Temperature regulation consists of coordinating the processes of heat production (chemical thermoregulation) and heat transfer (physical thermoregulation).
Heat production processes. Heat production occurs in all organs due to metabolic processes. Therefore, the blood that flows away from the organs, as a rule, has a higher temperature than that flowing in. But the role of different organs in heat production is different. At rest, the liver accounts for about 20% of the total heat production, other internal organs - 56%, 20%, during physical activity on skeletal muscles - up to 90%, internal organs - only 8%.
Thus, muscles during their contraction are a powerful reserve source of heat production. Changes in the activity of their metabolism during locomotion are the main mechanism of heat production. Among the various locomotions, several stages of muscle participation in heat production can be distinguished.
1. Thermoregulatory tone. In this case, the muscles do not contract. Only their tone and metabolism increase. This tone generally occurs in the muscles of the neck, torso and limbs. As a result, heat production increases by 50-100%.
2. Trembling occurs unconsciously and consists of periodic activity of high-threshold motor units against the background of thermoregulatory tone. When trembling, all energy is directed only to increasing heat generation, while during normal locomotion, part of the energy is spent on moving the corresponding limb, and part on thermogenesis. When shaking, heat production increases 2-3 times. 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.
3. Voluntary contractions consist of a conscious increase in muscle contraction. This is observed in conditions of low external temperature, when the first two stages are not sufficient. With voluntary contractions, heat production can increase 10-20 times.
Regulation of heat production in dovyazan muscles with the influence of a-motoneurons on the function and metabolism / muscles, in other tissues - sympathetic nervous system and catecholamines (increase metabolic rate by 50%) and the action of hormones, especially thyroxine, which almost doubles heat production.
A significant role in thermogenesis is lipids, which release significantly more energy during hydrolysis (9.3 kcal/g) than carbohydrates (4.1 kcal/g). Brown fat is of particular importance, particularly in children.
Heat transfer processes occurs in the following ways - radiation, convection, evaporation and thermal conductivity.
Radiation occurs using long-wave infrared radiation. This requires a temperature gradient between warm skin and cold walls and other objects. environment. Thus, the amount of radiation depends on the temperature and surface of the skin.
Thermal conduction occurs through direct contact of the body with objects (chair, bed, etc.). In this case, the rate of heat transfer from a more heated body to a less heated object is determined by the temperature gradient and their thermal providence. Heat transfer through this route increases significantly (14 times) when a person is in water. Partially by conduction, heat is transferred from internal organs to the surface of the body. But this process is slowed down due to the low thermal conductivity of fat.
Convection path. The air in contact with the surface of the body heats up in the presence of a temperature gradient. At the same time, it becomes lighter and, rising from the body, makes room for new portions of air. Thus, it takes away some of the heat. The intensity of natural convection can be increased due to additional air movement, reducing obstacles in its flow to the body (with appropriate clothing).
Evaporation of sweat. At room temperature, in an undressed person, about 20% of the heat is given off due to evaporation.
Thermal conductivity, 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 paths are not only ineffective, but the body heats up. Under these conditions, only one heat release mechanism is activated in the body, associated with the processes of sweating and steaming. Both physical laws (energy consumption for the evaporation process) and biological laws (sweating) are used here. Cooling of the skin is facilitated by the fact that 0.58 kcal is consumed to evaporate 1 ml of sweat. If it doesn't happen
evaporation of sweat, the efficiency of heat transfer decreases sharply. M
The rate of evaporation of water depends on the temperature gradient and the saturation of water vapor in the surrounding air. The higher the humidity, the less efficient this heat transfer path becomes. The efficiency of heat transfer decreases sharply when you are in water or wearing thick clothing. In this case, the body is forced to compensate for the lack of sweating by increasing sweating.
Evaporation has two mechanisms: a) perspiration - without the participation of sweat glands b) evaporation - with the active participation of sweat glands.
Perspiration- evaporation of water from the surface of the lungs, mucous membranes, skin, which is always wet. This evaporation is not regulated, it depends on the temperature gradient and humidity of the surrounding air, its value is about 600 ml/day. The higher the humidity, the less effective this type of heat transfer is.
