Pathophysiology
The body maintains the core temperature of 36.5-37.5oC by balancing heat production and heat loss.
Heat production
Heat is generated by cellular metabolism at a rate of 40-60 kcal/m2 of body surface area (BSA). This heat production can be increased by 30 kcal/m2 BSA by contraction of striated skeletal muscle. Shivering can generate 2-5 times the basal heat production rate.
Heat loss
Under normal circumstances, 55-65% of heat loss is from radiation. Conduction and convection account for about 15%; the rest is from respiration and evaporation.
Thermoregulation
The hypothalamus controls thermoregulation via increased heat conservation (peripheral vasoconstriction and behaviour responses) and heat production (shivering and increasing levels of thyroxine and adrenaline).
Learning bite
Once heat is lost, the body sacrifices the extremity with an intense peripheral vasoconstriction and counter-current exchange of heat (from the warm arteries to the cool veins) to preserve the core temperature – and the life of the patient.