How much heat is produced in the body The mechanism of heat regulation in the human body. Medical devices for measuring body temperature

The set of physiological mechanisms that regulate body temperature is called the physiological system of thermoregulation.
Generation of heat in the body. Heat in the body is formed as a result of the oxidation of nutrients during the breakdown of proteins, fats and carbohydrates. The energy that was previously in them in a latent state is released, consumed and ultimately given off by the body in the form of heat.
The place where the main generation of heat occurs is the muscles. This process goes on even when the person is at rest. Minor muscle movements already contribute to more heat generation, and when walking, its amount increases by 60-80%. During muscular work, the formation of heat increases by 4-5 times. In addition to skeletal muscles, heat generation occurs in the stomach, intestines, liver, kidneys and other organs.
The formation of heat in the body is accompanied by its return. The body loses as much heat as it generates, otherwise the person would die within a few hours.
These complex processes of regulation of the formation and release of heat by the body are called thermoregulation and are carried out by a number of adaptive mechanisms, which we will now consider.
Regulation of heat generation and heat transfer. Body temperature remains constant due to the fact that both the formation and release of heat are regulated in the body.
Heat is consumed by the body in different ways. The main way of heat transfer is the loss of heat by conduction, i.e., heating of the surrounding air and radiation; in addition, heat is consumed with exhaled air, for the evaporation of sweat, etc.
Consequently, the temperature of the human body remains constant due to the fact that, on the one hand, the intensity of oxidative processes, i.e., the formation of heat, is regulated, and on the other hand, the intensity and volume of heat transfer. These two methods of regulation are called chemical and physical thermoregulation.
Chemical thermoregulation is understood as a change in the intensity of metabolism under the influence of the environment. There is a certain relationship between air temperature and metabolism in the body. So, when the air temperature decreases, the formation of heat in the body increases.
Most of the heat is generated in the muscles. In the cold, the muscles tremble. When the ambient temperature drops, skin receptors that perceive temperature irritations are irritated: excitation occurs in them, which goes to the central nervous system and from there to the muscles, causing their contractions. Thus, the shivering and chills that we experience in the cold season or in a cold room are reflex acts that increase metabolism, and therefore increase heat generation. Increased metabolism occurs under the influence of cold, even when there are no muscle movements.
A significant amount of heat is also generated in the abdominal organs - in the liver and kidneys. This can be seen by measuring the temperature of the blood flowing to and from the liver. It turns out that the temperature of the outflowing blood is higher than the temperature of the inflowing blood. Therefore, the blood is heated as it flows through the liver.
As the air temperature rises, heat generation in the body decreases.
Physical thermoregulation. With an increase or decrease in the ambient temperature, not only the oxidative processes change, that is, heat generation, but also heat transfer, and with a decrease in temperature, heat transfer decreases, and with an increase, it increases.
Heat is given off by the body mainly by conduction and radiation, and only some part - in other ways. Thus, heat transfer by conduction is 31% of all heat generated in the body, by radiation - 44%, 10% is lost when water is evaporated by the skin, 12% is lost when water is evaporated by the lungs, 3% of heat is spent on heating the inhaled air and excreted urine and feces. .
By conduction, the body loses heat to heat the surrounding air and objects with which it comes into contact. Another way of heat transfer is heat radiation. At the same time, it happens
heating objects at some distance from the body.
How does the heat transfer change? An important role in heat transfer is played by the expansion and narrowing of skin vessels. Everyone knows that in cold, frosty air, a person's skin turns pale, and when the air is heated, red-hot, it turns red.
The change in skin color is due to the fact that under the influence of cold blood vessels, primarily arterioles, narrow. As a result, blood flow to the surface of the body decreases, and consequently, heat transfer through conduction and radiation also decreases.
Under the influence of heat, the vessels of the skin expand, blood flows abundantly to the surface of the body, which contributes to increased conduction and radiation of heat. In this way, heat is released to the environment only when the air temperature is below body temperature. The smaller the difference between skin temperature and air temperature, the less heat is given off to the environment. In this case, perspiration plays a significant role. When 1 g of sweat evaporates, 0.58 kcal is lost. Since sweating and evaporation occur continuously at any temperature, the amount of calories that a person loses in this case depends on the intensity of sweating. At an average temperature, a person loses about 800 ml of sweat per day. With the loss of such an amount of sweat, 450-500 kcal is consumed. As the temperature rises, sweat secretion increases and sometimes reaches several liters.
The greatest amount of sweat is released when the air temperature is equal to or higher than body temperature. Under these conditions, the transfer of heat by conduction of radiation is not possible, and therefore it is consumed mainly by perspiration.
In hot countries or hot rooms, where the air temperature is 37 ° C or slightly higher, heat is given off only by evaporation. At the same time, up to 4.5 liters of sweat are released from a person during the day, which provides a return of 2400-2800 kcal.
A large amount of sweat is lost during physical work, and this happens at any temperature. It is estimated that during particularly hard work a person loses up to 9 liters of sweat per day and, thus, gives off up to 5000 kcal by evaporation.
Sweating largely depends on the saturation of the air with water vapor. Under equal temperature conditions, greater sweat evaporation and, consequently, greater heat loss are provided under conditions of low water vapor content in the air. Therefore, the heat is easily tolerated in those places where the air is drier.
