Temperature is a measure of an organism’s ability to generate and get rid of heat. The human body has mechanisms to maintain its internal temperature within a relatively narrow, safe range despite relatively large variations in temperatures in which the body exists.
The purpose of maintaining body temperature within a relatively narrow range is to promote and sustain life.
Thermometers are used to measure body temperature. They are calibrated in either degrees Fahrenheit (°F) or degrees Celsius (°C). Temperatures in the United States are typically measured in degrees Fahrenheit. The standard in most countries of the world is degrees Celsius.
When humans become too warm, blood vessels in the skin increase in diameter (dilate). The purpose is to carry the excess heat to the surface of the skin. In turn, this causes the body to begin to perspire. As the perspiration evaporates, it helps to cool the body. When the body becomes too cold, the blood vessels decrease in diameter (contract) so that blood flow to the skin is reduced in an attempt to conserve body heat. This often causes people to start shivering. This involves rapid, involuntary contractions of muscles. Shivering helps to generate additional heat through muscle activity. Under normal conditions, these activities maintain human body temperature within a narrow range that is healthy for the organism.
Body temperature can be measured in many locations. The mouth, ear, armpit, and rectum are the most commonly used places. Temperature can also be measured on the forehead.
Body temperature is checked for several reasons.
- To detect fever.
- To document an abnormally low body temperature(hypothermia) in people who have been exposed tocold.
- To document an abnormally high body temperature(hyperthermia) in people who have been exposed toheat.
- To monitor the effectiveness of a fever-reducing medicine (antipyretic).
- To determine when a female is ovulating, thereby increasing the probability of becoming pregnant.
Preparation for taking a body temperature consists of ensuring that the thermometer is clean and disinfected.
Aftercare consists of ensuring that a thermometer is clean and disinfected. Electronic thermometers must be turned off to conserve their batteries.
Taking a body temperature involves little risk. Inserting a thermometer into the rectum can occasionally be painful. Breaking a thermometer that contains mercury causes exposure to a toxic substance (mercury).
Most people consider a normal body temperature to be an oral temperature of 98.6 degrees Fahrenheit. This is more correctly an average of body temperatures. A person’s body temperature varies during each 24 hour period. A normal range encompasses temperatures that are 1°F (0.6°C) above or below 98.6 degrees F. Some variation is due to fluctuations in physiology nd cellular metabolism. Bodily activities (or lack) can temporarily increase (or decrease) body temperature. Body temperature is very sensitive to hormone levels and may be higher or lower when a female is ovulating during her menstrual cycle.
A rectal or ear (tympanic membrane) temperature reading is 0.5 to 1 degree F (0.3 to 0.6 degrees C) higher than an oral temperature reading. A temperature taken in the armpit is 0.5 to 1 degree F (0.3 to 0.6 degrees C) lower than an oral temperature reading.
In adults, an oral temperature above 100 degrees F or a rectal or ear temperature above 101 degrees F is considered to be a fever. Children are considered to have a fever when their rectal temperature is 100.4 degrees F or higher.
Abnormally low body temperature is called hypothermia. It is always serious and can be life-threatening. Hypothermia can occur after exposure to cold, when a person is in shock, or after alcohol or drug usage. Metabolic disorders, such as hypothyroidism or diabetes can trigger hypothermia. An infection involving the entire body (sepsis) can cause hypothermia. Infections in older adults, newborn infants or other frail persons may be accompanied by hypothermia.
Perforations of the colon due to inserting a rectal thermometer too far have been reported. These are uncommon. The number of deaths associated with taking a temperature is essentially zero.
There are no alternatives to obtaining a body temperature.
Fever— An abnormally elevated body temperature, usually defined as being 101 degrees Fahrenheit or more
Hypothermia— An abnormally low body temperature, usually defined as being 90 degrees Fahrenheit or less
Sepsis— An infection involving the entire body
Bickley, L. S., and P. G. Szilagyi. Bates’ Guide to Physical Examination and History Taking. 9th ed. Philadelphia: Lippincott Williams and Wilkins, 2007.
Jarvis, C. Physical Examination and Health Assessment. 5th ed. Philadelphia: Saunders, 2007.
Seidel. H. M., J. Ball, J. Dains, and W. Bennedict. Mosby’s Physical Examination Handbook. 6th ed. St. Louis: MOsby, 2006.
Swartz, M. H. Textbook of Physical Diagnosis: History and Examination. 5th ed. Philadelphia: Saunders, 2005.
Bruel, C., J. J. Parienti, W. Marie et al. “Mild hypothermia during advanced life support: a preliminary study in out-of-hospital cardiac arrest.” Critical Care 12, no. 1 (2008): R31–R41.
