Flag_of_the_United_States_Public_Health_Service.svgThe normal body core temperature is 36.6℃ to 38.3℃ (via rectal), 36 to 37.7℃ (via oral), respectively. The temperature is fluctuating, with lowest in the early morning and highest in the late afternoon or early evening. Even in patients with fever, this diurnal variation of body temperature still exists.

The thermoregulation is based on heat production and heat loss, meanwhile the heat is produced by active tissues supplied by blood and redistributing of blood can increase or decrease heat loss from the body.


Heat Production

Body heat is produced by: 1.basic metabolic processes;2.food intake;and 3.muscular activity.

A variety of basic chemical reactions contribute to body heat production at all times. Ingestion of food increases heat production. But, the major source of heat is the contraction of skeletal muscle.


Heat Loss

Heat can lose from body via conduction, radiation, vaporization of sweat, respiration, urination, and defecation. Each way of heat loss take certain percentage of total heat loss, but the percentage is not always constant and change change, i.e., under different environment temperature, sporting like basketball, etc. At 21℃, vaporization is a minor component in humans at rest. As the environmental temperature approaches body temperature, radiation lossess decline and vaporization losses increase.

When the environmental temperature is below body temperature, heat can lose by conduction and radiation. Conduction is heat exchange between objects or substances at different temperatures that are in contact with one another. For example, when someone is at fever, care givers often put a wet wash on the patient's head. That is, to enhance the heat exchange from the patient to the wet wash via conduction. Note that heat must be exchanged between objects (the skin and environment) that in contact with each other.

In conduction, a basic characteristic of matter is that its molecules are in motion, with the amount of motion proportional to the temperature. These molecules collide with the molecules in cooler objects, transferring thermal energy to them. The amount of heat transferred is proportional to the temperature difference between the objects in contact (thermal gradient). Conduction is aided by convection, the movement of molecules away from the area of contact. Thus, for example, an object in contact with air at a different temperature changes the specific gravity of the air, and because warm air rises and cool air falls, a new supply of air is brought into contact with the object. Of course, convection is greatly aided if the object moves about in the medium or the medium moves past the object, for example, if a subject swims through water or a fan blows air through a room.

Note that heat that is transferred from skin to environment can be trapped by hair and clothing. That is, heat is conducted from the skin to the air trapped in the layer of hair or clothing, then from the trapped air to the exterior. When the thickness of the trapped layer is increased by erection of the hairs (horripilation), heat transfer across the layer is reduced and heat losses are decreased.

Because conduction occurs from the surface of one object to the surface of another, the temperature of the skin determines to a large extent the degree to which body heat is lost or gained. That is, the dilation or constriction of capillaries in the skin change the amount of blood flow from deep tissues to the skin, so resulting in varies of skin temperature and final the degree of heat conduction between body and environment. The rate at which heat is transferred from the deep tissues to the skin is called the tissue conductance.

Radiation is the transfer of heat by infrared electromagnetic radiation from one object to anther at a different temperature with which it is not in contact. When an individual is in a cold environment, heat is lost by conduction to the surrounding air and by radiation to cool objects in the vicinity. Conversely, of course, heat is transferred to an individual and the heat load is increased by these processes when the environmental temperature is above body temperature. Note that because of radiation, an individual can feel chilly in a room with cold walls even through the room is relatively warm. On a cold but sunny day, the heat of the sun reflected off bright objects exerts an appreciable warming effect. It is the heat reflected from the snow, for example, that in part makes it possible to ski in fairly light clothes even though the air temperature is below freezing.

Sweat is another major process transferring heat from the body in humans. Vaporization of water on the skin and mucous membranes of the mouth and respiratory passages takes away heat, by which vaporization of 1 g of water removes about 0.6 kcal of heat. A certain amount of water is vaporized at all times, which is insensible water vaporizing at a rate of approximately 50 mL/h in humans. When sweat secretion is increased, the degree to which the sweat vaporizes depends on the humidity of the environment, where in humidity environment the degree of vaporization of sweat is decreased.


Temperature-Regulating Mechanisms

Information of temperature to the brain and temperature receptors

The hypothalamus is said to integrate body temperature information from sensory receptors (primarily cold receptors) in the skin, deep tissues, spinal cord, extrahypothalamic portions of the brain, and the hypothalamus itself. Each of these five inputs contributes about 20% of the information that is integrated.

Responses

The reflex and semireflex thermoregulatory responses in humans include autonomic, somatic, endocrine, and behavioral changes. Generally, one group of responses increases heat loss and decrease heat production (stimulated by exposure to heat);whereas, the other group of responses decrease heat loss and increase heat production (stimulated by exposure to cold).

The main temperature-regulating responses are shown in the figure at left. The reflex responses activated by cold are controlled from the posterior hypothalamus. Those activated by warmth are controlled primarily from the anterior hypothalamus. Note that some thermoregulati `on against heat still occurs after decerebration at the level of the rostral midbrain.

The thermostat

In the hypothalamus there is a thermostat, which controls and maintains the temperature of the individual. If the thermostat has been reset to a new point different from the normal value, the body would sense the difference between true body temperature and the new thermostat via temperature receptors, and after the signal being transmitted into the hypothalamus, the ratio of heat production to heat loss will be changed accordingly via temperature-regulating responses to make the body core temperature the same as the new thermostat. For example, if the thermostat had been reset to above 37℃, the temperature receptors then signal that the actual temperature is below the new set point, and the temperature-raising mechanisms are activated. Then the ratio of heat production to heat loss would increases and the actual body temperature starts to increase until the value equaling the new set point.


Update on Dec 31st 2015

Mechanisms of Heat Loss or Gain

Most of the gain or loss of heat between the body and the environment is through the skin. Heat is mainly transferred to the skin from the internal environment by the circulatory system. There are four general mechanisms of heat transfer between the body and the environment. Radiation is the emission of heat to and from the skin by electromagnetic waves – the rate of the temperature transfer by radiation is proportional to the temperature difference between the body surface and the environment. Conduction is intermolecular thermal heat transfer and usually occurs between the skin and air. One loses heat more rapidly when immersed in water because conduction between the skin and water is faster than that between skin and air. Convection is the loss or gain of heat by the movement of air or water over the body. Because heat rises, air carries heat away from the body by convection. Finally, evaporation of water from the skin and the respiratory tract can carry a large amount of heat generated by the body because of the amount of heat required to transform water from the liquid to the gas phase.

Heat production in humans is usually by metabolism. The basal metabolic rate can be altered by circulating thyroid hormone, and by shivering thermogenesis, driven by innervation of skeletal muscle. Shivering is the rhythmic, involuntary contraction and relaxation of skeletal muscles that generates heat due to increase metabolic rate. Of course, one can voluntarily increase heat production from skeletal muscle with movement.