Thermoregulation, and the bodily system that performs this function, is the maintenance of a consistent internal body temperature, even when there are significant fluctuations in the external environmental temperature. The thermoregulatory system operates within two general boundaries: hypothermia, the condition where the body becomes so cold that its systems will not properly function, and hyper-thermia, the corresponding opposite physical state where the body is overheated.
The thermoregulatory system performs one of the three major homeostatic, or overall balancing functions within the body, all of which are interrelated. Osmoregulation is the internal mechanism that controls the level of water and mineral salts, chiefly sodium and potassium, within the body. The third of the key homeostatic functions is that performed by the liver, through the operation of the cardiovascular system in the maintenance of glucose (blood sugar) levels.
Thermoregulation is primarily achieved through physiological processes, as a function of the autonomic nervous system. The processes of the body are controlled involuntarily through various stimuli transmitted through the body that originate at the hypothalamus, the region of the brain that regulates much of the body's functions, such as heart rate and hormone production in the endocrine system. The brain processes the multitude of external signals that it receives through the sensory organs to direct the bodily systems in appropriate ways to control temperature.
Body temperature is automatically regulated in one of four ways: conduction, convection, evaporation, and radiation. Conduction is the process whereby a warm surface transfers heat to an adjacent cooler surface. If a warm body dives into a colder lake while swimming, there will be a conduction of some of the heat on the surface of the person's body to the surrounding water. Convection is created when a passing air current removes heat from the surface of the skin as it passes over it.
Evaporation occurs with respect to the perspiration produced through the actions of the capillaries, the small vessels of the cardiovascular system located near the surface of the skin. As the body releases the perspiration, a byproduct of the body's increased internal temperature raised by the energy created to produce movement, the conversion of the fluid perspiration from liquid into a gas as it evaporates on the surface of the skin tends to produce a cooling effect on the body. The extent of the cooling effect achieved through perspiration is subject to both the temperature and the level of the humidity in the surrounding air.
Radiation is the effect on body temperature as a result of heat received from external sources, primarily solar radiation. Heat may also radiate from the body to a limited extent.
Thermoregulation can also be achieved through the voluntary regulation of human behaviors; the seeking of shade on a warm day or shelter on a cold one are examples.
The mechanism of thermoregulation is centered on the fact that the ideal temperature for the healthy function of the internal organs of the body is approximately 98.6°F (37°C); the body will not tolerate significant variation from this standard, as hypothermia begins at approximately 95° F (35.5° C) or below; hyperthermia will begin at 103°F (40°C) and above. Both conditions can cause irreparable damage to the internal organs if not remedied quickly, as the body's involuntary response in each situation is to shut down organ function.
The involuntary mechanisms triggered when either hypothermic of hyperthermic conditions are sensed by the hypothalamus begin at the surface of the skin. When the body seeks to maintain body heat, the small hairs at the skin surface will be pushed into an upright position to better retain heat; if the body temperature is too high, the hairs will lie flat on the skin surface as a heat-release mechanism.
The subcutaneous glands (sweat glands) are located in the dermis, the second of the layers of the skin, a part of the endocrine system responsible for the release of perspiration. The sweat glands will be activated when the body seeks to cool itself.
The blood vessels located next to the surface of the skin are also activated whether the body seeks to cool or to warm itself. When the body is overheated, the cardiovascular system automatically directs the flow of additional blood into these vessels to permit the blood warmed by the body activity to be cooled. The vessels expand to accommodate the additional blood flow, a process known as vasodilation. In circumstances where the body senses an unhealthy low temperature, blood flow closest to the surface is restricted to permit all available blood to be directed to the internal organs and the brain, which is the contrasting process of vasoconstriction.
It is the action of vasoconstriction that renders the warming of the extremities for a person who has sustained hypothermia. When a person has suffered this cold weather illness, there is the temptation on the part of the helpers to quickly warm the feet and hands of the victim. This action upsets the body's thermoregulatory efforts and results in a potential wave of cold blood from those extremities flooding into the heart and internal organs. This cold blood can cause a shock to the heart function and trigger a heart attack. The thermoregulatory function is a powerful one and, in such circumstances, the entire body must be warmed slowly.