Evaporation of sweat is prevented by impenetrable clothing (rubber, anti-sweet suit, etc.). A person in such clothes sweats even in the cold, as a constant layer of air is created around him, which is not updated due to the lack of ventilation. This layer of air is saturated with vapors, which prevents further evaporation of sweat. Therefore, a long stay in these suits is impossible, as it causes an increase in body temperature.
In hot countries, hot workshops, during long hikes, a person loses a large amount of sweat. Thirst appears, but water does not quench it; on the contrary, the more water a person drinks, the more he sweats and the more thirsty becomes.
Along with sweat, salts are lost, so it becomes necessary to replenish not only the loss of water, but also the loss of salts. For this purpose, 0.5% salt is added to drinking water. Such slightly salted water is given in hot shops, during long hikes, etc. It quenches thirst and improves well-being.
Respiration plays a role in heat transfer. Heat is spent on the evaporation of water by the lungs and partly on the warming of the inhaled air. In the cold, reflex slowing of breathing occurs, and at high temperatures, breathing quickens, the so-called thermal shortness of breath occurs.
For better heat transfer, air circulation is of great importance. When the air is in motion, a constant layer of heated and vapor-saturated air is not created around the body. This is the significance of fans, fanning, etc. Clothing, on the other hand, creates a fixed layer of air and thereby impedes heat transfer.
Heat transfer is prevented by subcutaneous fat. The thicker the layer of fat, the worse it is. Therefore, people with a thick fat layer in the subcutaneous tissue tolerate cold more easily than thin people.
The human body temperature is constant. It is measured in the armpit or in the rectum (in infants). The average temperature in the armpit ranges from 36.5-36.9 ° C, in the rectum - slightly higher (37.2-37.5 C). The temperature of the internal organs is higher than the average body temperature, for example, the temperature of the liver is 38-38.5°C. The human body temperature fluctuates throughout the day. It is at its lowest at 3-4 p.m.
night, then gradually increases, reaching the highest point at 16 h, and again begins to decline. Temperature fluctuations occur within 0.5°C of the average value.
Body temperature can rise sharply during muscular work and reach up to 38-39°C or even up to 40°C. Upon termination of work, it quickly falls and reaches a normal value.
The constancy of body temperature is maintained by the two mechanisms already described: chemical and physical thermoregulation. However, the capabilities of the human body are limited, and under certain conditions, these mechanisms are insufficient. Then the constancy of temperature is violated and either its increase or decrease is observed. An increase in temperature above normal is called a fever. Fever can occur because heat generation increases with no change in heat loss, or, conversely, heat generation remains unchanged, and heat loss decreases.
Lowering the temperature to 32-33°C, as well as increasing it above 42-43°C, leads to death.
thermoregulation centers. The thermoregulatory center, called the thermal center, is located in the diencephalon. Its activity is determined by two factors: blood temperature and reflex effects. If the temperature of the blood washing the diencephalon is increased, then the thermoregulation center is excited, and changes occur in the body's activity that contribute to its decrease. With a decrease in blood temperature, the heat generation center reacts in such a way that the intensity of processes that contribute to an increase in temperature increases.
Another way of excitation is reflex influences. When exposed to temperature fluctuations on human skin, excitation occurs in the receptors, which enters the thermal center. From there, the impulses go already to the organs associated with heat generation (muscles, liver, etc.) and with heat transfer, and cause a change in their activity. Excitation from the centers of thermoregulation to the organs of heat generation and heat transfer is transmitted through the sympathetic nervous system.
The cerebral cortex plays an exceptionally large role in thermoregulation. Under normal conditions, the process of heat generation and heat transfer is under its influence.
The thermocomfortable temperature for a person in the air is usually + 19 ° C, in water - + 34 ° C. At such temperatures, the thermoregulation system does not turn on.
To maintain a constant body temperature of 36.6 ° C, a person needs to spend 200 kcal per day.
A decrease in body temperature even by 0.1 ° leads to a decrease in immunity.
Cold snaps in nature, as a rule, are very sharp. In order to painlessly endure climatic "surprises", a person must be tempered.
As you know, there are three levels of the body's response to stimuli of different strengths: training, activation and stress. Big cold is stress, including mental. If you are afraid of hypothermia in advance, freeze and wrap yourself up long before going out into the cold, then you urgently need to temper not only the body, but also the nerves. The survival experiment showed that people die, as a rule, not from the cold, but from fear of it.
The mood for hardening sets a strategic task for a person: to make friends with the cold for life. "The border of pleasure" allows you to solve a tactical problem: to dose the cold or heat. If the strategy encourages hardening, then tactics control the hardening load. Moreover, it does this in accordance with the individual physiological characteristics of the body and, of course, taking into account specific climatic conditions.
The need for a psychological attitude to tempering, interest in it - this is the most important principle. You can't waste time on it.
The essence of hardening is the training of thermoregulation processes, which include heat production and heat transfer. Cooling stimulates, on the one hand, an increase in the production of heat in the body, and on the other hand, the desire to preserve it, not to give it out. Training teaches the body to clearly respond to cold, quickly and actively respond to low environmental temperatures with increased heat production and reduced heat transfer. Thus, despite the cold, the normal body temperature is maintained. In an unhardened person, the mechanisms of thermoregulation work more weakly, the body temperature decreases, which leads to a weakening of the immune defense and an increase in the activity of pathogenic microorganisms. As a result of this - colds, flu, etc., which not only take them out of working condition, but also accumulate harmful effects, which inevitably undermines the overall potential of the body and reduces its vitality.

