Homeostasis, homeostasis (homeostasis; Greek homoios similar, the same + stasis state, immobility), - relative dynamic constancy internal environment(blood, lymph, tissue fluid) and the stability of the basic physiological functions (blood circulation, respiration, thermoregulation, metabolism, and so on) of the human and animal body. Regulatory mechanisms that maintain the physiological state or properties of cells, organs and systems of the entire organism at an optimal level are called homeostatic.
As is known, living cell represents a mobile, self-regulating system. Her internal organization supported active processes aimed at limiting, preventing or eliminating shifts caused by various influences from the surrounding and 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 an integral 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
participation of nervous, humoral, metabolic and other factors. Many individual mechanisms regulating intra- and intercellular relationships have, in some cases, mutually opposite (antagonistic) 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 arise during the life of the organism.
The term “homeostasis” was proposed in 1929 by physiologist W. Cannon, who believed that physiological processes, maintaining stability in the body, are so complex and diverse that it is advisable to combine them under common name homeostasis. However, back in 1878, C. Bernard wrote that all life processes have only one goal - maintaining the constancy of living conditions in our internal environment. Similar statements are found in the works of many researchers of the 19th and first half of the 20th centuries. (E. Pfluger, S. Richet, Frederic (L.A. Fredericq), I.M. Sechenov, I.P. Pavlov, K.M. Bykov and others). The works of L.S. were of great importance for the study of the problem of homeostasis. Stern (with colleagues), devoted to the role of barrier functions that regulate the composition and properties of the microenvironment of organs and tissues.
The very idea of homeostasis does not correspond to the concept of stable (non-fluctuating) equilibrium in the body - the principle of equilibrium does not apply to
complex physiological and biochemical
processes occurring in living systems. It is also incorrect to contrast homeostasis with rhythmic fluctuations in the internal environment. Homeostasis in a broad sense covers issues of the cyclic and phase course 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 flexible, changing depending on individual age, gender, social, professional and other conditions.
Of particular importance for the life of the body is the constancy of the composition of the blood - the fluid matrix of the body, as W. Cannon puts it. The stability of its active reaction (pH), osmotic pressure, ratio of electrolytes (sodium, calcium, chlorine, magnesium, phosphorus), glucose content, number of formed elements, and so on is well known. For example, blood pH, as a rule, does not go beyond 7.35-7.47. Even severe disorders of acid-base metabolism with pathology of acid accumulation in tissue fluid, for example in diabetic acidosis, have very little effect on the active blood reaction. Even though osmotic pressure blood and tissue fluid undergoes continuous fluctuations due to constant osmotically supplied active products interstitial metabolism, it remains at a certain level and changes only under certain severe pathological conditions.
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 characteristics, and the normal state of the organs depends on chemical composition, physicochemical, biological and other properties of this microenvironment. Its homeostasis is determined functional state histohematic barriers and their permeability in the directions blood→tissue fluid, tissue fluid→blood.
The constancy of the internal environment for the activity of the central nervous system is of particular importance: even minor chemical and physico-chemical changes that occur in the cerebrospinal fluid, glia and pericellular spaces can cause a sharp disturbance in the flow 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 optimal blood pressure levels. In this case, the upper limit of the blood pressure level is determined by the functionality of the baroreceptors of the body’s vascular system, and the lower limit is determined by the body’s blood supply needs.
The most advanced homeostatic mechanisms in the body of higher animals and humans include thermoregulation processes;
Homeostasis in classical meaning this word physiological concept, denoting the stability of the composition of the internal environment, the constancy of the components of its composition, as well as the balance of the biophysiological functions of any living organism.
The basis of such a biological function as homeostasis is the ability of living organisms and biological systems resist environmental changes; In this case, organisms use autonomous defense mechanisms.
This term was first used by the American physiologist W. Cannon at the beginning of the twentieth century.
Any biological object has universal parameters of homeostasis.
Homeostasis of the system and body
The scientific basis for such a phenomenon as homeostasis was formed by the Frenchman C. Bernard - it was a theory about the constant composition of the internal environment in the organisms of living beings. This scientific theory was formulated in the eighties of the eighteenth century and was widely developed.
