The concept of the internal environment of the body

Any organism - unicellular or multicellular - needs certain conditions of existence. These conditions are provided to organisms by the environment to which they have adapted in the course of evolutionary development.

The internal environment for human cells and organs are blood, lymph and tissue fluid.

If you cut your finger badly, blood will flow; if the cut is shallow and the vessels are not damaged, then instead of blood, a few drops of a clear liquid sometimes appear on the cut - this is tissue fluid. Tissue fluid constantly bathes the cells and serves as their living environment. Tissue fluid is constantly updated through the system of lymphatic vessels: tissue fluid is collected in these vessels (inside the lymphatic vessels it is called lymph), and then through the largest lymphatic vessel it enters the general circulation, where it mixes with blood.

The first living formations arose in the waters of the World Ocean, and sea water served as their habitat. As living organisms became more complex, some of their cells became isolated from the external environment. So part of the habitat was inside the organism, which allowed many organisms to leave aquatic environment and start living on land.

The "little sea", becoming more complex, gradually turned into the internal environment of animals. In this regard, it should not be surprising that the salt content in sea water and in the internal environment of the body is similar.

The internal environment of the body, in addition to salts, contains a lot of different substances - proteins, sugar, fat-like substances, hormones, etc. Each organ constantly releases the products of its activity into the internal environment and receives from it the substances it needs. And, despite such an active exchange, the composition of the internal environment remains virtually unchanged.

Homeostasis. Maintaining the constancy of living conditions in the internal environment is called homeostasis.

Individual cells and groups of cells in the human body are extremely sensitive to changes in their environment. As for the whole organism, the boundaries of changes in the external environment that it can endure are much wider than those of individual cells. Human cells function normally only at a temperature of 36-38 ° C. An increase or decrease in temperature beyond these limits leads to disruption of cell functions. A person, as is known, can normally exist with much wider fluctuations in the temperature of the external environment.

The cells maintain a constant amount of water and minerals. Many cells die almost instantly when placed in distilled water. The organism as a whole can endure both water starvation and excess intake of water and salts.

Individual cells are extremely sensitive to slight changes in the concentration of hydrogen ions. The whole organism is able to maintain a constant concentration of hydrogen ions, even when a lot of acidic or alkaline metabolic products enter the tissue fluid.

These examples are enough to make sure that organisms have special adaptations to ensure the constancy of the habitat of their cells.

A very important feature of the internal environment is that the content of substances in it is not absolutely the same, but varies within certain limits, i.e. for the content of each Substance, the norm is not just one number, but a certain range of indicators. For example, in the reference book you can read: the content of potassium ions in the blood of a healthy person is 16-20 mg% (i.e. 16-20 mg per 100 ml).

In practice, the content of any substance in the internal environment is never exactly the same - it constantly fluctuates, but within strictly defined limits.

The range of indicators for different substances different. Some indicators are maintained particularly accurately; they are called constants. Among the constants is, for example, the reaction of the blood (i.e., the concentration of hydrogen ions in it - pH).

In the body, indicators such as blood pressure, body temperature, osmotic pressure of blood and tissue fluid, the content of proteins and sugar, sodium, potassium, calcium, chlorine, and hydrogen ions are kept at a relatively constant level.

Not only the composition of the internal environment remains constant, but also its volume. However, the constancy of the volume of the internal environment is not absolutely unchanged. Part of the fluid from the internal environment is excreted from the body through the kidneys with urine, through the lungs with exhaled water vapor and into the digestive tract with digestive juices. Part of the water evaporates from the surface of the body in the form of sweat. These water losses are constantly replenished by the absorption of water from the digestive tract. There is a constant renewal of water with a general preservation of its volume. Cells also take part in maintaining a constant volume of fluid in the internal environment. The water inside the cells makes up about 50% of the body weight. If for some reason the amount of fluid in the internal environment decreases, then the movement of water from the cells into the intercellular space begins. This helps to maintain the constancy of the volume of the internal environment.

The constancy of the internal environment - homeostasis - is maintained by the continuous work of organs and tissues.

The role of various organs in maintaining homeostasis

The role of different organs in maintaining homeostasis is different. The digestive system ensures that nutrients enter the blood in the form in which they can be absorbed by the cells of the body.

The circulatory organs carry out a continuous movement of blood and deliver oxygen and nutrients to the cells, and the decay products are carried away from them. The respiratory organs provide oxygen to the blood and remove carbon dioxide.