Heat value
Heat sources
Heat production and heat supply
Use of heat
New technologies of heat supply

Heat value

Heat is one of the sources of life on Earth. Thanks to fire, the birth and development of human society became possible. From ancient times to this day, heat sources have served us faithfully. Despite the hitherto unprecedented level of technological development, a person, like many thousands of years ago, still needs warmth. With the growth of the world population, the need for heat increases.

Heat is among the most important resources of the human environment. It is necessary for a person to maintain his own life. Heat is also required for technologies, without which modern man cannot imagine his existence.

Heat sources

The oldest source of heat is the sun. Later, fire was at the disposal of man. Based on it, man created a technology for obtaining heat from fossil fuels.

Relatively recently, nuclear technologies have been used to produce heat. However, the combustion of fossil fuels is still the main method of heat production.

Heat production and heat supply

Developing technology, a person has learned to produce heat in large volumes and transfer it over quite considerable distances. Heat for large cities is produced at large thermal power plants. On the other hand, there are still many consumers who are supplied with heat by small and medium-sized boiler houses. In rural areas, households are heated by domestic boilers and stoves.

Heat generation technologies make a significant contribution to environmental pollution. By burning fuel, a person releases a large amount of harmful substances into the surrounding air.

Use of heat

In general, a person produces much more heat than he uses for his own benefit. We simply dissipate a lot of heat in the surrounding air.

Heat is lost
due to the imperfection of heat production technologies,
when transporting heat through heat pipelines,
due to the imperfection of heating systems,
due to the imperfection of housing,
due to imperfect ventilation of buildings,
when removing "excess" heat in various technological processes,
when burning production waste,
with vehicle exhaust gases on internal combustion engines.

To describe the state of affairs in the production and consumption of heat by a person, the word wastefulness is well suited. An example of, I would say, notorious wastefulness is the burning of associated gas in oil fields.

New technologies of heat supply

Human society spends a lot of effort and money to get heat:
extracts fuel deep underground;
transports fuel from deposits to enterprises and dwellings;
builds installations for heat generation;
builds heating networks for heat distribution.