So, homeostasis is the result of a complex mechanism of interaction in the field of regulation and coordination, which occurs both in the body as a whole and in its organs, cells and even at the molecular level.
The concept of homeostasis has received impetus additional development as a result of the use of cybernetics methods in the study of complex biological systems, such as biocenosis or population).
Functions of homeostasis
The study of objects with a feedback function has helped scientists learn about the numerous mechanisms responsible for their stability.
Even in conditions of serious changes, adaptation mechanisms do not allow the chemical and physiological properties of the body to change significantly. This is not to say that they remain absolutely stable, but serious deviations usually do not occur.
Mechanisms of homeostasis
The mechanism of homeostasis in higher animals is the most well developed. In the organisms of birds and mammals (including humans), the homeostasis function allows maintaining the stability of the number of hydrogen ions, regulates the constancy of the chemical composition of the blood, maintains pressure in the circulatory system and body temperature approximately at the same level.
There are several ways in which homeostasis affects organ systems and the body as a whole. This may be influenced by hormones, the nervous system, excretory or neuro-humoral systems of the body.
Human homeostasis
For example, the stability of pressure in the arteries is maintained by a regulatory mechanism that works in the manner of chain reactions in which the blood organs enter.
This happens because the vascular receptors sense a change in pressure force and transmit a signal about this to the human brain, which sends response impulses to the vascular centers. The consequence of this is an increase or decrease in tone circulatory system(heart and blood vessels).
In addition, organs of neurohumoral regulation come into play. As a result of this reaction, the pressure returns to normal.
Ecosystem homeostasis
An example of homeostasis in flora may serve to maintain constant leaf moisture by opening and closing stomata.
Homeostasis is also characteristic of communities of living organisms of any degree of complexity; for example, the fact that a relatively stable composition of species and individuals is maintained within a biocenosis is a direct consequence of the action of homeostasis.
Population homeostasis
This type of homeostasis, such as population homeostasis (its other name is genetic) plays the role of a regulator of the integrity and stability of the genotypic composition of a population under conditions of a variable environment.
It acts through the preservation of heterozygosity, as well as by controlling the rhythm and direction of mutational changes.
This type of homeostasis gives the population the opportunity to maintain optimal genetic composition, which allows the community of living organisms to maintain maximum viability.
The role of homeostasis in society and ecology
The need to manage complex systems of a social, economic and cultural nature has led to the expansion of the term homeostasis and its application not only to biological, but also to social objects.
An example of the work of homeostatic social mechanisms The following situation may serve: if there is a lack of knowledge or skills or professional deficiency in a society, then through a feedback mechanism this fact forces the community to develop and improve itself.
And if there is an excess number of professionals who are not actually in demand by society, negative feedback will occur and there will be fewer representatives of unnecessary professions.
Recently, the concept of homeostasis has found wide application in ecology, due to the need to study the state of complex ecological systems and the biosphere as a whole.
In cybernetics, the term homeostasis is used to refer to any mechanism that has the ability to automatically self-regulate.
Links on the topic of homeostasis
Homeostasis on Wikipedia
Homeostasis - maintaining the internal environment of the body
The world around us is constantly changing. Winter winds force us to put on warm clothes and gloves, and central heating encourages us to take them off. Summer sun reduces the need to conserve heat, 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 vary 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 maintained at a constant level of somewhere around 37 ° C, although it can rise and fall somewhat due to the daily rhythm.
Most people eat differently. Some people prefer a good breakfast, a light lunch and a hearty lunch with the obligatory dessert. Others don't eat for most of the day, but at midday they like to have a good snack and take a short nap. Some people do nothing but chew, while others don’t seem to care about food at all. And yet, if you measure the blood sugar level of the students in your class, they will all be close to 0.001 g (1 mg) per milliliter of blood, despite the large differences in diet and in the distribution of meals.
