The structure of the internal histohematic barriers of the body. Histohematic and blood-brain barriers of the brain. Functional groups of histohematic barriers

01.11.2019

Histohematic barrier - it is a set of morphological structures, physiological and physico-chemical mechanisms that function as a whole and regulate the flow of substances between the blood and organs.

Histohematic barriers are involved in maintaining the homeostasis of the body and individual organs. Due to the presence of histohematic barriers, each organ lives in its own special environment, which can differ significantly from the composition of individual ingredients. Particularly powerful barriers exist between the brain, the blood and tissue of the gonads, the blood and moisture of the chambers of the eye, the blood of the mother and fetus.

Histohematic barriers of various organs have both differences and a number of common structural features. Direct contact with blood in all organs has a barrier layer formed by the endothelium of blood capillaries. In addition, the HGB structures are the basement membrane (middle layer) and adventitial cells of organs and tissues (outer layer). Histohematic barriers, changing their permeability to various substances, can limit or facilitate their delivery to the organ. For a number of toxic substances, they are impenetrable, which manifests their protective function.

The most important mechanisms that ensure the functioning of histohematogenous barriers are further considered using the example of the blood-brain barrier, the presence and properties of which the doctor especially often has to take into account when using drugs and various effects on the body.

Blood-brain barrier

Blood-brain barrier is a set of morphological structures, physiological and physico-chemical mechanisms that function as a single whole and regulate the flow of substances between the blood and brain tissue.

The morphological basis of the blood-brain barrier is the endothelium and basement membrane of the cerebral capillaries, interstitial elements and glycocalyx, neuroglia astrocytes, covering the entire surface of the capillaries with their legs. The movement of substances across the blood-brain barrier involves the transport systems of the endothelium of the capillary walls, including vesicular transport of substances (pino- and exocytosis), transport through channels with or without the participation of carrier proteins, enzyme systems that modify or destroy incoming substances. It has already been mentioned that specialized water transport systems function in the nervous tissue using the aquaporin proteins AQP1 and AQP4. The latter form water channels that regulate the formation of cerebrospinal fluid and the exchange of water between the blood and brain tissue.

Brain capillaries differ from capillaries in other organs in that endothelial cells form a continuous wall. At the points of contact, the outer layers of endothelial cells merge, forming the so-called "tight junctions".

The blood-brain barrier performs protective and regulatory functions for the brain. It protects the brain from the action of a number of substances formed in other tissues, foreign and toxic substances, participates in the transport of substances from the blood to the brain and is an important participant in the mechanisms of homeostasis of the intercellular fluid of the brain and cerebrospinal fluid.

The blood-brain barrier is selectively permeable to various substances. Some biologically active substances, such as catecholamines, practically do not pass through this barrier. The only exceptions are small areas of the barrier on the border with the pituitary gland, pineal gland and some areas where the permeability of the blood-brain barrier for many substances is high. In these areas, channels and interendothelial gaps penetrating the endothelium were found, through which substances from the blood penetrate into the extracellular fluid of the brain tissue or into themselves. The high permeability of the blood-brain barrier in these areas allows biologically active substances (cytokines,) to reach those neurons of the hypothalamus and glandular cells, on which the regulatory circuit of the neuroendocrine systems of the body closes.

A characteristic feature of the functioning of the blood-brain barrier is the possibility of changing its permeability for a number of substances under different conditions. Thus, the blood-brain barrier is able, by regulating permeability, to change the relationship between the blood and the brain. Regulation is carried out by changing the number of open capillaries, blood flow velocity, changes in the permeability of cell membranes, the state of the intercellular substance, the activity of cellular enzyme systems, pino- and exocytosis. The permeability of the BBB can be significantly impaired in conditions of ischemia of the brain tissue, infection, the development of inflammatory processes in the nervous system, and its traumatic injury.

It is believed that the blood-brain barrier, while creating a significant obstacle to the penetration of many substances from the blood into the brain, at the same time well passes the same substances formed in the brain in the opposite direction - from the brain into the blood.

The permeability of the blood-brain barrier for various substances is very different. Fat-soluble substances tend to cross the BBB more easily than water-soluble substances.. Easily penetrate oxygen, carbon dioxide, nicotine, ethyl alcohol, heroin, fat-soluble antibiotics ( chloramphenicol and etc.)

