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The skin is like a respiratory organ. The main functions of the skin (protective, thermoregulatory, excretory, respiratory)

The main functions of the skin: providingprotective barrier between the body and the environment, including protection against mechanical damage, radiation, chemical irritants, bacteria, and immune, receptor. thermoregulatory, metabolic, resorption, secretory, excretory, respiratory.

Protective function of the skin includes mechanical protection against external influences.

The mechanical protection of the skin from pressure, bruises, ruptures, stretching, etc. is due to the density of the epidermis capable of repair, the elasticity and mechanical stability of the fibrous structures of the connective tissue of the dermis, and the buffering properties of the subcutaneous fatty tissue. The most important role in the implementation of the protective mechanisms of the skin belongs to the epidermis. The strength of its important component - the stratum corneum - is provided by proteins and lipids, and the elasticity is provided by proteins, lipids and low-molecular decomposition products of keratohyalin, which bind and retain water in the stratum corneum. In contrast, the dermo-epidermal junction in human skin is a relatively weak point. This explains the slight damage to the surface collagen of the papillary dermis in bullous dermatoses. The resistance of the skin to tearing in response to blunt force is predominantly associated with the dermis. At the same time, the elasticity of the skin is due to the straightening of collagen fibers along the axis of tension, and the return to its original state is due to elastic fibers. Violation of the structure of collagen fibers leads to excessive extensibility of the skin. The ability of the skin to compress with the formation of a fossa when a small object is pressed into the skin is due to the outflow of the intercellular gluing substance between the collagen fibers of the dermis.

Protection of the skin from radiation effects is realized primarily by the stratum corneum, which completely blocks infrared rays, and partially ultraviolet rays. Depending on the wavelength and biological effect on the body, there are: UV-A (320-400 nm), UV-B (290-320 nm) and UV-C (200-290 nm). UV-B affects mainly at the level of the epidermis, is the main cause of sunburn, premature skin aging, and in the future - precancer and skin cancer. UV-A can penetrate deep into the dermis, has the least erythematous potential, but can provoke increased sensitivity to the sun, and also play an important role in skin aging. The skin has two barriers that prevent the damaging effects of UV radiation: 1) a melanin barrier in the epidermis and 2) a proteoglycan barrier concentrated in the stratum corneum. The action of each of them is aimed at reducing its absorption by DNA and other components of the cell. Melanin is a large polymer capable of absorbing light in a wide wavelength range from 200 to 2400 nm and thereby protecting cells from the harmful effects of excessive insolation. Melanin is synthesized by melanocytes in the basal layer of the epidermis and transported to adjacent keratinocytes in melanosomes. Melanin synthesis is also influenced by the pituitary melanostimulating hormone. The protective mechanism of sunburn is associated with an increase in the number of functional melanocytes, an increase in the number of synthesized melanosomes and the rate of transfer of melanosomes to keratinocytes, as well as with the transition of the product of histidine metabolism in the epidermis - urocanic acid from the trans isomer to the cis isomer. Chronic exposure to sunlight over time leads to thickening of the epidermis, the development of solar elastosis and keratosis, precancer or skin cancer.

The normal stratum corneum of the skin provides protection against chemical irritants mainly due to keratin. Only chemicals that destroy the stratum corneum, as well as those soluble in the lipids of the epidermis, gain access to the deeper layers of the skin and then can spread throughout the body through the lymphatic and blood vessels.

Human skin serves as a natural and permanent habitat for numerous microorganisms: bacteria (Staphylococcus epidermidis diphteroidus, Propionbacterium acnes, Pityrosporum, etc.), fungi and viruses, since its surface contains many fatty and protein ingredients that create favorable conditions for their vital activity. At the same time, it is impenetrable to a variety of bacteria and pathogenic microorganisms, which rarely get on its surface.

The bactericidal property of the skin, which gives it the ability to resist microbial invasion, is due to the acidic reaction of keratin, the peculiar chemical composition of sebum and sweat, the presence on its surface of a protective water-lipid mantle with a high concentration of hydrogen ions (pH 3.5–6.7). The low molecular weight fatty acids included in its composition, primarily glycophospholipids and free fatty acids, have a bacteriostatic effect that is selective for pathogenic microorganisms. The mechanical obstacle to the invasion of pathogenic microorganisms into the skin, in addition to the integrity of the stratum corneum, is ensured by their removal with scales, the secretion of the sebaceous and sweat glands. On 1 cm2 of healthy human skin, there are from 115 thousand to 32 million different microorganisms, most of which belong to the permanent bacterial flora, which plays an important role in the antimicrobial protection of the skin and mucous membranes from pathogenic microorganisms. The ability of the skin to resist microbial invasion is reduced when the skin is traumatized. At the same time, the same microorganisms with a different nature of injury can cause different pathological processes. Thus, group A streptococci cause erysipelas after mechanical trauma to the epidermis or violation of its integrity due to the intertriginous form of mycosis of the feet, while streptococcal impetigo usually occurs at the site of scratching in atopic dermatitis.

The bactericidal properties of the skin are also reduced under the influence of skin pollution, hypothermia, overwork of the body, insufficiency of the gonads; they are also reduced in patients with skin diseases and in children. In particular, in infants, this is due to the tenderness and looseness of the stratum corneum of the epidermis, the morphological inferiority of elastic and collagen fibers, as a result of which children's skin is easily exposed to mechanical, radiation, thermal and chemical irritations. The survival of pathogenic microbial flora on the surface of the skin is also facilitated by a slightly alkaline or neutral environment of the water-lipid mantle with an insufficient amount of low molecular weight free fatty acids. The penetration of microbes through the upper layers of the epidermis is accompanied by the migration of leukocytes from the vessels and their penetration into the dermis and epidermis with the formation of a protective inflammatory reaction.

