA - have a border between the arteriole and the venule, which corresponds to the area where the tunica media of the arteriole ends. Regulation of blood flow is carried out by smooth muscle cells of the middle layer of the arteriole itself without special contractile apparatus;
ABA, which have special contractile devices in the form of rolls or pillows in the subepithelial layer, formed by longitudinally arranged smooth muscle cells. Contractions of the muscle cushions that protrude into the lumen of the anastomosis lead to cessation of blood flow.
This subgroup also includes ABA of the epithelioid type (simple and complex).
In simple ABAs of the epithelial type, the muscle cells are gradually replaced towards the venous end by short oval clear cells (E-cells), similar to epithelial ones. In complex and glomerular arterioles, the afferent arteriole divides into two to four branches, which pass into the venous segment.
atypical ABA (half shunts) are connections of arterioles and venules; through a short capillary-type vessel. Therefore, the blood discharged into the venous bed is not entirely arterial.
The connection of the arterial and venous systems, bypassing the capillaries, is of great importance for the regulation of blood pressure, blood supply to organs, arterialization of venous blood, mobilization of deposited blood, regulation of the flow of tissue fluid into the venous bed.
Venules. There are three types of venules:
postcapillary,
collective,
Muscular.
Postcapillary venules in their structure resemble the venous section of a capillary, but there are more pericytes in the wall of these venules than in capillaries.
In the collecting venules, individual smooth muscle cells appear and the outer membrane is more clearly defined.
Muscular venules have one or two layers of smooth myocytes in the middle tunica and a relatively well-developed outer tunica.
The venous section of the MCR, together with the lymphatic capillaries, performs a drainage function, regulating the hemolymphatic balance between blood and extravascular fluid, removing tissue metabolic products. Leukocytes migrate through the walls of venules, as well as through capillaries. Slow blood flow and low blood pressure, as well as the distensibility of these vessels, create conditions for blood deposition.
Vienna(venae) ensure the return of blood to the heart, blood deposition. The general structure of veins is the same as arteries, but has its own characteristics:
the vein wall is thinner than that of the corresponding artery;
collagen fibers predominate in the veins, and elastic fibers are poorly developed;
there is no outer elastic membrane, the inner elastic membrane is poorly developed;
The lumen of the vein on the specimen often has an irregular shape, whereas in the arteries it is round;
The outer membrane has the relatively greatest thickness in the veins, and the middle membrane in the arteries;
the presence of valves in some veins.
Veins are classified depending on the development of the muscular elements in its wall:
Non-muscular veins Muscular veins
Veins with weak development of muscle elements
Veins with strong development of muscular elements
Veins are of non-muscular type. Veins of this type include non-muscular veins of the dura and pia mater, veins of the retina, spleen, bones and placenta. The inside of the vessel wall is lined with endothelium on the basement membrane. The middle shell is missing. The outer shell is represented by a thin layer of loose fibrous connective tissue that fuses with the surrounding tissues, as a result of which these veins do not collapse and the outflow of blood through them occurs easily.
Veins with weak development of muscle elements. The structural features of their walls depend on hemodynamic conditions. The blood in them moves under the influence of gravity. These veins have a poorly defined subendothelial layer; the tunica media contains few smooth muscle cells. In the outer lining of the veins there are single muscle cells. This group of veins includes: veins of the upper body, neck, face, superior vena cava.
Veins with average development of muscle elements. An example is the brachial vein. Structural features: the inner membrane forms the valve apparatus, and also contains individual longitudinally directed myocytes, the internal elastic membrane is not expressed, the middle membrane is thin, smooth muscle cells are located in it circularly, the outer elastic membrane is absent, so the layers of connective tissue of the middle membrane are continuous directly into the loose fibrous connective tissue of the outer shell.
Veins with strong development of muscular elements. These veins are characterized by strong development of muscle cells in all three membranes. In the inner and outer membranes, smooth myocytes are located longitudinally, and in the middle membrane - circular. A characteristic feature of these veins is the presence of valves. These veins include: the veins of the lower half of the torso and legs.
Valves- these are pocket-like folds of the inner membrane, open towards the heart. They prevent the blood from flowing back. The basis of the valve is fibrous connective tissue. In this case, on the side facing the lumen of the vessel, predominantly elastic fibers lie under the endothelium, and on the opposite side there are many collagen fibers. There may be a small number of smooth myocytes at the base of the valve leaflet.
Inferior vena cava its structure differs sharply from the veins flowing into it. The inner and middle shells are poorly developed. The outer shell has a large number of longitudinally arranged bundles of smooth muscle cells and is 6-7 times thicker than the inner and middle shells combined. There are no valves in the inferior vena cava; their function is performed by transverse folds of the outer membrane that prevent the reverse flow of blood.
Veins are classified according to their caliber into large, medium and small.
Lymphatic vessels.
The lymphatic system conducts lymph from tissues into the venous bed. Functionally, lymphatic vessels are closely connected with blood vessels, especially in the area where microvasculature vessels are located. It is here that tissue fluid is formed and penetrates into the lymphatic channel.
Classification. Among the lymphatic vessels there are:
lymphatic capillaries,
intralymphatic vessels,
extralymphatic vessels,
thoracic duct,
right lymphatic duct.
Lymphatic capillaries They are blindly starting flattened tubules into which tissue fluid enters from the tissues along with metabolic products. Their wall is formed only by endothelium. There is no basement membrane or pericytes. The endothelium is connected to the surrounding connective tissue by bundles of anchor, or sling, filaments that prevent the collapse of capillaries. There are gaps between endothelial cells. The diameter of lymphatic capillaries can vary depending on the degree of their filling with lymph. Lymphatic capillaries perform a drainage function, participating in the absorption of blood plasma filtrate from connective tissue.
Lymphatic vessels. The structure of the wall of lymphatic vessels has much in common with veins, which is explained by similar conditions of lympho- and hemodynamics (low pressure, low flow rate, direction of outflow from tissues to the heart). There are vessels of muscular and non-muscular type. Medium and large lymphatic vessels have three well-developed membranes (inner, middle and outer) in their walls. The inner lining of the lymphatic vessels forms numerous folds - valves. The dilated areas of blood vessels between adjacent valves are called lymphangions. The tunica media is more pronounced in the vessels of the lower extremities. Lymph nodes are located along the lymphatic vessels. A feature of the structure of the wall of large lymphatic vessels (thoracic duct and right lymphatic duct) is a well-developed outer shell, which is 3-4 times thicker than the inner and middle ones combined. The outer shell contains longitudinal bundles of smooth muscle cells. There are up to 9 semilunar valves along the thoracic duct.
