Microcirculation system. What is microcirculatory circulation

Microcirculation(Greek mikros small + lat. circulatio circulation) - transport of biological fluids at the level of body tissues: movement of blood through capillary-type microvessels (capillary circulation), movement of interstitial fluid and substances through intercellular spaces and transport of lymph through lymphatic microvessels. The term was introduced by American researchers in 1954 . in order to integrate methodological approaches and information that related primarily to capillary blood flow (see. Circulation). The development of this direction led to ideas about microcirculation as a complex system that integrates the activities of three subsystems (compartments, or compartments): hemomicrocirculatory, lymphocirculatory and interstitial. The main task of the system microcirculation in the body is to maintain a dynamic balance of volume and mass parameters of fluid and substances in tissues - ensuring homeostasis of the internal environment. System microcirculation carries out the transport of blood and lymph through microvessels, the transfer of gases (see. Gas exchange), water, micro- and macromolecules through biological barriers (capillary walls) and the movement of substances in the extravascular space.

The central link of the system is the blood and lymphatic capillaries, the thinnest-walled vessels with a diameter of 3-5 to 30-40 µm (rice. 12 ). being an essential component of biological barriers. The walls of blood capillaries, formed mainly from specialized endothelial cells ( rice. 3 ), allow selective supply of working tissue elements with oxygen and ions. biologically active molecules, plasma proteins and other substances circulating in the blood. Lymphatic capillaries (see. Lymphatic system), the walls of which are also formed by endothelium, evacuate excess fluid, protein molecules and cell metabolism products from the tissues. The state of capillary circulation is determined by resistive microvessels - arterioles and precapillaries containing smooth muscle cells. The latter provide changes in the size of the working lumen of the vessels and, consequently, the volume of blood entering the capillaries. From capillaries, blood collects into capacitive vessels - postcapillaries and venules, which are also involved in the processes of transport of substances. Pathways of extracapillary blood flow (anastomoses, shunts) are involved in the blood supply to the capillaries. Transport of substances through the endothelial lining of blood and lymphatic vessels of the capillary type (vascular permeability) is carried out through intercellular contacts, open and diaphragmatic fenestrae and pores, as well as a system of plasmalemmal vesicles, or invaginations ( rice. 4 ). The numerous structures formed by the cell membrane (see. Biological membranes), serves hallmark endothelial cells. The main driving force that delivers blood to tissues and ensures the movement of interstitial fluid and lymph is the propulsive activity of the heart.

From a functional point of view, all transport processes in the system microcirculation interconnected and interdependent. This relationship is achieved due to gradients of forces (pressures) and concentrations at the level of the endothelial barriers separating the compartments and in each of them. Blood how a complex heterogeneous system of corpuscular nature has rheological properties, significantly differentiating it from other liquids. On hemodynamic conditions in the system microcirculation Not only the structural mechanisms of the microvasculature influence, but also the aggregative state of the blood, the interaction between the formed elements and the circulating plasma. Hemodynamic parameters in microvessels are closely related to the permeability of their walls, and the latter reflects force gradients and protein concentrations in the interstitium. In turn, the conditions existing in the interstitial environment lymphatic capillaries, form the mechanisms of lymph formation and promotion of lymph. Microcirculation as the main system that integrates the vital functions of tissues, it is regulated primarily by local control mechanisms - mediator, myogenic. Nervous and humoral influences are realized at the level of the smooth muscle apparatus of resistive microvessels and in the contraction of endothelial cells. In the activity of the system microcirculation The principle of self-regulation is very effectively manifested, according to which changes in functional parameters in each of the three compartments and at the boundaries between them significantly affect transport phenomena in neighboring compartments. The self-regulatory mechanism provides, in particular, protection of tissues from excess fluid intake and accumulation. The insufficiency of any link of this mechanism and the impossibility of its compensation leads to tissue edema - one of the most common syndromes in many pathological conditions.

Main parameters characterizing the functioning of the system microcirculation, are determined by hemodynamic conditions at the level of capillaries, the permeability of their walls, and the forces ensuring the movement of interstitial fluid and lymph. The speed of blood flow in capillaries usually does not exceed 1 mm/s, and red blood cells move slightly faster than plasma. Hydrostatic pressure in capillary-type vessels in different organs is recorded in the range of 18-40 mmHg st. As a rule, it slightly exceeds the colloid osmotic pressure of plasma proteins (19-21 mmHg st.), due to which the pressure gradient through the capillary walls is directed towards the tissue and fluid filtration dominates its reabsorption into plasma. The excess volume of fluid entering the tissue is reabsorbed by the roots lymphatic system or used for the formation of secretions, for example in the digestive glands. Hydraulic conductivity of the walls of blood microvessels, i.e. permeability to water varies depending on their nature (arterial or venous capillaries, venules) and organ affiliation. In capillaries with continuous endothelium (muscles, skin, heart, central nervous system) it varies within the range (1-130)× 10 -3 µm/s× mm Hg. st. The conductivity value of fenestrated endothelium (kidneys, intestinal mucosa, glands) is usually 2-3 orders of magnitude higher. Another important parameter characterizing the ability of the capillary wall to pass substances soluble in water, the osmotic reflection coefficient, is a dimensionless value and does not exceed 1. Its values ​​are especially important for assessing the permeability of the endothelium in relation to blood plasma proteins. In the capillary wall, the reflection coefficient of proteins such as albumin is 0.7-0.9. This means that the permeability of the capillary endothelium to macromolecules is low; for ions and small molecules, reflectance values ​​are close to 0.1. Another parameter is the permeability coefficient for K + ions, Na + has a value of the order of 10 -5 cm/s. For molecules of average mass (sugars, amino acids) it is somewhat less.

The value of hydrostatic pressure of the interstitial fluid (in the intercellular space) is usually estimated as close to zero, i.e. little different from the value of atmospheric pressure. Some measurement methods record values ​​less than atmospheric pressure: -6 -8 mmHg st. Although the permeability of capillary walls for proteins is limited, their content in tissues accounts for 30-40% of the total mass of protein circulating in the body. Colloid osmotic pressure in interstitial fluid reaches 10 mmHg Art. Low hydrostatic pressure and high colloid-osmotic pressure in the interstitial space contribute to the filtration of fluid into the tissue and the entry of substances dissolved in the blood plasma. Pressure gradients in the interstitium cause the movement of solutions in it and thereby the delivery necessary products to working cells. Plasma proteins, which also enter the intercellular environment, are evacuated mainly by lymphatic capillaries. The pressure in their lumen, apparently, differs little from atmospheric pressure, i.e., in relation to blood pressure, it is close to zero. As the lymph moves through the vessels, it increases slightly and at the exit from the system microcirculation can reach 14-16 mmHg Art. Although the mechanisms of lymph movement in microvessels are not yet clear enough, it has been shown that contractions of large lymphatic vessels (lymphangions), which have a developed muscular layer, play an important role.

Along with ensuring metabolic processes between plasma (lymph) and working tissue elements, the system microcirculation Performs other functions vital for the normal functioning of the body. The total mass of endothelial cells in the adult human body reaches 1.5-2 kg, and the size of the cell surface is generally extraordinary and, apparently, is close to 1000 m 2. A number of important biochemical reactions take place on this extensive surface, for example, the conversion of the inactive form of angiotensin I into the active form, angiotensin II. The converting enzyme is synthesized by endothelial cells (especially in the microvessels of the lungs) and then exposed on their surface. With the help of the capillary endothelium, biogenic amines - norepinephrine, serotonin - are deactivated; Almost all heparin and other biologically active molecules circulating in the plasma are sorbed on the endothelium. The role of the endothelium in the synthesis of prostaglandins, especially PGI 2 (prostacyclin), which maintains thromboresistance of the endothelial surface, is extremely important. In this way, as well as thanks to the synthesis by the endothelium of a number of factors of hemostasis and fibrinolysis, a close functional connection is achieved between microcirculation and the blood coagulation system (see. Blood coagulation system). Endothelial cells also synthesize a large class of connective tissue molecules - glycosaminoglycans, collagens, fibronectin, laminin, etc. A wide range cell receptors on the endothelial surface provides selective adsorption of substances and regulation of specific reactions of endothelial cells.

