Passage of newborn neurons through the GEB. Hematotesticular biological barrier. Physiology - how the BBB works

Blood-brain barrier(BBB) ​​is a physiological barrier that separates blood from cerebrospinal fluid and internal environment central nervous system in order to keep the latter constant. The concentration of many substances, such as amino acids, hormones, metal ions, in the blood is constantly changing, especially sharply after eating or physical activity. Most organs can tolerate such changes, however, they could be detrimental to the functioning of the central nervous system, leading to chaotic generation nerve impulses individual neurons, since many of the blood substances (for example, the amino acid glycine and the hormone norepinephrine) function as neurotransmitters, and some ions (for example, K +) can change the excitability of nerve cells.

Structure of the blood-brain barrier

The following structures are involved in creating the blood-brain barrier:

• Capillary endothelium, the cells of which are securely and closely connected to each other by tight junctions, as a result of which the capillaries of the CNS are less permeable throughout the body. This component is the most important in the creation of the BBB.
• Relatively thick basement membrane that surrounds each capillary on the outside.
• Cibulin-like "legs" of astrocytes, which tightly stick around the capillaries. Although these structures contribute to the formation of the BBB, their role is not so much to directly provide impermeability, but rather to stimulate endotheliocytes to form tight junctions.

Permeability of the blood-brain barrier

The blood-brain barrier has selective permeability: substances necessary for the nutrition of the nervous system can be transported from it by facilitated diffusion: glucose (with the participation of the GLUT 1 transporter), essential amino acids and some electrolytes. lipids (fats, fatty acid) and low molecular weight fat-soluble substances (oxygen, carbon dioxide, ethanol, nicotine, anesthetics) can passively diffuse through the BBB membranes. Substances such as proteins, most toxins and metabolic products cannot overcome it, and low molecular weight amino acids and potassium ions are even actively downloaded from the brain into the blood. In particular, a unique Na + -K + -2Cl co-transporter is used to maintain a low concentration of K +.

The passage of substances in the opposite direction - from the brain to the blood - is controlled much less, because the cerebrospinal substance flows into the venous bed through the villi of the arachnoid.

Distribution of the blood-brain barrier

The BBB is not the same in different parts of the central nervous system, for example, in plexus junctions (lat. Plexus choroidus) The capillaries of the brain ventricles are well permeable, but they are surrounded by ependymal cells, which are already interconnected by tight junctions. Sometimes the barrier in the plexus connections is distinguished from the blood-brain barrier and is called the hemato-spinal-cerebrospinal barrier, although they have much in common.

Some functional structures In the brain, the blood-brain barrier prevents them from doing their work, so they are deprived of it, these areas are united under the name of the navkolunochkovy organs, since they are located near the ventricles of the brain. For example, the center of vomiting in medulla oblongata at the fourth ventricle, must monitor the presence of toxic substances in the blood. And the hypothalamus, which is located at the bottom of the third ventricle, must constantly feel the chemical composition of the blood in order to regulate water-salt balance, body temperature and many other physiological indicators. In particular, it is active in response to blood proteins such as angiotensin II, which stimulates drinking, and interleukin-1, which causes fever.

The blood-brain barrier is also underdeveloped in newborns and infants, making them especially susceptible to toxic substances.

Clinical Significance

The ability of certain drugs to cross the BBB is important characteristic their pharmacokinetics. In particular, it is important to consider it in the treatment of the organs of the nervous system. For example, some antibiotics are actually unable to penetrate the tissues of the brain and spinal cord, while others do so quite easily. The BBB retains the amines dopamine and serotonin, but lets through their acidic precursors, L-DOPA and 5-hydroxytryptophan.

important clinical observation is that the blood-brain barrier is broken in the areas of tumor growth - again, the capillaries do not have normal contacts with astrocytes. This helps in the diagnosis of neoplasms in the CNS: if albumin labeled with 131 I is used, it will penetrate first of all into the tumor tissue, so that it can be localized.

