Homeostasis(ancient Greek ὁμοιοστάσις from ὅμοιος - the same, similar and στάσις - standing, immobility) - self-regulation, the ability of an open system to maintain the constancy of its internal state through coordinated reactions aimed at maintaining dynamic balance. The desire of the system to reproduce itself, to restore the lost balance, to overcome the resistance of the external environment. Population homeostasis is the ability of a population to maintain a certain number of its individuals for a long time.

General information

properties of homeostasis

• instability
• Striving for balance
• unpredictability

• Regulation of the level of basic metabolism depending on the diet.

Main article: Feedback

Ecological homeostasis

Biological homeostasis

Cellular homeostasis

The regulation of the chemical activity of the cell is achieved through a number of processes, among which the change in the structure of the cytoplasm itself, as well as the structure and activity of enzymes, is of particular importance. Autoregulation depends on temperature, the degree of acidity, the concentration of the substrate, the presence of certain macro- and microelements. Cellular mechanisms of homeostasis are aimed at restoring naturally dead cells of tissues or organs in case of violation of their integrity.

Regeneration-the process of updating the structural elements of the body and restoring their number after damage, aimed at providing the necessary functional activity

Depending on the regenerative response, tissues and organs of mammals can be divided into 3 groups:

1) tissues and organs that are characterized by cellular regeneration (bones, loose connective tissue, hematopoietic system, endothelium, mesothelium, mucous membranes of the gastrointestinal tract, respiratory tract and genitourinary system)

2) tissues and organs that are characterized by cellular and intracellular regeneration (liver, kidneys, lungs, smooth and skeletal muscles, autonomic nervous system, pancreas, endocrine system)

3) tissues, which are characterized mainly or exclusively by intracellular regeneration (myocardium and ganglion cells of the central nervous system)

In the process of evolution, 2 types of regeneration were formed: physiological and reparative.

Other areas

The actuary can talk about risk homeostasis in which, for example, people who have an anti-lock braking system installed in their car are not in a safer position than those who do not have it installed, because these people unconsciously compensate for a safer car by risky driving. This happens because some of the holding mechanisms - such as fear - stop working.

stress homeostasis

Examples

• thermoregulation
  • Skeletal muscle trembling may begin if the body temperature is too low.
• Chemical regulation

Sources

1. O.-Ya.L.Bekish. Medical biology. - Minsk: Urajay, 2000. - 520 p. - ISBN 985-04-0336-5.

Topic № 13. Homeostasis, mechanisms of its regulation.

The body as an open self-regulating system.

A living organism is an open system that has a connection with the environment through the nervous, digestive, respiratory, excretory systems, etc.

In the process of metabolism with food, water, during gas exchange, various chemical compounds enter the body, which undergo changes in the body, enter the structure of the body, but do not remain permanently. Assimilated substances decompose, release energy, decay products are removed into the external environment. The destroyed molecule is replaced by a new one, and so on.

The body is an open, dynamic system. In a constantly changing environment, the body maintains a stable state for a certain time.

The concept of homeostasis. General patterns of homeostasis of living systems.

homeostasis - the property of a living organism to maintain a relative dynamic constancy of the internal environment. Homeostasis is expressed in the relative constancy of the chemical composition, osmotic pressure, stability of the basic physiological functions. Homeostasis is specific and determined by the genotype.

The preservation of the integrity of the individual properties of an organism is one of the most general biological laws. This law is provided in the vertical series of generations by the mechanisms of reproduction, and throughout the life of the individual - by the mechanisms of homeostasis.

The phenomenon of homeostasis is an evolutionarily developed, hereditarily fixed adaptive property of the body to normal environmental conditions. However, these conditions can be short-term or long-term outside the normal range. In such cases, the phenomena of adaptation are characterized not only by the restoration of the usual properties of the internal environment, but also by short-term changes in function (for example, an increase in the rhythm of cardiac activity and an increase in the frequency of respiratory movements with increased muscular work). Homeostasis reactions can be directed to:

maintaining known steady state levels;

elimination or limitation of harmful factors;

development or preservation of optimal forms of interaction between the organism and the environment in the changed conditions of its existence. All these processes determine adaptation.

Therefore, the concept of homeostasis means not only a certain constancy of various physiological constants of the body, but also includes the processes of adaptation and coordination of physiological processes that ensure the unity of the body not only in the norm, but also under changing conditions of its existence.

The main components of homeostasis were defined by C. Bernard, and they can be divided into three groups:

A. Substances that provide cellular needs:

Substances necessary for the formation of energy, for growth and recovery - glucose, proteins, fats.

NaCl, Ca and other inorganic substances.

Oxygen.

internal secretion.

B. Environmental factors affecting cellular activity:

osmotic pressure.

Temperature.

Hydrogen ion concentration (pH).

B. Mechanisms that ensure structural and functional unity:

Heredity.

Regeneration.

immunobiological reactivity.

The principle of biological regulation ensures the internal state of the organism (its content), as well as the relationship between the stages of ontogenesis and phylogenesis. This principle has become widespread. When studying it, cybernetics arose - the science of purposeful and optimal control of complex processes in wildlife, in human society, industry (Berg I.A., 1962).

A living organism is a complex controlled system where many variables of the external and internal environment interact. Common to all systems is the presence input variables, which, depending on the properties and laws of the system's behavior, are transformed into weekends variables (Fig. 10).

Rice. 10 - General scheme of homeostasis of living systems

The output variables depend on the input variables and the laws of the system behavior.

The influence of the output signal on the control part of the system is called feedback , which is of great importance in self-regulation (homeostatic reaction). Distinguish negative andpositive feedback.

negative feedback reduces the influence of the input signal on the value of the output according to the principle: "the more (at the output), the less (at the input)". It helps to restore the homeostasis of the system.

At positive feedback, the value of the input signal increases according to the principle: "the more (at the output), the more (at the input)". It enhances the resulting deviation from the initial state, which leads to a violation of homeostasis.

However, all types of self-regulation operate on the same principle: self-deviation from the initial state, which serves as a stimulus for turning on correction mechanisms. So, normal blood pH is 7.32 - 7.45. A shift in pH by 0.1 leads to a violation of cardiac activity. This principle was described by Anokhin P.K. in 1935 and called the feedback principle, which serves to implement adaptive reactions.

General principle of homeostatic response(Anokhin: "Theory of functional systems"):

deviation from the initial level → signal → activation of regulatory mechanisms based on the feedback principle → correction of changes (normalization).

So, during physical work, the concentration of CO 2 in the blood increases → pH shifts to the acid side → the signal enters the respiratory center of the medulla oblongata → centrifugal nerves conduct an impulse to the intercostal muscles and breathing deepens → a decrease in CO 2 in the blood, pH is restored.

Mechanisms of regulation of homeostasis at the molecular-genetic, cellular, organismal, population-species and biospheric levels.

Regulatory homeostatic mechanisms function at the gene, cellular and systemic (organismic, population-species and biospheric) levels.

Gene mechanisms homeostasis. All phenomena of body homeostasis are genetically determined. Already at the level of primary gene products there is a direct connection - "one structural gene - one polypeptide chain". Moreover, there is a collinear correspondence between the DNA nucleotide sequence and the amino acid sequence of the polypeptide chain. The hereditary program of the individual development of an organism provides for the formation of species-specific characteristics not in constant, but in changing environmental conditions, within the limits of a hereditarily determined norm of reaction. The double helix of DNA is essential in the processes of its replication and repair. Both are directly related to ensuring the stability of the functioning of the genetic material.

From a genetic point of view, one can distinguish between elementary and systemic manifestations of homeostasis. Examples of elementary manifestations of homeostasis are: gene control of thirteen blood coagulation factors, gene control of histocompatibility of tissues and organs, which allows transplantation.

The transplanted area is called transplant. The organism from which tissue is taken for transplantation is donor , and to whom they transplant - recipient . The success of transplantation depends on the immunological reactions of the body. There are autotransplantation, syngeneic transplantation, allotransplantation and xenotransplantation.

Autotransplantation – transplantation of tissues in the same organism. In this case, the proteins (antigens) of the transplant do not differ from the proteins of the recipient. There is no immunological reaction.

Syngeneic transplant carried out in identical twins with the same genotype.

allotransplantation – transplantation of tissues from one individual to another belonging to the same species. The donor and recipient differ in antigens, therefore, in higher animals, long-term engraftment of tissues and organs is observed.

Xenotransplantation Donor and recipient belong to different types of organisms. This type of transplantation succeeds in some invertebrates, but such transplants do not take root in higher animals.

In transplantation, the phenomenon is of great importance immunological tolerance (tissue compatibility). Suppression of immunity in the case of tissue transplantation (immunosuppression) is achieved by: suppression of the activity of the immune system, radiation, administration of antilymphotic serum, hormones of the adrenal cortex, chemical preparations - antidepressants (imuran). The main task is to suppress not just immunity, but transplant immunity.

transplant immunity determined by the genetic constitution of the donor and recipient. The genes responsible for the synthesis of antigens that cause a reaction to the transplanted tissue are called tissue incompatibility genes.

In humans, the main genetic system of histocompatibility is the HLA (Human Leukocyte Antigen) system. Antigens are sufficiently well represented on the surface of leukocytes and are determined using antisera. The plan of the structure of the system in humans and animals is the same. A unified terminology has been adopted to describe the genetic loci and alleles of the HLA system. Antigens are designated: HLA-A 1 ; HLA-A 2 etc. New antigens that have not been finally identified are designated - W (Work). Antigens of the HLA system are divided into 2 groups: SD and LD (Fig. 11).

Antigens of the SD group are determined by serological methods and are determined by the genes of 3 subloci of the HLA system: HLA-A; HLA-B; HLA-C.

Rice. 11 - HLA main human histocompatibility genetic system

LD - antigens are controlled by the HLA-D sublocus of the sixth chromosome, and are determined by the method of mixed cultures of leukocytes.

Each of the genes that control HLA - human antigens, has a large number of alleles. So the HLA-A sublocus controls 19 antigens; HLA-B - 20; HLA-C - 5 "working" antigens; HLA-D - 6. Thus, about 50 antigens have already been found in humans.

The antigenic polymorphism of the HLA system is the result of the origin of one from the other and the close genetic relationship between them. The identity of the donor and recipient according to the antigens of the HLA system is necessary for transplantation. Transplantation of a kidney identical in 4 antigens of the system provides survival by 70%; 3 - 60%; 2 - 45%; 1 - 25%.

There are special centers that conduct the selection of a donor and recipient for transplantation, for example, in the Netherlands - "Eurotransplant". Typing by antigens of the HLA system is also carried out in the Republic of Belarus.

Cellular mechanisms homeostasis are aimed at restoring the cells of tissues, organs in case of violation of their integrity. The totality of processes aimed at restoring destructible biological structures is called regeneration. Such a process is characteristic of all levels: renewal of proteins, components of cell organelles, whole organelles and the cells themselves. Restoration of organ functions after an injury or rupture of a nerve, wound healing is important for medicine in terms of mastering these processes.

Tissues, according to their regenerative capacity, are divided into 3 groups:

Tissues and organs that are characterized cellular regeneration (bones, loose connective tissue, hematopoietic system, endothelium, mesothelium, mucous membranes of the intestinal tract, respiratory tract and genitourinary system.

Tissues and organs that are characterized cellular and intracellular regeneration (liver, kidneys, lungs, smooth and skeletal muscles, autonomic nervous system, endocrine, pancreas).

Fabrics that are predominantly intracellular regeneration (myocardium) or exclusively intracellular regeneration (ganglion cells of the central nervous system). It covers the processes of restoration of macromolecules and cell organelles by assembling elementary structures or by their division (mitochondria).

In the process of evolution, 2 types of regeneration were formed physiological and reparative .

Physiological regeneration - This is a natural process of restoring the elements of the body throughout life. For example, the restoration of erythrocytes and leukocytes, the change of the epithelium of the skin, hair, the replacement of milk teeth with permanent ones. These processes are influenced by external and internal factors.

Reparative regeneration is the restoration of organs and tissues lost due to damage or injury. The process occurs after mechanical injuries, burns, chemical or radiation injuries, as well as as a result of diseases and surgical operations.

Reparative regeneration is divided into typical (homomorphosis) and atypical (heteromorphosis). In the first case, it regenerates an organ that was removed or destroyed, in the second, another organ develops in place of the removed organ.

Atypical regeneration more common in invertebrates.

Hormones stimulate regeneration pituitary gland and thyroid gland . There are several ways to regenerate:

Epimorphosis or complete regeneration - restoration of the wound surface, completion of the part to the whole (for example, the growth of a tail in a lizard, limbs in a newt).

Morphollaxis - restructuring of the remaining part of the organ to the whole, only smaller. This method is characterized by the restructuring of the new from the remnants of the old (for example, the restoration of a limb in a cockroach).