Gunn, A. J., T. Hoehn, G. Hansmann et al. Hypothermia: an evolving treatment for neonatal hypoxic ischemic encephalopathy.” Pediatrics 121, no. 3 (2008): 648–650.
Mance, M. J. “Keeping Infants Warm: Challenges of Hypothermia.” Advances in Neonatal Care 8, no. 1 (2008): 6–12.
Salerian, A. J., and N. G. Saleri. “Cooling core body temperature may slow down neurodegeneration.” CNS Spectrums 13, no. 3 (2008): 227–229.
American College of Physicians. 190 N. Independence Mall West, Philadelphia, PA 19106-1572. (800) 523-1546, x2600, or (215) 351-2600. http://www.acponline.org.
HyperTextbook. “Information about body temperature.” 2008 [cited February 24, 2008]. http://hypertextbook.com/facts/LenaWong.shtml.
Kid’s Health. “Information about adolescent body temperature.” 2008 [cited February 24, 2008]. http://www.kidshealth.org/parent/general/body/fever.html.
Mayo Clinic. Information about low body temperature. 2008 [cited February 25, 2008]. http://www.mayoclinic.com/health/body-temperature/AN01513.
National Library of Medicine. “Information about body temperature.” 2008 [cited February 22, 2008]. http://www.nlm.nih.gov/medlineplus/ency/article/001982.htm.
L. Fleming Fallon, Jr, MD, DrPH
body temperature, internal temperature of a living organism. Mammals and birds are termed warm-blooded, or homeothermic, i.e., they are able to maintain a relatively constant inner body temperature, whereas other animals are cold-blooded, or poikilothermic, i.e., their body temperature varies according to the temperature of the environment.
Warm-blooded Animals (Homeotherms)
In humans and other mammals, temperature regulation represents the balance between heat production from metabolic sources and heat loss from evaporation (perspiration) and the processes of radiation, convection, and conduction. In a cold environment, body heat is conserved first by constriction of blood vessels near the body surface and later by waves of muscle contractions, or shivering, which serve to increase metabolism. Shivering can result in a maximum fivefold increase in metabolism. Below about 40°F (4°C) a naked person cannot sufficiently increase the metabolic rate to replace heat lost to the environment. Another heat-conserving mechanism, goose bumps, or piloerection, raises the body hairs; although not especially effective in humans, in animals it increases the thickness of the insulating fur or feather layer.
In a warm environment, heat must be dissipated to maintain body temperature. In humans, increased surface blood flow, especially to the limbs, acts to dissipate heat at the surface. At environmental temperatures above 93°F (34°C), or at lower temperatures when metabolism has been increased by work, heat must be lost through the evaporation of the water in sweat. People in active work may lose as much as 4 quarts per hour for short periods. However, when the temperature and humidity are both high, evaporation is slowed, and sweating is not effective. Most mammals do not have sweat glands but keep cool by panting (evaporation through the respiratory tract) and by increased salivation and skin and fur licking.
Temperature regulatory mechanisms act through the autonomic nervous system and are largely controlled by the hypothalamus of the brain, which responds to stimuli from nerve receptors in the skin. Continued exposure to heat or cold results in some slow acclimatization, e.g., more active sweating in response to continued heat and an increase in subcutaneous fat deposits in response to continued cold.
Environmental extremes may result in failure to maintain normal body temperature. In both increased body temperature, or hyperthermia, and decreased body temperature, or hypothermia, death may result (see heat exhaustion). Controlled hypothermia is used in some types of surgery to temporarily decrease the metabolic rate. Fever, caused by a resetting of the temperature regulatory mechanism, is a response to fever-causing, or pyrogenic, substances, such as bacterial endotoxins or leucocyte extracts. The upper limit of body temperature compatible with survival is about 107°F (42°C), while the lower limit varies.
In humans the inner body temperature alternates in daily activity cycles; it is usually lowest in early morning and is slightly higher at the late afternoon peak. In human females there is also a monthly temperature variation related to the ovulatory cycle. In many mammals and birds the body temperature shows more pronounced cyclic variations than in humans. For example, in hibernators the body temperature may lower to only a few degrees above the environmental temperature during the dormant periods; mammalian hibernators reawake spontaneously and in their active period are homeothermic.
Cold-blooded Animals (Poikilotherms)
Reptiles and other poikilothermic animals bask in warm weather and must hibernate in winter. The body temperature of fishes must remain close to that of the surrounding water, because heat is lost directly into the water during respiration. In the opah, bluefin tuna, and some other fishes, however, a special networks of blood vessels allow them to conserve metabolic heat and warm regions of their bodies. The mechanism of temperature regulation in homeotherms is considered an important evolutionary advance in that physical activity in such animals can be relatively independent of the environment.