The blood-brain barrier is extremely important for ensuring brain homeostasis, but many questions regarding its formation are still not fully elucidated. But it is already quite clear that the BBB is the most pronounced in terms of differentiation, complexity and density of the histohematological barrier. Its main structural and functional unit is the endothelial cells of the capillaries of the brain.

The metabolism of the brain, like no other organ, depends on substances coming from the bloodstream. Numerous blood vessels that ensure the functioning of the nervous system are distinguished by the fact that the process of penetration of substances through their walls is selective. The endothelial cells of the capillaries of the brain are interconnected by continuous tight junctions, so substances can only pass through the cells themselves, but not between them. To outer surface capillaries adjoin glial cells - the second component of the blood-brain barrier. In the choroid plexuses of the ventricles of the brain, the anatomical basis of the barrier is epithelial cells, which are also tightly interconnected. Currently, the blood-brain barrier is considered not as an anatomical and morphological, but as functional education, capable of selectively passing, and in some cases, delivering various molecules to nerve cells using active transport mechanisms. Thus, the barrier performs regulatory and protective functions.

There are structures in the brain where the blood-brain barrier is weakened. This is, first of all, the hypothalamus, as well as a number of formations at the bottom of the 3rd and 4th ventricles - the most posterior field (area postrema), subfornical and subcommissural organs, as well as pineal gland. The integrity of the BBB is disturbed in ischemic and inflammatory brain lesions.

The blood-brain barrier is considered to be finally formed when the properties of these cells satisfy two conditions. First, the rate of liquid-phase endocytosis (pinocytosis) in them should be extremely low. Secondly, specific tight contacts should be formed between cells, which are characterized by a very high electrical resistance. It reaches values ​​of 1000-3000 Ohm/cm2 for pia mater capillaries and from 2000 to 8000 0m/cm2 for intraparenchymal cerebral capillaries. For comparison: average value transendothelial electrical resistance of skeletal muscle capillaries is only 20 Ohm/cm2.

The permeability of the blood-brain barrier for most substances is largely determined by their properties, as well as the ability of neurons to synthesize these substances on their own. Substances that can overcome this barrier include primarily oxygen and carbon dioxide, as well as various metal ions, glucose, essential amino acids and fatty acids necessary for normal functioning brain. The transport of glucose and vitamins is carried out using carriers. At the same time, D- and L-glucose have different rates of penetration through the barrier - in the former it is more than 100 times higher. Glucose plays leading role both in the energy metabolism of the brain, and in the synthesis of a number of amino acids and proteins.

The leading factor determining the functioning of the blood-brain barrier is the level of metabolism of nerve cells.

Providing neurons with the necessary substances is carried out not only with the help of suitable blood capillaries, but also thanks to the processes of the soft and arachnoid membranes, through which cerebrospinal fluid circulates. Cerebrospinal fluid is found in the cranial cavity, in the ventricles of the brain and in the spaces between the meninges. In humans, its volume is about 100-150 ml. Thanks to the cerebrospinal fluid, the osmotic balance of nerve cells is maintained and metabolic products that are toxic to the body are removed. nervous tissue.

The passage of substances through the blood-brain barrier depends not only on the permeability of the vascular wall for them ( molecular weight, charge and lipophilicity of the substance), but also on the presence or absence of an active transport system.

Endothelial cells of brain capillaries are rich in stereospecific insulin-independent glucose transporter (GLUT-1), which ensures the transport of this substance across the blood-brain barrier. The activity of this transporter can ensure the delivery of glucose in an amount 2-3 times higher than that required by the brain in normal conditions.

Characteristics of transport systems of the blood-brain barrier (according to: Pardridge, Oldendorf, 1977)

In children with impaired functioning of this transporter, there is a significant decrease in the level of glucose in the cerebrospinal fluid and disturbances in the development and functioning of the brain.

Monocarboxylic acids (L-lactate, acetate, pyruvate), as well as ketone bodies transported by separate stereospecific systems. Although the intensity of their transport is lower than that of glucose, they are an important metabolic substrate in newborns and during starvation.

The transport of choline into the central nervous system is also mediated by the transporter and can be regulated by the rate of synthesis of acetylcholine in the nervous system.

