Temperature "core" and "shell". Body temperature in humans and isothermia of the Organism are all except

A. Human life can only occur in a narrow range of temperatures.

Temperature has a significant impact on the course of life processes in the human body and on its physiological activity. Life processes are limited to a narrow range of internal temperature within which basic enzymatic reactions can occur. For humans, a decrease in body temperature below 25°C and an increase above 43°C is usually fatal. Nerve cells are especially sensitive to temperature changes.

Heat causes intense sweating, which leads to dehydration of the body, loss of mineral salts and water-soluble vitamins. The consequence of these processes is blood thickening, disruption of salt metabolism, gastric secretion, and the development of vitamin deficiency. The acceptable weight loss due to evaporation is 2-3%. With 6% weight loss from evaporation, mental activity is impaired, and with 15-20% weight loss, death occurs. The systematic effect of high temperature causes changes in the cardiovascular system: increased heart rate, changes in blood pressure, weakening of the functional ability of the heart. Prolonged exposure to high temperatures leads to the accumulation of heat in the body, while the body temperature can rise to 38-41 ° C and heat stroke may occur with loss of consciousness.

Low temperatures may cause cooling and hypothermia of the body. When cooling, the body reflexively reduces heat transfer and increases heat production. A decrease in heat transfer occurs due to spasm (constriction) of blood vessels and an increase in the thermal resistance of body tissues. Prolonged exposure to low temperatures leads to persistent vascular spasm and disruption of tissue nutrition. The increase in heat production during cooling is achieved through the efforts of oxidative metabolic processes in the body (a decrease in body temperature by 1°C is accompanied by an increase in metabolic processes by 10°C). Exposure to low temperatures is accompanied by an increase in blood pressure, inspiratory volume and a decrease in respiratory rate. Cooling the body changes carbohydrate metabolism. Great cooling is accompanied by a decrease in body temperature, inhibition of the functions of organs and body systems.

B. Core and outer shell of the body.

From the point of view of thermoregulation, the human body can be imagined as consisting of two components - external shell and internal kernels.

Core- this is the part of the body that has a constant temperature (internal organs), and shell- a part of the body in which there is a temperature gradient (these are tissues of the surface layer of the body 2.5 cm thick). Through the shell there is heat exchange between the core and the environment, that is, changes in the thermal conductivity of the shell determine the constancy of the temperature of the core. Thermal conductivity changes due to changes in blood supply and blood filling of the membrane tissues.

The temperature of different parts of the core is different. For example, in the liver: 37.8-38.0°C, in the brain: 36.9-37.8°C. In general, the core temperature of the human body is 37.0°C. This is achieved through the processes of endogenous thermoregulation, the result of which is a stable balance between the amount of heat produced in the body per unit time ( heat production) and the amount of heat dissipated by the body during the same time into the environment ( heat transfer).

The temperature of human skin in different areas ranges from 24.4°C to 34.4°C. The lowest temperature is observed on the toes, the highest in the armpit. It is on the basis of measuring the temperature in the armpit that one usually judges the body temperature at a given time.

According to average data, the average skin temperature of a naked person under comfortable air temperature conditions is 33-34°C. There are daily fluctuations in body temperature. The amplitude of oscillations can reach 1°C. Body temperature is minimum in the pre-dawn hours (3-4 hours) and maximum in the daytime (16-18 hours).

The phenomenon of temperature asymmetry is also known. It is observed in approximately 54% of cases, and the temperature in the left armpit is slightly higher than in the right. Asymmetry is also possible in other areas of the skin, and the severity of asymmetry of more than 0.5°C indicates pathology.

B. Heat transfer. Balance of heat generation and heat transfer in the human body.

Human life processes are accompanied by continuous heat generation in his body and the release of the generated heat into the environment. The exchange of thermal energy between the body and the environment is called p heat exchange. Heat production and heat transfer are caused by the activity of the central nervous system, which regulates metabolism, blood circulation, sweating and the activity of skeletal muscles.

The human body is a self-regulating system with an internal heat source, in which, under normal conditions, heat production (the amount of heat generated) is equal to the amount of heat released to the external environment (heat transfer). Constancy of body temperature is called isothermal. It ensures the independence of metabolic processes in tissues and organs from fluctuations in ambient temperature.

The internal temperature of the human body is constant (36.5-37°C) due to the regulation of the intensity of heat production and heat transfer depending on the external temperature. And the temperature of human skin when exposed to external conditions can vary over a relatively wide range.

In 1 hour, the human body generates as much heat as is needed to boil 1 liter of ice water. And if the body were a heat-impermeable case, then within an hour the body temperature would rise by about 1.5 ° C, and after 40 hours it would reach the boiling point of water. During hard physical work, heat generation increases several times more. And yet our body temperature does not change. Why? It’s all about balancing the processes of formation and release of heat in the body.

The leading factor determining the level of heat balance is ambient temperature. When it deviates from the comfortable zone, a new level of heat balance is established in the body, ensuring isothermia in new environmental conditions. This constancy of body temperature is ensured by the mechanism thermoregulation, including the process of heat generation and the process of heat release, which are regulated by the neuroendocrine pathway.

D. The concept of thermoregulation of the body.

Thermoregulation- this is a set of physiological processes aimed at maintaining the relative constancy of the body’s core temperature in conditions of changing environmental temperatures by regulating heat production and heat transfer. Thermoregulation is aimed at preventing disturbances in the body's thermal balance or restoring it if such disturbances have already occurred, and is carried out through the neurohumoral route.

It is generally accepted that thermoregulation is characteristic only of homeothermic animals (these include mammals (including humans) and birds), whose body has the ability to maintain the temperature of the internal regions of the body at a relatively constant and fairly high level (about 37-38 ° C in mammals and 40-42°C in birds) regardless of changes in ambient temperature.

The thermoregulation mechanism can be represented as a cybernetic self-control system with feedback. Temperature fluctuations in the surrounding air affect special receptor formations ( thermoreceptors), sensitive to temperature changes. Thermoreceptors transmit information about the thermal state of the organ to the thermoregulation centers, in turn, the thermoregulation centers, through nerve fibers, hormones and other biologically active substances, change the level of heat transfer and heat production or parts of the body (local thermoregulation), or the body as a whole. When thermoregulation centers are turned off by special chemicals, the body loses the ability to maintain a constant temperature. This feature has been used in medicine in recent years for artificial cooling of the body during complex heart surgeries.

Skin thermoreceptors.

It is estimated that humans have approximately 150,000 cold and 16,000 heat receptors that respond to changes in the temperature of internal organs. Thermoreceptors are located in the skin, internal organs, respiratory tract, skeletal muscles and the central nervous system.

Skin thermoreceptors are quickly adaptable and react not so much to the temperature itself as to its changes. The maximum number of receptors is located in the head and neck, the minimum - on the limbs.

Cold receptors are less sensitive and their sensitivity threshold is 0.012°C (when cooled). The sensitivity threshold of thermal receptors is higher and amounts to 0.007°C. This is probably due to the greater danger to the body of overheating.

D. Types of thermoregulation.

Thermoregulation can be divided into two main types:

1. Physical thermoregulation:

Evaporation (sweating);

Radiation (radiation);

Convection.

2. Chemical thermoregulation.

Contractile thermogenesis;

Non-contractile thermogenesis.

Physical thermoregulation(a process that removes heat from the body) - ensures the preservation of constancy of body temperature by changing the release of heat by the body through conduction through the skin (conduction and convection), radiation (radiation) and evaporation of water. The release of heat constantly generated in the body is regulated by changes in the thermal conductivity of the skin, subcutaneous fat layer and epidermis. Heat transfer is largely regulated by the dynamics of blood circulation in heat-conducting and heat-insulating tissues. As the ambient temperature increases, evaporation begins to dominate in heat transfer.

Conduction, convection and radiation are passive heat transfer pathways based on the laws of physics. They are only effective if a positive temperature gradient is maintained. The smaller the temperature difference between the body and the environment, the less heat is given off. At the same indicators or at high ambient temperatures, the mentioned ways are not only ineffective, but the body also heats up. Under these conditions, only one heat release mechanism is activated in the body - sweating.

At low ambient temperatures (15°C and below), about 90% of daily heat transfer occurs due to heat conduction and heat radiation. Under these conditions, no visible sweating occurs. At an air temperature of 18-22°C, heat transfer due to thermal conductivity and heat radiation decreases, but heat loss by the body increases through the evaporation of moisture from the surface of the skin. When the ambient temperature rises to 35°C, heat transfer by radiation and convection becomes impossible, and body temperature is maintained at a constant level solely by the evaporation of water from the surface of the skin and alveoli of the lungs. When the air humidity is high, when water evaporation is difficult, the body may overheat and heat stroke may develop.

