Updated August 26, 2025 01:38PM
Brain Shutdown, Muscle Breakdown: What Heat Stroke Really Does to Your Body
Heat stroke strikes quickly. Avoid it with these tips from wilderness medicine expert David Fifer.
(Photo: FG Trade via Getty Images)
When outside temperatures soar, humidity swaddles you like a wet blanket, and the sun seems adamant about baking you to a crisp, being outdoors can feel downright miserable. Heat is the most lethal weather-related cause of death. It can turn from uncomfortable to life-threatening faster than you think, especially if you’re exerting yourself.
Heat exhaustion might tap you firmly on the shoulder to remind you to take it easy, but heat stroke can strike hard and fast and cause long-lasting internal damage and even death. But what’s actually happening in the body when someone suffers from this, the most severe type of heat illness? And how does heat become so deadly?
How Heat Stroke Happens
It comes down to physics, explains David Fifer, wilderness medicine expert and associate professor of emergency medical care at Eastern Kentucky University. Heat stroke occurs when your body can’t dissipate heat into the atmosphere, so it builds up inside to potentially disastrous effects.
That can happen because outside temps are soaring, you’re overexerting yourself, or a combination of both. However, triple-digit mercury readings aren’t required to suffer from heat stroke. Fifer once treated a marathoner who overheated when it was rainy and 58 degrees Fahrenheit.
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That said, the higher the heat index—and more importantly, the higher the wet bulb globe reading that considers factors like wind, sun intensity, longitude and latitude, and ambient moisture—the more danger you’re in, especially if you’re pushing yourself physically.
It all starts in your brain’s hypothalamus, which is responsible for helping your body stay cool when it’s hot out and you’re working hard. Sensations, such as heat on your skin, send signals that tell it that it’s time to initiate cooling protocols like sweating, the body’s primary heat regulating response.
But in order for sweat to be productive, it has to evaporate. And if the humidity level is higher than about 75 percent, that can’t happen effectively. Plus, when outside temperatures are higher than your body temperature, other regulating methods (convection, radiation and conduction) aren’t as effective. Your body then starts to overheat and triggers what medical professionals call “a clinical constellation of symptoms.”
Symptoms and Internal Systems
First, Fifer explains, as your body starts to accumulate and retain heat, your body temperature rises—heat stroke starts in earnest when your internal temp hits 104 degrees—and your blood vessels dilate. Your heart and respiratory rates increase and move blood from your vital organs to skin and extremities to encourage more heat loss through evaporation (sweat) and radiation.
Because your heart, muscles and lungs are now working harder, your body requires more oxygen, but you can only inhale so much, and you quickly develop an oxygen debt. Your blood pressure drops next, and you start to feel unwell, woozy, dizzy, and fatigued.
Now all this retained heat in your body causes muscle to break down, which clogs the kidneys with excess proteins. Blood vessels become inflamed and damaged, and your cells sound the alarm, which starts a full system inflammatory response. Proteins in your brain begin to denature (lose their structure and function) as the brain and central nervous system shut down.
Simultaneously, the heat damages liver cells, which prevents your body from metabolizing or detoxifying the bacteria that may be leaking from your gut into your bloodstream. All while, this system breakdown is preventing the body from effective thermoregulation and circulation.
You’re effectively in an all-too-literal death spiral as you experience confusion, disorientation, lethargy and agitation, plus a loss of coordination, balance and fine motor skills—similar to symptoms of medical stroke. Collapse and unconsciousness are the final symptoms.
The fatality rate of heat stroke is high, due to the immense amount of destruction to multiple internal systems, including renal, cardiovascular, and neural failure, Fifer states: If untreated, as high as 80 percent of people die, and if treated, up to 60 percent die. Even if you survive the initial event, the risk of death is still 60 to 70 percent for up to two years, Fifer says, because of the trauma on the body. Heat stroke is largely underreported, however, so it’s hard to say with accuracy how often it happens outdoors.
Call For Help
“People shouldn’t hesitate to call search and rescue if they suspect heat stroke,” Fifer says, because it comes with such high risks.
Keep an eye out for indications that companions are “hot and bothered,” as Fifer describes it: When it’s hot outside and someone becomes more irritable, combative, mentally foggy, or starts stumbling, you should worry. If you suspect heat exhaustion but symptoms don’t improve after resting in the shade, drinking electrolytes, and trying to cool down, assume heat stroke and call for help immediately. Hiking out won’t be an option.
The gold standard of care is to immerse the patient up to their neck in ice water until their body temperature drops to 102 degrees. In the backcountry, try an alpine lake or any cool body of water you’re near. If that’s not possible, while you wait for help, move the patient to the shade and cool them as best as possible by soaking their clothes if you have enough water and fanning them. Lower their temperature within 30 minutes of initiating treatment (which should start within 10 minutes of first symptoms), and they’ll have a better chance of survival, Fifer explains.
Prevention is the best medicine, Fifer states. So when it’s hot, wear sun-protective clothing, maintain a slower pace than usual, avoid hiking in the hottest part of the day, stay hydrated with water and electrolytes, and acclimatize to hot weather over a period of a few weeks before going hard.
Then take care, be vigilant, and stay safe in the heat.
