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Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when temperature surrounding is very different. This process is one aspect of homeostasis: a dynamic state of stability between an animal's internal environment and its external environment (the study of such processes in zoology has been called ecophysiology or physiological ecology). If the body is unable to maintain a normal temperature and it increases significantly above normal, a condition known as hyperthermia occurs. The opposite condition, when body temperature decreases below normal levels, is known as hypothermia.
Whereas an organism that thermoregulates is one that keeps its core body temperature within certain limits, a thermoconformer changes its body temperature with changes to the temperature outside of its body. It was not until the introduction of thermometers that any exact data on the temperature of animals could be obtained. It was then found that local differences were present, since heat production and heat loss vary considerably in different parts of the body, although the circulation of the blood tends to bring about a mean temperature of the internal parts. Hence it is important to determine the temperature of those parts which most nearly approaches to that of the internal organs. Also for such results to be comparable they must be made in the same situation. The rectum gives most accurately the temperature of internal parts, or in some cases of sex or species, the vagina, uterus or bladder.
Thermoregulation in humans
The skin assists in homeostasis (keeping different aspects of the body constant e.g. temperature). It does this by reacting differently to hot and cold conditions so that the inner body temperature remains more or less constant.
When in hot conditions:
- Sweat glands under the skin secrete sweat (fluid containing urea) which travels up the sweat duct, through the sweat pore and onto the surface of the skin. This causes heat loss by evaporation, however, a lot of essential water is lost.
- The hairs on the skin lie flat down preventing heat from being trapped between the hairs. This is caused by tiny muscles under the surface of the skin called arrector pili muscles relaxing so that its attached hair folicle is not erect.
- Blood capillaries under the surface of the skin can swell (dilate) so that more heat is carried by the blood and is lost to the air. This is called vasodilation.
Note: Urea is a waste product of the body. It is created by removing the poisonous portion of Amino Acids (the break down of protein) during digestion. Urea is also removed from the body in the form of urine by the kidneys. Urine is just water concentrated with urea and excess salts of the body.
When in cold conditions:
- Sweat stops being produced.
- Very minute muscles under the surface of the skin called erector muscles attached to an individual hair follicle erect, lifting the hair follicle upright. This makes our hairs stand on end which acts as an insulating layer, trapping heat. This is what also causes goose pimples since humans don't have very much hair.
- Blood capillaries under the surface of the skin can shrink (constrict) so that less heat is carried by the blood and lost to the surroundings. This process is called vasoconstriction. It is impossible to prevent any heat loss from the blood, only to reduce it.
- Muscles can also receive messages from the brain (the hypothalamus) to cause shivering. This produces heat as the cells in the muscle respire more as they do more work. Respiration is an exothermic reaction which gives out heat so more heat is produced which warms up the body.
Note: Messages from the brain that reach effectors (e.g. muscles and glands) are done so by motor neurons. Neurons are specialized cells that pass messages around the body in the form of electrical impulses. Motor neurons are the ones that pass messages from the brain directly to the effector, in this case muscles. A collection of thousands of neurons is termed a nerve.
The process explained above, in which the skin regulates body temperature, is called thermoregulation. This is also called homeostasis or is a part of homeostasis since it is keeping conditions in the body relatively constant. See Homeostasis
This can also been seen in Afriace
Thermoregulation in vertebrates
By numerous observations upon humans and other animals, John Hunter showed that the essential difference between the so-called warm-blooded and cold-blooded animals lies in observed constancy of the temperature of the former, and the observed variability of the temperature of the latter. Almost all birds and mammals have a high temperature almost constant and independent of that of the surrounding air. This is called homeothermy. Almost all other animals display a variation of body temperature, dependent on their surroundings. This is called poikilothermy.
There are, however, certain mammals which are exceptions, being warm-blooded during the summer or daytime, but cold-blooded during the winter when they hibernate or at night during sleep.
