From Big Medical Encyclopedia

HOMEOSTASIS, homeostasis (homeostasis; grech, homoios similar, the same + stasis a state, an immovability) — relative dynamic constancy of internal environment (blood, a lymph, an intercellular lymph) and stability of the main fiziol, functions (blood circulation, breath, thermal control, a metabolism etc.) human body of l of animals. The regulatory mechanisms supporting fiziol. the state or properties of cells, bodies and systems of a complete organism at optimum level, are called homeostatic.

It is known that living cell represents mobile, self-regulating system. Its internal organization is supported by the active processes directed to restriction, the prevention or elimination of the shifts caused by various influences from surrounding and internal environment. Ability to be returned to a reference state after a deviation from the nek-ry average level caused by this or that «revolting» factor is the main property of a cell. The metaphyte represents the complete organization, cellular elements a cut are specialized for performance of various functions. Interaction in an organism is carried out by the mechanisms difficult regulating, coordinating and correlating with participation of nervous, humoral, exchange and other factors. The set of the separate mechanisms regulating inside - and intercellular relationship, renders in some cases the mutually antithetical (antagonistic) influences counterbalancing each other. It leads to establishment in an organism mobile fiziol, a background (fiziol, balance) and allows live system to maintain relative dynamic constancy, despite changes in the environment and the shifts arising in the course of life activity of an organism.

The term «homeostasis» is offered in 1929 an amer. the physiologist U. Kennon who considered that fiziol, the processes maintaining stability in an organism are so difficult and diverse that it is reasonable to combine them under the general Name. However in 1878. To. Bernard wrote that all vital processes have only one purpose — maintenance of constancy of living conditions in our internal environment. Similar statements meet in works of many researchers of the 19 and first half of 20 century [E. Pflyuger, Sh. Richet, L. A. Fredericq, I. M. Sechenov, I. P. Pavlov, K. M. Bykov, etc.]. The great value for studying of a problem G. was played by works L. S. Stern (oh, sotr.), the devoted roles barrier functions (see), regulating structure and properties of a microenvironment of bodies and fabrics.

Idea of G. does not correspond to the concept of the steady (not fluctuating) balance in an organism — the principle of balance is not applicable to difficult fiziol, and the biochemical processes proceeding in live systems. Incorrectly also G.'s opposition to rhythmic fluctuations in internal environment (see. Biological rhythms ). In broad understanding covers questions of a cyclic and phase course of reactions, compensations (see. Compensatory processes ), regulation and self-regulation fiziol, functions (see. Self-control of physiological functions ), to loudspeaker of interdependence of nervous, humoral and other components of regulatory process. G.'s borders can be rigid and plastic, change depending on individual age, sexual, social, the prof. and other conditions.

Special value for life activity of an organism has constancy of composition of blood — a liquid basis of an organism (fluid matrix), according to U. Kennon. Stability of its active reaction (pH), osmotic pressure, a ratio of electrolytes (sodium, calcium, chlorine, magnesium, phosphorus), contents of glucose, number of uniform elements etc. is well-known. So, e.g., pH of blood, as a rule, does not go beyond 7,35 — 7,47. Even sharp frustration of acid-base metabolism with patol, accumulation of acids in an intercellular lymph, napr, at diabetic acidosis, influence active reaction of blood very little (see. Acid-base equilibrium ). In spite of the fact that the osmotic pressure of blood and an intercellular lymph is exposed to continuous fluctuations owing to constant receipt of osmotically active products of interstitial exchange, it remains at the certain level and changes only at some expressed patol, states (see. Osmotic pressure ). Preservation of constancy of osmotic pressure has paramount value for water exchange and maintenance of ionic balance in an organism (see. Water salt metabolism ). Ion concentration of sodium in internal environment differs in the greatest constancy. Content of other electrolytes fluctuates also in narrow borders. Existence of a large number osmoreceptors (see) in fabrics and bodies, including in the central nervous educations (a hypothalamus, a hippocampus), and coordinate system of regulators of water exchange and ionic structure allows an organism to eliminate quickly the shifts in the osmotic pressure of blood happening, e.g., at administration of water in an organism.

