Thermoregulation

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Thermoregulation

Definition

Thermoregulation refers to the mechanisms and control systems used by the body to balance thermal inputs and thermal losses so as to maintain its core temperature nearly constant.

Description

In a healthy individual, the temperature of the core of the body is regulated by feedback control mechanisms that maintain it nearly constant around 98.6°F (37°C) throughout the day, week, month, or year. This thermoregulation is efficiently coordinated by the central nervous system (CNS) as long as the temperature of the surroundings ranges between 68°F (20°C) and 130°F (54°C).

The body increases and lowers its core temperature using a temperature control system that works like a thermostat. Increased body temperature activates mechanisms promoting heat loss, and lowered body temperature activates mechanisms enabling the accumulation or production of heat. Such a system is called a feedback control system, because it uses as input the total or partial output of the system, meaning that the consequences of the process dictate how it will go on further. A feedback system has three components: sensors that register the change, a control center that receives the signals of the sensors, and an effector mechanism, meaning a pathway for the commands of the control center when it responds to the information received from the sensors. In thermoregulation, the control center is located in the hypothalamus, a tiny cluster of brain cells located in the brain just above the pituitary gland. It also contains the key temperature sensors. Other sensors, located all over the body, record whether the body temperature is too high or too low. There are three main effector mechanisms involved in thermoregulation. The first is the vasomotor system, which consists of the nerves that act on vascular smooth muscle to control blood vessel diameter; the second is provided by metabolic effectors, which are substances produced by the body to increase its activity. The third main effector mechanism is provided by the sweat glands. The vasomotor system is responsible for two physiological responses called vasodilation and vasoconstriction. The first increases blood flow in the tissues and the second decreases it.

Heat production, also called thermogenesis, is the result of several different body functions. One of them is the action of the thyroid gland, located in the neck. Hormones released by this gland increase the body's metabolism, meaning the activity of the body. Increased production of heat is thus achieved by increasing the metabolic processes in which energy is released in the form of heat. Other producers of heat are the skeletal muscles, the liver, the internal organs, and the brain. Muscles play a major role in thermogenesis. Because of their weight, they are able to produce very large amounts of heat very rapidly during increased physical activity. Digestion also results in an increased production of heat.

Heat is lost from the body in four different ways: by conduction, convection, evaporation, and radiation. Heat loss by conduction occurs because there is a gradient between the body temperature and the temperature of the surrounding environment. When the external temperature is lower, heat flows from the body to the colder external environment. The body also loses heat by evaporation, mainly through sweating. This mechanism occurs especially during phases of increased heat production, for example during physical exercise. The sweat glands are controlled by cholinergic impulses through the sympathetic nerve fibers. During intensive sweating, up to one liter of sweat may be formed. When the humidity of the environment is higher, heat loss through sweating is easier. When the body needs to accumulate heat, adrenergic impulses restrict the blood flow through the skin, with the result that the skin becomes an insulator, thus decreasing heat loss to a minimum. The body can also lose heat by convection, through the circulatory system. With this mechanism, heat flows from each cell to the surrounding extracellular fluid (ECF) and afterwards to the circulating blood. Heat loss is modulated by the amount of blood that circulates through the body surface. The high flow occurring through the subcutaneous area and the skin transfers the heat carried by the blood to the environment through the body surface. Finally, the body can lose heat by simply radiating it away.

Several conditions can influence body temperature, such as exercise, the time of day, the environmental temperature, digestion and the level of water consumption. For example, body temperature varies in the narrow range between 97.7° and 99.5°F (36.5° and 37.5°C). It slightly increases during the day, reaching a peak between 6:00 to 10:00 p.m. and a low between 2:00 and 4:00 a.m. This diurnal variation depends on the body activity throughout the day. Diurnal variations do not change in persons that work at night and sleep during the day and they also occur when fever is present. Fever reaches a peak in the evening, and decreases during the night so that, in the morning, even a very sick person may have an almost normal temperature. Body temperature changes are also more intensive in young people than in older people. Physical activity also increases body temperature, in some cases very significantly. For example, the average body temperature of marathon runners may increase to 102.2-105.8°F (39-41°C). The feedback control system responsible for thermoregulation is very complex, but overall, it can be summarized as follows:

