When a person is too hot, the blood vessels in her skin expand to carry the excess heat to her skin's surface. And for this reason the person begins to sweat. Then the sweat evaporates and this process helps to cool her body. When a person is too cold, his/her blood vessels narrow so that blood flow to her skin gets reduced to conserve body heat. As a result she starts shivering and it is an involuntary, rapid contraction of the muscles.