Precise regulation of body temperature and blood glucose are just two examples essential functions under the control of the nervous system. The composition of the fluids that surround all our cells is continuously regulated, allowing for amazing consistency.
Maintaining a constant internal environment of the body is called homeostasis (homeo - same, similar; stasis - stability, balance). The main responsibility for homeostatic regulation lies with the autonomic (autonomous) and intestinal parts 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. Acting together, these systems coordinate the body's needs with environmental conditions. (If this statement seems familiar to you, remember that we used exactly the same words to describe main function 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 regulation of blood flow, considered body fluids as an “internal environment” (milieu interne). Different organisms may have slightly different concentrations of certain salts and normal temperatures, but within a species, the internal environment of individuals meets the standards characteristic of that species. Only short-term and not very large deviations from these standards are allowed, otherwise the body cannot remain healthy and contribute to the survival of the species. Walter B. Cannon, the leading American physiologist of the mid-century, expanded Bernard's concept of the internal environment. He believed that the independence of the individual from continuous changes in external conditions is ensured by work homeostatic mechanisms, which maintain the constancy of the internal environment.
An organism's ability to cope with environmental demands varies greatly from species to species. A person who uses, in addition to the internal mechanisms of homeostasis, complex types of behavior, apparently, has the greatest independence from external conditions. However, 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 look at a number of structures that allow us to gain some independence from changing physical conditions outside world. We will also take a closer look at the regulatory mechanisms that maintain the constancy of our internal environment.
Astronauts put on special suits (space suits), which allow them to maintain normal temperature body, sufficient oxygen tension in the blood and blood pressure. Special sensors built into these suits record oxygen concentration, body temperature, cardiac performance and report this data to the spacecraft computers, and those in turn to the ground control computers. The computers of the controlled spacecraft can cope with almost any predictable situation regarding the needs of the body. If any unforeseen problem arises, computers located on Earth are connected to solve it, and they send new commands directly to the spacesuit’s devices.
In the body, the registration of sensory data and local control is carried out by the autonomic nervous system with the participation of the endocrine system, which takes on the function of overall coordination.
Autonomic nervous system
Some general principles of organization of sensory and motor systems will be very useful to us when studying systems internal regulation. All three departments autonomic (autonomic) nervous system have " sensory" And " motor» components. While the former record indicators of the internal environment, the latter enhance or inhibit the activity of those structures that carry out the regulation process 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.
Receptors involved in homeostasis operate 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 systems that collect information about the internal environment are very similar in 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 there are commands to bodies directly regulating the internal environment. These paths start with special autonomic preganglionic neurons
spinal cord. This organization is somewhat reminiscent of the organization of the spinal level of the motor system.
The main 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 innervate the glands, causing the process of secretion.
Autonomic nervous system consists of two large departments sympathetic And parasympathetic. Both departments have one structural feature, which 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 vegetative nervous system there is an additional link between spinal cord and the end working body (effector).
Autonomic neurons of the spinal cord combine sensory information coming from internal organs and other sources. On this basis they then regulate activity neurons of the autonomic ganglia. The connections between the ganglia and the spinal cord are called preganglionic fibers . A neurotransmitter used to transmit impulses from the spinal cord to ganglion neurons in both the sympathetic and parasympathetic divisions is almost always acetylcholine, the same transmitter with which spinal cord motor neurons directly control skeletal muscles. As in the fibers innervating skeletal muscles, the action of acetylcholine can be enhanced in the presence of nicotine and blocked by curare. Axons running from neurons of the autonomic ganglia, or postganglionic fibers , then go to the target organs, forming many branches there.
The sympathetic and parasympathetic divisions of the autonomic nervous system differ from each other
1) according to the levels at which preganglionic fibers exit the spinal cord;
2) according to the proximity of the ganglia to the target organs;
3) by neurotransmitter, which is used by postganglionic neurons to regulate the functions of these target organs.
We will now consider these features.
Sympathetic nervous system
In the sympathetic system, preganglionic fibers emerge from the thoracic and lumbar spinal cord. Its ganglia are located quite close to the spinal cord, and very long postganglionic fibers extend from them to the target organs (see Fig. 63). The main transmitter 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 effects on each organ.