Endomorphosis - recovery due to intracellular restructuring of tissue and organ. Due to the increase in the number of cells and their size, the mass of the organ approaches the initial one.

In vertebrates, reparative regeneration occurs in the following form:

Complete regeneration - restoration of the original tissue after its damage.

Regenerative hypertrophy characteristic of internal organs. In this case, the wound surface heals with a scar, the removed area does not grow back and the shape of the organ is not restored. The mass of the remaining part of the organ increases due to an increase in the number of cells and their size and approaches the original value. So in mammals, the liver, lungs, kidneys, adrenal glands, pancreas, salivary, thyroid glands regenerate.

Intracellular compensatory hyperplasia cell ultrastructures. In this case, a scar is formed at the site of damage, and the restoration of the original mass occurs due to an increase in the volume of cells, and not their number, based on the growth (hyperplasia) of intracellular structures (nervous tissue).

Systemic mechanisms are provided by the interaction of regulatory systems: nervous, endocrine and immune .

Nervous regulation carried out and coordinated by the central nervous system. Nerve impulses, entering cells and tissues, cause not only excitation, but also regulate chemical processes, the exchange of biologically active substances. Currently, more than 50 neurohormones are known. So, in the hypothalamus, vasopressin, oxytocin, liberins and statins are produced that regulate the function of the pituitary gland. Examples of systemic manifestations of homeostasis are the maintenance of a constant temperature, blood pressure.

From the standpoint of homeostasis and adaptation, the nervous system is the main organizer of all body processes. At the heart of adaptation, balancing organisms with environmental conditions, according to N.P. Pavlov, are reflex processes. Between different levels of homeostatic regulation there is a private hierarchical subordination in the system of regulation of the internal processes of the body (Fig. 12).

hemispheric cortex and parts of the brain

feedback self-regulation

peripheral neuro-regulatory processes, local reflexes

Cellular and tissue levels of homeostasis

Rice. 12. - Hierarchical subordination in the system of regulation of the internal processes of the body.

The most primary level is the homeostatic systems of the cellular and tissue levels. Above them are peripheral nervous regulatory processes such as local reflexes. Further in this hierarchy are the systems of self-regulation of certain physiological functions with various channels of "feedback". The top of this pyramid is occupied by the cerebral cortex and the brain.

In a complex multicellular organism, both direct and feedback connections are carried out not only by nervous, but also by hormonal (endocrine) mechanisms. Each of the glands that make up the endocrine system affects the other organs of this system and, in turn, is influenced by the latter.

Endocrine mechanisms homeostasis according to B.M. Zavadsky, this is a mechanism of plus or minus interaction, i.e. balancing the functional activity of the gland with the concentration of the hormone. With a high concentration of the hormone (above normal), the activity of the gland is weakened and vice versa. This effect is carried out by the action of the hormone on the gland that produces it. In a number of glands, regulation is established through the hypothalamus and the anterior pituitary gland, especially during a stress response.

Endocrine glands can be divided into two groups in relation to their relation to the anterior pituitary gland. The latter is considered central, and the other endocrine glands are considered peripheral. This division is based on the fact that the anterior pituitary gland produces the so-called tropic hormones, which activate certain peripheral endocrine glands. In turn, the hormones of the peripheral endocrine glands act on the anterior pituitary gland, inhibiting the secretion of tropic hormones.

The reactions that provide homeostasis cannot be limited to any one endocrine gland, but captures all glands to one degree or another. The resulting reaction acquires a chain flow and spreads to other effectors. The physiological significance of hormones lies in the regulation of other body functions, and therefore the chain character should be expressed as much as possible.

Constant violations of the body's environment contribute to the preservation of its homeostasis during a long life. If you create such conditions of life under which nothing causes significant changes in the internal environment, then the organism will be completely unarmed when it encounters the environment and will soon die.

The combination of nervous and endocrine regulatory mechanisms in the hypothalamus allows for complex homeostatic reactions associated with the regulation of the visceral function of the body. The nervous and endocrine systems are the unifying mechanism of homeostasis.

An example of a general response of nervous and humoral mechanisms is a state of stress that develops under adverse living conditions and there is a threat of homeostasis disturbance. Under stress, there is a change in the state of most systems: muscular, respiratory, cardiovascular, digestive, sensory organs, blood pressure, blood composition. All these changes are a manifestation of individual homeostatic reactions aimed at increasing the body's resistance to adverse factors. The rapid mobilization of the body's forces acts as a protective reaction to a state of stress.

With "somatic stress" the task of increasing the overall resistance of the organism is solved according to the scheme shown in Figure 13.

Rice. 13 - Scheme of increasing the overall resistance of the body when

Homeostasis - what is it? The concept of homeostasis

Homeostasis is a self-regulating process in which all biological systems strive to maintain stability during the period of adaptation to certain conditions that are optimal for survival. Any system, being in dynamic equilibrium, strives to achieve a stable state that resists external factors and stimuli.

The concept of homeostasis

All body systems must work together to maintain proper homeostasis within the body. Homeostasis is the regulation of body temperature, water content, and carbon dioxide levels. For example, diabetes mellitus is a condition in which the body cannot regulate blood glucose levels.

Homeostasis is a term that is used both to describe the existence of organisms in an ecosystem and to describe the successful functioning of cells within an organism. Organisms and populations can maintain homeostasis while maintaining stable birth and death rates.

Feedback

Feedback is a process that occurs when the body's systems need to be slowed down or completely stopped. When a person eats, food enters the stomach and digestion begins. In between meals, the stomach should not work. The digestive system works with a series of hormones and nerve impulses to stop and start acid production in the stomach.

Another example of negative feedback can be observed in the case of an increase in body temperature. The regulation of homeostasis is manifested by sweating, a protective reaction of the body to overheating. In this way, the rise in temperature is stopped and the problem of overheating is neutralized. In case of hypothermia, the body also provides for a number of measures taken in order to warm up.

Maintaining internal balance

Homeostasis can be defined as a property of an organism or system that helps it to maintain given parameters within the normal range of values. This is the key to life, and the wrong balance in maintaining homeostasis can lead to diseases such as hypertension and diabetes.

Homeostasis is a key element in understanding how the human body works. Such a formal definition characterizes a system that regulates its internal environment and seeks to maintain the stability and regularity of all processes occurring in the body.

Homeostatic regulation: body temperature

Body temperature control in humans is a good example of homeostasis in a biological system. When a person is healthy, their body temperature fluctuates around + 37°C, but various factors can affect this value, including hormones, metabolic rate, and various diseases that cause fever.

In the body, temperature regulation is controlled in a part of the brain called the hypothalamus. Through the bloodstream to the brain, temperature signals are received, as well as the analysis of the results of data on the frequency of respiration, blood sugar and metabolism. The loss of heat in the human body also contributes to reduced activity.

Water-salt balance

No matter how much water a person drinks, the body does not swell like a balloon, and the human body does not shrink like raisins if you drink very little. Probably, someone once thought about it at least once. One way or another, the body knows how much fluid needs to be stored to maintain the desired level.

The concentration of salt and glucose (sugar) in the body is maintained at a constant level (in the absence of negative factors), the amount of blood in the body is about 5 liters.

Blood sugar regulation

Glucose is a type of sugar found in the blood. The human body must maintain proper glucose levels in order for a person to remain healthy. When glucose levels get too high, the pancreas releases the hormone insulin.

If the blood glucose level drops too low, the liver converts the glycogen in the blood, thereby raising the sugar level. When pathogenic bacteria or viruses enter the body, it begins to fight the infection before the pathogenic elements can lead to any health problems.

Pressure under control

Maintaining healthy blood pressure is also an example of homeostasis. The heart can sense changes in blood pressure and send signals to the brain for processing. Next, the brain sends a signal back to the heart with instructions on how to respond correctly. If the blood pressure is too high, it must be lowered.

How is homeostasis achieved?

How does the human body regulate all systems and organs and compensate for the ongoing changes in the environment? This is due to the presence of many natural sensors that control temperature, blood salt composition, blood pressure and many other parameters. These detectors send signals to the brain, to the main control center, in case some values ​​deviate from the norm. After that, compensatory measures are launched to restore the normal state.

Maintaining homeostasis is incredibly important for the body. The human body contains a certain amount of chemicals known as acids and alkalis, and their proper balance is essential for the optimal functioning of all organs and body systems. The level of calcium in the blood must be maintained at the proper level. Because breathing is involuntary, the nervous system provides the body with much-needed oxygen. When toxins enter your bloodstream, they disrupt the body's homeostasis. The human body responds to this disturbance with the help of the urinary system.

It is important to emphasize that the body's homeostasis works automatically if the system functions normally. For example, a reaction to heat - the skin turns red, because its small blood vessels automatically dilate. Trembling is a response to being cold. Thus, homeostasis is not a set of organs, but the synthesis and balance of bodily functions. Together, this allows you to maintain the entire body in a stable state.

9.4. The concept of homeostasis. General patterns of homeostasis of living systems

Despite the fact that a living organism is an open system that exchanges matter and energy with the environment and exists in unity with it, it retains itself in time and space as a separate biological unit, retains its structure (morphology), behavioral reactions, specific physical -chemical conditions in cells, tissue fluid. The ability of living systems to withstand changes and maintain the dynamic constancy of composition and properties is called homeostasis. The term "homeostasis" was proposed by W. Cannon in 1929. However, the idea of ​​the existence of physiological mechanisms that ensure the maintenance of the constancy of the internal environment of organisms was expressed in the second half of the 19th century by C. Bernard.

Homeostasis has improved in the course of evolution. Multicellular organisms have an internal environment in which cells of various organs and tissues are located. Then specialized organ systems (circulation, nutrition, respiration, excretion, etc.) were formed, which are involved in ensuring homeostasis at all levels of organization (molecular, subcellular, cellular, tissue, organ and organism). The most perfect mechanisms of homeostasis were formed in mammals, which contributed to a significant expansion of the possibilities of their adaptation to the environment. Mechanisms and types of homeostasis evolved in the process of long-term evolution, being fixed genetically. The appearance in the body of alien genetic information, which is often introduced by bacteria, viruses, cells of other organisms, as well as its own mutated cells, can significantly disrupt the body's homeostasis. As a protection against alien genetic information, the penetration of which into the body and its subsequent implementation would lead to poisoning with toxins (foreign proteins), such a type of homeostasis arose as genetic homeostasis, which ensures the genetic constancy of the internal environment of the body. It is based on immunological mechanisms, including non-specific and specific protection of the body's own integrity and individuality. Non-specific mechanisms underlie innate, constitutional, species immunity, as well as individual nonspecific resistance. These include the barrier function of the skin and mucous membranes, the bactericidal action of the secretion of sweat and sebaceous glands, the bactericidal properties of the contents of the stomach and intestines, lysozyme secretion of the salivary and lacrimal glands. If the organisms penetrate into the internal environment, they are eliminated during the inflammatory reaction, which is accompanied by enhanced phagocytosis, as well as the virusostatic effect of interferon (a protein with a molecular weight of 25,000 - 110,000).

Specific immunological mechanisms form the basis of acquired immunity, carried out by the immune system, which recognizes, processes and eliminates foreign antigens. Humoral immunity is carried out through the formation of antibodies circulating in the blood. The basis of cellular immunity is the formation of T-lymphocytes, the appearance of long-lived T- and B-lymphocytes of "immunological memory", the occurrence of allergies (hypersensitivity to a specific antigen). In humans, protective reactions come into effect only at the 2nd week of life, reach their highest activity by the age of 10, decrease slightly from 10 to 20 years, remain approximately at the same level from 20 to 40 years, then gradually fade away.

Immunological defense mechanisms are a serious obstacle in organ transplantation, causing graft resorption. The most successful are currently the results of autotransplantation (transplantation of tissues within the body) and allotransplantation between identical twins. They are much less successful in interspecies transplantation (heterotransplantation or xenotransplantation).

Another type of homeostasis is biochemical homeostasis helps to maintain the constancy of the chemical composition of the liquid extracellular (internal) environment of the body (blood, lymph, tissue fluid), as well as the constancy of the chemical composition of the cytoplasm and plasmolemma of cells. Physiological homeostasis ensures the constancy of the processes of vital activity of the body. Thanks to him, isoosmia (the constancy of the content of osmotically active substances), isothermia (maintenance of the body temperature of birds and mammals within certain limits), etc., have arisen and are being improved. Structural homeostasis ensures the constancy of the structure (morphological organization) at all levels (molecular, subcellular, cellular, etc.) of the organization of the living.

Population homeostasis ensures the constancy of the number of individuals in the population. Biocenotic homeostasis contributes to the constancy of the species composition and number of individuals in biocenoses.