immune function. The skin plays an important role in immune processes. The main elements of the immune system of the skin arekeratinocytes, Langerhans cells, epidermal T-lymphocytes. Keratinocytes promote the maturation of T-lymphocytes by exposing them to the enzyme deoxynucleotidyl transferase. Most human skin T-lymphocytes are located in the dermis, usually around postcapillary venules and skin appendages. The share of intraepidermal T-lymphocytes is less than 10%. T-lymphocytes are able to recognize exogenous and endogenous antigens only after they are presented by antigen-presenting Langerhans cells, or helper cells. T cells recognize antigen only in a single structure with MHC. For recognition by T‑helper lymphocytes (CD4+), the antigen must be presented in combination with MHC class II (HLA‑DR, DP, DQ), while most T‑suppressor lymphocytes (CD8+) recognize the antigen in association with class I MHC molecules (HLA‑ A, B, C). During the immune response to exogenous or endogenous antigens, Langerhans cells involved in antigen presentation undergo phenotypic and functional changes, leave the epidermis and enter the lymphatic vessels of the dermis, and from there migrate to the paracortical layer of the lymph nodes. At this stage, Langerhans cells present an antigen located on their surface - MHC-complex to the T-cell antigen receptor on the surface of CD4+/CDD8- or CD4-/CD8+ T-cells. The antigen-specific T-cell response consists in the formation of blast forms of T-lymphocytes that return to areas of the skin containing the antigen.

Immune disorders play a pathogenetic role in various skin diseases, including bullous dermatoses, allergic dermatosis, psoriasis, and malignant T-cell lymphoma of the skin.

Receptor function of the skin It is realized by numerous nerve receptors that perceive pain, tactile (touch, pressure, vibration) and temperature (thermal, cold) irritation.

The skin is a huge receptor field, functionally connected through myelinated (A-fibers ) or unmyelinated (C-fiber ) sensory nerves with a central and autonomic nervous system and constantly responding to various stimuli from the environment, the central nervous system and internal organs.

Nerve endings are dispersed unevenly throughout the skin and are polyvalent in their function.

There are two types of functionally specific afferent units:mechanoreceptors and thermoreceptors , third -pain receptors - responds only to stimulation above the threshold (mechanical, thermal or chemical).

Only some of the receptors that differ functionally can be identified morphologically. Touch is perceived by mechanoreceptors located in the skin. Among them, hair follicle receptors are isolated on the skin covered with hair; on hairless skin (palms and soles). - located in the upper part of the dermis, fast-reacting Meissner bodies and slow-reacting Merkel receptors; in the dermis and subcutaneous tissue - Ruffini bodies; heat and cold are perceived by thermoreceptors.

Cold receptors are activated at a temperature approximately 1-20°C below normal skin temperature (34°C); thermal - at temperatures ranging from 32 to 35 ° C (at temperatures above 45 ° C, thermal pain is perceived not through thermal receptors, but through nociceptors).

Pain is mediated by nociceptors responsible for the perception of pain and itching, selectively responding to stimuli that can damage tissue. There are mechanical, thermal and polymodal (responding to several types of harmful effects, including mechanical, thermal and chemical) nociceptors. In particular, mechanical nocicentres are activated by sharp objects and are initially felt as a prick or quick, point, superficial and local pain, and then as a more diffuse burning or slow pain. The threshold for the perception of pain from heat is 45 °C.

Peripheral nerves, in addition to classical neurotransmitters such as norepinephrine and acetylcholine, contain neuropeptides that are released from nerve endings during depolarization and play a role in the regulation of synaptic transmission. A variety of neuropeptides have been found in human skin, including substance P, vasoactive intestinal peptide, somatostatin, calcitonin gene-related peptide, neuropeptide V, and bombesin. Neuropeptides not only act as neurotransmitters but also play a role in mediating skin inflammation.

Itching, like pain, is a nociceptive sensation perceived by cortical centers in response to exogenous and endogenous factors. It is closely related to pain, but unlike it, it occurs in the skin, and not in the internal organs. According to some researchers, it is a modified sensation of pain, and not an independent sensation. Itching and pain are conducted along unmyelinated C-fibers originating from the upper dermis of both the skin and mucous membranes. As a sensation, itching of the skin is a cortical process that occurs when irritants are exposed to the perceiving nervous apparatus, which consists of three sections: peripheral, embedded in the skin, central - in the upper sections of the central nervous system, and conductive, connecting both of these sections.

Thermoregulatory function of the skin carried out by the absorption and release of heat by the skin. Heat transfer through the surface of the skin is carried out by radiation, conduction, convection and evaporation. The implementation of the mechanisms of heat radiation in the form of energy of infrared rays and conduction, i.e. heat transfer in contact with the environment, occurs by changing the blood flow in the skin. Due to the higher vascularization of the skin, which significantly exceeds its need for nutrition, an increase in ambient temperature leads to an expansion of the skin vessels, an increase in the volume of blood flowing through it (sometimes up to 1 liter) and an increase in heat transfer. With a decrease in external temperature, the vessels narrow, a large mass of blood circulates through the internal organs, and heat transfer decreases sharply. An important role in thermoregulation is played by the system of arteriovenous shunts, especially in the acral regions (feet, hands, lips, nose, auricles), where the concentration of these shunts is highest and is controlled by noradrenergic sympathetic nerves. A decrease in sympathetic tone causes vasodilation of the skin. The skin becomes warmer than the surrounding air and increases heat transfer by convection, in which it gives off heat, heating the adjacent layer of air, which rises and is replaced by colder air. Sympathetic activity also regulates the diameter of arteriovenous anastomoses of the distal extremities. Heat transfer by radiation and convection is called "dry heat transfer", which accounts for up to 20–25% of heat transfer.