In spite of the fact that blood represents the general internal environment of an organism, cells of bodies and fabrics directly do not adjoin to it. In metaphytes each body has own internal environment (microenvironment) answering to its structural and functional features, and a normality of bodies depends on chemical structure, physical. - chemical, biol, and other properties of this microenvironment. Its G. is caused by a functional condition of gistogematichesky barriers (see. Barrier functions ) and their permeability in the directions blood -> an intercellular lymph, an intercellular lymph -> blood.

Especially constancy of internal environment is important for activity of c. N of page: even insignificant chemical and physical. - the chemical shifts arising in cerebrospinal liquid, a glia and pericellular spaces can cause sharp disturbance of a course of vital processes in separate neurons or in their ensembles (see. Blood-brain barrier ). The complex homeostatic system which is turning on various neurohumoral, biochemical, hemodynamic and other mechanisms of regulation is the system of ensuring optimum level arterial pressure (see). At the same time the upper limit of the ABP level is defined by functionality of baroreceptors of vascular system of a body (see. Angiotseptora ), and lower limit — the needs of an organism for blood supply.

Processes belong to the most perfect homeostatic mechanisms in an organism of the highest animals and the person thermal controls (see); at gomoyotermny animals of temperature variation in internal departments of a body at the jumps of temperature in the environment do not exceed the tenth shares of degree.

Various researchers on a miscellaneous explain mechanisms obshchebiol. character, the cornerstone G. Tak, U. Kennon special significance was attached by century of N of page, L. A. Orbeli considered one of major factors of G. adaptation and trophic function of a sympathetic nervous system. The organizing role of the nervous device (the principle of a nervosism) is the cornerstone of widely known ideas of essence of the principles. (I. M. Sechenov, I. P. Pavlov, A. D. Speransky, etc.). However neither the principle of a dominant (A. A. Ukhtomsky), nor the theory of barrier functions (L. S. Stern), nor the general adaptation syndrome (G. Selye), nor the theory of functional systems (P. K. Anokhin), nor hypothalamic regulation G. (N. I. Grashchenkov) and many other theories do not allow to solve Problem completely.

In certain cases idea of G. is not absolutely legally used for an explanation of the social phenomena isolated fiziol, states, processes and even. So there were terms «immunological», «electrolytic», «system», «molecular», «physical and chemical», «genetic homeostasis» which are found in literature, etc. Attempts to reduce a problem G. to the principle of self-regulation (see were made. Biological system , an autoregulyation in biological systems). An example of a solution of the problem of G. from positions of cybernetics is the attempt of Eshbi (W. R. Ashby, 1948) to design the self-regulating device modeling ability of live organisms to support the level of some sizes in fiziol, admissible borders (see. Homeostat ). Certain authors consider internal environment of an organism in the form of difficult and chain system with many «active entrances» (internals) and separate fiziol, indicators (a blood stream, the ABP, gas exchange, etc.), value of each of which is caused by activity of «entrances».

Before researchers and clinical physicians in practice there are questions of assessment of adaptive (adaptation) or compensatory opportunities of an organism, their regulation, strengthening and mobilization, forecasting of responses of an organism to the revolting influences. Some conditions of vegetative instability caused by insufficiency, surplus or inadequacy of regulatory mechanisms are considered as «a disease of a homeostasis». With the known convention the functional disturbances of normal activity of an organism connected with its aging, forced reorganization of biological rhythms, some phenomena of vegetative dystonia hyper - and hypocompensatory reactivity can be referred to them at stressful and extreme influences (see. Stress ) etc.

For assessment of a condition of homeostatic mechanisms in fiziol, an experiment and in a wedge, practice the various dosed functional trials (cold, thermal, adrenalinic, insulin, mezatonovy are applied, etc.) with definition in blood and urine of a ratio of biologically active agents (hormones, mediators, metabolites) etc.

