What is the difference between hyperthermia and hypothermia?

What happens to the human body when the temperature is too high or too low?

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The human body operates best at a temperature of around 37 degrees Celsius. We can tolerate a change of a few degrees in either direction, but any more than that and things start to go wrong.

Once body temperature drops below 35 degrees Celsius, mild hypothermia kicks in. To conserve heat, the body diverts blood away from the skin and hairs stand on end. The muscles contract and relax involuntarily, burning fuel to generate warmth. The colder the body gets, the more it starts to slow down. Nerve signals become sluggish, speech gets slurred and confusion starts to set in.

If the core temperature drops below 32 degrees Celsius, the situation becomes critical and medical attention is needed. At this point, shivering stops and the person may pass out. Below 30 degrees Celsius, the body loses its ability to warm itself up again, and this is often fatal.

The opposite of hypothermia is hyperthermia. The body has built-in mechanisms to lose heat, but sometimes it’s too warm for them to work properly. If the body can’t get rid of excess heat, core temperature starts to rise.

When sweating isn’t enough to lower body temperature, it can lead to dizziness and nausea. The loss of fluid triggers thirst and headaches. At the same time blood vessels dilate, bringing hot blood to the skin, but as the amount of fluid in the system drops, so too does blood pressure. This can cause dizziness and even fainting.

If the temperature climbs to over 40 degrees Celsius, molecules become misshapen and can no longer do their jobs properly, and cells start to die. Untreated, hyperthermia can lead to multiple organ failure.

Thankfully, the body has a built-in thermostat that normally keeps the temperature constant.

Too hot or too cold​

What are the signs of hyperthermia and hypothermia?


  • Dizziness: The combination of dilated blood vessels and fluid loss affects blood pressure, causing dizziness.
  • Thirst: Water is lost to sweating, lowering the amount of fluid in the blood and triggering thirst.
  • Sweating: Sweating cools the skin as water evaporates, which also removes some of the excess heat.
  • Dilated blood vessels: The blood vessels dilate, bringing warm blood to the surface of the skin.


  • Confusion: The cold affects cognitive function, making people feel drowsy and confused.
  • Altered breathing: At first, breathing speeds up, but as hypothermia progresses, both heart rate and breathing rate will slow down.
  • Shivering: An automatic shivering mechanism helps to generate extra heat by contracting and relaxing the muscles.
  • Pale skin: The blood vessels in the skin constrict, diverting blood to the core of the body and helping to conserve heat.

 This article was originally published in How It Works issue 109

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