Local or generalized disorders microcirculation occur in almost all diseases. In accordance with functional properties systems microcirculation These disorders are manifested by a complex of different syndromes. Yes, when shocked of different etiologies, the leading pathogenetic significance is acquired by the phenomena of tissue hypoperfusion, i.e. insufficiency of capillary circulation, and aggregation of erythrocytes - the formation of their conglomerates of different sizes and densities. Violations of the permeability of microvascular walls for fluid and protein, as well as leukocyte infiltration in the focus of acute inflammation, are the result of a specific response microcirculation under conditions of a complex balance of mediators: histamine, serotonin, complement system, arachidonic acid derivatives, reactive oxygen species and others (see. Inflammation). Persistent contraction of resistive microvessels - arterioles, and structural transformations of their walls serve as an effector mechanism for the development of hypertension syndrome. At the level microcirculation and with its direct participation such severe conditions as disseminated intravascular coagulation syndrome develop (see. Thrombohemorrhagic syndrome). During development pathological conditions syndromes of microcirculatory disorders are often combined into various combinations and appear with different intensities.

Study methods microcirculation include, in addition to traditional histological examination, study using an electron microscope, as well as intravital microscopic diagnosis of blood flow disorders (study of capillaries nail fold, conjunctiva, gums, mucous membranes). In ophthalmology, microscopy of the fundus vessels is widely used, which allows, when luminescent indicators are introduced into the blood, to evaluate not only appearance, but also vascular permeability. For this purpose, the subcutaneous Lendis test is also used - determining the permeability of capillaries by the amount of filtration of liquid and protein from capillary blood under conditions of increased hydrostatic pressure. Status indicator water balance in tissues the value of interstitial pressure can serve. For a summary assessment of tissue blood flow, extraction from blood and clearance of various substances, radionuclide methods are increasingly used. Viscometers are being introduced into clinical practice to study the aggregative state of blood at different shear rates. In medical and biological experimental research, methodological possibilities for studying microcirculation more extensive and informative. Almost all the most important parameters reflecting the functions of the system microcirculation, are available for quantitative analysis.

Bibliography: Johnson P. Peripheral Circulation, trans. from English, M., 1982; Kupriyanov V.V. Microcirculation system and microcirculatory bed, Arch. anat., histol. and embryol., t. 62, no. 3, p. 14, 1972; Kupriyanov V.V. and others. Microlymphology, M., 1953, bibliogr.; Levtov V.A., Regirer A. and Shadrina N.X. Blood rheology, M., 1982, bibliogr.; Orlov R.S., Borisov A.V. and Borisova R.P. Lymphatic vessels, L., 1983; Guide to Physiology. Physiology of blood circulation. Physiology of the vascular system, ed. P.G. Kostyuk, s. 5, 307, L., 1984. Vascular endothelium, ed. V.V. Kupriyanova et al., p. 44, Kyiv, 1986; Chernukh A. M., Alexandrov P. N. and Alekseev O.V. Microcirculation, M., 1975, bibliogr.

All systems, organs and tissues of the body function by receiving ATP energy, which, in turn, can be formed in sufficient quantities in the presence of oxygen. How does oxygen get into organs and tissues? It is transported by hemoglobin through blood vessels, which form a microcirculation or microhemodynamic system in organs.

Levels of the circulatory system

Conventionally, all blood supply to organs and systems of the body can be divided into three levels:

Microcirculation: what is it?

Microcirculation is the movement of blood through the microscopic, that is, the smallest, part of the vascular bed. There are five types of vessels that are part of it:

• arterioles;
• precapillaries;
• capillaries;
• postcapillaries;
• venules.

What’s interesting is that not all vessels in this channel function simultaneously. While some of them are actively working (open capillaries), others are in “sleep mode” (closed capillaries).

Regulation of blood movement through the smallest blood vessels is carried out by contraction of the muscular wall of arteries and arterioles, as well as by the work of special sphincters, which are located in the post-capillaries.

Structural features

The microvasculature has a different structure, depending on the organ in which it is located.

For example, in the kidneys, the capillaries are collected into a glomerulus, which is formed from the afferent artery, and from the glomerulus of capillaries itself the efferent artery is formed. Moreover, the diameter of the incoming one is twice as large as the outgoing one. This structure is necessary for the filtration of blood and the formation of primary urine.

And in the liver there are wide capillaries called sinusoids. Both the oxygen-rich arterial vein and the oxygen-poor arterial vein enter these vessels from the portal vein. deoxygenated blood. Special sinusoids are also present in the bone marrow.

Functions of microcirculation

Microcirculation is very an important part vascular bed, performing the following functions:

• exchange - exchange of oxygen and carbon dioxide between blood and cells of internal organs;
• heat exchange;
• draining;
• signal;
• regulatory;
• participation in the formation of the color and consistency of urine.

Pathological conditions

Blood flow in the microvasculature depends on the constancy of the internal environment of the body. Including on normal vascular function greatest influence affects the work of the heart and endocrine glands. However, other internal organs also have an influence. Therefore, the state of microcirculation reflects the functioning of the body as a whole.

Conventionally, all pathological conditions of microvasculature vessels can be divided into three groups:

Intravascular changes

Slowing of blood flow in the vessels, which can manifest itself as specific diseases, thrombocytopathies (impaired platelet function) and coagulopathies (blood clotting disorders), as well as pathologies that can occur in various diseases of the body. These conditions include red blood cell aggregation and sludge syndrome. In fact, these two processes are successive stages of one phenomenon.

First, temporary attachment of red blood cells occurs using surface contacts in the form of a column (erythrocyte aggregation). This condition is reversible and usually short-term. However, its progression can lead to strong gluing (adhesion) of blood cells, which is already irreversible.

This pathology is called the sludge phenomenon. This leads to a slowdown and complete cessation of blood flow in the vessel. Venules and capillaries are usually clogged. The exchange of oxygen and nutrients stops, which further causes ischemia and tissue necrosis.

Destruction of the vascular wall

Violation of the integrity of the vessel wall can occur both in pathological conditions of the whole organism (acidosis, hypoxia), and in direct damage to the vessel wall by biologically active agents. They act as such agents in vasculitis (inflammation vascular wall).

If the damage progresses, there is leakage (diapedesis) of red blood cells from the blood into surrounding tissues and the formation of hemorrhages.

Extravascular disorders

Pathological processes in the body can affect microcirculation vessels in two ways:

• The reaction of tissue basophils, which release into the environment biologically active agents and enzymes that directly affect the vessel and thicken the blood in the vessels.
• Impaired transport of tissue fluid.

Thus, microcirculation is a complex system that is in constant interaction with the entire body. It is necessary to know not only the main types of its disorders, but also methods for diagnosing and treating these diseases.

Microhemodynamic disorders: diagnosis

Depending on the organ affected, different methods can be used instrumental diagnostics, which can indirectly indicate the presence of microcirculation disorders through pathology internal organ:

Microhemodynamic disorders: treatment

To improve microcirculation, a group of drugs called angioprotectors is used. These are highly effective medicines, improving blood flow through the vessels and restoring the vessel itself. Their main properties are:

• reducing arterial spasm;
• ensuring vessel patency;
• improvement of blood rheology (viscosity);
• strengthening the vascular wall;
• anti-edematous effect;
• improving metabolism, that is, metabolism, in the vascular wall.