When the surroundings are hot or when the body is vigorously exercising:

The body core temperature starts to rise.
This increase in temperature is detected by heat sensors in the body.
These sensors send signals to the CNS.
The CNS stimulates the sweat glands.
This increases the production of sweat.
This activates the evaporation of sweat.
Evaporation promotes heat loss by evaporation.
The CNS also signals the vasomotor system to dilate the capillaries underlying the skin.
Vasodilation occurs and the capillaries become larger.
More blood flows underneath the skin surface.
Blood flow promotes heat loss by conduction, radiation, and convection.
The body core temperature returns to normal.

When the surroundings are cold or when the body is resting:

The body core temperature starts to drop.
This is detected by cold sensors in the body.
These sensors send signals to the CNS.
The CNS slows down the activity of the sweat glands.
This lowers the production of sweat.
Lowered production of sweat decreases the evaporation of sweat.
Lowered rates of evaporation reduces heat loss.
The CNS also signals the vasomotor system to constrict the capillaries underlying the skin.
Vasoconstriction occurs and the capillaries become narrower.
Less blood flows underneath the skin surface.
This reduces heat loss by conduction, radiation, and convection.
The body core temperature returns to normal.

Function

The major function of thermoregulation is to help maintain homeostasis, meaning the stability of the body's internal environment. A wide variety of body systems and organs interact to maintain the body's

KEY TERMS

Acetylcholine— Neurotransmitter produced by an enzyme in the body that stimulates muscle tissue.

Adrenaline— A hormone produced by the adrenal medulla that causes vasodilation of the small arteries in muscle and increases cardiac output.

Adrenergic— Substance that has an effect similar to that of adrenaline.

Antagonist— A substance that cancels or counteracts the action of another.

Capillaries— The smallest blood vessels of the body.

Central nervous system (CNS)— One of two major divisions of the nervous system. The CNS consists of the brain, the cranial nerves and the spinal cord.

Cholinergic— Substance that has an effect similar to that of acetylcholine.

Conduction— Heat transfer by means of molecular agitation within a material without any motion of the material as a whole. If one end of a metal piece is at a higher temperature, then heat will be transferred down the piece toward the colder end.

Convection— Heat transfer by motion of a fluid when the heated fluid is caused to move away from the source of heat, carrying energy with it.

Dermis— Layer of connective tissue underlying the skin. Contains smooth muscle tissue, nervous tissue and blood vessels.

Endocrine glands— Glands secreting substances that are released directly into the bloodstream and that regulate metabolism and other body functions.

Endocrine system— The system of glands in the body that secretes hormones directly into the circulatory system.

Enzyme— A type of protein produced by the body that speeds up chemical reactions. Some enzymes regulate certain functions due to their ability to change their activity by modifying their structure.

Extracellular fluid (ECF)— The fluid found outside of the cells and between the cells in body tissues.

Feedback system— A feedback system uses as input the total or partial output of the system. Feedback systems are used to control and regulate processes. They use the consequences of the process (for example, too much or too little produced) to regulate the rate at which the process occurs (decrease or increase the rate of the process).

Homeostasis— Stability of the body's internal environment, achieved by a system of integrated control systems activated by feedback systems. Homeostasis is thus the maintenance of a constant internal environment (the immediate surroundings of cells) in response to changes occurring in the conditions of the external environment and the internal body environment.

Hormone— A naturally occurring substance secreted by specialized cells that affects the metabolism or behavior of other cells possessing receptors for the hormone.

Hypothalamus— The hypothalamus is a tiny cluster of brain cells just above the pituitary gland that is involved in the regulation of body temperature.

Metabolic effectors— Substances, such as hormones, that can increase the metabolism of the body or of a target organ.

Metabolism— The sum of all the physical and biochemical processes occurring in the body to produce what is required to maintain life. This includes the transformation of nutrients into energy and the use of energy by the body.

