Thermoregulation Health Article

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 vaso-motor 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 sub-cutaneous 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 36.5°C 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 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.
• And this activates the evaporation of sweat.
• Which 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.
• Which 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.
• And it decreases the evaporation of sweat.
• Which reduces heat loss by evaporation.
• 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.