Due to the fact that the body functions and interacts with the environment as a single system, the processes underlying various types of homeostatic reactions are closely interconnected with each other. Separate homeostatic mechanisms are combined and implemented in a holistic adaptive reaction of the body as a whole. Such association is carried out due to the activity (function) of regulatory integrating systems (nervous, endocrine, immune). The most rapid changes in the state of the regulated object are provided by the nervous system, which is associated with the speed of the processes of occurrence and conduction of a nerve impulse (from 0.2 to 180 m/sec). The regulatory function of the endocrine system is carried out more slowly, as it is limited by the rate of release of hormones by the glands and their transfer in the bloodstream. However, the effect of hormones accumulating in it on a regulated object (organ) is much longer than with nervous regulation.

The body is a self-regulating living system. Due to the presence of homeostatic mechanisms, the body is a complex self-regulating system. The principles of existence and development of such systems are studied by cybernetics, while those of living systems are studied by biological cybernetics.

Self-regulation of biological systems is based on the principle of direct and feedback.

Information about the deviation of the controlled value from the set level is transmitted to the controller through the feedback channels and changes its activity in such a way that the controlled value returns to the initial (optimal) level (Fig. 122). Feedback can be negative(when the controlled value has deviated in a positive direction (synthesis of a substance, for example, has increased excessively)) and put-

Rice. 122. Scheme of direct and feedback in a living organism:

P - regulator (nerve center, endocrine gland); RO - regulated object (cell, tissue, organ); 1 – optimal functional activity of RO; 2 - reduced functional activity of RO with positive feedback; 3 - increased functional activity of RO with negative feedback

body(when the controlled value has deviated in the negative direction (the substance is synthesized in insufficient quantity)). This mechanism, as well as more complex combinations of several mechanisms, take place at different levels of organization of biological systems. As an example of their functioning at the molecular level, one can point to the inhibition of a key enzyme with excessive formation of the final product or the repression of enzyme synthesis. At the cellular level, the mechanisms of direct and feedback provide hormonal regulation and optimal density (number) of the cell population. A manifestation of direct and feedback at the level of the body is the regulation of blood glucose. In a living organism, the mechanisms of automatic regulation and control (studied by biocybernetics) are especially complex. The degree of their complexity contributes to an increase in the level of "reliability" and stability of living systems in relation to environmental changes.

The mechanisms of homeostasis are duplicated at different levels. This in nature realizes the principle of multi-loop regulation of systems. The main circuits are represented by cellular and tissue homeostatic mechanisms. They have a high degree of automatism. The main role in the control of cellular and tissue homeostatic mechanisms belongs to genetic factors, local reflex influences, chemical and contact interactions between cells.

The mechanisms of homeostasis undergo significant changes throughout human ontogenesis. Only 2 weeks after birth

Rice. 123. Options for loss and recovery in the body

biological defense reactions come into play (cells are formed that provide cellular and humoral immunity), and their effectiveness continues to increase by the age of 10. During this period, the mechanisms of protection against alien genetic information are improved, and the maturity of the nervous and endocrine regulatory systems also increases. The mechanisms of homeostasis reach the greatest reliability in adulthood, by the end of the period of development and growth of the organism (19-24 years). The aging of the body is accompanied by a decrease in the effectiveness of the mechanisms of genetic, structural, physiological homeostasis, a weakening of the regulatory influences of the nervous and endocrine systems.

5. Homeostasis.

An organism can be defined as a physicochemical system that exists in the environment in a stationary state. It is this ability of living systems to maintain a stationary state in a continuously changing environment that determines their survival. To ensure a steady state, all organisms - from the morphologically simplest to the most complex - have developed a variety of anatomical, physiological and behavioral adaptations that serve the same purpose - to maintain the constancy of the internal environment.

For the first time, the idea that the constancy of the internal environment provides optimal conditions for the life and reproduction of organisms was expressed in 1857 by the French physiologist Claude Bernard. Throughout his scientific activity, Claude Bernard was struck by the ability of organisms to regulate and maintain, within fairly narrow limits, such physiological parameters as body temperature or water content in it. He summarized this idea of ​​self-regulation as the basis of physiological stability in the form of the classic statement: "The constancy of the internal environment is a prerequisite for a free life."

Claude Bernard emphasized the difference between the external environment in which organisms live and the internal environment in which their individual cells are located, and understood how important it was for the internal environment to remain unchanged. For example, mammals are able to maintain body temperature despite fluctuations in ambient temperature. If it gets too cold, the animal may move to a warmer or more sheltered place, and if this is not possible, self-regulatory mechanisms come into play that increase body temperature and prevent heat loss. The adaptive significance of this lies in the fact that the organism as a whole functions more efficiently, since the cells of which it is composed are in optimal conditions. Self-regulation systems operate not only at the level of the organism, but also at the level of cells. An organism is the sum of its constituent cells, and the optimal functioning of the organism as a whole depends on the optimal functioning of its constituent parts. Any self-organizing system maintains the constancy of its composition - qualitative and quantitative. This phenomenon is called homeostasis, and it is common to most biological and social systems. The term homeostasis was introduced in 1932 by the American physiologist Walter Cannon.

homeostasis(Greek homoios - similar, the same; stasis-state, immobility) - the relative dynamic constancy of the internal environment (blood, lymph, tissue fluid) and the stability of basic physiological functions (blood circulation, respiration, thermoregulation, metabolism, etc. ) of humans and animals. Regulatory mechanisms that maintain the physiological state or properties of cells, organs and systems of the whole organism at an optimal level are called homeostatic. Historically and genetically, the concept of homeostasis has biological and biomedical prerequisites. There it is correlated as a final process, a period of life with a separate isolated organism or a human individual as a purely biological phenomenon. The finiteness of existence and the need to fulfill one's destiny - reproduction of one's own kind - allow one to determine the survival strategy of an individual organism through the concept of "preservation". "Preservation of structural and functional stability" is the essence of any homeostasis, controlled by a homeostat or self-regulating.

As you know, a living cell is a mobile, self-regulating system. Its internal organization is supported by active processes aimed at limiting, preventing or eliminating shifts caused by various influences from the environment and the internal environment. The ability to return to the original state after a deviation from a certain average level, caused by one or another "disturbing" factor, is the main property of the cell. A multicellular organism is a holistic organization, the cellular elements of which are specialized to perform various functions. Interaction within the body is carried out by complex regulatory, coordinating and correlating mechanisms with the participation of nervous, humoral, metabolic and other factors. Many individual mechanisms that regulate intra- and intercellular relationships, in some cases, have mutually opposite effects that balance each other. This leads to the establishment of a mobile physiological background (physiological balance) in the body and allows the living system to maintain relative dynamic constancy, despite changes in the environment and shifts that occur during the life of the organism.

As studies show, the methods of regulation existing in living organisms have many features in common with regulatory devices in non-living systems, such as machines. In both cases, stability is achieved through a certain form of management.

The very concept of homeostasis does not correspond to the concept of stable (not fluctuating) balance in the body - the principle of balance is not applicable to complex physiological and biochemical processes occurring in living systems. It is also wrong to oppose homeostasis to rhythmic fluctuations in the internal environment. Homeostasis in a broad sense covers the issues of cyclic and phase flow of reactions, compensation, regulation and self-regulation of physiological functions, the dynamics of the interdependence of nervous, humoral and other components of the regulatory process. The boundaries of homeostasis can be rigid and plastic, vary depending on individual age, gender, social, professional and other conditions.

Of particular importance for the life of the organism is the constancy of the composition of the blood - the liquid basis of the body (fluidmatrix), according to W. Cannon. The stability of its active reaction (pH), osmotic pressure, ratio of electrolytes (sodium, calcium, chlorine, magnesium, phosphorus), glucose content, number of formed elements, etc. is well known. For example, blood pH, as a rule, does not beyond 7.35-7.47. Even severe disorders of acid-base metabolism with pathological accumulation of acids in the tissue fluid, for example, in diabetic acidosis, have very little effect on the active reaction of the blood. Despite the fact that the osmotic pressure of blood and tissue fluid is subject to continuous fluctuations due to the constant supply of osmotically active products of interstitial metabolism, it remains at a certain level and changes only in some severe pathological conditions. Maintaining a constant osmotic pressure is of paramount importance for water metabolism and maintaining ionic balance in the body. The greatest constancy is the concentration of sodium ions in the internal environment. The content of other electrolytes also fluctuates within narrow limits. The presence of a large number of osmoreceptors in tissues and organs, including the central nervous formations (hypothalamus, hippocampus), and a coordinated system of regulators of water metabolism and ionic composition allows the body to quickly eliminate shifts in the osmotic blood pressure that occur, for example, when water is introduced into the body .

Despite the fact that blood represents the general internal environment of the body, the cells of organs and tissues do not directly come into contact with it. In multicellular organisms, each organ has its own internal environment (microenvironment) corresponding to its structural and functional features, and the normal state of organs depends on the chemical composition, physicochemical, biological and other properties of this microenvironment. Its homeostasis is determined by the functional state of histohematic barriers and their permeability in the directions of blood - tissue fluid; tissue fluid - blood.

Of particular importance is the constancy of the internal environment for the activity of the central nervous system: even minor chemical and physicochemical shifts that occur in the cerebrospinal fluid, glia, and pericellular spaces can cause a sharp disruption in the course of life processes in individual neurons or in their ensembles. A complex homeostatic system, including various neurohumoral, biochemical, hemodynamic and other regulatory mechanisms, is the system for ensuring the optimal level of blood pressure. At the same time, the upper limit of the level of arterial pressure is determined by the functionality of the baroreceptors of the vascular system of the body, and the lower limit is determined by the body's needs for blood supply.

The most perfect homeostatic mechanisms in the body of higher animals and humans include the processes of thermoregulation; in homoiothermic animals, fluctuations in temperature in the internal parts of the body during the most dramatic changes in temperature in the environment do not exceed tenths of a degree.

The organizing role of the nervous apparatus (the principle of nervism) underlies the well-known ideas about the essence of the principles of homeostasis. However, neither the dominant principle, nor the theory of barrier functions, nor the general adaptation syndrome, nor the theory of functional systems, nor the hypothalamic regulation of homeostasis, and many other theories can completely solve the problem of homeostasis.

In some cases, the concept of homeostasis is not quite rightly used to explain isolated physiological states, processes, and even social phenomena. This is how the terms “immunological”, “electrolyte”, “systemic”, “molecular”, “physico-chemical”, “genetic homeostasis”, etc., appear in the literature. Attempts have been made to reduce the problem of homeostasis to the principle of self-regulation. An example of solving the problem of homeostasis from the standpoint of cybernetics is Ashby's attempt (W.R. Ashby, 1948) to design a self-regulating device that simulates the ability of living organisms to maintain the level of certain quantities within physiologically acceptable limits.

In practice, researchers and clinicians face the issues of assessing the adaptive (adaptive) or compensatory capabilities of the body, their regulation, strengthening and mobilization, predicting the body's response to disturbing influences. Some states of vegetative instability, caused by insufficiency, excess or inadequacy of regulatory mechanisms, are considered as “diseases of homeostasis”. With a certain conventionality, they can include functional disturbances in the normal functioning of the body associated with its aging, forced restructuring of biological rhythms, some phenomena of vegetative dystonia, hyper- and hypocompensatory reactivity during stressful and extreme influences, etc.

To assess the state of homeostatic mechanisms in a physiological experiment and in clinical practice, various dosed functional tests are used (cold, thermal, adrenaline, insulin, mezaton, etc.) with the determination of the ratio of biologically active substances (hormones, mediators, metabolites) in blood and urine, etc. .d.

Biophysical mechanisms of homeostasis.

From the point of view of chemical biophysics, homeostasis is a state in which all processes responsible for energy transformations in the body are in dynamic equilibrium. This state is the most stable and corresponds to the physiological optimum. In accordance with the concepts of thermodynamics, an organism and a cell can exist and adapt to such environmental conditions under which a stationary flow of physicochemical processes can be established in a biological system, i.e. homeostasis. The main role in establishing homeostasis belongs primarily to cellular membrane systems, which are responsible for bioenergetic processes and regulate the rate of entry and release of substances by cells.

From these positions, the main causes of the disturbance are non-enzymatic reactions that are unusual for normal life activity, occurring in membranes; in most cases, these are chain reactions of oxidation involving free radicals that occur in cell phospholipids. These reactions lead to damage to the structural elements of cells and disruption of the regulatory function. Factors that cause homeostasis disorders also include agents that cause radical formation - ionizing radiation, infectious toxins, certain foods, nicotine, as well as a lack of vitamins, etc.

One of the main factors stabilizing the homeostatic state and functions of membranes are bioantioxidants, which inhibit the development of oxidative radical reactions.

Age features of homeostasis in children.

