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08.07.2012.

Body temperature regulation


The stresses of physical exertion often are complicated by environmental thermal conditions. Performing in extreme heat or cold places a heavy burden on the mechanisms that regulate body temperature. Although these mechanisms are amazingly effective in regulating body temperature under normal conditions, mechanisms of thermoregulation can be inadequate when we are subjected to extreme heat or cold. Fortunately, our bodies are able to adapt to such environmental stresses with continued exposure over time, a process known as acclimation(which refers to a short-term adaptation, e.g. days to weeks) or acclimatization(the proper term when we are referring to adaptations gained over long periods of time, e.g. months to years).

Body temperature regulation

Humans are homeothermic, which means that internal body temperature is physiologically regulated to keep it nearly constant even when environmental temperature changes. Although a person’s temperature varies from day to day, and even from hour to hour, these fluctuations are usually no more than about 1.0 °C(1.8 °F). Only during prolonged heavy exercise, fever due to illness, or extreme conditions of heat or cold do body temperatures deviate from the normal range of 36.1 to 37.8 °C(97.0 – 100.0 °F). Body temperatures reflects a careful balance between heat production and heat loss. Whenever this balance is disturbed, body temperature changes.

Metabolic heat production

Only a small part(usually less than 25%) of the energy(adenosine triphosphate – ATP) the body produces is used for physiological functions such as muscle contraction: the rest is converted to heat. All active tissues produce metabolic heat(M) that must be intricately offset by heat loss to the environment to maintain the internal temperature of the body. If the body’s heat production exceeds its heat loss, as it often does during moderate- to heavy-intensity aerobic activity, the internal temperature increases. People’s ability to maintain a constant internal temperature depends on their ability to balance the metabolic heat they produce and the heat they gain from the environment with the heat their body loses.







Transfer of body heat to and from the environment

For the body to transfer heat to the environment, the heat produced in the body must move from deep in the body(the core) to the skin(the shell), where it has access to the outside environment. The heat is moved from the core to the skin by the blood. Only when heat reaches the skin can it be transferred to the environment by any of four mechanisms: conduction, convection, radiation and evaporation.

Conduction and convection

Heat conduction(K) involves the transfer of heat from one solid material to another through direct molecular contact. As an example, heat can be lost from the body when the skin is in contact with a cold object, as when one sits on cold metal bleachers at a ball game. Conversely, if a hot object is pressed against the skin, heat from the object will be conducted to the skin, warming it. If the contact is prolonged, heat from the skin surface can be transferred to the core, raising internal(core) temperature. During exercise, conduction is usually negligible as a source of heat exchange because the body surface area in contact with solid objects(for example, soles of the feet on hot playing fields) is small.
Convection(C), on the other hand, involves moving heat from one place to another by the motion of a gas or a liquid across the heated surface. Although we’re not always aware of it, the air around us is is constant motion. As it circulates around us, passing over the skin, heat is exchanged with the air molecules. The greater the movement of the air(the liquid, such as water), the greater the rate of heat exchange by convection. Thus, in an environment that is cooler than the skin temperature, convection permits the transfer of heat from the skin to the air(heat loss); however, if air temperature is higher than skin temperature, heat is gained by the body through convection. We often consider these processes to be mechanisms of heat loss, forgetting that when the environmental temperature exceeds skin temperature, the gradient works in the opposite direction.
Convection constantly removes metabolically generated heat from the body when the air temperature is lower than the skin temperature. However, if a person is submerged in cold water, the amount of heat dissipated from the body to the water by convection can be nearly 26 times greater than when the person is exposed to a similar cold air temperature.

Radiation

In a body at rest, radiation(R) is the primary method of the discharging the resting body’s excess heat. At normal room temperature(typically 21-25 °C, or 69.8-77 °F), the nude body losses about 60% of its excess heat by radiation. The heat is given off in the form of infrared rays, which are a type of electromagnetic wave. Figure below shows two infrared thermograms of an individual.