The main drugs that improve microcirculation include the following:

It can be concluded that, despite their small size and diameter, microhemodynamic vessels perform very important function in organism. Therefore, microcirculation is a self-sufficient system of the body, the condition of which can and should be given special attention.

I Microcirculation (Greek mikros small + Latin circulatio rotation)

transport of biological fluids at the level of body tissues: movement of blood through capillary-type microvessels (capillary circulation), movement of interstitial fluid and substances through intercellular spaces and transport of lymph through lymphatic microvessels. The term was introduced by American researchers in 1954 . in order to integrate methodological approaches and information that related primarily to capillary blood flow (see Blood circulation) . The development of this direction has led to the idea of ​​\u200b\u200bM. as a complex system that integrates the activity of three subsystems (compartments, or compartments): hemomicrocirculatory, lymphocirculatory, and interstitial. The main task of the M. system in the body is to maintain a dynamic balance of volume and mass parameters of fluid and substances in tissues - ensuring homeostasis of the internal environment. The M. system transports blood and lymph through microvessels and transports gases (see Gas exchange) , water, micro- and macromolecules through biological barriers (capillary walls) and the movement of substances in the extravascular space.

The central link of the system is the blood and lymphatic capillaries, the thinnest-walled vessels with a diameter of 3-5 to 30-40 µm (rice. 12 ). being an essential component of biological barriers. The walls of blood capillaries, formed mainly from specialized endothelial cells ( rice. 3 ), allow selective supply of working tissue elements with oxygen and ions. biologically active molecules, plasma proteins and other substances circulating in the blood. Lymphatic capillaries (see Lymphatic system) , the walls of which are also formed by endothelium, evacuate excess fluid, protein molecules and cell metabolism products from the tissues. The state of capillary circulation is determined by resistive microvessels - arterioles and precapillaries containing smooth muscle cells. The latter provide changes in the size of the working lumen of the vessels and, consequently, the volume of blood entering the capillaries. From capillaries, blood collects into capacitive vessels - postcapillaries and venules, which are also involved in the processes of transport of substances. Pathways of extracapillary blood flow (anastomoses, shunts) are involved in the blood supply to the capillaries. Transport of substances through the endothelial lining of blood and lymphatic vessels of the capillary type (vascular permeability) is carried out through intercellular contacts, open and diaphragmatic fenestrae and pores, as well as a system of plasmalemmal vesicles, or invaginations ( rice. 4 ). Numerous structures formed by the cell membrane (see Biological membranes) , serves as a hallmark of endothelial cells. The main driving force that delivers blood to tissues and ensures the movement of interstitial fluid and lymph is the propulsive activity of the heart.

From a functional point of view, all transport processes in the transport system are interconnected and interdependent. This relationship is achieved due to gradients of forces (pressures) and concentrations at the level of the endothelial barriers separating the compartments and in each of them. Blood, as a complex heterogeneous system of corpuscular nature, has rheological properties that significantly distinguish it from other liquids. Hemodynamic conditions in the M. system are influenced not only by the structural mechanisms of the microvasculature, but also by the aggregate state of the blood and the interaction between the formed elements and circulating plasma. Hemodynamic parameters in microvessels are closely related to the permeability of their walls, and the latter reflects force gradients and protein concentrations in the interstitium. In turn, the conditions existing in the interstitial environment of the lymphatic capillaries form the mechanisms of lymph formation and movement of lymph. M., as the main system that integrates the vital functions of tissues, is regulated primarily by local control mechanisms - mediator and myogenic. Nervous and humoral influences are realized at the level of the smooth muscle apparatus of resistive microvessels and in the contraction of endothelial cells. The activity of the M system very effectively manifests the principle of self-regulation, according to which changes in functional parameters in each of the three compartments and at the boundaries between them significantly affect transport phenomena in neighboring compartments. The self-regulatory mechanism provides, in particular, protection of tissues from excess fluid intake and accumulation. The insufficiency of any link of this mechanism and the impossibility of its compensation leads to tissue edema - one of the most common syndromes in many pathological conditions.

The main parameters characterizing the functioning of the M. system are determined by hemodynamic conditions at the level of capillaries, the permeability of their walls, and the forces that ensure the movement of interstitial fluid and lymph. The speed of blood flow in capillaries usually does not exceed 1 mm/s, and red blood cells move slightly faster than plasma. Hydrostatic pressure in capillary-type vessels in different organs is recorded in the range of 18-40 mmHg st. As a rule, it slightly exceeds the colloid osmotic pressure of plasma proteins (19-21 mmHg st.), due to which the pressure gradient through the capillary walls is directed towards the tissue and fluid filtration dominates its reabsorption into plasma. The excess volume of fluid entering the tissue is reabsorbed by the roots of the lymphatic system or used to form secretions, for example in the digestive glands. Hydraulic conductivity of the walls of blood microvessels, i.e. permeability to water varies depending on their nature (arterial or venous capillaries, venules) and organ affiliation. In capillaries with continuous endothelium (muscles, skin, heart, central nervous system) it varies within the range (1-130)․10 -3 µm/s․mmHg st. The conductivity value of fenestrated endothelium (kidneys, intestinal mucosa, glands) is usually 2-3 orders of magnitude higher. Another important parameter characterizing the ability of the capillary wall to pass substances soluble in water, the osmotic reflection coefficient, is a dimensionless value and does not exceed 1. Its values ​​are especially important for assessing the permeability of the endothelium in relation to blood plasma proteins. In the capillary wall, the reflection coefficient of proteins such as albumin is 0.7-0.9. This means that the permeability of the capillary endothelium to macromolecules is low; for ions and small molecules, reflectance values ​​are close to 0.1. Another parameter is the permeability coefficient for K + ions, Na + has a value of the order of 10 -5 cm/s. For molecules of average mass (sugars, amino acids) it is somewhat less.

The value of hydrostatic pressure of the interstitial fluid (in the intercellular space) is usually estimated as close to zero, i.e. little different from the value of atmospheric pressure. Some measurement methods record values ​​less than atmospheric pressure: -6 -8 mmHg st. Although the permeability of capillary walls for proteins is limited, their content in tissues accounts for 30-40% of the total mass of protein circulating in the body. Colloid osmotic pressure in interstitial fluid reaches 10 mmHg Art. Low hydrostatic pressure and high colloid-osmotic pressure in the interstitial space contribute to the filtration of fluid into the tissue and the entry of substances dissolved in the blood plasma. Pressure gradients in the interstitium cause the movement of solutions in it and thereby the delivery of necessary products to the working cells. Plasma proteins, which also enter the intercellular environment, are evacuated mainly by lymphatic capillaries. The pressure in their lumen, apparently, differs little from atmospheric pressure, i.e., in relation to blood pressure, it is close to zero. As the lymph moves through the vessels, it increases slightly and at the exit from the system M. can reach 14-16 mmHg Art. Although the mechanisms of lymph movement in microvessels are not yet clear enough, it has been shown that contractions of large lymphatic vessels (lymphangions), which have a developed muscular layer, play an important role.

Along with ensuring metabolic processes between plasma (lymph) and the working elements of tissue, the metabolic system also performs other functions that are vital for the normal functioning of the body. The total mass of endothelial cells in the adult human body reaches 1.5-2 kg, and the size of the cell surface is generally extraordinary and, apparently, is close to 1000 m 2. A number of important biochemical reactions take place on this extensive surface, for example, the conversion of the inactive form of angiotensin I into the active form, angiotensin II. The converting enzyme is synthesized by endothelial cells (especially in the microvessels of the lungs) and then exposed on their surface. With the help of the capillary endothelium, biogenic amines - norepinephrine, serotonin - are deactivated; Almost all heparin and other biologically active molecules circulating in the plasma are sorbed on the endothelium. The role of the endothelium in the synthesis of prostaglandins, especially PGI 2 (prostacyclin), which maintains thromboresistance of the endothelial surface, is extremely important. In this way, as well as thanks to the synthesis by the endothelium of a number of factors of hemostasis and fibrinolysis, a close functional connection is achieved between M. and the blood coagulation system (see Blood coagulation system (Blood coagulation system)) . Endothelial cells also synthesize a large class of connective tissue molecules - glycosaminoglycans, collagens, fibronectin, laminin, etc. A wide range of cellular receptors on the endothelial surface ensures selective adsorption of substances and regulation of specific reactions of endothelial cells.