Lipid-insoluble glucose and some essential amino acids cannot pass into the brain by simple diffusion. Carbohydrates are recognized and transported by special transporters GLUT1 and GLUT3. This transport system is so specific that it distinguishes between stereoisomers of D- and L-glucose: D-glucose is transported, but L-glucose is not. Glucose transport into the brain tissue is insensitive to insulin, but is inhibited by cytochalasin B.

Carriers are involved in the transport of neutral amino acids (for example, phenylalanine). For the transfer of a number of substances, active transport mechanisms are used. For example, due to active transport against concentration gradients, Na + , K + ions, the amino acid glycine, which acts as an inhibitory mediator, are transported.

Thus, the transfer of substances using various mechanisms is carried out not only through plasma membranes, but also through the structures of biological barriers. The study of these mechanisms is necessary to understand the essence of regulatory processes in the body.

Histohematic barrier It is the barrier between blood and tissue. They were first discovered by Soviet physiologists in 1929. The morphological substrate of the histohematic barrier is the capillary wall, which consists of:

1) fibrin film;

2) endothelium on the basement membrane;

3) a layer of pericytes;

4) adventitia.

In the body, they perform two functions - protective and regulatory.

Protective function associated with the protection of tissue from incoming substances (foreign cells, antibodies, endogenous substances, etc.).

Regulatory function is to ensure a constant composition and properties of the internal environment of the body, the conduction and transmission of molecules of humoral regulation, the removal of metabolic products from cells.

The histohematic barrier can be between tissue and blood and between blood and fluid.

The main factor affecting the permeability of the histohematic barrier is permeability. Permeability- the ability of the cell membrane of the vascular wall to pass various substances. It depends on:

1) morphofunctional features;

2) activities of enzyme systems;

3) mechanisms of nervous and humoral regulation.

In the blood plasma there are enzymes that can change the permeability of the vascular wall. Normally, their activity is low, but in pathology or under the influence of factors, the activity of enzymes increases, which leads to an increase in permeability. These enzymes are hyaluronidase and plasmin. Nervous regulation is carried out according to the non-synaptic principle, since the mediator enters the capillary walls with a fluid current. The sympathetic division of the autonomic nervous system reduces permeability, while the parasympathetic division increases it.

Humoral regulation is carried out by substances that are divided into two groups - increasing permeability and decreasing permeability.

The mediator acetylcholine, kinins, prostaglandins, histamine, serotonin, and metabolites that shift the pH to an acidic environment have an increasing effect.

Heparin, norepinephrine, Ca ions can have a lowering effect.

Histohematic barriers are the basis for the mechanisms of transcapillary exchange.

Thus, the structure of the vascular wall of capillaries, as well as physiological and physicochemical factors, greatly influence the work of histohematic barriers.

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Between the blood and the extracellular space there are formations called histohematic barriers that separate the blood plasma from the extracellular fluid of various tissues of the body. The latter is separated from the intracellular fluid by cell membranes. Histohematic barriers and cell membranes are selectively permeable to ions and organic compounds. Therefore, the electrolyte and organic compositions of blood plasma, extracellular and intracellular fluid differ from each other.
According to the peculiarities of permeability for proteins at the blood-tissue level, all histohematic barriers are divided into three groups: insulating, partially insulating and non-insulating. The insulating barriers include hematoliquor (between CSF and blood), hematoneuronal, hematotesticular (between blood and testicles), hematoencephalic (between blood and brain tissue) and hematoophthalmic (between blood and intraocular fluid), barrier of the lens of the eye. Partially insulating barriers include barriers at the level of the bile capillaries of the liver, the adrenal cortex, the pigment epithelium of the eye between the vascular and retinal membranes, the thyroid gland, the end lobes of the pancreas, and the hemato-ophthalmic barrier at the level of the ciliary processes of the eye. Although non-isolating barriers allow the protein to penetrate from the blood into the interstitial fluid, they limit its transport into the microenvironment and cytoplasm of parenchymal cells. Such barriers exist in the myocardium, skeletal muscles, adrenal medulla, and parathyroid glands.
The structural element of histohematic barriers is the wall of blood capillaries. Morphological and functional features of capillary endothelial cells - the size of pores in their membrane, the presence of fenestra, the presence of an intercellular basic substance that cements the gaps between capillary endotheliocytes and the thickness of the basement membrane determine the permeability of the barrier to water and molecules of substances of various sizes and structures dissolved in it. The substances contained in the blood (water, oxygen, CO2, glucose, amino acids, urea, etc.) can penetrate the barrier in two ways (Fig. 1.2): transcellularly (through endothelial cells) and paracellularly (through the intercellular basic substance).
Transcellular transport of substances can be passive (i.e., along a concentration or electrochemical gradient without consuming energy