The constancy of the internal environment of the body and the relative stability of physicochemical parameters in childhood are provided with a pronounced predominance of anabolic metabolic processes over catabolic ones. This is an indispensable condition for growth and distinguishes the child's body from the body of adults, in which the intensity of metabolic processes is in a state of dynamic equilibrium. In this regard, the neuroendocrine regulation of the homeostasis of the child's body is more intense than in adults. Each age period is characterized by specific features of homeostasis mechanisms and their regulation. Therefore, in children much more often than in adults, there are severe violations of homeostasis, often life-threatening. These disorders are most often associated with the immaturity of the homeostatic functions of the kidneys, with disorders of the functions of the gastrointestinal tract or respiratory function of the lungs.

The growth of the child, expressed in an increase in the mass of his cells, is accompanied by distinct changes in the distribution of fluid in the body. The absolute increase in the volume of extracellular fluid lags behind the rate of overall weight gain, so the relative volume of the internal environment, expressed as a percentage of body weight, decreases with age. This dependence is especially pronounced in the first year after birth. In older children, the rate of change in the relative volume of extracellular fluid decreases. The system for regulating the constancy of the volume of liquid (volume regulation) provides compensation for deviations in the water balance within fairly narrow limits. A high degree of tissue hydration in newborns and young children determines a significantly higher need for water than in adults (per unit body weight). Losses of water or its limitation quickly lead to the development of dehydration due to the extracellular sector, i.e., the internal environment. At the same time, the kidneys - the main executive organs in the system of volume regulation - do not provide water savings. The limiting factor of regulation is the immaturity of the tubular system of the kidneys. The most important feature of the neuroendocrine control of homeostasis in newborns and young children is the relatively high secretion and renal excretion of aldosterone, which has a direct effect on the state of tissue hydration and the function of the renal tubules.

Regulation of the osmotic pressure of blood plasma and extracellular fluid in children is also limited. The osmolarity of the internal environment fluctuates over a wider range ( 50 mosm/l) , than adults

( 6 mosm/l) . This is due to the greater body surface area per 1 kg. weight and, consequently, with more significant losses of water during respiration, as well as with the immaturity of the renal mechanisms of urine concentration in children. Homeostasis disorders, manifested by hyperosmosis, are especially common in children during the neonatal period and the first months of life; at older ages, hypoosmosis begins to predominate, associated mainly with gastrointestinal or kidney disease. Less studied is the ionic regulation of homeostasis, which is closely related to the activity of the kidneys and the nature of nutrition.

It was previously believed that the main factor determining the value of the osmotic pressure of the extracellular fluid is the concentration of sodium, but more recent studies have shown that there is no close correlation between the sodium content in the blood plasma and the value of the total osmotic pressure in pathology. The exception is plasmatic hypertension. Therefore, homeostatic therapy by administering glucose-salt solutions requires monitoring not only the sodium content in serum or plasma, but also changes in the total osmolarity of the extracellular fluid. Of great importance in maintaining the total osmotic pressure in the internal environment is the concentration of sugar and urea. The content of these osmotically active substances and their effect on water-salt metabolism can increase sharply in many pathological conditions. Therefore, for any violations of homeostasis, it is necessary to determine the concentration of sugar and urea. In view of the foregoing, in children of early age, in violation of the water-salt and protein regimes, a state of latent hyper- or hypoosmosis, hyperazotemia may develop.

An important indicator characterizing homeostasis in children is the concentration of hydrogen ions in the blood and extracellular fluid. In the antenatal and early postnatal periods, the regulation of acid-base balance is closely related to the degree of blood oxygen saturation, which is explained by the relative predominance of anaerobic glycolysis in bioenergetic processes. Moreover, even moderate hypoxia in the fetus is accompanied by the accumulation of lactic acid in its tissues. In addition, the immaturity of the acidogenetic function of the kidneys creates the prerequisites for the development of "physiological" acidosis (a shift in the acid-base balance in the body towards a relative increase in the number of acid anions.). In connection with the peculiarities of homeostasis in newborns, disorders often occur that stand on the verge between physiological and pathological.

The restructuring of the neuroendocrine system during puberty (puberty) is also associated with changes in homeostasis. However, the functions of the executive organs (kidneys, lungs) reach their maximum degree of maturity at this age, so severe syndromes or diseases of homeostasis are rare, but more often we are talking about compensated changes in metabolism, which can only be detected by a biochemical blood test. In the clinic, to characterize homeostasis in children, it is necessary to examine the following indicators: hematocrit, total osmotic pressure, sodium, potassium, sugar, bicarbonates and urea in the blood, as well as blood pH, p0 2 and pCO 2.

Features of homeostasis in the elderly and senile age.

The same level of homeostatic values ​​in different age periods is maintained due to various shifts in the systems of their regulation. For example, the constancy of blood pressure at a young age is maintained due to a higher cardiac output and low total peripheral vascular resistance, and in the elderly and senile - due to a higher total peripheral resistance and a decrease in cardiac output. During the aging of the body, the constancy of the most important physiological functions is maintained in conditions of decreasing reliability and reducing the possible range of physiological changes in homeostasis. Preservation of relative homeostasis with significant structural, metabolic and functional changes is achieved by the fact that at the same time not only extinction, disturbance and degradation occurs, but also the development of specific adaptive mechanisms. Due to this, a constant level of sugar in the blood, blood pH, osmotic pressure, cell membrane potential, etc. is maintained.

Changes in the mechanisms of neurohumoral regulation, an increase in the sensitivity of tissues to the action of hormones and mediators against the background of a weakening of nervous influences, are essential in maintaining homeostasis during the aging process.

With the aging of the body, the work of the heart, pulmonary ventilation, gas exchange, renal functions, secretion of the digestive glands, the function of the endocrine glands, metabolism, etc., change significantly. These changes can be characterized as homeoresis - a regular trajectory (dynamics) of changes in the intensity of metabolism and physiological functions with age in time. The value of the course of age-related changes is very important for characterizing the aging process of a person, determining his biological age.

In the elderly and senile age, the general potential of adaptive mechanisms decreases. Therefore, in old age, with increased loads, stress and other situations, the likelihood of disruption of adaptive mechanisms and homeostasis disturbances increase. Such a decrease in the reliability of homeostasis mechanisms is one of the most important prerequisites for the development of pathological disorders in old age.

Thus, homeostasis is an integral concept, functionally and morphologically uniting cardiovascular system, respiratory system, renal system, water-electrolyte metabolism, acid-base balance.

Main purpose of cardio-vascular system – supply and distribution of blood in all pools of microcirculation. The amount of blood ejected by the heart in 1 minute is the minute volume. However, the function of the cardiovascular system is not just to maintain a given minute volume and its distribution among the pools, but to change the minute volume in accordance with the dynamics of tissue needs in different situations.

The main task of the blood is the transport of oxygen. Many surgical patients experience an acute drop in minute volume, which impairs oxygen delivery to tissues and can lead to cell, organ, and even whole-body death. Therefore, the assessment of the function of the cardiovascular system should take into account not only the minute volume, but also the supply of oxygen to the tissues and their need for it.

Main purpose respiratory systems - ensuring adequate gas exchange between the body and the environment at a constantly changing rate of metabolic processes. The normal function of the respiratory system is to maintain a constant level of oxygen and carbon dioxide in the arterial blood with normal vascular resistance in the pulmonary circulation and with the usual expenditure of energy for respiratory work.

This system is closely connected with other systems, and primarily with the cardiovascular system. The function of the respiratory system includes ventilation, pulmonary circulation, diffusion of gases across the alveolar-capillary membrane, transport of gases by the blood, and tissue respiration.

Functions renal system : The kidneys are the main organ designed to maintain the constancy of the physicochemical conditions in the body. The main of their functions is excretory. It includes: regulation of water and electrolyte balance, maintenance of acid-base balance and removal of metabolic products of proteins and fats from the body.

Functions water and electrolyte metabolism : water in the body plays a transport role, filling cells, interstitial (intermediate) and vascular spaces, is a solvent of salts, colloids and crystalloids and takes part in biochemical reactions. All biochemical fluids are electrolytes, since salts and colloids dissolved in water are in a dissociated state. It is impossible to list all the functions of electrolytes, but the main ones are: maintaining osmotic pressure, maintaining the reaction of the internal environment, participating in biochemical reactions.

Main purpose acid-base balance It consists in maintaining the constancy of the pH of the liquid media of the body as the basis for normal biochemical reactions and, consequently, life. Metabolism occurs with the indispensable participation of enzymatic systems, the activity of which closely depends on the chemical reaction of the electrolyte. Together with water-electrolyte metabolism, acid-base balance plays a decisive role in the ordering of biochemical reactions. Buffer systems and many physiological systems of the body take part in the regulation of acid-base balance.

homeostasis

Homeostasis, homeoresis, homeomorphosis - characteristics of the state of the body. The system essence of the organism is manifested primarily in its ability to self-regulate in continuously changing environmental conditions. Since all organs and tissues of the body consist of cells, each of which is a relatively independent organism, the state of the internal environment of the human body is of great importance for its normal functioning. For the human body - a land creature - the environment is the atmosphere and the biosphere, while it interacts to a certain extent with the lithosphere, hydrosphere and noosphere. At the same time, most of the cells of the human body are immersed in a liquid medium, which is represented by blood, lymph and intercellular fluid. Only integumentary tissues directly interact with the human environment, all other cells are isolated from the outside world, which allows the body to largely standardize the conditions for their existence. In particular, the ability to maintain a constant body temperature of about 37 ° C ensures the stability of metabolic processes, since all the biochemical reactions that make up the essence of metabolism are very temperature dependent. It is equally important to maintain a constant tension of oxygen, carbon dioxide, concentration of various ions, etc. in the liquid media of the body. Under normal conditions of existence, including during adaptation and activity, small deviations of such parameters occur, but they are quickly eliminated, the internal environment of the body returns to a stable norm. Great French physiologist of the 19th century. Claude Bernard said: "The constancy of the internal environment is a prerequisite for a free life." The physiological mechanisms that ensure the maintenance of the constancy of the internal environment are called homeostatic, and the phenomenon itself, which reflects the body's ability to self-regulate the internal environment, is called homeostasis. This term was introduced in 1932 by W. Cannon, one of those physiologists of the 20th century, who, along with N.A. Bernstein, P.K. Anokhin and N. Wiener, stood at the origins of the science of control - cybernetics. The term "homeostasis" is used not only in physiological, but also in cybernetic research, since it is precisely the maintenance of the constancy of any characteristics of a complex system that is the main goal of any control.

Another remarkable researcher, K. Waddington, drew attention to the fact that the body is able to maintain not only the stability of its internal state, but also the relative constancy of dynamic characteristics, i.e., the flow of processes over time. This phenomenon, by analogy with homeostasis, was called homeoresis. It is of particular importance for a growing and developing organism and consists in the fact that the organism is able to maintain (within certain limits, of course) the "channel of development" in the course of its dynamic transformations. In particular, if a child, due to an illness or a sharp deterioration in living conditions caused by social causes (war, earthquake, etc.), lags significantly behind his normally developing peers, this does not mean that such a lag is fatal and irreversible. If the period of adverse events ends and the child receives adequate conditions for development, then both in terms of growth and the level of functional development, he soon catches up with his peers and in the future does not differ significantly from them. This explains the fact that children who have had a serious illness at an early age often grow up into healthy and proportionately built adults. Homeoresis plays an important role both in the management of ontogenetic development and in the processes of adaptation. Meanwhile, the physiological mechanisms of homeoresis are still insufficiently studied.

The third form of self-regulation of body constancy is homeomorphosis - the ability to maintain the invariance of the form. This characteristic is more characteristic of an adult organism, since growth and development are incompatible with the invariance of form. Nevertheless, if we consider short periods of time, especially during periods of growth inhibition, then in children it is possible to detect the ability to homeomorphosis. We are talking about the fact that in the body there is a continuous change of generations of its constituent cells. Cells do not live long (the only exception is nerve cells): the normal lifespan of body cells is weeks or months. Nevertheless, each new generation of cells almost exactly repeats the shape, size, arrangement and, accordingly, the functional properties of the previous generation. Special physiological mechanisms prevent significant changes in body weight in conditions of starvation or overeating. In particular, during starvation, the digestibility of nutrients increases sharply, and during overeating, on the contrary, most of the proteins, fats and carbohydrates that come with food are "burned" without any benefit to the body. It has been proven (N.A. Smirnova) that in an adult, sharp and significant changes in body weight (mainly due to the amount of fat) in any direction are sure signs of a breakdown in adaptation, overstrain and indicate a functional dysfunction of the body. The child's body becomes especially sensitive to external influences during periods of the most rapid growth. Violation of homeomorphosis is the same unfavorable sign as violations of homeostasis and homeoresis.