Probably, one should think: is everything reasonable here, is everything justified?

The so-called technical and economic advantages of modern heat supply systems are inherently momentary. They are associated with significant environmental pollution and irrational use of resources.

There is heat that does not need to be extracted. This is the warmth of the sun. It must be used.

One of the ultimate goals of heat supply technology is the production and delivery of hot water. Have you ever used an outdoor shower? A container with a tap installed in an open place under the rays of the Sun. A very simple and affordable way to supply warm (even hot) water. What's stopping you from using it?

With the help of heat pumps, a person uses the heat of the Earth. A heat pump does not need fuel, it does not need an extended heating main with its heat losses. The amount of electricity required to run a heat pump is relatively small.

The benefits of the most modern and advanced technology will be nullified if its fruits are used stupidly. Why produce heat away from consumers, transport it, then distribute it to dwellings, heating the Earth and the surrounding air along the way?

It is necessary to develop distributed heat production as close as possible to the places of consumption, or even combined with them. A method of heat production called cogeneration has long been known. Cogeneration plants produce electricity, heat and cold. For the fruitful use of this technology, it is necessary to develop the human environment as a single system of resources and technologies.

It seems that in order to create new technologies for heat supply, one should
review existing technologies,
try to get away from their shortcomings,
collect on a single basis for interaction and complement each other,
take full advantage of their strengths.
This implies understanding

Man, as you know, belongs to homoiothermic, or warm-blooded, organisms. Does this mean that the temperature of his body is constant, i.e. body does not respond to changes in environmental temperature? Reacts, and even very sensitively. The constancy of body temperature is, in fact, the result of continuously occurring reactions in the body that maintain its thermal balance unchanged.

From the point of view of metabolic processes, heat production is a side effect of chemical reactions of biological oxidation, during which nutrients entering the body - fats, proteins, carbohydrates - undergo transformations, ending in the formation of water and carbon dioxide. The same reactions with the release of thermal energy also occur in the organisms of poikilothermic, or cold-blooded, animals, but due to their significantly lower intensity, the body temperature of poikilothermic animals only slightly exceeds the ambient temperature and changes in accordance with the latter.

All chemical reactions occurring in a living organism depend on temperature. And in poikilothermic animals, the intensity of energy conversion processes, according to the van't Hoff rule *, increases in proportion to the external temperature. In homeothermic animals, this dependence is masked by other effects. If a homoiothermic organism is cooled below a comfortable ambient temperature, the intensity of metabolic processes and, consequently, the production of heat in it increase, preventing a decrease in body temperature. If thermoregulation is blocked in these animals (for example, during anesthesia or damage to certain parts of the central nervous system), the curve of heat production versus temperature will be the same as for poikilothermic organisms. But even in this case, significant quantitative differences remain between metabolic processes in poikilothermic and homoiothermic animals: at a given body temperature, the intensity of energy exchange per unit body mass in homoiothermic organisms is at least 3 times higher than the intensity of metabolism in poikilothermic organisms.

Many non-mammalian and non-avian animals are able to alter their body temperature to some extent through "behavioral thermoregulation" (e.g. fish can swim in warmer water, lizards and snakes can "sunbathe"). Truly homoiothermic organisms are able to use both behavioral and autonomous methods of thermoregulation, in particular, they can produce additional heat if necessary due to the activation of metabolism, while other organisms are forced to focus on external heat sources.

The temperature of most warm-blooded mammals lies in the range from 36 to 40 ° C, despite significant differences in body size. At the same time, the intensity of metabolism (M) depends on body weight (m) as its exponential function: M = k x m 0.75, i.e. the value of M/m 0.75 is the same for the mouse and for the elephant, although the metabolic rate per 1 kg of body weight in the mouse is much higher than that of the elephant. This so-called law of decreasing the intensity of metabolism depending on body weight reflects the fact that heat production corresponds to the intensity of heat transfer to the surrounding space. For a given temperature difference between the internal environment of the body and the environment, the heat loss per unit of body mass is the greater, the greater the ratio between the surface and volume of the body, and the latter ratio decreases with increasing body size.

Body temperature and heat balance

When additional heat is required to maintain a constant body temperature, it can be generated by:

1) voluntary motor activity;
2) involuntary rhythmic muscle activity (trembling caused by cold);
3) acceleration of metabolic processes not associated with muscle contraction.

