Lesson 1, Topic 1
In Progress

07-Body Temperature

March 28, 2021

Considering that more than 60% of the energy released from nutrient molecules during catabolism is converted to heat rather than being transferred to ATP, it is no wonder that maintaining a constant body temperature can be a challenge. Maintaining homeostasis of body temperature, or thermoregulation, is the function of the hypothalamus. The hypothalamus operates a variety of negative-feedback mechanisms that keep body temperature in its normal range (36.2° C to 37.6° C, or 97° F to 100° F).
The skin is often involved in negative-feedback loops that maintain body temperature. When the body is overheated, blood flow to the skin increases (see the box Exercise and the Skin on p. 156). Warm blood from the body’s core can then be cooled by the skin, which acts as a radiator. At the skin, heat can be lost from blood by the following mechanisms, which are also illustrated in Figure 19-9:

1. Radiation—flow of heat waves from the blood and skin
2. Conduction—transfer of heat energy to the skin and then to cooler external environment
3. Convection—transfer of heat energy to cooler air that is continually flowing away from the skin
4. Evaporation—absorption of heat from blood and skin by water (sweat) vaporization



Abnormal body temperature
Maintenance of body temperature within a narrow range is necessary for the normal functioning of the body. As Figure 19-11 shows, straying too far out of the normal range of body temperatures can have very serious physiological consequences. The following sections identify a few of these important conditions related to body temperature.


Fever—A fever or febrile state is an unusually high body temperature associated with a systemic inflammation response. In the case of infections, chemicals called pyrogens (literally, “fire-makers”) cause the thermostatic control centers of the hypothalamus to produce a fever. Because the body’s “thermostat” is reset to a higher setting, a person feels a need to warm up to this new temperature and often experiences “chills” as the febrile state begins.
The high body temperature associated with infectious fever is thought to enhance the body’s immune responses, helping to eliminate the pathogen. Strategies aimed at reducing the temperature of a febrile person are most often normally counteracted by the body’s heat-generating mechanisms and thus have the effect of further weakening the infected person. Under ordinary circumstances, it is best to let the fever “break” on its own after the pathogen is destroyed.

Malignant hyperthermia
Malignant hyperthermia (MH) is an inherited condition characterized by an abnormally increased body temperature 546(hyperthermia) and muscle rigidity when exposed to certain anesthetics. The drug dantrolene (Dantrium), which inhibits heat-producing muscle contractions, has been used to prevent or relieve effects of this condition.

Heat exhaustion
Heat exhaustion occurs when the body loses a large amount of fluid resulting from heat-loss mechanisms. This usually happens when environmental temperatures are high. Although a normal body temperature is maintained, the loss of water and electrolytes can cause weakness, vertigo, nausea, and possible loss of consciousness. Heat exhaustion may also be accompanied by skeletal muscle cramps that are often called heat cramps. Heat exhaustion is treated with rest (in a cool environment) accompanied by fluid replacement.

Also called sunstroke, heatstroke is a severe, sometimes fatal condition resulting from the inability to maintain a normal body temperature in an extremely warm environment. Such thermoregulatory failure may result from factors such as old age, disease, drugs that impair thermoregulation, or simply be caused by overwhelming elevated environmental temperatures.
Heatstroke is characterized by body temperatures of 41° C (105° F) or higher; tachycardia; headache; and hot, dry skin. Confusion, convulsions, or loss of consciousness may occur. Unless the body is cooled and body fluids are replaced immediately, death may result.

Hypothermia is the inability to maintain a normal body temperature in extremely cold environments. Hypothermia is characterized by body temperatures lower than 35° C (95° F); shallow and slow respirations; and a faint, slow pulse. Hypothermia is usually treated by slowly warming the affected person’s body.

Frostbite is local damage to tissues caused by extremely low temperatures. Damage to tissues results from formation of ice crystals accompanied by a reduction in local blood flow. Necrosis (tissue death) and even gangrene (decay of dead tissue) can result from frostbite.