Physiology of thermoregulation

Normal oral temperature has been demonstrated to be 33.2–38.2 °C [2]. Rectal temperature averages 34.4–37.8 °C, while the normal tympanic temperature is 35.4–37.8 °C.

The human body generates heat through metabolism. Basal metabolic rate is a measure of the number of calories expended at rest while sedentary. The maintenance of body temperature to a narrow window, or thermoregulation, is a complex task controlled by the hypothalamus. The body’s extremities have a greater variation in temperature, dependent on environmental factors, including clothing. Generally, the extremities tend to be cooler than the rest of the body, while the core temperature fluctuates very little.

While metabolism of the various body tissues generates heat, the majority of body heat comes from skeletal muscle activity. Endocrine function can also affect metabolic rate and increase temperature. For example, epinephrine and norepinephrine can increase basal metabolic rate and subsequent heat generation. Similarly, thyroid hormones can produce an elevation of metabolic rate as well as an increase in body temperature.

Body temperature homeostasis occurs with heat generation in balance with heat dissipation. The narrow homeostatic range prevents enzymatic and cellular dysfunction or injury. Several reflexes or semi-reflexes help to maintain temperature. The reflexes or semi-reflexes activated by cold include shivering, hunger, increased voluntary activity, curling up, decreased heat loss, cutaneous vasoconstriction, epinephrine and norepinephrine release, and erection of the short body airs (i.e. “goose bumps”). The reflexes or semi-reflexes activated by heat include cutaneous vasodilation, sweating, decreased voluntary movement, anorexia, decreased heat production, and increased respiration. The posterior hypothalamus controls the reflexes activated by cold, while those for warmth are located in the anterior hypothalamus. Sweating and cutaneous vasodilation occur with activation of the anterior hypothalamus [3]. Sensors in the spinal cord, skin, deep tissues, hypothalamus, and extrahypothalamic regions of the brain provide feedback to the hypothalamus on body temperature [3]. The hypothalamus is activated by core temperature increases of less than 1 ºC [4]. Lesions of the anterior area of the hypothalamus cause hyperthermia.

Heat is dispersed from the body by several different mechanisms including conduction, convection, evaporation, and radiation. Heat loss by conduction occurs through direct contact with an object or environment that is cooler. Skin temperature greatly affects the amount of body heat lost or gained through conduction. The amount of heat dispersed from the body’s core to the skin is reliant on cutaneous blood flow. When the cutaneous vessels dilate, more blood flows to the skin, allowing for greater heat transfer from the deeper tissues (tissue conductance). Horripilation is the erection of the cutaneous hairs which helps to trap air near the skin and inhibit heat transfer from the skin. Clothing also inhibits tissue conductance by limiting transfer of heat from the skin to the environment. Convection refers to the removal of heat as cooler air passes over exposed skin. The more air passes over the skin (e.g. from a fan), the more heat can be dispersed. Evaporation is the heat lost via converting a liquid to a gas. In the human body, about 600 kcal/hour in ideal conditions can be removed through evaporation [5]. Radiation is the transfer of heat by infrared electromagnetic waves. Approximately 250–300 kcal/hour can be transferred to the human body by solar radiation, with clothing acting as a barrier and providing some protection, estimated at 100 kcal/hour [6]. Additionally, the body disperses heat by radiating to cool objects in the vicinity [3].