61 - Chapter 61 THERMOREGULATORY MECHANISMS A Heat...

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THERMOREGULATORY MECHANISMS A complete analysis of interactions between animals and their environment involves consideration of metabolism, homeostasis, homeorhesis, climatology, thermodynamics, behavior, stress, and adaptation. The subdiscipline called thermoregulatory physiology attempts to deal with all of these aspects. Some ways in which animals satisfy their homeostatic drives in the face of thermal challenges and potential abuses from the environment will be highlighted. The discussion is intentionally selective and merely touches on factors to be considered when assessing animal functions under widely differing climatic conditions. Heat production is an inevitable part of catabolism. The highly efficient oxidation of glucose to carbon dioxide, water, and salvageable energy in the form of adenosine triphosphate (ATP) still results in about 50% energy loss as heat. This seemingly inefficient conversion of chemical energy from nutrients into heat production is nevertheless essential for homeotherms that are attempting to maintain deep body, or core , temperatures in the broad range of 35 to 40 C. Excessive heat produced is lost to the environment, so that core temperature can be closely regulated. However, some heat loss from animals is obligatory, especially in colder, less humid environments. This need for heat generation must be considered in the energy economy of animals. At a practical level, this aspect is factored into the design of animal housing and in formulating rations to meet the nutritive requirements of livestock. A. Heat exchanges Animals exchange heat with the environment through two basic processes: sensible and insensible . Sensible heat loss occurs when thermal energy is directly transferred through the physical processes of radiation , conduction , and convection . Insensible or evaporative losses occur when thermal energy is used to change the physical state of water by evaporation , or vaporization . Many animals are able to sweat, thereby secreting aqueous solutions onto the skin surface, from which evaporation can take place. Sweating is regulated centrally by what is known as sudomotor control. Other animals achieve the same result behaviorally by wetting their surface by wallowing or related activities. Additional insensible heat loss occurs because inspired air is brought to saturation by the evaporation of water off the mucosal surfaces in the buccal and nasal cavities, and upper trachea, as described in Chapter 36. 1. Sensible exchanges Radiation , which can cause both heat loss and heat gain, is the transfer of heat by electromagnetic waves without appreciable heating of the space between the emitter and absorber . This mechanism is the basis for the warming effect of the sun, either directly or with the benefit of reflection. This also occurs during the winter, even though air temperatures are quite cold. Numerous factors affect radiant heat transfer, and some generalization is needed. Radiant heat losses
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This note was uploaded on 03/23/2009 for the course ANSCI 1110 taught by Professor Brucecurrie during the Fall '08 term at Cornell.

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61 - Chapter 61 THERMOREGULATORY MECHANISMS A Heat...

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