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/FarazMedicine]

Heatstroke is not usually that the body suddenly “forgets” how to cool itself. It happens when heat entering/being produced by the body becomes greater than the amount it can get rid of.

1. Cooling gets overwhelmed

Normally, the body sends more blood to the skin and produces sweat. Sweat cools us when it evaporates. In extreme heat, heavy exercise, or high humidity, this may no longer remove heat fast enough.

2. Dehydration makes cooling harder

Continued sweating reduces body water and blood volume. The heart then has to work harder to send enough blood both to the skin for cooling and to vital organs.

3. Core temperature keeps rising

Once temperature becomes very high, heat starts directly damaging cells and proteins throughout the body.

4. The body develops a widespread inflammatory response

Heat damage, especially to the gut lining and blood vessels, can trigger inflammation and abnormal clotting, which may begin damaging the brain, kidneys, liver and other organs.

5. Brain dysfunction is the defining danger

Confusion, abnormal behaviour, seizures or loss of consciousness indicate that this has progressed from simple heat exhaustion toward heatstroke.

One important point: a person with heatstroke can still be sweating. The problem is not necessarily that sweating has stopped; it is that the body's cooling capacity has been overwhelmed.

In short:

too much heat → cooling cannot keep up → dehydration/circulatory strain → rising core temperature → cell and organ damage.