Also, from work done by J. O. Wakelin Barratt, it has been shown that under certain pathological conditions a warm-blooded (homeothermic) animal may become for a time cold-blooded (poikilothermic). He has shown conclusively that this condition exists in rabbits suffering from rabies during the last period of their life, the rectal temperature being then within a few degrees of the room temperature and varying with it. He explains this condition by the assumption that the nervous mechanism of heat regulation has become paralysed. The respiration and heart-rate being also retarded during this period, the resemblance to the condition of hibernation is considerable. Again, Sutherland Simpson has shown that during deep anaesthesia a warm-blooded animal tends to take the same temperature as that of its environment. He demonstrated that when a monkey is kept deeply anaesthetized with ether and is placed in a cold chamber, its temperature gradually falls, and that when it has reached a sufficiently low point (about 25 °C in the monkey), the employment of an anaesthetic is no longer necessary, the animal then being insensible to pain and incapable of being roused by any form of stimulus; it is, in fact, narcotized by cold, and is in a state of what may be called "artificial hibernation." Once again this is explained by the fact that the heat-regulating mechanism has been interfered with. Similar results have been obtained from experiments on cats.
- Main article: Ectotherm
- Getting wet in a river, lake or sea.
- Climbing to lower ground from trees, into valleys, burrows, etc.
- Entering a cold water or air current.
- Building a nest that allows natural or generated air/water flow for cooling.
- Lie on cold ground.
- Staying wet in a river, lake or sea.
- Covering in cool mud.
- Find shade.
- Enter a burrow shaped for radiating heat (Black box effect).
- Expand folds of skin.
- Expose wing surfaces.
Ectothermic heating (or minimising heat loss)
- Climb to higher ground up trees, ridges, rocks.
- Entering a warm water/air current.
- Building an insulated nest or burrow.
- Lie on hot rock.
- Lie in sun.
- Fold skin to reduce exposure.
- Conceal wing surfaces.
Even though fish and other ectotherms have developed the ability to remain functional even when the water temperature is below freezing and some even use natural antifreeze or antifreeze proteins to resist ice crystal formation in their tissues; amphibians (also ectotherms) must cope with the loss of heat through their moist skins by evaporative cooling; reptiles, like amphibians must warm their bodies by behavioral adaptations; the stratum corneum they possess limits heat loss by evaporative cooling.
- Main article: Endotherm
To regulate body temperature, an organism may need to prevent heat gains in arid environments. Evaporation of water, either across respiratory surfaces or across the skin in those animals possessing sweat glands, helps in cooling body temperature to within the organism's tolerance range. Animals with a body covered by fur have limited ability to sweat, relying heavily on panting to increase evaporation of water across the moist surfaces of the lungs and the tongue and mouth. Birds also avoid overheating by panting since their thin skin has no sweat glands. Down feathers trap warm air acting as excellent insulators just as hair in mammals acts as a good insulator; mammalian skin is much thicker than that of birds and often has a continuous layer of insulating fat beneath the dermis — in marine mammals like whales this is referred to as blubber. Dense coats found in desert endotherms also aid in preventing heat gain. Another cold weather strategy is to temporarily decrease metabolic rate and body temperature regulated decrease in body temperature decreases the temperature difference between the animal and the air and therefore minimizes heat loss. Furthermore, having a lower metabolic rate is less energetically expensive. Many animals survive cold frosty nights through torpor, a short-term temporary drop in body temperature. Organisms when presented with the problem of regulating body temperature not only have behavioural, physiological and structural adaptations, but also a feedback system to trigger these adaptations to regulate temperature accordingly. The main features of this system are; Stimulus, Receptor, Modulator, Effector and then the feedback of the now adjusted temperature to the Stimulus. This cyclical process aids in homeostasis.
Heat production in birds and mammals
In cold environments, birds and mammals employ the following adaptations and strategies to minimize heat loss:
- using small smooth muscles (erector pili in mammals) which are attached to feather or hair shafts; this shivering thermogenesis distorts the surface of the skin as the feather/hair shaft is made more erect (called goose bumps or pimples)
- increasing body size to more easily maintain core body temperature (warm-blooded animals in cold climates tend to be larger than similar species in warmer climates (see Bergmann's Rule))
- having the ability to store energy as fat for metabolism
- have shortened extremities
- have countercurrent blood flow in extremities (e.g. Arctic Wolf or penguins) to avoid freezing of tissues
In warm environments, birds and mammals employ the following adaptations and strategies to maximize heat loss:
- behavioral adaptations like living in burrows during the day and being nocturnal
- evaporative cooling by perspiration and panting
- storing fat reserves in one place (e.g. camel's hump) to avoid its insulating effect
- elongated, often vascularized extremities to conduct body heat to the air
Behavioral temperature regulation
In addition to human beings, a number of other animals also maintain their body temperature with physiological and behavioral adjustments. For example, a desert lizard is an ectotherm and is therefore unable to control its temperature through metabolic regulation. However, by altering its location continuously, it is able to maintain a crude form of temperature control. In the morning, only its head will emerge from its burrow. Later, the entire body is exposed. The lizard basks in the sun, absorbing solar heat. When the temperature reaches higher levels, the lizard will hide under rocks or return to its burrow. When the sun goes down or the temperature falls, it emerges again.