The most efficient way to release heat is to evaporate the sweat generated. Sweating is regulated by the central nervous system (psychogenic sweating) and cholinergic sympathetic fibers, so parasympathomimetic substances (acetylcholine, pilocarpine, etc.) increase sweat secretion, and atropine, blocking this mechanism, inhibits sweating. The hypothalamus, in response to temperature changes, receives impulses from central and peripheral (skin) thermoreceptors. Heat and cold thermoreceptors are located on heat and cold thermoreceptor cells unevenly scattered throughout the body. The strongest stimulus for the appearance of sweat is an increase in body temperature, while skin thermoreceptors are 10 times less effective. The temperature factor regulates mainly the activity of the sweat glands of the trunk, rear of the hands, neck, forehead, and nasolabial folds. Despite the fact that skin thermoreceptors do not play an important role in changing body temperature, changes in skin temperature have an impact on human life. In particular, its reduction requires the use of warmer clothes, room heating, etc.

Heat transfer of the skin in a number of dermatoses is significantly impaired. In particular, in psoriasis, toxidermia, mycosis fungoides, Cesari's syndrome, the inflammatory reaction of the skin can lead to generalized skin vasodilation involving up to 10–20% of circulating blood into the skin bloodstream.

The exchange function of the skin unitessecretory, excretory, resorption and respiratory activity . The skin is involved in the metabolism of carbohydrates, proteins, lipids, water, minerals and vitamins. In terms of the intensity of water, mineral and carbon dioxide metabolism, the skin is only slightly inferior to the liver and muscles. It accumulates and releases large amounts of water much faster and easier than other organs. The processes of metabolism and acid-base balance depend on human nutrition (for example, when acidic foods are abused, the sodium content in the skin decreases) and other factors. The skin and subcutaneous adipose tissue are powerful depots of nutrients consumed during the fasting period.

resorption function of the skin. The skin is a multilayered membrane with three anatomically distinct layers: the stratum corneum, 10 µm thick, the germ (Malpighian) layer, 100 µm thick, and the papillary dermis, 100‑200 µm thick; each of them has different diffusion constants. Even healthy skin has some permeability for almost any substance, and the levels of penetration of various substances can vary by 10 thousand times. The degree of skin resistance is different for water- and fat-soluble chemicals, for compounds with small and large molecular weight. It differs depending on the localization of the skin area, the thickness of the stratum corneum, the degree of its hydration, the presence or absence of lipid lubrication of the skin and its qualitative composition. Many chemicals enter the skin through the relatively impermeable stratum corneum (transdermal route) and remain there for long periods of time. Some chemicals with small molecular sizes can penetrate through the hair follicles, as well as the excretory ducts of the sebaceous and sweat glands. A significant increase in the permeability of the skin occurs after its treatment with organic solvents (acetone, chloroform, etc.), which lead to a local decrease in the amount of lipids. Upon contact of the skin with water, not only part of the lipid mantle is removed, but also the barrier functions of the skin change as a result of its hydration, which also leads to an increase in its permeability. The composition of the chemical substance significantly affects the permeability. Fats and substances dissolved in them penetrate better through the skin. The permeability of the skin also changes with the development of dermatoses; substances that have not previously penetrated the stratum corneum of intact skin begin to freely overcome this barrier. As for the delivery of drugs by the transdermal route, its advantage over their administration through the mouth or parenterally is due to the fact that this route does not depend on the pH value, stomach contents, time after eating, etc. The drug with this method of administration can be delivered directly to the affected organ, and its dosage eliminates large concentration fluctuations, as with parenteral administration. It should be especially noted that most parenteral drugs do not have a pronounced ability to accumulate selectively in the skin. i.e., they are not dermatotropic. Attempts to increase the concentration of the drug in the skin by increasing its parenteral doses leads to an increase in the frequency of side effects. Local application of drugs is devoid of such disadvantages.

secretory function carried out by sebaceous and sweat glands Sebum is a complex fatty substance of a semi-liquid consistency, which includes free lower and higher fatty acids, associated fatty acids in the form of cholesterol esters and other stearins and high molecular weight aliphatic alcohols and glycerin, small amounts of hydrocarbons, free cholesterol , traces of nitrogenous and phosphorus compounds. The sterilizing effect of sebum is due to the significant content of free fatty acids in it. The function of the sebaceous glands is regulated by the nervous system, as well as hormones of the endocrine glands (sex, pituitary and adrenal cortex). On the surface of the skin, sebum, mixing with sweat, forms a thin film of water-fat emulsion, which plays an important role in maintaining the normal physiological state of the skin.

excretory function combined with secretory and is carried out by the secretion of sweat and sebaceous glands. The amount of organic and inorganic substances released by them, mineral metabolism products, carbohydrates, vitamins, hormones, enzymes, microelements and water depends on gender, age, topographic features of the skin. In case of insufficiency of liver or kidney function, the excretion through the skin of substances that are usually removed with urine (acetone, bile pigments, etc.) increases.

Respiratory function of the skin It takes in oxygen from the air and releases carbon dioxide. Skin respiration intensifies with an increase in ambient temperature, during physical work, during digestion, the development of acute inflammatory processes in the skin, etc .; it is closely related to redox processes and is controlled by enzymes, the activity of sweat glands rich in blood vessels and nerve fibers.

Skin deficiency is a condition associated with severe loss or dysfunction of the skin (similar to the failure of other systems - cardiovascular, respiratory, renal, hepatic, etc.). Skin deficiency is the loss of normal control over thermoregulation, water-electrolyte and protein balance of the body, loss of mechanical, chemical and microbial barrier. It requires special treatment as an emergency and, in addition to thermal burns, can occur with Lyell and Stevens-Johnson syndromes, pustular psoriasis, erythroderma, pemphigus vulgaris, graft-versus-host disease, epidermolysis bullosa.

This complex and important organ plays a huge role in the human body. Without healthy skin, good health and appearance are unthinkable. What are the functions of the skin and what is its purpose, read further in the article.

What are the functions of the skin?

The main functions of the skin:

providing a protective barrier between the body and the environment, including protection against mechanical damage, radiation, chemical irritants, bacteria,

as well as the immune function of the skin,

receptor,

thermoregulatory function of the skin,

metabolic function of the skin,

resorption,

secretory,

excretory function of the skin,

respiratory.