Local or generalized M. disorders occur in almost all diseases. In accordance with the functional properties of the M. system, these disorders are manifested by a complex of various syndromes. Thus, with Shock of various etiologies, the phenomena of tissue hypoperfusion acquire leading pathogenetic significance, i.e. insufficiency of capillary circulation, and aggregation of erythrocytes - the formation of their conglomerates of different sizes and densities. Impairments in the permeability of microvascular walls for fluid and protein, as well as leukocyte infiltration in the focus of acute inflammation, are the result of a specific reaction of M. in conditions of a complex balance of mediators: histamine, serotonin, the complement system, arachidonic acid derivatives, reactive oxygen species and others (see Inflammation ) . Persistent contraction of resistive microvessels - arterioles, and structural transformations of their walls serve as an effector mechanism for the development of hypertension syndrome. At the level of M. and with its direct participation, such severe conditions as disseminated intravascular coagulation syndrome develop (see Thrombohemorrhagic syndrome) . With the development of pathological conditions, syndromes of microcirculatory disorders are often combined in various combinations and manifest themselves with different intensities.

Methods for studying M. include, in addition to traditional histological examination, study using an electron microscope, as well as intravital microscopic diagnosis of blood flow disorders (study of capillaries of the nail fold, conjunctiva, gums, mucous membranes). In ophthalmology, microscopy of the fundus vessels is widely used, which allows, when luminescent indicators are introduced into the blood, to evaluate not only the appearance, but also the permeability of blood vessels. For this purpose, the subcutaneous Lendis test is also used - determining the permeability of capillaries by the amount of filtration of liquid and protein from capillary blood under conditions of increased hydrostatic pressure. An indicator of the state of water balance in tissues can be the value of interstitial pressure. For a summary assessment of tissue blood flow, extraction from blood and clearance of various substances, radionuclide methods are increasingly used. Viscometers are being introduced into clinical practice to study the aggregative state of blood at different shear rates. In medical and biological experimental research, the methodological possibilities for studying M. are more extensive and informative. Almost all the most important parameters reflecting the functions of the metabolic system are available for quantitative analysis.

Bibliography: Johnson P. Peripheral Circulation, trans. from English, M., 1982; Kupriyanov V.V. Microcirculation system and microcirculatory bed, Arch. anat., histol. and embryol., t. 62, no. 3, p. 14, 1972; Kupriyanov V.V. and others. Microlymphology, M., 1953, bibliogr.; Levtov V.A., Regirer A. and Shadrina N.X. Blood rheology, M., 1982, bibliogr.; Orlov R.S., Borisov A.V. and Borisova R.P. Lymphatic vessels, L., 1983; Guide to Physiology. Physiology of blood circulation. Physiology vascular system, ed. P.G. Kostyuk, s. 5, 307, L., 1984. Vascular endothelium, ed. V.V. Kupriyanova et al., p. 44, Kyiv, 1986; Chernukh A.M., Alexandrov P.N. and Alekseev O.V. Microcirculation, M., 1975, bibliogr.

Microcirculation is the most important physiological basis of metabolism in human body. Enriching the blood with oxygen from the lungs and regularly supplying nutrients through the intestines is meaningless if all these molecules do not reach the organs and tissues. It is through the smallest vessels oxygen and nutrients are exchanged in the body.

A little physiology

The microvasculature is an amazing network of small arterioles, venules and capillaries that distribute blood throughout the body. To better understand the physiological basis of blood circulation, it is necessary to consider the entire system as a whole. Blood circulation includes the following important links:

1. The heart is a biological pump, under the influence of which blood moves through the vessels and is distributed throughout the body. Passing through the lungs, the blood is enriched with oxygen and releases carbon dioxide into the exhaled air.
2. Arteries are muscular vessels through which, under the influence of the heart, blood enriched with oxygen and nutrients moves throughout the body.
3. Veins are elastic vessels that collect blood from organs and ensure its flow back to the heart.
4. Between the arterial and venous there is a microcirculatory bed. It consists of tiny capillaries, through the wall of which metabolism occurs, each cell receives oxygen and nutrients. At the same time, elimination of metabolic products and carbon dioxide occurs.

Regulation of capillary blood flow is a complex physiological process. Not all the smallest vessels are equally filled with blood at the same time. The body redistributes the volume of blood flow depending on its needs.

Microcirculation

When you eat, the brain and autonomic nervous system stimulate blood flow to the gastrointestinal tract. In severe illnesses and shock conditions, the so-called centralization of blood flow occurs. All the body’s forces are directed to maintaining microcirculation in vital important organs: brain, heart. The blood flow of other organs is at basic level necessary to maintain life.

Problems with microcirculation

Disruption of the capillary bed underlies most pathological processes. At the microscopic level, there is a spasm of arterioles or their blockage with microthrombi from shaped elements blood. This leads to a lack of oxygen and a transition of cells to the anaerobic (without the participation of oxygen) process of glucose breakdown.

As a result, the body accumulates sour foods metabolism, in particular lactic acid or lactate, which greatly aggravates metabolic disorders.

Some diseases whose pathogenesis is based on microcirculatory disorders:

1. Diabetes. One of the main complications is microangiopathy, that is, pathology of the capillary bed. Poor glycemic control leads to thickening of capillary walls and impaired transport across membranes. The nutrition of tissues is disrupted, and lesions appear on the legs. The lesion affects almost all vessels, even the retinal arterioles in the eyes.
2. (IHD). Main cause of ischemic heart disease– deposition of cholesterol on the walls of blood vessels, the formation of atherosclerotic plaques. These factors disrupt normal peripheral blood flow, and the vessels become rigid. Not only the myocardium suffers, but also other organs. Trophic disturbances on lower limbs often caused obliterating atherosclerosis vessels.
3. Stroke or disorder cerebral circulation. Thrombosis or rupture cerebral vessel leads to ischemic or respectively. Damage nerve cells(neurons) occurs due to blockade of the smallest vascular bed.
4. Kidney diseases. Renal pathology associated with impaired elimination of fluid and nitrogen metabolism products. The gradual accumulation of urea also negatively affects vascular perfusion, disrupting normal tissue trophism.

Not all pathological processes whose pathogenesis is based on microcirculatory disorders are listed here. The presence of systemic atherosclerosis always aggravates the situation. For patients with a large number of cholesterol plaques and thickening of the vascular walls, for example, it is much more difficult.

Maintenance therapy

To assess the condition of the microvasculature, doctors use a special device - a blood microcirculation analyzer. Using skin sensors, he evaluates the filling of capillaries with blood, the tone of peripheral arterioles, and blood oxygen saturation (saturation).

Medicine today has a wide range of medicines, eliminating vascular spasm and improving microcirculation. Assign similar drugs various specialists can: when diabetes mellitus- an endocrinologist, in case of coronary artery disease - a therapist or cardiologist, in case of a stroke or transient ischemic attack - a neurologist, a surgeon will take care of it.

