|Human Physiology 6/e Fox|
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Hyperthermia and Hypothermia
If heat gain exceeds the ability of the body to lost heat, then body temperature increases above normal levels, a condition called hyperthermia. Hyperthermia can result from exposure to hot environments, exercise, fever, and anesthesia.
Exposure to a hot environment normally results in the activation of heat loss mechanisms, and body temperature is maintained at normal levels. This is an excellent example of a negative-feedback mechanism. However, prolonged exposure to a hot environment can result in heat exhaustion. The normal negative-feedback mechanisms for controlling body temperature are operating, but they are unable to maintain a normal body temperature. Heavy sweating results in dehydration, decreased blood volume, decreased blood pressure, and increased heart rate.
Individuals suffering from heat exhaustion have a wet, cool skin because of the heavy sweating. They usually feel weak, dizzy, and nauseated. Treatment includes reducing heat gain by moving to a cooler environment, reducing heat production by muscles by ceasing activity, and restoring blood volume by drinking fluids.
Heat stroke is a breakdown of the normal negative-feedback mechanisms of temperature regulation. If the temperature of the hypothalamus becomes too high, it no longer functions appropriately. Sweating stops, and the skin becomes dry and flushed. The person becomes confused, irritable, or even comatose. In addition to the treatment for heat exhaustion, heat loss from the skin should be increased. This can be accomplished by increasing evaporation from the skin by applying wet cloths or by increasing conductive heat loss by immersing the person in a cool bath.
Exercise increases body temperature because of the heat produced as a by-product of muscle activity. Normally vasodilation and increased sweating prevent body temperature increases that are harmful. In a hot, humid environment the evaporation of sweat is decreased, and exercise levels have to be reduced to prevent overheating.
Fever is the development of a higher-than-normal body temperature following the invasion of the body by microorganisms or foreign substances. Lymphocytes, neutrophils, and macrophages release chemicals called pyrogens (pi'ro-jenz) that raise the temperature set point of the hypothalamus. Consequently body temperature and metabolic rate increase. Fever is believed to be beneficial because it speeds up the chemical reactions of the immune system and inhibits the growth of some microorganisms. Although beneficial, body temperatures greater than 41 degrees C (106 degree F) can be harmful. Aspirin lowers body temperature by affecting the hypothalamus, resulting in dilation of skin blood vessels and sweating.
Malignant hyperthermia is an inherited muscle disorder. Drugs used to induce general anesthesia for surgery cause sustained, uncoordinated muscle contractions in some individuals. Consequently body temperature increases.
Therapeutic hyperthermia is an induced local or general body increase in temperature. It is a treatment sometimes used on tumors and infections.
If heat loss exceeds the ability of the body to produce the heat, body temperature decreases below normal levels. Hypothermia is a decrease in body temperature to 35 degrees C (95 Degree F) or below. Hypothermia usually results from prolonged exposure to cold environments. At first, normal negative-feedback mechanisms maintain body temperature. Heat loss is decreased by constricting blood vessels in the skin, and heat production is increased by shivering. If body temperature decreases despite these mechanisms, hypothermia develops. The individual's thinking becomes sluggish, and movements are uncoordinated. heart, respiratory, and metabolic rates decline, and death results unless body temperature is restored to normal. Rewarming should occur at a rate of a few degrees per hour.
Frostbite is damage to the skin and deeper tissues resulting from prolonged exposure to the cold. Damage results from cold injury to cells, injury from ice crystal formation, and reduced blood flow to affected tissues. The fingers, toes, ears, nose, and cheeks are most commonly affected. Damage from frostbite can range from redness and discomfort to loss of the affected part. The best treatment is immersion in a warm water bath. Rubbing the affected area and local dry heat should be avoided.
Therapeutic hypothermia is sometimes used to slow metabolic rate during surgical procedures such as heart surgery. Because metabolic rate is decreased, tissues do not require as much oxygen as normal and are less likely to be damaged.
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