Heat Balance
Homeothermic animals maintain a stable core body temperature by balancing the heat generated by metabolism and lost to the environment. The heat balance is accomplished by physiological, morphological and behavioural thermoregulatory mechanisms.
The animal losses heat continuously as sensible heat from the body surface through conduction, convection and radiation during stressful environmental conditions. Further, the animal losses heat incessantly as evaporative (insensible) heat through the respiratory tract and skin surface. The exact quantity of heat loss depends upon the thermal demand of the surrounding environment and the resistance to heat flow in the tissue, skin and its plumage. The environmental heat demand is a function of meteorological factors, reflecting the cooling power of the surroundings which equals the rate of heat flow from an animal to a specific environment. The thermal balance of any animal is determined by the net heat exchange by heat transfer mechanisms, together with metabolic heat production (M). The general equation describing heat balance is (Willmer et al. 2005):M = hcond (T b - T a)+ hconv (T s - T a) + had (T s - T sur) + E + S
where h values are the various heat transfer coefficients (conduction, convection and radiation), Tb is body temperature, Ta is ambient temperature, Ts is surface temperature of the body, Tsur is surrounding surface temperature, E is the evaporative heat loss and S is heat storage. This equation may be shortened for a constant condition where there is no change in animal body temperature, no heat storage and no difference between Ts and Tsur, as:
M = h (Tb - T a)+ E
where h is the total heat transfer coefficient.
In a thermally stable environment, in the ectotherm, E and M are negligible and the equation is simplified as Tb = Ta.The thermal balance of an animal is determined by metabolic heat production and its dissipation via evaporation, radiation, convection and conduction. The autonomic thermoeffectors play the vital physiological mechanisms to maintain normothermia in the absence of behavioural thermoregulations. Autonomic mechanisms activate the thermoregulatory pathways in the anterior hypothalamus to retain equity between heat generation and loss. The hypothalamus activates the body heat production or dissipation in response to environmental conditions. The adaptive thermoregulation maintains a balance between thermogenesis and thermolysis processes which are increased during cold or heat responses, respectively. Thermogenesis is the process by which body heat is produced by the alterations in metabolism, muscle activity and hormones levels. However, thermolysis refers to the overall process to dissipate excessive body heat by vasodilatation, sweating and panting.
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