In a person at rest, at an air temperature of about 20°C and a total heat transfer of 419 kJ (100 kcal) per hour, 66% is lost through radiation, water evaporation - 19%, convection - 15% of the total heat loss by the body.

Chemical thermoregulation(the process that ensures the formation of heat in the body) - is realized through metabolism and through the heat production of tissues such as muscles, as well as the liver, brown fat, that is, by changing the level of heat generation - by increasing or weakening the intensity of metabolism in the cells of the body. When organic substances are oxidized, energy is released. Part of the energy goes to the synthesis of ATP (adenosine triphosphate is a nucleotide that plays an extremely important role in the exchange of energy and substances in the body). This potential energy can be used by the body in its further activities. All tissues are a source of heat in the body. Blood flowing through tissue heats up. An increase in ambient temperature causes a reflex decrease in metabolism, as a result of which heat generation in the body decreases. When the ambient temperature decreases, the intensity of metabolic processes reflexively increases and heat generation increases.

The activation of chemical thermoregulation occurs when physical thermoregulation is insufficient to maintain a constant body temperature.

Let's consider these types of thermoregulation.

Physical thermoregulation:

Under physical thermoregulation understand the set of physiological processes leading to changes in the level of heat transfer. There are the following ways for the body to release heat into the environment:

Evaporation (sweating);

Radiation (radiation);

Thermal conduction (conduction);

Convection.

Let's look at them in more detail:

1. Evaporation (sweating):

Evaporation (sweating)- is the release of thermal energy into the environment due to the evaporation of sweat or moisture from the surface of the skin and mucous membranes of the respiratory tract. In humans, sweat is constantly secreted by the sweat glands of the skin (“palpable,” or glandular, loss of water), and the mucous membranes of the respiratory tract are moisturized (“imperceptible” loss of water). At the same time, the “perceptible” loss of water by the body has a more significant impact on the total amount of heat given off by evaporation than the “imperceptible” one.

At an ambient temperature of about 20°C, moisture evaporation is about 36 g/h. Since 0.58 kcal of thermal energy is spent on the evaporation of 1 g of water in a person, it is easy to calculate that through evaporation, the adult human body releases about 20% of the total dissipated heat into the environment under these conditions. Increasing external temperature, performing physical work, and staying in heat-insulating clothing for a long time increase sweating and it can increase to 500-2,000 g/h.

A person does not tolerate relatively low ambient temperatures (32°C) in humid air. A person can remain in completely dry air without noticeable overheating for 2-3 hours at a temperature of 50-55°C. Clothing that is impervious to air (rubber, thick, etc.), which prevents the evaporation of sweat, is also poorly tolerated: the layer of air between the clothing and the body is quickly saturated with vapor and further evaporation of sweat stops.

The process of heat transfer through evaporation, although it is only one of the methods of thermoregulation, has one exceptional advantage - if the external temperature exceeds the average skin temperature, then the body cannot transfer heat to the external environment by other methods of thermoregulation (radiation, convection and conduction), which we will look at below. Under these conditions, the body begins to absorb heat from the outside, and the only way to dissipate heat is to increase the evaporation of moisture from the surface of the body. Such evaporation is possible as long as the ambient air humidity remains less than 100%. With intense sweating, high humidity and low air speed, when drops of sweat, without having time to evaporate, merge and flow from the surface of the body, heat transfer by evaporation becomes less effective.

When sweat evaporates, our body releases its energy. Actually, thanks to the energy of our body, liquid molecules (i.e. sweat) break molecular bonds and pass from liquid to gaseous state. Energy is spent on breaking bonds, and, as a result, body temperature decreases. A refrigerator works on the same principle. He manages to maintain a temperature inside the chamber much lower than the ambient temperature. It does this thanks to the electricity consumed. And we do this by using the energy obtained from the breakdown of food products.

Control over the selection of clothing can help reduce heat loss from evaporation. Clothing should be selected based on weather conditions and current activity. Don't be lazy to take off excess clothing as your load increases. You will sweat less. And don’t be lazy to put it on again when the load stops. Remove water and wind protection if there is no rain or wind, otherwise your clothes will get wet from the inside from your sweat. And when we come into contact with wet clothes, we also lose heat through thermal conductivity. Water conducts heat 25 times better than air. This means that in wet clothes we lose heat 25 times faster. This is why it is important to keep your clothes dry.

Evaporation is divided into 2 types:

A) Imperceptible perspiration(without the participation of sweat glands) is the evaporation of water from the surface of the lungs, mucous membranes of the respiratory tract and water seeping through the epithelium of the skin (evaporation from the surface of the skin occurs even if the skin is dry).

Up to 400 ml of water evaporates through the respiratory tract per day, i.e. the body loses up to 232 kcal per day. If necessary, this value can be increased due to thermal shortness of breath. On average, about 240 ml of water seeps through the epidermis per day. Consequently, in this way the body loses up to 139 kcal per day. This value, as a rule, does not depend on regulatory processes and various environmental factors.

b) Perceived perspiration(with the active participation of sweat glands) - This is the transfer of heat through the evaporation of sweat. On average, per day at a comfortable ambient temperature, 400-500 ml of sweat is released, therefore, up to 300 kcal of energy is released. The evaporation of 1 liter of sweat in a person weighing 75 kg can lower body temperature by 10°C. However, if necessary, the volume of sweating can increase to 12 liters per day, i.e. You can lose up to 7,000 kcal per day through sweating.

The efficiency of evaporation largely depends on the environment: the higher the temperature and lower the humidity, the greater the effectiveness of sweating as a heat transfer mechanism. At 100% humidity, evaporation is impossible. With high atmospheric humidity, high temperatures are more difficult to tolerate than with low humidity. In air saturated with water vapor (for example, in a bathhouse), sweat is released in large quantities, but does not evaporate and flows off the skin. Such sweating does not contribute to heat transfer: only that part of the sweat that evaporates from the surface of the skin is important for heat transfer (this part of the sweat constitutes effective sweating).

2. Radiation (radiation):

Radiation (radiation)- this is a method of transferring heat to the environment by the surface of the human body in the form of electromagnetic waves in the infrared range (a = 5-20 microns). Due to radiation, all objects whose temperature is above absolute zero give off energy. Electromagnetic radiation passes freely through a vacuum; atmospheric air can also be considered “transparent” for it.

As you know, any object that is heated above the ambient temperature emits heat. Everyone felt it sitting around the fire. A fire emits heat and heats up objects around it. At the same time, the fire loses its heat.

The human body begins to radiate heat as soon as the ambient temperature drops below the surface temperature of the skin. To prevent heat loss by radiation, you need to protect exposed areas of the body. This is done using clothing. Thus, we create a layer of air in clothing between the skin and the environment. The temperature of this layer will be equal to body temperature and heat loss by radiation will decrease. Why won't heat loss stop completely? Because now the heated clothes will radiate heat, losing it. And even if you put on another layer of clothing, you will not stop the radiation.

The amount of heat dissipated by the body into the environment by radiation is proportional to the surface area of ​​the radiation (the surface area of ​​the body not covered by clothing) and the difference in the average temperatures of the skin and the environment. At an ambient temperature of 20°C and a relative air humidity of 40-60%, the adult human body dissipates about 40-50% of the total heat given off by radiation. If the ambient temperature exceeds the average skin temperature, the human body, absorbing infrared rays emitted by surrounding objects, warms up.

Heat transfer by radiation increases as the ambient temperature decreases and decreases as it increases. Under conditions of constant ambient temperature, radiation from the body surface increases as the skin temperature increases and decreases as it decreases. If the average temperatures of the surface of the skin and the environment are equalized (the temperature difference becomes zero), then the transfer of heat by radiation becomes impossible.

It is possible to reduce the heat transfer of the body by radiation by reducing the surface area of ​​the radiation - change in body position. For example, when a dog or cat is cold, they curl up into a ball, thereby reducing the heat transfer surface; when it is hot, animals, on the contrary, take a position in which the heat transfer surface increases as much as possible. A person who “curls up into a ball” while sleeping in a cold room is not deprived of this method of physical thermoregulation.

3. Thermal conduction (conduction):

Thermal conduction (conduction)- this is a method of heat transfer that occurs during contact, contact of the human body with other physical bodies. The amount of heat given off by the body to the environment in this way is proportional to the difference in the average temperatures of the contacting bodies, the area of ​​the contacting surfaces, the time of thermal contact and the thermal conductivity of the contacting body.