The mechanism of sweat secretion. The sweat gland consists of two parts: the gland itself, which is located in the subdermal layer, and excretory ducts that open on the surface of the skin. A primary secretion is formed in the gland, and a secondary secretion, sweat, is formed in the ducts due to reabsorption.
The primary secretion is similar to blood plasma. The difference is that this secretion contains no proteins and glucose, and less Na +. Thus, in initial sweat the concentration of sodium is about 144 nmol/l, chlorine - 104 nmol/l. These ions are actively absorbed as sweat passes through the excretory ducts, providing water absorption. The absorption process largely depends on the rate of formation and progression of sweat; as these processes are active, the more Na + and Cl- remains. With heavy sweating, up to half the concentration of these ions can remain in the sweat. Severe sweating is accompanied by an increase in the concentration of urea (up to 4 times higher than in plasma) and potassium (up to 1.2 times higher than in plasma). The total high concentration of ions, forming a high level of osmotic pressure, ensures a decrease in reabsorption and the release of large amounts of water through sweat.
With heavy sweating, a lot of NaCl can be wasted (up to 15-30 g/day). However, the body has mechanisms to ensure that these important ions are retained during heavy sweating. They are involved in adaptation processes, in particular, aldosterone enhances Na + reabsorption.
The functions of the sweat glands are regulated by special mechanisms. Their activity is influenced by the sympathetic nervous system, but the mediator here is acetylcholine. Secretory cells, in addition to M-cholinergic receptors, also have adrenergic receptors that respond to blood hormone catecholamines. Activation of the function of the sweat glands is accompanied by an increase in its blood supply.
The amount of sweat secreted can reach 1.5 l/h, and in adapted people - up to 3 l/h.
At room temperature in an undressed person, about 60% of the heat is given off due to radiation, about 12-15% - air convection, about 20% - evaporation, 2-5% - thermal conductivity. But this ratio depends on a number of conditions, in particular on the ambient temperature.
The main role in the regulation of heat transfer processes is played by changes in the blood supply to the skin. The narrowing of skin vessels and the opening of arteriovenous anastomoses contribute to less heat flow from the core to the membrane and its retention in the body. On the contrary, with the dilation of skin vessels, its temperature can increase by 7-8 ° C. At the same time, heat transfer also increases.
Conventionally, the skin can be called the radiator system of the body. Blood flow in the skin can vary from 0 to 30% IOC. Skin vascular tone is controlled by the sympathetic nervous system.
Thus, body temperature is a balance between the processes of heat production and heat transfer. When heat production prevails over heat loss, body temperature rises and, conversely, if heat loss is higher than heat production, body temperature decreases.
THERMOREGULATION AND HEALTH
The human habitat extends from the pole zones, where the air temperature sometimes reaches -86°C, to equatorial savannahs and deserts, in the hottest areas of which it approaches +50°C in the shade! Nevertheless, in such a wide temperature range, a person maintains active vitality and sufficient performance due to his thermal stability, when body temperature fluctuates within relatively narrow limits - from 36 to 37 ° C.
Homeothermy – constancy of body temperature - makes a person independent of the temperature conditions of residence, since the biochemical reactions that ensure his life continue to be carried out at an optimal level due to the preservation of adequate activity of the tissue enzymes and vitamins that provide them, catalyzing and activating certain aspects of metabolism, tissue hormones, neurotransmitters and other substances , on which the normal functioning of the body depends. A shift in temperature in one direction or another sharply changes the activity of these substances, and to varying degrees for each of them - as a result, a disconnection occurs in the activity of individual aspects of metabolism. In poikilothermic, cold-blooded animals, whose body temperature is determined by the ambient temperature (increases or decreases with the latter), the activity of their tissue enzymes as biological catalysts changes along with changes in external thermal conditions. That is why, when the temperature decreases, the degree of manifestation of their vital activity decreases until it stops completely - the so-called suspended animation, and at a very high temperature, either death or drying occurs, which in some of the poikilotherms is also a type of suspended animation. Thus, with a change in external temperature, the vital activity of some insects (locusts) can be restored both after freezing to the temperature of liquid nitrogen (–189°C) and after drying. A case has been described of the revival, albeit short-lived, of a giant newt that had been frozen in a glacier, according to experts, at least about 5,000 years ago.