Heart(cor) – the central organ of blood and lymph circulation. Thanks to its ability to contract, the heart moves the blood.
The heart wall is formed by three membranes:
endocardium, (internal);
myocardium, (medium);
epicardium, (external).
Endocardium consists of four layers:
endothelium on the basement membrane;
subendothelial layer - loose connective tissue rich in poorly differentiated cells;
muscle-elastic layer - formed by smooth myocytes and elastic fibers;
The outer connective tissue layer consists of loose fibrous connective tissue containing elastic, collagen and reticular fibers.
Valves.
Valves are located between the atria and ventricles of the heart, as well as the ventricles and large vessels. They are thin fibrous plates of dense fibrous connective tissue covered with endothelium with a small number of cells. The cells covering the valve partially overlap each other in a tiling pattern or form finger-like depressions of the cytoplasm of one cell into another. The valve walls do not have blood vessels. The structure of the atrial and ventricular parts of the valve leaflets is not the same. The atrial side has a smooth surface; here in the subendothelial layer there is a dense plexus of elastic fibers and bundles of smooth muscle cells. The number of muscle bundles increases noticeably at the base of the valve. The ventricular side has an uneven surface. It is equipped with outgrowths from which tendon threads begin. In this area, only a small number of elastic fibers are located under the endothelium.
Myocardium consists of cardiac muscle tissue and layers of loose fibrous connective tissue with blood vessels and nerves. There are typical contractile muscle cells - cardiomyocytes and atypical - conductive cardiac myocytes, which are part of the so-called cardiac conduction system. Contractile myocytes are rectangular cells with a centrally located nucleus. In the cytoplasm, myofibrils are located longitudinally. The basement membrane is involved in the formation of T-tubules. Striated cardiac muscle tissue, described in the section "Muscle Tissue".
The conduction system of the heart unites muscle cells that form and conduct impulses to contractile cardiomyocytes. It consists of: the sinus-atrial node, the atrioventricular node, the atrioventricular bundle of His. There are three types of conducting muscle cells:
1. The first type is pacemakers or pacemaker cells capable of spontaneous contraction. They are distinguished by their small size, polygonal shape, small number of randomly arranged myofibrils. There are no T-systems.
2. Transitional - thin, elongated cells, myofibrils are more developed, oriented in parallel, but not always.
3. The cells of the Hiss bundle are large, there are no T-systems, the myofibrils are thin, located in no particular order along the periphery of the cell, the nuclei are localized eccentrically.
Epicardium and pericardium. The outer layer of the heart, or epicardium, is the visceral layer of the pericardium. The epicardium consists of a thin layer of connective tissue that is covered with mesothelium.
Between the epicardium and pericardium there is a slit-like space containing a small amount of fluid that acts as a lubricant. In the pericardium, the connective base is more developed than in the epicardium.
Structure of blood vessels The cardiovascular system (CVS) consists of the heart, blood and lymphatic vessels. Vessels in embryogenesis are formed from mesenchyme. They are formed from the mesenchyme of the marginal zones of the vascular strip of the yolk sac or the mesenchyme of the embryo. In late embryonic development and after birth, vessels are formed by budding from capillaries and post-capillary structures (venules and veins). Blood vessels are divided into great vessels (arteries, veins) and microvasculature vessels (arterioles, precapillaries, capillaries, postcapillaries and venules). In the main vessels, blood flows at high speed and there is no exchange of blood with tissues; in the vessels of the microvasculature, blood flows slowly for better exchange of blood with tissues. All organs of the cardiovascular system are hollow and, in addition to the vessels of the microcirculatory system, contain three membranes: 1. The inner membrane (intima) is represented by the internal endothelial layer. Behind it is the subendothelial layer (PBST). The subendothelial layer contains a large number of poorly differentiated cells migrating into the tunica media and delicate reticular and elastic fibers. In muscular-type arteries, the inner tunica is separated from the middle tunica by an internal elastic membrane, which is a collection of elastic fibers. 2. The middle membrane (media) in arteries consists of smooth myocytes arranged in a gentle spiral (almost circular), elastic fibers or elastic membranes (in elastic-type arteries); In the veins, it may contain smooth myocytes (in veins of the muscular type) or predominate connective tissue (veins of the non-muscular type). In veins, unlike arteries, the middle membrane (media) is much thinner compared to the outer membrane (adventitia).
3. The outer shell (adventitia) is formed by the RVST. In arteries of the muscular type there is an outer elastic membrane that is thinner than the internal one.
Arteries Arteries have 3 membranes in their wall structure: intima, media, adventitia. Arteries are classified depending on the predominance of elastic or muscular elements on the artery: 1) elastic, 2) muscular and 3) mixed type.
In arteries of elastic and mixed types, in comparison with arteries of muscular type, the subendothelial layer is much thicker. The middle shell in elastic arteries is formed by fenestrated elastic membranes - an accumulation of elastic fibers with zones of their sparse distribution (“windows”). Between them there are layers of PBST with single smooth myocytes and fibroblastic cells. Muscular-type arteries contain many smooth muscle cells. The farther from the heart, the located are the arteries with a predominance of the muscular component: the aorta is of the elastic type, the subclavian artery is of the mixed type, and the brachial artery is of the muscular type. An example of a muscular type is also the femoral artery.
Veins Veins have 3 membranes in their structure: intima, media, adventitia. Veins are divided into 1) non-muscular and 2) muscular (with weak, medium or strong development of the muscular elements of the middle shell). Veins of a muscleless type are located at the level of the head, and vice versa - veins with a strong development of the muscular membrane on the lower extremities. Veins with a well-developed muscular layer have valves. Valves are formed by the inner lining of the veins. This distribution of muscle elements is associated with the effect of gravity: it is more difficult to lift blood from the legs to the heart than from the head, therefore in the head there is a muscleless type, in the legs there is a highly developed muscle layer (for example, the femoral vein). The blood supply to the vessels is limited to the outer layers of the tunica media and the adventitia, while in the veins the capillaries reach the inner tunica. Innervation of blood vessels is provided by autonomic afferent and efferent nerve fibers. They form the adventitial plexus. Efferent nerve endings reach mainly the outer regions of the tunica media and are predominantly adrenergic. Afferent nerve endings of baroreceptors that respond to pressure form local subendothelial accumulations in the great vessels.
An important role in the regulation of vascular muscle tone, along with the autonomic nervous system, is played by biologically active substances, including hormones (adrenaline, norepinephrine, acetylcholine, etc.).