Heat loss by conduction occurs when there is direct contact with a cold object. At this moment, our body gives up its heat. The rate of heat loss greatly depends on the thermal conductivity of the object with which we come into contact. For example, the thermal conductivity of stone is 10 times higher than that of wood. Therefore, sitting on a stone, we will lose heat much faster. You've probably noticed that sitting on a rock is somehow colder than sitting on a log.

Solution? Insulate your body from cold objects using poor heat conductors. Simply put, for example, if you are traveling in the mountains, then when you take a break, sit on a tourist rug or a bundle of clothes. At night, be sure to place a travel mat under your sleeping bag that is appropriate for the weather conditions. Or, in extreme cases, a thick layer of dry grass or pine needles. The earth conducts (and therefore “takes”) heat well and cools greatly at night. In winter, do not handle metal objects with bare hands. Use gloves. In severe frosts, metal objects can cause local frostbite.

Dry air and adipose tissue are characterized by low thermal conductivity and are heat insulators (poor heat conductors). Clothing reduces heat transfer. Heat loss is prevented by the layer of still air that is located between clothing and skin. The finer the cellularity of its structure containing air, the higher the thermal insulating properties of clothing. This explains the good heat-insulating properties of wool and fur clothing, which allows the human body to reduce heat dissipation through thermal conductivity. The air temperature under clothes reaches 30°C. And, conversely, the naked body loses heat, since the air on its surface is constantly changing. Therefore, the skin temperature of naked parts of the body is much lower than that of clothed parts.

Humid air saturated with water vapor is characterized by high thermal conductivity. Therefore, a person’s stay in an environment with high humidity and low temperature is accompanied by increased heat loss from the body. Wet clothing also loses its insulating properties.

4. Convection:

Convection- this is a method of heat transfer from the body, carried out by transferring heat by moving particles of air (water). To dissipate heat by convection, a flow of air with a lower temperature than the temperature of the skin is required over the surface of the body. In this case, the layer of air in contact with the skin heats up, reduces its density, rises and is replaced by colder and more dense air. Under conditions when the air temperature is 20°C and the relative humidity is 40-60%, the body of an adult dissipates about 25-30% of heat into the environment through heat conduction and convection (basic convection). As the speed of air flow (wind, ventilation) increases, the intensity of heat transfer (forced convection) also increases significantly.

The essence of the convection process is as follows- our body heats the air near the skin; heated air becomes lighter than cold air and rises, and it is replaced by cold air, which heats up again, becomes lighter and is replaced by the next portion of cold air. If the heated air is not captured with clothing, then this process will be endless. In fact, it is not our clothes that warm us, but the air they trap.

When the wind blows, the situation gets worse. The wind carries huge portions of unheated air. Even when we put on a warm sweater, the wind doesn’t cost anything to drive the warm air out of it. The same thing happens when we move. Our body “slams” into the air, and it flows around us, acting like wind. This also increases heat loss.

What solution? Wear a windproof layer: a windbreaker and windproof pants. Don't forget to protect your neck and head. Due to active blood circulation in the brain, the neck and head are the hottest areas of the body, so heat loss from them is very large. Also, in cold weather, you need to avoid drafty places both while driving and when choosing a place to spend the night.

Chemical thermoregulation:

Chemical thermoregulation heat generation is carried out due to changes in the level of metabolism (oxidative processes) caused by microvibration of muscles (oscillations), which leads to a change in the formation of heat in the body.

The source of heat in the body is the exothermic reactions of oxidation of proteins, fats, carbohydrates, as well as the hydrolysis of ATP (adenosine triphosphate is a nucleotide that plays an extremely important role in the metabolism of energy and substances in the body; first of all, this compound is known as a universal source of energy for all biochemical processes occurring in living systems). When nutrients are broken down, part of the released energy is accumulated in ATP, and part is dissipated in the form of heat (primary heat - 65-70% of energy). When using high-energy bonds of ATP molecules, part of the energy is used to perform useful work, and part is dissipated (secondary heat). Thus, two heat flows - primary and secondary - are heat production.

Chemical thermoregulation is important for maintaining a constant body temperature both under normal conditions and when the ambient temperature changes. In humans, increased heat generation due to an increase in metabolic rate is observed, in particular, when the ambient temperature becomes lower than the optimal temperature, or comfort zone. For a person wearing ordinary light clothing, this zone is within 18-20°C, and for a naked person it is 28°C.

The optimal temperature while in water is higher than in air. This is due to the fact that water, which has a high heat capacity and thermal conductivity, cools the body 14 times more than air, therefore, in a cool bath, metabolism increases significantly more than during exposure to air at the same temperature.

The most intense heat generation in the body occurs in the muscles. Even if a person lies motionless, but with tense muscles, the intensity of oxidative processes, and at the same time heat generation, increases by 10%. Small physical activity leads to an increase in heat generation by 50-80%, and heavy muscular work - by 400-500%.

The liver and kidneys also play a significant role in chemical thermoregulation. The blood temperature of the hepatic vein is higher than the blood temperature of the hepatic artery, which indicates intense heat generation in this organ. When the body cools, heat production in the liver increases.

If it is necessary to increase heat production, in addition to the possibility of receiving heat from the outside, the body uses mechanisms that increase the production of thermal energy. Such mechanisms include contractile And non-contractile thermogenesis.

1. Contractile thermogenesis.

This type of thermoregulation works if we are cold and need to raise our body temperature. This method consists of muscle contraction. When muscles contract, the hydrolysis of ATP increases, therefore the flow of secondary heat used to warm the body increases.

Voluntary activity of the muscular system mainly occurs under the influence of the cerebral cortex. In this case, an increase in heat production is possible by 3-5 times compared to the value of the basal metabolism.

Usually, when the ambient temperature and blood temperature decrease, the first reaction is increase in thermoregulatory tone(the hair on the body “stands on end”, “goosebumps” appear). From the point of view of the mechanics of contraction, this tone is a microvibration and allows you to increase heat production by 25-40% of the initial level. Usually the muscles of the neck, head, torso and limbs take part in creating tone.

With more significant hypothermia, the thermoregulatory tone turns into a special type of muscle contraction - cold muscle tremors, in which the muscles do not perform useful work and their contraction is aimed solely at generating heat. Cold shivering is an involuntary rhythmic activity of superficially located muscles, as a result of which the metabolic processes of the body are significantly enhanced, the consumption of oxygen and carbohydrates by muscle tissue increases, which entails increased heat generation. Trembling often begins in the muscles of the neck and face. This is explained by the fact that, first of all, the temperature of the blood that flows to the brain must increase. It is believed that heat production during cold shivering is 2-3 times higher than during voluntary muscle activity.

The described mechanism works at a reflex level, without the participation of our consciousness. But you can also raise your body temperature with conscious motor activity. When performing physical activity of varying intensity, heat production increases 5-15 times compared to the resting level. During the first 15-30 minutes of prolonged operation, the core temperature rises quite quickly to a relatively stationary level, and then remains at this level or continues to rise slowly.

2. Non-contractile thermogenesis:

This type of thermoregulation can lead to both an increase and a decrease in body temperature. It is carried out by accelerating or slowing down catabolic metabolic processes (oxidation of fatty acids). And this, in turn, will lead to a decrease or increase in heat production. Due to this type of thermogenesis, the level of heat production in a person can increase 3 times compared to the level of basal metabolism.

Regulation of the processes of non-contractile thermogenesis is carried out by activating the sympathetic nervous system, the production of thyroid hormones and the adrenal medulla.

E. Thermoregulation control.

Hypothalamus.

The thermoregulation system consists of a number of elements with interrelated functions. Information about temperature comes from thermoreceptors and travels to the brain through the nervous system.

Plays a major role in thermoregulation hypothalamus. It contains the main centers of thermoregulation, which coordinate numerous and complex processes that ensure the maintenance of body temperature at a constant level.

Hypothalamus- this is a small area in the diencephalon, which includes a large number of groups of cells (over 30 nuclei) that regulate the neuroendocrine activity of the brain and homeostasis (the ability to maintain the constancy of its internal state) of the body. The hypothalamus is connected by nerve pathways to almost all parts of the central nervous system, including the cortex, hippocampus, amygdala, cerebellum, brain stem and spinal cord. Together with the pituitary gland, the hypothalamus forms the hypothalamic-pituitary system, in which the hypothalamus controls the release of pituitary hormones and is the central link between the nervous and endocrine systems. It secretes hormones and neuropeptides, and regulates functions such as hunger and thirst, body thermoregulation, sexual behavior, sleep and wakefulness (circadian rhythms). Recent studies show that the hypothalamus also plays an important role in the regulation of higher functions, such as memory and emotional state, and thereby participates in the formation of various aspects of behavior.