Protective function of the skin

Protective function of the skin includes mechanical protection against external influences.

The mechanical protection of the skin from pressure, bruises, ruptures, stretching, etc. is due to the density of the epidermis capable of repair, the elasticity and mechanical stability of the fibrous structures of the connective tissue of the dermis, and the buffering properties of the subcutaneous fatty tissue. The most important role in the implementation of the protective function of the skin belongs to the epidermis. The strength of its important component - the stratum corneum - is provided by proteins and lipids, and the elasticity is provided by proteins, lipids and low-molecular decomposition products of keratohyalin, which bind and retain water in the stratum corneum. In contrast, the dermo-epidermal junction in human skin is a relatively weak point. This explains the slight damage to the surface collagen of the papillary dermis in bullous dermatoses. The resistance of the skin to tearing in response to blunt force is predominantly associated with the dermis. At the same time, the elasticity of the skin is due to the straightening of collagen fibers along the axis of tension, and the return to its original state is due to elastic fibers. Violation of the structure of collagen fibers of the skin function leads to excessive extensibility of the skin. The ability of the skin to compress with the formation of a fossa when a small object is pressed into the skin is due to the outflow of the intercellular gluing substance between the collagen fibers of the dermis.

The protective function of the skin and its barriers

In the protective function of the skin, there are two barriers that prevent the damaging effects of UV radiation:

melanin barrier in the epidermis

proteoglycan barrier concentrated in the stratum corneum.

The action of each of them is aimed at reducing its absorption by DNA and other components of the cell. Melanin is a large polymer capable of absorbing light in a wide wavelength range from 200 to 2400 nm and thereby protecting cells from the harmful effects of excessive insolation. Melanin is synthesized by melanocytes in the basal layer of the epidermis and transported to adjacent keratinocytes in melanosomes. Melanin synthesis is also influenced by the pituitary melanostimulating hormone. The protective function of sunburn is associated with an increase in the number of functional melanocytes, an increase in the number of synthesized melanosomes and the rate of transmission of melanosomes to keratinocytes, as well as with the transition of the product of histidine metabolism in the epidermis - urocanic acid from the trans-isomer to the cis-isomer. Chronic exposure to sunlight over time leads to thickening of the epidermis, the development of solar elastosis and keratosis, precancer or skin cancer.

The bactericidal function of the skin

The bactericidal function of the skin, which gives it the ability to resist microbial invasion, is due to the acidic reaction of keratin, the peculiar chemical composition of sebum and sweat, the presence on its surface of a protective water-lipid mantle with a high concentration of hydrogen ions (pH 3.5–6.7). The low molecular weight fatty acids included in its composition, primarily glycophospholipids and free fatty acids, have a bacteriostatic effect that is selective for pathogenic microorganisms. The mechanical obstacle to the invasion of pathogenic microorganisms into the skin, in addition to the integrity of the stratum corneum, is ensured by their removal with scales, the secretion of the sebaceous and sweat glands. On 1 cm2 of healthy human skin, there are from 115 thousand to 32 million different microorganisms, most of which belong to the permanent bacterial flora, which plays an important role in the antimicrobial protection of the skin and mucous membranes from pathogenic microorganisms. The ability of the skin to resist microbial invasion is reduced when the skin is traumatized. At the same time, the same microorganisms with a different nature of injury can cause different pathological processes. Thus, group A streptococci cause erysipelas after mechanical trauma to the epidermis or violation of its integrity due to the intertriginous form of mycosis of the feet, while streptococcal impetigo usually occurs at the site of scratching in atopic dermatitis.

The bactericidal functions of the skin of the skin are also reduced under the influence of skin pollution, with hypothermia, overwork of the body, insufficiency of the sex glands; they are also reduced in patients with skin diseases and in children. In particular, in infants, this is due to the tenderness and looseness of the stratum corneum of the epidermis, the morphological inferiority of elastic and collagen fibers, as a result of which children's skin is easily exposed to mechanical, radiation, thermal and chemical irritations. The survival of pathogenic microbial flora on the surface of the skin is also facilitated by the slightly alkaline or neutral environment of the water-lipid mantle with an insufficient amount of low molecular weight free fatty acids. The penetration of microbes through the upper layers of the epidermis is accompanied by the migration of leukocytes from the vessels and their penetration into the dermis and epidermis with the formation of a protective inflammatory reaction.

Secretory function of the skin

secretory function carried out by sebaceous and sweat glands Sebum is a complex fatty substance of a semi-liquid consistency, which includes free lower and higher fatty acids, associated fatty acids in the form of cholesterol esters and other stearins and high molecular weight aliphatic alcohols and glycerin, small amounts of hydrocarbons, free cholesterol , traces of nitrogenous and phosphorus compounds. The sterilizing functions of sebum are due to the significant content of free fatty acids in it. The function of the sebaceous glands is regulated by the nervous system, as well as hormones of the endocrine glands (sex, pituitary and adrenal cortex). On the surface of the skin, sebum, mixing with sweat, forms a thin film of water-fat emulsion, which plays an important role in maintaining the normal physiological state of the skin.

excretory function of the skin

excretory function combined with the secretory function of the skin and is carried out by the secretion of sweat and sebaceous glands. The amount of organic and inorganic substances released by them, mineral metabolism products, carbohydrates, vitamins, hormones, enzymes, microelements and water depends on gender, age, topographic features of the skin. In case of insufficiency of liver or kidney function, the excretion through the skin of substances that are usually removed with urine (acetone, bile pigments, etc.) increases.

Respiratory function of the skin

The respiratory function of the skin is to absorb oxygen from the air and release carbon dioxide. Skin respiration intensifies with an increase in ambient temperature, during physical work, during digestion, the development of acute inflammatory processes in the skin, etc .; it is closely related to redox processes and is controlled by enzymes, the activity of sweat glands rich in blood vessels and nerve fibers.

