No fire inside you.
So where does the heat come from?
feels the cold in winter
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The full narration of the video.
Lie in bed all day without moving, and your body gives off enough heat to boil eight big kettles of cold water. Yet there's no fire inside you. So where does all that heat come from? Trace it back one step: the heat comes from food.
Rice, bread and meat all hold stored energy. You eat them, break them down, and your blood carries the pieces to more than thirty trillion cells. Each cell works like a tiny kitchen. A kitchen needs a flame: gas meets air, catches all at once, and gives off light and heat.
In a cell, food meets oxygen too, but there's no flame. The cell takes food apart in tiny steps, each one releasing just a little energy, so it never sets itself on fire. On a stove, only part of the flame's energy goes into the pot. The rest escapes and warms up the kitchen.
Cells are the same: only about a third of that energy gets packed into tiny energy packets, saved for work. The other two thirds turns into heat on the spot. And those packets get spent: every heartbeat, every breath, every time you lift a hand breaks one open. Once the job is done, that energy ends up as heat too.
So while you lie still, almost all the energy your body uses ends up as heat. So which kitchens run hottest? At rest, your liver, brain, heart and kidneys weigh only about a twentieth of your body, yet make more than half the heat.
Blood flows past them and carries that heat to your hands and feet, like bowls of hot soup carried from the stove to every table. Once you move, your muscles become the biggest stove: running fast, you make about ten times the heat you make lying down. When it's cold, your body has two more tricks.
Trick one is shivering: muscles twitch over and over, lifting nothing, just making heat, up to five times the usual amount. Trick two is brown fat: its kitchens don't even bother with a pot, so all the energy turns straight into heat. Newborn babies can't shiver well yet, so they rely on it to stay warm.
Adults have brown fat too. Several research teams only confirmed it in 2009. But how much it really helps us in winter is still being argued over. Your guess: does that bit of brown fat in grown-ups help a lot, or just a little?
A day in bed
boils eight kettles
An adult who lies still in bed all day gives off enough heat to boil eight big kettles of cold water.
Yet there is no flame anywhere in the body. Go back one step: that heat comes from the food you eat.
Every cell is
a tiny kitchen
and bursts into flame
and is taken apart step by step
Once food is broken down, the blood carries it to the 30-plus trillion cells in your body, and each one “burns” it slowly.
Only part of the fire
goes into the pot
Only part of a stove’s fire goes into the pot. The rest warms the kitchen. Cells work the same way:
So when you lie still, almost all the energy your body uses ends up as heat.
The four
hottest stoves
At rest, the liver, brain, heart and kidneys make up only about one-twentieth of body weight, yet they give off more than half the heat.
Blood flows past them and carries the heat to your hands and feet, like bowls of hot soup carried from the stove to every table. Once you run, muscles become the biggest stove: running fast makes about ten times the heat of lying down.
When it is cold,
the body has two tricks
confirmed in 2009
is still debated.
Your guess: a lot, or a little?
Quick check
When you lie perfectly still, what does most of the energy your body uses end up as?
Each cell is a tiny kitchen: only part of the fire goes into the pot, and even the work energy ends up as heat.
