KEY POINTS

1. Temperature is a major factor affecting tissue function.

2. Homeotherms and poikilotherms use different strategies to regulate body temperature.

3. Body temperature depends on the balance between heat input and heat output.

Heat Production

1. Heat is a byproduct of all metabolic processes.

2. Shivering produces heat by muscle contraction.

3. Nonshivering thermogenesis is an increase in basal metabolic rate, caused especially by the oxidation of fats, to produce heat.

HeatTransfer in the Body

1. Because tissues are poor conductors, heat is most effectively transferred in the blood.

2. Countercurrent heat exchange mechanisms are used both to conserve and to lose heat.

Heat Exchange with the Environment

1. Heat loss by convection occurs when the body warms air or water.

2. Heat loss by conduction occurs when the body is in contact with a cooler surface.

3. Heat loss by radiation occurs when infrared radiation emitted by the body is absorbed by cooler objects.

4. Heat loss by evaporation occurs when the water in sweat, saliva, and respiratory secretions is converted into water vapor.

Temperature Regulation

1. Mammals and birds regulate the input and output of heat to maintain body temperature within a narrow limit.

2. Temperature-sensitive receptors are located in the central nervous system, the skin, and some internal organs.

3. Information from central and peripheral heat-sensitive neurons is integrated in the hypothalamus to regulate heat-losing or heat-conserving mechanisms.

Integrated Responses

1. The responses to heat stress are peripheral vasodilation and increased evaporative cooling.

2. The responses to cold stress are peripheral vasoconstriction, piloerection, and increased metabolic heat production by shivering and nonshivering thermogenesis.

3. Fever is an elevation of body temperature that results from an increase in the thermoregulatory set point.

Heat Stroke, Hypothermia, and Frostbite

1. Heat stroke occurs when heat production or input exceeds heat output, so body temperature rises to dangerous levels.

2. Hypothermia occurs when heat output exceeds heat production, so body temperature decreases to dangerous levels.

3. Frostbite occurs when ice crystals form in the tissues of the extremities.