The human body, like all mammals, generate its own internal heat. Despite large fluctuations in the temperature of the environment, the body’s temperature is maintained within a narrow range in order to optimize cell and organ function. Although it may vary slightly between individuals, the body’s core temperature averages about 37°C.
If the body maintains a constant temperature, why do we shiver uncontrollably when we are extremely cold, and sweat profusely in extremely hot conditions?
In cold conditions the body can increase its temperature by decreasing heat loss, and/or by increasing heat production. Heat loss is decreased by minimizing heat loss to the environment through vasoconstriction of the arterioles in the skin. (An arteriole is a small-diameter blood vessel that branches out from an artery and leads to capillaries.) Constriction of the arterioles reduces blood flow and leads to a reduction in heat transfer from the skin to the environment. The body increases heat production by involuntary contractions and relaxation of skeletal muscles (shivering) – as muscle movement generates heat through an increased metabolic rate.
In hot conditions the body temperature can be reduced by increasing heat loss, and/or by decreasing heat production. Heat loss is increased through vasodilation (opening) of the arterioles in the skin, which increases blood flow and results in an increase in heat transfer from the blood to the skin and into the environment. At the same time sweat glands in the skin increase the production of sweat in order to lose heat through evaporation. Heat production is basically reduced by a reduction in voluntary movement.
Body temperature is regulated by the body’s main frame computer – the brain – through various feedback and control mechanisms. Temperature sensitive sensors are located in the skin (peripheral thermoreceptors that provide information about environmental temperature) and in the hypothalamus in the brain (central thermoreceptors that sense core temperature). The temperature control center is also situated in the hypothalamus. The control center regulates body temperature through interaction with the sweat glands; the arterioles of the skin; the skeletal muscle; and to a minor degree by the adrenal medulla part of the adrenal glands, which releases adrenaline and noradrenaline under conditions such as anxiety, pain, upright posture, exercise, and cold.
The hormones produced by the thyroid gland, and the physiologically active thyroid hormone, T3 in particular, play a vital role in body temperature. The T3 initiates a cascade of chemical reactions within each cell, which affects each cell’s metabolic rate. The metabolic rate of the cells determines the metabolic rate of the body, which, together with the surface area, activity level, sweating, environmental conditions, and other factors, determines the body’s temperature.
Shivering or sweating with fever: Fever is characterized by an increase in body temperature, usually due to the body fighting an infection. The body temperature temporarily increases to a new plateau, resulting in an increase in the hypothalamic set point for temperature in the control center of the hypothalamus. When the body temperature is lower than the new hypothalamic set point, the brain then activates the body’s mechanisms to increase body temperature, such as involuntary shivering and a tendency to curl up in order to minimize heat loss. When the fever “breaks” and the hypothalamic set point returns to normal, the body dissipates excess heat through profuse sweating during recovery from the acute phase of an infection.
Sources:
Control of body temperature. Chapter 70. Medical Physiology. A systems approach. (Physiology Handbook). By Hershell Raff, Michael Levitzky, et al.
Arteriole. Information last edited 27 November 2018. Wikipedia. (www.wikipedia.org)
Background: Function of the adrenal glands. Patient Education Site. The American Association of Endocrine Surgeons. (www.endocrinediseases.org)
HEALTH INSIGHT
January 2019