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Lesson 2 -- Temperature


a. To Determine the Cause of the Patient's Condition. When a person becomes ill, one of the first things that must be done is to determine what disease, injury, or other factor is responsible. Some problems, such as pneumonia and heatstroke, cause the body to become warmer than normal. Some problems, such as generalized hypothermia and some forms of shock, cause the body to become cooler than normal. Many other problems will have little or no effect upon the body's temperature. Determining whether the patient's temperature is normal, higher than normal, or lower than normal can be important in determining what is wrong with the patient.

b. To Determine the Effectiveness of Treatment. If the patient's condition has caused his body to become warmer or cooler than normal, then his temperature should return to normal as he becomes healthier. Change or lack of change in the patient's temperature may indicate if the treatment being used is actually working.


The average body temperature (measured orally) of the average healthy human being is 98.6F (37.0 C). This does not mean that everyone should always have a temperature of 98.6 F. An individual's normal temperature may be slightly higher or slightly lower. A body temperature between 96.8 F and 100.4 F is considered to be within the normal temperature range.


Why is it necessary to have a normal temperature range? Why isn’t a temperature of 98.6 F normal and everything else is abnormal? The reason is that a person can have a body temperature that is slightly above or below 98.6 F and still be normal and healthy. Some of the factors that make it necessary that "normal" temperature be defined as a range are given below.

a. People Have Different "Normal Temperatures." Remember that 98.6 F is the average normal temperature. Some people have normal temperature that is slightly higher than average while others have normal temperature that is slightly lower than average.

(1) A person whose normal body temperature is above average (such as 99.0 F) is said to have a "high-normal" body temperature.
(2) A person whose normal body temperature is below average (such as 97.6 F) is said to have a "low-normal" body temperature.

b. Menstrual Cycle Affects Body Temperature. A woman's body temperature drops slightly before ovulation, rises about 1 F above normal during ovulation, and then returns to her normal level.

c. Pregnancy Affects Body Temperature. During pregnancy, a woman's body temperature stays above her regular normal temperature.

d. Physical Activity Affects Body Temperature. When a person exercises or does hard work, his muscles change stored energy supplies in the body (mainly glucose and fat) into usable energy. When the body’s muscles change stored energy into usable energy, heat is given off. This is why you can warm up in cold weather by doing exercises.

e. Age Affects Body Temperature.

(1) A newborn baby has some difficulty in adjusting his body temperature. His temperature may be slightly high one time and slightly low the next. By the time, the baby is one year old, the parts of his body that control his body temperature are fully developed and his normal body temperature has been established.

(2) An elderly person will usually have a low-normal body temperature. The lower body temperature is caused by changes within his body and by a decrease in physical activity.

f. Weather Affects Body Temperature. When a person's body is exposed to hot weather, his body temperature rises. When a person's body is exposed to cold weather, his body temperature drops. In the cold environment the body loses heat in the following five ways.

(1) Conduction. Conduction is the direct transfer of heat from a part of the body to a colder object. For example, when a warm hand touches cold metal or ice, or when a person's hand is immersed in water with a temperature below his body temperature. Heat passes directly from the body to the colder object.
(2) Convection. Convection occurs when heat is transferred to circulating air, as when cool air moves across the surface of a person's body. A person who is standing outside in windy winter weather and who is wearing lightweight clothing is losing heat to the environment mostly by convection.
(3) Evaporation. Evaporation is the conversion of any liquid to a gas. The evaporation process requires energy (heat). Evaporation is the natural mechanism by which sweating cools the body. This is why swimmers coming out of the water feel a sensation of cold as the water evaporates from their skin. Individuals who exercise vigorously in a cool environment may sweat and feel warm at first, but later, as their sweat evaporates, they can become exceedingly cool.
(4) Radiation. Radiation is the loss of body heat directly to colder objects in the environment. Because heat always travels from a warm object to a cooler one, a person standing in a cold room will lose heat by radiation.
(5) Breathing. Breathing causes body heat to be lost as warm air in the lungs is exhaled into the atmosphere and cooler air is inhaled.