The concept of biological constants. The body is a complex of a huge number of a wide variety of substances. In the process of vital activity of body cells, the concentration of these substances can change significantly, which means a change in the internal environment. It would be unthinkable if the control systems of the body were forced to monitor the concentration of all these substances, i.e. have a lot of sensors (receptors), continuously analyze the current state, make management decisions and monitor their effectiveness. Neither the information nor the energy resources of the body would be enough for such a regime of control of all parameters. Therefore, the body is limited to monitoring a relatively small number of the most significant indicators that must be maintained at a relatively constant level for the well-being of the vast majority of body cells. These most rigidly homeostatic parameters thus turn into "biological constants", and their invariance is ensured by sometimes quite significant fluctuations of other parameters that do not belong to the category of homeostatic ones. Thus, the levels of hormones involved in the regulation of homeostasis can change tenfold in the blood, depending on the state of the internal environment and the impact of external factors. At the same time, homeostatic parameters change only by 10-20%.

The most important biological constants. Among the most important biological constants, for the maintenance of which at a relatively unchanged level, various physiological systems of the body are responsible, we should mention body temperature, blood glucose level, content of H+ ions in body fluids, partial tension of oxygen and carbon dioxide in tissues.

Disease as a symptom or consequence of homeostasis disorders. Almost all human diseases are associated with a violation of homeostasis. So, for example, in many infectious diseases, as well as in the case of inflammatory processes, temperature homeostasis is sharply disturbed in the body: fever (temperature rise), sometimes life-threatening, occurs. The reason for such a violation of homeostasis may lie both in the features of the neuroendocrine reaction, and in violations of the activity of peripheral tissues. In this case, the manifestation of the disease - fever - is a consequence of a violation of homeostasis.

Usually, feverish conditions are accompanied by acidosis - a violation of the acid-base balance and a shift in the reaction of body fluids to the acid side. Acidosis is also characteristic of all diseases associated with the deterioration of the cardiovascular and respiratory systems (diseases of the heart and blood vessels, inflammatory and allergic lesions of the bronchopulmonary system, etc.). Often, acidosis accompanies the first hours of a newborn's life, especially if normal breathing did not begin immediately after birth. To eliminate this condition, the newborn is placed in a special chamber with a high oxygen content. Metabolic acidosis with heavy muscular exertion can occur in people of any age and manifests itself in shortness of breath and increased sweating, as well as painful sensations in the muscles. After completion of work, the state of acidosis can persist from several minutes to 2-3 days, depending on the degree of fatigue, fitness and the effectiveness of homeostatic mechanisms.

Very dangerous diseases that lead to a violation of water-salt homeostasis, such as cholera, in which a huge amount of water is removed from the body and tissues lose their functional properties. Many kidney diseases also lead to a violation of water-salt homeostasis. As a result of some of these diseases, alkalosis can develop - an excessive increase in the concentration of alkaline substances in the blood and an increase in pH (shift to the alkaline side).

In some cases, minor but long-term disturbances in homeostasis can cause the development of certain diseases. So, there is evidence that excessive consumption of sugar and other sources of carbohydrates that disrupt glucose homeostasis leads to damage to the pancreas, as a result, a person develops diabetes. Also dangerous is the excessive consumption of table and other mineral salts, hot spices, etc., which increase the load on the excretory system. Kidneys May not cope with the abundance of substances that need to be removed from the body, resulting in a violation of water-salt homeostasis. One of its manifestations is edema - the accumulation of fluid in the soft tissues of the body. The cause of edema usually lies either in the insufficiency of the cardiovascular system, or in violations of the kidneys and, as a result, mineral metabolism.

Homeostasis is:

homeostasis

Homeostasis(ancient Greek ὁμοιοστάσις from ὁμοιος - the same, similar and στάσις - standing, immobility) - self-regulation, the ability of an open system to maintain the constancy of its internal state through coordinated reactions aimed at maintaining dynamic balance. The desire of the system to reproduce itself, to restore the lost balance, to overcome the resistance of the external environment.

Population homeostasis is the ability of a population to maintain a certain number of its individuals for a long time.

The American physiologist Walter B. Cannon proposed the term in 1932 in his book The Wisdom of the Body as a name for "the coordinated physiological processes that maintain the body's most stable states." Later, this term was extended to the ability to dynamically maintain the constancy of its internal state of any open system. However, the concept of the constancy of the internal environment was formulated as early as 1878 by the French scientist Claude Bernard.

General information

The term "homeostasis" is most often used in biology. For multicellular organisms to exist, it is necessary to maintain the constancy of the internal environment. Many ecologists are convinced that this principle also applies to the external environment. If the system is unable to restore its balance, it may eventually cease to function.

Complex systems - for example, the human body - must have homeostasis in order to maintain stability and exist. These systems not only have to strive to survive, they also have to adapt to environmental changes and evolve.

properties of homeostasis

Homeostatic systems have the following properties:

• instability system: tests how it can best adapt.
• Striving for balance: all the internal, structural and functional organization of systems contributes to maintaining balance.
• unpredictability: The resultant effect of a certain action can often be different from what was expected.

Examples of homeostasis in mammals:

• Regulation of the amount of micronutrients and water in the body - osmoregulation. Carried out in the kidneys.
• Removal of waste products of the metabolic process - isolation. It is carried out by exocrine organs - kidneys, lungs, sweat glands and the gastrointestinal tract.
• Body temperature regulation. Lowering the temperature through sweating, a variety of thermoregulatory reactions.
• Regulation of blood glucose levels. It is mainly carried out by the liver, insulin and glucagon secreted by the pancreas.

It is important to note that although the body is in balance, its physiological state can be dynamic. Many organisms exhibit endogenous changes in the form of circadian, ultradian, and infradian rhythms. So, even while in homeostasis, body temperature, blood pressure, heart rate and most metabolic indicators are not always at a constant level, but change over time.

Mechanisms of homeostasis: feedback

Main article: Feedback

When there is a change in variables, there are two main types of feedback that the system responds to:

1. Negative feedback, expressed as a reaction in which the system responds in such a way as to reverse the direction of change. Since the feedback serves to maintain the constancy of the system, it allows you to maintain homeostasis.
   • For example, when the concentration of carbon dioxide in the human body increases, the lungs are signaled to increase their activity and exhale more carbon dioxide.
   • Thermoregulation is another example of negative feedback. When body temperature rises (or falls), thermoreceptors in the skin and hypothalamus register the change, triggering a signal from the brain. This signal, in turn, causes a response - a decrease in temperature (or increase).
2. Positive feedback, which is expressed as an increase in the change in a variable. It has a destabilizing effect, so it does not lead to homeostasis. Positive feedback is less common in natural systems, but also has its uses.
   • For example, in nerves, a threshold electrical potential causes the generation of a much larger action potential. Blood clotting and birth events are other examples of positive feedback.

Stable systems need combinations of both types of feedback. While negative feedback allows you to return to a homeostatic state, positive feedback is used to move to a completely new (and quite possibly less desirable) state of homeostasis, a situation called "metastability". Such catastrophic changes can occur, for example, with an increase in nutrients in rivers with clear water, which leads to a homeostatic state of high eutrophication (algae overgrowth of the channel) and turbidity.

Ecological homeostasis

Ecological homeostasis is observed in climax communities with the highest possible biodiversity under favorable environmental conditions.

In disturbed ecosystems, or sub-climax biological communities - such as the island of Krakatoa, after a strong volcanic eruption in 1883 - the state of homeostasis of the previous forest climax ecosystem was destroyed, like all life on this island. Krakatoa went through a chain of ecological changes in the years following the eruption, in which new plant and animal species succeeded each other, which led to biodiversity and, as a result, a climax community. Ecological succession in Krakatoa took place in several stages. A complete chain of successions leading to a climax is called a preserie. In the Krakatoa example, this island developed a climax community with 8,000 different species recorded in 1983, a hundred years after the eruption wiped out life on it. The data confirm that the position is maintained in homeostasis for some time, while the emergence of new species very quickly leads to the rapid disappearance of old ones.

The case of Krakatoa and other disturbed or intact ecosystems shows that the initial colonization by pioneer species occurs through positive feedback reproduction strategies in which the species disperse, producing as many offspring as possible, but with little or no investment in the success of each individual. . In such species, there is a rapid development and an equally rapid collapse (for example, through an epidemic). As an ecosystem approaches climax, such species are replaced by more complex climax species that adapt through negative feedback to the specific conditions of their environment. These species are carefully controlled by the potential capacity of the ecosystem and follow a different strategy - the production of smaller offspring, in the reproductive success of which in the microenvironment of its specific ecological niche, more energy is invested.

Development begins with the pioneer community and ends with the climax community. This climax community is formed when flora and fauna come into balance with the local environment.

Such ecosystems form heterarchies in which homeostasis at one level contributes to homeostatic processes at another complex level. For example, the loss of leaves on a mature tropical tree makes room for new growth and enriches the soil. Equally, the tropical tree reduces the access of light to lower levels and helps prevent other species from invading. But the trees also fall to the ground and the development of the forest depends on the constant change of trees, the cycle of nutrients carried out by bacteria, insects, fungi. Similarly, such forests contribute to ecological processes, such as the regulation of microclimates or ecosystem hydrological cycles, and several different ecosystems may interact to maintain river drainage homeostasis within a biological region. The variability of bioregions also plays a role in the homeostatic stability of a biological region, or biome.

Biological homeostasis

Further information: Acid-base balance

Homeostasis acts as a fundamental characteristic of living organisms and is understood as maintaining the internal environment within acceptable limits.

The internal environment of the body includes body fluids - blood plasma, lymph, intercellular substance and cerebrospinal fluid. Maintaining the stability of these fluids is vital for organisms, while its absence leads to damage to the genetic material.

With regard to any parameter, organisms are divided into conformational and regulatory. Regulatory organisms keep the parameter at a constant level, regardless of what happens in the environment. Conformational organisms allow the environment to determine the parameter. For example, warm-blooded animals maintain a constant body temperature, while cold-blooded animals exhibit a wide temperature range.

We are not talking about the fact that conformational organisms do not have behavioral adaptations that allow them to regulate the given parameter to some extent. Reptiles, for example, often sit on heated rocks in the morning to raise their body temperature.

The advantage of homeostatic regulation is that it allows the body to function more efficiently. For example, cold-blooded animals tend to become lethargic in cold temperatures, while warm-blooded animals are almost as active as ever. On the other hand, regulation requires energy. The reason why some snakes can only eat once a week is that they use much less energy to maintain homeostasis than mammals.

Cellular homeostasis

The regulation of the chemical activity of the cell is achieved through a number of processes, among which the change in the structure of the cytoplasm itself, as well as the structure and activity of enzymes, is of particular importance. Autoregulation depends on temperature, the degree of acidity, the concentration of the substrate, the presence of certain macro- and microelements.

Homeostasis in the human body

Further information: Acid-base balance See also: Blood buffer systems

Various factors affect the ability of body fluids to sustain life. These include parameters such as temperature, salinity, acidity, and the concentration of nutrients - glucose, various ions, oxygen, and waste products - carbon dioxide and urine. Since these parameters affect the chemical reactions that keep the organism alive, there are built-in physiological mechanisms to keep them at the required level.

Homeostasis cannot be considered the cause of the processes of these unconscious adaptations. It should be taken as a general characteristic of many normal processes acting together, and not as their root cause. Moreover, there are many biological phenomena that do not fit this model - for example, anabolism.

Other areas

The concept of "homeostasis" is also used in other areas.

The actuary can talk about risk homeostasis, in which, for example, people who have non-stick brakes on their cars are not in a safer position than those who do not, because these people unconsciously compensate for the safer car by risky driving. This happens because some of the holding mechanisms - such as fear - stop working.

Sociologists and psychologists can talk about stress homeostasis- the desire of a population or individual to remain at a certain stress level, often artificially causing stress if the "natural" level of stress is not enough.

Examples

• thermoregulation
  • Skeletal muscle trembling may begin if the body temperature is too low.
  • Another type of thermogenesis involves the breakdown of fats to release heat.
  • Sweating cools the body through evaporation.
• Chemical regulation
  • The pancreas secretes insulin and glucagon to control blood glucose levels.
  • The lungs take in oxygen and release carbon dioxide.
  • The kidneys excrete urine and regulate the level of water and a number of ions in the body.

Many of these organs are controlled by hormones from the hypothalamic-pituitary system.

see also

Categories:

• homeostasis
• open systems
• Physiological processes

Wikimedia Foundation. 2010.

Homeostasis is a process that takes place independently in the body and is aimed at stabilizing the state of human systems when internal conditions (changes in temperature, pressure) or external conditions (changes in climate, time zone) change. This name was proposed by the American physiologist Cannon. Subsequently, homeostasis began to be called the ability of any system (including the environment) to maintain its internal constancy.

