Thermal/Heat Exchange Mechanisms
Approximately, 75% of heat losses are continuous by sensible means of convection, conduction and radiation, and 25% as latent or hidden/insensible ways of evaporative heat loss through the respiratory system and skin.
During variation in environmental temperature, initially animals manage to maintain homeothermy by regulating its surface temperature through the process of vasodilatation or vasoconstriction. In addition, the animal modifies its exposed surface area through behavioural responses by changing its posture. Heat dissipation depends on the surrounding conditions of an animal and is primarily determined by peripheral temperature with the underlying changes in temperature gradient. The body surface area of the endotherms is poorly covered with coats or even lacks insulation with a higher range of cutaneous blood flow, and these areas are referred to as ‘thermal windows’, such as ears, feet and nose of mammals, and bills, feet, comb and wattles of birds. Further, heat transfer or dissipation mechanisms greatly vary among animals, depending on several internal and external factors like estrus, pregnancy, parturition, lactation and environmental conditions. The Ta, RH, wind speed, solar radiation and shade also highly in fluence the body temperature. As soon as the temperature balance between the surface of the animal and the environment is decreased during hot environmental conditions, the animal enhances its evaporative heat loss to make up the reduced sensible heat loss. The feed intake and metabolism by the animal increase with reduced environmental temperature. In addition to this, the enhanced shivering or non-shivering thermogenesis are vital adaptive responses adopted by animals. Therefore, to maintain a constant core body temperature, heat produced and gained must be equal to the heat lost from the body. There are four major pathways (convection, conduction, radiation and evaporative heat loss) that aids in maintaining homeothermy in animals.28.9.1 Radiation
Radiation is the movement of heat through release of electromagnetic energy from warm body to cold environment without physical contact. These electromagnetic waves are divided into short-waves or waves from the sun and long waves which radiate from the environment. Radiant energy heats the air indirectly by heating solid surfaces, for example soil, clouds, water, trees and animals. The loss or gain of heat ensued from the body by infrared waves depends on temperature and received infrared thermal radiation from its surroundings. However, heat transfer by the radiation is influenced by surface area of the animal, skin temperature, surrounding air and the emissivity of the animal’s skin. The degree of radiant heat transfer is very complex where exposure is a function of direct sunlight to the surface of the animal and the level of reflection from the surroundings. Further, the intensity of radiation depends on the colour and presence or absence of vegetation on the surface. In animals, solar radiation is a function of surface area exposed to the radiation and the colour and structure of their coat. The black coat colour absorbs more heat (absorbance ~1.00) than red (0.65) and white fur (0.37).
28.9.2 Conduction
The dissipation of heat from warmer to cooler objects through direct physical contact can be termed as conduction. This depends on the temperature gradient between the contacting surfaces and also on thermal conductivity. The process of conduction transfers the core heat to the skin surface and from the periphery to surroundings. If an animal is lying on a cool surface, the conductive heat transfer is significantly higher than standing depending upon the thermal conductance, temperature gradient and area of contact. Conduction plays a minimal role in heat dissipation mechanism during thermal stress in livestock due to the reduced thermal gradient.
28.9.3 Convection
It is transfer of heat from one molecule to another or could be considered as a specialized conduction where the heat from warm body is moved away from the area by a current of air or water. The efficiency of convective heat loss is influenced by velocity of air, air temperature, body surface area and surface temperature. The factors that resist flow of air will reduce the rate of convective heat transfer because animal lose heat to the environment through air. The animal’s fur entraps a layer of air close to the skin which prevents the passive convection. However, convection facilitates in respiratory heat loss by increased air flow via nasal routes where the upper respiratory tract takes away extra quantity of body heat. In addition, convective heat transfer is one of the major approaches adopted by animals to transfer core body heat to the periphery.
28.9.4 Evaporation
Evaporation is the process of removal of adequate heat into a liquid which turns it into a gas. The level of heat transferred during hot environmental conditions through conduction, convection and radiation is inadequate. The evaporation is the principal way of heat loss mechanism in animals when environmental temperature and radiant heat are high or equivalent to skin temperature.
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