Vitamins are not synthesized by the brain and are supplied from the blood using special transport systems. Despite the fact that these systems have a relatively low transport activity, under normal conditions they can provide the transport of the amount of vitamins necessary for the brain, but their deficiency in food can lead to neurological disorders. Some plasma proteins can also cross the blood-brain barrier. One way they enter is through receptor-mediated transcytosis. This is how insulin, transferrin, vasopressin and insulin-like growth factor penetrate the barrier. Endothelial cells of brain capillaries have specific receptors for these proteins and are able to carry out endocytosis of the protein-receptor complex. It is important that, as a result of subsequent events, the complex breaks down, the intact protein can be released into opposite side cells, and the receptor is reintegrated into the membrane. For polycationic proteins and lectins, transcytosis is also the way of penetration through the BBB, but it is not associated with the work of specific receptors.

Many neurotransmitters present in the blood are unable to cross the BBB. So, dopamine does not have this ability, while L-DOPA penetrates the BBB using the neutral amino acid transport system. In addition, capillary cells contain enzymes that metabolize neurotransmitters (cholinesterase, GABA transaminase, aminopeptidases, etc.), drugs and toxic substances, which protects the brain not only from neurotransmitters circulating in the blood, but also from toxins.

Carrier proteins also participate in the work of the BBB, transporting substances from the endothelial cells of the capillaries of the brain into the blood, preventing their penetration into the brain, for example, b-glycoprotein.

During ontogeny, the transport rate various substances through the BBB changes significantly. Thus, the rate of transport of b-hydroxybutyrate, tryptophan, adenine, choline, and glucose in newborns is significantly higher than in adults. This reflects a relatively higher demand developing brain in energy and macromolecular substrates.

Blood-brain barrier It is a functional barrier that prevents the penetration of a number of substances such as antibiotics, toxic chemical and bacterial compounds from the blood into the nervous tissue.

Question51. The blood-brain barrier and its functions

At the heart of functioning blood-brain barrier lies reduced permeability, which is characteristic of blood capillaries in the nervous tissue. The main structural component of this barrier is the trailing junctions that ensure the continuity of the endothelial cells of these capillaries.

Cytoplasm their endothelial cells does not contain fenestra, which are found in many other areas, and pinocytic vesicles are very few. The low permeability of these capillaries is partly due to the expanded areas of neuroglial cell processes surrounding them.

Vascular plexus composed of folds of the pia mater high content dilated fenestrated capillaries that penetrate deep into the ventricles of the brain. It is found in the roof of the III and IV ventricles and in part of the walls of the lateral ventricles. The choroid plexus is formed by loose connective tissue of the pia mater, covered with a single layer of cuboidal or low columnar epithelium, the cells of which transport ions.

Home function choroid plexus is the production cerebrospinal fluid, which contains only a small amount of solids and completely fills the ventricles, the central canal of the spinal cord, the subarachnoid space and the perivascular space. The cerebrospinal fluid is important for the metabolism of the central nervous system and acts as a mechanism to protect it from mechanical shocks.

cerebrospinal fluid- transparent, with low density (1.004-1.008 g/ml) and very low protein concentration. In one milliliter of this fluid, single desquamated cells and two to five lymphocytes are also found. Cerebrospinal fluid is continuously produced and circulated in the ventricles, from which it is directed to the subarachnoid space.

Vascular plexus.
The basis of the choroid plexus is formed by loose connective tissue With large quantity blood capillaries (CC), it is covered with a single layer of cubic epithelium

In it in villi arachnoid membrane is the main absorption of cerebrospinal fluid into the venous circulation. (In the nervous tissue of the brain lymphatic vessels missing.)

decline suction cerebrospinal fluid or blockage of its outflow from the ventricles leads to a condition known as hydrocephalus (Greek hydro - water + kephale - head). Hydrocephalus is any disorder in which the cavities of the central nervous system contain excess amount cerebrospinal fluid, which causes an increase in intracranial pressure.

congenital hydrocephalus leads to an increase in the head, accompanied by a violation mental activity and muscle weakness. Adults have numerous neurological symptoms, also caused by damage to the nervous tissue of the brain.

