Does the Human Body Really Replace Itself Every 7 Years?

The short answer is “no.” Tune in to learn how long it really takes, plus how nuclear weapons led scientists to the solution.

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain Stuff from How Stuff Works. Hey, brain Stuff,
it's Christian Seger. Today's question, does your body really replace
itself every seven years? The short answer is no, But
don't worry. This isn't a case of chicanerous researchers pulling
the wool of shoddy science over your eyes. Your body

(00:24):
mostly replaces itself every seven to fifteen years. Some bits
are never replaced. Others, like the lining of your stomach
and intestines, are renewed much faster due to constant wear
and tear from the process of digestion. These cells have
an average lifespan of just five days. Yes, the organs

(00:45):
that work the hardest have the fastest change over. You
get a whole new skin every two to four weeks,
and your red blood cells last less than half a year.
That's not bad considering that they're route through your circulatory
system is about a thousand miles. The average American car
doesn't even travel that far, and your liver renews itself

(01:07):
at least once every couple of years. As the human
bodies detoxifier, it goes through a lot. Other tissues take
longer to completely replenish themselves, like your bones, for instance.
Skeletal cells die and new ones grow constantly, but the
complete process takes about ten years, and the process slows

(01:30):
down as we get older, which is why our bones
tend to get weaker as we age. And like I said,
some parts of your body stay with you for life.
The cells of the inner lens of your eye formed
when you were just an embryo. Your tooth enamel wears
down with use, never to return, and evidence indicates that

(01:52):
you can't regrow the neurons of your cerebral cortex. It's
loss can lead to diseases like dementia. Luckily, other parts
of your brain do regenerate, like the hippocampus, which helps
us create memories, and the olive factory bulb, which helps
us smell. So how do we know all this? Well,

(02:13):
it turns out it's thanks to our old pal nuclear
weapons testing high fives for radioactive stuff being released into
the atmosphere no really above ground nuclear detonations during World
War Two and the Cold War spiked Earth's air supply
with extra carbon fourteen. It's been declining back toward the

(02:35):
norm at a predictable rate since the nineteen sixties, which
means that you can use the amount of it present
in any given tissue sample to determine when those cells
were born. More carbon fourteen means older cells. Check out

(02:56):
the brain stuff channel on YouTube, and for more on
this and thousands of other topics, visit how staff works
dot com