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Normal human body temperature, also known as normothermia or euthermia, is a concept that depends upon the place in the body at which the measurement is made. The value of 36.8 °C ±0.7 °C, or 98.2 °F ±1.3 °F is the common oral measurement. Rectal measurements, or measurements taken directly inside the body cavity, are typically about a half degree Celsius (1 °F) higher. The core body temperature of an individual also tends to vary during the day and with activity level, with the lowest value in the second half of the sleep cycle; this low point, called nadir, is one of the primary markers for circadian rhythms.
In 1861, Carl Reinhold August Wunderlich claimed to measure the temperatures of one million people, and reported the average to be 37 °C. In the United States, normal human body temperature is commonly quoted as 98.6 °F, which is an inappropriately exact conversion of Wunderlich's 19th century announcement that the human body temperature is 37 °C. In Russia and former Soviet countries, the commonly quoted value is Template:Convert/LoffAoffDbSoffTTemplate:Convert/test/A, based on an armpit reading.
Temperature control (thermoregulation) is part of a homeostatic mechanism that keeps the organism at optimum operating temperature, as it affects the rate of chemical reactions. In humans the average temperature is Template:Convert/CTemplate:Convert/test/A, though it varies among individuals, as well as cycling regularly through the day, as controlled by one's circadian rhythms with the lowest temperature occurring about two hours before one normally wakes up.
Body temperature normally fluctuates over the day, with the lowest levels around 4 a.m. and the highest in the late afternoon, between 4:00 and 6:00 p.m. (assuming the person sleeps at night and stays awake during the day). Therefore, an oral temperature of 37.2 °C (99.0 °F) would, strictly speaking, be normal in the afternoon but not in the morning. An individual's body temperature typically changes by about 0.5 °C (0.9 °F) between its highest and lowest points each day.
Temperature is increased after eating, and psychological factors also influence body temperature.
Many outside factors affect the measured temperature as well. "Normal" values are generally given for an otherwise healthy, non-fasting adult, dressed comfortably, indoors, in a room that is kept at a normal room temperature (22.7 to 24.4°C or 73 to 76 °F), during the morning, but not shortly after arising from sleep. Furthermore, for oral temperatures, the subject must not have eaten, drunk, or smoked anything in at least the previous fifteen to twenty minutes, as the temperature of the food, drink, or smoke can dramatically affect the reading.
Children develop higher temperatures with activities like playing, but this is not fever because their set-point is normal. Elderly patients may have a decreased ability to generate body heat during a fever, so even a low-grade fever can have serious underlying causes in geriatrics.
Normal body temperature may differ as much as 1.0 °F between individuals or from day to day.
Specific temperature concepts
- Main article: fever
A temperature setpoint is the level at which the body attempts to maintain its temperature. When the setpoint is raised, the result is a fever. Most fevers are caused by infectious disease.
An organism at optimum temperature is considered afebrile or apyrexic, meaning "without fever".
- Main article: Hyperthermia
Hyperthermia is an acute condition which occurs when the body produces or absorbs more heat than it can dissipate. It is usually caused by prolonged exposure to high temperatures. The heat-regulating mechanisms of the body eventually become overwhelmed and unable to effectively deal with the heat, causing the body temperature to climb uncontrollably. Hyperthermia at or above about 40 °C (104 °F) is a life-threatening medical emergency and requires immediate treatment. Common symptoms include headache, confusion, and fatigue. If sweating has resulted in dehydration, then the affected person may have dry, red skin.
In a medical setting, mild hyperthermia is commonly referred to as heat exhaustion or heat prostration; severe hyperthermia is called heat stroke. Heat stroke may come on suddenly, but it usually follows the untreated milder stages. Treatment involves cooling and rehydrating the body. This may be done through moving out of direct sunlight to a cooler and shaded environment, drinking water, removing clothing that might keep heat close to the body, or sitting in front of a fan. Bathing in tepid or cool water, or even just washing the face and other exposed areas of the skin, can be helpful.
With fever, the body raises the core temperature to a higher temperature through the action of the part of the brain that controls the body temperature; with hyperthermia, the body temperature is raised without the consent of the heat control centers.
- Main article: Hypothermia
In hypothermia, the body temperature drops below that required for normal metabolism and bodily functions. In humans, this is usually due to excessive exposure to cold air or water, but it can be deliberately induced as a medical treatment. Symptoms usually appear when the body's core temperature drops by 1-2°C (1.8-3.6°F) below normal temperature.
Basal body temperature
- Main article: Basal body temperature
Basal body temperature is the lowest temperature attained by the body during rest (usually during sleep). It is generally measured immediately after awakening and before any physical activity has been undertaken, although the temperature measured at that time is somewhat higher than the true basal body temperature. In women, temperature differs at various points in the menstrual cycle, and this can be used for family planning.
Core temperature, also called core body temperature, is the operating temperature of an organism, specifically in deep structures of the body such as the liver, in comparison to temperatures of peripheral tissues. Core temperature is normally maintained within a narrow range so that essential enzymatic reactions can occur. Prolonged core temperature elevation (hyperthermia) or depression (hypothermia) is incompatible with life.