Here are some medications and their mechanism of action:

1. Antiplatelet agents (Aspirin, Clopidogrel) and anticoagulants (Warfarin, Heparin) prevent the aggregation of blood cells and impair organ circulation. Prescribed by the attending physician solely according to indications. It is unacceptable to take such medications on your own.
2. Angioprotectors have proven themselves well - drugs that strengthen the vascular and capillary walls and improve the transport of oxygen and nutrients through membranes. This group includes drugs such as Trental, Curantil.
3. Nootropic agents (Piracetam, Memotropil) optimize brain microcirculation and are used as maintenance therapy and for the prevention of strokes.
4. Vasodilators are medications that eliminate spasm of arterioles and improve perfusion (Vinpocetine, Cinnarizine).
5. Biogenic stimulants activate metabolism, energy metabolism between the capillary and the cell. Drugs in this group – Actovegin, Solcoseryl.

There are not only tablet forms. Surgeons often prescribe various ointments, increasing blood flow to the skin, which is the prevention of trophic perfusion disorders.

Correction of microcirculatory disorders should be carried out in conjunction with treatment of the underlying disease. In diabetes mellitus, it is necessary to maintain glycemia within normal limits; coronary artery disease involves lowering cholesterol levels and monitoring. Only under such conditions can stable remission of the disease be achieved.

MICROCIRCULATION SYSTEM

Academician Academy of Medical Sciences of the USSR V.V. K u p r i a n o v, Ph.D. V. V. Banin

On issues close to the topic covered, articles P1 Blood vessels, Lymphatic vessels, Microcirculation, Permeability, etc. have been published in the BME.

Currently, the microcirculation system is understood as a set of ways of moving fluids in the body at the microscopic level, methods of transporting ions, molecules, cells, as well as metabolic processes necessary for the life support of the body. This is an open, living system that has the property of self-organization, depending on homeostasis and influencing it. The microcirculation system plays important role in a living organism. The existence of living matter at all levels and in all forms of organization (cells, tissues, organs) is possible only if they are supplied with the necessary nutrients, plastic, regulatory substances and oxygen through the microcirculation system.

The primary basis of the microcirculation system in phylogenesis is prevascular microcirculation in lower invertebrates. With the release of the endothelium, a system of intravascular microcirculation arose, the edges were widely connected with tissue lacunae, and then became more and more isolated. Closed microcirculation exists in annelids. In fish, the circulatory and lymphatic systems are separated. Simultaneously with the isolation of intravascular microcirculation, extravascular microcirculation is also preserved; both systems communicate through submicroscopic openings in the walls of the capillaries.

In the human embryo early stages development, extravascular microcirculation is also observed, thanks to the cut, histotrophic nutrition is carried out. In a 21-day embryo, the heart begins to contract; by this time will be formed blood vessels and intravascular microcirculation develops. The endothelial lining, arising from mesenchymal cells, in the primary capillaries is not continuous and continuous. Lymphatic capillaries also appear based on tissue crevices. Extravascular microcirculation, which ensures the delivery of substances to cells and tissue drainage, is subsequently maintained in the form of interstitial transport.

The term “microcirculation” was first used in 1954 and was initially considered as a synonym for capillary circulation. However, researchers who joined forces in studying microcirculation gradually became clear that focusing only on the transport of blood through microvessels and through their walls does not allow them to cover the content of the problem as a whole. In the USSR, a point of view was formulated according to which microcirculation should be understood as all transport and metabolic processes at the microscopic level. This point of view was discussed at the VII All-Union Congress of Anatomists, Histologists and Embryologists (1966). Continuation of work in the chosen direction and a systematic approach to accumulated knowledge led to

Identification by V.V. Kupriyanov (1972) of the microcirculation system.

The intensification of research on the physiology and pathology of microcirculation in the USSR is associated with Ch. arr. with the activities of A. M. Chernukh, his students and employees. In their research, new techniques were used (television technology, intravital studies using fluorescent microscopy, etc.). The issues of membrane permeability, methods of regulating the transport of substances, in particular the role of the mast cell system in these processes, were illuminated in a new way. After the All-Union Conferences on Microcirculation (1972, 1977 and 1984), the use of data concerning microcirculation in practical medicine expanded.

Structure of the microcirculation system

Any living system that expresses a certain unity of an organic substrate presupposes the presence of subsystems, elements, their connections and interactions, i.e., the structure of the system. In the microcirculation system, a material basis was initially identified - a very sensitive and mobile mosaic of microcirculation pathways - the microcirculatory bed. It connects the arterial part of the bloodstream with the venous one, and therefore can be called hemomicrocirculatory. At the same time, it includes lymphatic pathways at the microscopic level. Pathways for intervascular transport of fluids connecting hemomicrocirculation vessels and lymphatic microvessels, and vascular-tissue communications are also components of the microvasculature. Thus, the composition of the microcirculatory bed includes all parts of the hemomicrocirculation (arterioles, precapillaries, true capillaries, postcapillaries, venules and atheriovenular anastomoses), micro lymphatic

pathways (lymphatic capillaries, postcapillaries, initial and collecting lymphatic vessels) and interstitial, along which tissue fluid moves. The microvasculature is the morphological basis of the microcirculation system, divided into three subsystems (compartments, compartments): circulatory, lymphatic and interstitial.

In contrast to classical angiology, which considers the blood capillary as the central object of study, the study of microcirculation based on the three-compartment model shifts the focus of researchers' attention to the analysis of the relationships and interactions between blood, interstitial fluid and lymph. Such an analysis is extremely important for understanding the main function of the microcirculation system - ensuring the vital activity of cells. Since the 50s. 20th century - the period of the origin and formation of the doctrine of microcirculation - successive stages of studying the patterns of organization of the microcirculatory bed and the performance of hemodynamic and transport functions by it are traced. The basis for a fruitful study of hemodynamics and related trans-

The fluid port through the walls of the capillaries was established by the work of the famous American pathologist Zweifach (B. W. Zweifach).

The system-structural approach has become the theoretical basis for understanding the microcirculation system as a universal life support system on the scale of the entire organism. The microcirculatory bed is currently considered as a kind of “organ” of circulatory and tissue homeostasis, responsible for the metabolic and fluid (water) balance in the body.

I. P. Pavlov saw progress in the physiology of blood circulation “in the systematic study of the relationships in which the individual components of a complex hemodynamic machine are located during its life activity.” Such research includes the study of blood microcirculation through the thinnest vessels, which are also parts of a complex “hemodynamic machine”.

But hemodynamics at the micro level is determined not only by the internal forces of blood circulation; it is naturally subordinated to the metabolic needs of tissues, the conditions of the environment surrounding the capillaries (interstitium), and even the level of lymph formation. Thus, only with a comprehensive coverage of all elements of the microcirculation system can the processes of macro- and microcirculation along vascular and extravascular pathways be explained. There is a need to jointly consider blood circulation, the formation and transport of lymph, the movement of fluid and substances through the walls of metabolic vessels and in the interstitium. Although each element of the microcirculatory system plays a certain, specific role in transport interactions, the final, cumulative result of the functioning of the entire microcirculatory bed of an organ is decisive, since the activity of the elements is subordinated to the general task of ensuring tissue homeostasis.

Micro circulatory

bloodstream

Ideas about the structure and functions of all departments circulatory system have changed radically over the past decades due to new approaches and the development of more advanced techniques. Dramatic changes have occurred in the study of the terminal section of the circulatory system. As a result of an in-depth study of the hemomicrocirculatory bed, the old understanding of the capillary network as a single structure on the path of the transition of arterial blood to venous blood has been abolished. Blood capillaries are not the only components of the microvasculature; there are also precapillaries and postcapillaries, arterioles and venules, as well as arteriovenular anastomoses. Latest methods Angiological studies made it possible to clearly and accurately differentiate all parts of the microcirculatory bloodstream, determine their histotopography, the nature of the diffusion of substances between vessels and working cells. It was found that in various organs In addition to the previously known features of arterial and venous architectonics, various variants of the form of organization of the microvasculature are widely represented. V.V. Kupriyanov proposed a classification, according to the cut, such forms are distinguished as network with varieties depending on the contours of the network cells (round, oval, rectangular, square, polygonal); arcade, or lace; looped with elongated loops of vessels in the form of clubs or hairpins; basket-shaped, etc. Of particular importance is the morphological analysis of indicators of the density of vessels per unit of area, the size of the distances between them, length and diameter. In particular, the distance from the capillary to the working cell (diffusion distance) ranges from several micrometers in intensively blood-supplied organs (for example, in the kidney) to 50 microns and more

She is in connective tissue structures. All these indicators are included in the formula for organ specificity of the microvasculature.