Blood capillaries Blood capillaries contain endothelial cells lying on the basement membrane. The endothelium has a metabolic apparatus capable of producing a large number of biologically active factors, including endothelins, nitric oxide, anticoagulant factors, etc., which control vascular tone and vascular permeability. Adventitial cells are closely adjacent to the vessels. Pericytes, which may be involved in membrane cleavage, take part in the formation of capillary basement membranes. Capillaries are distinguished: 1. Somatic type. The lumen diameter is 4-8 microns. The endothelium is continuous, not fenestrated (i.e. not thinned, the fenestra is a window in translation). The basement membrane is continuous and well defined. The pericyte layer is well developed. There are adventitial cells. Such capillaries are located in the skin, muscles, bones (what is referred to as the soma), as well as in organs where cells need to be protected - as part of histohematic barriers (brain, gonads, etc.) 2. Visceral type. Clearance up to 8-12 microns. The endothelium is continuous, fenestrated (in the area of the windows there is practically no cytoplasm of the endothelial cell and its membrane is adjacent directly to the basement membrane). All types of contacts predominate between endothelial cells. The basement membrane is thinned. There are fewer pericytes and adventitial cells. Such capillaries are found in internal organs, for example, in the kidneys, where urine must be filtered.
3. Sinusoidal type. The lumen diameter is more than 12 microns. The endothelial layer is discontinuous. Endotheliocytes form pores, hatches, fenestrae. The basement membrane is discontinuous or absent. There are no pericytes. Such capillaries are necessary where not only the exchange of substances between blood and tissues occurs, but also “cell exchange”, i.e. in some blood-forming organs (red bone marrow, spleen), or large substances - in the liver.
Arterioles and precapillaries. Arterioles have a lumen diameter of up to 50 microns. Their wall contains 1-2 layers of smooth myocytes. The endothelium is elongated along the vessel. Its surface is smooth. The cells are characterized by a well-developed cytoskeleton, an abundance of desmosomal, hinge, and imbricated contacts. In front of the capillaries, the arteriole narrows and becomes a precapillary. Precapillaries have a thinner wall. The muscular layer is represented by individual smooth myocytes. Postcapillaries and venules. Postcapillaries have a lumen of smaller diameter than that of venules. The structure of the wall is similar to the structure of the venule. Venules have a diameter of up to 100 µm. The inner surface is uneven. The cytoskeleton is less developed. The contacts are mostly simple, butt-to-end. Often the endothelium is higher than in other vessels of the microvasculature. Cells of the leukocyte series penetrate through the wall of the venule, mainly in the areas of intercellular contacts. The outer layers are similar in structure to capillaries. Arteriolo-venular anastomoses.
Blood can flow from the arterial systems to the venous system, bypassing the capillaries, through arteriole-venular anastomoses (AVA). There are true AVA (shunts) and atypical AVA (half-shunts). In half-shunts, the afferent and efferent vessels are connected through a short, wide capillary. As a result, mixed blood enters the venule. In true shunts, there is no exchange between the vessel and the organ, and arterial blood enters the vein. True shunts are divided into simple (one anastomosis) and complex (several anastomoses). It is possible to distinguish shunts without special locking devices (the role of the sphincter is played by smooth myocytes) and with a special contractile apparatus (epithelioid cells, which, when swollen, compress the anastomosis, closing the shunt).
Lymphatic vessels. Lymphatic vessels are represented by microvessels of the lymphatic system (capillaries and postcapillaries), intraorgan and extraorgan lymphatic vessels. Lymphatic capillaries begin blindly in tissues, contain thin endothelium and a thinned basement membrane.
The wall of medium and large lymphatic vessels contains endothelium, subendothelial layer, muscular layer and adventitia. According to the structure of the membranes, the lymphatic vessel resembles a muscular vein. The inner lining of the lymphatic vessels forms valves, which are an integral attribute of all lymphatic vessels after the capillary section.
Clinical significance. 1. In the body, arteries, especially elastic and muscular-elastic types, are most sensitive to atherosclerosis. This is due to hemodynamics and the diffuse nature of the trophic supply of the inner membrane, its significant development in these arteries. 2. In the veins, the valve apparatus is most developed in the lower extremities. This greatly facilitates the movement of blood against the hydrostatic pressure gradient. Violation of the structure of the valve apparatus leads to gross disruption of hemodynamics, edema and varicose veins of the lower extremities. 3. Hypoxia and low molecular weight products of cell destruction and anaerobic glycolysis are among the most powerful factors stimulating the formation of new blood vessels. Thus, areas of inflammation, hypoxia, etc., are characterized by subsequent rapid growth of microvessels (angiogenesis), which ensures the restoration of the trophic supply of the damaged organ and its regeneration.
4. Antiangiogenic factors that prevent the growth of new vessels, according to a number of modern authors, could become one of the effective antitumor groups of drugs. By blocking the growth of blood vessels into rapidly growing tumors, doctors could thereby cause hypoxia and death of cancer cells.
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Particular histology of the cardiovascular system
Vascular development.
The first vessels appear in the second – third week of embryogenesis in the yolk sac and chorion. A cluster is formed from the mesenchyme - blood islands. The central cells of the islets round off and become blood stem cells. Peripheral islet cells differentiate into the vascular endothelium. Vessels in the body of the embryo are formed a little later; blood stem cells do not differentiate in these vessels. Primary vessels are similar to capillaries, their further differentiation is determined by hemodynamic factors - pressure and blood flow speed. Initially, a very large part is deposited in the vessels, which is reduced.
Structure of blood vessels.
In the wall of all vessels, 3 membranes can be distinguished:
1. internal
2. average
3. external
Arteries
Depending on the ratio of muscle elastic components, arteries of the following types are distinguished:
Elastic
Large main vessels are the aorta. Pressure – 120-130 mm/Hg/st, blood flow speed – 0.5 1.3 m/sec. The function is transport.
Inner shell:
A) endothelium
flattened polygonal cells
B) subendothelial layer (subendothelial)
It is represented by loose connective tissue and contains stellate-shaped cells that perform combial functions.
Middle shell:
It is represented by fenestrated elastic membranes. Between them there is a small number of muscle cells.
Outer shell:
It is represented by loose connective tissue and 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 shells 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 the taps of the vascular system.
The middle shell is represented by 1-2 layers of smooth muscle cells.