In adults, shivering is the most important involuntary mechanism of thermogenesis. "Non-shivering thermogenesis" occurs in newborn animals and children, as well as in small, cold-adapted animals and hibernating animals. The main source of "non-shivering thermogenesis" is the so-called brown fat - a tissue characterized by an excess of mitochondria and a "multilacular" distribution of fat (numerous small droplets of fat surrounded by mitochondria). This tissue is found between the shoulder blades, in the armpits and in some other places.

In order for the body temperature not to change, heat production must equal heat loss. According to Newton's law of cooling, the heat given off by the body (minus the losses associated with evaporation) is proportional to the temperature difference between the inside of the body and the surrounding space. In humans, heat transfer is zero at an ambient temperature of 37 ° C, and when the temperature drops, it increases. Heat transfer also depends on the conduction of heat within the body and peripheral blood flow.

Thermogenesis associated with metabolism at rest (Fig. 1) is balanced by heat transfer processes in the ambient temperature zone T 2 -T 3 if cutaneous blood flow gradually decreases as temperature decreases from T 3 to T 2 . At temperatures below T 2 the constancy of body temperature can only be maintained by increasing thermogenesis in proportion to heat loss. The highest heat production provided by these mechanisms in humans corresponds to a metabolic level 3–5 times higher than the intensity of basal metabolism and characterizes the lower limit of the thermoregulation range T 1 . If this limit is exceeded, hypothermia develops, which can lead to death from hypothermia.

At an ambient temperature above T 3 temperature equilibrium could be maintained by weakening the intensity of metabolic processes. In fact, the temperature balance is established due to an additional heat transfer mechanism - the evaporation of the released sweat. Temperature T 4 corresponds to the upper limit of the thermoregulation range, which is determined by the maximum intensity of sweating. At medium temperature above T 4 hyperthermia occurs, which can lead to death from overheating. Temperature range T 2 -T 3 , within which the body temperature can be maintained at a constant level without the participation of additional mechanisms of heat production or sweating, is called thermoneutral zone. In this range, the intensity of metabolism and heat production are, by definition, minimal.

human body temperature

The heat produced by the body in the norm (i.e., under equilibrium conditions) is given off to the surrounding space by the surface of the body, so the temperature of body parts near its surface should be lower than the temperature of its central parts. Due to the irregularity of the geometric shapes of the body, the temperature distribution in it is described by a complex function. For example, when a lightly dressed adult is in a room with an air temperature of 20 ° C, the temperature of the deep muscle of the thigh is 35 ° C, the deep layers of the calf muscle is 33 ° C, the temperature in the center of the foot is only 27–28 ° C, and the rectal temperature is approximately 37 °C. Fluctuations in body temperature caused by changes in external temperature are most pronounced near the surface of the body and at the ends of the limbs (Fig. 2).

Rice. 2. The temperature of different areas of the human body in conditions of cold (A) and heat (B)

The internal temperature of the body itself is not constant either in space or in time. Under thermoneutral conditions, temperature differences in the internal regions of the body are 0.2–1.2 °C; even in the brain, the temperature difference between the central and outer parts reaches more than 1 °C. The highest temperature is noted in the rectum, and not in the liver, as previously thought. In practice, temperature changes over time are usually of interest, so it is measured in any one specific area.

For clinical purposes, it is preferable to measure rectal temperature (the thermometer is inserted through the anus into the rectum to a standard depth of 10–15 cm). Oral, more precisely sublingual, temperature is usually 0.2–0.5 ° C lower than rectal. It is influenced by the temperature of the inhaled air, food and drink.

In sports medicine research, esophageal temperature (above the entrance to the stomach) is often measured, which is recorded using flexible thermal sensors. Such measurements reflect changes in body temperature faster than recording rectal temperature.

Axillary temperature can also serve as an indicator of core body temperature, because when the arm is pressed tightly against the chest, temperature gradients shift so that the boundary of the inner layer reaches the axilla. However, this takes some time. Especially after being in the cold, when the superficial tissues were cooled and vasoconstriction occurred in them (this is especially common with a cold). In this case, to establish thermal equilibrium in these tissues, about half an hour should pass.

In some cases, core temperature is measured in the external auditory canal. This is done using a flexible sensor, which is placed near the eardrum and protected from external temperature influences with a cotton swab.