Through the lungs, kidneys, skin, end products of metabolism and some other substances are removed from the body.

The nervous system plays an important role in maintaining homeostasis. Quickly responding to various changes in the external or internal environment, the nervous system changes the activity of organs in such a way that shifts or disturbances in the body are leveled.

Thanks to the development of devices that ensure the constancy of the internal environment of the body, its cells are less susceptible to changing influences of the external environment.

Violation of homeostasis leads to significant changes in the functioning of organs and to various diseases. That is why the measurement of indicators such as body temperature, physical and chemical composition of blood, blood pressure, has great importance for diagnosis, i.e., recognition of diseases.

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Among inorganic compounds living organisms, water plays a special role. Water is the main medium in which the processes of metabolism and energy conversion take place.

The water content in most living organisms is 60-70%. Water forms the basis of the internal environment of living organisms (blood, lymph, intercellular fluid). Unique properties water is determined by the structure of its molecules. In a water molecule, one oxygen atom is covalently bonded to two hydrogen atoms. The water molecule is polar (dipole). The positive charge is concentrated on the hydrogen atoms because oxygen is more electronegative than hydrogen. A negatively charged oxygen atom of one water molecule is attracted to a positively charged hydrogen atom of another molecule, forming a hydrogen bond, which is 15-20 times weaker than a covalent one. Therefore, hydrogen bonds are easily broken, which is observed, for example, during the evaporation of water. Due to the thermal movement of molecules in water, some hydrogen bonds are broken, some are formed.

Thus, the molecules are mobile in the liquid state, which is very important for metabolic processes. Water molecules easily penetrate cell membranes. Due to the high polarity of the molecules, water is a solvent for other polar compounds. Depending on the ability to dissolve certain compounds in water, they are conditionally divided into hydrophilic, or polar, and hydrophobic, or non-polar. Hydrophilic compounds soluble in water include most salts. Hydrophobic compounds (almost all fats, some proteins) contain non-polar groups, do not form hydrogen bonds, so these compounds do not dissolve in water. It has a high heat capacity and at the same time a high thermal conductivity for liquids. These properties make water ideal for maintaining the body's thermal equilibrium.

To support the vital processes of individual cells and the body as a whole importance have mineral salts. Living organisms contain both dissolved salts (in the form of ions) and salts in the solid state. Ions are divided into positive (cations of metal elements K +, Na +, Ca2 +, M2 +, etc.) and negative (anions of hydrochloric acids - Cl -, sulfuric - HSO4 -, SO42 -, carbonate - HCO3 -, phosphate - H2PO4 - , HPO42 - etc.). Different concentrations of K + and Na + cations in the cell and intercellular fluid cause a potential difference on the cell membrane; a change in the permeability of the membrane for K + and Na + under the influence of irritation ensures the occurrence of nerve and muscle excitation. Phosphoric acid anions maintain a neutral reaction of the intracellular environment (pH = 6.9), anions carboxylic acid- slightly alkaline reaction of blood plasma (рН = 7.4). Calcium compounds (CaCO3) are part of the shells of molluscs and protozoa, shells of crayfish. Hydrochloric acid creates an acidic environment in the stomach of vertebrates and humans, thereby ensuring the activity of gastric juice enzymes. Residues of sulfuric acid, joining water-insoluble compounds, ensuring their solubility, which contributes to the removal of these compounds from cells and the body.

Water - the most common substance. Seas and oceans occupy 71% of the surface of the globe. However, in Lately there was a shortage of fresh water, because. saline waters are used by people little, and fresh water used for irrigation and in industry.

Density. In water, the weight of all organisms is lightened, and many organisms float in the water without sinking to the bottom. But the density of water makes it difficult to move, so organisms must have well-developed muscles for fast swimming. With depth, pressure increases greatly - deep-sea inhabitants endure pressure.

Light. Penetrates to a shallow depth. Therefore, plants exist only in the upper horizons. At great depths, animals live in complete darkness.

Temperature regime. Temperature fluctuations in the water are smoothed out, aquatic inhabitants do not adapt to severe frost and heat.

Limited oxygen. Its solubility is not very high and decreases with contamination or heating. Therefore, in reservoirs there are deaths from a lack of oxygen.

Salt composition.