gie) and active (against the gradient with energy costs). Transcellular transfer of substances is also carried out with the help of pinocytosis, i.e., the process of active absorption by cells of fluid bubbles or colloidal solutions. Paracellular transport, or the transfer of substances through intercellular gaps filled with the main substance that envelops the fibrous structures of fibrillar protein, is possible for molecules of different sizes (from 2 to 30 nm), since the sizes of intercellular gaps in capillaries are not the same. The basement membrane of the capillaries of different organs has an unequal thickness, and in some tissues it is discontinuous. This barrier structure plays the role of a molecular filter that allows molecules of a certain size to pass through. The basal membrane contains glycosaminoglycans that can reduce the degree of polymerization and adsorb enzymes that increase the permeability of the barrier. Outside, in the basement membrane, there are process cells - pericytes. There is no exact information about the function of these cells; it is assumed that they play a supporting role and produce the main substance of the basement membrane.
The main functions of histohematic barriers are protective and regulatory. The protective function consists in delaying the transition of harmful substances of an endogenous nature, as well as foreign molecules from the blood into the interstitial environment and the cell microenvironment, by barriers. At the same time, not only the vascular wall itself with its selective permeability, but also the cellular-colloidal structures of the interstitium, adsorbing such substances,
prevent their entry into the microenvironment of cells. If there was a penetration of large molecular foreign substances into the interstitial space and they did not undergo adsorption, phagocytosis and decay here, then such substances enter the lymph, and not the cellular microenvironment. In this regard, lymph is like a “second line of defense”, since the antibodies, lymphocytes and monocytes contained in it ensure the neutralization of foreign substances.
Due to the regulatory function, the histohematic barriers control the composition and concentration of molecules of various compounds in the interstitial fluid, changing the permeability of the barriers to ions, nutrients, mediators, cytokines, hormones, and cell metabolism products. Thus, histohematic barriers regulate the flow of various substances from the blood into the interstitial fluid and the timely outflow of cellular metabolic products from the intercellular space into the blood.
The permeability of histohematic barriers changes under the influence of the autonomic nervous system (for example, sympathetic influences reduce their permeability). Hormones circulating in the blood (for example, corticosteroids reduce the permeability of the blood-brain barrier), tissue biologically active substances (biogenic amines - serotonin, histamine, heparin, etc.), enzymes (hyaluronidase, etc.) ), formed both by the endothelial cells themselves and by the cellular elements of the interstitial space. For example, hyaluronidase is an enzyme that causes the depolymerization of hyaluronic acid, the main substance of the intercellular spaces. Therefore, when it is activated, the permeability of barriers increases sharply; serotonin - reduces their permeability, histamine increases it; heparin - inhibits hyaluronidase and, as a result of a decrease in its activity, reduces the permeability of barriers; cytokinases - activate plasminogen, and increasing the dissolution of fibrin fibers, increase the permeability of the barrier. Metabolites increase the permeability of barriers, causing a shift in pH to the acid side (for example, lactic acid).
The permeability of histohematic barriers also depends on the chemical structure of the molecules of the transferred substances, their physicochemical properties. So, for lipid-soluble substances, histohematic barriers are more permeable, since such molecules more easily pass through the lipid layers of cell membranes.

Histohematic barriers are a combination of morphological, physiological and physicochemical mechanisms that function as a whole and regulate the interactions of blood and organs. Histohematic barriers are involved in the creation of homeostasis of the body and individual organs. Due to the presence of HGB, each organ lives in its own special environment, which can differ significantly from blood plasma in terms of the composition of individual ingredients. Particularly powerful barriers exist between the blood and the brain, the blood and tissue of the gonads, the blood and the chamber moisture of the eye. The barrier layer formed by the endothelium of the blood capillaries has direct contact with the blood, followed by the basement membrane with pericytes (middle layer) and then adventitial cells of organs and tissues (outer layer). Histohematic barriers, changing their permeability to various substances, can limit or facilitate their delivery to the organ. For a number of toxic substances, they are impenetrable. This is their protective function.

