Regulating Body Temperature

Organisms regulate body temperature by manipulating heat gain and loss. An equation, used by K. Schmidt-Nielsen (1983), can help us undertand the components of heat that may be manipulated:
Hs = Hm + Hcd + Hcv + Hr - He
Here Hs, the total heat stored in the body of an organism, is made up of Hm, heat gained from metabolism; Hcd, heat gained or lost through conduction; Hcv, heat lost or gained by convection; Hr, heat gained or lost through electromagnetic radiation; and Hc, heat lost through evaporation. These heat components represent ways that heat is transferred between an organism and its environment. Metabolic heat, Hm is the energy released within an organim during the process of celular respiration. Conduction is the transfer of heat between objects in direct physical contact, as occurs when you sit on a stone bench on a cold winter’s day; convection is the process of heat flow between a solid body and a moving fluid, such as between you and wind on a cold day. During the process of conductin or convection, Hcd and Hcv, the direction of heat flow is always from the warmer region to the colder. Heat may also be transfered throgh electromagnetic radiation. This transfer of heat, Hr s often called simply radiation. All objects above absolute ), above -273ºC, give off electromagnetic radiation, but the most obvious source in our environment is the sun. Curiously, we are blind to most of this heat flux, because at sea level over half of the energy content of sunlight falls outside our visible range. Much of this radiation that wi cannot see is in the infrared part of the spectrum. The electromagnetic radiation emitted by most objects in our environment, including our own bodies, is also infrared light. Infrared light is responsible for most of the warmth you feel when standing in front of a fire or that you feel radiating from the sunny side of a building on a winter’s day. The chilling effect of standung outdoors under a clear, cold night sky with no wind is also mainly due to radiative heat flux, in this case from your body to the surroundings, including the night sky. (Moles, 2005)

 Heat may be lost by an organism through evaporation, He. In general we need only consider the heat lost as water evaporates from the surface of an organism. The ability of water to absorb a large amount of heat as it evaporates makes cooling systems based on the evaporation of water very effective. The body temperature of this organisms, called poikilotherms, varies directly with environmental temperatures. Of the organisms that regulate body temperature, most use external sources of energy and a combination of anatomy and behavior to manipulate Hc, Hr, and He. Animals that rely mainly on external sources of energy for regulating body temperature are called ectotherms. Organisms that rely heavily on internally derived metabolic heat energy, are called endotherms. Among endotherms, birds and mammals use metabolic energy to selectively heat critical organs. Endotherms that use metabolicenergy to maintain relatively constant body temperature are called homeotherms. The only homeothermic organisms are birds and mammals. Temperature regulation presents both plants and ecthotermic animals with a similar problem. Both groups of organisms rely primarily on external sources of energy. Despite the much greater mobility of most ecthothermic animals, the ways in which plants and ectothermic animals solve these problems are similar. (Moles, 2005)

 Reference: 
Moles, M.CV. 2005. Ecology: Concept and Application. McGraw-Hill Inc., New York.

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