To understand what organs the sympathetic nervous system affects, it is easiest to imagine what happens to an excited animal ready for a fight-or-flight response.
Pupils dilate to let in more light; The heart rate increases and each contraction becomes more powerful, which leads to increased overall blood flow. Blood flows from the skin and internal organs to the muscles and brain. The motility of the gastrointestinal system weakens, digestion processes slow down. Muscles located along airways, leading to the lungs, relax, which allows the breathing rate to increase and gas exchange to increase. Liver and fat cells release more glucose and fatty acids, high-energy fuels, into the blood, and the pancreas is instructed to produce less insulin. This allows the brain to receive a larger share 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 that carries out all these changes is norepinephrine.
Exists additional system, which has an even more generalized effect in order to more accurately ensure all these changes. On the tops of the buds they sit like two small caps, adrenal glands . In their inner part - the medulla - there are special cells innervated by preganglionic sympathetic fibers. During embryonic development, these cells 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, medullary 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 regulation is carried out by endocrine organs (see p. 89).
Parasympathetic nervous system
In the parasympathetic department preganglionic fibers are coming 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, as they ganglia, as a rule, are located near or within the tissues they innervate.
A transmitter is used at the endings of the fibers of the parasympathetic system acetylcholine. The response of the corresponding target cells to acetylcholine is insensitive to the effects of nicotine or curare. Instead, acetylcholine receptors are activated by muscarine and blocked by atropine.
The predominance of parasympathetic activity creates the conditions for “ rest and recovery» organism. In its extreme manifestation general character parasympathetic activation resembles the state of rest that occurs after a hearty meal. Increased blood flow to the digestive tract speeds up the movement of food through the intestines and increases the secretion of digestive enzymes. The frequency and strength of heart contractions decrease, the pupils narrow, respiratory tract decreases, and the formation of mucus in them increases. The bladder contracts. Taken together, these changes return the body to the peaceful state that preceded the fight-or-flight response. (All this is presented in Fig. 63; see also Chapter 6.)
Comparative characteristics of the parts 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 (have, as they say, “ double innervation"). Other tissues and organs, such as muscle arteries, receive only sympathetic innervation. In general it can be said that two departments work alternately: depending on the activity of the body and on the commands of the higher vegetative centers, first 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 parts simply reflects their complementary roles. 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 brain stem and known as the nucleus 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 suppressed, counteracting the increase in blood pressure.
How important is the functioning of each department for adaptive reactions? Surprisingly, not only animals, but also people can tolerate almost complete shutdown of the sympathetic nervous system without visible bad consequences. This switch-off 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 find 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 disrupted, and with any intense muscle load Fatigue develops quickly.
Diffuse nervous system of the intestine
Recent research has revealed the existence third important division of the autonomic nervous system - diffuse nervous system of the intestine . 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 intestinal walls. Axons from these ganglion cells cause circular and longitudinal muscle contractions that push food through the gastrointestinal tract, a process called peristalsis. Thus, these ganglia determine the characteristics 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 intestine and relaxation of the area located just 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 increases peristalsis, and the sympathetic system weakens it.
Acetylcholine serves as a mediator that stimulates the smooth muscles of the intestine. However, the inhibitory signals leading to relaxation appear to be transmitted by a variety of substances, of which only a few have been studied. Among the intestinal 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 exerts much less control over the autonomic system than over the sensory or skeletal motor systems. Areas of the brain most associated with vegetative functions, - This hypothalamus And brain stem, especially the part that is located directly above the spinal cord - the medulla oblongata. It is from these areas that the main pathways to the sympathetic and parasympathetic preganglionic autonomic neurons at the spinal level come.
Hypothalamus. The hypothalamus is one of the areas of the brain general structure and the organization of which is more or less similar in representatives of different classes of vertebrates.
