Pyrogens cause the hypothalamus to raise the body's temperature by creating additional heat and reducing heat loss.
Shivering and rigors are triggered to generate heat from muscle activity. Peripheral vasoconstriction reduces heat loss by shunting blood internally, hence making the patient look pale.
These mechanisms continue until the the blood reaching the hypothalamus reaches the new setting. The hypothalamus maintains the raised temperature at the new setting until a lower level is set when heat loss is achieved by sweating, vasodilation and the evoparation of insensible fluid loss.
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