At different environmental conditions, the internal parts of the body are maintained at a constant temperature. These internal parts are together referred to as the body core. A layer of fatty tissue under the skin surrounds the core, fat is a good insulator of heat and slows down heat exchange between the core and the environment. In other words, the fat layer acts like a blanket around the body. Humans are endothermic and strive to maintain a constant body temperature regardless of the temperature of the environment. The optimum temperature for chemical reactions to take place in the body is 37 degrees.

Above this temperature many body enzymes become denatured and chemical reactions cannot take place. Keeping the body temperature relatively constant is a classic example of homeostasis. Body temperature is controlled by balancing heat production against heat loss to the environment (Rasch 590-95). Body temperature averages 36.

2 degrees and is usually maintained within a narrow range of 35. 6-37. 8 degrees, despite considerable change in external temperature. A healthy individual's body temperature fluctuates approximately 1 degree in 24 hours.

The adaptive value of this precise temperature homeostasis becomes obvious when we consider the effect of temperature on the rate of biochemical reactions, specifically enzyme activity. At normal body temperature, conditions are optimal for enzymatic activity. As body temperature rises, catalysis is accelerated: With each rise of 1 degree, the rate of chemical reactions is about 10% faster. As the temperature spikes upward beyond the homeostatic range, neurons are depressed and proteins begin to denature (Rasch 590-95) Most heat produced in the body is generated in deep organs such as the liver, brain, and heart, as well as in skeletal muscles.

The heat energy transfer from the core of the body to skin. The skin controls the heat loss to the environment. Human body temperature is regulated in a special area of the brain called the hypothalamus and the nerve cells within the hypothalamus are called the core temperature sensors. The hypothalamus contains a number of both heat sensitive and cold sensitive neurons. These receptors respond to the temperature of arterial blood flowing through the region of the brain. Heat loss from the body may occur by four processed: (1) radiation, (2) conduction, (3) convection, and / or (4) evaporation.

The first three of these heat loss mechanisms require a temperature gradient to exist between the skin and the environment. Radiation is heat loss in the form of infrared rays. This involves the transfer of heat from the surface of one object to the surface of another, with no physical contact being involved. At rest in a comfortable environment, 60% of the heat loss occurs via radiation. This is possible because skin temperature is greater than the temperature of surrounding objects, and a net loss of body hat occurs due to the thermal gradient. Note that on a hot, sunny day when surface temperatures are greater than skin temperature, the body can also gain heat via radiation (Nielson 452-56).

Therefore, it is important to remember that radiation is heat transfer by infrared rays and can result in either heat loss or heat gain depending on the environmental conditions. Conduction is defined as the transfer of heat from the body into molecules of cooler objects in contact with its surface. In general, the body loses only small amounts of heat due to this process. An example from the body to a metal chair while a person is sitting on it. The heat loss occurs as long as the chair is cooler than the body surface in contact with it. Convection is a form of conductive heat loss in which heat is transmitted to either air or water molecules in contact with the body.

In convective heat loss, air or water molecules are warmed and move away from the source of heat and are replaced by cooler molecules. An example of forced convection is a fan moving large quantities of air past the skin; this would increase the number of air molecules coming in contact with the skin and thus promote heat loss. Practically speaking, the amount of heat loss due to convection is dependent on the airflow over the skin. Therefore, under the same wind conditions, cycling at high speeds would improve convective cooling when compared to cycling at slow speeds or running. Swimming in cool water also results in convective heat loss. In fact, waters effectiveness in cooling is about 25 times greater than that of air at the same temperature.

The final means of heat loss is evaporation. Evaporation accounts for approximately 25% of the heat loss at rear, but under most environmental conditions it is the most important means of heat loss during exercise (Nadel 125-52). In evaporation, heat is transferred from the body to water on the surface of the skin. When this water gains sufficient heat, it is converted to a gas, taking the heat away from the body.

Not that evaporation occurs due to a vapour pressure is the pressure exerted by water molecules that have been converted to gas. Evaporative cooling during exercise occurs in the following way. When body temperature rises above normal, the nervous system stimulates sweat glands to secrete sweat onto the surface of the skin. As sweat evaporates, heat is lost to the environment, which in turn lowers skin temperature.

Evaporation of sweat from the skin is dependent on three factors: (1) the temperature and relative humidity, (2) the convective currents around the body, and (3) the amount of skin surface exposed to the environment. At high environmental temperatures, relative humidity is the most important factor by far in determining the rate of evaporative heat loss, High relative humidity reduces the rate of evaporation, In fact, when the relative humidity is near 100%, evaporation is limited. Therefore, cooling by way of evaporation is most effective under conditions of low humidity (Nadel 4-6). An increase in core temperature above the set point results in the hypothalamus initiating a series of physiological actions aimed at increasing the amount of heat loss (Johnson 59-98).

First, the hypothalamus stimulates the sweat glands, which results in an increase in evaporation of heat loss. In addition, the vasomotor control centre withdraws the normal vasoconstrictor tone to the skin, promoting increased skin blood flow and therefore allowing increased heat loss. When core temperature returns too moral, the stimulus to promote both sweating and vasodilatation is removed. This is an example of a control system using "Negative feedback." During sub maximal constant-load exercise in a cool / moderate environment, heat production increases due to muscular contraction and is directly proportional to the exercise intensity. The venous blood draining the exercising muscle distributes the excess heat throughout the body core.

