warmer than optimal Impulses from this area increase and are conducted through autonomic pathways to the spinal cord and then, through cholinergic fibers to the sweat glands, where sweat is released Exercise and certain emotional states release circulating epinephrine and norepinephrine to increase sweat production A 0.6°C increase in core temperature causes a 10% elevation in basal metabolic rate There are four different ways for the body to reduce excess heat: convection, conduction, radiation, and evaporation, with the latter being the most important thermoregulatory mechanism Clinical Recognition Three types of heat illnesses are recognized and represent different physiologic disturbances ( Table 90.4 ) Heat cramps refer to the sudden onset of brief, intermittent, and excruciating cramps in muscles after they have been subjected to severe work stress Cramps tend to occur after the work is done, on relaxing, or on taking a cool shower Occasionally, abdominal muscle cramps may simulate an acute abdomen The usual victim is highly conditioned and acclimatized Typically, these individuals can produce sweat in large quantities and provide themselves with adequate fluid replacement but inadequate salt replacement Electrolyte depletion is probably the cause of heat cramps Most spasms last less than a minute, but some persist for several minutes, during which a rock-hard mass may be palpated in the affected muscle Cramps often occur in clusters Rapid voluntary contraction of a muscle, contact with cold air or water, or passive extension of a flexed limb may reproduce a cramp Laboratory investigation reveals hyponatremia and hypochloremia and virtually absent urine sodium The blood urea nitrogen (BUN) level is usually normal but may be mildly elevated Heat exhaustion is less clearly demarcated from heat stroke than are heat cramps There are two types of heat exhaustion with significant overlap: water depleted and sodium depleted In most cases, water depletion predominates because individuals who live and work in a hot environment not always voluntarily replace their total water deficit Progressive lethargy, intense thirst, and inability to work or play progress to headache, vomiting, CNS dysfunction (including hyperventilation, paresthesias, agitation, incoordination, or actual psychosis), hypotension, and tachycardia Hemoconcentration, hypernatremia, hyperchloremia, and urinary concentration are typical Body temperature may rise but rarely to higher than 39°C (102.2°F) If unattended, heat exhaustion may progress to frank heat stroke Heat exhaustion may also occur because of predominant salt depletion As in heat cramps, water losses are replaced but without adequate electrolyte