Evaporative heat dissipation refers to the water content of body fluidskinandmucosa (Mainlyrespiratory tractMucous membrane)liquid stateConvert toGaseousIt is a heat dissipation method that takes away a lot of heat at the same time.Every 1g of water evaporated can take away 2.44 kJ of heat, so evaporation is a very effective way to dissipate heat.
Evaporation can be divided into non perspiration and perspiration.Non perspiration (also called non sensible evaporation) is a continuous form of heat dissipation that is evaporated before obvious sweat drops are gathered.Generally not noticed by people.Under moderate room temperature (below 30 ℃) and humidity conditions, about 25% of the heat is emitted by sweating in this way, which is the activity process of sweat glands to actively secrete sweat. Sweat evaporation can effectively carry away the heat.Because sweating can be felt, it is also called sensible evaporation.
There are two forms of human evaporation: insensible evaporation and sweating.Even when the human body is in low temperature and there is no juice secretion, the skin and respiratory tract are constantly saturated with water and evaporated. This water evaporation is called imperceptible evaporation, in which the skin water evaporation is also called non perspiration, that is, this water evaporation is not perceived by people and is connected with the activity switch of the juice glands.When the room temperature is below 30 ℃, the water that does not evaporate is quite constant, and 12-15g/h · m2 of water is evaporated, half of which is the water evaporated from the respiratory tract;The other half of the water is evaporated from the tissue gap of the skin.The unconscious evaporation of human body in 24h is 400-600ml.Infants' unconscious evaporation rate is higher than that of adults. Therefore, infants are more likely to suffer from severe dehydration in the absence of water.Unsensible evaporation is a very effective way to dissipate heat. Some animals, such as dogs, have juice glands, but they cannot secrete juice under high temperature. At this time, they must passHot wheezingBreathing enhances evaporation and heat dissipation by respiratory tract.
Sweating is the activity that sweat glands secrete juice.Sweating is an obvious perspiration secretion that can be realized. Therefore, the evaporation of juice is also called sensible evaporation.
People start to sweat when the ambient temperature reaches about 30 ℃ in a quiet state.If the air humidity is high and clothes are worn more, the temperature of 25 ℃ can cause human sweating.When people work or exercise, although the temperature is below 20 ℃, they can also sweat, and the amount of sweat is often more.
The moisture in sweat accounts for 99%, while the solid component is less than 1%. Most of the solid components aresodium chloride, and a small amountpotassium chloride、ureaEtc.Compared with plasma, sweat has the following characteristics: the concentration of sodium chloride is generally lower than that of plasma;People who sweat in high temperature operation may lose more sodium chloride in sweat;In sweatglucoseThe concentration of is almost zero;lactic acidThe concentration is mainly in plasma;proteinThe concentration of is zero.In the experiment, the pressure in the secretory lumen was measured to be above 37.3 kPa (250 mmHg) when the sweat gland secreted.This indicates that sweat is not a simple plasma filtrate, but is actively secreted by sweat gland cells.A lot of lactic acid isGlandular cellA product that enters secretory activity.The sweat just secreted from sweat gland cells is isotonic to plasma, but when flowing through the sweat gland lumen, sodium and chlorine areReabsorptionTherefore, the sweat discharged finally is hypotonic.The amount of sodium excreted in sweat is also affected byAldosterolAdjustment of.Because sweat is hypotonic, dehydration caused by excessive sweating can lead toHypertonic dehydration。
Sweating is a reflex activity.Human sweat glands accept sympatheticCholinergic fibreDominate, soacetylcholineIt can promote the secretion of eccrine glands.Sweating centers are distributed fromspinal cordreachCerebral cortexOfcentral nervous systemMedium.Under normal circumstances, the main function ishypothalamusIt is probably located in or near the thermoregulatory center.
Warm sweating refers to the sweat secreted by sweat glands in various parts of the body in a warm environment.The main factors of initiating tepid sweating are: ① the tepid environment stimulates the temperature receptors in the skin, and the impulse is transmitted to the sweating center, which causes sweating reflexively; ②The warm environment warms the skin blood, and the heated blood flows to the heat sensitivity of the hypothalamus sweating centerneuron, can cause sweating.The physiological significance of tepid sweating is to dissipate heat.If 1.7L sweat is evaporated every hour, the body heat can emit about 4200kJ of heat.However, if the sweat rolls off from the body or is wiped off without evaporation, there is no evaporation and heat dissipation effect.
Sweating speed is affected by ambient temperature and humidity.The higher the ambient temperature, the faster the sweating speed.If the time is too long in the high temperature environment, the sweating speed will be significantly slowed down due to sweat gland fatigue.With high humidity, sweat is not easy to evaporate, so body heat is not easy to evaporate, and body heat is not easy to dissipate.In addition, when the wind speed is high, the sweat is easy to evaporate, and the sweat evaporates quickly, which is easy to dissipate heat and reduce the sweating speed.
Labor intensity also affects sweating speed.The labor intensity is high, and the more heat is produced, the more sweat is produced.
Sweating caused by mental tension or emotional excitement is called mental sweating.It is mainly found in palm, sole of foot and armpit.The central nerve of mental sweating may be in the motor area of cerebral cortex.Psychogenic sweating has little effect on thermoregulation.
Species specificity
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Evaporative heat dissipation has obvious species specificity.
Horses sweat a lot, and cattle sweat moderately;
Sheep and dogs strengthen evaporation and heat dissipation from the respiratory tract through hot panting breathing. The respiratory rate of animals rises to 200~400 times · min-1, showing mouth opening breathing, and the amount of saliva secretion increases significantly;Rodents apply saliva or water to their hair to evaporate heat.
When the process of heat generation and heat dissipation reaches a dynamic balance, the temperature of animals can be maintained at a certain level.In addition, a small amount of heat is lost with the excretion of exhaled gas, urine, feces, etc. Fish belongs to aquatic metathermal animals, because there are similarCountercurrent heat exchangeThe temperature of muscle is 14 ℃ higher than that of water, so tuna swimming in water is not limited by water temperature;At the same time, the digestive organs and liver of tuna can maintain a higher temperature to improve the digestibility, supply more heat to muscles and improve work efficiency.