Thus, the ability to maintain a constant body temperature under different living conditions makes warm-blooded animals independent of the circumstances of nature and capable of maintaining a high level of vitality. This ability is due to a complex system of thermoregulation, which ensures a decrease in heat production and active heat transfer in case of danger of overheating and activation of thermogenesis when heat transfer is limited - in case of danger of hypothermia.
Statistics show that in Russia, of all cases of temporary disability, more than 40% are due to colds, which gives the average person reason to consider the thermoregulation system to be imperfect. However, there are many facts indicating high natural human resistance to low temperatures. Thus, yogis-resps compete at temperatures below –20°C in the speed of drying wet sheets with the heat of their bodies, sitting naked on the ice of a frozen lake. It has become traditional for specially trained swimmers to swim through the Bering Strait from Alaska to Chukotka (more than 40 km) at a water temperature of +4°C - +6°C. The Yakuts rub newborns with snow, and the Ostyaks and Tungus immerse them in snow, pour cold water over them and then wrap them in reindeer skins... In this case, apparently, we should rather talk about the perversion of the perfect mechanisms of human thermoregulation by conditions that are far from the conditions that formed them in evolution life modern man than about the imperfection of the mechanisms themselves.
While most vital functions - blood circulation, respiration, digestion, etc. - have some specific structural and functional apparatus, thermoregulation does not have such an organ and is a function of the entire organism as a whole.
According to the scheme proposed by I.P. Pavlov, a warm-blooded organism can be represented as a relatively thermostable “core” and a “shell” with a wide temperature range. The core, whose temperature ranges from 36.8–37.5 ° C, includes mainly vital internal organs: heart, liver, stomach, intestines, etc. Particularly noteworthy is the role of the liver, which has a relatively high temperature - above 37.5 ° C, and the large intestine, the microflora of which, in the process of their vital activity, produces a lot of heat, ensuring the maintenance of the temperature of adjacent tissues. The thermolabile shell consists of limbs, skin and subcutaneous tissues, muscles, etc. The temperature of different parts of the shell varies widely. Thus, the temperature of the toes is about 24°C, the ankle joint is 30–31°C, the tip of the nose is 25°C, the armpit, rectum is 36.5–36.9°C, etc. However, the temperature of the shell is very flexible, which is determined by living conditions and the state of the body, therefore its thickness can change from very thin in the heat to very powerful, compressing the core in the cold. Such a relationship between the core and the shell is due to the fact that the first primarily produces heat (at rest), and the second must ensure the conservation of this heat. This is precisely what explains the circumstance that in hardened people the shell quickly and reliably envelops the core in the cold, maintaining optimal conditions for maintaining the activity of vital organs and systems, while in unhardened people the shell remains thin even under these conditions, creating a threat of hypothermia of the core (for example, when the temperature drops lungs at just 0.5°C there is a threat of pneumonia).
The body's thermal stability is ensured mainly by two complementary regulatory mechanisms - physical and chemical. Physical thermoregulation It is mainly activated when there is a danger of overheating and involves releasing heat into the environment. In this case, all possible heat transfer mechanisms are included: heat radiation, heat exchange, convection and evaporation. Thermal radiation is carried out due to infrared rays emanating from the skin having a high temperature. Heat conduction is realized due to the temperature difference between the skin and the surrounding air. This difference increases due to hyperemia - dilation of skin vessels and the influx of more warm blood from the internal organs, which is why the color of the skin becomes pink in the heat. In this case, the efficiency of heat transfer is determined by the thermal conductivity and heat capacity of the external environment: thus, these indicators at the corresponding temperatures for water are 20–27 times higher than for air. This makes it clear why the thermocomfortable air temperature for humans is about 18°C, and water temperature is 34°C. Heat transfer due to the evaporation of sweat is very effective, since when 1 ml of sweat evaporates from the surface of the body, the body loses 0.56 kcal of heat. If we consider that an adult produces about 800 ml of sweat even in conditions of low physical activity, the effectiveness of this method becomes clear.
