The body's temperature has to strictly regulated, because all of the reactions taking place in the body rely on a stable temperature. The body has different mechanisms to control its temperature to make sure it does not get too cold or too hot.
When the temperature of the body decreases, there are two main responses that occur. The muscles start to contract and release rapidly, this creates heat to warm the body (this is also known as shivering), and the blood flow is somewhat restricted in that the capillaries constrict so that less heat can be transferred to the cold air, and thus keep the heat within the body.
