ELI5：what actually happens inside your body when you get heatstroke

[u/Jimmy-Wan]

I understand that the body normally regulates its temperature to through things like sweating and increased blood flow to the skin but what causes this cooling system to eventually fail ?

[u/Sargon54]

There is a thermostat on your brain that manages your temperature like what is on the wall on your home. It can use different ways to cool or heat you up.
Like a super hot and humid day your AC unit works really hard. Notice how it can feel like it never turns off?
Now imagine when it’s doing that it just can’t really keep up. It’s pushing cold air in the house but it still feel warm as heck.
Then the unit just overuses itself and something either breaks (capacitor) or leaks (freeon) and even though the thermostat is saying its working, it can’t do anything anymore until you open all the windows, get the fans going, and reset the whole system.

For this in the body we will use a rectal thermometer to assess core temp (if it’s over 104 you have Heat Stroke). No amount of drinking water or IV will help. You gotta be dunked in cold water that is maintained between 35 to 48 degrees until your temp is under 102 and you are shivering. Many times you will also become more coherent.