The components of the microvasculature of an organ interact according to the principles of integration, with each component (arteriole, venule, capillary, shunt) performing specific functions. As a result, any changes in microvessels lead to corresponding changes in other vessels, which affects the overall function of the vascular system of the organ. This is how the law of organizational integrity and functional synergism of all components of the microcirculatory bed manifests itself.

Using modern methods, the preservation of the optimal balance of microcirculation during fluctuations in homeostasis has been confirmed, and at the same time, deviations from this state in pathology have been clarified. Thus, with normal permeability of the walls of microcirculatory vessels, the volume of fluid leaving the blood and the volume of fluid returning to it are equal. The predominance of fluid filtration not only causes hydration of tissues and organs, but is also accompanied by a temporary decrease in the volume of blood circulating in the vessels. When the body is dehydrated, plasma deficiency is compensated by intense resorption of interstitial fluid.

Disorders of peripheral hemomicrocirculation are accompanied by fluctuations in blood flow in a given region or in the whole organ. More often, the amount of blood flowing through the microvasculature decreases (hypotransfusion). An increase in blood flow is observed with a decrease in peripheral resistance to blood flow and with hypertension. The permeability of their walls also depends on the degree of filling of the vessels.

The microvasculature is the main link in organ vascular plasticity. It follows from this that its functional state should be the focus of attention of the clinician-angiologist, since microcirculation disorders in most cases are the root cause of further vascular (and not only vascular) disorders.

The plasticity of the microvasculature as one of the adaptation mechanisms is based on three types of structural devices: the first type is devices that regulate the reservoir functions of blood vessels, capable of increasing the capacity of the vascular bed of an organ; the second type - devices necessary for the redistribution of blood and lymph, regulating the direction and speed of blood and lymph flow; the third type is devices used to change the permeability of the walls of blood vessels. Further development of the concept of plasticity and reactivity of the microvasculature in clinical practice is carried out using biomicroscopy of the blood vessels of the conjunctiva of the eyeball and the vessels of the nail bed.

The reserve for increasing the capacity of the vascular bed is mobilized under conditions of increasing functional loads. Only due to the extensibility of the walls of microvessels, the capacity of the organ blood pool can be doubled. Under the influence of accumulating blood, the vessels of the microcirculatory bed become tortuous, loops and glomeruli of capillaries are formed, vascular lacunae, venous lakes, and sinusoids appear. If it is necessary to increase blood supply to tissues, the number of capillaries per unit area increases, and hypertrophy of the muscular elements of the vascular wall develops. Both in the case of expansion of the lumens of blood vessels and new formation of capillaries, the cross-sectional area of ​​the total mass of the vessels of the organ changes. Structural devices that redistribute blood also ensure reliable vascularization of the organ as a whole.

Regulation of the permeability of vascular barriers is based on a variety of forms and ways of organizing pathways

Transport of fluid and substances dissolved in it through the endothelium. This is what formed the basis for the existing classifications of blood capillaries. The most common and convenient in practical terms is the division of capillaries into somatic, visceral and sinusoidal. Somatic capillaries have a solid, continuous and non-fenestrated endothelial lining. Endotheliocytes most often connect through tight junctions, although the latter can vary markedly in the degree of permeability to fluid and macromolecules. N. Si-mionescu et al. (1975), Ya. L. Karaganov et al. (1985) suggest that these differences are likely due to variations in the development of the network of contact fibrils consisting of intramembrane particles of plasma-molemma cells. The basement membrane of somatic capillaries is usually well defined; it is continuous and, splitting, surrounds pericytes - special connective tissue cells, which are part of the capillary wall. Somatic capillaries are typical of muscles, skin, heart, lungs, brain and spinal cord, as well as other organs and tissues.

The endothelial lining of the visceral (fenestrate) capillaries is also continuous, but in the peripheral zones of the cells its thickness is minimal. As a result, windows (fenestrae) are formed, connecting the lumen of the vessel with the pericapillary space. In the capillaries of the mucous membrane of the intestinal wall and pancreas, the fenestrae are covered with thin single-layer diaphragms, which are considered as derivatives of the extremely thin plasmalemma of the cell. In other tissues, for example, in the glomeruli of the kidney, fenestrae do not have such diaphragms and are numerous rounded pores covered from the interstitial surface of the cells by a well-developed thickened basement membrane.

The endothelium of sinusoidal type capillaries, characteristic of the liver, bone marrow, and spleen, is not continuous, discontinuous, with extensive pores (“defects”). The basement membrane of such capillaries is fenestrated, and their walls allow the free exchange of not only macromolecules, but also cellular forms.

A distinctive feature of the capillary endothelium, like vascular endothelium in general, there are numerous plasmalemmal (micropinocytotic) vesicles, sometimes constituting up to 30-40% of the cell volume. It is believed that these vesicles are the main route through which plasma proteins are transported into the interstitium. In recent years, reasonable doubts have emerged about the universal transport function of the vesicular apparatus of endothelial cells, although its significance cannot be completely denied. Merging with each other and with cell surfaces, vesicles are capable of forming continuous communications - transendothelial channels, which allow the transport of proteins into the interstitial space due to transfer by fluid current.

Differences in the organization of transendothelial transport pathways in capillaries different types, as well as in individual vascular segments (arterial and venous capillaries, postcapillaries, venules) correlate with the permeability of their walls to fluid and blood plasma proteins. Thus, the endothelium of visceral capillaries is characterized by 30-50 times greater hydraulic conductivity (filtration coefficient) than somatic endothelium. The permeability of the vascular wall for water and protein increases towards the venous sections of the microvasculature.

The wall of sinusoidal capillaries offers virtually no resistance to the transfer of any macromolecules circulating in the blood into the interstitium. That is why the protein content in the lymph flowing from the liver is almost no different from its content in plasma.

The conditions for blood delivery to the capillaries depend on the structure of the arterioles. Their diameter reaches 100 microns, while precapillaries have a diameter of about 16-25 microns. The wall of arterioles consists of three membranes, which are called the same as the membranes of muscular arteries, but in their structure they are more likely to resemble monocellular layers. So, outer shell characterized by a relative richness of fibrillar elements, between which fibroblasts are scattered, surrounded by the ground substance. In the middle shell of arterioles, myocytes, as a rule, lie in a dense layer. In the wall of the precapillary, several myocytes are localized at the site where the precapillary branches into capillaries. Noteworthy is the spiral twisting of the muscle cell around the lumen of the microvessel, which contributes to more efficient pumping of blood. At the same time, with spontaneous contractions of muscle cells in the walls of microvessels, peripheral resistance sharply increases precisely at the level of arterioles and thin arteries. Myoendothelial contacts in the arteriolar wall, identified by electron microscopy, are considered as ways of exchanging information and as a means of initiating myogenic reactions.

The issue of precapillary sphincters has not been completely resolved. There are two opinions: first, the sphincter should be called the accumulation of myocytes in the branching zone of the arterioles, since the muscleless area does not participate in the occlusion of the lumen; second, the entire precapillary arteriole, regardless of the distribution of muscle cells, is a precapillary sphincter. However, the difference of opinion is not of a fundamental nature, since in both cases the instructions of I. II remain in force. Pavlov about the presence of “taps” in peripheral vessels that regulate blood flow. Such “faucets” may well be precapillary sphincters, since, firstly, they have a narrow lumen, secondly, this lumen is covered by circularly located muscle cells, and thirdly, in places where these cells are concentrated there are multiple myoendothelial contacts.

