It is thought that in this stage the thermoregulatory center is reset. The thermoregulatory center has probably two compartments. The impulses from the sympathetic compartment that are sent by sympathetic fibers to the whole organism are operating in this stage. In cutaneous and subcutaneous vessels, they cause vasoconstriction, thus they decrease the the heat outcome. On the other hand, muscles, liver, and heart, under the influence of sympathetic compartment, increase production of heat that forms, together with decreased outcome of heat, the optimal situation for heat accumulation in an organism.
Body temperature increases, but the sick person has a feeling of cold.
Thermogenesis participates in this process through thyroxin and triiodine thyronine. In consequence of thyroxin thermogenesis and the activation of sympathicus, the effect of cardiovascular and respiratory systems increases together the basal metabolism. These changes may be measured by increased utilization of oxygen in the organism. Climax means that the body temperature culminates. At culmination of fever, such a temperature is achieved to which the thermoregulatory center is reset.