Hypothermia

Your body is a furnace running out of fuel

Your body is like a house furnace that needs three things: fuel (glucose, fat), oxygen to burn it, and a thermostat (hypothalamus) to keep the flame going. In normal cold, the thermostat cranks the burner up (shivering muscles burn fuel fast) and closes the vents to your skin (vasoconstriction keeps heat in the core). Hypothermia is what happens when the furnace loses fuel (hypoglycemia, malnutrition), the thermostat breaks (sepsis damages the hypothalamus), or the house sits in a freezing wind so strong that even a roaring furnace cannot keep up (severe cold exposure, water immersion). At first the furnace roars (violent shivering, fast heart), but once it runs out of fuel or oxygen, the burner dies (shivering stops, heart slows), and the house temperature plummets. The cold is like putting the whole furnace on ice — it preserves everything instead of destroying it — which is why you can revive a frozen furnace after hours if you restart it gently and give it fuel and oxygen again.