Usually, skin temperature is measured to determine the temperature of the surface layer of the body. In this case, the measurement at one point gives an inadequate result. Therefore, in practice, the average skin temperature is usually measured in the forehead, chest, abdomen, shoulder, forearm, back of the hand, thigh, lower leg and dorsal surface of the foot. When calculating, the area of ​​the corresponding body surface is taken into account. The “average skin temperature” found in this way at a comfortable ambient temperature is approximately 33–34 °C.

The human body temperature fluctuates during the day: it is minimal in the early morning hours and maximal (often with two peaks) in the daytime (Fig. 3). The amplitude of diurnal fluctuations is approximately 1 °C. In animals active at night, the temperature maximum is observed at night. It would be easiest to explain these facts by saying that the increase in temperature occurs as a result of increased physical activity, but this explanation turns out to be incorrect.

Temperature fluctuations are one of many daily rhythms. Even if we exclude all orienting external signals (light, temperature changes, meal times), body temperature

continues to fluctuate rhythmically, but the period of oscillation in this case is from 24 to 25 hours. Thus, daily fluctuations in body temperature are based on an endogenous rhythm (“biological clock”), usually synchronized with external signals, in particular with the rotation of the Earth. During travels connected with the crossing of the earth's meridians, it usually takes 1-2 weeks for the temperature rhythm to come into line with the lifestyle determined by the new local time for the body.

Rhythms with longer periods are superimposed on the rhythm of daily temperature changes, for example, a temperature rhythm synchronized with the menstrual cycle.

During walking, for example, heat production is 3-4 times, and during strenuous physical work even 7-10 times higher than at rest. It also increases in the first hours after eating (by about 10-20%). Rectal temperature during a marathon run can reach 39-40°C, and in some cases almost 41°C. The average skin temperature, however, is reduced by exercise-induced perspiration and evaporation. During sub-maximal work, as long as perspiration occurs, the increase in core temperature is almost independent of the ambient temperature in the range of 15-35°C. Dehydration of the body leads to a rise in internal temperature and significantly reduces performance.

Heat dissipation

How does the heat that has arisen in the bowels of the body leave it? Partially with secretions and with exhaled air, but the role of the main cooler is played by blood. Due to its high heat capacity, blood is very well suited for this purpose. It takes heat from the cells of tissues and organs washed by it and carries it through the blood vessels to the skin and mucous membranes. This is where the heat transfer takes place. Therefore, the blood flowing from the skin is approximately 3 °C colder than the inflowing blood. If the body is deprived of the ability to remove heat, then in just 2 hours its temperature rises by 4 ° C, and a rise in temperature to 43–44 ° C is, as a rule, incompatible with life.

Heat transfer in the extremities is to some extent determined by the fact that the blood flow here occurs according to the countercurrent principle. The deep large vessels of the limbs are arranged in parallel, due to which the blood following the arteries to the periphery gives off its heat to the nearby veins. Thus, the capillaries located at the ends of the limbs receive pre-cooled blood, so the fingers and toes are most sensitive to low temperatures.

The terms of heat transfer are: conduction of heat H P, convection H To, radiation H izl and evaporation H Spanish. The total heat flux is determined by the sum of these components:

H bunk= H P+ H To+ H izl+ H Spanish .

Heat transfer by conduction occurs when the body is in contact (whether standing, sitting, or lying down) with a dense substrate. The magnitude of the heat flux is determined by the temperature and thermal conductivity of the adjacent substrate.

If the skin is warmer than the surrounding air, the layer of air adjacent to it heats up, rises and is replaced by colder and denser air. The driving force of this convective flow is the difference between the temperatures of the body and the environment near it. The more movements that occur in the outside air, the thinner the boundary layer becomes (maximum thickness 8 mm).

For the range of biological temperatures, heat transfer due to radiation H rad can be described with sufficient accuracy using the equation:

H izl= h izl x (T skin- T izl) x A,

where T skin– average skin temperature, T izl– average radiation temperature (temperature of surrounding surfaces, e.g. room walls),
A is the effective surface area of ​​the body and
h izl is the coefficient of heat transfer due to radiation.
coefficient h izl takes into account the emissivity of the skin, which for long-wave infrared radiation is approximately 1 regardless of pigmentation, i.e. the skin radiates almost as much energy as a completely black body.