Some animals living in cold environments maintain their body temperature by preventing heat loss. Their fur grows more densely to increase the amount of insulation. Some animals are regionally heterothermic and are able to allow their less insulated extremities to cool to temperatures much lower than their core temperature -- nearly to 0 °C. This minimizes heat loss through less insulated body parts, like the legs, feet (or hooves), and nose.
Hibernation, estivation, and daily torpor
To cope with limited food resources and low temperatures, some mammals hibernate in underground burrows. In order to remain in "stasis" for long periods, these animals must build up brown fat reserves and be capable of slowing all body functions. True hibernators (e.g. groundhogs) keep their body temperature down throughout their hibernation while the core temperature of false hibernators (e.g. bears) varies with them sometimes emerging from their dens for brief periods. Some bats are true hibernators which rely upon a rapid, non-shivering thermogenesis of their brown fat deposit to bring them out of hibernation.
Daily torpor occurs in small endotherms like bats and humming birds which temporarily reduce their high metabolic rates to conserve energy.
Variations in the temperature of human beings and some animals
Normal human temperature
Previously, average oral temperature for healthy adults had been considered 37.0 °C (98.6 °F), while normal ranges are 36.1 °C (97.0 °F) to 37.8 °C (100.0 °F). In Russia, the temperature had been measured axillary. 36.6 °C was considered "ideal" temperature, while normal ranges are 36 °C to 36.9 °C.
Recent studies suggest that the average temperature for healthy adults is 98.2 °F or 36.8 °C (same result in three different studies). Variations (one standard deviation) from three other studies are:
- 36.4 - 37.1 °C
- 36.3 - 37.1 °C for males, 36.5 - 37.3 °C for females
- 36.6 - 37.3 °C 
Variations from thermometer placement
According to a study of children aged 4-14 from St. Luke's Hospital, Guardamangia, Malta, the mean difference between oral and axillary temperature was 0.56 °C, while the mean difference between rectal and axillary temperature for children under 4 years old was 0.38 °C. 
Variations associated with development
Of the lower warm-blooded animals, there are some that appear to be cold-blooded at birth. Kittens, rabbits and puppies, if removed from their surroundings shortly after birth, lose their body heat until their temperature has fallen to within a few degrees of that of the surrounding air. But such animals are at birth blind, helpless and in some cases naked. Animals who are born when in a condition of greater development can maintain their temperature fairly constant. In strong, healthy infants a day or two old the temperature rises slightly, but in that of weakly, ill-developed children it either remains stationary or falls. The cause of the variable temperature in infants and young immature animals is the imperfect development of the nervous regulating mechanism.
The average temperature falls slightly from infancy to puberty and again from puberty to middle age, but after that stage is passed the temperature begins to rise again, and by about the eightieth year is as high as in infancy.
Variations due to circadian rhythms
In humans, a diurnal variation has been observed dependent on the periods of rest and activity, the maximum ranging from 10 a.m. to 6 p.m., the minimum from 11 p.m. to 3 a.m. Sutherland Simpson and J.J. Galbraith did much work on this subject. In their first experiments they showed that in a monkey there is a well-marked and regular diurnal variation of the body temperature, and that by reversing the daily routine this diurnal variation is also reversed (Simpson & Galbraith, 1905). The diurnal temperature curve follows the periods of rest and activity, and is not dependent on the incidence of day and night; in monkeys which are active during the night and resting during the day, the body temperature is highest at night and lowest through the day. They then made observations on the temperature of animals and birds of nocturnal habit, where the periods of rest and activity are naturally the reverse of the ordinary through habit and not from outside interference. They found that in nocturnal birds the temperature is highest during the natural period of activity (night) and lowest during the period of rest (day), but that the mean temperature is lower and the range less than in diurnal birds of the same size. That the temperature curve of diurnal birds is essentially similar to that of man and other homoiothermal animals, except that the maximum occurs earlier in the afternoon and the minimum earlier in the morning. Also that the curves obtained from rabbit, guinea pig and dog were quite similar to those from man.
These observations indicate that body temperature is partially regulated by circadian rhythms.