The polarity of the molecules and the ability to form hydrogen bonds make water a good solvent for a huge amount of inorganic and organic substances. Majority chemical reactions is the interaction between water-soluble substances. Under the action of enzymes, water enters into hydrolysis reactions, in which OH - and H + water are added to the free valences of various molecules. Water forms the basis of the internal environment of living organisms. Water provides the influx of substances into the cell and their removal through the outer cell membrane (transport function). Water is a heat regulator. Due to the good thermal conductivity and greater heat capacity of water, when the environment t changes, inside the cell t remains unchanged or its fluctuations are much smaller than in environment. Water is a donor of electrons and protons in energy exchange. Water is involved in the formation of higher structures of biological macromolecules. Cellular metabolism depends on the balance of free and bound water. Water has a high heat capacity. The specific heat capacity of water is the amount of heat required to raise the temperature of 1 kg of water by 10. Water is the only substance that has a higher density in the liquid state than in the solid state. There is surface tension on the surface of water.

Water- a complex living system inhabited by plants, animals and microorganisms that constantly multiply and die, which ensures self-purification of water bodies.

Water has the highest density at t 4 0 C (1 g / cm 3), therefore, water bodies do not freeze in winter. Water molecules have polarity and are attracted to each other by opposite poles, forming associations due to hydrogen bonds. The most stable are doubled water molecules that have 2 hydrogen bonds. Water molecules are resistant to heating, only at t 1000 0 С steam begins to dissociate into H and O 2. Composition natural water. 5 groups of substances: 1. the main ions (cations: Na +, Ca 2+, Mg 2+, Mn 2+, Fe 2+, Fe 3+, K +), 2. anions (HCO 3-, SO 4 2- , Cl -, CO 3 2-, SO 3 2-, S 2 O 3-), 3. dissolved gases (CO 2 O 2 N 2 H 2 S CH 4), 4. nutrients (NH 3 - ammonia, nitrites , nitrates, P, Si), 5. trace elements (I, F, Cu, Br, CO, Ni). According to the content of anions, natural waters are divided into carbonate, bicarbonate, sulfate, chloride. According to the content of cations: calcium, magnesium and sodium water. The content of salts in water affects the corrosion of metal, concrete and stone materials. Mineralization of river water - 200-1000 mg/l, lake water - 15-300 mg/l, sea water - 3500 mg/l. Chlorides, ammonia and nitrates are indicators of organic matter entering the water. Water pollution with organic matter is accompanied by an increase in anaerobic and aerobic bacteria and fungi. Ammonia (MPC - 2 mg/l) indicates fresh water pollution. In deep underground waters, the presence of ammonia is possible, which is formed due to the reduction of nitrates in the absence of O 2 . In swampy and peaty waters, the ammonia content is not an indicator of pollution (ammonia of plant origin). Nitrites (KNO 2 , HNO 2), products of ammonia oxidation during nitrification, indicate the age of pollution. Nitrates (maximum concentration limit - 10 mg/l) - the end product of mineralization. If ammonia, nitrates and nitrites are present at the same time, the water is dangerous in an epidemic sense. Nitrates (Ca (NO 3) 2, NaNO 3, KNO 3) can be contained due to the dissolution of soil salts, mineral fertilizers, saltpeter. Nitrates are the precursors of the formation of carcinogenic substances - nitrosamines. They reduce the body's resistance to the effects of mutagenic and carcinogenic factors. Chlorides - an indicator of domestic pollution (MPC - 20-30 mg / l). In places with saline soil, groundwater contains chlorides of saline origin. Wells and pits must not be contaminated organic matter. They should be located in uncontaminated elevated areas, at least 50 m away from latrines, latrines, sewerage networks, cattle yards, cemeteries, warehouses for fertilizers and pesticides.

Life forms of hydrobionts. In the water column (pelagial): 1. plankton - organisms that are not capable of active movement (algae, protozoa, crustaceans), are not able to withstand water currents. Cryoplankton (flagellates) - the population of melt water, is formed under the rays of the sun in ice cracks and snow voids. 2. nekton - large animals whose motor activity is sufficient to overcome water currents (fish, squid, mammals). 3. pleuston - organisms, part of the body of which is in the water, and part above the surface (duckweed, gastropods, fish). 4. benthos (bacteria, actinomycetes, algae and fungi, protozoa, sponges, corals, annelids, crustaceans, echinoderms, insect larvae) lives on the surface of the soil (epibenthos) and in its thickness (endobenthos). Pelagobenthos is located in the zone of contact between the water column and the bottom. 5. periphyton - foulers - all organisms that live on dense substrates outside the bottom layer of water (bivalves and barnacles, sponges). 6. neuston - organisms living in the surface layer of water. On the surface of the water film - epineuston (water strider bugs, flies) or under it - hyponeuston (copepods, juvenile fish, insects, mollusk larvae).