Capillaries
Classification:
Depending on the diameter:
narrow 4.5-7 microns - muscles, nerves, musculoskeletal tissue
average 8-11 microns – skin, mucous membranes
sinusoidal up to 20-30 microns – endocrine glands, kidneys
lacunae up to 100 microns – found in the corpora cavernosa
Depending on the structure:
Somatic – continuous endothelium and continuous basement membrane – muscles, lungs, central nervous system
Capillary structure:
3 layers, which are analogues of 3 shells:
A) endothelium
B) pericytes enclosed in a basement membrane
B) adventitial cells
2. Finished - 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.
similar to capillaries, but have more pericytes
Classification:
● fibrous (muscleless) type
Found in the spleen, placenta, liver, bones, and meninges. In these veins, the subendothelial layer continues into the surrounding connective tissue
● muscular type
There are three subtypes:
● Depending on the muscle component
A) veins with weak development of muscle elements are located above the level of the heart, blood flows passively due to its heaviness.
B) veins with average development of muscle elements - brachial vein
C) veins with strong development of muscle elements, large veins lying below the level of the heart.
Muscular elements are found in all three membranes
Structure
Inner shell:
Endothelium
The subendothelial layer is a longitudinally directed bundle 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.
DEVELOPMENT
The heart is formed at the end of the 3rd week of embryogenesis. Under the visceral leaf of the splanchnotome, an accumulation of mesenchymal cells is formed, which turn into elongated tubes. These mesenchymal accumulations protrude into the cilomic cavity, bending the visceral layers 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 a duplicate of the endocardium.
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Discipline: Histology | CommentThe importance of the cardiovascular system (CVS) in the life of the body, and therefore the knowledge of all aspects of this area for practical medicine, is so great that the study of this system has been separated into two independent areas, cardiology and angiology. The heart and blood vessels are systems that function not periodically, but constantly, therefore more often than other systems they are susceptible to pathological processes. Currently, cardiovascular diseases, along with cancer, occupy a leading place in 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, and the deposition of blood.
Classification: I. The central organ is the heart. II. Peripheral section: A. Blood vessels: 1. Arterial link: a) arteries of elastic type; b) arteries of the muscular type; c) arteries of mixed type. 2. Microcirculatory bed: a) arterioles; b) hemocapillaries; c) venules; d) arteriolo-venular anastomoses 3. Venous link: a) veins of the muscular type (with weak, medium, strong development of muscle elements; b) veins of the non-muscular type. B. Lymphatic vessels: 1. Lymphatic capillaries. 2. Intraorgan lymphatic vessels. 3. Extraorgan lymphatic vessels. In the embryonic period, the first blood vessels are formed in 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 form from the surrounding mesenchyme connective tissue and smooth muscle elements of the vascular wall. Soon, blood vessels are formed from the mesenchyme in the body of the embryo, which connect with 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 during autopsy, researchers often found these vessels empty (not containing blood) and thought that vital “pneuma” or air was distributed through them throughout the body. Arteries of elastic, muscular and mixed types have a common principle of structure: there are 3 membranes in the wall - inner, middle and outer adventitia. The inner shell consists of layers: 1. Endothelium on the basement membrane. 2. The subendothelial layer is a snout fibrous tissue with a high content of poorly differentiated cells. 3. Internal elastic membrane - a plexus of elastic fibers. The middle layer contains smooth muscle cells, fibroblasts, elastic and collagen fibers. At the border of the middle and outer adventitia there is an outer elastic membrane - a plexus of elastic fibers. The outer adventitia of the arteries is histologically represented by loose fibrous tissue with vascular vessels and vascular nerves. Features in the structure of types of arteries are due to differences in the hemadynamic conditions of their functioning. Differences in structure mainly concern the middle shell (different ratios of the constituent elements of the shell): 1. Arteries of the elastic type - these include the aortic arch, pulmonary trunk, thoracic and abdominal aorta. Blood enters these vessels in spurts under high pressure and moves at high speed; There is a large pressure drop during the transition from systole to diastole. The main difference from arteries of other types is in the structure of the tunica media: in the tunica media, elastic fibers predominate from the above components (myocytes, fibroblasts, collagen and elastic fibers). Elastic fibers are located not only in the form of individual fibers and plexuses, but also form elastic fenestrated membranes (in adults, the number of elastic membranes reaches up to 50-70 words). Thanks to their increased elasticity, the wall of these arteries not only withstands high pressure, but also smoothes out large differences (jumps) in pressure during the systole-diastole transition. 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 speed. Arteries of the muscular type differ from arteries of other types by the predominance of myocytes in the medial shell over other structural components; The inner and outer elastic membranes are clearly defined. Myocytes are oriented spirally in relation to the lumen of the vessel and are found even in the outer lining of these arteries. Thanks to the powerful muscular component of the middle shell, these arteries control the intensity of blood flow to individual organs, maintain the falling pressure and further push the blood, which is why muscular arteries are also called the “peripheral heart”.
3. Arteries of mixed type - these include large arteries extending from the aorta (carotid and subclavian arteries). In structure and function they occupy an intermediate position. The main structural feature: in the tunica media, myocytes and elastic fibers are represented approximately equally (1: 1), there is a small amount of collagen fibers and fibroblasts.
The microcirculatory bed is a link located between the arterial and venous links; ensures regulation of blood supply to the organ, metabolism between blood and tissues, deposition of blood in organs. Composition: 1. Arterioles (including precapillary). 2. Hemocapillaries. 3. Venules (including postcapillary). 4. Arteriolo-venular anastomoses. Arterioles are vessels connecting arteries with hemocapillaries. They retain the principle of the structure of 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 one layer of myocytes, and closer to the capillaries - single myocytes. As the diameter in the tunica media increases, the number of myocytes increases; first one, then two or more layers of myocytes are formed. Due to the presence of myocytes in the wall (in the precapillary arterioles in the form of a sphincter), the arterioles regulate the blood supply to the hemocapillaries, thereby the intensity of exchange between the blood and the tissues of the organ. Hemocapillaries. The wall of hemocapillaries has the smallest thickness and consists of 3 components - endothelial cells, basement membrane, pericytes in the thickness of the basement membrane. There are no muscle elements in the capillary wall, however, the diameter of the internal lumen may vary slightly as a result of changes in blood pressure, the ability of the nuclei of pericytes and endothelial cells to swell and contract. The following types of capillaries are distinguished: 1. Type I hemocapillaries (somatic type) - capillaries with continuous endothelium and a continuous basement membrane, diameter 4-7 µm. They are present in skeletal muscles, in the skin and mucous membranes.. 2. Type II hemocapillaries (fenestrated or visceral type) - the basement membrane is solid, the endothelium has fenestrae - thinned areas in the cytoplasm of endothelial cells. Diameter 8-12 microns. They are found in the capillary glomeruli of the kidneys, in the intestines, and in the endocrine glands. 3. Type III hemocapillaries (sinusoidal type) - the basement membrane is not continuous, is absent in places, and gaps remain between the endothelial cells; diameter 20-30 microns or more, not constant throughout - there are expanded and narrowed areas. The blood flow in these capillaries is slowed down. Found in the liver, hematopoietic organs, and endocrine glands. Around the hemocapillaries there is a thin layer of loose fibrous tissue with a large 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 endothelial cells and the basement membrane. Capillaries can change structure, transform into vessels of a different type and caliber; New branches can form from existing hemocapillaries. Precapillaries differ from hemocapillaries in that in the wall, in addition to endothelial cells, basement membrane, and pericytes, there are single or groups of myocytes.