Under different living conditions, the ratio of heat loss in one way or another changes noticeably. Thus, at rest and at optimal air temperature, the body loses 31% of the generated heat by conduction, 44% by radiation, 22% by evaporation (including due to moisture from the respiratory tract) and 3% by convection. With a strong wind, the role of convection increases, with an increase in air humidity - conduction, and with intense work - evaporation (for example, with intense physical activity, the evaporation of sweat sometimes reaches 3-4 liters per hour!).
The efficiency of heat transfer from the body is exceptionally high. Biophysical calculations show that disruption of these mechanisms, even in a person at rest, would lead to an increase in his body temperature within an hour to 37.5 ° C, and after 6 hours - to 46–48 ° C, when irreversible destruction of protein structures begins.
Chemical thermoregulation becomes especially important when there is a risk of hypothermia. The loss of fur by man relative to animals has made him especially sensitive to the effects of low temperatures, as evidenced by the fact that humans have almost 30 times more cold receptors than heat receptors. At the same time, the improvement of adaptation mechanisms to cold has led to the fact that a person tolerates a decrease in body temperature much more easily than an increase. Thus, infants can easily tolerate a decrease in body temperature of 3–5°C, but have difficulty with an increase of 1–2°C. An adult tolerates hypothermia up to 33–34°C without any consequences, but loses consciousness when overheated from external sources to 38.6°C, although with fever from an infection he can retain consciousness even at 42°C. At the same time, there have been cases of revival of frozen people whose skin temperature dropped below freezing point.
The essence of chemical thermoregulation is to change the activity of metabolic processes in the body: at high external temperatures it decreases, and at low temperatures it increases. Studies show that when the ambient temperature decreases by 1°C, metabolic activity in a naked person at rest increases by 10%. (However, the switching off of the higher regulatory mechanisms of thermostability in warm-blooded animals by anesthesia and so-called neuroleptics makes them dependent on the ambient temperature, and when their body temperature is cooled to 32°C, their oxygen consumption decreases to 50%, at 20°C - to 20%, and when +1°С – up to 1% of the initial level.)
Of particular importance for maintaining body temperature is the tone of skeletal muscles, which increases with a decrease in ambient temperature and decreases with warming. It is significant that these processes proceed more actively, the more dangerous the impending violation of thermal stability is. Thus, at an air temperature of 25–28°C (and especially in combination with high humidity), the muscles are largely relaxed, and the thermal energy they produce is negligible. On the contrary, when there is a danger of hypothermia, tremors become increasingly important - uncoordinated contractions of muscle fibers, when external mechanical work is almost completely absent, and almost all the energy of the contracting fibers goes into thermal energy(this phenomenon is called non-contractile thermogenesis). It is not surprising, therefore, that during trembling, the body’s heat production can increase more than three times, and during strenuous physical work - 10 times or more.
The lungs also play an undoubted role in chemical thermoregulation, which, due to changes in the metabolic activity of the high-calorie fats included in their structure, maintain a relatively constant temperature - which is why at high external temperatures the blood flowing from the lungs is cooler, and at low temperatures it is warmer than the inhaled air.
The physical and chemical mechanisms of thermoregulation work with a high degree of coordination due to the presence in the central nervous system of the corresponding center in the diencephalon (hypothalamus). That is why, at high ambient temperatures, on the one hand, heat transfer increases (due to increased skin temperature, increased respiration, increased processes evaporation of sweat, etc.), and on the other hand, heat production decreases (due to a decrease in muscle tone, a transition to the body’s absorption of less energy-containing products); at low temperatures, the opposite is true: heat production increases and heat transfer decreases.