In general, it is generally accepted that hypothalamus - this is the focus of visceral integrative functions. Signals from the neural systems of the hypothalamus directly enter networks that excite the preganglionic portions 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 from the anterior pituitary gland, and the axons of other hypothalamic neurons terminate in the posterior pituitary gland. Here these endings release mediators that circulate in the blood as hormones: 1) vasopressin, which increases blood pressure in emergency cases when fluid or blood loss occurs; it also reduces the excretion of water in the urine (this is why vasopressin is also called antidiuretic hormone); 2) oxytocin, stimulating uterine contractions at the final stage of labor.
Rice. 65. Hypothalamus and pituitary gland. The main functional areas of the hypothalamus are shown schematically.
Although there are several clearly demarcated nuclei among the clusters of hypothalamic neurons, most of the hypothalamus is a collection of zones with blurred boundaries (Fig. 65). However, in three zones there are quite pronounced nuclei. 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. The cells lining the ventricle transmit information to the neurons of the periventricular zone about important internal parameters that may require regulation - for example, temperature, salt concentration, levels of hormones secreted by the thyroid gland, adrenal glands or gonads in accordance with instructions from the pituitary gland.
2. Medial zone contains most of the pathways through which the hypothalamus carries out endocrine control through the pituitary gland. Very roughly, we can say 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. Via lateral zone cells the hypothalamus is controlled by higher levels of the cortex big brain and 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, there is comparison of information coming from two sources - internal and external environment. If, say, the cortex judges that the time and circumstances are not suitable for eating, the sensory report of low blood sugar and an empty stomach will be put aside until a more favorable moment. The limbic system is less likely to ignore the hypothalamus. Rather, this system may add emotional and motivational coloration to the interpretation of external sensory signals or compare the representation of the environment based on these signals with similar situations that occurred 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 when carrying out short-term regulatory functions. The hypothalamus “knows” in advance what needs the body will have during the normal daily rhythm of life. For example, it brings the endocrine system into full readiness to action as soon as we wake up. It also monitors the hormonal activity of the ovaries throughout the menstrual cycle; takes measures to prepare the uterus for the arrival of a fertilized egg. In migratory birds and hibernating mammals, the hypothalamus, with its ability to determine length, daylight hours coordinates the vital functions of the body during cycles lasting several months. (About these aspects of centralized regulation internal functions will be discussed in chapters 5 and 6.)
Medulla oblongata(thalamus and hypothalamus)
The hypothalamus makes up less than 5% of the total brain mass. However, in this small quantity tissues contain centers that support all body functions, with the exception of spontaneous respiratory movements, regulation of blood pressure and heart rate. These latter 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 by the hypothalamus and medulla oblongata is lost, although with the help of artificial respiration it is still possible to maintain sufficient saturation of the circulating blood with oxygen.
continuation
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Internal environment of the body- a set of body fluids located inside it, as a rule, in certain reservoirs and natural conditions and never in contact with the outside environment. The term was proposed by the French physiologist Claude Bernard.
Cells can only function in a liquid environment. Blood, tissue fluid and lymph form the internal environment of the body. The basis of the internal environment of the body is blood, which delivers oxygen to cells, nutrients and metabolic products are removed. However, blood does not come into direct contact with the cells of the body. In the tissues, part of the blood plasma leaves the blood capillaries and turns into tissue fluid. Excess tissue fluid is absorbed by lymphatic capillaries and flows back into the blood in the form of lymph through the lymphatic vessels. Thus, blood, tissue fluid and lymph directly circulate within the body, ensuring the exchange of substances between the cells of the body and the environment. Scientists from many countries around the world have tried to find out the nature of the mechanisms that maintain the constancy of the internal environment of humans and higher animals.
The set of factors and mechanisms that ensure this constancy is called homeostasis. Homeostasis– the ability of biological systems to resist changes and maintain the dynamic constancy of the composition and properties of the organism.
Homeostasis is the relatively dynamic constancy of the internal environment of the body, ensuring the stability of its basic physiological functions.
Claude Bernard (1878) – formulation of the concept of homeostasis.
Walter Cannon coined the term homeostasis, his hypothesis - individual parts of the body are stable, since the internal environment surrounding them is stable.
