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Thermoregulation in homeothermic and poikilothermic organisms
Citation:
GORDON, C. J. Thermoregulation in homeothermic and poikilothermic organisms. Presented at Hypothermia-From Threat to Cure, sponsored by NY Academy of Sciences, Manhattan, NY, September 19, 2008.
Impact/Purpose:
Homeothermic organisms (birds and mammals) have evolved autonomic and behavioral thermoeffectors to maintain a relatively constant core temperature over a wide range of environmental temperatures. Poikilotherms, including reptiles, amphibians, fish, and insects have internal temperatures that generally vary with environmental temperature but many species are capable of achieving a homeothermic state by using behavioral mechanisms. The body temperature of homeotherms and poikilotherms is susceptible to infectious and chemical agents as well as to many types of traumatic insults. The change in body temperature can be forced or regulated. Forced changes in temperature result from impairment of thermoeffector function for the control of heat gain or heat loss but with a normal set-point. Forced hyperthermia and hypothermia also occur when the thermoregulatory system is overwhelmed by acute exposure to heat or cold stress, respectively. On the other hand, a regulated change in temperature is mediated centrally when a change in the set-point temperature activates thermoeffector responses to raise or lower body temperature. Fever from exposure to infectious agents is a common type of regulated hyperthermia and is characterized by a behavioral preference for warmer ambient temperatures and activation of autonomic responses to raise heat production and reduce heat loss. Regulated hypothermia is a typical thermoregulatory response in rodents subjected to chemical insults, hypoxia, hemorrhage, and other insults. During regulated hypothermia, a preference for cooler ambient temperature, suppression in metabolic rate, and activation of thermoeffectors to increase heat loss mediates a controlled reduction in core temperature. Regulated hypothermia is an adaptive response to improve the ability to recover from the insult. A variety of poikilothermic species also show a regulated like reduction in body temperature when subjected to hypoxia. The hypothermic response is inversely proportional to body size. The reduction in surface area:body mass ratio with an increase in body size is a key factor that limits how quickly the body can cool. Rodents are metabolic specialists, meaning that they thermoregulate primarily by modulations in heat production. Their rapid hypothermic response results from the ability to reduce metabolic rate abruptly. Relatively large mammals rely more on their control of surface temperature to thermoregulate and they are unable to lower core temperature as quickly. These fundamental differences in thermoregulation between small and large mammals should be taken into consideration when extrapolating hypothermic responses from rodents to humans. This is an abstract of a proposed presentation and does not necessarily reflect EPA policy
Description:
Homeothermic organisms (birds and mammals) have evolved autonomic and behavioral thermoeffectors to maintain a relatively constant core temperature over a wide range of environmental temperatures. Poikilotherms, including reptiles, amphibians, fish, and insects have internal temperatures that generally vary with environmental temperature but many species are capable of achieving a homeothermic state by using behavioral mechanisms. The body temperature of homeotherms and poikilotherms is susceptible to infectious and chemical agents as well as to many types of traumatic insults. The change in body temperature can be forced or regulated. Forced changes in temperature result from impairment of thermoeffector function for the control of heat gain or heat loss but with a normal set-point. Forced hyperthermia and hypothermia also occur when the thermoregulatory system is overwhelmed by acute exposure to heat or cold stress, respectively. On the other hand, a regulated change in temperature is mediated centrally when a change in the set-point temperature activates thermoeffector responses to raise or lower body temperature. Fever from exposure to infectious agents is a common type of regulated hyperthermia and is characterized by a behavioral preference for warmer ambient temperatures and activation of autonomic responses to raise heat production and reduce heat loss. Regulated hypothermia is a typical thermoregulatory response in rodents subjected to chemical insults, hypoxia, hemorrhage, and other insults. During regulated hypothermia, a preference for cooler ambient temperature, suppression in metabolic rate, and activation of thermoeffectors to increase heat loss mediates a controlled reduction in core temperature. Regulated hypothermia is an adaptive response to improve the ability to recover from the insult. A variety of poikilothermic species also show a regulated like reduction in body temperature when subjected to hypoxia. The hypothermic response is inversely proportional to body size. The reduction in surface area:body mass ratio with an increase in body size is a key factor that limits how quickly the body can cool. Rodents are metabolic specialists, meaning that they thermoregulate primarily by modulations in heat production. Their rapid hypothermic response results from the ability to reduce metabolic rate abruptly. Relatively large mammals rely more on their control of surface temperature to thermoregulate and they are unable to lower core temperature as quickly. These fundamental differences in thermoregulation between small and large mammals should be taken into consideration when extrapolating hypothermic responses from rodents to humans. This is an abstract of a proposed presentation and does not necessarily reflect EPA policy