5 Maintenance of Thermal Balance Because the mechanical efficiency of the human body is mostly below 25%, more than 75% of the total energy used is converted into heat. The greater the exercise intensity, the greater the total amount of heat produced. This excess heat has to be removed and dissipated to prevent overheating and hyperthermia. During maximal exercise, the rectal temperature can exceed 40 ℃ and the muscle temperature 41 ℃ without causing any discomfort for the exercising person.

6 Factors  Magnitude of Metabolic Rate Human beings can be considered tropical animals inasmuch as they require an ambient temperature of 28 to 30 ℃ to maintain thermal balance at rest. Muscular exercise is associated with an increase in metabolic rate.

2 Thirst 1 Water Balance 3 Water deficit Water Balance SECTION 3 WATER BALANCE SECTION 3 WATER BALANCE

1 Water Balance Reasonable figures for the daily water loss are as follows: from gastrointestinal trace, 200 ml; respiratory tract, 400 ml; skin, 500 ml; kidneys, 1,500 ml; total, 2,600 ml. this loss is balanced by an intake as follows: fluid, 1,300 ml; water in the food, 1,000 ml; water liberated during the oxidation in the cells, 300 ml; total, 2,600 ml. however, the water loss can increase considerably when the individual exercises or is exposed to a hot environment.

2 Thirst In adults, about 70% of the lean body weight is water, so there is a substantial buffer to cover water losses over limited periods of time. However, in the long run, water intake must balance water loss by the several routes mentioned. Hypothalamus and adjacent preoptic regions play the essential role in the thirst mechanism (Andersson 1967; Stevenson 1965). Osmoreceptors react to an increase in the osmolarity of the intracellular fluid. Any change in the internal environment leading to cellular hypohydration will elicit thirst.

3 Water deficit We have concluded that high sweat rates with excessive loss of body fluids can cause a deficit of body water (hypohydration or dehydration). Hypohydration causes greater heat storage in the body, which reduces the heat strain tolerance. Prolonged exposure to heat or prolonged exercise certainly causes hypohydration.

3 Water deficit Dehydration also affects crew members of high-performance aircraft. Nunneley and Stribley (1979) showed that dehydration tends to lower G-tolerance and increase the variability of response to heat. The simplest method of determining whether the fluid intake has been adequate is by weighing the individual under standard conditions. Excessive fluid loss can occur in cold environments.

3 Water deficit Dehydration also affects crew members of high-performance aircraft. Nunneley and Stribley (1979) showed that dehydration tends to lower G-tolerance and increase the variability of response to heat. Well-trained subjects are less affected in their performance by hypohydration than are untrained subjects (Buskirk, Iampietro, and Bass 1958; Saltin 1964). Acclimatization to heat does not seem to protect people from the deteriorating effect of hypohydration. The simplest method of determining whether the fluid intake has been adequate is by weighing the individual under standard conditions.

SECTION 4 TEMPERATURE REGULATION  Physiology of Temperature Regulation  Failure to Tolerate Heat  Factors  Factors related to maintain temperature

 Temperature-Sensitive Receptors In the hypothalamus and the adjacent preoptic region, as shown in animal experiments, there are nerve cells which by local heating and cooling elicit the same reactions that occur during exposure to heat or cold (Hammel 1965; Hardy 1967). 1 Physiology of Temperature Regulation

 Thermoregulatory Centers The anterior hypothalamus and the preoptic region are sensitive to changes in the local temperature. 1 Physiology of Temperature Regulation

2 Failure to Tolerate Heat The vastly increasing number of participants in long-distance running races under hot conditions has markedly increased heat casualties and heat illnesses, such as heat exhaustion, heat syncope, and heat stroke, often in combination with dehydration, especially in marathon runners (J.R.Sutton 1984) brain function is particularly vulnerable to heat (M.A.Baker 1982). Tolerance to elevated deep body temperature is extended if the brain is kept cool (Carithers and Seagrave 1976).

2 Failure to Tolerate Heat The most serious consequence of exposure to intense heat is heat stroke, which can be fatal. It is caused by a sudden collapse of temperature regulation leading to a marked increase in body heat content. The rectal temperature can be 41 ℃ or higher, and the skin is hot and dry. Another type of temperature regulation failure is the so-called anhidrotic heat exhaustion.

2 Failure to Tolerate Heat A third type of serious disturbance caused by heat exposure is excessive loss of fluid and salt, usually because of failure to replace fluid and salts lost through sweating. Heat syncope is a less serious affliction caused by heat exposure.

2 Failure to Tolerate Heat A third type of serious disturbance caused by heat exposure is excessive loss of fluid and salt, usually because of failure to replace fluid and salts lost through sweating. Heat syncope is a less serious affliction caused by heat exposure.

 Age 3 Factors It is generally believed that children cannot tolerate hot environments as well as adults and that the greatest risk of heat sickness for children is heat exhaustion The old start to sweat later than do young individuals. Following heat exposure, it takes longer for their for their body temperature to return to normal levels. Older people react with a higher peripheral blood flow, but their maximal capacity is probably lower.

 Sex 3 Factors The available evidence shows that women require lower evaporative cooling both in hot and wet environments and in hot and dry environments (Shapiro, Pandolf, et al. 1981). Women have a lower tissue conductance in cold and a higher tissue conductance in heat than do men. This indicates a greater variation in the peripheral reaction to climatic stress in women. Physical fitness is an important factor when men and women are compared in the heat.

4 Factors related to maintain temperature  Heat Protection Brain function appears to be especially vulnerable to heat (M.A.Baker 1982). The temperature inside the head (i.e., in the brain) does not rise as high as in the rest of the body. The use of helmets can cause heating of the head.

4 Factors related to maintain temperature  Clothing In a moist, hot climate where the temperature of the environment is lower than that of the skin, it is advisable to wear as little clothing as possible. If the ambient temperature is higher than that of the skin, clothing can protect the individual from the radiant heat of the environment. Loose-fitting clothing that permits free circulation of air between the skin and the clothing is preferable. To remain in heat balance, a person sleeping outdoors at -40 ℃ needs protective clothing with an insulation value of about 12 Clo units.

4 Factors related to maintain temperature  Microclimate Preferred environmental temperatures range from 17 ℃ up to 31 ℃, depending on the climate and clothing worn. However, 21 to 24 ℃ represents the comfort zone for a large majority of individuals. Edholm and Weiner (1981) concluded that the most effective environmental temperature for mental effort is 28 ℃ in terms of number of signals missed in a particular set of experiments.

4 Factors related to maintain temperature  Warm-up for Physical Performance It is a relatively old observation that physical performance is improved following warm-up The benefit of the higher temperature during exercise lies in the fact that the metabolic processes in the cell can proceed at a higher rate, because these processes are temperature- dependent. For each degree of temperature increase, the metabolic rate of the cell increases by about 13%. At the higher temperature, the exchange of oxygen from the blood to the tissues is also much more rapid.

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