The phrase "internal environment of the body" appeared thanks to a French physiologist who lived in the 19th century. In his works, he emphasized that a necessary condition for the life of an organism is to maintain constancy in the internal environment. This provision became the basis for the theory of homeostasis, which was formulated later (in 1929) by the scientist Walter Cannon.

Homeostasis is the relative dynamic constancy of the internal environment,

As well as some static physiological functions. The internal environment of the body is formed by two fluids - intracellular and extracellular. The fact is that each cell of a living organism performs a specific function, so it needs a constant supply of nutrients and oxygen. She also feels the need for the constant removal of metabolic products. The necessary components can penetrate the membrane only in a dissolved state, which is why each cell is washed by tissue fluid, which contains everything necessary for its vital activity. It belongs to the so-called extracellular fluid, and it accounts for 20 percent of body weight.

The internal environment of the body, consisting of extracellular fluid, contains:

• lymph (an integral part of tissue fluid) - 2 l;
• blood - 3 l;
• interstitial fluid - 10 l;
• transcellular fluid - about 1 liter (it includes cerebrospinal, pleural, synovial, intraocular fluids).

All of them have different composition and differ in their functionality.

properties. Moreover, the internal environment may have little difference between the consumption of substances and their intake. Because of this, their concentration fluctuates constantly. For example, the amount of sugar in the blood of an adult can range from 0.8 to 1.2 g/L. In the event that the blood contains more or less of certain components than necessary, this indicates the presence of a disease.

As already noted, the internal environment of the body contains blood as one of the components. It consists of plasma, water, proteins, fats, glucose, urea and mineral salts. Its main location is (capillaries, veins, arteries). Blood is formed due to the absorption of proteins, carbohydrates, fats, water. Its main function is the relationship of organs with the external environment, the delivery of necessary substances to the organs, the removal of decay products from the body. It also performs protective and humoral functions.

Tissue fluid consists of water and nutrients dissolved in it, CO 2 , O 2 , as well as dissimilation products. It is located in the spaces between tissue cells and is formed due to tissue fluid being intermediate between blood and cells. It transfers from the blood to the cells O 2, mineral salts,

Lymph consists of water and dissolved in it. It is located in the lymphatic system, which consists of vessels merged into two ducts and flowing into the vena cava. It is formed due to tissue fluid, in sacs that are located at the ends of the lymphatic capillaries. The main function of the lymph is to return tissue fluid to the bloodstream. In addition, it filters and disinfects tissue fluid.

As we can see, the internal environment of an organism is a combination of physiological, physico-chemical, respectively, and genetic conditions that affect the viability of a living being.

The environment is a set of living conditions for living beings. Allocate the external environment, i.e. a complex of factors that are outside the body, but necessary for its life, and the internal environment.

The internal environment of the body is called the totality of biological fluids (blood, lymph, tissue fluid) that bathe cells and tissue structures and take part in metabolic processes. Claude Bernard proposed the concept of "internal environment" in the 19th century, thereby emphasizing that, in contrast to the changing external environment in which a living organism exists, the constancy of the vital processes of cells requires a corresponding constancy of their environment, i.e. internal environment.

A living organism is an open system. An open system is a system whose existence requires a constant exchange of matter, energy and information with the external environment. The interconnections of the body and the external environment ensure the entry of oxygen, water and nutrients into the internal environment, the removal of carbon dioxide and unnecessary, and sometimes harmful, metabolites from it. The environment provides the body great amount information perceived by numerous sensitive formations of the nervous system.

The external environment has not only beneficial, but also harmful effects on the life of the organism. However, a healthy organism functions normally if the influence of the environment does not exceed the limits of admissibility. Such dependence of the organism's vital activity on the external environment, on the one hand, and the relative stability and independence of life processes from changes in the environment, on the other hand, is provided by the property of the organism, called homeostasis (homeostasis). The organism is an ultra-stable system that itself searches for the most stable and optimal state, keeping various parameters of functions within the boundaries of physiological (“normal”) fluctuations.