As core temperature increases, thermal sensors in eh hypothalamus sense the increase in blood temperature, and the thermal integration centre in the hypothalamus compares this increase in temperature with the set point temperature and finds a difference between the two. The response is to direct the nervous system to commence sweating and to increase body flow to the skin. These acts serve to increase in body temperature. At this point, the internal temperature reaches a new, elevated steady state core temperature does not represent a change in the set-point temperature, as occurs in fever. Instead, the thermal regulatory centre attempts to return the core temperature back to resting levels, but I incapable of doing so in the face of the sustained heat production associated with exercise (Savage 1249-1257). Heat production increases in proportion to the exercise intensity.

Also there is a linearly increase in energy output, heat production, and total heat loss as a function of exercise work rate. Further, convective and radiative heat loss does not increase as a function of work rate. This is due to a relatively constant temperature gradient between the skin and the environment. In contrast, there is a consistent rise in evaporative heat loss with increments in exercise intensity.

This emphasizes the point that evaporation is the primary means of losing heat during exercise. If the athlete has dressed as cool as the sport will allow, and he has allowed for maximum evaporation, convection, conduction, and radiation of heat away from the body and is still experiencing heat stress, the next step is to become acclimated. The body is capable of adjusting to this stress through repeated exposure to the heat. Acclimatization is usually best accomplished by progressively increasing the level of heat of intensity of work done in the hot environment (Strydom 636). Part of this is mental adjustment, but the major portions are a true physiological adjustment by which the body adjusts itself to cool more effectively. Most athletes will require between 4-10 days of exposure to heat for acclimatization (Wyndham 1586).

Acclimatization is best accomplished by progressively increasing amounts of work to be accomplished by progressively increasing amounts of work to be accomplished each day in the heat stress situation. An athlete is heat acclimatized when he can perform the required work, such as running for that hour and maintains his body temperature within 1 degree of its normal range. The primary adoptions that occur during heat acclimation are and increase plasma volume, earlier onset of sweating, higher sweat rate, reduced salt loss in sweat, a reduced skin blood flow, and increased synthesis of heat shock proteins. Depending on the climatic conditions, the relative contributions of evaporative and dry heat exchange to the total hat loss will vary. The hotter the climate, the greater the dependence on evaporative heat loss and, thus, on sweating. Therefore, a substantial volume of body water may be lost via sweating to enable evaporative cooling.

Generally, the individual dehydrates during exercise because of fluid non-availability or a mismatch between thirst and body water requirements. In these instances, the individual starts the exercise task as dehydrated but incurs an exercise-heat mediated dehydration over a prolonged period of time. A person's sweating rate is dependent on the climatic conditions, clothing worn, and exercise intensity. Athletes performing height-intensity exercise commonly have sweating rates of 1. 0-2.

5 L per hour while in the heat. Fluid requirements will vary in relation to climatic heat stress, clothing worn, acclimation state, and physical activity levels. Daily fluid requirements might range from 2-4 L per day in temperate climates. Electrolytes, primarily sodium chloride and o a lesser extent potassium, are lost in sweat. During exercise-heat stress, a principal problem is to avoid dehydration by matching fluid consumption to seat loss. Since thirst provides a poor index of body water needs, person will dehydrate by 2-8 percent of their body weight during exercise of prolonged sweat loss.

Hypohydration increases core temperature responses during exercise in temperate and hot climates. A critical deficit of 1 percent of body weight elevates core temperature during exercise. As the magnitude of water deficit increases, there is a concomitant graded elevation of core temperature during exercise heat stress. The magnitude of core temperature elevation ranges from 0. 10 to 0. 23 degrees for every percent body weight lost, and this elevation is greater during exercise in hot than in temperate climates.

Hypohydration not only elevates core temperate response, but it also negates the core temperature advantages conferred by high-aerobic fitness and heat acclimation. Therefore, heat-acclimated person who does not drink adequately may more rapidly experience the adverse effects of hypo hydration than their no acclimated counterparts. Hypohydration impairs both dry and evaporative heat loss. Hypohydratin delays sweating onset and skin vasodilatation. It also reduces sweating sensitivity.

Hypohydratin may be associated with either reduced or unchanged sweating rates at a given metabolic rate in the heat. The physiological mechanisms mediating the reduced dry and evaporated heat loss from hypohydratin in clude both the separate and combined effects of plasma hyperosmolality and reduced blood volume (Morris 677-86). Hyper hydration, or greater than normal body water, had been suggested to improve, above eu hydration levels, thermoregulation and exercise-heat performance. The concept that hyper hydration might be beneficial for exercise performance arose from the adverse consequences of hypo hydration. It was theorized that body water expansion might reduce the cardiovascular and thermal strain of exercise by expanding blood volume and reducing blood tonicity, thereby improving exercise performance. Studies that have directly expanded blood volume have usually reported decreased cardiovascular strain during exercise, but have reported disparate results on heat dissipation and exercise-heat performance.

Studies that have attenuated plasma hyperosmolality during exercise heat stress generally report improved heat dissipation, but have not addressed exercise performance. Ten studies have been published that evaluated hyper hydration effects on thermoregulation in the heat. Briefly, 6 of 10 studies observed smaller core temperature increases during exercise with hyper hydration. Tougher, these studies support the notion that hyper hydration can provide a thermoregulatory benefit (Morris 677-86).

The human body is a marvellous collection of systems, all working to keep us alive and in a stable and healthy state. The thermoregulatory system is just one of the many systems we studied. Exercise in the heat can be one of the most severe physiological stressor someone faces. This is why we have such a complex mechanism for maintaining the core body temperature under stressors, like the increasing of normal temperature during a run.

I discussed many mechanisms to help maintain normal body temperature such as radiation, convection, conduction, and evaporation that help maintain homeostasis in our bodies, and this is important for life and death matters.