Variations due to women's menstrual cycles
During the follicular phase (which lasts from the first day of menstruation until the day of ovulation), the average basal body temperature in women ranges from 36.45 - 36.7 °C (97.6 - 98.6 °F). Within 24 hours of ovulation, women experience an elevation of 0.15 - 0.45 °C (0.2 - 0.9 °F) due to the increased metabolic rate caused by sharply elevated levels of progesterone. The basal body temperature ranges between 36.7 - 37.3°C (97.6 - 99.2°F) throughout the luteal phase, and drops down to pre-ovulatory levels within a few days of menstruation. Women can chart this phenomenon to determine whether and when they are ovulating, or to aid conception or contraception.
Variations due to fever
Fever is a regulated elevation of the set point of core temperature in the hypothalamus, caused by circulating pyrogens produced by the immune system. To the subject, a rise in core temperature due to fever may result in feeling cold in an environment that people without fever do not.
Variations due to biofeedback
Variations due to other factors
In Simpson's & Galbraith's work, the mean temperature of the female was higher than that of the male in all the species examined whose sex had been determined.
Meals sometimes cause a slight elevation, sometimes a slight depression—alcohol seems always to produce a fall. Exercise and variations of external temperature within ordinary limits cause very slight change, as there are many compensating influences at work, which are discussed later. The core temperature of those living in the tropics is within a similar range to those dwelling in the Arctic regions.
Low body temperature increases lifespan
It was long theorised that low body temperature may prolong life. On November 2006, a team of scientists from the Scripps Research Institute reported that transgenic mice which had body temperature 0.3-0.5 C lower than normal mice (due to overexpressing the uncoupling protein 2 in hypocretin neurons (Hcrt-UCP2), which elevated hypothalamic temperature, thus forcing the hypothalamus to lower body temperature) indeed lived longer than normal mice. The lifespan was 12% longer for males and 20% longer for females. Mice were allowed to eat as much as they wanted.    The effects of body temperature on longevity have not been studied in humans.
Limits compatible with life
There are limits both of heat and cold that a warm-blooded animal can bear, and other far wider limits that a cold-blooded animal may endure and yet live. The effect of too extreme a cold is to lessen metabolism, and hence to lessen the production of heat. Both catabolic and anabolic changes share in the depression, and though less energy is used up, still less energy is generated. This diminished metabolism tells first on the central nervous system, especially the brain and those parts concerned in consciousness. Both heart rate and respiration rate become diminished, drowsiness supervenes, becoming steadily deeper until it passes into the sleep of death. Occasionally, however, convulsions may set in towards the end, and a death somewhat similar to that of asphyxia takes place.
In some experiments on cats performed by Sutherland Simpson and Percy T. Herring, they found them unable to survive when the rectal temperature was reduced below 16°C. At this low temperature respiration became increasingly feeble, the heart-impulse usually continued after respiration had ceased, the beats becoming very irregular, apparently ceasing, then beginning again. Death appeared to be mainly due to asphyxia, and the only certain sign that it had taken place was the loss of knee jerks.
On the other hand, too high a temperature hurries on the metabolism of the various tissues at such a rate that their capital is soon exhausted. Blood that is too warm produces dyspnea and soon exhausts the metabolic capital of the respiratory centre. Heart rate is increased, the beats then become arrhythmic and finally cease. The central nervous system is also profoundly affected, consciousness may be lost, and the patient falls into a comatose condition, or delirium and convulsions may set in. All these changes can be watched in any patient suffering from an acute fever. The lower limit of temperature that man can endure depends on many things, but no one can survive a temperature of 45°C (113°F) or above for very long. Mammalian muscle becomes rigid with heat rigor at about 50°C, and obviously should this temperature be reached the sudden rigidity of the whole body would render life impossible.
H.M. Vernon has done work on the death temperature and paralysis temperature (temperature of heat rigor) of various animals. He found that animals of the same class of the animal kingdom showed very similar temperature values, those from the Amphibia examined being 38.5°C, Fish 39°C, Reptilia 45°C, and various Molluscs 46°C. Also in the case of Pelagic animals he showed a relation between death temperature and the quantity of solid constituents of the body, Cestus having lowest death temperature and least amount of solids in its body. In higher animals, however, his experiments tend to show that there is greater variation in both the chemical and physical characters of the protoplasm, and hence greater variation in the extreme temperature compatible with life.