— Return to the section « Histology"

1. The body of a nerve cell - a neuron: structure, histology
2. Dendrites of nerve cells: structure, histology
3. Axons of nerve cells: structure, histology
4. Membrane potentials of nerve cells. Physiology
5. Synapse: structure, functions
6. Glial cells: oligodendrocytes, Schwann cells, astrocytes, ependymal cells
7. Microglia: structure, histology
8. Central nervous system (CNS): structure, histology
9. Histology meninges. Structure
10. Blood-brain barrier: structure, histology

The blood-brain barrier (BBB)- a physiological barrier between the circulatory system and the central nervous system.

Blood-brain barrier

The BBB is present in all vertebrates; its main function is to maintain brain homeostasis.

The blood-brain barrier protects the nervous tissue from microorganisms circulating in the blood, toxins, cellular and humoral factors immune system that perceive brain tissue as foreign. It performs the function of a highly selective filter through which nutrients, and the products of its vital activity are excreted into the bloodstream.

The human body and higher animals have a number of specific physiological systems providing adaptation (adaptation) to constantly changing conditions of existence. This process is closely related to the need for the obligatory preservation of the constancy of essential physiological parameters, the internal environment of the body, physical chemical composition tissue fluid of the intercellular space.

Among the homeostatic adaptive mechanisms designed to protect organs and tissues from foreign substances and regulate the constancy of the composition of tissue interstitial fluid, the leading place is occupied by the blood-brain barrier. By definition, L. S. Stern, the blood-brain barrier combines a set of physiological mechanisms and corresponding anatomical formations in the central nervous system involved in regulating the composition of cerebrospinal fluid (CSF).

In the ideas about the blood-brain barrier, the following are emphasized as the main provisions: 1) the penetration of substances into the brain is carried out mainly not through the cerebrospinal fluid, but through circulatory system at the capillary level - a nerve cell; 2) the blood-brain barrier is to a greater extent not an anatomical formation, but functional concept characterizing a certain physiological mechanism. Like any physiological mechanism existing in the body, the blood-brain barrier is under the regulatory influence of the nervous and humoral systems; 3) among the factors controlling the blood-brain barrier, the leading one is the level of activity and metabolism of the nervous tissue.

The blood-brain barrier regulates the penetration of biologically active substances, metabolites, chemical substances, affecting the sensitive structures of the brain, prevents foreign substances, microorganisms, toxins from entering the brain.

The main function that characterizes the blood-brain barrier is the permeability of the cell wall. The necessary level of physiological permeability, adequate to the functional state of the body, determines the dynamics of the flow of physiologically active substances into the nerve cells of the brain.

The functional scheme of the blood-brain barrier includes neuroglia and the system of cerebrospinal fluid spaces along with the histohematological barrier. The histohematic barrier has a dual function: regulatory and protective. The regulatory function ensures the relative constancy of physical and physical and chemical properties, chemical composition, physiological activity of the intercellular environment of an organ, depending on its functional state. The protective function of the histohematic barrier is to protect organs from the ingress of foreign or toxic substances endo- and exogenous nature.

The leading component of the morphological substrate of the blood-brain barrier, which ensures its functions, is the wall of the brain capillary. There are two mechanisms for the penetration of a substance into brain cells: through the cerebrospinal fluid, which serves as an intermediate link between the blood and the nerve or glial cell, which performs a nutritional function (the so-called cerebrospinal fluid pathway), and through the capillary wall. In an adult organism, the main route of movement of a substance into nerve cells is hematogenous (through the walls of capillaries); the cerebrospinal fluid path becomes auxiliary, additional.

The permeability of the blood-brain barrier depends on the functional state of the body, the content of mediators, hormones, and ions in the blood. An increase in their concentration in the blood leads to a decrease in the permeability of the blood-brain barrier for these substances.