Regulation of the permeability of the vascular wall is carried out at the submicroscopic level cellular structures: the size and number of vesicles and fenestrae increase, transendothelial channels are formed, the cytoskeleton of endothelial cells changes. Thanks to these adaptive transformations, the stability of exchange between blood and tissues is maintained. Thus, the structure of the microvasculature reflects the unity of hemodynamic constants and metabolic functions.

As a result of the fusion of capillaries, the first venular tubes are formed, called postcapillary venules, or postcapillaries. They actually stand closer to the capillaries, and to the collecting veins, which (if there are myocytes in their walls) are also called muscle venules. As a rule, the venule wall is thin, easily permeable not only to water with crystalloids dissolved in it, but also to macromolecules. Postcapillary venules differ little from capillaries in diameter (on average 8-15 µm), collecting venules are larger (diameter up to 80 µm). According to Rodin (J. A. G. Rhodin), the ratio of the diameter of the lumen of postcapillaries to the thickness of their wall is 10:1, and for collecting venules this figure is 50:1. Due to the increase in the diameter of venules, a larger number of endothelial cells should participate in the formation of their walls.

The endothelial lining of postcapillaries and venules is distinguished by certain features of the organization of transmural channels that serve to transport water and various substances. The permeability of intercellular contacts for plasma proteins such as albumin in venules is noticeably greater than in capillaries; transendothelial channels are more common. In certain organs, e.g. V

In lymph nodes, postcapillary venules are lined with high endothelium and serve as the main site of migration of immunocompetent cells. The walls of postcapillary venules of the peritoneum have 1.5-1.8 times greater hydraulic conductivity (permeability to water) than the walls of other metabolic blood microvessels.

Venules collect blood from the microvasculature and direct it to the venous collectors. As capacitive vessels, venules have drainage, reservoir and storage functions. Their share of participation in peripheral resistance to blood flow is 20% of the total vascular resistance. The remaining 80% falls on resistive vessels - arteries and arterioles. According to V.I. Kozlov (1975), up to 40% of the blood flowing through the vessels is concentrated in the venous section of the microcirculatory bed. The sum of capillary and venular capacity in the peripheral bloodstream reaches 85% of the capacity of the entire bloodstream. B.I. Tkachenko rightly believes that capacitive vessels play a very significant role in blood circulation and maintaining normal organ function.

Among the many structural mechanisms regulating microhemodynamics, arteriole-venular anastomoses occupy a special place. Their importance as shunt devices is currently beyond doubt. But the role of arteriovenular anastomoses is much more significant. We are talking about the existence of two ways of blood transport in the microcirculatory bed: the main (transcapillary) and the additional (juxtacapillary). Shunts allow some relief from capillary blood flow and prevent hemostasis. Under functional loads and under pathological conditions, arteriovenular anastomoses expand.

Previously, arteriovenous anastomoses of the closing and glomus type were identified. Studies conducted in recent decades have shown that the discharge of blood from the arterial to the venous link through shortened paths, or shunts, occurs at the micro level, i.e. at the level of arterioles and venules. Such vascular formations, called arteriovenular anastomoses, are recognized as natural components of the microvasculature. It is necessary to distinguish between the arterial section of the arteriolovenular anastomosis, equipped with muscle cells (obturator devices), and the venular, muscleless one. Arteriovenular anastomoses without locking devices are designated as half-shunts due to the fact that they do not discharge arterial blood, but mixed. Half-shunts are detected in the dura mater of the brain and spinal cord, in the serous membranes, in endocrine organs. Preferential flow channels can also be likened to half-shunts.

Initial sections of the lymphatic system

At any blood supply area There are also lymphatic microvessels. The exception is various departments c. n. pp., retina, bone tissue. Liquid and various substances, including plasma proteins, which are transported through the walls of blood microvessels, together with soluble waste products of cells, form tissue or interstitial fluid. Part of the tissue fluid, including water and low molecular weight compounds, is reabsorbed into the blood vessels. However, this volume is always less than the volume of fluid filtered into the tissue from the plasma. The total mass of protein, which is transported into the tissue through the walls of blood microvessels, is almost 50% of the amount circulating in the plasma, and the tissues contain more protein than the blood. The main route through which excess filtered fluid and most of the protein are returned to the plasma is lymphatic microvessels. Thus, interstitial fluid containing proteins constitutes lymph. The protein concentration in it varies

It varies widely (from 30 to 90% of plasma concentration) depending on the region, and therefore on the permeability of blood microvessels, functional state organ, filtration intensity, lymph formation, etc.

The mechanisms of entry of interstitial fluid into the lumen of resorbing lymphatic vessels have not yet been fully elucidated. It is believed that the main force promoting lymphatic resorption and the movement of lymph to the collecting vessels is the difference in hydrostatic pressure in the interstitial space and the lumen of the lymphatic capillaries. Kasley-Cmht (J. R. Gasley-Smith, 1983) also allows for the possibility of “sucking” fluid from tissues due to more high concentrations protein in lymph.

The microlymphatic bed is a complex complex of interconnected lymphatic capillaries, postcapillaries, primary and collecting lymphatic vessels. Various lymphatic segments are topographically and functionally closely connected with blood microvessels, and this connection determines the differentiated participation of lymphatic capillaries and postcapillaries in the resorption of various components of interstitial fluid.

Lymphatic capillaries are thin-walled wide endothelial channels, the diameter of which can reach 200 microns. They begin either as blind finger-like protrusions or as fragments of a network devoid of valves. The wall of lymphatic capillaries is formed by thinned endothelial cells, and in some tissues - by a fragmented basement membrane. Tissue fluid and macromolecules penetrate into the lumen of the capillaries through intercellular gaps; Some of them can be open very widely - from 50 nm to 1-2 microns. The frequency of the appearance of “open” contacts in the lymphatic endothelium correlates with the intensity of resorption and, consequently, hematolymphatic exchange. “Open” contacts, freely allowing macromolecules, particles (chylomicrons) and even cells to pass through, are quite often found in the lymphatic capillaries of the diaphragm, villi of the small intestine, etc. It is believed that the degree of opening of intercellular gaps is regulated by the tension of anchor and slender filaments - thin connective tissue fibers that are fixed to the plasmalemma of endothelial cells. The lymph accumulating in the lumen of the capillaries moves to the next segments due to the periodically occurring pressure difference, which opens the valves. Lymphodynamics are also stimulated by the pressure of surrounding tissues, for example, during muscle contraction, and by the mechanism of suction into the collector lymphatic vessels.

Lymphatic postcapillaries. As soon as lymph flow occurs in the lumen of the lymphatic capillaries, the resorbed fluid moves to other segments of the lymphatic tract. Traditionally it was believed that lymph flows from the capillaries into the lymphatic vessels. V.V. Kupriyanov (1969) established that in lymphatic networks and plexuses, cells are formed mainly by such endothelial channels, which contain valves, - lymphatic postcapillaries. The valve leaflets in them are folds (duplicates) of the endothelium with a few collagen fibrils. Thanks to the valves, the cells or chains of lymphatic postcapillaries have clearly defined contours. Lymphatic postcapillaries are typical resorbing microvessels. The structure of their walls is almost no different from the structure of lymphatic capillaries. Only as you approach the level of the lymphatic vessels, the basement membrane is more clearly and regularly revealed and in its immediate surroundings the content of connective tissue fibers increases. Endothelial cells forming the walls of postcapillaries are smaller: 1 mm2 of the surface of the capillary

There are 25% more nuclei than in capillaries.