Insufficiency of skin functions

Insufficiency of skin functions is a condition associated with severe loss or dysfunction of the skin (similar to the insufficiency of other systems - cardiovascular, respiratory, renal, hepatic, etc.). Skin deficiency is the loss of normal control over thermoregulation, water-electrolyte and protein balance of the body, loss of mechanical, chemical and microbial barrier. Insufficiency of skin function requires special treatment as an emergency and, in addition to thermal burns, can occur with Lyell and Stevens-Johnson syndromes, pustular psoriasis, erythroderma, pemphigus vulgaris, graft-versus-host disease, epidermolysis bullosa.

The skin is the outer covering of the body and performs a complex set of physiological functions. It is actively involved in the process of metabolism, especially water, mineral, fat, carbohydrate, vitamin and energy. The skin is a huge depot of carbohydrates, toxins, circulating immune complexes, antigens, antibodies and other products of general and tissue metabolism. Participating in all vital processes of the body, the skin performs a number of important special functions. functions: immune, protective, secretory, receptor, etc.

The skin is an immune organ. Healthy skin and intact mucous membranes are a barrier to most microorganisms, with the exception of those with a special penetration apparatus. This protective function of the skin was previously explained only by mechanical factors - the stratum corneum, water-lipid mantle, high elasticity and subcutaneous fatty tissue. However, at present, there is information about the immune activity of the main structures of the skin that implement the immune response: the epidermis, dermis, and subcutaneous fatty tissue.

Due to the fact that T-lymphocytes are the main element of the immune system, the anatomical, molecular and functional similarity of epidermal keratinocytes with epithelial cells of the thymus has been proven. These include epidermal thymocyte-activating factor (ETAF), interleukins-1, 2 (T-cell growth factors), interleukin-3 (mast cell proliferation and degranulation factor), natural killer activating factor (FANK), epidermal granulocyte activity factor . In addition to them, keratinocytes produce a number of nonspecific mediators, biologically active factors involved in the immune and inflammatory reactions of the skin. Among them, the most studied are fatty acid metabolites (prostaglandins, leukotrienes, fatty acid hydroxides), plasminogen activator and inhibitor.

Keratinocytes promote the maturation of T-lymphocytes by the action of deoxynucleotidyltransferase. epidermal cells

able to induce the expression of this enzyme, as well as the secretion of thymopoietin in the process of T-lymphocyte differentiation. The important role of epidermal cells in immune processes in the skin is also confirmed by their ability to express immunoassociative antigens (HLA-DR) on their surface. Some researchers believe that these receptors facilitate the migration of white process epidermocytes into the skin, others believe that with their help, keratinocytes can present antigen and interact directly with lymphocytes.

The similarity of keratinocytes to thymic epithelial cells is confirmed by common heteroantigens found in the basal cells of the epidermis and the hormonal epithelium of the thymus. The common morphological features of these organs were established during the cultivation of the thymus epithelium. It turned out that thymus cells, when cultivated in the medium, turn into typical epidermal keratinocytes. Subsequently, an antigen characteristic of the cells of the basal layer of the epidermis was found in the receptors of the thymus bodies (Hassal bodies). In the deeper structures of the thymus bodies, antigens characteristic of the prickly, granular and stratum corneum of the epidermis were identified, which allows us to consider the epidermis as an organ functionally similar to the thymus gland.

In the dermis, immune activity is mediated by lymphocytes around postcapillary venules of the superficial choroid plexus and skin appendages. Immunomorphological methods have established that T-lymphocytes make up 90% of all skin lymphocytes and are located mainly in the epidermis and upper layers of the dermis. B-lymphocytes are found in the middle and deep layers of the dermis. The lymphocytes of the perivascular areas consist of almost the same number of helpers and suppressors, and the helper-suppressor index is 0.93-0.96. Most of these cells are in an activated form, which is confirmed by the detection of immunoassociative antigens (HLA-DR) and interleukin-2 receptors on their surface.

Endothelial cells of postcapillary venules of the superior vascular plexus and the macrophage system play a significant role in the development and formation of skin immune responses. The macrophage system is represented in the dermis and subcutaneous adipose tissue by fibroblasts, phagocytic macrophages (histiocytes), and dendritic cells. Morphologically differentiated tissue histiocyte is a process cell with a large number of

microvilli. Histiocytes contain RNA and enzymes in the cytoplasm. On the surface of histiocytes, like all macrophages, there are receptors for C3 and the Fc fragment of lgG. The macrophage system of the skin also includes mast cells involved in the migration of T-lymphocytes in antigen-antibody reactions of the type of immediate hypersensitivity. The implementation of immune processes in the skin also involves blood cells migrating into the skin (monocytes, eosinophils, neutrophils, basophils, erythrocytes), which perform various immune functions, the basis of which is the interaction of T-lymphocytes with nonspecific defense factors.

The immune function is also performed by white process epidermocytes, which are an altered variety of the population of tissue macrophages. Like mast cells, fibrocytes and macrophages, these cells do not have immune specificity, but when activated by antigens or cytokines, they exhibit physiological activity with the release of biologically active substances.

protective function. The barrier properties of the skin as an organ of mechanical protection are provided by significant electrical resistance, the strength of collagen and elastic fibers, and elastic subcutaneous fatty tissue. The skin is protected from drying out by a compact stratum corneum and a water-lipid mantle located on the surface of the skin. The stratum corneum is resistant to many chemical and physical damaging effects.

The protective function of the skin against microbial flora is very important. This is facilitated by the rejection of keratinized epithelium and the secretion of sebaceous and sweat glands. In addition, the skin has sterilizing properties due to the acidic reaction of the water-lipid film, which simultaneously inhibits the absorption of foreign substances. At the same time, the water-lipid mantle of the skin prevents the penetration of microorganisms, and the low molecular weight fatty acids contained in it have a depressing effect on the growth of pathogenic flora (“its own sterilizer”).