Thus, the perfect mechanisms of human thermoregulation make it possible to maintain optimal vitality in a wide range of external temperatures.
Thermoregulation is a process that ensures the body’s ability to maintain body temperature at a certain level, regardless of the ambient temperature.
The thermoregulatory center can be excited both humorally (by the temperature of the blood flowing through it) and reflexively (by irritation of skin receptors with heat or cold). Excitation of the thermoregulatory center activates all thermoregulatory mechanisms: the intensity of oxidative processes, skeletal muscle tone, vasomotor reactions, secretion of sweat glands, respiratory movements. The intensity of oxidative processes can change either through the autonomic nervous system or by changing the secretion of thyroid hormones and the adrenal medulla. Changes in muscle function, dilation or constriction of blood vessels, secretion of sweat, changes in respiratory movements occur reflexively through the vasomotor, respiratory and sweating centers.
Cortex
The thermoregulation center is, in turn, under the control of the cerebral cortex. If an animal is exposed to overheating in a certain environment and the corresponding regulatory reactions occur, then after some time the environment alone (without overheating) will cause in it the same reactions as overheating. Thus, a conditioned reflex reaction takes place here, occurring with the participation of the cerebral cortex.
The temperature limits of life are very wide. Spores of many bacteria can withstand heating up to 150°, and some of them do not lose viability at temperatures close to absolute zero. On the other hand, in the hot springs of the island of Ischia (Italy) at a temperature of about 85°, some ciliates live. There is still much that remains insufficiently studied. Fish, insects and even mammals can be frozen and then carefully thawed. For example, carp were frozen to 15 degrees below zero and again, gradually rotting, brought back to life, but freezing even one degree below 15 is already fatal for the animal. However, it is also known that when sperm are frozen to a temperature close to minus 200° and stored for a long time at this temperature, a significant part of them retains normal viability and fertilizing power.
On this page there is material on the following topics:
Mechanisms of heat transfer from the body in conditions of cold and heat ">
Mechanisms of heat transfer from the body in cold and warm conditions: a) redistribution of blood between the vessels of internal organs and the vessels of the skin surface; b) redistribution of blood in the vessels of the skin.
Physical thermoregulation appeared at later stages of evolution. Its mechanisms do not affect the processes of cellular metabolism. The mechanisms of physical thermoregulation are activated reflexively and, like any reflex mechanism, have three main components. Firstly, these are receptors that perceive changes in temperature inside the body or the environment. The second link is the thermoregulation center. The third link is effectors, which change the processes of heat transfer, maintaining body temperature at a constant level. The body, apart from the sweat gland, does not have its own effectors of the reflex mechanism of physical thermoregulation.
The importance of physical thermoregulation
Physical thermoregulation is the regulation of heat transfer. Its mechanisms ensure that body temperature is maintained at a constant level both in conditions where the body is in danger of overheating and when cooling.
Physical thermoregulation is carried out by changes in heat transfer by the body. It becomes especially important in maintaining a constant body temperature while the body is in conditions of elevated ambient temperature.
Heat transfer is carried out by heat radiation (radiation heat transfer), convection, i.e. movement and mixing of air heated by the body, heat conduction, i.e. heat transfer from a substance in contact with the surface of the body. The nature of heat transfer from the body changes depending on the intensity of metabolism.
Heat loss is prevented by the layer of still air that is located between clothing and skin, since air is a poor conductor of heat. The layer of subcutaneous fatty tissue largely prevents heat transfer due to the low thermal conductivity of fat.
Temperature regulation
Skin temperature, and therefore the intensity of heat radiation and heat conduction, can change in cold or hot environmental conditions as a result of blood redistribution in the vessels and when the volume of circulating blood changes.
In the cold, the blood vessels of the skin, mainly arterioles, narrow; a larger amount of blood enters the vessels of the abdominal cavity and thereby limits heat transfer. The superficial layers of the skin, receiving less warm blood, emit less heat, so heat transfer decreases. In addition, when the skin is strongly cooled, arteriovenous anastomoses open, which reduces the amount of blood entering the capillaries and thereby prevents heat transfer.
