[038] Homeostasis of the core body temperature (GB#107A01) | 基礎医学教育研究会(KIKKEN)Lab

● Hot or cold, the internal temperature is almost constant
When the temperature goes down, humans do their best to prevent them from getting cold by wearing clothes and raising the temperature of the room. When the temperature rises, humans try to take off clothes, put air conditioners, and alleviate the heat. However, even if it is in a naked state, the body is equipped with a mechanism that keeps the core temperature of the central part of the body almost constant against changes in the outside air temperature. If deep body temperature is constant, humans are somehow alive. (If deep body temperature moves greatly you can not live.)
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Core body temperature is somewhat different between humans, centered around 37 °C to 38 °C, and it is known that there is a slight periodic variation in one day. A physiological almost one day cycle “circadian rhythm” that triggers the discovery of the body clock is found from a careful observation of this body temperature. However, taking the average of the day, each human’s temperature keeps nearly constant throughout the year (if healthy) even if the season changes.

● heat source in the body

Humans have a heat source of body temperature in their bodies. If the cell is alive, some kind of heat will be generated, so it will be a heat source anywhere in the body, but the part with a high degree of heat production is somewhat restricted. It is said to be mainly visceral, brain and skeletal muscle. However, in the data hit on the Internet, the order and proportion of the data varies, and it is unknown what the original data is, and it can not be determined which is true. Even so, it seems certain that the proportion of internal organs, especially the liver, is high and high. In quiet state, it accounts for about a quarter of the body’s overall fever (this also varies depending on the material). There is no doubt that the liver will be the top among the whole body in terms of heat output per weight of tissue(← Actually, heart and kidney seem to be top). It seems that the brain is also responsible for about one-eighth of the whole fever. Skeletal muscle is a vessel with a great difference between resting and active. There are also materials that account for half of the whole body’s fever even at rest, but from the ratio of the blood flow rate, I think that the total of about 4/1 is reasonable.

● Metabolism proceeds by heat

In order to live, cells perform metabolism such as making, destroying and moving each. Metabolic activities require a range of temperature environments. Since metabolic reactions of living organisms are carried out using water as a medium, reactions can not be made if water freezes. Basically, the higher the temperature, the faster the chemical reaction proceeds. However, when the temperature exceeds 40 degrees Celsius, proteins that work as enzymes begin to degenerate and the reaction system dies. Although the optimum temperature may be different depending on each metabolic reaction, since the body is a group, it is most efficient to match the temperature to a reasonable temperature that can be harmonized as a whole. When advancing the chemical reaction in the laboratory beaker, it is soaked in hot water, heated with a lamp, or heat is applied from the outside. In that respect, heat is generated by the metabolism itself in the human body, so if you use the heat to keep the temperature, it is convenient at one stone. That just temperature is a healthy human body temperature.

● Body temperature is maintained in balance between heat generation and heat dissipation

As long as the human body is alive, we can not stop metabolism. Depending on age, sex and physique, if you are a 20-year-old man with a weight of 60 kg, energy of about 1,500 kcal disappears even if you do not do anything for the whole day just lying down. The physical work to put it on the outside is about breathing movement and some turning behaviors (stirring the surrounding air, deforming the futon (laugh)), the rest should change to heat entirely. Since 1 calorie (1 cal) is the amount of energy to raise the temperature of 1 g of water by 1 °C, the energy for raising 1 kg of water by 1 °C is 1 kcal. 1500 kcal per day will be the amount of energy that will raise 25 °C over the course of one day, (1500/60 = 25) for 60 kg of water. If the body had a thermal insulation structure like a thermos, this heat accumulates more and more and the temperature of the body will continue to rise by approximately 1 °C per hour (No, not a day, die a few hours!). However, in the human body the amount of heat generated in one day is diverged all the way out in a day, it is irresistible to the body. Body temperature is almost constant every day, so it is natural to take it from theory, but its nature is actually wonderfully subtle and exquisite due to the ability to adjust living bodies.