The concept and characteristics of homeostasis

Wikipedia characterizes this term as the desire to survive, adapt and develop. In order for homeostasis to be correct, the coordinated work of all organs and systems is needed. In this case, all parameters in a person will be normal. If some parameter is not regulated in the body, this indicates a violation of homeostasis.

The main characteristics of homeostasis are as follows:

• analysis of the possibilities of adapting the system to new conditions;
• the desire to maintain balance;
• the impossibility of predicting the results of the regulation of indicators in advance.

Feedback

Feedback is the actual mechanism of action of homeostasis. Thus the body reacts to any changes. The body functions continuously throughout a person's life. However, individual systems must have time to rest and recover. During this period, the work of individual organs slows down or stops altogether. This process is called feedback. Its example is a break in the work of the stomach, when food does not enter it. Such a break in digestion provides a stop in the production of acid due to the action of hormones and nerve impulses.

There are two types of this mechanism, which will be described next.

negative feedback

This type of mechanism is based on the fact that the body reacts to changes, trying to direct them in the opposite direction. That is, it strives again for stability. For example, if carbon dioxide accumulates in the body, the lungs begin to work more actively, breathing quickens, due to which excess carbon dioxide is removed. And also it is thanks to the negative feedback that thermoregulation is carried out, due to which the body avoids overheating or hypothermia.

positive feedback

This mechanism is directly opposite to the previous one. In the case of its action, the change in the variable is only amplified by the mechanism, which brings the organism out of equilibrium. This is a rather rare and less desirable process. An example of this is the presence of electrical potential in nerves., which instead of decreasing the action, leads to its increase.

However, thanks to this mechanism, development and transition to new states occur, which means that it is also necessary for life.

What parameters does homeostasis regulate?

Despite the fact that the body is constantly trying to maintain the values ​​of parameters important for life, they are not always stable. Body temperature will still change within a small range, as will heart rate or blood pressure. The task of homeostasis is to maintain this range of values, as well as help in the functioning of the body.

Examples of homeostasis are the excretion of waste products from the human body, carried out by the kidneys, sweat glands, gastrointestinal tract, as well as the dependence of metabolism on diet. A little more about the adjustable parameters will be discussed later.

Body temperature

The clearest and simplest example of homeostasis is the maintenance of normal body temperature. Overheating of the body can be avoided by sweating. The normal temperature range is 36 to 37 degrees Celsius. An increase in these values ​​\u200b\u200bcan be triggered by inflammatory processes, hormonal and metabolic disorders, or any diseases.

The part of the brain called the hypothalamus is responsible for controlling body temperature in the body. There are signals about the failure of the temperature regime, which can also be expressed in rapid breathing, an increase in the amount of sugar, an unhealthy acceleration of metabolism. All this leads to lethargy, a decrease in the activity of the organs, after which the systems begin to take measures to regulate temperature indicators. A simple example of the body's thermoregulatory response is sweating..

It is worth noting that this process also works with an excessive decrease in body temperature. So the body can warm itself due to the breakdown of fats, in which heat is released.

Water-salt balance

Water is necessary for the body, and everyone knows this well. There is even a norm of daily fluid intake, in the amount of 2 liters. In fact, each organism needs its own amount of water, and for some it may exceed the average value, while for others it may not reach it. However, no matter how much water a person drinks, the body will not accumulate all the excess fluid. Water will remain at the required level, while all the excess will be removed from the body due to osmoregulation carried out by the kidneys.

Blood homeostasis

In the same way, the amount of sugar, namely glucose, which is an important element of the blood, is regulated. A person cannot be completely healthy if the sugar level is far from normal. This indicator is regulated by the functioning of the pancreas and liver. In the case when the glucose level exceeds the norm, the pancreas acts, in which insulin and glucagon are produced. If the amount of sugar becomes too low, glycogen from the blood is processed into it with the help of the liver.

normal pressure

Homeostasis is also responsible for the normal blood pressure in the body. If it is broken, signals about this will come from the heart to the brain. The brain reacts to the problem and, with the help of impulses, helps the heart to reduce high pressure.

The definition of homeostasis characterizes not only the correct functioning of the systems of one organism, but can also apply to entire populations. Depending on this, there are types of homeostasis described below.

Ecological homeostasis

This species is present in a community provided with the necessary living conditions. It arises through the action of a positive feedback mechanism, when organisms that begin to inhabit an ecosystem multiply rapidly, thereby increasing their numbers. But such a rapid settlement can lead to an even faster destruction of a new species in the event of an epidemic or a change in conditions to less favorable ones. So organisms need to adapt and stabilize, which is due to negative feedback. Thus, the number of inhabitants decreases, but they become more adapted.

Biological homeostasis

This type is just typical for individuals whose body strives to maintain internal balance, in particular, by regulating the composition and amount of blood, intercellular substance and other fluids necessary for the normal functioning of the body. At the same time, homeostasis does not always oblige to keep the parameters constant, sometimes it is achieved by adapting and adapting the body to changing conditions. Due to this difference, organisms are divided into two types:

• conformational - those who strive to preserve values ​​(for example, warm-blooded animals, whose body temperature should be more or less constant);
• regulatory, which adapt (cold-blooded, having a different temperature depending on the conditions).

At the same time, the homeostasis of each of the organisms is aimed at compensating for the costs. If warm-blooded animals do not change their lifestyle when the ambient temperature drops, then cold-blooded animals become lethargic and passive so as not to waste energy.

Besides, Biological homeostasis includes the following subspecies:

• cellular homeostasis is aimed at changing the structure of the cytoplasm and the activity of enzymes, as well as the regeneration of tissues and organs;
• homeostasis in the body is ensured by regulating temperature indicators, the concentration of substances necessary for life, and the removal of waste.

Other types

In addition to use in biology and medicine, the term has found application in other areas.

Maintenance of homeostasis

Homeostasis is maintained due to the presence in the body of so-called sensors that send impulses to the brain containing information about pressure and body temperature, water-salt balance, blood composition and other parameters important for normal life. As soon as some values ​​begin to deviate from the norm, a signal about this enters the brain, and the body begins to regulate its performance.

This complex adjustment mechanism incredibly important to life. The normal state of a person is maintained with the correct ratio of chemicals and elements in the body. Acids and alkalis are necessary for the stable functioning of the digestive system and other organs.

Calcium is a very important structural material, without the right amount of which a person will not have healthy bones and teeth. Oxygen is essential for breathing.

Toxins can interfere with the smooth functioning of the body. But so that health is not harmed, they are excreted due to the work of the urinary system.

Homeostasis works without any human effort. If the body is healthy, the body will self-regulate all processes. If people are hot, the blood vessels dilate, which is expressed in reddening of the skin. If it's cold - there is a shiver. Thanks to such responses of the body to stimuli, human health is maintained at the right level.

Homeostasis (Greek homoios - the same, similar, stasis - stability, balance) is a set of coordinated reactions that maintain or restore the constancy of the internal environment of the body. In the middle of the nineteenth century, the French physiologist Claude Bernard introduced the concept of the internal environment, which he considered as a collection of body fluids. This concept was expanded by the American physiologist Walter Cannon, who meant by the internal environment the totality of fluids (blood, lymph, tissue fluid) that are involved in metabolism and maintaining homeostasis. The human body adapts to constantly changing environmental conditions, but the internal environment remains constant and its indicators fluctuate within very narrow limits. Therefore, a person can live in various environmental conditions. Some physiological parameters are regulated especially carefully and finely, for example, body temperature, blood pressure, glucose, gases, salts, calcium ions in the blood, acid-base balance, blood volume, its osmotic pressure, appetite, and many others. Regulation is carried out according to the principle of negative feedback between the receptors f , which detect changes in the indicated indicators and control systems. Thus, a decrease in one of the parameters is captured by the corresponding receptor, from which impulses are sent to one or another brain structure, at the command of which the autonomic nervous system turns on complex mechanisms to equalize the changes that have occurred. The brain uses two main systems to maintain homeostasis: autonomic and endocrine. Recall that the main function of the autonomic nervous system is to maintain the constancy of the internal environment of the body, which is carried out due to a change in the activity of the sympathetic and parasympathetic parts of the autonomic nervous system. The latter, in turn, is controlled by the hypothalamus, and the hypothalamus by the cerebral cortex. The endocrine system regulates the function of all organs and systems through hormones. Moreover, the endocrine system itself is under the control of the hypothalamus and pituitary gland. Homeostasis (Greek homoios - the same and stasis - state, immobility)

As our understanding of normal, and even more pathological, physiology became more complex, this concept was refined as homeokinesis, i.e. mobile equilibrium, the balance of constantly changing processes. The body is woven from millions of "homeokinesics". This huge living galaxy determines the functional status of all organs and cells that are bound by regulatory peptides. Like the world economic and financial system - many firms, industries, factories, banks, stock exchanges, markets, shops ... And between them - "convertible currency" - neuropeptides. All body cells constantly synthesize and maintain a certain, functionally necessary, level of regulatory peptides. But when deviations from "stationarity" occur, their biosynthesis (in the body as a whole or in its individual "loci") either increases or weakens. Such fluctuations occur constantly when it comes to adaptive reactions (getting used to new conditions), performance of work (physical or emotional actions), the state of pre-illness - when the body "turns on" increased protection against functional imbalance. The classic case of maintaining balance is the regulation of blood pressure. There are groups of peptides between which there is constant competition - to increase / decrease pressure. In order to run, climb a mountain, bathe in a sauna, perform on stage, and finally think, a functionally sufficient increase in blood pressure is necessary. But as soon as the work is over, the regulators come into action, ensuring the “calming” of the heart and normal pressure in the vessels. Vasoactive peptides constantly interact to "allow" to increase the pressure to such and such a level (no more, otherwise the vascular system will go "peddling"; a well-known and bitter example is a stroke) and so that after the completion of physiologically necessary work

2. Learning goals:

Know the essence of homeostasis, the physiological mechanisms of maintaining homeostasis, the basics of homeostasis regulation.

To study the main types of homeostasis. Know the age-related features of homeostasis

3. Questions for self-preparation for mastering this topic:

1) Definition of the concept of homeostasis

2) Types of homeostasis.

3) Genetic homeostasis

4) Structural homeostasis

5) Homeostasis of the internal environment of the body

6) Immunological homeostasis

7) Mechanisms of regulation of homeostasis: neurohumoral and endocrine.

8) Hormonal regulation of homeostasis.

9) Organs involved in the regulation of homeostasis

10) General principle of homeostatic reactions

11) Species specificity of homeostasis.

12) Age-related features of homeostasis

13) Pathological processes, accompanied by a violation of homeostasis.

14) Correction of the homeostasis of the body is the main task of the doctor.

__________________________________________________________________

4. Type of lesson: extracurricular

5. Duration of the lesson- 3 hours.

6. Equipment. Electronic presentation "Lectures on biology", tables, dummies

homeostasis(gr. homoios - equal, stasis - state) - the property of an organism to maintain the constancy of the internal environment and the main features of its inherent organization, despite the variability of the parameters of the external environment and the action of internal disturbing factors.

The homeostasis of each individual is specific and determined by its genotype.

The body is an open dynamic system. The flow of substances and energy observed in the body determines self-renewal and self-reproduction at all levels from molecular to organismic and population.

In the process of metabolism with food, water, during gas exchange, a variety of chemical compounds enter the body from the environment, which, after transformations, are likened to the chemical composition of the body and are included in its morphological structures. After a certain period, the absorbed substances are destroyed, releasing energy, and the destroyed molecule is replaced by a new one, without violating the integrity of the structural components of the body.

Organisms are in a constantly changing environment, despite this, the main physiological indicators continue to be carried out in certain parameters and the body maintains a stable state of health for a long time, thanks to self-regulation processes.

Thus, the concept of homeostasis is not related to the stability of processes. In response to the action of internal and external factors, some change in physiological parameters occurs, and the inclusion of regulatory systems ensures the maintenance of a relative constancy of the internal environment. Regulatory homeostatic mechanisms function at the cellular, organ, organismic and supraorganismal levels.

In evolutionary terms, homeostasis is a hereditarily fixed adaptation of an organism to normal environmental conditions.

There are the following main types of homeostasis:

1) genetic

2) structural

3) homeostasis of the liquid part of the internal environment (blood, lymph, interstitial fluid)

4) immunological.