Destruction of the hypothalamic centers or disruption of nerve connections leads to loss of the ability to regulate body temperature.

The anterior hypothalamus contains neurons that control heat transfer processes.(they provide physical thermoregulation - vasoconstriction, sweating). When the neurons of the anterior hypothalamus are destroyed, the body does not tolerate high temperatures, but physiological activity in cold conditions remains.

Neurons of the posterior hypothalamus control the processes of heat generation(they provide chemical thermoregulation - increased heat generation, muscle tremors). If they are damaged, the ability to increase energy exchange is impaired, so the body does not tolerate cold well.

Thermosensitive nerve cells of the preoptic region of the hypothalamus directly “measure” the temperature of arterial blood flowing through the brain and are highly sensitive to temperature changes (able to distinguish a difference in blood temperature of 0.011 ° C). The ratio of cold- and heat-sensitive neurons in the hypothalamus is 1:6, so central thermoreceptors are preferentially activated when the temperature of the “core” of the human body increases.

Based on the analysis and integration of information about the temperature of the blood and peripheral tissues, the average (integrated) value of body temperature is continuously determined in the preoptic region of the hypothalamus. These data are transmitted through intercalary neurons to a group of neurons in the anterior hypothalamus, which set a certain level of body temperature in the body - the “set point” of thermoregulation. Based on the analysis and comparisons of the average body temperature and the set point temperature to be regulated, the “set point” mechanisms, through the effector neurons of the posterior hypothalamus, influence the processes of heat transfer or heat production to bring the actual and set temperature into correspondence.

Thus, due to the function of the thermoregulation center, a balance is established between heat production and heat transfer, which allows maintaining body temperature within optimal limits for the body’s vital functions.

Endocrine system.

The hypothalamus controls the processes of heat production and heat transfer, sending nerve impulses to the endocrine glands, mainly the thyroid, and adrenal glands.

Participation thyroid gland in thermoregulation is due to the fact that the influence of low temperature leads to increased release of its hormones (thyroxine, triiodothyronine), which accelerate metabolism and, consequently, heat formation.

Role adrenal glands is associated with their release of catecholamines into the blood (adrenaline, norepinephrine, dopamine), which, by increasing or decreasing oxidative processes in tissues (for example, muscle), increase or decrease heat production and narrow or enlarge skin vessels, changing the level of heat transfer.

human life:

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QUESTION N 7. The development of heat stroke is possible at the following body temperature:

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QUESTION No. 8. During the stage of decompensation of hypothermia, the body develops:

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1. Bradycardia and bradypnea

2. Suppression of the activity of the cerebral cortex

3. Progressive decrease in basal metabolic rate

4. All answers are correct

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QUESTION N 9. When the ambient temperature increases, compensatory reactions

organisms are all EXCEPT:

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1. Bradycardia and bradypnea

2. Hyperpnea

3. Peripheral vascular dilatation

4. Tachycardia and tachypnea

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QUESTION N 10. A characteristic sign of a second degree thermal burn is:

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1. Erythema

2. Formation of bubbles

3. Necrosis of all layers of skin

4. All answers are correct

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QUESTION N 11. The stage of compensation for hyperthermia is characterized by everything EXCEPT:

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1. Increasing the level of gas exchange

2. Increase in minute volume of blood circulation

3. Reduced level of gas exchange

4. Increased sweating

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QUESTION N 12. The stage of decompensation of hypothermia is characterized by:

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1. Narrowing of the lumen of peripheral vessels

2. Dysfunction and mismatch of different structures of the central nervous system

3. Progressive decrease in the level of basal metabolism

4. All of the above

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QUESTION N 13. Hyperthermia of the body develops as a result of everything EXCEPT:

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1. Activation of heat transfer processes with normal or reduced heat production

2. Inhibition of heat transfer processes during normal heat production

3. Inhibition of heat transfer processes with increased heat production

4. Dissociation of oxidation and phosphorylation processes

The concept of thermal homeostasis

Thermal homeostasis is the ability to maintain a constant body (core) temperature at a certain level. This ability served as the basis for the division of animals into warm-blooded and cold-blooded (from the Greek “homoyos” - equal, “poikilos” - diverse, “therme” - heat).

9.1. “Core” and “shell” of the body as concepts of thermal homeostasis

The idea of ​​homeothermicity in higher animals and humans has been greatly shaken in the last ten years. It turned out that: a) the superficial tissue of the body - the “shell” (skin, subcutaneous fat, superficial muscles, tissues of the extremities) have poikilothermic properties, i.e. depending on the ambient temperature, their temperature can fluctuate up to 10° C; b) at the same time, the organs of the chest, abdominal cavity, and the brain - the “core” of the body - are in homeothermal conditions, their temperature changes by no more than 2 ° C.

The temperature in the cavities close to it (on the eardrum of the ear canal, in the sublingual fossa, rectum and vagina, armpits) most closely reflects the temperature of the “core” of the body.

Based on the above, thermal homeostasis can be defined as maintaining the temperature of the “core” of the body at a constant level.

9.2. Mechanisms of thermal homeostasis

The mechanisms of thermal homeostasis are the mechanisms of heat generation (thermogenesis) and heat transfer.

9.2.1. Mechanisms of thermogenesis: contractile and non-contractile

Due to the “combustion” of ATP, both the 1st and 2nd mechanisms work. The versatility is manifested in the fact that part of the energy of ATP, in addition to the perfect work, is necessarily dissipated in the form of heat.

Contractile thermogenesis provides 70% of heat production and is carried out due to the contractile activity of muscles (voluntary movements, thermoregulatory muscle tone). If skeletal muscles (fast muscle fibers) are associated primarily with motor function, then contractions (trembling) of slow postnotonic muscles (extensor muscles), muscles of hair follicles (hair erection, goose bumps), etc. have thermogenic significance.

Non-contractile thermogenesis accounts for 30% of heat production and is ensured by the release of heat during the operation of Na,K-ATPase and the uncoupling of oxidative phosphorylation.

The formation of heat in newborns in response to cooling is 80% due to a special tissue - brown fat. They do not exhibit trembling (no contractile thermogenesis). Brown fat is deposited around the heart, neck and between the shoulder blades, on the chest (Fig. 18). A white fat cell contains one large drop of fat, while a brown fat cell contains many small droplets of fat and a lot of mitochondria. The iron-containing pigment of mitochondrial cytochromes gives it a brown color. Due to the large number of mitochondria, the oxidative capacity of brown fat cells exceeds that of white fat. Removing a few grams of brown fat from newborn rabbits eliminated their ability to increase heat production in response to cooling.

The system for turning on brown fat thermogenesis can be presented as follows. Thermoreceptors that perceive cold exposure send impulses to the brain. The thermoregulation center of the hypothalamus switches these impulses through the reticular formation to the sympathetic nerves going to the adipose tissue, where norepinephrine is released at their synapses. Through 3,5-AMP, it increases the activity of triglyceride lipase, which breaks down fats into glycerol and fatty acids. Fatty acids cause uncoupling of the processes of oxidation and phosphorylation, and the energy of oxidation is not converted into ATP, but is immediately dissipated in the form of heat.

Another mechanism of non-contractile thermogenesis is based on an increase in the activity of Na-K-ATPase under the influence of T3/T4 (25-30% of ATP is normally converted into heat during the operation of ATPase).

Conclusion: chemical catabolism of food substrates leads to the generation of ATP, most of which goes into work, and a smaller part necessarily into heat. Thus, the processes of heat formation in the body are of a chemical nature.

Now let's look at the components of thermal homeostasis associated with various heat transfer pathways.

9.2.2. Heat transfer mechanisms

It should be said right away that they are basically of a physical nature.

1. Heat transfer by evaporation from the skin and upper respiratory tract. When 1 ml of H 2 O is evaporated, 0.58 kcal is subtracted from the evaporating area. During a day, a person at an ambient temperature of 35° C loses about 5 liters of sweat, which corresponds to 2900 kcal. Evaporation from the body surface depends on sweating and the temperature and humidity of the environment. At high temperatures and humidity, the evaporation of moisture may stop and the skin will remain wet, causing a feeling of stuffiness and poor health (in the summer on a bus).
2. Heat transfer by convection is the absorption of heat by the environment around a person (air - fan, water - swimming).
3. Heat transfer by radiation is the transfer of heat from the body due to the infrared radiation of the body (exposing parts of the body or, conversely, covering them with clothing).