Chlorides are present in the skin in a significant amount, more than 2 times the content of this anion in muscle tissue. It is believed that this is a means of protection against pathogenic microorganisms. In the presence of myeloperoxidase, localized in the azurophilic granules of neutrophils and monocytes, hypochlorite is formed from chlorine and hydrogen peroxide, which destroys the structure of the microbial membrane, which leads to the death of the organism.

The protective function of the skin is also carried out by proteoglycans, which consist of polysaccharides (95%) and protein (5%) units. These polyanions, which are very large in size, bind water and cations, forming the basic substance of the connective tissue. Proteoglycans act as a molecular sieve for substances diffusing in the extracellular matrix: small molecules penetrate the network, while large ones are retained.

The mucous membrane of the mouth, whose structure is similar to the structure of the skin, also performs protective functions, although to a lesser extent. This is facilitated by the constant wetting of the oral mucosa with saliva, which leads to its supersaturation with water, a decrease in the sweating of the interstitial fluid, and thus makes it difficult for the penetration of microbial flora and foreign substances. The bactericidal properties of lysozyme contained in saliva enhance the protective role of the oral mucosa.

Under the influence of high-energy ultraviolet rays of the sun, free radicals form in the skin. Such molecules easily enter into chemical reactions, including chain ones. Violation of the function of biological membranes, built mainly of proteins and lipids, is one of the most important biological effects of ultraviolet rays. Protection of the body from the damaging effects of the ultraviolet rays of the sun, which lie outside the light visible to the human eye (less than 400 nm), is carried out using several mechanisms. The stratum corneum thickens in the skin, skin pigmentation increases, urocanic acid passes from the trans-isomer to the cis-isomer, enzymatic and non-enzymatic systems of antiradical protection are mobilized. The shielding layer of the pigment either absorbs light of all wavelengths or filters out particularly dangerous rays. Melanin, in particular, absorbs visible light and ultraviolet rays in the entire range.

The more melanin in the skin, the more fully it provides protection from rays harmful to the body. In the skin, a rapid renewal of melanin occurs, which is lost during the desquamation of the epidermis, and then re-synthesized by melanoblasts. The synthesis of melanin is influenced by the hypozysis hormone (melanin-stimulating hormone), an important role is played by tyrosinase, which catalyzes the oxidation of tyrosine, and doxyphenylalanine (DOPA). Biochemical mechanisms of antioxidant defense provide inhibition of free radical reactions at the stages of initiation, branching and termination of oxidation chains.

secretory function. This function is carried out as a result of the secretory activity of keratinocytes, immunoregulatory cells, as well as the functional activity of the sebaceous and sweat glands.

The formation of keratin - the main protein of the epidermis - is a complex secretory process, it is carried out by keratinocytes. The initial stage takes place in the cells of the basal layer, where keratin fibrils appear in the form of tonofilaments. In the cells of the spiny layer, the protein of tonofilaments is converted into α-keratin, similar to prekeratin - actomyosin.

More specific structures are observed in the cells of the granular layer. Keratohyalin granules appear in them, which contain fibrils. Fibrils turn into eleidin, and then into filaments of keratin, which forms the basis of the cells of the stratum corneum. As cells move from the basal layer to the upper layers of the epidermis, the nuclei and other cell organelles keratinize into tonofilaments, which gradually form protoplasm protein into keratin.

The growth and reproduction of epidermal cells under normal physiological conditions are influenced by complex mutually competing extracellular and intracellular factors. Intracellular mediators mediating the action of hormones and other biologically active substances on cell mitosis include cyclic nucleotides, prostaglandins, chalones, leukotrienes, interleukins (especially IL-1 and IL-2) and calcium ions, which affect the activity of phosphodiesterase and cAMP to cGMP ratio. The epidermal growth factor significantly affects the intracellular control of mitosis. This polypeptide has a hyperplastic effect on epithelial tissues. Its activity depends on the function of the pituitary-adrenal system.

Thus, the state of a complex physiological system - corticosteroid hormones and adrenaline in cooperation with intracellular mediators, including phosphodiesterase, adenylate cyclase, cAMP and cGMP - determines the activity of the epidermal growth factor and its effect on the secretion of keratin by epidermocytes. An important role in the implementation of the secretory function of the skin is played by the sebaceous and sweat glands.

The sebaceous glands produce sebum, which is composed of fatty acids, cholesterol esters, aliphatic alcohols, small amounts of carbohydrates, free cholesterol, glycerol, and small amounts of nitrogenous and phosphate compounds. In the sebaceous glands

the secret is in a liquid or semi-liquid state. Standing out on the surface of the skin and mixing with sweat, sebum forms a water-lipid mantle. It protects the skin, has bactericidal and fungistatic activity. It is believed that the sterilizing effect of sebum is due to the content of free fatty acids in it. In addition to secretory, the sebaceous glands also perform an excretory function. With sebum, toxic substances formed in the intestines, medium molecular weight peptides, as well as many medicinal substances - iodine, bromine, antipyrine, salicylic acid, ephedrine, etc.

The amount of sebum produced is different for each person, it is uneven in different parts of the skin. So, the largest amount of sebum is released on the skin of the scalp, forehead, cheeks, nose (up to 1000 sebaceous glands per 1 cm 2), in the central part of the chest, interscapular region, upper back and perineum. The function of the sebaceous glands is regulated by the endocrine and nervous systems. Testosterone and related substances stimulate, while estrogens suppress the secretion of sebum.

The sweat secreted by the eccrine sweat glands is slightly acidic. In addition to water, it contains a small amount of dissolved inorganic (sulfates, phosphates, sodium chloride, potassium chloride) and organic (urea, uric acid, ammonia, amino acids, creatinine, etc.) substances.

The chemical composition of sweat is not constant and can vary depending on the amount of fluid drunk, emotional stress, mobility, general condition of the body, ambient temperature, and also depends on the topography of the sweat glands. Sweat from the forehead contains 6-7 times more iron than sweat from the skin of the hands or feet. The content of chlorides in sweat depends on the rate of sweating, metabolic rate, skin temperature and age of the person. Medicinal substances - iodine, quinine, antibiotics - can also be excreted from the body with sweat. On average, 750-1000 ml of sweat is released per day, but at high temperatures several liters of sweat can be excreted. In the regulation of the activity of the sweat glands, the leading role belongs to the central and autonomic nervous system. The main stimulator of the activity of these glands is an increase in external temperature.