Genetic homeostasis- preservation of genetic stability due to the strength of the physicochemical bonds of DNA and its ability to recover after damage (DNA repair). Self-reproduction is a fundamental property of the living, it is based on the process of DNA reduplication. The very mechanism of this process, in which a new DNA strand is built strictly complementary around each of the constituent molecules of the two old strands, is optimal for accurate information transfer. The accuracy of this process is high, but reduplication errors can still occur. Violation of the structure of DNA molecules can also occur in its primary chains without regard to reduplication under the influence of mutagenic factors. In most cases, the cell genome is restored, the damage is corrected, due to repair. When repair mechanisms are damaged, genetic homeostasis is disrupted both at the cellular and organismal levels.

An important mechanism for maintaining genetic homeostasis is the diploid state of somatic cells in eukaryotes. Diploid cells are more stable in functioning, because the presence of two genetic programs in them increases the reliability of the genotype. Stabilization of the complex system of the genotype is provided by the phenomena of polymerization and other types of gene interaction. Regulatory genes that control the activity of operons play an important role in the process of homeostasis.

Structural homeostasis- this is the constancy of the morphological organization at all levels of biological systems. It is advisable to single out the homeostasis of a cell, tissue, organ, body systems. The homeostasis of the underlying structures ensures the morphological constancy of the higher structures and is the basis of their vital activity.

The cell, as a complex biological system, is inherent in self-regulation. The establishment of homeostasis of the cellular environment is provided by membrane systems, which are associated with bioenergetic processes and regulation of the transport of substances into and out of the cell. In the cell, the processes of change and restoration of organelles are continuously going on, the cells themselves are destroyed and restored. Restoration of intracellular structures, cells, tissues, organs in the course of the life of the organism occurs due to physiological regeneration. Restoration of structures after damage - reparative regeneration.

Homeostasis of the liquid part of the internal environment- the constancy of the composition of blood, lymph, tissue fluid, osmotic pressure, the total concentration of electrolytes and the concentration of individual ions, the content of nutrients in the blood, etc. These indicators, even with significant changes in environmental conditions, are kept at a certain level, thanks to complex mechanisms.

For example, one of the most important physicochemical parameters of the internal environment of the body is the acid-base balance. The ratio of hydrogen and hydroxide ions in the internal environment depends on the content in body fluids (blood, lymph, tissue fluid) of acids - proton donors and buffer bases - proton acceptors. Usually, the active reaction of the medium is evaluated by the H+ ion. The pH value (the concentration of hydrogen ions in the blood) is one of the stable physiological indicators and varies in humans within narrow limits - from 7.32 to 7.45. The activity of a number of enzymes, membrane permeability, protein synthesis processes, etc. largely depend on the ratio of hydrogen and hydroxyl ions.

The body has various mechanisms that ensure the maintenance of acid-base balance. Firstly, these are the buffer systems of blood and tissues (carbonate, phosphate buffers, tissue proteins). Hemoglobin also has buffering properties, it binds carbon dioxide and prevents its accumulation in the blood. The activity of the kidneys also contributes to the maintenance of a normal concentration of hydrogen ions, since a significant amount of acidic metabolites is excreted in the urine. If these mechanisms are insufficient, the concentration of carbon dioxide in the blood increases, there is some shift in pH to the acid side. In this case, the respiratory center is excited, pulmonary ventilation is enhanced, which leads to a decrease in the content of carbon dioxide and the normalization of the concentration of hydrogen ions.

The sensitivity of tissues to changes in the internal environment is different. So a pH shift of 0.1 in one direction or another from the norm leads to significant disturbances in the activity of the heart, and a deviation of 0.3 is life-threatening. The nervous system is particularly sensitive to low oxygen levels. For mammals, fluctuations in the concentration of calcium ions exceeding 30% are dangerous, etc.

Immunological homeostasis- maintaining the constancy of the internal environment of the body by maintaining the antigenic individuality of the individual. Immunity is understood as a way of protecting the body from living bodies and substances bearing signs of genetically alien information (Petrov, 1968).

Bacteria, viruses, protozoa, helminths, proteins, cells, including altered cells of the organism itself, carry alien genetic information. All of these factors are antigens. Antigens are substances that, when introduced into the body, are capable of causing the production of antibodies or another form of immune response. Antigens are very diverse, most often they are proteins, but these are also large molecules of lipopolysaccharides, nucleic acids. Inorganic compounds (salts, acids), simple organic compounds (carbohydrates, amino acids) cannot be antigens, because have no specificity. The Australian scientist F. Burnet (1961) formulated the position that the main significance of the immune system is the recognition of "own" and "foreign", i.e. in maintaining the constancy of the internal environment - homeostasis.

The immune system has a central (red bone marrow, thymus gland) and a peripheral (spleen, lymph nodes) link. The protective reaction is carried out by lymphocytes formed in these organs. Type B lymphocytes, when they encounter foreign antigens, differentiate into plasma cells that secrete specific proteins, immunoglobulins (antibodies), into the blood. These antibodies, connecting with the antigen, neutralize them. This reaction is called humoral immunity.

T-type lymphocytes provide cellular immunity by destroying foreign cells, such as transplant rejection, and mutated cells of their own body. According to the calculations given by F. Burnet (1971), in each genetic change of dividing human cells, about 10 - 6 spontaneous mutations accumulate within one day, i.e. at the cellular and molecular levels, processes that disrupt homeostasis are continuously occurring. T-lymphocytes recognize and destroy mutant cells of their own body, thus ensuring the function of immune surveillance.

The immune system controls the genetic constancy of the organism. This system, consisting of anatomically separated organs, represents a functional unity. The property of immune defense has reached its highest development in birds and mammals.

homeostasis regulation carried out by the following organs and systems (Fig. 91):

1) central nervous system;

2) neuroendocrine system, which includes the hypothalamus, pituitary gland, peripheral endocrine glands;

3) diffuse endocrine system (DES), represented by endocrine cells located in almost all tissues and organs (heart, lung, gastrointestinal tract, kidneys, liver, skin, etc.). The bulk of DES cells (75%) is concentrated in the epithelium of the digestive system.

It is now known that a number of hormones are simultaneously present in the central nervous structures and endocrine cells of the gastrointestinal tract. So the hormones enkephalins and endorphins are found in nerve cells and endocrine cells of the pancreas and stomach. Cholecystokinin was found in the brain and duodenum. Such facts gave grounds for creating a hypothesis about the presence in the body of a single system of cells of chemical information. The peculiarity of nervous regulation is the speed of the onset of the response, and its effect manifests itself directly in the place where the signal arrives along the corresponding nerve; reaction is short.

In the endocrine system, regulatory influences are associated with the action of hormones carried with the blood throughout the body; the effect of the action is long-lasting and does not have a local character.

The unification of the nervous and endocrine mechanisms of regulation occurs in the hypothalamus. The general neuroendocrine system allows for complex homeostatic reactions associated with the regulation of the visceral functions of the body.

The hypothalamus also has glandular functions, producing neurohormones. Neurohormones, getting into the anterior lobe of the pituitary gland with blood, regulate the release of tropic hormones of the pituitary gland. Tropic hormones directly regulate the work of the endocrine glands. For example, thyroid-stimulating hormone from the pituitary stimulates the thyroid gland by increasing the level of thyroid hormone in the blood. When the concentration of the hormone rises above the norm for a given organism, the thyroid-stimulating function of the pituitary gland is inhibited and the activity of the thyroid gland is weakened. Thus, to maintain homeostasis, it is necessary to balance the functional activity of the gland with the concentration of the hormone in the circulating blood.

This example shows the general principle of homeostatic reactions: deviation from the initial level --- signal --- activation of regulatory mechanisms on the feedback principle --- correction of change (normalization).

Some endocrine glands are not directly dependent on the pituitary gland. These are the pancreatic islets that produce insulin and glucagon, the adrenal medulla, the pineal gland, the thymus, and the parathyroid glands.

The thymus occupies a special position in the endocrine system. It produces hormone-like substances that stimulate the formation of T-lymphocytes, and a relationship is established between immune and endocrine mechanisms.

The ability to maintain homeostasis is one of the most important properties of a living system that is in a state of dynamic equilibrium with environmental conditions. The ability to maintain homeostasis is not the same in different species, it is high in higher animals and humans, which have complex nervous, endocrine and immune mechanisms of regulation.

In ontogeny, each age period is characterized by the peculiarities of metabolism, energy and mechanisms of homeostasis. In the child's body, the processes of assimilation predominate over dissimilation, which causes growth, an increase in body weight, the mechanisms of homeostasis are not yet mature enough, which leaves an imprint on the course of both physiological and pathological processes.

With age, there is an improvement in metabolic processes, regulatory mechanisms. In adulthood, the processes of assimilation and dissimilation, the system of normalization of homeostasis provide compensation. With aging, the intensity of metabolic processes decreases, the reliability of regulatory mechanisms weakens, the function of a number of organs fades, and at the same time new specific mechanisms develop that support the preservation of relative homeostasis. This is expressed, in particular, in an increase in the sensitivity of tissues to the action of hormones, along with a weakening of nervous influences. During this period, adaptive features are weakened, therefore, an increase in load and stressful conditions can easily disrupt homeostatic mechanisms and often become the cause of pathological conditions.

Knowledge of these patterns is necessary for a future doctor, since the disease is a consequence of a violation of the mechanisms and ways of restoring homeostasis in humans.

Homeostasis - maintenance of the body's internal environment

The world around us is constantly changing. Winter winds force us to put on warm clothes and gloves, while central heating encourages us to take them off. The summer sun reduces the need for heat retention, at least until efficient air conditioning does the opposite. And yet, regardless of the ambient temperature, the individual body temperature of healthy people you know is unlikely to differ by much more than one tenth of a degree. In humans and other warm-blooded animals, the temperature of the internal regions of the body is kept at a constant level somewhere around 37 ° C, although it may rise and fall somewhat in connection with the daily rhythm.

Most people eat differently. Some prefer a good breakfast, a light lunch and a hearty lunch with the obligatory dessert. Others don't eat most of the day, but at noon they like to have a good snack and a little nap. Some only do what they chew, others seem not to care about food at all. And yet, if you measure the blood sugar content of the students in your class, then it will all be close to 0.001 g (1 mg) per milliliter of blood, despite the large difference in the diet and distribution of meals.

Precise regulation of body temperature and blood glucose are just two examples of the most important functions under the control of the nervous system. The composition of the fluids that surround all our cells is continuously regulated, which allows for its amazing constancy.

Maintaining a constant internal environment is called homeostasis (homeo - the same, similar; stasis - stability, balance). The main responsibility for homeostatic regulation is borne by the autonomic (autonomous) and intestinal sections of the peripheral nervous system, as well as the central nervous system, which gives orders to the body through the pituitary gland and other endocrine organs. Working together, these systems coordinate the needs of the body with environmental conditions. (If this statement sounds familiar to you, remember that we used exactly the same words to describe the main function of the brain.)

The French physiologist Claude Bernard, who lived in the 19th century and devoted himself entirely to the study of the processes of digestion and the regulation of blood flow, considered body fluids as an “internal environment” ( milieu interne). In different organisms, the concentration of certain salts and the normal temperature may be somewhat different, but within a species, the internal environment of individuals corresponds to the standards characteristic of this species. Only short-term and not very large deviations from these standards are allowed, otherwise the organism cannot remain healthy and contribute to the survival of the species. Walter B. Cannon, the foremost American physiologist of the middle of this century, extended Bernard's concept of the internal environment. He believed that the independence of the individual from continuous changes in external conditions is ensured by the work homeostatic mechanisms that maintain the constancy of the internal environment.

The ability of an organism to cope with the demands of its environment varies greatly from species to species. A person who uses complex types of behavior in addition to the internal mechanisms of homeostasis, apparently, has the greatest independence from external conditions. Nevertheless, many animals surpass it in certain species-specific capabilities. For example, polar bears are more resistant to cold; some species of spiders and lizards living in deserts tolerate heat better; camels can go longer without water. In this chapter, we will consider a number of structures that allow us to gain some degree of independence from the changing physical conditions of the external world. We will also take a closer look at the regulatory mechanisms that maintain the constancy of our internal environment.

Astronauts wear special suits (suits) that allow them to maintain normal body temperature, sufficient oxygen tension in the blood and blood pressure when working in an environment close to vacuum. Special sensors built into these suits record oxygen concentration, body temperature, and heart rate indicators and report these data to spacecraft computers, which in turn report to ground control computers. The computers of a controlled spacecraft can cope with almost any of the predictable situations regarding the needs of the organism. If any unforeseen problem arises, computers located on Earth are connected to solve it, which send new commands directly to the suit's instruments.
In the body, registration of sensory data and local control is carried out by the autonomic nervous system with the participation of the endocrine system, which assumes the function of general coordination.

autonomic nervous system

Some general principles of organization of sensory and motor systems will be very useful to us in the study of systems of internal regulation. All three divisions autonomic (autonomous) nervous system have " sensory" and " motor" Components. While the former register indicators of the internal environment, the latter enhance or inhibit the activity of those structures that carry out the process of regulation itself.