Human temperature variation effects
Fevers are not to be confused with heat stroke. In fever the person can feel cold at high body temperatures since the body is fooled into thinking it is cold by the point that the body thermostat is set at. It is literally set higher than usual.
- 37°C (98.6°F) - Normal body temperature (which varies between about 36.12-37.5°C (96.8-99.5°F)
- 38°C (100.4°F) - Sweating, feeling very uncomfortable, slightly hungry.
- 39°C (102.2°F) (Pyrexia) - Severe sweating, flushed and very red. Fast heart rate and breathlessness. There may be exhaustion accompanying this. Children and people with epilepsy may be very likely to get convulsions at this point.
- 40°C (104°F) - Fainting, dehydration, weakness, vomiting, headache and dizziness may occur as well as profuse sweating.
- 41°C (105.8°F) - (Medical emergency) - Fainting, vomiting, severe headache, dizziness, confusion, hallucinations, delirium and drowsiness can occur. There may also be palpitations and breathlessness.
- 42°C (107.6°F) - Subject may turn pale or remain flushed and red. They may become comatose, be in severe delirium, vomiting, and convulsions can occur. Blood pressure may be high or low and heart rate will be very fast.
- 43°C (109.4°F) - Normally death, or there may be serious brain damage, continuous convulsions and shock. Cardio-respiratory collapse will occur.
- 44°C (111.2°F) or more - Almost certainly death will occur; however, patients have been known to survive up to 46.5°C (115.7°F).
- 37°C (98.6°F) - Normal body temperature (which varies between about 36-37.5°C (96.8-99.5°F)
- 36°C (96.8°F) - Mild to moderate shivering (it drops this low during sleep). May be a normal body temperature.
- 35°C (95.0°F) - (Hypothermia) is less than 35°C (95.0°F) - Intense shivering, numbness and bluish/grayness of the skin. There is the possibility of heart irritability.
- 34°C (93.2°F) - Severe shivering, loss of movement of fingers, blueness and confusion. Some behavioural changes may take place.
- 33°C (91.4°F) - Moderate to severe confusion, sleepiness, depressed reflexes, progressive loss of shivering, slow heart beat, shallow breathing. Shivering may stop. Subject may be unresponsive to certain stimuli.
- 32°C (89.6°F) - (Medical emergency) Hallucinations, delirium, complete confusion, extreme sleepiness that is progressively becoming comatose. Shivering is absent (subject may even think they are hot). Reflex may be absent or very slight.
- 31°C (87.8°F) - Comatose, very rarely conscious. No or slight reflexes. Very shallow breathing and slow heart rate. Possibility of serious heart rhythm problems.
- 28°C (82.4°F) - Severe heart rhythm disturbances are likely and breathing may stop at any time. Patient may appear to be dead.
- 24-26°C (75.2-78.8°F) or less - Death usually occurs due to irregular heart beat or respiratory arrest; however, some patients have been known to survive with body temperatures as low as 14.2°C (57.5°F).
- Swan, K. G.; R. E. Henshaw (March, 1973). "Lumbar sympathectomy and cold acclimatization by the arctic wolf.". Analysis of Surgery 177: 286-292. Retrieved on 03/29/2007.
- Adaptations for an Aquatic Environment. SeaWorld/Busch Gardens Animal Information Database, 2002. Last accessed November 27, 2006.
- Introduction to Penguins. Mike Bingham, International Penguin Conservation Work Group. Last accessed November 27, 2006.
- Rectal, ear, oral, and axillary temperature comparison
- Comparison of Axillary Temperature with Rectal or Oral Temperature and Determination of Optimum Placement Time in Children
- How reliable is axillary temperature measurement?
- Transgenic Mice with a Reduced Core Body Temperature Have an Increased Life Span, by Bruno Conti et al. Science, 3, November 2006
- Reduced Body Temperature Extends Lifespan, Study Finds
- Bee cool, live long
- Excerpt: Humans, Body Extremes. Guinness World Records, 2004. Last accessed November 27, 2006.
- Handbook of Physiology, Kirkes, (Philadelphia, 1907)
- Simpson, S. & Galbraith, J.J. (1905) Observations on the normal temperatures of the monkey and its diurnal variation, and on the effects of changes in the daily routine on this variation. Transactions of the Royal Society of Edinburgh 45: 65-104.
- Weldon Owen Pty Ltd. (1993). Encyclopedia of animals - Mammals, Birds, Reptiles, Amphibians. Reader's Digest Association, Inc. Pages 567-568. ISBN 1875137491.