In the first, ice crystals form in the space outside of the cells. Water is lost from the cell s interior, and dehydration promotes the destruction of the cell. In the second, the damaged lining of the blood vessels is the main culprit. As blood flow returns to the extremities upon rewarming, it finds that the blood vessels themselves are injured, also by the cold. Holes appear in vessel walls and blood leaks out into the tissues.
Flow is impeded and turbulent, and small clots form in the smallest vessels of the extremities.
Because of these blood flow problems, complicated interactions occur, and inflammation causes further tissue damage. This injury is the primary determinant of the amount of tissue damage you will have in the end. It is rare for the inside of the cells themselves to be frozen. This phenomenon is only seen in very rapid freezing injuries, such as those produced by frozen metals. A variety of frostbite classification systems have been proposed.