About 20% of the heat transfer of the human body under neutral temperature conditions is due to the evaporation of water from the surface of the skin or from the mucous membranes of the respiratory tract. Heat transfer by evaporation occurs even at 100% relative humidity of the ambient air. This happens as long as the skin temperature is higher than the ambient temperature and the skin is completely hydrated due to sufficient perspiration.

When the ambient temperature exceeds body temperature, heat transfer can only be carried out by evaporation. The efficiency of cooling due to sweating is very high: with the evaporation of 1 liter of water, the human body can give off a third of the total heat generated in rest conditions for the whole day.

Influence of clothes

The effectiveness of clothing as a heat insulator is due to the smallest volumes of air in the structure of the woven fabric or in the pile, in which no noticeable convective currents arise. In this case, heat is transferred only by conduction, and air is a poor conductor of heat.

Environmental factors and thermal comfort

The influence of the environment on the thermal regime of the human body is determined by at least four physical factors: air temperature, humidity, radiation temperature and air (wind) speed. It depends on these factors whether the subject feels “thermal comfort”, whether it is hot or cold. The comfort condition is that the body does not need the work of thermoregulation mechanisms, i.e. he would not need to tremble or sweat, and the blood flow in the peripheral organs could maintain an intermediate speed. This condition corresponds to the thermoneutral zone mentioned above.

These four physical factors are somewhat interchangeable in terms of comfort and need for thermoregulation. In other words, the sensation of cold caused by a low air temperature can be attenuated by a corresponding increase in the radiation temperature. If the atmosphere feels stuffy, the feeling can be alleviated by lowering the humidity or temperature of the air. If the radiation temperature is low (cold walls), an increase in air temperature is required to achieve comfort.

According to recent studies, the value of a comfortable temperature for a lightly dressed (shirt, underpants, long cotton trousers) seated subject is approximately 25–26 ° C at 50% air humidity and equal air and wall temperatures. The corresponding value for a naked subject is 28 °C. The average skin temperature is approximately 34°C. During physical work, as the subject expends more and more physical effort, the comfortable temperature decreases. For example, for light office work, the preferred air temperature is approximately 22°C. Oddly enough, during heavy physical work, room temperature, at which sweating does not occur, is felt as too low.

The diagram in fig. 4 shows how the values ​​of comfort temperature, humidity and ambient air temperature correlate during light physical work. Each degree of discomfort can be associated with one temperature value - the effective temperature (ET). The numerical value of ET is found by projecting onto the X-axis the point at which the line of discomfort intersects the curve corresponding to 50% relative humidity. For example, all combinations of temperature and humidity values ​​in the dark gray area (30°C at 100% RH or 45°C at 20% RH, etc.) correspond to an effective temperature of 37°C, which in turn corresponds to a certain degree of discomfort. In the range of lower temperatures, the effect of humidity is smaller (the slope of the discomfort lines is steeper), since in this case the contribution of evaporation to the total heat transfer is insignificant. Discomfort increases with an increase in the average temperature and moisture content of the skin. When the parameter values ​​that determine the maximum skin moisture (100%) are exceeded, the heat balance can no longer be maintained. Thus, a person is able to withstand conditions outside this boundary only for a short time; sweat at the same time flows in streams, since it is released more than it can evaporate. The lines of discomfort shift, of course, depending on the thermal insulation provided by clothing, wind speed, and the nature of the exercise.

Comfortable water temperatures

Water has a much higher thermal conductivity and heat capacity than air. When water is in motion, the resulting turbulent flow near the surface of the body takes away heat so quickly that at a water temperature of 10 ° C, even strong physical stress does not allow maintaining thermal equilibrium, and hypothermia occurs. If the body is completely at rest, to achieve thermal comfort, the water temperature should be 35-36 ° C. Depending on the thickness of the insulating adipose tissue, the lower maximum comfortable temperature in the water ranges from 31 to 36 °C.

To be continued

* According to the van't Hoff rule, when the temperature changes by 10 °C (in the range from 20 to 40 °C), oxygen consumption by tissues changes in the same direction by 2–3 times.

WARM-BLOODED AND COLD-BLOODED ANIMALS

In the process of evolutionary development, mammals, birds and humans have developed the ability to constantly maintain the same body temperature. Regardless of the temperature of the external environment, that is, both in heat and in cold, the body temperature of this group of animals and humans does not change, but is maintained at the same level. This ability to maintain a constant temperature creates more constant conditions that are important for the normal functioning of the organism, and makes it relatively less dependent on environmental conditions.

