Maintaining a constant temperature throughout the body of a warm-blooded animal would require completely unjustified expenditure of energy. Evolution has placed cells and tissues in the peripheral parts of the body that are capable of performing their functions at different temperatures. And, conversely, in the internal parts of the body there are organs that are extremely sensitive to changes in their temperature conditions. For example, the needs of liver cells and especially brain neurons for a constant temperature level are not comparable with the similar needs of skin epithelial cells and subcutaneous fat. It follows that the subject of regulation is the thermal state not of the entire body as a whole, but only of its internal organs.
There is no consensus among experts regarding the specific parameter by which constancy is maintained. Some consider the object of regulation to be body temperature, others - its heat content, and still others - the amount of heat flow emitted by it.
The body of a homeothermic animal is divided into two parts: core And shell. Heat is generated in the “core”, and the “shell” dissipates it into the environment. In a hot environment and/or with a high level of physical activity, the boundaries of the “core” expand. In a cold environment and at rest, on the contrary, a narrowing of the “core” of the body occurs and, accordingly, an expansion of its “shell”. Therefore, the “core” of the body in all cases includes internal organs (a lot of heat is generated in the liver, intestines, brain) and sometimes skeletal muscles. The “shell” consists of: skin and subcutaneous fatty tissue (always) and sometimes skeletal muscles. Thus, “core” and “shell” (hereinafter these terms are given without quotation marks) are not morphological concepts, but functional ones. The boundary between them is not constant and moves depending on the ambient temperature, the thermal insulating properties of clothing or fur, and the level of physical activity (Fig. 11.8).
In an unfavorable external environment, the body sacrifices the temperature regime of the shell (skin, subcutaneous layer, even skeletal muscles), concentrating all its efforts on maintaining a constant temperature level of the core (brain and mediastinum, in which the blood vessels carrying blood to the brain pass).
Restoring the normal level of temperature (shell) (+34 ° C) is a secondary task that the body performs as soon as the opportunity arises. Therefore, the temperature of the peripheral parts of the body of a warm-blooded animal fluctuates significantly without any damage to its health and activity. Thus, in a person, the temperature of the fingers can, without causing pain, vary within a range of at least three tens of degrees Celsius (approximately from +15 to +45 ° C). Therefore, the concept of constancy should be associated with core temperature to a greater extent than with shell temperature. This ratio reflects Barton's formula for rate average body temperature person:
Ttela = 2/3 T core + 1/3 shell(1).
Coefficients 2/3 and 1/3 are given for the so-called thermoneutral environment(air temperature +22 °C, person is lightly dressed) and resting states. In cold conditions, the contraction of the body core will lead to a decrease in the first coefficient and a corresponding increase in the second. In a hot environment and/or during heavy muscular work, the first coefficient may approach 1, and the second - 0.
Core body temperature is measured in different ways: by immersing a thermometer in the rectum or sigmoid colon - rectal or colonial temperature (in humans it is 37.0-37.2 °C); into the esophagus, to the level of the heart - esophageal; under the tongue - oral, or sublingual(in humans it is 0.2-0.5 o C lower than rectal); into the external auditory canal near the eardrum, with mandatory obturation of the passage with a tampon - auricular temperature. The most informative is measuring the temperature of the hypothalamic region of the brain.
The temperature of the body shell is measured only on open areas of the skin and mucous membranes. To accurately assess it in a person, numerous temperature sensors (from 7 to 20-30) are recorded on all main areas of the body surface: on the forehead, cheeks, neck, chest, stomach, back, etc. The measurement result at each point is multiplied by a coefficient , reflecting the share of a given area in the total surface area of the body, and obtain weighted average skin temperature (WAT), which as average body temperature used in Barton's formula. For a person in a thermoneutral environment, the TTC is approximately 33-34 °C. In experiments on animals, the temperature of the shell is taken to be the temperature of any part of the body not covered with thick fur (rat tail, rabbit ear, etc.).
The most common indicator of thermal state in clinical practice is the temperature of the axillary region. (axillary temperature). It is influenced by both core temperature and shell temperature, so the axillary temperature is close to the average body temperature (see Barton's formula). To accurately measure axillary temperature, the armpit must be closed (the arm is pressed against the body) for at least 10 minutes to allow sufficient heat to accumulate in this area.
Valuable information is provided by measuring temperature in specific organs, directly or non-invasively (from the area of projection of the organ onto the surface of the body - see Fig. 11.13), but to date this approach has not received proper development.
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 temperature of the body surface 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 rates of various biochemical reactions, as well as disruption of the membrane structure as a result of thermal changes 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.
