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Key Points (continued)

radiation. Sympathetic cholinergic nerves regulate sweat production by the cutaneous eccrine glands and regulate heat loss by evaporation.

• Changes in body temperature are sensed by receptors in the preoptic anterior hypothalamus, spinal cord, abdomen, skin, and other organs. Information from these receptors is integrated in the hypothalamus, which compares the temperature sensed to a desired setpoint. When body temperature deviates from the set-point, the hypothalamus integrates appropriate responses to conserve or dissipate heat.

• Heat pyrexia (heat stroke) occurs when body temperature exceeds ^40°C. At this point, hypothalamic regulation ceases and body temperature rises uncontrollably.

• Heat exhaustion occurs after excessive loss of salt and water due to sweating, and/or when venous return is compromised by a combination of heavy exertion and maximal cutaneous vasodilation. It can be accompanied by heat cramps and circulatory collapse, although the body temperature is not necessarily significantly elevated.

• Hypothermia results in a general slowing of metabolism and of cardiac and neural excitability. The thermoregulatory ability of the hypothalamic integrative center is lost when the body core temperature falls below ~30°C.

• Fever is most often seen in the presence of infection. The body's immune cells react to exogenous pyrogens from the infecting organisms by releasing cytokines that act as endogenous pyrogens. These endogenous pyrogens cause the release of arachidonic acid metabolites such as prostaglandin E2 in the tissues, including the hypothalamus. These autacoids have an effect equivalent to raising the temperature set-point. The response is chills, shivering, cutaneous vasodilation, and increased heat production, often followed in cycles by sweating and cutaneous vasodilation.

The temperature of the human body is maintained within 0.6°C (1°F) of its normal value of 37°C (98.6°F) over a relatively wide range of environmental temperatures and during activity and rest. Body temperature is regulated in this range by means of homeostatic feedback mechanisms that are analogous to those that maintain the balance between the intake and output of solutes and water; however, in this case it is the body temperature rather than a solute concentration or the volume of a fluid compartment that is sensed and regulated. Also, in analogy to the mass-balance relationships that establish the constancy of an electrolyte concentration, the constancy of body temperature depends on a balance of input, output, and production, but, in the case of temperature, the balance is between the input, output, and production of heat, rather than mass. Body temperature is determined by the total quantity of heat in the body and the thermal capacity of the body, which is the rate at which the body temperature rises or falls as its heat content increases or decreases. The thermal capacity varies little from person to person, and it is slightly less than 1°C for every kcal of heat lost or gained per kilogram of body weight. In other words, if a 70-kg man were to retain 70 kcal of heat energy, his body temperature would rise by almost 1°C; if he lost that much heat, it would fall by almost 1° C. This is a comparatively high heat capacity and is due to the fact that 60 to 70% of the body mass is water, which has a very high heat capacity.

Nevertheless, it can be seen that even the metabolic production of heat at rest (70 to 80 kcal/hr) would rapidly raise the body temperature to lethal levels if it were not for the ability of the body to dissipate this heat to the environment.

Because the thermal capacity is fixed, body temperature is directly proportional to the total body heat content, i.e., is the total quantity of heat in the body. As long as this quantity remains constant, the body temperature remains constant; however, this constancy requires a balance of heat input, loss, and production, as shown in Fig. 1. Heat is continually produced by the exothermic biochemical reactions that take place in all

Heat Production

From Metabolism

Heat Input

Total Body Heat Content

(Temperature)

Heat Output

Figure 1 Balance of heat input, output, and production. The total body heat content directly determines the body temperature. Heat is constantly being produced in the body by metabolism. To maintain a constant body temperature, this heat production must be matched by a net loss to the environment. This is accomplished by altering heat input from and output to the environment.

body cells, and heat can be both lost and gained by exchange with the environment. The balance that maintains a constant total body heat content can be expressed by the relation:

Heat Loss = Heat Input + Heat Production (1)

This equation is completely analogous to the equation for mass balance (Eq. [2] in Chapter 25). The only difference is that the entity that is taken in, produced, or lost by the body is heat energy rather than matter. A constant body temperature is maintained by balancing the rate of heat loss from the body so that it is precisely equal to the sum of the metabolic heat production and any heat input from the environment. Heat exchange with the environment can result in either loss or gain, and both can occur simultaneously. For example, the body may be heated by the visible and infrared radiation from the sun at the same time as it is being cooled by a cold wind. However, because the body is constantly producing heat, even at complete rest (because of the basal metabolic rate), heat loss from the body must always exceed heat input if body temperature is to remain constant. In other words, there must always be a net heat loss to the environment that matches the metabolic heat production.

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