Figure 5 Production of fever. Endogenous pyrogens released from leukocytes in response to exogenous pyrogens released by infectious microorganisms cause the local release of products of arachidonic acid metabolism such as prostaglandin E2 and thromboxanes. In the hypothalamus, these products cause an increase in the set-point temperature. Because the core temperature is then less than the set-point, the integrative center responds by increasing heat production through shivering, decreasing heat loss by cutaneous vasoconstriction and suppressing sweating. The increased heat production and decreased heat loss lead to an increase in core temperature until the new set-point is reached.

arachidonic acid metabolites, including prostaglandin E2 and thromboxanes in many tissues, including the hypothalamus. The latter agents act locally in the hypothalamus to produce the change in the temperature setpoint; for this reason, aspirin and other drugs that block prostaglandin synthesis reduce fever. However, there are indications that the development of fever is of benefit as a normal body defense in combating some infections.

It is of special importance to note the increased vulnerability of children and the elderly to heat-related illnesses. Children produce more heat from metabolism per kg of body weight than adults, and, because their total body mass is smaller, their core temperature can change very rapidly. Infants and toddlers are also unable to regulate their own fluid intake and they may become rapidly dehydrated in a hot environment. Elderly individuals often have an impaired thirst response to dehydration and/or may be subject to medical conditions and medications that interfere with the normal regulation of salt and water balance. Thus, the elderly are more susceptible to heat exhaustion and heat stroke, and over half of the emergency room visits for these conditions in summer hot spells are typically for patients over 65.

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