Patients enter pre-op mildly to moderately dehydrated. They've had no recent food to metabolize. We strip them of their insulating layers and replace their clothes with a thin gown that flaps open in the back. We drip cold fluid through their veins, lay them on cold surgical tables in breezy rooms, put their brains to sleep and breathe dry air into their lungs. We expose their skin, paint volatile fluids onto the surgical site and expose their warm cavities to room air. These factors are enough to overwhelm normal thermoregulation, and they explain why all patients undergoing any surgical procedure face the risk of hypothermia.