● Heat always escapes from the surface of the body

The amount of heat that disappears only by living even without doing anything during the day corresponds to the basal metabolic rate (BMR). Heat produced in the human body escapes through the body’s surface, the skin. If it is not skin, it is thrown away as breathing exhalation, urine or feces. After all, the airway and the wall of the gastrointestinal tract are also the surface of the body tissue, so eventually there is no place for heat escape other than the surface of the body. The rate at which heat escapes from the skin of a certain area is constant if the temperature difference between the skin and the environment does not change, and when the area doubles the amount of heat escapes doubles. Considering that body temperature is maintained, the speed of heat release in the body and the rate of heat release from the skin should be just balanced through the day. Although the basal metabolic rate varies depending on the human physique, age, sex, ambient temperature and the like, basically it is said that “basal metabolism is proportional to the surface area of the body”.

● Is the body surface area proportional to 2/3 th power of body weight?

Then, how much is the surface area of the body? It seems that the tape has been stretched around the body of the examinee and the total area was actually measured. According to the data, it is about 1.69 m² for men and about 1.51 m² for girls. Because we can not use these methods one by one, we usually estimate by calculation. If it is a sphere, a cube, or a simple three-dimensional object, the surface area can be determined from the dimensions by a simple calculation, and if it is the same shape, it is proportional to the 2/3 th power of the volume. If there is only a difference in size even in some uneven shape, its surface area is still proportional to the 2/3 th power of the volume. It is troublesome to measure the volume if it is a human body, but since the volume will be almost proportional to the body weight, the surface area can be determined from the body weight. Therefore, roughly speaking, it is considered that the surface area of a human is proportional to the 2/3 th power of the body weight. However, this is the case if the same shape, so the surface area is not the same between a person with the same weight but being round and shorter and a tall person with a cramp and a tall person. The rugged and tall person should have a large surface area. Therefore, in computing the surface area of a human being, mathematical expression that calculates not only the body weight but also the height is calculated is used. (However, in actuality assessment of basal metabolism, it is often based on the daily amount per kg body weight in many cases.)

●Heat escape depends on temperature difference

The way of escape of heat from the body is proportional to the difference between the body temperature and the temperature of the environment (in short, the temperature of the skin surface). If the environmental temperature on the skin surface is higher than the body temperature, the body will receive heat from the outside. Usually, the world we live in is much lower than body temperature. Because heat of metabolism must always escape, this is of course naturally, but the manner of escape of heat will change considerably depending on the temperature of the environment. Do not escape easily when the temperature of the environment is high, escape quickly when the temperature is low. When escaping faster than the rate of heat production, the amount of heat maintained by the body decreases, so the body temperature of the core body gradually decreases. If the temperature of the environment is high and heat can not escape, deep body temperature will gradually increase. However, with this change, body temperature will be swayed by changes in the seasons and changes in the temperature of the day. The creatures that are said to be “poikilotherm” are in that state.

● There is a mechanism to keep the temperature inside the body constant

Humans have a mechanism to keep deep body temperature constant even if the temperature outside the body changes if it is a certain temperature range. We are colleagues of creatures that are said to be “homoiotherm”. This animation shows how it looks like, based on an old but famous figure thought by Jurgen Aschoff (1913 – 1998), a German scientist who discovered the existence of a physiological body clock. The lower one shows roughly 20 °C, the higher one shows the change when placed in an environment of approximately 35 °C. (I do not know if it is air temperature or water temperature. I think that it is water temperature as a usual experimental method of experiment, but I am not sure.) “homoiotherm” means temperature is constant. Although it is said that the whole body is not always constant, when the ambient temperature is low the temperature of the limbs falls below 30 °C, but the temperature in the center of the body is kept at 37 °C. Conversely, when the ambient temperature is 35 °C, the temperature of the limbs rises, but the body temperature also maintains 37 °C. According to Aschoff, the body temperature has a core part that keeps a constant temperature even if the ambient temperature changes, and the shell temperature whose temperature changes depending on the ambient temperature is surrounding the core body part. The core temperature is the temperature of the internal organs part of the trunk and the part of the head brain, which corresponds to the so-called deep body temperature.
In the center part, there is a heat source that physically generates constant heat per time, and if the heat physically escapes from the surface, the temperature of the central part also easily changes when the environmental temperature changes. The core body temperature of human does not become so. That is because when the ambient temperature is low, the speed of escaping heat is reduced as much as possible, and when the ambient temperature is high, the speed of escaping heat is adjusted as much as possible. The point of the adjustment is to change the thermal conductivity of this part by adjusting the blood flow of the blood vessel running under the skin. At high temperatures, of course, do not forget the cooling effect by sweat. Also, the basal metabolic rate itself also rises slightly when it is colder than when it is hot, trying to balance the heat to escape as much as possible. (Let’s take these mechanisms on another occasion.)