The redistribution of blood that occurs in the cold - a decrease in the amount of blood circulating through the superficial vessels and an increase in the amount of blood passing through the vessels of the internal organs - helps to preserve heat in the internal organs, the temperature of which is maintained at a constant level.
As the ambient temperature rises, the blood vessels of the skin dilate, and the amount of blood circulating in them increases. The volume of circulating blood throughout the body also increases due to the transfer of water from tissues to vessels, and also because the spleen and other blood depots release additional amounts of blood into the general bloodstream. Increasing the amount of blood circulating through the vessels of the body surface promotes heat transfer through radiation and convection. To maintain a constant body temperature at high ambient temperatures, sweating, which occurs due to heat transfer during the evaporation of water, is also important.
The body temperature of humans and higher animals is maintained at a relatively constant level, despite fluctuations in ambient temperature. This constancy of body temperature is called isothermia.
Isothermia is characteristic only of the so-called homeothermic, or warm-blooded, animals and is absent from poikilothermic, or cold-blooded, animals whose body temperature is variable and differs little from the ambient temperature.
Isothermia develops gradually during ontogenesis. A newborn baby's ability to maintain a constant body temperature is far from perfect. As a result, cooling may occur (hypothermia) or overheating (hyperthermia) body at ambient temperatures that do not affect an adult. Likewise, even minor muscular work, such as a child's prolonged cry, can lead to an increase in his body temperature. The body of premature babies is even less able to maintain a constant body temperature, which for them largely depends on the temperature of their environment.
Heat generation occurs as a result of continuously occurring exothermic reactions. These reactions occur in all organs and tissues, but with different intensities. In tissues and organs that perform active work - in muscle tissue, liver, kidneys - a greater amount of heat is released than in less active ones - connective tissue, bones, cartilage.
Heat loss from organs and tissues depends to a large extent on their location: superficially located organs, such as skin, skeletal muscles, give off more heat and cool more strongly than internal organs, which are more protected from cooling.
The body temperature of a healthy person is 36.5-36.9 °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. For workers who work long night shifts, temperature fluctuations may be reversed.
A person's body temperature can remain constant only if heat generation and heat loss from the whole organism are equal. This is achieved through physiological mechanisms of thermoregulation. manifests itself as a result of the interaction of the processes of heat generation and heat transfer, regulated by neuroendocrine mechanisms. Thermoregulation is usually divided into chemical and physical.
Chemical thermoregulation carried out by changing the level of heat generation, i.e. strengthening or weakening the intensity of metabolism in the cells of the body, and is important for maintaining a constant body temperature both under normal conditions and when the ambient temperature changes.
The most intense heat generation in the body occurs in the muscles. Even if a person lies motionless, but his muscles are tense, the intensity of oxidative processes, and at the same time heat generation, increases by 10%. Light physical activity leads to an increase in heat generation by 50-80%, and heavy muscular work - by 400-500%.
In cold conditions, heat generation in the muscles increases, even if the person is stationary. This is due to the fact that cooling of the body surface, acting on receptors that perceive cold stimulation, reflexively excites random involuntary muscle contractions, manifested in the form of trembling (chills). At the same time, the body's metabolic processes are significantly enhanced, the consumption of oxygen and carbohydrates by muscle tissue increases, which entails an increase in heat generation. Even voluntary imitation of trembling increases heat generation by 200%. If muscle relaxants are introduced into the body - substances that disrupt the transmission of nerve impulses from nerve to muscle and thereby eliminate reflex muscle tremors, even with an increase in ambient temperature, a decrease in body temperature occurs much faster.
In chemical thermoregulation significant role The liver and kidneys also play a role. 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.
The release of energy in the body occurs due to the oxidative breakdown of proteins, fats and carbohydrates; therefore, all mechanisms that regulate oxidative processes also regulate heat generation.
Physical thermoregulation carried out by changes in heat transfer by the body. It becomes especially important in maintaining a constant body temperature while the body is in conditions of elevated ambient temperature.