Conclusion. Heat transfer mechanisms use the “shell” of the body, the temperature of which is maintained mainly by the transfer of heat with arterial blood from the “core” (internal organs) to the “shell” of the body.

9.3. Thermal homeostasis regulation system

The neuroendocrine system controls physiological and behavioral shifts in the heat-regulatory behavior of mammals and cold-blooded animals.

As agreed earlier, by thermal homeostasis we mean maintaining the temperature of the “core” of the body at a constant level; in this case, the “shell” of the body is given the functions of a working organ, all of whose activities are aimed only at maintaining the thermal homeostasis of the internal organs, the “core”.

9.3.1. The concept of a "set point"

The question of turning on and off the mechanisms of heat generation and heat transfer under the influence of the neuroendocrine control system is closely related to the concept of the “set point”. It is proposed to understand this controlled variable as the temperature of the deep structures of the brain.

The best idea about it is the temperature of the eardrum (about 37.1 ° C).

A deviation of brain temperature from 37.1° C causes changes in the activity of the central parts of the neuro-endocrine system (hypothalamus), which trigger either heat production reactions (increased catabolism, tremors) or reactions of increased heat loss (vasodilation).

The way to turn on the heat transfer and heat generation systems is common (Fig. 19). Impulses from the cold receptors of the skin, direct washing of the hypothalamus with cold blood, or the action of pyrogens on it lead to a deviation in the temperature of the neurons of the thermoregulation nucleus from the set point. Excitation of the hypothalamus switches through the reticular formation to the SAS or through the primary stress mediator system to the pituitary gland. As a result, the work of the heat transfer system (the play of skin microvessels) and the thermogenesis system (contractile and non-contractile) is enhanced or weakened.

9.3.2. The role of feedback in thermoregulation

After turning on the indicated systems and approaching the brain temperature to 37.1 ° C, the effect of excitation of physical and chemical thermoregulation according to the principle of negative feedback is suppressed.

If there is excessive retention or release of heat, the temperature of the “core” can no longer be compensated by physiological thermoregulation and is prevented only by changing the behavior of humans, animals, i.e. the 2nd system of thermal homeostasis turns on.

9.3.3. Social thermoregulation

Thus, maintaining the thermal homeostasis of the “core” with the help of physical and chemical mechanisms of thermoregulation is possible only within certain normal limits of environmental influence. To maintain the thermal homeostasis of the “core” under adverse influences outside these limits, it is necessary to include other mechanisms - behavioral shifts, conscious human actions or changes in the behavioral reactions of living beings.

Returning to the topic of heredity, we can say that the creation of a human thermoregulation system is also a phenotypic manifestation of a person’s genotype, its species characteristic. This new trait, which appeared in evolution, provided the species with enormous advantages in survival. A specific example in this case is the conscious thermoregulatory activity of a person, which allows him to survive under such temperature influences under which the life of other living organisms is generally impossible (space). This way of adaptation of the body is associated with maintaining the temperature of the “shell” of the body at a normal or slightly reduced level due to fires, clothing, and the construction of dwellings.

9.4. Daily (circadian) rhythm of body temperature changes

The temperature curve at the patient's head is a reflection of the state of thermal homeostasis. Maintaining thermal homeostasis of the “core” of the body is reflected in the fact that a person’s temperature normally fluctuates within very narrow limits during the day (from 0.8 to 1.2 ° C).

The dynamics of these fluctuations reflect the daily (circadian) rhythm of fluctuations in the activity functions of higher animals and humans, associated with the change of day and night.

The circadian rhythm and its changes are of great importance for occupational health and problems of human adaptation to unusual (extreme) environmental conditions. Its study is also important for pathology, because In many diseases, daily temperature fluctuations are maintained to some extent. In particular, daily temperature dynamics (in various forms) persist during fever.

The practical significance of the temperature curve is a reflection of the state of thermal homeostasis. Sharp rhythm disturbances in febrile patients have significant diagnostic - before the use of drugs - and prognostic significance.

9.5. What is fever?

Fever - Febris - has been known for a long time, since the time of Hippocrates, who identified some diseases and called them febrile (typhoid, malaria, Pappataci fever and others). Throughout the history of medicine, these diseases have passed under this term. From the middle to the beginning of the 20th century, fever was considered a disease that could occur without an increase in temperature.

Now we understand fever as a complex of symptoms characterized by an increase in body temperature, characteristic of many infectious diseases. That is, an increase in temperature is the essence of a fever. The Russian name “dashing” (bad) quite fully reflects the patient’s condition with this pathology.

9.5.1. Etiology: infectious and non-infectious effects that cause the formation of pyrogens in the body (interleukin-1)

An increase in temperature usually occurs due to a bacterial or viral infection. Sometimes high temperature also accompanies non-infectious effects. However, the bacteria that cause a fever do not have to be alive. “Devouring” bacteria, leukocytes produce a special protein - interleukin-1. It is he who apparently informs the hypothalamus about the need to increase body temperature, increasing the content of prostaglandin E in the thermoregulation center of the hypothalamus, which leads to an increase in the set point (Fig. 20). It has also been established that aspirin brings the temperature to normal by suppressing the formation of prostaglandin E in the thermoregulatory center. Aspirin is known to reduce high fevers but has no effect on normal ones.

9.5.2. The role of the relationship between thermogenesis and heat transfer in the pathogenesis of fever

Previously, the cause of fever was considered to be increased heat production. But Galen also showed that the cause of fever is heat retention in the body. We still believe that this is one of the main factors. Further, many researchers assumed that an increase in heat production alone could not cause a fever; it was also necessary to limit heat loss. Liebermeister, A.A. Likhachev, P.P. Avrorov proved with the help of V.V. Pashutin’s calorimetric system that during physical work, heat generation increases by 200-300%, and the temperature increases slightly.

During fever, heat generation increases by 63%, but heat transfer is delayed, as a result, body temperature rises to 40° C.

Later, P.R. Veselkin showed the role of shifting the set point in raising temperature homeostasis to a higher level.

9.5.2.1. Stages of fever

Fever develops in 3 stages as a result of changes in the ratio of heat transfer and heat generation:

1. Temperature rise [show]

   A change in thermoregulation occurs, characterized by the following changes: a sharp limitation of heat transfer and the beginning of an increase in thermogenesis. An increase in the tone of the sympathetic nervous system (activation of the SAS) leads to a narrowing of superficial vessels and thus limiting heat transfer due to evaporation, radiation, and convection. Activation of both non-contractile and contractile thermogenesis begins. The latter begins with the chewing muscles (“teeth chatter”), then other muscles are involved, which is accompanied by the production of energy. The main thing in increasing the temperature at this stage is not so much an increase in heat generation, but a decrease in heat transfer.

2. Temperature at a high level [show]

   It is characterized by a sharp increase in heat production. At the same time, heat transfer increases, but compared with stage 1, and not with the norm. Mainly due to the expansion of the superficial vessels of the skin (hyperemia), sweating is reduced.

3. Temperature drop [show]

   Heat transfer is turned on at full power due to the expansion of surface vessels and a sharp increase in sweating, radiation, and convection. The result is a drop in temperature: critical (fast) or lytic (slow). The latter option is more favorable for the patient.

Complications. A temperature crisis (tipping point) may be accompanied by acute cardiovascular failure - collapse. The doctor should be careful due to the fact that heat formation may remain elevated.

9.6. Changes in the functions of a number of organs during fever

The main event at the cellular level - an increase in temperature to 40 ° C causes an increase in the fluidity of membrane lipids with disruption of the functions of proteins - receptors, transporters, ATP generation, and detoxification.

9.6.1. Changes in the functions of the central nervous system covers all formations, from the cortex to the spinal cord and is clinically manifested:

• the predominance of the inhibitory process (inhibition of the patient’s reactions, lethargy, drowsiness, apathy). Changes in the nervous system are determined not only by changes in temperature, but also by other factors: intoxication, etc.;
• the predominance of excitatory processes (cases of delirium, hallucinations, and violence have been described in patients with typhoid fever and lobar pneumonia).

9.6.2. Changes in the cardiovascular system

The activity of the cardiovascular system changes in stages. Initially, there is an increase in rhythm, in most cases proportional to the rise in temperature. There is a contraction of superficial vessels and a flow of blood to the internal organs. In the second stage, the heart rate is also increased, but the superficial vessels may dilate, leading to a drop in blood pressure. In the third stage, there is a decrease in heart rate, a drop in blood pressure until collapse.