Temperature examination in the rectum is the traditional gold standard measurement used to estimate core temperature (oral temperature is affected by hot or cold drinks and mouth-breathing). Rectal temperature is expected to be approximately one Fahrenheit degree higher than an oral temperature taken on the same person at the same time. Ear thermometers measure eardrum temperature using infrared sensors. The blood supply to the tympanic membrane is shared with the brain. However, this method of measuring body temperature is not as accurate as rectal measurement and has a low sensitivity for fevers, missing three or four out of every ten fevers in children. Ear temperature measurement may be acceptable for observing trends in body temperature but is less useful in consistently identifying fevers.
Direct measurement of core body temperature would require invasive insertion of a probe and is not clinically possible, so a variety of indirect methods are used. Whilst the rectal temperature is generally considered to give the most accurate assessment of core body temperature, particularly in hypothermia, though its recording is disliked by patients and medical staff alike.
Methods of measurement
The temperature reading depends on which part of the body is being measured. Measurements are commonly taken in the mouth, the ear, the anus, or the armpit. In females, the vagina can also be used. The median daytime temperature among healthy adults are as follows:
- Temperature in the anus (rectum/rectal), vagina, or in the ear (otic) is about 37.6 °C (99.6 °F)
- Temperature in the mouth (oral) is about 36.8 °C (98.2 °F)
- Temperature under the arm (axillary) is about 36.4 °C (97.6 °F) 
Normal human body temperature varies slightly from person to person and by the time of day. Consequently, each type of measurement has a range of normal temperatures. The range for normal human body temperatures, taken orally, is 36.8±0.7 °C (98.2±1.3 °F). This means that any oral temperature between 36.1 and 37.5 °C (96.9 and 99.5 °F) is likely to be normal.
Problems of various measurements
It is claimed that changes in core body temperature are reflected sooner and more accurately in the ear than at other sites.
Influencing factors on other areas where temperatures are taken:
- Oral temperatures are influenced by drinking, eating and breathing.
- Rectal temperatures lag behind changes in core body temperature and there is a risk of cross-contamination.
- Skin temperatures, measured under the arm or at the forehead, are not always reliable indicators of core body temperature, especially during those critical times when core body temperature is increasing or decreasing. This is because the skin is a tool the body uses to control core body temperature. For example, when fever is increasing people are likely to react by shivering and drawing in heat from the increased core body temperature. Skin temperatures are further influenced by factors such as fever-lowering medication, clothing and external temperature.
There is a risk of injury from cracking the original glass thermometers if too much force is applied by the teeth to hold them in place and the alcohol or mercury contents are poisonous. This is avoided by the use of electronic thermometers which are made from solid plastic and use a metal (thermocouple) sensor.
A plastic thermometer strip placed on the forehead gives an approximate local reading, which depends to a great extent on ambient air temperature and local circulation effects. Using a thermometer to record the temperature under the armpit is less affected by surrounding air temperature, but is still prone to diverge from true core temperature if there are alterations in blood circulation.
Recently small ear thermometers have become available and it is thought that the eardrum closely mirrors core temperature values. These work by detecting the infrared heat emission from the tympanic membrane and a measurement is quickly taken within one second making them popular for use with children. Whilst the electronic display of the temperature value is easier to read than interpreting the graduation marks on a thermometer, there are some concerns for the accuracy of ear thermometers in home use.
Diurnal variation in body temperature
Body temperature and ovulation
Animal body temperature
Types of temperature control
Examples of temperature control include:
- Snakes and lizards sunning themselves on rocks.
- Fish changing depths in the water column to find a suitable temperature.
- Desert animals burrowing beneath the sand during the day.
- Insects that warm their flight muscles by vibrating them in place.
- Dilating or constricting peripheral blood vessels to adapt more or less quickly to the ambient temperature.
Many homeothermic, or warm-blooded, animals also make use of these techniques at times. For example, all animals are at risk of hypothermia on cold days, and most homeothermic animals can shiver to get warmer.
Poikilotherms often have more complex metabolisms than homeotherms (homopathics). For an important chemical reaction, poikilotherms may have four to ten enzyme systems that operate at different temperatures. As a result, poikilotherms often have larger, more complex genomes than homeotherms in the same ecological niche. Frogs are a notable example of this effect.
Because their metabolism is so variable, poikilothermic animals do not easily support complex, high-energy organ systems such as brains or wings. Some of the most complex adaptations known involve poikilotherms with such organ systems. One example is the swimming muscles of Tuna, which are warmed by a heat exchanger. In general, poikilothermic animals do not use their metabolisms to heat or cool themselves. For the same body weight poikilotherms need ⅓ to 1/10 of the energy of homeotherms. They therefore eat only ⅓ to 1/10 of the food needed by homeothermic animals.
Some larger poikilotherms, by virtue of their substantial volume to surface area ratio, are able to maintain relatively high body temperatures and high metabolic rates. This phenomenon, known as gigantothermy (inertial homeothermy), has been observed in sea turtles and great white sharks, and was most likely present in many dinosaurs and ancient sea reptiles (such as ichthyosaurs and plesiosaurs). For example, some species of sea turtles are homeothermic some of the time. They float on the surface of the ocean to absorb heat and then, after submerging again, stay homeothermic for periods of time because of their sheer size. During long periods of time underwater their body temperature may decrease, depending on the temperature of the surrounding water. Their body temperature may also decrease when they float on the surface of the ocean at night, depending on the surrounding temperature.
However, large dinosaurs were probably not poikilotherms, but homeotherms (homeothermic all the time) due to the overwhelming mass of their bodies.[How to reference and link to summary or text]
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