The functional system of the blood-brain barrier appears to be important component neurohumoral regulation. In particular, the principle of chemical feedback in the body is realized through the blood-brain barrier. It is in this way that the mechanism of homeostatic regulation of the composition of the internal environment of the body is carried out.

The regulation of the functions of the blood-brain barrier is carried out by the higher parts of the central nervous system and humoral factors. A significant role in the regulation is assigned to the hypothalamic-pituitary adrenal system. In the neurohumoral regulation of the blood-brain barrier importance have metabolic processes especially in brain tissue.

At various types cerebral pathology, for example, injuries, various inflammatory lesions of the brain tissue, there is a need to artificially reduce the level of permeability of the blood-brain barrier. Pharmacological influences can increase or decrease the penetration into the brain of various substances introduced from the outside or circulating in the blood.

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HEMATO-ENCEPHALIC BARRIER(Greek, haima, haimat blood + lat. encephalon, from Greek, enkephalos brain) - a physiological mechanism that selectively regulates the metabolism between the blood and the central nervous system. G.-e.

BBB. Its importance for the structure and function of the brain

b. also carries out protective function, preventing the penetration into the cerebrospinal fluid and the brain (head and spinal) of some foreign substances that enter the bloodstream, and intermediate metabolic products formed in the body in some patol, conditions. Therefore, the closely related protective and regulatory functions of G.-e are conventionally distinguished. b., providing a relative invariability of the composition, fiz.-chem. and biol, properties of cerebrospinal liquid and adequacy of the microenvironment of separate nervous elements.

On the existence of a mechanism that limits the transition of some chemical. compounds, mainly dyes, from the blood to the brain, indicated P. Earl them (1885), M. Lewandowski, (1900), Goldmann (E. Goldmann, 1913) and others. The term "blood-brain barrier" was proposed by L. S. Stern and Gauthier (R. Gauthier) in 1921. Stern, based on the analysis of a large experimental material, for the first time formulated fiziol, the foundations of the doctrine of G.-e. b. also has defined G.'s value - e. b. for activity c. n. With.

Morfol, G.'s substrate - e. b. are anatomical elements located between blood and neurons: capillary endothelium, cell basement membrane, glia, choroid plexus, membranes of the brain. Great importance in G.'s structures - e. b. has a so-called the main substance, the composition of which includes complexes of protein and polysaccharides - mucopolysaccharides. Many authors play a special role in the implementation of G.'s function - e. b. attributed to neuroglial cells. The terminal perivascular (sucker) legs of astrocytes, adjacent to the outer surface of the capillaries, can selectively extract from the bloodstream substances necessary for nourishing neurons and return their metabolic products to the blood [J. B. Brierley, 1957]. At the same time in all G.'s structures - e. b. enzymatic reactions can occur that contribute to the restructuring, oxidation, neutralization and destruction of substances coming from the blood (A. Labori, 1964).

The regulatory function is assessed by determining the permeability coefficient (more precisely, the distribution coefficient), i.e. the ratio of the concentration of a substance in the cerebrospinal fluid to its concentration in the blood serum. For most of the studied blood elements, the permeability coefficient is less than one, and only for magnesium and chlorine ions is it greater than one. The value of the coefficient depends on the composition of the blood and cerebrospinal fluid.

The use of radioisotope indication (see Radioisotope diagnostics) led to a certain revision of the concept of G.-e. b. It is established that G.'s permeability - e. b. unequal in various departments brain and, in turn, can change in different ways. The theory of the multiplicity of barrier formations (a system of brain barriers), functioning depending on the chemistry and changing needs of certain nervous structures, has become widespread. It has been established that there are “barrierless” zones in the brain (area postrema, neurohypophysis, pituitary stalk, epiphysis, gray tubercle), where substances introduced into the blood enter almost unhindered. In some departments of a brain (eg, in a hypothalamus) G.'s permeability - e. b. in relation to biogenic amines, electrolytes, some foreign substances, it is higher than in other parts of the brain, which ensures the timely receipt of humoral information in higher autonomic centers; emergence of some patol, processes (violation of mechanisms of regulation of functions, autonomic disorders, diencephalic syndromes, etc.) may be associated with an increase or decrease in G.'s permeability - e. b.