Changes in body temperature under the influence of physical activity. Muscle activity, more than an increase in any other physiological function, is accompanied by the breakdown and resynthesis of ATP - this is one of the main sources of contraction energy in the muscle cell. But a small part of the potential energy of macroergs is spent on external work, the rest is released in the form of heat - from 80 to 90% - and is “washed out” from the muscle cells by venous blood. Consequently, with all types of muscle activity, the load on the thermoregulatory apparatus sharply increases. If he were unable to cope with the release of more heat than at rest, then the human body temperature would increase by about 6°C in an hour of hard work. Increased heat transfer in humans is ensured during work due to convection and radiation, due to an increase in the temperature of the skin and increased exchange of the skin layer of air due to body movement. But the main and most effective way of heat transfer is the activation of sweating. Increased activation of the sweating apparatus is accompanied by the release of bradykinin by sweat gland cells, which has a vasodilatory effect on nearby muscles and counteracts the systemic vasoconstrictor effect of adrenaline. An increase in body temperature is beneficial during work: the excitability, conductivity, and lability of nerve centers increase, the viscosity of muscles decreases, and the conditions for the separation of oxygen from hemoglobin in the blood flowing through them improve. A slight increase in temperature can be noted even in the pre-start state and without warming up (it occurs conditionally).
Human body temperature at rest. Temperature "core" and "shell" of the body.
The possibility of vital processes is limited by a narrow temperature range of the internal environment in 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. From the point of view of thermoregulation, the human body can be imagined as consisting of two components: the outer, shell, and the inner, core. The core is the part of the body that has a constant temperature, and the shell is the part of the body that has a temperature gradient. Through the shell there is heat exchange between the core and the environment. 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. The temperature of human skin in different areas ranges from 24.4 to 34.4°C. The lowest temperature is observed on the toes, the lowest 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 circadian - daily - fluctuations in body temperature. The amplitude of vibrations can reach 1°. Body temperature is minimum in the pre-dawn hours (3-4 hours) and maximum in the daytime (16-18 hours). These shifts are caused by fluctuations in the level of regulation, i.e. associated with changes in the activity of the central nervous system. In conditions of movement associated with the intersection of hour meridians, it takes 1-2 weeks for the temperature rhythm to come into line with the new local time. Rhythms with longer periods may be superimposed on the circadian rhythm. The temperature rhythm synchronized with the menstrual cycle is most clearly manifested. The phenomenon of axillary 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° indicates pathology. Constancy of a person's body temperature can only be maintained if the processes of heat generation and heat transfer from the whole organism are equal. In the thermoneutral (comfortable) zone there is a balance between heat production and heat transfer. The leading factor determining the level of heat balance is the 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. The optimal ratio of heat production and heat transfer is ensured by a set of physiological processes called thermoregulation. It should be noted that a person can begin to perform even heavy work at normal body temperature, and only gradually, much slower than pulmonary ventilation, does the core temperature reach values corresponding to the level of general metabolism. Thus, an increase in the core temperature of the body is a necessary condition not for starting work, but for its continuation for a more or less long time. Perhaps, therefore, the main adaptive significance of this reaction is the restoration of performance during the muscular activity itself.
Isothermia and thermoregulation.
For normal life activity and functioning of the internal systems of the body, the temperature of the internal environment must remain at a relatively constant level, despite fluctuations in ambient temperature. This constancy of body temperature is called isothermia.
This constant temperature is maintained by a special process - thermoregulation.
Despite the constant temperature of the internal environment of the body, the human body temperature can be different. In the body, there are conventionally two halves: the outer - “ shell" and internal - "core".
"Core" includes the spinal cord and brain, organs of the chest and abdominal cavity and pelvis. Their temperature is almost always constant and depends to a small extent on the temperature of the external environment.
"Shell" includes organs and tissues located on the periphery of the body. These include the skin and skeletal muscles. The temperature of the shell is not constant and depends on the temperature of the environment. Under normal conditions, the membrane makes up approximately 25-30% of body weight. But its volume is not constant. When the external temperature decreases, the volume of the shell increases, and when it increases, it decreases. This serves as an important mechanism for regulating core temperature. The shell acts as a buffer, softening sudden temperature fluctuations.
The fundamental difference between the core and the shell lies in the nature of their reactions to changes in external temperature. The nucleus reacts in a “counteraction” way: to cooling – by increasing blood supply and heat generation, and to heating – by decreasing blood supply and heat generation. The shell reacts according to the method of passive “adaptation”: to heating - by increasing the blood supply to the heated organs, and to cooling - by decreasing the blood supply to the cooled areas.
A person’s body temperature is usually judged on the basis of its measurement in the armpit. Here the temperature of a healthy person is 36.5-36.9˚C. This temperature range is most favorable for the occurrence of all chemical reactions, for the activity of the brain and the entire body.
Different areas of the skin surface have different temperatures. Usually the temperature of the skin of the torso and head is relatively higher (33-34˚C). The temperature of the hands and feet is lower. The temperature difference between the torso and limbs is 10˚C or more. The highest skin temperature is in the neck area, and the lowest is on the fingers and toes.
The temperature of the external environment at which a person does not experience sensations of either cold or heat is called thermoneutral zone of the environment. For a person in ordinary clothes at rest, the thermoneutral air temperature is 19 - 22˚С, and for a naked person 28 - 31˚С. Neutral water temperature is 35˚C.
Body temperature does not remain constant, but fluctuates throughout the day within 0.5-0.7˚C. Rest and sleep decrease, and muscle activity increases body temperature. The maximum temperature is observed at 16-18 hours in the evening, the minimum – at 3-4 hours in the morning. Temperature fluctuations may be reversed for night shift workers.
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 off 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 thermal insulation 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.