The skin constantly radiates heat in all directions to objects around it, such as clothing, furniture, and walls, objects that are warmer. If the temperature of the surrounding objects is greater than that of the skin, the body will experience a net heat gain via radiation. A tremendous amount of radiant heat is received from exposure to the sun.
Taken together, conduction, convection, and radiation are considered avenues of dry heat exchange. Resistance to dry heat exchange is called insulation, a concept known to everyone as it relates to clothing and home heating and cooling. The ideal insulator is a layer of still air(remember that moving air causes convective heat loss), which we achieve by trapping layers of air in fibers(down, fiberglass, etc.). Adding insulation in this way minimizes unwanted heat loss in cold environments. However, during exercise scenarios we want to dissipate heat to the environment, which is best accomplished by wearing light-colored clothing(to limit radiant heat absorption) that allows for maximally exposed skin surface area.

Evaporation

Evaporation(E) is the primary for heat dissipation during exercise. As a fluid evaporates, heat is lost. Evaporation accounts for about 80% of the total heat loss when one is physically active and is therefore an extremely important avenue for heat loss. Even at rest, evaporation accounts for 10% to 20% of body heat loss, since some evaporation accounts for 10% to 20% of body heat loss, since some evaporation occurs without our awareness(insensible water loss).
As body core temperature increases, sweat production increases. As sweat reaches the skin, it is converted from a liquid to a vapor, and heat is lost from the skin in the process. Thus, sweat evaporation becomes increasingly important as body temperature increases.
Evaporation of 1L of sweat in an hour results in the loss of 680W(2,428kJ) of heat. An important concept to remember is that sweat must evaporate for any heat loss to occur. Some sweat drips off the body or stays on the skin or in the clothing. This nonevaporated sweat contributes nothing to body cooling and simply represents a wasteful loss of body water.
Analogous to insulation, which limits dry heat exchange, clothing adds resistance to sweat evaporation. While some cooling of the skin does occur as sweat evaporates from wet clothing surfaces, the cooling power is less than for evaporation directly from skin to air. Clothing that fits loosely and comprises fabrics that promote wicking or free movement of water vapor molecules through the fabric enhance evaporative cooling.
Figure below shows the complex interaction between the mechanisms of body heat balance(production and loss) and environmental conditions. Using the symbols defined in the previous paragraphs, we can represent the state of heat balance by a simple equation:

M – W ± R ± C ± K – E = 0,

where W represents any useful work being performed as a result of muscle contraction. Notice that while R, C, and K can be either positive(heat gain) or negative(heat loss), E can only be negative. When M – W ± R ± C ± K – E > 0, heat is stored in the body and core temperature rises.



Humidity and heat loss

The water vapor pressure of the air(the pressure exerted by water vapor molecules suspended in the air) plays a major role in evaporative heat loss. Relative humidity is a more commonly used term that relates the water vapor pressure of the air that of fully saturated air(100% humidity). When humidity is high, the air already contains many water molecules. This decreases its capacity to accept more water because the vapor pressure gradient between the skin and the air is decreased. Thus, high humidity limits sweat evaporation and heat loss, while low humidity offers an ideal opportunity for sweat evaporation and heat loss. But this efficient cooling mechanism can also pose a problem. If sweating is prolonged without adequate fluid replacement, dehydration can occur.

Thermoregulatory control of heat exchange

We live our entire lives within a very small, fiercely protected range of internal body temperatures. If sweating and evaporation were unlimited, we could withstand extreme ambient heat(e.g., even oven temperatures >200°C for short periods!) if we were protected from contact with hot surfaces. On the other hand, the temperature limits for living cells range from about 0°C(where ice crystals form) to about 45°C(where intracellular proteins start to unravel), and humans can tolerate internal temperatures below 35°C for only brief periods of time. To maintain internal temperature within these limits, we have developed very effective and, in some instances specialized, physiological responses to heat and cold. These responses involve the finely controlled coordination of several body systems.
Internal body temperature at rest is regulated at approximately 37°C(98.6°F). During exercise, the body is often unable to dissipate heat as rapidly as it is produced. In rare circumstances, a person can develop an internal temperature exceeding 40°C(104°F), with a temperature above 42°C(107.6°F) in active muscles. The muscles’ energy systems become more chemically efficient with a small increase in muscle temperature, but internal body temperatures above 40°C can adversely affect the nervous system and reduce further efforts to unload excess heat. How does the body regulate its internal temperature? The hypothalamus plays a central role.