Venules begin with postcapillary venules, which differ from capillaries by the large content of pericytes in the wall and the presence of valve-like folds of endothelial cells. As the diameter of the venules increases, the content of myocytes in the wall increases - first single cells, then groups and finally continuous layers.
Arteriolo-venular anastomoses (AVA) are shunts (or anastomoses) between arterioles and venules, i.e. carry out direct communication 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 membranes; There are myocytes, especially many in the middle shell, which act as a sphincter.
VEINS. A feature of the 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. The veins have valves - a duplication of the inner lining. The number of muscle elements in the wall of these vessels depends on whether the blood moves with or against gravity. Veins of the muscleless type are found in the dura mater, bones, retina, placenta, and red bone marrow. The wall of muscleless veins is lined internally with endothelial cells on the basement membrane, followed by a layer of fibrous SDT; there are no smooth muscle cells. Veins of the muscular type 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 in a collapsed state. The tunica media contains 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 membrane in the longitudinal direction, in the middle - circular direction.
LYMPHATIC VESSELS begin with lymphatic capillaries (LC). LCs, unlike hemocapillaries, begin blindly and have a larger diameter. The inner surface is lined with endothelium; there is no basement membrane. Under the endothelium there is a loose fibrous tissue with a high content of reticular fibers. The diameter of the LC is not constant - there are narrowings and expansions. Lymphatic capillaries merge to form intraorgan lymphatic vessels - their structure is close to veins, because are under 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. Extraorgan lymphatic vessels are also similar in structure to veins, but the basal endothelial membrane is poorly defined and absent in places. The internal elastic membrane is clearly visible in the wall of these vessels. The middle shell receives special development in the lower extremities.
HEART. The heart is formed 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 layer of splanchnotomes. Paired cords are formed from the mesenchyme, which soon turn into tubes, from which the inner lining of the heart - the endocardium - is ultimately formed. The areas of the visceral layer of splanchnotomes that encircle these tubes are called myoepicardial plates, which subsequently differentiate into the myocardium and epicardium. As the embryo develops, with the appearance of the trunk fold, the flat embryo folds into a tube - the body, while the 2 heart buds end up in the chest cavity, come closer and finally merge into one tube. Next, this heart tube begins to quickly grow in length and, not fitting into the chest, forms several bends. Neighboring loops of the bending tube grow together and a 4-chambered heart is formed from a simple tube. HEART is the central organ of the cardiovascular system, has 3 membranes: internal - endocardium, middle (muscular) - myocardium, external (serous) - epicardium. The endocardium consists of 5 layers: 1. Endothelium on the basement membrane. 2. Subendothelial layer of loose fibrous tissue with a large number of poorly differentiated cells. 3. Muscle-elastic layer (myocytes, elastic fibers). 4. Elastic-muscular layer (myocyte-elastic fibers). 5. Outer SDT layer (loose fibrous SDT). In general, the structure of the endocardium resembles the structure of the wall of a blood vessel. The muscular layer (myocardium) consists of 3 types of cardiomyocytes: contractile, conductive and secretory (for structural features and functions, see the topic “Muscle tissue”). The endocardium is a typical serous membrane and consists of layers: 1. Mesothelium on the basement membrane. 2. Superficial collagen layer. 3. Layer of elastic fibers. 4. Deep collagen layer. 5. Deep collagen-elastic layer (50% of the total thickness of the epicardium). Under the mesothelium, in all layers between the fibers there are fibroblasts. Regeneration of the cardiovascular system. Vessels, endocardium and epicardium regenerate well. Reparative regeneration of the heart is poor, the defect is replaced by a scar; physiological regeneration - well expressed, due to intracellular regeneration (renewal of worn-out organelles). Age-related changes in cardiovascular system. In the vessels of elderly and senile age, a thickening of the inner lining is observed, and deposits of cholesterol and calcium salts (atherosclerotic plaques) are possible. In the middle layer of blood vessels, the content of myocytes and elastic fibers decreases, the amount of collagen fibers and acidic mucopolysaccharides increases.
PRIVATE HISTOLOGY.
Cardiovascular system.
The system includes the heart, arterial and venous vessels and lymphatic vessels. The system is formed at the 3rd week of embryogenesis. Vessels are formed from mesenchyme. According to their diameter, vessels are divided into
Large
Average
Small ones.
The wall of blood vessels is divided into inner, outer and middle membranes.
Arteriesaccording to their structure they are divided into
1. Elastic arteries
2. Arteries of the muscular-elastic (mixed) type.
3. Arteries of the muscular type.
TO elastic arteries These include large vessels such as the aorta and pulmonary artery. They have a thick, developed wall.
ü Inner shell contains the endothelium layer, which is represented by flat endothelial cells on the basement membrane. It creates conditions for blood flow. Next is the subendothelial layer of loose connective tissue. The next layer is a weave of thin elastic fibers. There are no blood vessels. The inner lining is nourished diffusely from the blood.
ü Middle shell powerful, wide, occupies the main volume. It contains thick elastic fenestrated membranes (40-50). They are built from elastic fibers and connected to each other by the same fibers. They occupy the main volume of the membrane; individual smooth muscle cells are located obliquely in their windows. The structure of the vessel wall is determined by hemodynamic conditions, the most important of which are blood flow speed and blood pressure level. The wall of large vessels is highly extensible, since the blood flow speed (0.5-1 m/s) and pressure (150 mm Hg) are high here, so it returns well to its original state.
ü Outer shell built of loose fibrous connective tissue, and it is denser in the inner layer of the outer shell. The outer and middle shells have their own vessels.
TO arteries of the muscular-elastic type include the subclavian and carotid arteries.
They have inner shell plexuses of muscle fibers are replaced by an internal elastic membrane. This membrane is thicker than fenestrated membranes.
In the middle shell the number of fenestrated membranes decreases (by 50%), but the volume of smooth muscle cells increases, that is, the elastic properties - the ability of the wall to stretch - decrease, but the contractility of the wall increases.
Outer shell the same structure as that of large vessels.
