Ingestion of food leads into increase in heat production which varies greatly with the type of nutrients ingested. Also in ruminants the rumen micro-organism constitutes an auxiliary source of heat (about 10 per cent) in addition to the animal’s moral heat production. The foetuses as well as lactation add up to the amount of heat production by the dam.
During severe cold or active physical exercise, the heat generation by muscular activities increases while the heat from abdominal organ decreases. In cattle and sheep, heat production is about 10 per cent greater in standing position than in lying position. In pregnant animal, foetus metabolism together with the acceleration of body processes of the dam result in an increment in the total heat production.
The presence of brown fat is another source of heat production. Brown fat is found in rodents but has been found in other animals including man. It is especially useful in homeotherms exposed to cold and hibernators.
The brown adipose tissue is distributed around vital organs of the thorax, along sympathetic ganglia of the central nervous system, around the cervical and thoracic segment of the spinal cord to prevent loss of heat and excessive cold from inactivating the function of the vital organs.
Both the metabolic and thermo-genic actions of the brown fat are by stimulation from sympathetic nervous system under cold condition.
Heat Loss: Heat loss from animal body is by two means:
(1) sensible heat loss i.e. through radiation, convection and conduction
(2) insensible heat loss which is through evaporation of water.
Heat loss by means of sensible heat loss offers little or no control for the animal to regulate unlike insensible heat loss in which animal exerts marked control. Heat transfer by sensible heat can be in either direction of loss or gain, while insensible heat transfer is only along one direction i.e. through loss from animal to the environment.
Heat transfer involves two forms of gradient:
• Inner gradient and out gradient. Inner gradient concerns with heat flows from the core of the body to the surface of the body.
• Outer gradient on the other hand refers to the heat that goes from the surface of the body to the environment. Heat transport along the inner gradient is affected by conduction across the tissues and by convection by the blood. Along the outer gradient, heat transport is by the following:
1. Convection across the hair coat and boundary layer of still air surrounding the body
2. Convection from the boundary layer of air to the fully moving air
3. By radiation from the tips of the air across the boundary layer
4. Evaporation across the hair coat and boundary layer.