9.6.3. Change in breathing manifest themselves in the form of tachypnea, but minute volume does not increase, because shallow breathing. At the same time, this is one of the ways to compensate for the increase in heat transfer by evaporation.

9.6.4. Digestive system changes are largely related to the action of interleukin-1. In particular, there is a slowdown in the secretion of gastrointestinal juices, a decrease in the acidity of gastric juice, and a slowdown in peristalsis, accompanied by an increase in the absorption of the liquid part of the contents of the digestive tube.

9.6.5. Metabolic changes

The 1st stage is mainly manifested by the acceleration of oxidative processes. In general, there is an increase in catabolism at the beginning and disorganization of metabolism at the end with a violation of the body’s resistance to stress factors. Protein metabolism is characterized by a negative nitrogen balance, i.e. the excess of nitrogen excretion from the body over its intake. An artificial increase in protein intake does not normalize the nitrogen balance, which returns to normal in the 3rd stage. Carbohydrate metabolism is characterized by an increase in the breakdown of glucose stores - glycogen. Lipids are also mobilized from storage to form non-esterified fatty acids - energy material.

The 3rd stage is characterized by a sharp loss of water and minerals through sweat. First, isotonic, then hypotonic hypohydration develops.

9.7. Biological role of fever

In the process of evolution, fever has developed as a protective mechanism, but it can also be pathological in nature when the temperature rises to levels at which there is a significant increase in the fluidity of lipids of cell biomembranes.

Do cold-blooded animals get fevers? American researcher Dr. M.J. Kluger was able to confirm the protective role of fever experimentally. For his experiments, the researcher used cold-blooded animals, whose body temperature changes easily - it adapts to the temperature of their environment. A large lizard (length without tail up to 15 cm), the iguana under natural conditions can change its body temperature in the range from 15 ° C at night to 29-50 ° C during the day, depending on whether it is in the shade or in the sun.

The laboratory created a climate regime similar to that which an iguana can find in its desert. In different places of the cage and in accordance with the time of day, the lamps were turned on in such a way that the lizard could choose the temperature it needed. A tiny thermometer in the iguana's rectum made it possible to monitor its body temperature at all times. Healthy lizards moved in various “microclimates” that were at their disposal, and maintained their temperature consistently at 38-39 ° C.

How will sick lizards behave? The iguanas were infected with bacteria that caused inflammation in their paws. In this case, the lizards began to choose a warmer place in the cage and their temperature increased to 40-42 ° C. In other words, they deliberately caused themselves to feel hot.

The question immediately arose: was it useful for them? The answer was given by the following simple experiment. The iguanas were placed in 5 boxes, each of which had a constant temperature: 34 and 36 ° C - low temperature, but still within normal limits for iguanas: 38 ° C - normal: 40 and 42 ° C - above normal.

After three days, 96% of the lizards kept at the highest temperature were alive. In boxes with a normal temperature, 34% of animals survived, and in boxes with a temperature of 34 ° C - only 10%.

In other words, the high temperature helped the animals resist infection. Of course, one could say that high temperature inhibits the division of bacteria. This is true. Scientists (in particular, Dr. A. Lvov from the Pasteur Institute in Paris) have long shown that some infectious organisms reproduce more slowly at elevated temperatures than at normal temperatures (bacteriostatic effect). However, there is reason to believe that in addition to this, with increasing temperature, many components of homeostasis are activated: the production of phagocytes and helper T-lymphocytes increases under the influence of interleukin-1, and the amount of microelements (in particular, iron) required by infectious microorganisms decreases. This last phenomenon has been observed by other scientists. In addition, fever also causes stress syndrome.

Conclusion: The doctor’s task is to assess the patient’s condition at a given time. The very frequent desire to necessarily relieve a high temperature is incorrect. To substantiate the position that the fever of a sick person differs primarily in that he regulates his temperature at a higher level, the following data can be cited:

1. A febrile patient, like a healthy one, trembles when placed in a cold bath.
2. An increase in body temperature during fever does not directly depend on the ambient temperature, but can be observed both with an increase and a decrease in ambient temperature.
3. During fever, the temperature, having risen to a certain value, continues to remain at this level for a long time, even if heat generation remains high. This indicates that heat generation and heat transfer are balanced and the temperature is steadily regulated at a new level.

9.8. Differences between fever and overheating

General: increased body temperature.

Differences:

1. The febrile reaction does not depend on the ambient temperature, i.e. thermal homeostasis is preserved.
2. With fever, an active increase in temperature is observed, because under the influence of pyrogens the set point shifts. Overheating is passive, thermal homeostasis is disturbed and when body temperature rises due to increased ambient temperature. This is no longer a protective phenomenon, but a consequence of a breakdown of the thermoregulatory system. The cause of overheating may be a prolonged stay in an environment with a higher temperature or difficulty in heat transfer processes (working in a spacesuit).
3. Fever is protective, overheating is not.

9.9. The use of fever for therapeutic purposes

Used to convert a chronic process into an acute one (stressor). For example, artificial fever is used in the treatment of chronic, sluggish infections such as dysentery and gonorrhea. For this purpose, a ready-made pyrogen is administered, or inflammation is caused (intramuscular injection of milk, autologous blood). In all variants, the common link is the increased formation of interleukin-1.

Body temperature

Body temperature is a complex indicator of the thermal state of the body of animals and humans.

Maintaining body temperature within certain limits is one of the most important conditions for the normal functioning of the body. Poikilothermic animals, which include invertebrates, fish, amphibians, and reptiles, have a body temperature close to the ambient temperature. Homeothermic animals - birds and mammals - in the process of evolution acquired the ability to maintain a constant body temperature when the ambient temperature fluctuates.

In a homeothermic organism, two temperature zones are conventionally distinguished - the shell and the core. The shell consists of superficial structures and tissues - skin, connective tissue, core - blood, internal organs and systems. The temperature of the core is higher than that of the shell and is relatively stable: the temperature difference between the internal organs is several tenths of a degree, with the liver having the highest temperature (about 38°). The temperature of other internal organs, including the brain, is close to the temperature of the blood in the aorta, which determines the average core temperature. In the brain of rabbits and some other animals, a difference in temperature between the cerebral cortex and the hypothalamus was noted, reaching 1°.

The temperature of the shell is 5-10° lower than the temperature of the core and is not the same in different parts of the body, which is due to differences in their blood supply, the size of the subcutaneous fat layer, etc. The body surface temperature depends significantly on the ambient temperature. When the body is heated for a short time (for example, in a Finnish sauna at an air temperature of 80-100°), the temperature of the skin of the extremities, which is normally about 30°, can rise to 45-48°, and when cooled, drop to 5-10°.

The presence of zones with different temperatures in the body does not allow one to unambiguously determine body temperature. To characterize it, the concept of weighted average temperature is often used, which is calculated as the average of the temperatures of all parts of the body. More precisely, body temperature can be characterized by a temperature pattern - the distribution of temperature over the surface of the body (Fig. 1.) or in its core. The characteristic of body temperature is also used by the temperature gradient, which is represented by a vector directed towards the highest temperature value, and the magnitude of the vector corresponds to the change in temperature per unit length. The image of the temperature diagram of the body in the form of isotherms and gradient values ​​complement each other: the closer the isotherms are located, the greater the temperature gradient of the body parts.

Body temperature is measured using various thermometers and temperature sensors. Core temperature can be measured quite accurately (with an error of less than 0.5°) by placing a thermometer in the armpit, under the tongue, in the rectum or external auditory canal. Normal human body temperature, measured in the rectum, is close to 37°. The temperature measured under the tongue is 0.2-0.3° less, in the armpit it is 0.3-0.4° less.

Most people have well-defined daily fluctuations in body temperature, lying in the range of 0.1-0.6°. The highest body temperature is observed in the second half of the day, the lowest at night. There are also seasonal fluctuations in body temperature: in summer it is 0.1-0.3° higher than in winter. Women also have a pronounced monthly rhythm of changes in body temperature: during ovulation, it increases by 0.6-0.8°. An increase in body temperature is observed during intense muscle work and strong emotional experiences.

Maintaining life in homeothermic animals and humans is possible only within a certain range of body temperature. The interval between normal and upper lethal temperature of internal organs is about 6°. In humans and higher mammals the upper lethal temperature is approximately 43°, in birds it is 46-47°. The causes of death of homeothermic animals and humans when body temperature exceeds the upper critical limit are considered to be a violation of the biochemical equilibrium in the body due to the influence of temperature changes on the speed of various biochemical reactions, as well as disruption of the membrane structure as a result of a thermal change in the conformation of macromolecules, thermal inactivation of enzymes occurring at a rate exceeding the rate of their synthesis, denaturation of proteins as a result of heating, lack of oxygen. The lower lethal body temperature is 15-23°. With artificial cooling of the body (see Artificial hypothermia), when special measures are taken to preserve its viability, body temperature can be lowered to lower values ​​without risk to life.

