The body’s natural reaction to cold is self-preservation. To keep the core warm, the muscles that control the arteries clench tightly and restrict the flow of blood only to vital areas. The process is known as vasoconstriction. This is why frostbite starts in the extremities: the lack of blood flow to those areas makes them cool much faster than if they were flush with warm blood. The sudden change to heat has the opposite effect. Arteries suddenly pop open and blood surges back into those cold areas, generating an excruciating wave of pain.