The excretory function of the skin is combined with the secretory. In addition to the secretion of organic and inorganic substances by the sebaceous and sweat glands,

substances, products of mineral metabolism, carbohydrates, vitamins, hormones, enzymes, trace elements and a significant amount of water are removed from the body. Sweat is released constantly and continuously. Distinguish invisible sweating in the form perspiratio insensibilis and profuse, occurring with increased thermoregulation.

The function of the apocrine glands is related to the activity of the gonads. They begin to function with the onset of puberty and cease their function in menopause. Apocrine glands, as well as sebaceous and sweat glands, respond to emotional, endocrine dysfunctions, stressful situations and changes in the thermal regime.

Respiratory and resorption functions. The resorption properties of the skin depend on the functional activity of the sebaceous hair follicles, the state of the water-fat mantle, and the strength of the stratum corneum. The surface of the palms and soles has a weak resorption capacity as a result of physiological hyperkeratosis. In places where the sebaceous and sweat glands are abundant, the stratum corneum is weakly expressed, the resorption properties of the skin are enhanced: fat-soluble drugs are absorbed - iodine, phenol, pyrogallol, resorcinol, salicylic acid, boric acid, etc. In inflammatory changes in the skin, resorption processes are activated, therefore drugs for external use should not exceed therapeutic concentrations. Participation of the skin in respiration, i.e. uptake of oxygen and release of carbon dioxide is negligible. The skin absorbs 1/180 of the oxygen and releases 1/90 of the pulmonary carbon dioxide exchange.

thermoregulatory function. Adaptive mechanisms that maintain a constant body temperature are diverse. In addition to the reduced thermal conductivity of the stratum corneum of the epidermis, the fibrous substances of the dermis and subcutaneous fatty tissue are essential. An even more significant influence on thermoregulation is exerted by the state of blood and lymph circulation and the excretory capacity of the sebaceous and sweat glands.

The sweat glands that produce sweat cool the skin by evaporating it to maintain a constant body temperature. Evaporation of sweat is an energy-intensive process: the evaporation of 1 liter requires 2400 kJ, which corresponds to 1/3 of the total heat generated at rest for the whole day. The activity of the sweat glands is mainly regulated by the temperature factor in the skin of the trunk, the back surface of the hands,

extensor surface of the forearms and shoulders, neck, forehead, nasolabial folds. Heat transfer by heat radiation and evaporation is increased with vegetodistonic and dyscirculatory disorders.

exchange function. The role of the skin in metabolism is especially significant because of its deposition ability. The hydrophilicity of connective tissue cells, elastic, collagen and argyrophilic fibers, subcutaneous adipose tissue causes a delay in intracellular and extracellular fluid and minerals, vitamins, microelements. Carbohydrates, cholesterol, iodine, bromine, amino acids, bile acids and slags formed in the process of lipid peroxidation are deposited in the skin. In this regard, long before general metabolic disorders in the skin, a number of pathological processes occur in the form of persistent itching in case of impaired liver function or persistent pyogenic elements in latent diabetes mellitus.

Many chemicals that have penetrated into the stratum corneum remain in it for a long time. The administration of prednisolone labeled with a radionuclide by percutaneous iontophoresis made it possible to detect the drug even 2 weeks after local iontophoresis, and when taken orally, it is detected only for 24 hours.

vitamins have a great impact on the condition of the skin. In particular, vitamins of group B, which support the normal course of redox processes, vitamin PP (nicotinic acid), which promotes the removal of metabolites and detoxification, vitamins A, E, D, being anti-infective factors, activate protein metabolism, normalize the process of keratoplasty in the epidermis, contribute to regeneration of the epithelium in inflammatory processes.

receptor function. The skin not only protects the body from various influences, but is also a multifactorial analyzer, as it is an extensive receptor field. The receptor functions of the skin are provided by a wide variety of sensitive nerve endings and sensory bodies, dispersed unevenly throughout the skin. There is tactile (sense of touch and pressure), pain and temperature (sense of cold and heat) skin sensitivity. Tactile sensitivity is most characteristic of the skin of the terminal phalanges of the fingers, the skin in large folds and on the mucous membrane of the tongue. Such sensitivity includes sensations of density, softness and other features of the consistency of objects. Nerve formations that perceive cold and heat (it is assumed that these are Ruffini's bodies and Krause's flasks) are located

in the skin is uneven, so the perception of heat and cold is different in certain areas of the skin.

The mucous membrane of the mouth is also rich in a variety of nerve endings that perceive heat, cold, pain and touch. However, unlike the skin, the sensitivity of all types to less intense stimuli is more pronounced.

The receptor field of the skin functionally interacts with the central and autonomic nervous systems, constantly participates in dermoneurotropic, dermovisceral connections. The skin continuously responds to a variety of stimuli coming from the environment, as well as their central nervous system and internal organs. It is logical to imagine that the skin is like a screen on which functional and organic changes in the activity of internal organs, the central nervous system, the endocrine and immune systems are projected. Often, even with a slight disorder in the activity of the body and its individual functions and systems, changes occur in the skin, sometimes allowing one to confidently assume one or another visceral or endocrine pathology.

The skin performs 5 main functions - protective, heat-regulating, secretory, respiratory, metabolic, etc.

protective function. The epidermis protects the vessels and nerves, as well as the tissues located under it, from the harmful effects of the external environment. The pigment produced protects the skin from excessive exposure to sunlight.

The sebaceous glands located to the skin (about 300 thousand sebaceous glands) secrete from 500-800 g of sebum within a month. By lubricating the surface of the skin with a thin layer, sebum protects the skin from the harmful effects of water, sweat, and chemical compounds.

cleaning function. By releasing sweat, the skin frees the body from metabolic products harmful to it - toxic substances that have entered the body along with food or medicines.

