The human body has a homeostatic temperature regulation system, which means it tries to regulate temperature by producing heat or losing heat as needed. How the body adapts to cold depends on the intensity of the exposure as well as body composition, age and overall fitness.
The chemical reactions that occur as part of metabolism are one of the major sources of heat production in the human body. When exposed to cold, the body increases metabolic heat production to maintain a normal temperature. Shivering is another way the body adapts to cold weather. This response occurs when the muscles rapidly contract and relax in an attempt to warm up the body.
The glands of the endocrine system also play a role in heat production. Thyroxine, norepinephrine and epinephrine secretion results in an increased basal metabolic rate, which is the minimum amount of energy needed for the body to maintain all of its essential functions.
As a person acclimates to cold temperatures, the body becomes more efficient at conserving heat and minimizing heat loss. Instead of shivering, the body is able to generate heat by other means. As the body acclimates to the cold, it also does a better job of delivering heat to the upper and lower extremities.