Intramuscular receptors, along with receptors located in tendons and some other places, respond to pressure and stretch. Together, they make up a special kind of internal sensory system that helps control our movements.
The receptors involved in homeostasis act in a different way: they sense changes in blood chemistry or pressure fluctuations in the vascular system and in hollow internal organs such as the digestive tract and bladder. These sensory systems, which collect information about the internal environment, are very similar in their organization to systems that receive signals from the surface of the body. Their receptor neurons form the first synaptic switches inside the spinal cord. Along the motor pathways of the autonomic system go commands to the bodies directly regulating the internal environment. These paths begin with special autonomic preganglionic neurons spinal cord. Such an organization is somewhat reminiscent of the organization of the spinal level of the motor system.

The focus of this chapter will be on those motor components of the autonomic system that innervate the muscles of the heart, blood vessels, and intestines, causing them to contract or relax. The same fibers also innervate the glands, causing the process of secretion.

autonomic nervous system consists of two large sections sympathetic and parasympathetic. Both divisions have one structural feature that we have not encountered before: the neurons that control the muscles of the internal organs and glands lie outside the central nervous system, forming small encapsulated clusters of cells called ganglia. Thus, in the autonomic nervous system there is an additional link between the spinal cord and the terminal working organ (effector).

Autonomic neurons of the spinal cord combine sensory information from internal organs and other sources. On this basis, they then regulate the activity autonomic ganglion neurons. The connections between the ganglia and the spinal cord are called preganglionic fibers . The neurotransmitter used to transmit impulses from the spinal cord to ganglion neurons in both the sympathetic and parasympathetic regions is almost always acetylcholine, the same neurotransmitter by which the motor neurons of the spinal cord directly control the skeletal muscles. As in the fibers that innervate skeletal muscles, the action of acetylcholine can be enhanced in the presence of nicotine and blocked by curare. Axons going from autonomic ganglion neurons, or postganglionic fibers , then go to the target organs, forming many branches there.

The sympathetic and parasympathetic divisions of the autonomic nervous system are different
1) according to the levels at which preganglionic fibers exit the spinal cord;
2) by the proximity of the location of the ganglia to the target organs;
3) by the neurotransmitter that postganglionic neurons use to regulate the functions of these target organs.
We will now consider these features.

Sympathetic nervous system

In the sympathetic system, preganglionic fibers exit from the thoracic and lumbar spinal cord. Its ganglia are located quite close to the spinal cord, and very long postganglionic fibers run from them to the target organs (see Fig. 63). The main mediator of the sympathetic nerves is norepinephrine, one of the catecholamines, which also serves as a mediator in the central nervous system.

Rice. 63. The sympathetic and parasympathetic divisions of the autonomic nervous system, the organs they innervate, and their effect on each organ.

To understand which organs are affected by the sympathetic nervous system, it is easiest to imagine what happens to an excited animal, ready for a fight or flight response.
The pupils dilate to let in more light; the frequency of heart contractions increases, and each contraction becomes more powerful, which leads to an increase in overall blood flow. Blood drains from the skin and internal organs to the muscles and brain. Motility of the gastrointestinal system weakens, digestion processes slow down. Muscles along the airways leading to the lungs relax, allowing for faster breathing and increased gas exchange. The cells of the liver and adipose tissue give more glucose and fatty acids into the blood - high-energy fuel, and the pancreas is instructed to produce less insulin. This allows the brain to receive a greater proportion of the glucose circulating in the bloodstream, since unlike other organs, the brain does not require insulin to utilize blood sugar. The mediator of the sympathetic nervous system, which carries out all these changes, is norepinephrine.

There is an additional system that has an even more generalized effect in order to better ensure all these changes. They sit on the tops of the kidneys like two small caps, adrenal glands . In their inner part - the medulla - there are special cells innervated by preganglionic sympathetic fibers. These cells in the process of embryonic development are formed from the same neural crest cells from which the sympathetic ganglia are formed. Thus, the medulla is a component of the sympathetic nervous system. When activated by preganglionic fibers, medulla cells release their own catecholamines (norepinephrine and epinephrine) directly into the blood for delivery to target organs (Fig. 64). Circulating hormone mediators - serve as an example of how the regulation of endocrine organs is carried out (see p. 89).

parasympathetic nervous system

In the parasympathetic preganglionic fibers go from the brain stem("cranial component") and from the lower, sacral segments of the spinal cord(see Fig. 63 above). They form, in particular, a very important nerve trunk called vagus nerve , whose numerous branches carry out all the parasympathetic innervation of the heart, lungs and intestinal tract. (The vagus nerve also transmits sensory information from these organs back to the central nervous system.) Preganglionic parasympathetic axons very long, because ganglia are usually located near or within the tissues they innervate.

At the ends of the fibers of the parasympathetic system, a neurotransmitter is used acetylcholine. The response of the respective target cells to acetylcholine is insensitive to the action of nicotine or curare. Instead, acetylcholine receptors are activated by muscarine and blocked by atropine.

The predominance of parasympathetic activity creates conditions for " rest and recovery» organism. At its extreme, the general pattern of parasympathetic activation is reminiscent of the resting state that comes after a hearty meal. Increased blood flow to the digestive tract accelerates the movement of food through the intestines and enhances the secretion of digestive enzymes. The frequency and strength of heart contractions decrease, the pupils constrict, the lumen of the airways decreases, and the formation of mucus in them increases. The bladder contracts. Taken together, these changes return the body to that peaceful state that preceded the "fight or flight" response. (All of this is illustrated in Figure 63; see also Chapter 6.)

Comparative characteristics of the departments of the autonomic nervous system

The sympathetic system, with its extremely long postganglionic fibers, is very different from the parasympathetic system, in which, on the contrary, the preganglionic fibers are longer and the ganglia are located near or inside the target organs. Many internal organs, such as the lungs, heart, salivary glands, bladder, gonads, receive innervation from both parts of the autonomic system (they are said to have " double innervation"). Other tissues and organs, such as muscle arteries, receive only sympathetic innervation. On the whole, it can be said that two departments work alternately: depending on the activity of the organism and on the commands of the higher vegetative centers, one or the other of them dominates.

This characterization, however, is not entirely correct. Both systems are constantly in a state of varying degrees of activity.. The fact that target organs such as the heart or the iris can respond to impulses from both areas simply reflects their complementary role. For example, when you are very angry, your blood pressure rises, which excites the corresponding receptors located in the carotid arteries. These signals are received by the integrating center of the cardiovascular system, located in the lower part of the brainstem and known as the nuclei of the solitary tract. Excitation of this center activates the preganglionic parasympathetic fibers of the vagus nerve, which leads to a decrease in the frequency and strength of heart contractions. At the same time, under the influence of the same coordinating vascular center, sympathetic activity is inhibited, counteracting an increase in blood pressure.

How essential is the functioning of each of the departments for adaptive reactions? Surprisingly, not only animals, but also people can endure almost complete shutdown of the sympathetic nervous system with no visible ill effects. This shutdown is recommended for some forms of persistent hypertension.

But it's not so easy to do without the parasympathetic nervous system. People who have undergone such an operation and found themselves outside the protective conditions of a hospital or laboratory adapt very poorly to the environment. They cannot regulate body temperature when exposed to heat or cold; with blood loss, their blood pressure regulation is disturbed, and with any intense muscle load, fatigue quickly develops.

Diffuse intestinal nervous system

Recent studies have revealed the existence third important division of the autonomic nervous system - diffuse intestinal nervous system . This department is responsible for the innervation and coordination of the digestive organs. Its work is independent of the sympathetic and parasympathetic systems, but can be modified under their influence. This is an additional link that connects the autonomic postganglionic nerves with the glands and muscles of the gastrointestinal tract.

The ganglia of this system innervate the walls of the intestines. Axons from the cells of these ganglia cause contractions of the annular and longitudinal muscles, pushing food through the gastrointestinal tract, a process called peristalsis. Thus, these ganglia determine the features of local peristaltic movements. When the food mass is inside the intestine, it slightly stretches its walls, which causes a narrowing of the area located slightly higher along the course of the intestine, and relaxation of the area located slightly below. As a result, the food mass is pushed further. However, under the influence of parasympathetic or sympathetic nerves, the activity of the intestinal ganglia can change. Activation of the parasympathetic system enhances peristalsis, and activation of the sympathetic system weakens it.

Acetylcholine serves as a mediator that excites the smooth muscles of the intestine. However, inhibitory signals leading to relaxation appear to be transmitted by various substances, of which only a few have been studied. Among the gut neurotransmitters, there are at least three that also act in the central nervous system: somatostatin (see below), endorphins, and substance P (see Chapter 6).

Central regulation of the functions of the autonomic nervous system

The central nervous system exercises control over the autonomic system to a much lesser extent than over the sensory or skeletal motor system. Areas of the brain that are most associated with autonomic functions are hypothalamus and brain stem, especially that part of it that is located directly above the spinal cord - the medulla oblongata. It is from these areas that the main pathways go to sympathetic and parasympathetic preganglionic autonomic neurons at the spinal level.

Hypothalamus. The hypothalamus is one of the areas of the brain, the general structure and organization of which is more or less similar in representatives of various classes of vertebrates.

In general, it is considered that hypothalamus is the focus of visceral integrative functions. Signals from the neuronal systems of the hypothalamus directly enter the networks that excite the preganglionic sections of the autonomic nerve pathways. In addition, this region of the brain exercises direct control over the entire endocrine system through specific neurons that regulate the secretion of hormones of the anterior pituitary gland, and the axons of other hypothalamic neurons terminate in the posterior pituitary gland. Here, these endings secrete mediators that circulate in the blood as hormones: 1) vasopressin, which increases blood pressure in emergency cases, when there is a loss of fluid or blood; it also reduces the excretion of water in the urine (which is why vasopressin is also called antidiuretic hormone); 2) oxytocin, stimulating uterine contractions at the final stage of childbirth.

Rice. 65. Hypothalamus and pituitary gland. Schematically shows the main functional areas of the hypothalamus.

Although among the clusters of hypothalamic neurons there are several clearly demarcated nuclei, most of the hypothalamus is a collection of zones with blurred boundaries (Fig. 65). However, there are quite pronounced nuclei in three zones. We will now consider the functions of these structures.

1. Periventricular zone directly adjacent to the third cerebral ventricle, which passes through the center of the hypothalamus. Cells lining the ventricle relay information to neurons in the periventricular zone about important internal parameters that may need to be regulated, such as temperature, salt concentration, and levels of hormones secreted by the thyroid, adrenals, or gonads, as instructed by the pituitary gland.

2. Medial zone contains most of the pathways by which the hypothalamus exercises endocrine control through the pituitary gland. It can be said very approximately that the cells of the periventricular zone control the actual execution of commands given to the pituitary gland by the cells of the medial zone.

3. Through lateral zone cells control over the hypothalamus from the higher instances of the cerebral cortex and the limbic system. It also receives sensory information from the centers of the medulla oblongata, which coordinate respiratory and cardiovascular activity. The lateral zone is where higher brain centers can make adjustments to the reactions of the hypothalamus to changes in the internal environment. In the cortex, for example, comparison of information coming from two sources - internal and external environment. If, say, the cortex decides that the time and circumstances are not suitable for eating, the sensory reports of low blood sugar and an empty stomach will be put aside until a more favorable moment. Ignoring the hypothalamus by the limbic system is less likely. Rather, this system can add emotional and motivational coloring to the interpretation of external sensory cues, or compare perceptions of the environment based on these cues with similar situations in the past.

Together with the cortical and limbic components, the hypothalamus also performs many routine integrating actions, and over much longer periods of time than during the implementation of short-term regulatory functions. The hypothalamus “knows” in advance what needs the body will have in a normal daily rhythm of life. He, for example, brings the endocrine system into full readiness for action as soon as we wake up. It also monitors the hormonal activity of the ovaries throughout the menstrual cycle; takes steps to prepare the uterus for the arrival of a fertilized egg. In migratory birds and hibernating mammals, the hypothalamus, with its ability to determine the length of daylight hours, coordinates the life of the organism during cycles lasting several months. (These aspects of centralized regulation of internal functions will be discussed in Chapters 5 and 6.)

Medulla(thalamus and hypothalamus)

The hypothalamus makes up less than 5% of the entire brain mass. However, this small amount of tissue contains centers that support all the functions of the body, with the exception of spontaneous respiratory movements, the regulation of blood pressure and heart rhythm. These last functions depend on the medulla oblongata (see Fig. 66). With traumatic brain injuries, the so-called “brain death” occurs when all signs of electrical activity of the cortex disappear and control from the hypothalamus and medulla oblongata is lost, although artificial respiration can still maintain sufficient saturation of the circulating blood with oxygen.

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