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Awake ground squirrel

Onset of hibernation

18 Hours

Onset of arousal Reentry

Arousal from hibernation begins with a large rise in metabolic heat production, followed by body warming.

Arousal from hibernation begins with a large rise in metabolic heat production, followed by body warming.

Hibernating ground squirrel

41.19 A Ground Squirrel Enters Repeated Bouts of Hibernation during Winter At the beginning of each bout of hibernation, the ground squirrel's metabolic rate and body temperature fall. Its body temperature may come into equilibrium with the temperature of its nest and stay at that level for days.The bout is ended by a rise in metabolic heat production that returns body temperature to a normal level.

► Homeostasis is the maintenance of a constant internal environment.

Tissues, Organs, and Organ Systems

► Cells that have a similar structure and function make up a tissue. There are four general types of tissues: epithelial, connective, muscle, and nervous. Review Figure 41.2

► Organs consist of multiple tissue types and make up organ systems. Review Table 41.1. See Web/CD Activity 41.1

Physiological Regulation and Homeostasis

► Homeostasis depends on the ability to control and regulate the functions of organs and organ systems.

► Regulatory systems have set points and respond to feedback information. Negative feedback corrects deviations from the set point, positive feedback amplifies responses, and feedforward information changes the set point. Review Figure 41.4. See Web/CD Tutorial 41.1

Temperature and Life

► Living systems require a range of temperatures between the freezing point of water and the temperatures that denature proteins.

► Most biological processes and reactions are temperature-sensitive. Q10 is a measure of temperature sensitivity. Review Figure 41.5

► Animals that cannot avoid seasonal changes in body temperature have biochemical adaptations that compensate for those changes. These adaptations enable animals to acclimatize to seasonal changes. Review Figure 41.6

Maintaining Optimal Body Temperature

► Homeotherms maintain a fairly constant body temperature most of the time; poikilotherms do not.

► Endotherms produce metabolic heat to elevate body temperature; ectotherms depend mostly on environmental sources of heat. Review Figure 41.7

► Ectotherms and endotherms can regulate body temperature through behavior. Review Figure 41.8

► Heat exchange between a body and the environment occurs via radiation, conduction, convection, and evaporation. Review Figure 41.10

► Ectotherms and endotherms can control heat exchange with the environment by altering blood flow to the skin. Review Figure 41.11

► Some ectotherms can produce metabolic heat to raise their body temperatures.

► Some fishes have circulatory systems that function as coun-tercurrent heat exchangers to conserve heat produced by muscle metabolism. Review Figure 41.12

Thermoregulation in Endotherms

► Endotherms have high basal metabolic rates. Over a range of environmental temperatures called the thermoneutral zone, the metabolic rate of resting endotherms remains at basal levels. The basal metabolic rate per gram of tissue decreases as endotherms get bigger. Review Figures 41.13, 41.14. See Web/CD Activity 41.2

► When the environmental temperature falls below the lower critical temperature, endotherms maintain their body temperatures through shivering and nonshivering metabolic heat production. Review Figure 41.15

► Endotherms that live in cold climates have adaptations that minimize heat loss, including a reduced surface area-to-volume ratio and increased insulation.

► When the environmental temperature rises above an upper critical temperature, metabolic rate increases as a consequence of active evaporative water loss through sweating or panting.

The Vertebrate Thermostat

► The vertebrate thermostat is located in the hypothalamus. It has set points for activating thermoregulatory responses.

► In mammals, cooling the hypothalamus induces the constriction of blood vessels and increased metabolic heat production. Heating the hypothalamus induces the dilation of blood vessels and panting. Review Figure 41.17

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