The heart rate begins to slow to protect the vital organs when an individual's internal temperature drops below 95 degrees Fahrenheit, causing hypothermia. Hypothermia is caused by the body losing heat to the external environment faster than it can produce heat.
The heart and liver produce most of the heat in the human body, therefore the extremities farthest from the core are the first to be affected by hypothermia. That is why fingers and toes are most commonly lost to frostbite. As the body continues to cool, blood circulation is progressively restricted to the core and head to protect the liver, lungs, brain and heart as long as possible.