Heart rate can rise or fall, but the temperature of the blood must remain constant. A severe drop in body heat can damage cells by inhibiting critical enzyme reactions.
Even a mild rise in temperature makes us feverish, and we cannot survive for long is our temperature shoots above 108*F. The innate intelligence of the body monitors its temperature through a thermostat that measures the heat of blood flowing through the brain.
If air temperature drops even a fraction of a degree and our blood cools, the autonomic nervous system responds instantly. Parasympathetic nerves slow the heart; sympathetic nerves constrict vessels in the skin.
When weather turns hot or when we exercise — combusting more oxygen and thus generating heat — blood changes course. The sympathetic nerves open arteriole valves, and the blood vessels in our skin act like radiators, cooling the body by casting off heat to the surrounding air.
Amazing isn’t it?