Nervous system— The entire system of nerve tissue in the body. It includes the brain, the brainstem, the spinal cord, the nerves and the ganglia and is divided into the peripheral nervous system (PNS) and the central nervous system (CNS).

Peripheral nervous system (PNS)— One of the two major divisions of the nervous system. The PNS consists of the somatic nervous system (SNS), which controls voluntary activities, and of the autonomic nervous system (ANS), which controls regulatory activities. The ANS is further divided into sympathetic and parasympathetic systems.

Radiation— Heat transfer that occurs by the emission of electromagnetic waves which carry energy away from the emitting object.

Thermogenesis— Production of heat.

Thermoregulation— Regulation of body temperature so as to maintain it nearly constant at 98.6°F (37° C).

Thyroid gland— A butterfly-shaped endocrine gland located in the neck on both sides of the windpipe. It controls the rate at which the body produces energy from nutrients. It secretes the hormones triiodothyronine (T3) and thyroxine (T4) which increase the rate of metabolism and cardiac output.

Vasoconstriction— The decrease in the internal diameter of a blood vessel resulting from tightening the smooth muscle located in the walls of the vessel. Vasoconstriction decreases the blood flow.

Vasodilation— The increase in the internal diameter of a blood vessel resulting from relaxation of the smooth muscle located in the walls of the vessel. Vasodilation increases the blood flow.

Vasomotor system— The neural systems which act on vascular smooth muscle to control blood vessel diameter.

internal environment (the immediate surroundings of cells) constant in response to changes that occur either in the conditions of the external environment or in the conditions of the internal body environment. Thermoregulation is one of these essential homeostatic mechanisms.

Role in human health

Thermoregulation is of the utmost importance in maintaining health, because human life is only compatible with a narrow range of temperatures. Core temperature changes of the order of 6°F (3°C) will not interfere with physiological functions, but any variation outside that range has very serious effects. For example, at 82.4°F (28°C), the muscles can no longer respond; at 86°F (30°C), confusion occurs and the body can no longer control its temperature; at 91.4°F (33°C), loss of consciousness occurs; at 107.6°F (42°C), the CNS breaks down with irreversible brain damage; and at 111.2°F (44°C), death occurs, the result of the body proteins starting to denature.

Common diseases and disorders

Fever—Increase in body core temperature. Fever is not an illness but a natural reaction to a number of illnesses.
Hyperthermia—Overheating of the body caused only by an external factor, as for example a hot environment, or a hot bath.
Hypothermia—A low body temperature, as caused by exposure to cold weather or a state of low temperature of the body induced by decreased metabolism.
Hypothyroidism—Hypothyroidism refers to a condition in which the amount of thyroid hormones in the body is below normal. Since the thyroid hormones are important in thermoregulation, hypothyroidism affects the body's capacity to control temperature.

Resources

BOOKS

Imber, G. Body Temperature. Scranton, PA: William Morrow & Co, 2001.

Jessen, K. Temperature Regulation in Humans and Other Mammals. New York: Springer Verlag, 2001.

PERIODICALS

Boulant, J. A. "Role of the preoptic-anterior hypothalamus in thermoregulation and fever." Clinical Infectious Diseases 31: Suppl. 5 (October 2000): S157-S161.

Febbraio, M. A.; R. Wallis; D. A. Schoeller; J. Nedergaard; V. Golozoubova; A. Matthias; A. Asadi; A. Jacobsson; B. Cannon; T. B. VanItallie. "Alterations in energy metabolism during exercise and heat stress." Sports Medicine 31 (2001): 47-59.

Sawka, M. N.; E. F. Coyle. "Influence of body water and blood volume on thermoregulation and exercise performance in the heat." Exercise & Sport Science Review 27 (1999): 167-218.

Schoeller, D. A.; J. Nedergaard; V. Golozoubova; A. Matthias; A. Asadi; A. Jacobsson; B. Cannon; T. B. VanItallie. "The importance of clinical research: the role of thermogenesis in human obesity." American Journal of Clinical Nutrition 73 (March 2001): 511-516.