Homeostasis is the relative dynamic constancy of the internal environment and the stability of physiological functions. This is precisely dynamic, and not static constancy, since it implies not only the possibility, but the necessity of fluctuations in the composition of the internal environment and the parameters of functions within physiological boundaries in order to achieve the optimal level of vital activity of the organism.

The activity of cells requires an adequate function of supplying them with oxygen and effectively flushing out carbon dioxide and other waste substances or metabolites from them. To restore collapsing protein structures and extract energy, cells must receive plastic and energy material that enters the body with food. All this cells receive from their microenvironment through tissue fluid. The constancy of the latter is maintained through the exchange of gases, ions and molecules with blood. Consequently, the constancy of blood composition and the state of barriers between blood and tissue fluid, the so-called histohematic barriers, are the conditions for homeostasis of the microenvironment of cells. The selective permeability of these barriers provides a certain specificity of the composition of the microenvironment of cells, which is necessary for their functions.

On the other hand, the tissue fluid participates in the formation of lymph, exchanges with the lymphatic capillaries draining the tissue spaces, which makes it possible to effectively remove large molecules from the cellular microenvironment that are unable to diffuse through the histohematological barriers into the blood. In turn, the lymph flowing from the tissues through the thoracic lymphatic duct enters the bloodstream, ensuring the maintenance of the constancy of its composition. Consequently, in the body between the fluids of the internal environment there is a continuous exchange, which is a prerequisite for homeostasis.

The relationship of the components of the internal environment with each other, with the external environment and the role of the main physiological systems in the implementation of the interaction of the internal and external environment are shown in Fig. 2.1. The external environment affects the body through the perception of its characteristics by the sensitive apparatus of the nervous system (receptors, sensory organs), through the lungs, where gas exchange takes place, and through the gastrointestinal tract, where water and food ingredients are absorbed. The nervous system exerts its regulatory effect on cells by releasing special mediators at the ends of nerve conductors - mediators that enter through the microenvironment of cells to special structural formations of cell membranes - receptors. The influence of the external environment perceived by the nervous system can also be mediated through the endocrine system, which secretes special humoral regulators, hormones, into the blood. In turn, the substances contained in the blood and tissue fluid to a greater or lesser extent irritate the receptors of the interstitial space and bloodstream, thereby providing nervous system information about the composition of the internal environment. The removal of metabolites and foreign substances from the internal environment is carried out through the excretory organs, mainly the kidneys, as well as the lungs and the digestive tract.

The constancy of the internal environment is the most important condition for the vital activity of the organism. Therefore, deviations in the composition of the liquids of the internal environment are perceived by numerous receptors. Fig. 2.1. Scheme of interconnections of the internal environment of the body.

structures and cellular elements, followed by the inclusion of biochemical, biophysical and physiological regulatory reactions aimed at eliminating the deviation. At the same time, the regulatory reactions themselves cause changes in the internal environment in order to bring it into line with the new conditions of the organism's existence. Therefore, the regulation of the internal environment is always aimed at optimizing its composition and physiological processes in the body.

The boundaries of homeostatic regulation of the constancy of the internal environment can be rigid for some parameters and plastic for others. Accordingly, the parameters of the internal environment are called hard constants, if the range of their deviations is very small (pH, ion concentration in the blood), or plastic constants (levels of glucose, lipids, residual nitrogen, interstitial fluid pressure, etc.), i.e. subject to relatively large fluctuations. The constants change depending on age, social and professional conditions, time of year and day, geographical and natural conditions, and also have gender and individual characteristics. Environmental conditions are often the same for more or less people living in a certain region and belonging to the same social and age group, but the internal environment constants may differ in different healthy people. Thus, the homeostatic regulation of the constancy of the internal environment does not mean the complete identity of its composition in different individuals. However, despite the individual and group characteristics, homeostasis ensures the maintenance of normal parameters of the internal environment of the body.

Usually, the average values ​​of the parameters and characteristics of the vital activity of healthy individuals, as well as the intervals within which the fluctuations of these values ​​correspond to homeostasis, are called the norm, i.e. able to keep the body at the level of optimal functioning.

Accordingly, for general characteristics The internal environment of the body is normally given the intervals of fluctuations of its various indicators, for example, the quantitative content of various substances in the blood in healthy people. At the same time, the characteristics of the internal environment are interrelated and interdependent quantities. Therefore, shifts in one of them are often compensated by others, which is not necessarily reflected in the level of optimal functioning and human health.

