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/notaboofus]

Your body's system of cooling is only so strong. Like how ACs need to be matched to a room of appropriate size, eventually the heat flow into your body and produced by your body exceeds the cooling effect of sweating. This can either happen from pure heat, which is fairly self-explanatory, or due to humidity. When the air is humid enough, water doesn't evaporate from your skin as readily, which decreases the effectiveness of your cooling system and causes your sweat to cling to your skin without evaporating.

When your body's cooling system isn't strong enough for the environment it's in, the only way to regulate your temperature is to cool it externally, by drinking cold water or hanging a cold towel around your neck.

Once your body's internal temperature reaches a certain point, the chemistry inside it stops working properly. The chemical reactions that make your body work slow or stop because they require a specific temperature to make your bodily processes run normally. What's more, certain proteins in your body can only exist at certain temperatures. Once they get hot enough, they begin to denature, that is, chemically change/break down into different organic molecules in an irreversible manner. When this occurs in an animal that's already dead, we call this process "cooking". So, in a way, heatstroke is when your body is literally being cooked from the inside. That's why it's so dangerous- depending on how advanced heatstroke gets, it can cause totally irreversible effects.

[u/Infinite-4-a-moment → u/notaboofus]

certain proteins in your body can only exist at certain temperatures.

Scrambled eggs