Protective and regulating functions of G. - e. b. are studied in humans and animals in onto- and phylogeny, as well as in different states body - during menstruation and pregnancy, with changes in body temperature and environment, in conditions of malnutrition, starvation and beriberi, with fatigue, insomnia, endocrine and autonomic dysfunctions, asphyxia, nervous disorders and disorders internal organs, infections, anesthesia, traumatic brain injury, shock, the introduction of various pharmacol, drugs, exposure to ionizing radiation, etc. So, in particular, it was found that in the process of phylogenesis, nerve cells become more sensitive to changes in the composition and properties of their environment . It conducts to improvement of barrier mechanisms of c. n. With. So, for example, some substances easily penetrate from the blood into the brain in low-organized, but are retained by G.-e. b. in more highly organized organisms. Besides, G. - e. b. differs in high permeability at embryos and newborns in comparison with an adult organism. There is an assumption that the high lability of the nervous system in children to a certain extent depends on the increased permeability of their G.-e.

Of great theoretical and practical importance is the question of selectivity (selective permeability) G.-e. b. in relation to substances that are often close to each other in chemical terms. structure and biol, properties. So, for example, L-dopa in c. n. With. penetrates easily, and D-dopa and dopamine are delayed. Selectivity G.-e. b. during the transition of substances from the blood into the cerebrospinal fluid and c. n. With. much more pronounced than during the transition from the cerebrospinal fluid into the blood. G.-e. b. in this case similar to a selective filter in the direction of blood - c. n. With. or a safety valve in the opposite direction (L. S. Stern and Gauthier, 1918).

According to modern concepts, G.-e. b. is a self-regulating system, the state of a cut depends on the needs of nerve cells and the level of metabolic processes not only in the brain itself, but also in other organs and tissues of the body. G.'s permeability - e. b. regulated by nerves and humoral mechanisms. At the same time, there is still no theory that fully explains the regularity of the transition of various substances from the blood into the cerebrospinal fluid and brain tissue.

Studying of G.'s protective function - e. b. It has special meaning for identification of a pathogeny and in therapy of diseases of c. n. With. Reducing the permeability of the barrier promotes penetration into c. n. With. not only foreign substances, but also products of disturbed metabolism; at the same time increase in G.'s resistance - e. b. closes (partially or completely) the way to protective bodies, hormones, metabolites, mediators. G.'s extremely limited permeability - e. b. in relation to some chemotherapeutic drugs used in wedges, practice (compounds of arsenic, bismuth, mercury, etc.), antibiotics (eg, penicillin, streptomycin), antibodies (antitoxins, agglutinins, hemolysins) is often an obstacle in the treatment diseases c. n. With. Suggested various methods increase in G.'s permeability - e. b. (overheating or hypothermia of the body, exposure to x-rays, malaria vaccination, etc.), but they are not always effective. In these cases introduction pharmakol is possible. drugs, treatment serums, biologically active substances directly into the cerebrospinal fluid (lumbar or suboccipital injection according to Stern).

For studying of G.'s function - e. b. are usually used substances that penetrate into the cerebrospinal fluid and the brain in small quantities. For this purpose, in experiments on animals, acidic (primarily trypan blue) or basic dyes, salts of hydroiodide, picric or salicylic acid and determine their content (quantitative or qualitative test) in the cerebrospinal fluid and brain tissue. Wide application found methods of an autoradiography (see), gistol., chemistry, electron microscopy. ; In a wedge, practice bromine, iodine, salicylic, nitrate, uranine, hemolysin, glucose and other methods of a research of G. are offered. b. According to Walter (F. Walter, 1929), the substances used for this purpose must meet the following requirements: distributed in the blood and cerebrospinal fluid before they are released, do not break down in the body and do not bind to proteins; they should not change G.'s state - e. b. and harm the body. An indicator that can be accurately quantified should be chosen.

With the known precautions for a research of a condition of G. - e. b. radioisotope method can also be used in humans.

See also Barrier functions, Cerebrospinal fluid.