Heat transfer is carried out by heat radiation (radiative heat transfer), or convection, those. movement and movement of air heated by heat, heat conduction, those. transfer of heat to substances in direct contact with the surface of the body, and water evaporation from the surface of the skin and lungs.
Under normal conditions, a person's heat loss by conduction is small, since air and clothing are poor heat conductors. Radiation, evaporation and convection occur at varying rates depending on the ambient temperature. In a person at rest, at an air temperature of about 20 ° C and a total heat transfer equal to 419 kJ (100 kcal) per hour, 66% is lost by radiation, 19% by water evaporation, and 15% by convection of the total heat loss by the body . 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.
Clothing reduces heat transfer. Heat loss is prevented by the layer of still air that is located between clothing and skin, since air is a poor conductor of heat. 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. The air temperature under clothes is 30 °C. On the contrary, a 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.
In the cold, the blood vessels of the skin, mainly arterioles, narrow: more blood enters the vessels of the abdominal cavity, thereby limiting heat transfer. The surface layers of the skin, receiving less warm blood, emit less heat - heat transfer decreases. With strong cooling of the skin, in addition, arteriovenous anastomoses open, which reduces the amount of blood entering the capillaries and thereby prevents heat transfer.
The redistribution of blood that occurs in the cold - a decrease in the amount of blood circulating through the superficial vessels and an increase in the amount of blood passing through the vessels of the internal organs - helps to maintain heat in the internal organs.
As the ambient temperature rises, the blood vessels of the skin dilate, and the amount of blood circulating in them increases. The volume of circulating blood throughout the body also increases due to the transfer of water from tissues to vessels, and also because the spleen and other blood depots release additional amounts of blood into the general bloodstream. Increasing the amount of blood circulating through the vessels of the body surface promotes heat transfer through radiation and convection.
To maintain a constant human body temperature at high ambient temperatures, the evaporation of sweat from the surface of the skin, which depends on the relative humidity of the air, is of primary importance. In air saturated with water vapor, water cannot evaporate. Therefore, 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 into 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 sweat is called effective sweating).
Air-tight clothing (rubber, etc.) that prevents the evaporation of sweat is poorly tolerated: the layer of air between the clothing and the body is quickly saturated with vapor and further evaporation of sweat stops.
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.
Since some of the water is evaporated by the lungs in the form of vapors that saturate the exhaled air, breathing is also involved in maintaining body temperature at a constant level. At high ambient temperatures, the respiratory center is reflexively excited, at low temperatures it is depressed, breathing becomes less deep.
Thus, the constancy of body temperature is maintained through the combined action, on the one hand, of mechanisms regulating the intensity of metabolism and heat generation dependent on it (chemical regulation of heat), and on the other, mechanisms regulating heat transfer (physical regulation of heat) (Fig. 9.10) .
Rice. 9.10.
Regulation of isothermia. Regulatory reactions that ensure the maintenance of constant body temperature are complex reflex acts that occur in response to temperature stimulation of skin receptors, skin and subcutaneous vessels, as well as the central nervous system itself. These receptors that sense cold and heat are called thermoreceptors. At a relatively constant ambient temperature, rhythmic impulses are sent from the receptors to the central nervous system, reflecting their tonic activity. The frequency of these impulses is maximum for cold receptors of the skin and skin vessels at a temperature of 20-30 ° C, and for skin thermal receptors - at a temperature of 38-43 ° C. With sudden cooling of the skin, the frequency of impulses in cold receptors increases, and with rapid warming it becomes smaller or stops. Thermal receptors react in the exact opposite way to the same temperature changes. Warm and cold receptors of the central nervous system respond to changes in the temperature of the blood flowing to the nerve centers (central thermoreceptors). The bulk of the heat is produced by skeletal muscles and internal organs, which form the core, and the skin creates a shell aimed at retaining or removing heat from the body (Fig. 9.11).
Rice. 9.11.