Muscular arteries predominate in the body among the arteries. They make up the bulk of blood vessels.
Their inner shell corrugated, contains endothelium. The subendothelial layer of loose connective tissue is well developed. There is a powerful elastic membrane.
Middle shell contains elastic fibers in the form of arcs, the ends of which are attached to the inner and outer elastic membranes. And their central sections seem to interlock. Elastic fibers and membranes form a single connected elastic frame that occupies a small volume. In the loops of these fibers there are bundles of smooth muscle cells. They predominate sharply and go circularly and in a spiral. That is, the contractility of the vessel wall increases. When this membrane contracts, the section of the vessel shortens, narrows and spirals.
Outer shell contains an outer elastic membrane. It is not as convoluted and thinner than the inner one, but is also built of elastic fibers, and loose connective tissue is located along the periphery.
The smallest muscular type vessels are arterioles.
They retain three thinner shells.
In the inner shell contains endothelium, subendothelial layer and a very thin internal elastic membrane.
In the middle shell smooth muscle cells run circularly and spirally, with the cells arranged in 1-2 rows.
In the outer shell there is no outer elastic membrane.
Arterioles break down into smaller ones hemocapillaries. They are located either in the form of loops or in the form of glomeruli, and most often form networks. Hemocapillaries are most densely located in intensively functioning organs and tissues - skeletal muscle fibers, cardiac muscle tissue. The diameter of the capillaries is not the same - from 4 to 7 µm. These are, for example, blood vessels in muscle tissue and brain substances. Their size corresponds to the diameter of the red blood cell. Capillaries diameter 7-11 microns found in mucous membranes and skin. Sinusoidal capillaries (20-30 microns) are present in the hematopoietic organs and lacunar- in hollow organs.
The hemocapillary wall is very thin. Includes a basement membrane that regulates capillary permeability. The basement membrane is split in sections, and cells are located in the split sections pericytes. These are process cells; they regulate the lumen of the capillary. On the inside of the membrane there are flat endothelial cells. Outside the blood capillary lies loose, unformed connective tissue, which contains tissue basophils(mast cells) and adventitial cells that participate in capillary regeneration. Hemocapillaries perform a transport function, but the leading one is the trophic = metabolic function. Oxygen easily passes through the walls of the capillaries into the surrounding tissues, and metabolic products return. The implementation of the transport function is helped by slow blood flow, low blood pressure, a thin capillary wall and loose connective tissue located around.
Capillaries merge into venules . The venous system of capillaries begins with them. Their wall has the same structure as that of capillaries, but the diameter is several times larger. Arterioles, capillaries and venules make up the microvasculature, which performs an metabolic function and is located inside the organ.
Venules merge into veins. In the wall of the vein there are 3 membranes - internal, middle and external, but the veins differ in the content of smooth muscle elements of connective tissue.
Highlight non-muscular veins . They have only an inner membrane, which contains the endothelium, subendothelial layer, connective tissue, which passes into the stroma of the organ. These veins are located in the dura mater, spleen, and bones. Blood is easily deposited in them.
Distinguish veins of the muscular type with underdeveloped muscle elements . They are located in the head, neck, and torso areas. They have 3 shells. The inner layer contains the endothelium, the subendothelial layer. The middle shell is thin, poorly developed, and contains individual circularly arranged bundles of smooth muscle cells. The outer shell consists of loose connective tissue.
Veins with moderately developed muscle elements located in the middle part of the body and in the upper extremities. In their inner and outer membranes, longitudinally arranged bundles of smooth muscle cells appear. In the middle shell, the thickness of the circularly located muscle cells increases.
Veins with highly developed muscular elements are found in the lower torso and lower extremities. In them, the inner shell forms folds-valves. In the inner and outer shells there are longitudinal bundles of smooth muscle cells, and the middle shell is represented by a continuous circular layer of smooth muscle cells.
In veins of the muscular type, unlike arteries, the smooth inner surface has valves, there are no outer and inner elastic membranes, there are longitudinal bundles of smooth muscle cells, the middle membrane is thinner, smooth muscle cells are located in it circularly.
Regeneration.
Hemocapillaries regenerate very well. As the diameter of the vessels increases, the ability to regenerate worsens.
Histophysiology of the heart.
There are 3 membranes: endocardium, myocardium, and pericardium. The endocardium develops from mesenchyme, the myocardium from mesoderm, the connective tissue plate of the epicardium from mesenchyme, the mesothelium (pericardium) from mesoderm. Laid in the 4th week of embryogenesis.
Endocardium-relatively thin. Contains endothelium, a subendothelial layer of loose connective tissue. The muscular-elastic layer is thin, it is formed by individual smooth muscle cells braided with elastic fibers. There is also an outer connective tissue layer. The endocardium is nourished diffusely.
The main mass of the wall is myocardium, which is represented by cardiac muscle tissue, a structural and functional unit, which is the contractile cardiomyocyte. They form cardiac muscle fibers and, due to anastomotic processes, they are connected to adjacent parallel muscle fibers and form a three-dimensional network of muscle fibers. Muscle fibers run in several directions. Between them there are thin layers of loose connective tissue with a high density of hemocapillaries.
In the myocardium, at the border with the endocardium, there are fibers of the cardiac conduction system, which regulates the contractile activity of the myocardium. It is built from conducting cardiomyocytes.
The main mechanism of myocardial regeneration is intracellular regeneration, which leads to compensatory cell hypertrophy and compensation for the function of dead cardiomyocytes. A connective tissue scar forms in place of dead cardiomyocytes.
Epicard. Its main component is a plate of loose connective tissue, which is covered on the surface with mesothelium. It secretes a mucous secretion. Due to this, there is free sliding between the outer and inner layers of the pericardium during contraction and relaxation of the heart muscle.
Lymphatic system.
Lymphatic vessels have the same structure as blood vessels, however, lymphatic capillaries have structural features. They start blindly, they are wider than the blood vessels, and the basement membrane in their wall is less developed. There are gaps between the endothelial cells, and on the outside there is loose connective tissue. Its tissue fluid, saturated with toxins, lipids and blood cells (mainly lymphocytes), penetrates through the cracks into the lumen of the lymphatic capillaries and forms lymph, which then enters the bloodstream system.
The main function is detoxification.
Blood system.
It includes blood and hematopoietic organs. They develop from mesenchyme, which is formed at the 3rd week of embryogenesis mainly from the mesoderm, in small quantities from the ectoderm and is represented by process cells that are located between the germ layers. During embryogenesis, all types of connective tissue are formed from mesenchyme, including blood, lymph and smooth muscle tissue. After birth, there is no mesenchyme; it is transformed into derivatives, but they retain a large number of stem cells, that is, these tissues have a high ability to regenerate through cell proliferation and differentiation.
