system
Given the following post, respond as if you're commenting on Reddit:
human
What actually kills a person when their body temperature is too low?

What is the first fatal effect of lowering a person's body temperature? I bet a lot of bodily functions cease to function at too low temperature on different scales. Is there a big one? Maybe one that occurs way before the rest. Thanks.
gpt
There are a strange set of physiological responses to hypothermia. In one sense, hypothermia can be preservative - the body's functions slow to a minimum - and is in fact used in some critical situations in hospitals. Humans can also survive a hugely depressed core body temperature. In fact, people are not supposed to be declared dead when they are hypothermic because recovery may be possible. There is a common adage among doctors that "you're not dead until you're warm and dead". 

However, there are a number of physiological responses that could potentially cause death in hypothermia:

1. Low temperatures affect the electrical system of the heart. The channels that allow ions through them respond slowly and incompletely. So the heart doesn't get the signals to beat as fast as it should. If this continues, you can get what's called bradyarrhythmia (slow heart rate) to such an extent that the heart cannot maintain proper blood pressure. This could be fatal.
2. The cough and gag reflexes are suppressed in cold temperatures. You could potentially die from aspirating saliva or vomit. 
3. Carbon dioxide production by the body decreases because the body is not metabolically active. Carbon dioxide is one of the major triggers for your brain to stimulate breathing. This is generally protective in normal physiological situations. This control system is broken in hypothermia and the brain may stop triggering breathing.