NOTE: If a person is working in an area that is hot due to the type of work being done, such as in a steel mill, his body temperature will rise since the hot environment is his "weather."

g. Time of Day Affects Body Temperature. A person's body temperature is usually lower in the morning than in the afternoon. This change is mainly due to warmer weather and more physical activity occurring later in the day.

h. Emotions Affect Body Temperature. A person that is excited (joyful, scared, angry, and so forth.) will have an increase in body temperature. The excitement causes the body to increase the rate at which it changes stored food (glucose and fat) into usable energy. As the energy output increases, so does the amount of heat produced by the body.

i. Place of Measurement Affects Measurement. The three locations normally used in determining the body temperature are the mouth (oral temperature), the rectum (rectal temperature), and the armpit (axillary temperature). If you measured a person's body temperature using all three of these methods, you would obtain three slightly different temperatures. The axillary (armpit) temperature would be slightly lower than the oral (mouth) temperature while the rectal (rectum) temperature would be slightly higher than the oral temperature. The rectal temperature is considered an essential measurement in the hypothermic (cold injury) or hyperthermic (heat injury) patient. All other methods of obtaining the body temperature are not considered accurate for the pre-hospital or battlefield environment when dealing with environmental injuries. The oral and axillary methods should be used for the clinical or field sick call settings only.


a. Hypothalamus. Human beings, like other mammals, have bodies that stay about the same temperature even when the outside (environmental) temperature changes. The body's temperature stays constant because it is constantly monitored by a small area inside the brain called the hypothalamus (hi-po-THAL-ah-mus). When the body begins to cool, the hypothalamus causes the body to produce more heat. When the body becomes too warm, the hypothalamus causes the body to loose heat faster. These heating and cooling actions are very important since tissue damage and even death can result if the body gets too cold or too hot. The hypothalamus receives information concerning the body's temperature from several sources.

(1) Skin. One source of temperature information is the skin. The skin contains many nerves that have special functions. Some nerves protect the body by providing information in the form of pain. Other nerves provide the sense of touch. The hypothalamus uses two other types of nerves. One type senses heat while the other senses cold. These nerves provide information concerning the temperature of the environment.

(2) Hypothalamus. An important source of information concerning the body's actual temperature comes from the hypothalamus itself. Part of the hypothalamus can sense the temperature of the blood flowing through the hypothalamus. During a hyperthermic emergency such as heatstroke, the hypothalamus can be overwhelmed and temporarily shut down, causing the body to lose its ability to cool the body.

b. Cooling Reactions.

(1) Perspiration increases. When perspiration (sweat) on the skin evaporates, the process uses some of the body's heat. The hypothalamus causes the body to perspire more. This increased rate of perspiration then results in more body heat being lost through evaporation.

(2) Blood vessels enlarge. Blood vessels near the surface of the skin loose heat to the environment. The hypothalamus causes these blood vessels to become larger (dilate) when the body is too warm. When the blood vessels enlarge, they loose heat faster. This enlargement causes the skin to have a reddish (flushed) appearance.

c. Warming Reactions.

(1) Muscle activity increases. When the large muscles of the body are active, heat is produced. When the body becomes too cool, the hypothalamus causes the large muscles to contract and relax. These contractions and relaxations cycles, called shivers, produce body heat.

(2) Blood vessels contract. When the body is loosing too much heat, the hypothalamus causes the blood vessels near the surface of the skin to contract (become smaller). The blood vessels' decrease in size causes the vessels to loose heat slower than normal. The contraction of the blood vessels causes the skin to look pale.


When a person's body temperature is not within the normal temperature range, the cause is usually an infection or a dangerous environmental condition.

a. Infection. An infection occurs when the body is invaded by harmful microorganisms. When an infection occurs, the body attempts to destroy the invading microorganisms. One method used by the body is raising the body's temperature to a point where the invading microorganisms will be weakened or destroyed. When a person has a body temperature above his normal body temperature, he is said to have a fever. Another name for fever is pyrexia. Pyrexia comes from the Greek word "pyr," which means "on fire."

b Environmental Conditions. Even though the body can maintain a normal body temperature under most weather conditions, very hot or very cold conditions can cause the hypothalamus to work improperly.

(1) Heatstroke. Sometimes the body cannot get rid of body heat fast enough and the body temperature rises. This condition is most likely to happen when a person is performing hard work in a hot climate. The rising body temperature is too much for the hypothalamus to handle and it begins to lose control of the body's cooling mechanisms. When the body stops perspiring, the body temperature continues to rise. This condition is known as heatstroke. The person will usually lose consciousness. Rapid cooling of the body is essential in the preventing major organ damage and death in the heatstroke patient.

(2) Generalized hypothermia. Generalized hypothermia refers to the lowering of the body temperature below normal levels. Death will result if the body temperature is not restored. For example, suppose a person falls off a ship into cold ocean water. The body looses heat to the cold water faster than the body can produce heat. If the person is removed from the cold conditions (pulled out of the water onto a boat, for example), the body may not be able to warm itself. In such a case, an outside source of heat (such as another person lying body to body with the victim) is needed to restore normal body temperature to the victim until the hypothalamus is functioning properly again. The body must be warmed slowly and in a controlled environment to prevent overwhelming of the body's system.


This course is derived from the original work, Taking Vital Signs, Academy of Health Sciences, Fort Sam Houston, Texas

David L. Heiserman, Editor

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All Rights Reserved

Revised: May 18, 2013