● Do you consume energy when you drink cold water

When you drink cold water, your body feels cold. When swallowing 500 ml of 10 °C water into the body at body temperature 37 °C, the heat of 27 °C x 0.5 L = 13.5 kcal is roughly calculated and absorbed by the drinking water. If the body weight is 60 kg and the influence of deprived heat is uniformly applied, the body temperature will drop by 13.5/60 = 0.225 °C. You may feel that actually it might have been a bit cold. Since the body automatically tries to keep the body temperature constant, in order to make it the original body temperature, we will also bring the heat of 13.5 kcal from somewhere. At this time, will it burn a little more heat sources inside the body? Then drinking cold water will make it a little diet, but rather than just slightly lowering the way of escaping heat, it may not be difficult to procure this amount of heat. I do not know how it actually is, but in any case, if you think that you drank cold water in the midsummer and chilled, then temporarily, you will have to experience a bit hotter experience than before. Although it is a body structure, it is a bit disappointing.

● “Poikilotherm” also has a comfortable temperature

It seems that poikilotherm animals are also called external temperature animals (ectotherm). They do not metabolize enough to warm the body. Heat gets from outside, and metabolism activities according to the heat. If it is cold it is quiet and if it gets warm they can act actively. Since they do not use extra energy in the first place, they can live as much as possible at low temperatures that human beings will die. However, it does not mean that it is OK at any temperature, and the range of the optimum temperature for each living thing to live is not wide except for it. Animals living in cold water are comfortable with their cold water temperature. A warm and comfortable temperature for humans may be like hot water for water creatures. We sometimes hear the news that a large amount of fish has died or no longer as the water temperature and the seawater temperature of the pond are a little high. Attention that you should not touch the fish you casually with bare hands, but forgot about it, but it is very important.

● The generation of heat is proportional to the number of active cells

As animals are usually not only sleeping all day, they actually consume more energy than the basal metabolic rate. This energy is also destined to be discharged from the body surface, mostly as heat. The main source of heat is muscle, in vertebrates, especially skeletal muscles. Since heat is generated from each cell level, as the number of active cells increases, the calorific value on the whole is large. In the case of humans, the amount of skeletal muscle occupies approximately 40 % of body weight. When resting, the blood flow rate from 20 % to 30 % of the whole body rises to 80% at the time of maximum maximum body movement. Because the stroke volume of the heart also becomes five times, then the energy consumption of the skeletal muscle will jump up to nearly 40 times when it is resting. Of course, the body attempts to maximize the efficiency of the heat dissipation from the body surface, but it does not catch up so much, so the body temperature rises sharply. Therefore, in terms of temperature regulation, activities with strong exercise can not be continued for a long time. However, heat is generated even at normal level muscle exercise necessary for ordinary life. On average, energy consumption (mostly heat generation) due to activity excluding basal metabolic rate will be proportional to muscle mass. As a result, muscular humans will have more average heat release than non-muscular humans. Furthermore, further roughly summarized, it is said that ‘energy consumption by activity is proportional to body weight’.

○ Referenced sites

・Why is not the hot bath at 38 °C while the temperature is 38 °C, the 43rd prize prize film of junior high school prize promotion prize, Ministry of Education, Science and Technology award, 2002.
・Why is the air temperature hot and cold at the same 33 °C? Pages of everyday chemical engineering, chemical engineering materials.
・What is body temperature? How to measure correct body temperature, Terumo Body Temperature Institute.
・Easy-to-understand high school physics “Thermal motion”
・Easy-to-understand high school physics “Work equivalence of heat”
・Japanese body surface area, exposure index handbook, National Institute of Advanced Industrial Science and Technology.
・How to measure elephant’s body temperature, Animal Cross, 2011.

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○ Referenced books

・Lehninger Principles of Biochemistry 6th, International Edition, Macmillan Higher Education, England.
・カラー版 ボロン ブールペープ 「生理学」， 西村書店
・肉単―ギリシャ語・ラテン語 (語源から覚える解剖学英単語集 (筋肉編))
・カラー図解 人体の正常構造と機能 全10巻縮刷版，坂井 建雄，日本医事新報社
・トートラ人体解剖生理学　原書8版，丸善
・柔道整復学校協会編「生理学」，南江堂
・東洋療法学校協会編「生理学」，医歯薬出版株式会社

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