Living organism– an open self-regulating system that develops in close interaction with the environment. Changes in the environment directly or indirectly affect the components, causing corresponding changes in them.
Thanks to self-regulation mechanisms, these changes occur within the normal reaction range and do not cause serious disturbances in physiological functions.
Violation of regulatory mechanisms leads to breakdown compensatory possibilities organism, reducing its resistance to constantly changing environmental conditions, disturbances of homeostasis and the development of pathologies.
Homeostasis mechanisms should be aimed at maintaining the level of a steady state, coordinating processes to eliminate or limit the influence of harmful factors, optimal interaction between the body and the environment in changed conditions of existence.
Components of homeostasis:
Components that provide cellular needs: proteins, fats, carbohydrates; inorganic substances; water, oxygen, internal secretion.
Components affecting cellular activity: osmotic pressure, temperature, hydrogen ion concentration.
Types of homeostasis:
Genetic homeostasis . The genotype of the zygote, when interacting with environmental factors, determines the entire complex of variability of the organism, its adaptive ability, that is, homeostasis. The body reacts to changes in environmental conditions specifically, within the limits of a hereditarily determined reaction norm. The constancy of genetic homeostasis is maintained on the basis matrix syntheses, and the stability of the genetic material is ensured by a number of mechanisms (see mutagenesis).
Structural homeostasis. Maintaining the constancy of the composition and integrity of the morphological organization of cells and tissues. The multifunctionality of cells increases the compactness and reliability of the entire system, increasing its potential capabilities. The formation of cell functions occurs through regeneration.
Regeneration:
1. Cellular (direct and indirect division)
2. Intracellular (molecular, intraorganoid, organoid)
Physico-chemical homeostasis.
Gas homeostasis: the concentration of oxygen and carbon dioxide in the body is ensured by the external respiration system. Factors regulating external respiration: minute volume of respiration of alveolar air, depending on the activity of the respiratory center; gas content in the blood and pulmonary capillaries; diffusion of gases through the membrane of blood cells, uniform pulmonary blood flow and adequate ventilation.
Acid-base balance of the body: blood pH = 7.32-7.45, the ratio of hydrogen and hydroxyl ions depends on the content of acids, which act as proton donors, and amphoteric bases, which are acceptors. Its regulation is ensured by buffer systems, tissue proteins, and the collagen substance of connective tissue, which is capable of adsorbing acids.
Osmotic properties of blood: the osmotic pressure of blood depends on the concentration of the solution and temperature, but does not depend on the nature of the solute and solvent. The constancy of the osmotic properties of blood is ensured by water balance. Water balance The body is supported by mechanisms for the supply of water and salts. Redistribution of water and salts between cells and intracellular organelles, release of water and salts into the environment. The basis for the integration of all physicochemical homeostasis is neuroendocrine regulation.
Physiological homeostasis.
Thermal homeostasis: maintaining heat content. An important condition heat balance serves as the movement of the environment that washes the body and its parts, in which heat exchange occurs; the regulation of thermal insulation is ensured by the flow of warm blood from the deep areas of the body to its surface
Hemostasis system: activation of the blood coagulation system, the required level of blood cells, restoration of the properties of the vascular wall.
Biochemical homeostasis: maintaining the level of metabolic processes, in particular anabolism and catabolism, the balance of synthesis and decay processes is carried out by changing the activity of enzymes, the rate of enzymatic reactions, inducing the biosynthesis of proteins and enzymes and regulating the rate of decay of biologically active substances.
Immunological homeostasis.
The immune system protects the body from exogenous substances, infectious agents that carry genetically foreign information, as well as from pathologically altered cells. Recognition - destruction - elimination. Central authorities immune system - bone marrow and thymus. Peripheral organs – spleen and lymphoid tissue. The bone marrow produces a stimulator of antibody producers, which activates the system of B-lymphocytes, which provide the humoral component of immunity, and the thymus produces thymosin, which activates the production of T-lymphocytes. Maintaining immunological homeostasis must be ensured by the required concentration of T and B lymphocytes.
