Hi, this is Kate from MinuteEarth.
My temperature is just under 37 degrees Celsius - 98.6, for you Fahrenheit-ers - normal for a human.
But here’s something weird about our temperature: the highest body temp we know someone survived
isn't that far above 'normal', but the lowest is more than twice as far in the
other direction. Why can we get so much colder than we can hot, before our body calls it quits?
It turns out that we’re already playing with fire, as far as what’s happening inside our bodies.
Proteins we rely on to perform critical biological jobs work better when they’re toasty because heat
loosens up their coiled structure, making them flexible enough to interact with other molecules.
Plus, warmer temps boost our immune system, making it tough for germs to gain a foothold.
But there is a limit to how hot any given molecule in our body can get before it breaks.
Over 40 degrees or so, certain proteins get so loose that they start to uncoil completely.
Not only can they no longer do their critical jobs; uncoiled proteins start tangling up in tight
clumps, which can gum up cells. This tangling is what you’re seeing when you fry an egg,
although the egg's hardy proteins can tolerate way more heat than the delicate proteins in our cells.
Other things start breaking above 40 degrees too; membranes made of fatty molecules start to liquify
and channels in and between our cells malfunction, so stuff leaks where it isn’t supposed to. By the
time our body reaches about 47 degrees, enough critical components have broken down that we’re
toast. Yet because of the benefits of warmth, the human body - like the bodies of lots of
other warm-blooded critters - has evolved to operate pretty close to this breaking point,
with only a little wiggle room for the occasional fever or overexertion.
There’s way more wiggle room in the other direction. As molecules get colder,
they become less prone to interacting, which slows chemical reactions down. Your body can’t
metabolize food as readily, electrical activity in your brain decreases, and less blood gets
pumped through your body. This slow-down isn’t inherently deadly - it can actually be helpful,
because the slower reactions happen, the less energy and oxygen organs require. That’s why we
chill organs before transplanting them and induce hypothermia in people with brain injuries, and
it’s part of how bears and other animals survive huge wintertime drops in their body temperature.
But there is a limit to how slow - and therefore how low - you can go. Even cold,
slowed-down organs require some blood flow to supply life-sustaining resources and take
away life-threatening toxins...without that, we die. But as long as the temperature of a
person - or a snoozing bear - stays just high enough to keep our organs working,
we can generally reverse the problems caused by cold...just
warm up again (slowly!) and things will go more or less back to normal.
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