There are about 2 million sweat glands in the human skin, they are unevenly distributed throughout the body. The allocation of 0.4 to 2 liters of sweat per day is considered normal for the body.

regulatory function. The skin can cool the blood by exposing it to an external environment that is cooler than the temperature inside the body. If the temperature of the environment is increased, the skin muscles relax, the vessels expand, the body's heat transfer increases and blood flow increases. The result is profuse sweating. At low temperatures, heat transfer decreases sharply, as the vessels narrow, and blood flow decreases. This may explain why some people blush in the heat, and turn pale in extreme cold. Both sweating and heat loss occur under the constant control of the central nervous system. And if the thermoregulatory function of the skin is disturbed, this affects the state of the whole organism.

Power function. The skin is able to absorb some substances. Despite the fact that most of these substances linger on the surface due to their protective function, some substances (mercury, alcohol, ether) can sink deep into the skin. Through the skin, the body also absorbs vegetable and animal fats. It is thanks to this that our skin absorbs creams and solutions.

Respiratory function consists in the exchange of gases. About 2% of carbon dioxide is released through the skin, and about 1% of all oxygen exhaled by a person is absorbed. During the day, the skin removes up to 800 g of water vapor. This is more than 2 times higher than the performance of the lungs.

In addition, the skin contributes to the formation of vitamin D. It is in the skin that the substance from which vitamin D is formed is concentrated. Under the influence of sunlight, the process is activated - the substance turns into an active vitamin, which is distributed throughout the body through the blood vessels. To speed up this reaction, it is not necessary to lie in the sun, just exposure to daylight on small areas of the skin is enough.

Proper and regular skin care, cosmetic massage, creams, masks strengthen the skin of the face, increase its resistance to diseases, and also improve the general condition of the skin of the face.

The skin performs many functions, the main of which are the following: protective, immune, receptor, thermoregulatory, metabolic, resorption, secretory, excretory, respiratory.

Protective function of the skin represents the protection of the skin from mechanical external influences: pressure, bruises, tears, stretching, radiation exposure, chemical irritants, etc. The epidermis protects the skin from mechanical damage, and the degree of protection depends on the thickness and strength of its stratum corneum. Collagen and elastic fibers protect the skin from injuries with blunt objects, the first of which stretch along the axis of tension, and the second return the skin to its original state. Excessive extensibility of the skin is due to a violation of the structure of collagen fibers. The epidermis also protects the skin from radiation exposure by completely blocking infrared rays and partially blocking ultraviolet rays. In the epidermis, there are two "protective" barriers: melanin, responsible for an increase in the increase in the number of functional melanocytes and, as a result, the appearance of a tan during prolonged insolation, and a protein barrier located in the stratum corneum of the epidermis. A healthy stratum corneum of the epidermis protects the skin from many chemical irritants, with the exception of those that can destroy the stratum corneum or dissolve in the lipids of the epidermis, gaining access to the deeper layers of the skin. The skin protects the body from the penetration of bacteria due to the peculiar chemical composition of sebum and sweat, the presence of a protective water-lipid mantle on its surface, as well as the presence of microorganisms belonging to the permanent bacterial flora and preventing the penetration of pathogenic microorganisms. Accordingly, when the skin is traumatized, hypothermia, overwork of the body, etc., its ability to resist the penetration of microbes is impaired.

Immune function of the skin . T-lymphocytes present in the skin recognize exogenous and endogenous antigens; Largenhans cells deliver antigens to the lymph nodes, where they are neutralized.

Receptor function of the skin - the ability of the skin to perceive pain, tactile and temperature irritation. There are the following types of functional units that transmit nerve impulses: mechanoreceptors, thermoreceptors. There are also pain receptors, but they respond only to that stimulation (thermal, mechanical, chemical), the degree of which exceeds the pain threshold.

Stimulation of cold receptors occurs when exposed to temperatures below normal skin temperature (34 degrees) by 1-20 degrees; thermal - at a temperature of 32-35 degrees. The temperature above 45 degrees goes beyond the pain threshold of a person and therefore is perceived not by thermal receptors, but by nociceptors. Nociceptors are responsible for the perception of pain and itching; among them, mechanical, temperature and polymodal (i.e., perceiving several types of stimuli) nociceptors are distinguished.

Thermoregulatory function of the skin lies in its ability to absorb and release heat. The increase in heat transfer occurs due to the expansion of skin vessels for various reasons (for example, an increase in ambient temperature), and a decrease in heat transfer, respectively, occurs with vasoconstriction. The release of heat is carried out by radiation, conduction, convection and evaporation, and the release of heat with sweat released by the skin is the most effective way.

The metabolic function of the skin combines a group of private functions: secretory, excretory, resorption and respiratory activity. Resorption function - the ability of the skin to absorb various substances, including drugs. This is the advantage of local medicines over oral ones, because. the use of the former does not depend on side factors (for example, the acidity of the medium and the contents of the stomach), and there is also no possibility of overdose. The secretory function is carried out by the sebaceous and sweat glands of the skin, which secrete lard and sweat, which, when mixed, form a thin film of water-fat emulsion on the surface of the skin. This film plays an important role in maintaining the physiologically normal condition of the skin. The excretory function is closely related to the secretory function and is carried out by the secretion of sweat and sebaceous glands, which secrete organic and inorganic substances, mineral metabolism products, carbohydrates, hormones, enzymes, etc. Respiratory function - the ability of the skin to absorb oxygen and release carbon dioxide, which increases with an increase in ambient temperature, during physical work, during digestion, and the development of inflammatory processes in the skin.

Some skin diseases can cause a malfunction of the skin (so-called "skin deficiency"), this is an emergency and requires special treatment. Such possible disorders include loss of normal control over thermoregulation, water-salt and protein balance of the body, loss of the mechanical, chemical and microbial barrier.

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