In accordance with the laws of thermodynamics, metabolic and energy processes are associated with the production of heat. In some animals (and humans), body temperature remains at a constant level, which significantly exceeds the temperature of the environment due to intensive heat production controlled by special regulatory mechanisms. This - homeothermic (warm-blooded)) organisms. Another group of animals (fish, amphibians) is characterized by a significantly lower intensity of heat production; their body temperature only slightly exceeds the temperature of the environment and undergoes the same fluctuations ( poikilothermic, cold-blooded animals).

Heat production and body temperature. All chemical reactions in the body depend on temperature. In poikilotherms, the intensity of energy processes increases in proportion to the external temperature in accordance with Van Hoff's rule. In homeothermic animals, this rule is masked by another effect (regulatory thermogenesis) and appears only when thermoregulation is blocked (anesthesia, damage to the nervous system). Even after blockade of the regulatory component, significant quantitative differences remain between metabolic processes in cold-blooded and warm-blooded animals: at the same body temperature, the intensity of energy exchange per unit of body mass in warm-blooded animals is 3 times greater. Anesthesia, together with a decrease in body temperature, can cause a noticeable decrease in the degree of oxygen consumption and a delay in the processes of tissue destruction - this is used in surgery.

Heat production and body size. The body temperature of most warm-blooded animals lies in the range of 36-39 ° C, despite significant differences in weight and size. In contrast, metabolic rate (M) is a power function of body weight (m): M = km 0.75. The coefficient k is approximately the same for both a mouse and an elephant. This law of the dependence of metabolism on body weight reflects the tendency to establish a correspondence between heat production and the intensity of heat transfer into the environment. The greater the ratio between the surface and volume of the body, the greater the heat loss per unit mass, and this ratio decreases with increasing body size. In addition, in small animals the insulating layer of the body is thinner. If you arrange some animals in order of decreasing intensity of metabolic processes, you get the following: mouse, rabbit, dog, human, elephant.

Thermoregulatory thermogenesis. When additional heat is needed to maintain body temperature, it can be generated in the following ways:

1. Voluntary activity of the muscular system.

2. Involuntary tonic or rhythmic (tremor) activity. These two pathways are called contractile thermogenesis.

3. Acceleration of metabolic processes not associated with muscle contraction (not contraction)

body thermogenesis).

In an adult, shivering is the most significant involuntary manifestation of thermogenesis mechanisms. In a newborn baby, it is not contractile thermogenesis (the combustion of brown fat in the “metabolic cauldron”) that is of greater importance. Accumulations of brown fat with a large number of mitochondria are located between the shoulder blades, in the armpit. As the body cools, its temperature increases and blood flow increases. By increasing thermogenesis, body temperature is maintained at a constant level.

Environmental factors and thermal comfort. The effect of environmental temperatures on the body depends on at least four physical factors: air temperature, humidity, radiation temperature and air speed (wind). These factors determine whether a person feels "thermal comfort" or feels hot or cold. The condition of comfort is that the body does not need the functioning of thermoregulation mechanisms: it does not require any trembling or sweating, and the blood flow in the peripheral areas maintains an average speed. This is the so-called thermoneutral zone.

These four factors are to some extent interchangeable.

The comfort temperature value for a lightly dressed (shirt, shorts, long cotton trousers) seated person is 25-26 o C with a humidity of 50% and equal air and wall temperatures. For a naked person = 28 o C. Under conditions of thermal comfort, the average skin temperature = 34 o C. As physical work is performed, the comfort temperature drops. For light office work it is 22 o C.

Discomfort increases with the average temperature and humidity of the skin (the part of the body surface covered with sweat).

Heat dissipation.

1. Internal heat flow. Less than half of all heat generated inside the body spreads to the surface by conduction through tissue. Most of it goes by convection into the bloodstream. Blood has a high heat capacity. The blood flow of the extremities is organized according to the principle of a rotary-countercurrent mechanism, which facilitates heat exchange between the vessels.

2. External heat flow. Heat is transferred outwards through conduction, convection, radiation and evaporation. Heat transfer by conduction is when a body comes into contact with a dense substrate. When body contact occurs with air - convection, radiation or evaporation. If the skin is warmer than the air, the adjacent layer heats up and moves upward, being replaced by colder air. Forced convection (blowing) significantly increases the intensity of heat transfer. The radiation occurs in the form of long-wave infrared radiation. About 20% of the heat transfer of the human body in neutral temperature conditions occurs due to the evaporation of water from the skin and mucous membranes of the respiratory tract.

The influence of clothing - from a physiological point of view, it is a form of thermal resistance or insulation. The effectiveness of clothing is determined by the smallest volumes of air in the structure of the fabric or in the pile, where external currents do not penetrate. In this case, heat is transferred only by conduction, and air is a poor conductor of heat.

Body temperature and heat balance. If it is necessary to maintain a constant body temperature, a stable balance must be achieved between heat production and heat transfer. When the environmental temperature decreases, a constant body temperature can be maintained only if regulatory mechanisms ensure an increase in thermogenesis in proportion to heat loss. The highest heat production provided by these mechanisms in humans corresponds to basal metabolic rates 3–5. This indicator characterizes the lower limit of the thermoregulation range (0-5 o C in the external environment for adults, 23 o C for newborns). If this limit is exceeded, hypothermia and cold death develop.

When the temperature of the environment increases, temperature equilibrium is maintained due to a decrease in exchange, due to additional heat transfer mechanisms. The upper limit of the thermoregulation range is determined by the mechanisms of intense sweat secretion, which increases by 60% at 100% skin humidity and can reach 4 l/hour.

With an increase in the temperature of the environment, the skin vessels dilate, the total amount of circulating blood increases due to its exit from the depot, due to the entry of water from the tissues. This promotes increased heat transfer. But the main thing is still evaporation. The average heat generation per day during vigorous activity is about 2500-2800 kcal. To maintain body temperature at a constant level under these conditions, it is necessary to evaporate 4.5 liters of water. For heavy muscular work - up to 12 liters. in a day. Water evaporation depends on the relative humidity of the air in the room and is impossible at 100% humidity. Therefore, high humidity at high temperatures is poorly tolerated. In this case, sweat does not evaporate, but flows off the skin. This type of sweating does not contribute to the transfer of heat. Clothing that is impervious to air (leather, rubber) is also poorly tolerated, as it prevents evaporation. In completely dry air, a person does not overheat in 2-3 hours at T 55 o C.

Human body temperature. The heat generated in the body is transferred to the surrounding space by the surface of the body. Therefore, T about the surface is less than T about the core of the body, and T about the distal part of the limbs is less than the proximal one. In this regard, the spatial distribution of body temperature has a complex three-dimensional shape. For example, when a lightly dressed adult is in a room with an air temperature of 20 o C, in the deep muscles of his thigh the temperature is 35 o C, in the calf muscle - 33 o C, on the foot - 27 o C, in the rectum -37 o C.

Fluctuations in body temperature with changes in external temperature are more pronounced near the surface of the body and in the end parts of the limbs. There is a “homeothermic core” and a “poikilothermic shell”.

Core body temperature itself is not constant, either spatially or temporally. The differences are 0.2-1.2 o C. Even in the brain, the temperature of the center and cortex differs by 1 o C. As a rule, the highest T o is observed in the rectum (and not in the liver, as was previously believed!). In this regard, it is impossible to express the T about the body in one number. For practice, it is enough to find a certain area in which T o can be considered as representative of the entire internal layer. Clinical measurements require an easily accessible area with minor spatial temperature variations. In this sense, it is preferable to use rectal temperature. In this case, a special rectal thermometer is inserted at 10-15 cm. Normally, it is 37 o C.

Oral temperature (sublingual) is also used clinically. Usually it is 0.2-0.5 o less than rectal.

Axillary temperature (most often used in Russia) is 36.5-36.6 o. Can serve as an indicator of core body temperature because when the arm is pressed tightly against the chest, the temperature gradient shifts so that the boundary of the body's core reaches the armpit. However, you have to wait quite a long time (10 minutes) until enough heat accumulates in these areas. If the superficial tissues were initially cold in conditions of low ambient temperature and vasoconstriction occurred in them, then about half an hour should pass for the appropriate equilibrium to be established in these tissues.