The internal environment is a reflection of the most complex integration of the vital activity of different cells, tissues, organs and systems with the influences of the external environment.

This determines the importance individual features internal environment that distinguish each person. The basis of the individuality of the internal environment is genetic individuality, as well as long-term exposure to certain conditions of the external environment. Accordingly, the physiological norm is an individual optimum of vital activity, i.e. the most coordinated and effective combination of all life processes in real environmental conditions.

2.1. Blood as the internal environment of the body.

Fig.2.2. The main constituents of blood.

Blood consists of plasma and cells (shaped elements) - erythrocytes, leukocytes and platelets, which are in suspension (Fig. 2.2.). Since plasma and cellular elements have separated sources of regeneration, blood is often isolated into an independent type of tissue.

The functions of the blood are diverse. These are, first of all, in a generalized form, the functions of transport or transfer of gases and substances necessary for the vital activity of cells or to be removed from the body. These include: respiratory, nutritional, integrative-regulatory and excretory functions (see Chapter 6).

Blood also performs a protective function in the body, due to the binding and neutralization of toxic substances that enter the body, the binding and destruction of foreign protein molecules and foreign cells, including those of infectious origin. Blood is one of the main environments where the mechanisms of specific protection of the body from foreign molecules and cells are carried out, i.e. immunity.

Blood is involved in the regulation of all types of metabolism and temperature homeostasis, is the source of all fluids, secrets and excretions of the body. The composition and properties of blood reflect the shifts that occur in other fluids of the internal environment and cells, and therefore blood tests are the most important diagnostic method.

The amount or volume of blood in a healthy person is within 68% of body weight (4 - 6 liters). This condition is called normovolemia. After excessive water intake, blood volume may increase (hypervolemia), and in severe physical work in hot workshops and excessive sweating - to fall (hypovolemia).

Fig.2.3. Determination of hematocrit.

Since blood is made up of cells and plasma, the total volume of blood is also the sum of the volume of plasma and the volume of cellular elements. Part of the blood volume attributable to the cellular part of the blood is called the hematocrit (Fig. 2.3.). In healthy men, the hematocrit is in the range of 4448%, and in women - 4145%. Due to the presence of numerous mechanisms for regulating blood volume and plasma volume (volumoreceptor reflexes, thirst, nervous and humoral mechanisms for changing the absorption and excretion of water and salts, regulation of blood protein composition, regulation of erythropoiesis, etc.), hematocrit is a relatively rigid homeostatic constant and its long and persistent a change is possible only in high altitude conditions, when adaptation to a low partial pressure of oxygen enhances erythropoiesis and, accordingly, increases the proportion of blood volume per cellular elements. Normal values ​​of hematocrit and, accordingly, the volume of cellular elements are called normocythemia. An increase in the volume occupied by blood cells is called polycythemia, and a decrease is called oligocythemia.

Physicochemical properties of blood and plasma. The functions of blood are largely determined by its physicochemical properties, among which highest value have osmotic pressure, oncotic pressure and colloidal stability, suspension stability, specific gravity and viscosity.

The osmotic pressure of blood depends on the concentration of molecules of substances dissolved in it (electrolytes and non-electrolytes) in the blood plasma and is the sum of the osmotic pressures of the ingredients contained in it. In this case, over 60% of the osmotic pressure is created by sodium chloride, and in total, inorganic electrolytes account for up to 96% of the total osmotic pressure. Osmotic pressure is one of the rigid homeostatic constants and in a healthy person averages 7.6 atm with a possible range of fluctuations of 7.38.0 atm. If the liquid of the internal environment or an artificially prepared solution has the same osmotic pressure as normal blood plasma, such a liquid medium or solution is called isotonic. Accordingly, a fluid with a higher osmotic pressure is called hypertonic, and a fluid with a lower osmotic pressure is called hypotonic.

Osmotic pressure ensures the transition of the solvent through a semi-permeable membrane from a less concentrated solution to a more concentrated solution, therefore it plays an important role in the distribution of water between the internal environment and body cells. So, if the tissue fluid is hypertonic, then water will enter it from two sides - from the blood and from the cells, on the contrary, when the extracellular medium is hypotonic, water passes into the cells and blood.