Lymphatic postcapillaries, according to V.V. Banin (1981), are capable of intensively resorbing macromolecules from their environment. Their functional significance is very great, since postcapillaries are located in the tissues next to the venules, through the walls of which the transport of proteins into the interstitium occurs most actively. In certain tissues, for example, in the peritoneum of mammals, the total surface area of ​​lymphatic postcapillaries is 2-6 times greater than the surface of capillaries.

Interstitial fluid and proteins penetrate into the lumen of lymphatic postcapillaries through intercellular contacts. Numerous micropinocytotic vesicles take part in the transfer of proteins through the endothelium of postcapillaries and capillaries. They constitute a significant part of the total cellular volume. Through transendothelial channels in the endothelium of lymphatic microvessels are formed much less frequently than in the endothelium of blood microvessels.

As lymph accumulates in the lumen of the postcapillary, hydrostatic pressure increases, and when a certain threshold value is reached, the valve opens to the next segment. Thus, lymphodynamics and resorptive activity in the chains or cells of lymphatic postcapillaries are regulated by the developed valve apparatus. Periodically, lymph is retained in individual postcapillaries (intervalvular segments), and then some of the water can be filtered out of the lumen back into the tissue. During the next phase of expulsion, more concentrated lymph moves in a centripetal direction. The proteins contained in it are capable of creating a higher colloid osmotic pressure than in the surrounding tissue fluid, and thereby attract water into the lumen of the vessel. This mechanism, together with the topographical features of lymphatic capillaries and postcapillaries, ensures a subtle and precise adaptation of the processes of lymph formation to the intensity of filtration of fluid and protein from blood microvessels.

Initial and collecting lymphatic vessels. These lymphatic segments show signs of additional, non-endothelial vascular membranes- connective tissue fibers and single cells surrounding the basement membrane and closely adjacent to it. As the lymph moves in a centripetal direction, the walls of the vessels thicken, myocytes appear in their composition, which subsequently form a continuous layer. The valve flaps of the lymphatic vessels are thicker than those in the lymphatic postcapillaries. They have a well-developed connective tissue fibrous base, including cellular forms (fibroblasts). In the area where the valve leaflets are fixed, and immediately in front of them, a thickened cuff of the wall is formed, formed by a condensation of fibers and myocytes. Such microvessels perform predominantly drainage functions: intercellular contacts in the endothelial lining are formed by dense complexes, the endothelium is noticeably thickened, and the number of vesicles is reduced.

Histophotometric comparison of the protein content in the resorbing segments (capillaries and postcapillaries) and in the lumen of the collecting lymph vessels indicates an increase in the protein concentration in the lymph as it passes to the regional lymph nodes.

Interstitial space

In parenchymal and hollow organs, the blood and lymphatic pathways are immersed in an interstitial gel. This basic substance of connective tissue forms, together with the fibrillar components, the interstitial space. It concentrates 3 times the pain-

A higher volume of water than in blood plasma. Interstitial fluid, being the most important component of the internal environment of the body, is capable of maintaining fairly constant composition and physicochemical properties under physiological conditions. However, tissue homeostasis not only does not exclude, but also provides for constant renewal and movement of the intercellular environment. Since blood and lymphatic microvessels are primarily involved in the formation of interstitial fluid, hematolymphatic transfer is an important factor of homeostasis.

In contrast to the pathways of blood and lymph, there appear to be no anatomically clearly defined pathways for the transport of interstitial fluid. In some modern hypotheses, however, the possibility of preferential movement of tissue fluid, including macromolecules, is discussed, according to the so-called. interstitial channels - spaces in the matrix containing relatively few glycosaminoglycans. Data have also been obtained indicating the distribution of proteins along connective tissue fibers or near the walls of lymphatic microvessels.

The movement of interstitial fluid in tissues can fundamentally be associated with two processes: convection, which occurs as a result of gradients of hydrostatic or colloid-osmotic pressure, and diffusion, which depends on the difference in the concentrations of a particular substance. There is no firm certainty yet that hydrostatic pressure at different points in space can vary noticeably. It is possible that this difference is due to unequal hydration of the matrix due to variations in the intensity of fluid filtration from blood microvessels.

Specific values ​​of interstitial pressure in different tissues can differ very significantly, from -2 to -6 mm Hg. Art. in the subcutaneous connective tissue, according to A. F. Scholander, up to +4 - 15 mm Hg. Art. in the kidney, spleen, myocardium, according to R. G. Grainger. Differences in measured quantities may also be associated with the measurement method itself. The lack of a consensus not only regarding the specific values ​​of tissue pressure, but also its nature makes it difficult to understand the mechanisms of such important processes as lymph formation.

As already noted, the content of plasma proteins in the interstitial space depends on the permeability of the microvascular walls for macromolecules. In tissues whose capillaries have a somatic type of endothelium, for example, in muscles, the protein concentration is at least 30% of the concentration in the blood plasma. As Wiederhielm's research has shown (S. A. Wiederhielm, 1972), the osmotic effect of proteins, mainly albumin, is noticeably enhanced due to their interaction with “fixed” biopolymers of the interstitial space - glycosaminoglycans and collagen. The value of interstitial colloid osmotic pressure is usually estimated in the range of 7-11 mm Hg. Art. It significantly depends on the water content in the interstitial space and is regulated by the resorptive activity of the roots of the lymphatic system. Due to the fact that the permeability of different blood microvessels for proteins is not the same, the protein content in the interstitial space can vary significantly. Photometric analysis shows that the concentration of albumin and other proteins of average mass near the walls of veins is 3-4 times higher than their concentration in other sections. The resulting concentration gradients are capable of moving interstitial fluid and orienting its flows to the resorbing lymphatic microvessels. The diffusion of protein molecules in tissue is limited by the matrix of the ground substance, and the degree of this restriction is related to the hydration of the tissue. Conditions that promote the filtration of fluid into tissue from

Plasma (venous stagnation, the action of vasoactive substances such as histamine, inflammation, etc.) usually lead to increased hydration of the interstitial gel, increased pressure in it, increased protein transport and, as a result, stimulation of lymph formation. The combination of these processes, important for maintaining water balance, is figuratively called the safety factor against edema.

Structural and functional units of the microvasculature

The spatial orientation, structural parameters and hemodynamic characteristics of the microvasculature in various organs have their own characteristics depending on their structure, functions performed and the energy (metabolic) needs of their constituent tissues. The unifying factor in the structural organization of the microcirculatory bed, in all likelihood, should be some kind of “basic cell” - a unit reflecting the general principle of the structure of the microcirculation system. Attempts to identify such a basic unit were made in the studies of A. Krog (1927), who proposed the “tissue cylinder” model. Subsequently, such units as capillaron, segment, microdistrict, and functional element were discussed. The degree of their structural complexity, as well as the breadth of coverage of the entire variety of transport processes in tissues, are very different. The most widespread idea is that of a segment, or module, that unites a complex of blood microvessels and allows effective analysis microhemodynamics in them. However, the movement of blood through microvessels is only a part, albeit a very important one, of the activity of the microcirculation system. It is difficult to study phenomena such as permeability, interstitial transport and lymph formation within the hemodynamic model. Therefore, as a structural and functional unit of the microvasculature, it is advisable to consider the entire complex of vascular (blood and lymphatic) and extravascular communications that take part in providing the metabolic needs of a certain area of ​​tissue. The formal boundaries of such an area can be structures formed from anastomosing arterioles and accompanying venules, or other naturally recurring vascular associations. It is very important that such complexes also include the lymphatic pathways found

Living in certain topographical relationships with blood microvessels. The interstitial space of such a tissue area acts as a universal mediator, a connecting link not only between blood and lymphatic microvessels, but also between microvessels and any cellular elements. In such a miniature unit, which assimilates any transport processes occurring in a given tissue area, a model of the entire microvasculature is embodied. In fact, the module is a kind of equivalent to the structural and functional unit of an organ and reflects organ specificity to the same extent as the specificity of the organization and functioning of the entire microcirculation system.

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