Thermoregulation in Hot Environments

Maintaining core temperature is a balance between heat production and loss. Heat is produced by muscular exercise, digestion, and cellular processing of glucose. The body absorbs heat from the environment through convection and radiation, especially from sunlight. Heat is lost from the body by radiation, conduction, convection, and vaporization of water in the respiratory passages and on the skin through perspiration. The balance between heat production and heat loss determines the body temperature, a process called thermoregulation (1).

Paradoxically, cold environments can also cause heat gain. The body's defense mechanisms against cold include shivering. Shivering can cause a net heat gain through muscle contractions, hence heat-related injuries can occur in cold weather, especially if accompanied by other causes of heat gain (3).

Factors that can impact the increased production of heat include muscular conditioning, timing and type of food intake, and the factors that impact basal metabolism. These factors include thyroid hormone status, gender, age, race, and the presence of illness. Pregnancy can increase heat production and impact regulation as well. Drugs, such as cocaine or other stimulants, can increase the metabolic rate and create heat. Infections can cause fever that not only creates heat but also decreases the body's ability to adjust for heat (3,4).

A rise in the blood temperature of less than 1°C (2°F) activates peripheral and brain heat receptors that signal the hypothalamus thermoregulatory center. In turn, the afferent signals from this center increase the delivery of heated blood to the surface of the body. These signals activate nerve endings that act on blood vessel smooth muscles to cause vasodilatation that increases blood flow in the skin by up to 8 L per minute. An increase in the blood temperature also initiates perspiration. If the air surrounding the surface of the body is not saturated with water, perspiration will evaporate and cool the body surface. At maximal efficiency in a dry environment, sweating can dissipate about 600 kcal of heat per hour. The evaporation of sweat is critical for the transfer of heat from the body to the environment. An elevated blood temperature also causes tachycardia, increased cardiac output, and increased respiratory rate. As blood flow is transferred from the core circulation to the muscles and skin to facilitate heat dissipation, blood flow to the internal organs is reduced, particularly in the intestines and kidneys (4).

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