Biophysical mechanisms of a homeostasis

From the point of view of chemical biophysics the homeostasis is a state, at Krom all processes responsible for power turning into an organism, are in a dynamic equilibrium. This state has the greatest stability and corresponds fiziol, to an optimum. According to representations thermodynamics (see) an organism and a cell can exist and adapt to such conditions of the environment under which in biol, system possibly establishment of a stationary current physical. - chemical processes, i.e. a homeostasis. The main role in G.'s establishment belongs first of all to cellular membrane systems which are responsible for biopower processes and regulate the speed of receipt and release of substances cells (see. Membranes biological ).

From these positions the non-enzymatic reactions, unusual to normal life activity, proceeding in membranes are the main reasons for disturbance; in most cases it is the chain reactions of oxidation with participation of free radicals arising in phospholipids of cells. These reactions lead to damage of structural elements of cells and dysfunction of regulation (see. Radicals , Chain reactions ). Also the agents causing a radikaloobrazovaniye — ionizing radiation, infectious toxins, some food stuffs, nicotine, and also a lack of vitamins etc. treat the factors which are a cause of infringement of G.

One of the major factors stabilizing a homeostatic state and functions of membranes are bioantioxidants which constrain development of oxidizing radical reactions (see. Antioxidants ).

Age features of a homeostasis at children

Constancy of internal environment of an organism and relative stability physical. - chemical indicators at children's age are provided at the expressed dominance of anabolic processes of exchange over catabolic. It is an indispensable condition growth (see) also distinguishes a children's organism from an organism of adults at whom intensity of metabolic processes is in a condition of a dynamic equilibrium. In this regard neuroendocrinal regulation of G. of a children's organism appears more intense, than at adults. Each age period is characterized by specific features of mechanisms G. and their regulation. Therefore at children considerably more often than heavy disturbances of G., quite often life-threatening occur at adults. These disturbances are most often connected with immaturity of homeostatic functions of kidneys, with disorders of functions went. - kish. a path or respiratory function of lungs (see. Breath ).

Growth of the child which is expressed in increase in mass of its cells is followed by clear changes of distribution of liquid in an organism (see. Water salt metabolism ). Absolute increase in volume of extracellular liquid lags behind rates of the general increase of weight therefore the relative volume of internal environment expressed percentage of body weight decreases with age. This dependence is especially brightly expressed on the first year after the birth. At children of more advanced ages rates of changes of relative volume of extracellular liquid decrease. The system of regulation of constancy of volume of liquid (volyumoregulyation) provides compensation of deviations in a water balance in rather narrow limits. High extent of hydration of fabrics at newborns and children of early age defines much higher, than at adults, need of the child for water (counting on a mass unit of a body). Losses of water or its restriction quickly lead to development of dehydration at the expense of the extracellular sector, i.e. internal environment. At the same time kidneys — the principal executive bodies in system of a volyumoregulyation — do not provide economy of water. The limiting factor of regulation is immaturity of canalicular system of kidneys. The most important feature of neuroendocrinal control of G. at newborns and children of early age consists in rather high secretion and renal excretion Aldosteronum (see) that exerts a direct impact on a condition of hydration of fabrics and function of renal tubules.

Regulation of osmotic pressure of a blood plasma and extracellular liquid at children is also limited. The Osmomolyarnost of internal environment fluctuates with broader range (+ 50 mosm/l), than at adults (+ 6 mosm/l). It is connected with the bigger size of body surface at 1 kg of weight and, therefore, with more essential losses of water at breath, and also with immaturity of renal mechanisms of concentration of urine at children. G.'s disturbances which are shown hyper osmosis especially often occur at children of the period of a neonatality and the first months of life; at more advanced ages the hypoosmosis connected by hl begins to prevail. obr. with went. - kish. disease or diseases of kidneys. The ionic regulation of G. which is closely connected with activity of kidneys and character of food is less studied.

