The body’s response to cooling
The concept of constancy of human body temperature, as research has shown, is to a certain extent relative. For example, open skin at low temperatures cools faster than closed ones. But the temperature of the closed areas of the body and internal organs with fluctuations in ambient temperature remains almost unchanged.
Temperature fluctuations depend on the time of day, the activity of the body, the ambient temperature, the insulating properties of clothing. During heavy physical work, training and sporting events, body temperature may rise by 1-2 ° C or more.
They affect body temperature and changes in the physiological state of the body, nervous excitement, and pregnancy.
A person can tolerate deviations of internal body temperature from normal by 4 ° C to both sides: the lower limit is 33 ° C, the upper one is 41 ° C. During the day, the body temperature varies slightly: its maximum values (37.0-37.1 ° C) are observed at 16-18 h, the minimum (36.2-36.0 ° C) at 3-4 am. In older people, body temperature can drop to 35-36 ° C.
Constancy of body temperature is possible only if the amount of heat generated is equal to the amount of heat given off by the body to the environment. In other words, the constancy of body temperature is provided by a combination of two interrelated processes – heat production and heat transfer. If the heat input is equal to its consumption, then the body temperature “remains at a constant level. If heat production prevails over heat transfer, the body temperature rises. In cases where heat generation lags behind heat transfer, a decrease in body temperature is observed.
Heat formation for a person is the most important way to maintain a constant body temperature. The continuous flow of metabolic processes in the body is accompanied by the formation of heat and the cost of vital energy.
An unequal amount of heat is formed in various organs of the body. The main regulator of heat production is muscles. With intense physical exertion, they supply up to 90% of heat. Under normal conditions, the share of muscles accounts for 65-70% of heat production. The second most important source of heat production is the liver and the digestive tract. They give 20-30% of heat.
In addition to the heat generated in the body, a person in the hot season receives the heat of the environment. Thus, with a decrease in the ambient temperature below 15 ° C, the heat generation increases significantly, and with an increase above 30 ° C it decreases.
However, with a significant increase in the ambient temperature (above 37 ° C) there is a disturbance of heat transfer and the body temperature rises again. When the air temperature drops, cold shivering often occurs — an involuntary contraction of skeletal muscles. This body reaction is protective in nature: it enhances the heat buildup in the muscles and thereby maintains normal body temperature.
Thus, the amount of heat in the body is determined, firstly, by the heat generated by metabolic processes, and secondly, coming from the external environment.
Along with the formation of heat in the body is constantly consumed by heat transfer. Otherwise, a person would die from overheating. The calculation shows that if for any reason the heat transfer ceases, the temperature of the human body will increase by 2.5 ° C every hour. By the end of the day it will rise above 60 ° С.
Heat is mainly released through the skin, as well as through breathing. Heat release occurs according to the laws of physics in the following ways: by the radiation of heat by the heated surface of the body; conducting heat by heating colder air and body-contacting objects; heat consumption by evaporation from the surface of the skin and lungs.
Heat radiation is a property of a surface heated to a certain temperature to radiate heat in the form of radiant energy — infrared rays. Heat conduction and heat radiation alone account for about 70–80% of the total heat transfer.
Heat conduction is the direct release of heat from the skin to adjacent objects or particles of air or water. Conduction is facilitated by convection, that is, the change of heated air or water particles by other, colder ones.
Convection is enhanced by the presence of wind, water flow, as well as running, swimming, sports and outdoor games. However, it should be remembered that convection cools the body only in cases where the external temperature is below the temperature of the human body. The hot desert wind does not cool, but, on the contrary, heats the body. The presence of clothing also changes the intensity of convection. A scuba diver suit, for example, protects a swimmer from overcooling during long swimming in cold water.
Conduction depends on the thermal conductivity of the medium. Thus, the thermal conductivity of air is small, and the water is large. That is why cooling in water is much faster than in air. Cold moist air cools the body faster than dry air of the same temperature.