The preoptic-anterior hypothalamus: the body’s thermostat

A simple way to envision the mechanisms that control internal body temperature is to compare them to the thermostat that controls the air temperature in a home, although the body’s mechanisms function in a more complex manner and generally with greater precision than a home heating and cooling system. Sensory receptors called thermoreceptors detect changes in temperature and relay this information to the body’s thermostat, located in a region of the brain called the preoptic-anterior hypothalamus(POAH). In response, the hypothalamus activates mechanisms that regulate the heating or cooling of the body. Like a home thermostat, the hypothalamus has a predetermined temperature, or set point, that it tries to maintain. This is the normal body temperature. The smallest deviation from this set point signals this thermoregulatory center to readjust the body temperature.
Thermoreceptors are located throughout the body but especially in the skin and central nervous system. The peripheral receptors located in the skin monitor the skin temperature, which varies with changes in the temperature around a person. They provide information not only to the POAH but also to the cerebral cortex, which allows one to consciously perceive temperature and voluntarily control one’s exposure to heat or cold. Because the skin temperature changes long before core temperature, these receptors serve as an early warning system for impeding thermal challenges.
Central receptors are located in the hypothalamus, other brain regions, and the spinal cord and monitor the temperature of the blood as it circulates through sensitive areas. These central receptors are responsive to blood temperature changes as small as 0.01°C(0.018°F). Because of this exquisite sensitivity, very small changes in the temperature of the blood passing through the hypothalamus quickly trigger reflexes that help one conserve or eliminate body heat as needed.



Effectors that alter body temperature

When body temperature fluctuates, the normal temperature usually can be restored by the actions of four sets of effectors:
  • Skin arterioles. When skin or core temperature changes, the POAH sends signals via the sympathetic nervous system(SNS) to the smooth muscle in the walls of the arterioles that supply the skin, causing them to dilate or constrict. This either increases or decreases skin blood flow. Skin vasoconstriction results primarily from SNS release of the neurotransmitter norepinephrine, although other neurotransmitters are involved as well, and facilitates heat conservation by minimizing dry heat exchange. Skin vasodilation in response to heat stress is a more complex and less well understood process. Increased skin blood flow aids in heat dissipation to the environment through conduction, convection, and radiation(and indirectly, evaporation as skin temperature goes up). Fine-tuning of skin blood flow is the mechanism by which minute-to-minute adjustments are made in heat balance and exchange. These adjustment are rapid and occur with no real energy cost to the body.
  • Eccrine sweat glands. When either the skin or core temperature is elevated sufficiently, the POAH also sends impulses through the SNS to the eccrine sweat glands, resulting in active secretion of sweat onto the skin surface. The primary neurotransmitter involved is acetylcholine; thus we refer to sweat gland activation as sympathetic cholinergic stimulation. Like skin arterioles, sweat glands are about 10 times more responsive to increases in core temperature than to similar increases in skin temperature. The evaporation of this moisture removes heat from the skin surface.
  • Skeletal muscle. Skeletal muscle is called into action when a person needs to generate more body heat. In a cold environment, thermoreceptors in the skin sense cold and send signals to the hypothalamus. Similarly, whenever the blood temperature decreases, the change is sensed by central thermoreceptors. In response to this combined neural input, the hypothalamus activates the brain centers that control muscle tone. These centers stimulate shivering, which is a rapid, involuntary cycle of contraction and relaxation of skeletal muscles. This increased muscle activity is ideal for generating heat to either maintain or increase the body temperature, because no useful work results from the shivering, only heat production.
  • Endocrine glands. The effects of several hormones cause the cells to increase their metabolic rates. Increased metabolism affects heat balance because it increases heat production. Cooling the body stimulates the release of thyroxine from the thyroid gland. Thyroxine can elevate the metabolic rate throughout the body by more than 100%. Also, recall that epinephrine and norepinephrine(the catecholamines) mimic and enhance the activity of the SNS. Thus, they directly affect the metabolic rate of virtually all body cells.

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