Periodic fluctuations in core temperature. During the day, a person's minimum temperature is observed in the pre-dawn hours, and the maximum in the afternoon. The amplitude of the oscillations is 1 o C. The daily (circadian) rhythm is based on an energy mechanism (biological clock), which is usually synchronized with the rotation of the earth. In conditions of travel associated with crossing the earth's meridians, it takes 1-2 weeks for the temperature regime to come into line with the conditions of the new local time. Circadian rhythms are superimposed on others (menses in women, etc.).

The temperature during physical activity can increase by 2 ° C or more, depending on the intensity of the activity. At the same time, the average skin temperature decreases, as sweat is released due to the work of the muscles, which cools the skin. Rectal temperature during work can reach 41 o (for marathon runners).

Skin blood vessels can respond directly to changes in T - so-called. cold expansion, which is due to the local thermosensitivity of the vascular muscles. Cold dilation of blood vessels is usually observed in the form of the following reaction. When a person is exposed to extreme cold, he first experiences maximum vasoconstriction, which manifests itself in pallor and a feeling of cold in exposed areas. However, after some time, blood suddenly rushes into the vessels of the cooled parts of the body, which is accompanied by redness and warming of the skin. If exposure to cold continues, the events repeat periodically.

Cold vasodilation is thought to be a protective mechanism to prevent frostbite, especially in cold-adapted individuals. However, this mechanism can precipitate the death of general hypothermia in those who are forced to swim in cold water for a long time.

When water plays the role of the environment, since it has greater thermal conductivity and heat capacity than air, more heat is removed from the body by convection. If the water is in motion, then heat is removed so quickly that at an ambient temperature of +10 o C, even strong physical work does not allow maintaining thermal equilibrium, and hypothermia occurs. If the body is at complete rest, then to achieve temperature comfort, the temperature of the water should be 35-36 o. The lower limit of the thermoneutral zone depends on the thickness of the adipose tissue.

Mechanisms of thermoregulation. Thermoregulatory reactions are reflexes carried out by the central nervous system. They arise in response to stimulation of thermoreceptors in the periphery and in the central nervous system itself. There are two types of thermoreceptors - some perceive heat (heat receptors), others perceive cold (cold receptors). Both react with the appearance of a flash of impulses in response to adequate stimulation (a corresponding change in the temperature of the environment), and what matters is the rate of temperature change and the magnitude of the stimulus (the difference between the initial and new temperatures in the tissues).

Temperature receptors in the central nervous system are located in the preoptic zone of the anterior part of the hypothalamus, in the reticular formation of the midbrain and in the spinal cord. The presence of such receptors is proven by the appearance of tremors in the dog when the denervated limb cools. Local cooling of different parts of the brain causes bursts of impulses.

Thermoregulation centers are located in the hypothalamus. Its destruction makes the animal poikilothermic. Removal of other parts of the brain does not significantly affect the processes of heat generation and heat transfer. There are cores for heat transfer and heat production. It has been shown that the processes of physical thermoregulation are regulated mainly by the anterior hypothalamus, and chemical thermoregulation by the caudal nuclei. Both centers are in complex reciprocal relationships.

The executive mechanisms of the functional system for maintaining a constant body temperature (FST) are all those organs that can provide two normally mutually balanced processes of heat production and heat transfer, as well as special adaptive behavior.

The endocrine system is also involved in temperature regulation. Thus, thyroxine increases the intensity of metabolism, increasing heat production. Adrenaline constricts blood vessels, maintaining core body temperature.

Ontogenesis of thermoregulation. In immature-bearing animals, newborns are not capable of thermoregulation and are actually poikilothermic (gophers, hamsters, etc.). In other animals and in humans, all themoregulatory reactions (increased thermogenesis, vasomotor activity, sweat, behavior) can be turned on immediately after birth to one degree or another. This applies even to premature babies weighing about 1000 g. It is widely believed that newborns have an immature hypothalamus, responsible for thermoregulation. However, the newborn meets its needs through non-contractile thermogenesis. Children's heat production increases by 200% without shivering.

The small size of the newborn is a disadvantage in terms of thermoregulation. The ratio between body surface and volume is 3 times that of an adult, and the fat layer is small. Therefore, per unit mass of heat, children produce 4-5 times more heat. The upper limit of the thermoneutral zone of newborns is 32-34 o, the lower limit is 23 o C. Within this limited range, a newborn is able to maintain a constant temperature.

Thermal adaptation. The most important feature that occurs during thermal adaptation is the change in the intensity of sweat secretion, which can increase 3 times and reach 4 l/h for short periods. During adaptation to high temperatures, the electrolyte content in sweat decreases significantly to avoid loss of salts.

One of the main adaptive changes is the increase in thirst for a given level of water loss as thermal adaptation develops. This is necessary to maintain water balance.

In addition, threshold temperatures for associated vasomotor responses and sweating vary in different directions depending on whether the heat exposure is acute, chronic, moderate, or severe. Thus, 4-6 days after a daily 2-hour heat stress with maximum sweat production (sauna), reactions of sweat secretion and vasodilation occur at internal temperatures 0.5 o lower than before. The biological significance of the threshold shift is that, due to adaptation, the body temperature at a given heat load decreases, so that the body is protected from a critical increase in heart rate and blood flow - reactions that can lead to heat syncope.

In contrast, in persons living long-term in the tropics (chronic mild heat shift), the core temperature at rest is higher, and the reactions of sweating and vasodilation begin at a body temperature 0.5 ° C higher than in a temperate climate. This type of thermal adaptation is called adaptive endurance.

Hyperthermia. Hyperthermia occurs when the temperature in the armpit rises to more than 37 o C. The maximum body temperature for survival is + 42 o C (very briefly 43 o). At the same time, all thermoregulatory processes are extremely tense. Under conditions of prolonged heat stress at temperatures above 40-41 o, severe brain damage occurs - “heat or sunstroke”. Heat syncope with relatively mild overheating in people with impaired cardiovascular system functions is more dependent on circulatory failure than on thermoregulation mechanisms.

Fever. Fever develops as a result of increased heat production through shivering and maximum vasoconstriction in the peripheral parts of the body, i.e. the body behaves as if it were at a low ambient temperature. During the recovery period, the opposite process occurs - with the help of sweat and vasodilation, the body temperature drops in the same way as when a person has a fever. In this case, a person can correctly respond to true changes in external temperature. The mechanism for the appearance of a febrile reaction is associated with the release of leukocyte and bacterial pyrogens onto the central thermoregulatory apparatus.

Cold adaptation. Fur, fat layer, brown fat are all types of cold adaptation mechanisms in different animals. These mechanisms are not characteristic of an adult, so you can often hear the opinion that adults are not capable of any physiological adaptation to cold; they should rely only on behavioral adaptation (clothing and warm homes). It is said that man is a “tropical creature” who can survive in the Arctic only due to his civilization.

However, it has been shown that in cases of prolonged exposure to cold, people develop tolerance (endurance) to the cold. The threshold for the development of tremors and changes in metabolic thermoregulatory reactions shifts towards lower temperatures. In this case, even moderate hypothermia may occur. Similar tolerance is observed among the aborigines of Australia, who can spend a whole night almost naked without shivering at an ambient temperature of about 0 o C, as well as among Japanese divers, who spend several hours in water of about 10 o C. The same applies to ours. walruses."

It was shown that the shivering threshold could be shifted toward lower temperatures over just a few days during which subjects were subjected to repeated cold stress. With prolonged exposure (Eskimos, residents of Patagonia), the intensity of the basal metabolism increases by 25-50% - this is a metabolic adaptation.

Local adaptation. If the hands of a warmly dressed person are regularly cooled, then pain in the hands decreases. This is due to the fact that cold expansion of blood vessels occurs at a higher room temperature.

Hypothermia. Hypothermia occurs when the armpit temperature drops below 35°. This happens faster when immersed in cold water. In this case, a state similar to anesthesia is observed - the disappearance of sensitivity, weakening of reflex reactions, decreased excitability of the central nervous system, metabolic rate, slowing of breathing and heart rate, and a drop in blood pressure. This is the basis for the use of artificial hypothermia, which reduces the brain's need for oxygen, making longer bleeding during operations on the heart and large vessels tolerable. There are now known cases of heart shutdown during hypothermia for 40-60 minutes (Vereshchagin). Hypothermia is stopped by quickly warming the body. Artificial hypothermia is carried out when the thermoregulatory mechanisms are turned off.

In old age, hypothermia develops due to overregulation of temperature reactions - normally the body temperature reaches 35 o (a phenomenon opposite to fever).

A decrease in body temperature to 26-28 o causes death from cardiac fibrillation.