The blood-brain barrier (BBB) is a combination of morphological structures, physiological and physico-chemical mechanisms that function as a whole and regulate the interaction of blood and brain tissue. The morphological basis of the BBB is the endothelium and the basement membrane of the cerebral capillaries, interstitial elements and glycocalyx, neuroglia, the peculiar cells of which (astrocytes) cover the entire surface of the capillary with their legs. The barrier mechanisms also include transport systems of the endothelium of the capillary walls, including pino- and exocytosis, endoplasmic reticulum, channel formation, enzyme systems that modify or destroy incoming substances, as well as proteins that act as carriers.

In the structure of brain capillary endothelial membranes, as well as in a number of other organs, aquaporin proteins were found that create channels that selectively let water molecules through.

Among the functions of the BBB are protective and regulatory. It protects the brain from the action of foreign and toxic substances, participates in the transport of substances between the blood and the brain, and thereby creates homeostasis of the intercellular fluid of the brain and cerebrospinal fluid.

The blood-brain barrier is selectively permeable to various substances. Some biologically active substances (for example, catecholamines) practically do not pass through this barrier. The only exceptions are small sections of the barrier on the border with the pituitary gland, epiphysis and some parts of the hypothalamus, where the permeability of the BBB for all substances is high.

In these areas, gaps or channels penetrating the endothelium were found, through which substances from the blood penetrate into the extracellular fluid of the brain tissue or into the neurons themselves.

The high permeability of the BBB in these areas allows biologically active substances to reach those neurons of the hypothalamus and glandular cells, on which the regulatory circuit of the neuroendocrine systems of the body closes.

A characteristic feature of the functioning of the BBB is the regulation of the permeability for substances adequately to the prevailing conditions. Regulation comes from:

1) changes in the area of open capillaries,

2) changes in blood flow,

3) changes in the state of cell membranes and intercellular substance, activity of cellular enzyme systems, pino- and exocytosis.

It is believed that the BBB, while creating a significant obstacle to the penetration of substances from the blood into the brain, at the same time well passes these substances in the opposite direction from the brain to the blood.

The permeability of the BBB for various substances varies greatly. Fat-soluble substances, as a rule, penetrate the BBB more easily than water-soluble substances. Oxygen, carbon dioxide, nicotine, ethyl alcohol, heroin, fat-soluble antibiotics (chloramphenicol, etc.) penetrate relatively easily.

Lipid-insoluble glucose and some essential amino acids cannot pass into the brain by simple diffusion. They are recognized and transported by special carriers. The transport system is so specific that it distinguishes stereoisomers of D- and L-glucose. D-glucose is transported, but L-glucose is not. This transport is provided by carrier proteins built into the membrane. Transport is insulin insensitive, but inhibited by cytocholasin B.

Large neutral amino acids (eg, phenylalanine) are transported similarly.

There is also active transport. For example, due to active transport against concentration gradients, Na + , K + ions, the amino acid glycine, which acts as an inhibitory mediator, are transported.

The given materials characterize the methods of penetration of biologically important substances through biological barriers. They are essential for understanding humoral regulation in the body.

Histohematic barriers (HGB): purpose and functions

Histohematic barriers are a combination of morphological, physiological and physicochemical mechanisms that function as a whole and regulate the interactions of blood and organs. Histohematic barriers are involved in the creation of homeostasis of the body and individual organs. Due to the presence of HGB, each organ lives in its own special environment, which can differ significantly from blood plasma in terms of the composition of individual ingredients. Particularly powerful barriers exist between the blood and the brain, the blood and tissue of the gonads, the blood and the chamber moisture of the eye. Physiology and pathology of histohematic barriers / Ed. L.S. Stern.- M., 1968.- S. 67. Direct contact with blood has a barrier layer formed by the endothelium of blood capillaries, then comes the basement membrane with pericytes (middle layer) and then adventitial cells of organs and tissues (outer layer). Histohematic barriers, changing their permeability to various substances, can limit or facilitate their delivery to the organ. For a number of toxic substances, they are impenetrable. This is their protective function. Human Physiology: Textbook / Ed. V.M. Smirnova.- M.: Medicine, 2001.- S. 132.