Earlier was considered that the major factor determining the size of osmotic pressure of extracellular liquid is concentration of sodium, however later researches showed that close correlation between the content of sodium in a blood plasma and the size of the general osmotic pressure at pathology does not exist. The exception is made by a plasmatic hypertension. Therefore, performing homeostatic therapy by administration of glyukozosolevy solutions demands control not only of the content of sodium in serum or a blood plasma, but also behind changes of the general osmomolyarnost of extracellular liquid. In maintenance of the general osmotic pressure in internal environment concentration of sugar and urea is of great importance. Content of these osmotically active agents and their influence on a water salt metabolism at many patol, states can sharply increase. Therefore at any disturbances of G. it is necessary to define concentration of sugar and urea. Owing to the above at children of early age at disturbance of the water-salt and proteinaceous modes the state hidden hyper - or hypoosmosis, a hyperazotemia can develop (E. Kerpel-Froniush, 1964).

The important indicator characterizing G. at children is concentration of hydrogen ions in blood and extracellular liquid. In antenatal and early post-natal the periods regulation of acid-base equilibrium is closely connected with saturation rate of blood oxygen that is explained by relative dominance of anaerobic glycolysis in biopower processes. At the same time even the moderate hypoxia at a fruit is followed by accumulation in its fabrics milk to - you. Besides, immaturity of atsidogenetichesky function of kidneys creates premises for development of «physiological» acidosis (see). Due to G.'s features newborns quite often have frustration standing on a side between physiological and pathological.

Reorganization of neuroendocrinal system in the pubertal period is also accompanied by G. Odnako's changes functions of executive bodies (kidneys, lungs) reach at this age of the maximum degree of a maturity therefore heavy syndromes or G.'s diseases meet seldom, it is

about the compensated shifts more often in a metabolism which can be revealed only at biochemical, a blood analysis. In clinic for G.'s characteristic at children it is necessary to investigate the following indicators: a hematocrit, the general osmotic pressure, content of sodium, potassium, sugar, bicarbonates and urea in blood, and also pH of blood, pO 2 and pCO 2 .

Features of a homeostasis at advanced and senile age

the Same level of homeostatic sizes during various age periods is supported due to various shifts in systems of their regulation. E.g., constancy of the ABP level at young age is maintained due to higher minute cordial emission and low general peripheric resistance of vessels, and in elderly and senile — at the expense of more high general peripheric resistance and reduction of size of minute cordial emission. During the aging of an organism constancy of the major fiziol, functions is maintained in the conditions of reduction of reliability and reduction of possible range fiziol, changes of. Relative G.'s preservation at essential structural, exchange and functional changes is reached by the fact that at the same time there is not only a fading, disturbance and degradation, but also development of specific adaptive mechanisms. At the expense of it the invariable level of a sugar content in blood, pH of blood, osmotic pressure, membrane potential of cells etc. is supported.

In G.'s preservation in the course of aging of an organism changes of mechanisms have essential value neurohumoral regulation (see), increase in sensitivity of fabrics to effect of hormones and mediators against the background of weakening of nervous influences.

During the aging of an organism cardiac performance, lung ventilation, gas exchange, renal functions, secretion of digestive glands, function of hemadens, a metabolism, etc. significantly changes. These changes can be characterized as a homeorhesis — a natural trajectory (dynamics) of change of intensity of exchange and fiziol. functions with age in time. Value of the course of age changes is very important for the characteristic of process of aging of the person, his definition biol, age.

At advanced and senile age the general potentialitys of adaptive mechanisms decrease. Therefore in old age at the raised loadings, stresses and other situations the probability of failure of adaptable mechanisms and G.'s disturbance increase. Such reduction of reliability of mechanisms G. is one of the most important premises of development patol, disturbances in old age.

See also Internal environment of an organism .

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