Functions blood .
1. Transport. The respiratory, trophic, and excretory functions are realized through the blood.
2. Protective function.
3. Homeostatic function is maintaining a constant environment in the body.
Blood is a liquid tissue and an organ at the same time (5-6 liters). Its intercellular substance is liquid and has a special name - plasma. Plasma occupies 50-60% of the total blood volume. The rest is formed elements of blood.
Plasma.In plasma, water predominates (90-93%), the remaining 7-10% (the so-called dry residue) is represented by proteins (6-8.5%). These are fibrinogen, globulin, albumin.
Among the formed elements of blood, erythrocytes, leukocytes and platelets are distinguished.
Red blood cellsdominate in quantitative terms. For men 4-5.5· 10 12 in a liter. For women 4-5· 10 12 per liter.
Red blood cells are non-nuclear cells. 80% of the total number are discocytes, 20% are erythrocytes of other shapes (spike-shaped, spherical). 75% of red blood cells reach 7-8 microns in diameter. These are normocytes. Of the remaining 12.5% are microcytes, the remaining 12.5% are macrocytes.
Reticulocytes are found among red blood cells. Their number is 2-12% . In their cytoplasm they contain remnants of organelles in the form of a network. An increase in the number of reticulocytes occurs when the red bone marrow is irritated.
Red blood cells lack organelles and contain hemoglobin, which has a high affinity for oxygen and carbon dioxide.
Main function -transport = respiratory. They transport oxygen to tissues and carbon dioxide in the opposite direction. On their surface they transport antibodies, proteins, antigens, and drugs.
Red blood cells are formed in the red bone marrow, circulate and function in the blood (4 months), and die in the spleen.
Leukocytes(white blood cells). Their number is 4-9· 10 9 per liter of blood. Leukocytes are divided into 2 groups.
1. Granular leukocytes or granulocytes. They contain a segmented nucleus; the cytoplasm has a specific granularity, which is perceived by different dyes. Based on this feature, leukocytes are divided into neutrophilic leukocytes, eosinophilic leukocytes and basophilic leukocytes.
2. Non-granular leukocytes or agranulocytes. These include lymphocytes and immunocytes. They have no specific granularity in the cytoplasm; the nucleus is round and spherical in shape. They are mobile, capable of passing through the wall of hemocapillaries and moving in tissues. Movement occurs according to the principle of chemotaxis.
The life cycle of all leukocytes contains formation and maturation phase(in the hematopoietic organs). Then they come out into the blood and circulate. This is a short-term phase. IN tissue phase leukocytes exit into loose connective tissue, where they are activated and perform their functions and die there.
Granular leukocytes.
Neutrophil leukocytes or neutrophils make up 50-75% of the total. Diameter 10-15 microns. To stain blood cells, azure-eosin or the so-called Romanovsky-Ginza method is used. In their cytoplasm, neutrophils contain small, thread-like, abundant neutrophil granules. It contains bactericidal substances.
Neutrophils, according to the degree of maturity and the structure of the nucleus, are divided into segmented (45-70% of the total number). These are mature neutrophils. Their nucleus contains 3-4 segments connected by thin chromatin filaments. By function they are microphages. They phagocytose toxic substances and microorganisms. Their phagocytic activity is 70-99%, and the phagocytic index is 12-25.
In addition to segmented ones, band neutrophils are isolated - younger cells with S-shaped core.
Young neutrophils are also released. They are 0-0.5%. These are functionally active cells and have a curved bean-shaped nucleus.
The number of neutrophils is expressed by the term neutrophilia. An increase in the number of mature forms is called a shift to the right, an increase in the number of young forms is a shift to the left. The number of neutrophils increases in acute inflammatory diseases. Neutrophils are formed in the red bone marrow. They circulate in the blood for a short period - 2-3 hours. Move to the surface of the epithelium. The tissue phase lasts 2-3 days.
Eosinophils . There are significantly fewer of them than neutrophils. Their number is 1-5% of the total. The diameter is 12-14 microns. The core contains 2 large segments. The cytoplasm is filled with large eosinophilic granules and contains large acidophilic granules. Grains are lysosomes. Their content increases in allergic conditions, and they are able to phagocytose antigen-antibody complexes.
Basophil granulocytes are 0-0.5%. Diameter 10-12 microns. They contain a large lobed nucleus, their cytoplasm contains large basophilic granules. These cells are formed in the red bone marrow and circulate in the blood for a short period. The tissue phase is long. It is assumed that tissue basophils-mast cells are formed from blood basophils, since their grains also contain heparin and histamine. The number of basophils increases in the blood during chronic diseases and is an unfavorable prognostic sign. Eosinophils are formed in the red bone marrow, and perform their functions within 5-7 days in loose connective tissue.
Non-granular leukocytes.
Lymphocytes make up 20-35% of all leukocytes. Among lymphocytes, small lymphocytes predominate (diameter less than 7 µm). They have a round basophilic nucleus, a narrow basophilic rim of the cytoplasm, and poorly developed organelles. They also distinguish medium lymphocytes (7-10 µm) and large lymphocytes (more than 10 µm) - they are not normally found in the blood, only in leukemia.
All lymphocytes according to their immunological properties are divided into T-lymphocytes (60-70%), B-lymphocytes (20-30%) and zero lymphocytes.
T lymphocytes- These are thymus-dependent lymphocytes. They are formed in the thymus and according to their properties are divided into Killer T lymphocytes(they provide cellular immunity). They recognize foreign cells, approach them, and release cytotoxic substances that destroy the cytolemma of the foreign cell. Defects appear in the cytolemma into which fluid rushes and the foreign cell is destroyed. Also distinguished Helper T-lymphocytes. They stimulate B lymphocytes, turning them into plasma cells in response to an antigenic stimulus, their production of antibodies that neutralize antigens, they stimulate humoral immunity. Also distinguished Suppressor T lymphocytes. They inhibit humoral immunity. They also highlight T lymphocyte amplifiers. They regulate relationships among all varieties of T lymphocytes. Also distinguished Memory T-lymphocytes. They remember information about the antigen the first time they encounter it and, when they encounter it again, they provide a rapid immune response. Memory T lymphocytes determine lasting immunity.
B lymphocytesare formed in the red bone marrow. Final differentiation occurs in the lymph nodes of the mucous membrane, mainly of the digestive canal. They provide humoral immunity. When an antigen arrives, B lymphocytes are transformed into plasma cells, which produce antibodies (immunoglobulins) and the latter neutralize the antigens. Among B lymphocytes there are also Memory B lymphocytes. B lymphocytes are relatively short-lived cells.