Endocrine homeostasis: synthesis and secretion of hormones, transport of hormones, specific metabolism of hormones in the periphery and their excretion, interaction of hormones with target cells, regulation and self-regulation of gland functions internal secretion.
All homeostases as a whole constitute biological homeostasis , whole system various functions and indicators that ensure the preservation and maintenance of normal functioning of the body in changing environmental conditions.
Regulation biological homeostasis:
Local: carried out through positive and negative feedback, when a change in one indicator leads to a change in another, is characterized by autonomy; this property is inherent in any component of a living system.
Humoral regulation , is associated with the entry into the internal environment of the body of humoral factors - mediators, hormones, biologically active substances, etc. the humoral system reacts to external influences slowly, because has no connection with the environment, but gives a more stable and long-lasting effect, provided by the endocrine glands. Based on humoral regulation adaptive reactions develop to changes in the internal environment of the body.
Nervous regulation: the main coordinator of all biological processes, which is due to structural and functional features nervous system: presence in all organs and tissues, direct contact with the external environment through receptors, high excitability, lability and precise targeting nerve impulses and high speed of information transfer. The regulation of adaptive reactions is based on reflex processes. Nervous regulation ensures changes in the functional activity of organs or functions in response to external influence and adaptation of the body to the external environment.
Levels of neuroendocrine regulation:
1. Cell membrane
2. Endocrine glands
3. Pituitary gland
4. Hypothalamus
Enabling different levels neurohumoral regulation is determined by the intensity of the influence of the factor, the degree of deviation of physiological parameters and the lability of adaptive systems.
Question 54.
The body of higher animals has developed adaptations that counteract many influences of the external environment, providing relatively constant conditions for the existence of cells. This is of utmost importance for the functioning of the whole organism. We illustrate this with examples. The cells of the body of warm-blooded animals, i.e. animals with a constant body temperature, function normally only within narrow temperature limits (in humans, within 36-38°). A temperature shift beyond these boundaries leads to disruption of cell activity. At the same time, the body of warm-blooded animals can normally exist with significantly wider fluctuations in environmental temperature. For example, a polar bear can live at temperatures of -70° and +20-30°. This is due to the fact that in the whole organism its heat exchange with the environment is regulated, i.e. heat generation (intensity, chemical processes, occurring with the release of heat) and heat transfer. Thus, at low ambient temperatures, heat generation increases and heat transfer decreases. Therefore, when the external temperature fluctuates (within certain limits), the body temperature remains constant.
The functions of the body's cells are normal only when the osmotic pressure is relatively constant, due to the constant content of electrolytes and water in the cells. Changes in osmotic pressure - its decrease or its increase - lead to sudden disturbances in the functions and structure of cells. The organism as a whole can exist for some time even with an excess supply and deprivation of water, and with large and small amounts of salts in food. This is explained by the presence in the body of devices that help maintain
constancy of the amount of water and electrolytes in the body. In case of excess water intake, significant amounts of it are quickly excreted from the body by the excretory organs (kidneys, sweat glands, skin), and if there is a lack of water, it is retained in the body. Equally excretory organs regulate the content of electrolytes in the body: they quickly remove excess amounts or retain them in body fluids when there is insufficient salt intake.
The concentration of individual electrolytes in the blood and tissue fluid, on the one hand, and in the protoplasm of cells, on the other, is different. The blood and tissue fluid contain more sodium ions, and the protoplasm of cells contains more potassium ions. The difference in ion concentrations inside and outside the cell is achieved by a special mechanism that retains potassium ions inside the cell and does not allow sodium ions to accumulate in the cell. This mechanism, the nature of which is not yet clear, is called the sodium-potassium pump and is associated with the metabolic process of the cell.