Bibliography: Kassil G. N. Hemato-brain barrier, M., 1963; Stern L. S. Direct nutrient medium of organs and tissues, Physiological mechanisms, which determine its composition and properties, M., 1960; In a k a in L. The blood-brain barrier, with special regard to the use of radioactive isotopes, Springfield, 1956; Brain-barrier systems ed. by A. Lajtha, Amsterdam, 1968; Dob-b i n g J. The blood-brain barrier, Physiol. Rev., v. 41, p. 130, 1961; Handbook of physiology, sec. 1 - Neurophysiology, ed. by J. Field a. o., v. 3, Washington, 1960.

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 body's homeostasis and individual bodies. Thanks to the presence histohematic barriers each organ lives in its own special environment, which can differ significantly from the composition of individual ingredients. Particularly strong barriers exist between the brain, the blood and tissue of the gonads, the blood and moisture of the chambers of the eye, and the blood of the mother and fetus.

Histohematic barriers of various organs have both differences and a number of common features buildings. Direct contact with blood in all organs has a barrier layer formed by the endothelium of the blood capillaries. In addition, the structures of HGB 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 histohematological barriers are further considered on the example of the blood-brain barrier, the presence and properties of which the doctor especially often has to take into account when applying medicines 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 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 biological active substances(cytokines,) 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 in various conditions. Thus, the blood-brain barrier is able, by regulating the 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 permeability cell membranes, states intercellular substance, 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, ethanol, 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.

A person is occupied with injuries. And only a small part of the lesions are caused directly by diseases of the central nervous system.

Due to some of its features, the nervous system is very interesting from the point of view of science. The thing is that anatomy is extremely difficult to understand. Forming its basis nerve fibers have their own, different from other tissues of the human body, structure.

One of the main features is the extremely low ability to regenerate. This is not to say that damaged nerves do not recover, but their recovery is very slow and requires certain conditions.

Another feature of the nervous system in general, and the central nervous system in particular, is the blood-brain barrier (BBB).

It's no secret that the head and spinal cord are in a special liquid, similar in composition to but differing from it in the content of various fractions of proteins and microelements. The cerebrospinal (or cerebrospinal) fluid is formed from the blood and lymph under the action of a special "filter", the role of which is performed by the blood-brain barrier.

Special cages with interendothelial contacts prevent penetration into this fluid. Today, scientists have not fully figured out how the regulation of the filtering ability of the barrier occurs, but it is reliably known that its throughput changes with changes in the metabolic activity of the brain. In addition, the blood-brain barrier has differences in different departments brain, which determines its different ability to filter fluids (blood and lymph).

Studies have shown that some of the substances penetrate the BBB mainly from blood vessels, another part of them - from the system, and the rest are able to come from both environments at the same rate. Own, unique and unexplored so far, the system of self-regulation of the composition of the cerebrospinal fluid ensures the supply of substances in the amount that the central nervous system needs. This happens with the regulation of the volume of the liquid part, the amount and composition of proteins, as well as the composition of incoming ions (the latter are represented by potassium and sodium).

What is the blood-brain barrier for?

First of all, its action is aimed at creating a relatively isolated environment for the central nervous system, but it also performs a protective function, preventing the penetration of bacteria and viruses into the cerebrospinal fluid from the blood or lymph flow. It is important to understand that in case of violations in the functioning of the BBB, the consequences will be very serious. So, bacteria that have penetrated into the cerebrospinal fluid lead to meningitis, encephalitis and other inflammatory processes meninges and brain tissues.

A number of studies conducted by experts have demonstrated the ability to influence throughput blood-brain barrier various drugs. In addition, the previously used medicines began to identify this feature. Today, doctors are well aware of what medications and how they affect the BBB. Moreover, we have learned to use these properties for the benefit of man.

Thus, the blood-brain barrier performs a number of very significant functions that support the optimal condition of the internal organs of the human body. However, it should be understood that such features of the barrier make it very sensitive both to injuries and to various pathological conditions, which is why it is so important to understand and take into account these aspects in the prevention and treatment of diseases.