critical temperature

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the degree of sensible heat or cold, expressed in terms of a specific scale. See Table of Temperature Equivalents in the Appendices. Body temperature is measured by a clinical thermometer and represents a balance between the heat produced by the body and the heat it loses. Though heat production and heat loss vary with circumstances, the body regulates them, keeping a remarkably constant temperature. An abnormal rise in body temperature is called fever.

Normal Body Temperature. Body temperature is usually measured by a thermometer placed in the mouth, the rectum, or the auditory canal (for tympanic membrane temperature). The normal oral temperature is 37° Celsius (98.6° Fahrenheit); rectally, it is 37.3° Celsius (99.2° Fahrenheit). The tympanic membrane temperature is a direct reflection of the body's core temperature. These values are based on a statistical average. Normal temperature varies somewhat from person to person and at different times in each person. It is usually slightly higher in the evening than in the morning and is also somewhat higher during and immediately after eating, exercise, or emotional excitement. Temperature in infants and young children tends to vary somewhat more than in adults.
Temperature Regulation. To maintain a constant temperature, the body must be able to respond to changes in the temperature of its surroundings. When the outside temperature drops, nerve endings near the skin surface sense the change and communicate it to the hypothalamus. Certain cells of the hypothalamus then signal for an increase in the body's heat production. This heat is conducted to the blood and distributed throughout the body. At the same time, the body acts to conserve its heat. The arterioles constrict so that less blood will flow near the body's surface. The skin becomes pale and cold. Sometimes it takes on a bluish color, the result of a color change in the blood, which occurs when the blood, flowing slowly, gives off more of its oxygen than usual. Another signal from the brain stimulates muscular activity, which releases heat. Shivering is a form of this activity—a muscular reflex that produces heat.

When the outside temperature goes up, the body's cooling system is ordered into action. Sweat is released from sweat glands beneath the skin, and as it evaporates, the skin is cooled. Heat is also eliminated by the evaporation of moisture in the lungs. This process is accelerated by panting.

An important regulator of body heat is the peripheral capillary system. The vessels of this system form a network just under the skin. When these vessels dilate, they allow more warm blood from the interior of the body to flow through them, where it is cooled by the surrounding air.
Abnormal Body Temperature. Abnormal temperatures occur when the body's temperature-regulating system is upset by disease or other physical disturbances. fever usually accompanies infection and other disease processes. In most cases when the oral temperature is 37.8°C (100°F) or over, fever is present. Temperatures of 40°C (104°F) or over are common in serious illnesses, although occasionally very high fever accompanies an illness that causes little concern. Temperatures as high as 41.7°C (107°F) or higher sometimes accompany diseases in critical stages. Subnormal temperatures, below 35.6°C (96°F) occur in cases of collapse; see also symptomatic hypothermia.
absolute temperature (T) that reckoned from absolute zero (−273.15°C), expressed on an absolute scale.
basal body temperature (BBT) the temperature of the body under conditions of absolute rest; it has a slight sustained rise during the luteal phase of the menstrual cycle and can be used as an indirect indicator of when ovulation has occurred.
body temperature the temperature of the body of a human or animal; see temperature.
core temperature the temperature of structures deep within the body, as opposed to peripheral temperature such as that of the skin.
critical temperature that below which a gas may be converted to a liquid by increased pressure.
normal temperature the body temperature usually registered by a healthy person, averaging 37°C (98.6°F).
risk for imbalanced body temperature a nursing diagnosis accepted by the North American Nursing Diagnosis Association, defined as a state in which an individual is at risk of failure to maintain body temperature within the normal range.
subnormal temperature temperature below the normal. See also symptomatic hypothermia.

crit·i·cal tem·per·a·ture

the temperature of a gas above which it is no longer possible by use of any pressure, however great, to convert it into a liquid.

critical temperature

the highest temperature at which a substance can exist as a liquid.

crit·i·cal tem·per·a·ture

(krit'i-kăl tem'pĕr-ă-chŭr)
The temperature of a gas above which it is no longer possible by use of any pressure, however great, to convert it into a liquid.


1. a point at which one property or state changes to another property or state.
2. pertaining to a crisis in a disease.

critical care
care of a patient in a life-threatening situation of an illness. Includes artificial life support system.
critical care unit
see intensive care unit.
embryological critical period
the period during the life of the embryo, specific for each body system, during which organ genesis takes place.
critical distance
critical point drying
the technique used in preparing tissues for electron microscopy; to eliminate distortion due to surface tension.
critical temperature
the body temperature above which the animal is said to be fevered. See body temperature.


the degree of sensible heat or cold, expressed in terms of a specific scale. See also hypothermia, hyperthermia.

absolute temperature
that reckoned from absolute zero (−459.67°F or −273.15°C).
air temperature
the temperature of the surrounding air as measured by a dry-bulb thermometer.
ambient temperature
temperature of the immediate environment.
body temperature
a prime technique for assessing health status of a patient. Always a rectal temperature. Average temperatures above which hyperthermia, pyrexia or fever can be said to occur are listed under pyrexia.
critical temperature
1. that below which a gas may be converted to a liquid by pressure.
2. the environmental temperature at which the body is unable to maintain a constant body temperature and at which heat production must be increased (cold temperatures) or at which heat loss must be increased (high temperatures).
effective temperature
the combination of air temperature, humidity and wind speed. See also temperateness index.
environmental temperature
air temperature.
nonpermissive temperature
one at which a conditional gene mutation is nonfunctional. See also temperature-sensitive mutation.
normal body temperature
that usually registered by a healthy animal. See pyrexia.
permissive temperature
one at which a conditional gene mutation can express its normal function. See also temperature-sensitive mutation.
premortal temperature fall
the sudden fall in body temperature of a previously fevered animal just before death.
rectal temperature
the body temperature as measured by a rectal thermometer which has been in situ and in contact with the mucosa of the rectum with the anal sphincter tightly closed for at least 30 seconds. Alternative equipment is a dipolar electrode in a rectal probe.
temperature stress
exposure to excessively high or low environmental temperature.
windchill temperature
a combination of wind velocity and air temperature. See also effective temperature (above).
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