The hypothalamus contains the main thermoregulation centers, which coordinate numerous and complex processes that ensure the maintenance of body temperature at a constant level. This is proven by the fact that the destruction of the hypothalamus entails a loss of the ability to regulate body temperature and makes the animal poikilothermic, while the removal of the cerebral cortex, striatum and visual thalamus does not noticeably affect the processes of heat generation and heat transfer.
The hypothalamic regulation of body temperature involves the endocrine glands, mainly the thyroid and adrenal glands.
The participation of the thyroid gland in thermoregulation is proven by the fact that the introduction into the blood of an animal of the blood serum of another animal that has been in the cold for a long time causes an increase in metabolism in the first. This effect is observed only when the thyroid gland is preserved in the second animal. Obviously, while staying in cooling conditions, there is an increased release of thyroid hormone into the blood, which increases metabolism and, consequently, the formation of heat.
The participation of the adrenal glands in thermoregulation is due to their release of adrenaline into the blood, which, by enhancing oxidative processes in tissues, in particular in muscles, increases heat generation and constricts skin vessels, reducing heat transfer. Therefore, adrenaline can cause an increase in body temperature ( adrenaline hyperthermia).
Hypothermia and hyperthermia. If a person is in conditions of significantly increased or decreased ambient temperature for a long time, then the mechanisms of physical and chemical thermoregulation of heat, due to which the body temperature is maintained under normal conditions, may turn out to be insufficient: hypothermia of the body occurs - hypothermia or overheating - hyperthermia.
Hypothermia - a condition in which the body temperature drops below 35 °C. Hypothermia occurs most quickly when immersed in cold water. In this case, excitation of the sympathetic nervous system is first observed, heat transfer is reflexively limited and heat production is enhanced. The latter is facilitated by muscle contraction - muscle tremors. After some time, the body temperature begins to decrease. In this case, a state similar to anesthesia is observed: disappearance of sensitivity, weakening of reflex reactions, decreased excitability of nerve centers. The metabolic rate sharply decreases, breathing slows down, heart contractions slow down, cardiac output decreases, and blood pressure decreases (at a body temperature of 24-25 ° C it can be 15-20% of the original).
IN last years artificially created hypothermia with body cooling to 24-28 ° C is used in surgical clinics performing operations on the heart and central nervous system. The point of this event is that hypothermia significantly reduces the metabolism of the brain and, consequently, the need for oxygen in this organ. As a result, longer bleeding of the brain becomes possible (instead of 3-5 minutes at normal temperature to 15-20 minutes at 25-28 ° C), which means that with hypothermia, patients can more easily tolerate temporary shutdown of cardiac activity and respiratory arrest.
Cryotherapy is also used for some other diseases.
Hyperthermia - a condition in which the body temperature rises above 37 °C. It occurs with prolonged exposure to high ambient temperatures, especially with humid air and, therefore, little effective sweating. Hyperthermia can also occur under the influence of some endogenous factors that increase heat generation in the body (thyroxine, fatty acids, etc.). Severe hyperthermia, in which the body temperature reaches 40-41 ° C, is accompanied by a severe general condition of the body and is called heat stroke.
One should distinguish from hyperthermia such a change in temperature when external conditions are not changed, but the process of thermoregulation itself is disrupted. An example of such a disorder is an infectious fever. One of the reasons for its occurrence is the high sensitivity of the hypothalamic centers for the regulation of heat exchange to certain chemical compounds, in particular to bacterial toxins.
Thus, the balance of factors responsible for heat production and heat transfer is the main mechanism of thermoregulation.
Questions and tasks
- 1. What is the role of proteins in the body? What is the essence of regulation of protein metabolism?
- 2. What is the role of carbohydrates in the body? What is the essence of regulation of carbohydrate metabolism?
- 3. What is the role of fats in the body? What is the essence of the regulation of fat metabolism?
- 4. What is the importance of vitamins in human life?
- 5. The importance of physical and chemical thermoregulation in the body. Explain your answer.
- 6. In recent years, artificially created hypothermia with body cooling to 24-28 ° C has been used in practice in surgical clinics performing operations on the heart and central nervous system. What is the point of this event?