Animals whose body, due to the presence of a number of adaptations, maintains a constant temperature, are called warm-blooded (homeothermic). Humans are also warm-blooded.

Invertebrates and a significant part of vertebrates do not have a constant temperature. The body temperature of these animals depends on the temperature of the environment where they are. If the ambient temperature decreases, the body temperature of these animals decreases, and, conversely, an increase in the ambient temperature entails an increase in the body temperature of these animals. This group of animals is called cold-blooded (poikilothermic). Their body is devoid of adaptations that would make it possible to regulate their own temperature.

The intensity of life processes occurring in the body of these animals is subject to fluctuations and depends on the ambient temperature. The significance of this circumstance can be shown by the example of a frog: in winter, when its body temperature approaches 0 °, it jumps over a distance of 10-15 cm; in summer, when her body temperature rises to 20-25 °, her jumps even exceed 100 cm.

HEAT GENERATION IN THE ORGANISM

Heat in the body is formed as a result of the oxidation of nutrients to the final products of their breakdown. The place where the generation of heat mainly takes place ismuscles. In the muscles, the formation of heat occurs even when a person is at complete rest. Minor muscle movements already contribute to more heat generation, and when walking, heat generation increases by 60-80%. During muscular work, the formation of heat increases by 4-5 times. In addition to skeletal muscles, heat generation occurs in the liver, kidneys and other organs. Above all, the temperature of the liver. In it, compared with other organs (per unit weight), more heat is generated.

The formation of heat in the body is accompanied by its return. The body loses as much heat as it generates. Heat does not linger in the human body, otherwise he would die within a few hours.

These complex processes of regulation of the formation and release of heat by the body are called thermoregulation and are carried out by a number of adaptive mechanisms, to be consideredwhich we will pass.

REGULATION OF HEAT GENERATION AND HEAT TRANSFER

Body temperature remains constant due to the fact that with the help of a number of mechanisms in the body, the central nervous system regulates both the production and release of heat.

In the cells and organs of our body, oxidative processes occur, which are accompanied by the release of energy. A change in the intensity of oxidative processes, and, consequently, in the intensity of energy release, entails a change in heat generation.

Heat is consumed by the body in different ways. The main ways of heat transfer are: heat loss by conduction, i.e. heating, of the surrounding air and radiation; in addition, heat is consumed with exhaled air, during the evaporation of sweat, etc.

Consequently, the body temperature of warm-blooded animals remains constant due to the fact that the nervous system regulates, on the one hand, the intensity of oxidative processes, i.e., the formation of heat, and, on the other hand, the intensity of heat transfer. These interrelated processes, called chemical and physical thermoregulation, are due to the activity of the central nervous system.

Chemical thermoregulation. Chemical thermoregulation is understood as a change in the intensity of metabolism that occurs under the influence of the environment. A change in the temperature of the external environment is captured by the skinnymi receptors and reflectively there is a change in the intensity of metabolism, i.e., heat generation. There is, for example, a certain relationship between air temperature and metabolism in the body. So, when the air temperature decreases, the formation of heat in the body increases.

Most of the heat is generated in the muscles. One of the adaptive mechanisms is muscle trembling that occurs in the cold. The shivering that occurs when the body cools is the result of a reflex. When the ambient temperature drops, skin receptors that perceive temperature irritations are irritated; excitation arises in them, which goes to the central nervous system and from there to the muscles, causing their periodic contractions.

Thus, the shivering and chills that we experience in the cold season or in a cold room are reflex acts that increase metabolism, and therefore increase heat generation.

Increased metabolism occurs under the influence of cold, even when there are no muscle movements. This was shown in the experiment when the animal was cooled. It turned out that if the animal is cooled, it intensifies regardless of whether the trembling has come or not.

A significant amount of heat is also formed in the abdominal organs - the liver and kidneys. This can be seen by measuring the temperature of the blood flowing to the liver and the temperature of the outflowing blood. It turns out that the temperature of the outflowing blood is higher than the temperature of the inflowing blood. Therefore, warmed up when flowing through the liver

As the air temperature rises, heat generation in the body decreases.

Article on the topic Formation and release of body heat