Memory T lymphocytes and memory B lymphocytes are recycling cells. From tissues they enter lymph, from lymph into blood, from blood into tissue, then back into lymph, and so on throughout their lives. When they encounter the antigen again, they undergo blast transformation, that is, they turn into lymphoblasts, which proliferate and this leads to the rapid formation of effector lymphocytes, the action of which is directed towards a specific antigen.
Null lymphocytes - these are lymphocytes that have the properties of neither T-lymphocytes nor B-lymphocytes. It is believed that blood stem cells and natural killer cells circulate among them.
Monocytes - these are the largest cells, diameter 18-20 microns. They have a large bean-shaped, strongly basophilic nucleus and a wide, weakly basophilic cytoplasm. Organelles are moderately developed, of which lysosomes are the best developed. Monocytes are produced in red bone marrow. Up to several days they circulate in the blood and in tissues and organs they turn into macrophages, which have a special name in each organ.
The structure of arterioles
Topic: Microcirculatory bed: arterioles, capillaries, venules and arteriolo-venular anastomoses. Features of the structure of the walls of blood vessels. Types of capillaries, structure, localization. Heart. Sources of development. The structure of the membranes of the heart. Age characteristics.
The vessels of the microvasculature include: arterioles, capillaries, venules and arteriolo-venular anastomoses.
The functions of the vessels of the microvasculature are:
1. Exchange of substances and gases between blood and tissues.
2. Regulation of blood flow.
3. Blood deposition.
4. Drainage of tissue fluid.
The microcirculatory bed begins with arterioles, into which arteries become as the lumen diameter and wall thickness decrease.
Arterioles– these are small vessels with a diameter of 100 to 50 microns. They are similar in structure to muscular arteries.
The arteriole wall consists of three membranes:
1. The inner lining is represented by endothelium located on the basement membrane. Underneath it are single cells of the subendothelial layer and a thin internal elastic membrane that has holes (perforations) through which endothelial cells contact the smooth myocytes of the middle layer to transmit signals from endothelial cells about changes in the concentration of biologically active substances that regulate arteriolar tone.
2. The middle membrane is represented by 1 – 2 layers of smooth myocytes.
3. The outer shell is thin and merges with the surrounding connective tissue.
The smallest arterioles with a diameter of less than 50 microns are called precapillary arterioles or precapillaries. Their wall consists of endothelium lying on the basement membrane, individual smooth myocytes and outer adventitial cells.
At the site where precapillaries branch into capillaries, there are sphincters, which are several layers of smooth myocytes that regulate blood flow into the capillaries.
Functions of arterioles:
· Regulation of blood flow in organs and tissues.
· Regulation of blood pressure.
Capillaries- these are the thinnest-walled vessels of the microcirculatory bed, through which blood is transported from the arterial bed to the venous bed.
The capillary wall consists of three layers of cells:
1. The endothelial layer consists of polygonal cells of various sizes. There are villi on the luminal (facing the lumen of the vessel) surface, covered with glycocalyx, which adsorbs and absorbs metabolic products and metabolites from the blood.
Endothelial functions:
Atrombogenic (synthesize prostaglandins that prevent platelet aggregation).
Participation in the formation of the basement membrane.
Barrier (it is carried out by the cytoskeleton and receptors).
Participation in the regulation of vascular tone.
Vascular (synthesize factors that accelerate the proliferation and migration of endothelial cells).
Synthesis of lipoprotein lipase.
1. A layer of pericytes (process-shaped cells containing contractile filaments and regulating the lumen of capillaries), which are located in the splits of the basement membrane.
2. A layer of adventitial cells embedded in an amorphous matrix, in which thin collagen and elastic fibers pass.
Classification of capillaries
1. By lumen diameter
Narrow ones (4-7 microns) are found in transversely striated muscles, lungs, and nerves.
Wide (8-12 microns) are found in the skin and mucous membranes.
Sinusoidal (up to 30 microns) are found in the hematopoietic organs, endocrine glands, and liver.
Lacunae (more than 30 microns) are located in the columnar zone of the rectum and cavernous bodies of the penis.
2. According to the structure of the wall
Somatic, characterized by the absence of fenestrae (local thinning of the endothelium) and holes in the basement membrane (perforations). Located in the brain, skin, muscles.
Fenestrated (visceral type), characterized by the presence of fenestrae and the absence of perforations. They are located where molecular transfer processes occur especially intensively: glomeruli of the kidneys, intestinal villi, endocrine glands).
Perforated, characterized by the presence of fenestrae in the endothelium and perforations in the basement membrane. This structure facilitates the passage through the wall of the capillary cells: sinusoidal capillaries of the liver and hematopoietic organs.
Capillary function– the exchange of substances and gases between the lumen of the capillaries and surrounding tissues is carried out due to the following factors:
1. Thin wall of capillaries.
2. Slow blood flow.
3. Large area of contact with surrounding tissues.
4. Low intracapillary pressure.
The number of capillaries per unit volume varies in different tissues, but in each tissue there are 50% non-functioning capillaries that are in a collapsed state and only blood plasma passes through them. When the load on the organ increases, they begin to function.
There is a capillary network that is enclosed between two vessels of the same name (between two arterioles in the kidneys or between two venules in the portal system of the pituitary gland); such capillaries are called the “miraculous network.”
When several capillaries merge, they form postcapillary venules or postcapillaries, with a diameter of 12 -13 microns, in the wall of which there is fenestrated endothelium, more pericytes. When postcapillaries merge, they form collecting venules, in the middle membrane of which smooth myocytes appear, the adventitial membrane is better expressed. Collecting venules continue into muscle venules, the middle shell of which contains 1-2 layers of smooth myocytes.
Function of venules:
· Drainage (receipt of metabolic products from the connective tissue into the lumen of the venules).
· Blood cells migrate from the venules into the surrounding tissue.
The microvasculature consists of arteriolo-venular anastomoses (AVA)- these are vessels through which blood from arterioles enters venules bypassing capillaries. Their length is up to 4 mm, diameter more than 30 microns. AVAs open and close 4 – 12 times per minute.
ABAs are classified into true (shunts), through which arterial blood flows, and atypical (half shunts) through which mixed blood is discharged, because When moving along the half-shunt, a partial exchange of substances and gases occurs with the surrounding tissues.
Functions of true anastomoses:
· Regulation of blood flow in capillaries.
· Arterialization of venous blood.
· Increased intravenular pressure.
Functions of atypical anastomoses:
· Drainage.
· Partially exchangeable.