Body cells are very sensitive to shifts in the concentration of hydrogen ions. A change in the concentration of these ions in one direction or another sharply disrupts the vital activity of cells. The internal environment of the body is characterized by a constant concentration of hydrogen ions, depending on the presence of so-called buffer systems in the blood and tissue fluid (p. 48) and on the activity of the excretory organs. When the content of acids or alkalis in the blood increases, they are quickly eliminated from the body and in this way the constancy of the concentration of hydrogen ions in the internal environment is maintained.
Cells, especially nerve cells, are very sensitive to changes in blood sugar levels, which serve as an important nutrient. That's why great value for the life process has a constant blood sugar level. It is achieved by the fact that when the blood sugar level increases in the liver and muscles, the polysaccharide deposited in the cells, glycogen, is synthesized from it, and when the blood sugar level decreases, glycogen is broken down in the liver and muscles and grape sugar is released into the blood.
Constancy of chemical composition and physical and chemical properties the internal environment is important feature organisms of higher animals. To denote this constancy, W. Cannon proposed a term that has become widespread - homeostasis. The expression of homeostasis is the presence of a number of biological constants, i.e., stable quantitative indicators that characterize the normal state of the body. Such constant indicators are: body temperature, osmotic pressure of blood and tissue fluid, the content of sodium, potassium, calcium, chlorine and phosphorus ions, as well as proteins and sugar, the concentration of hydrogen ions and a number of others.
Noting the constancy of the composition, physicochemical and biological properties of the internal environment, it should be emphasized that it is not absolute, but relative and dynamic. This constancy is achieved by the continuously performed work of a number of organs and tissues, as a result of which the shifts in the composition and physico-chemical properties of the internal environment that occur under the influence of changes in the external environment and as a result of the vital activity of the body are leveled out.
Role different organs and their systems in maintaining homeostasis are different. Thus, the digestive system ensures that nutrients enter the bloodstream in the form in which they can be used by the cells of the body. The circulatory system carries out continuous blood movement and transport various substances in the body, as a result of which nutrients, oxygen and various chemical compounds formed in the body itself are supplied to the cells, and breakdown products, including carbon dioxide, released by the cells, are transferred to the organs that remove them from the body. The respiratory organs ensure the supply of oxygen to the blood and the removal of carbon dioxide from the body. The liver and a number of other organs carry out a significant number of chemical transformations - the synthesis and breakdown of many chemical compounds that are important in the life of cells. Excretory organs - kidneys, lungs, sweat glands, skin - remove waste products from the body organic matter and maintain a constant content of water and electrolytes in the blood, and therefore in tissue fluid and in the cells of the body.
In maintaining homeostasis vital role belongs to the nervous system. Responsive to various changes external or internal environment, it regulates the activity of organs and systems in such a way that shifts and disturbances that occur or could occur in the body are prevented and leveled out.
Thanks to the development of devices that ensure the relative constancy of the internal environment of the body, its cells are less susceptible to the changing influences of the external environment. According to Cl. Bernard, “constancy of the internal environment is a condition for free and independent life.”
Homeostasis has certain boundaries. When an organism stays, especially for a long time, in conditions that differ significantly from those to which it is adapted, homeostasis is disrupted and changes may occur that are incompatible with normal life. Thus, with a significant change in external temperature in the direction of either increasing or decreasing, body temperature may increase or decrease and overheating or cooling of the body may occur, leading to death. Likewise, with a significant restriction of the intake of water and salts into the body or a complete deprivation of these substances, the relative constancy of the composition and physicochemical properties of the internal environment is disrupted after some time and life ceases.
A high level of homeostasis occurs only at certain stages of the species and individual development. Lower animals do not have sufficiently developed adaptations to mitigate or eliminate the effects of changes in the external environment. For example, relative constancy of body temperature (homeothermy) is maintained only in warm-blooded animals. In so-called cold-blooded animals, body temperature is close to the temperature of the external environment and is variable (poikilothermia). A newborn animal does not have the same constancy of body temperature, composition and properties of the internal environment as an adult organism.
Even small disturbances of homeostasis lead to pathology, and therefore the determination of relatively constant physiological indicators, such as body temperature, blood pressure blood, composition, physicochemical and biological properties of blood, etc., is of great diagnostic importance.
