Hi, I'm Adam Cole.
This week we've got a question from Lulu Miller, the host of NPR's Invisibilia podcast:
How old are our bodies actually?
Because if you think about how we're constantly being regenerated
our physical matter is being shed and replenished ... what actually remains?
Good question, Lulu!
Well, your hair obviously doesn't remain the same
and I'm not just talking about the style
each hair on your head is replaced every 2 to 7 years
A hundred hairs fall out every day and new ones grow back in their place
And look at your fingernails - they're completely new every six months or so
it turns out it's just a matter of time before almost every part of your body
refreshes itself in a similar way
The lining of your stomach and intestines
gets pretty beat up -- it's constantly exposed to acid and bile
and so those cells get replaced every few days
Every few weeks, your outer layer of skin is completely renewed
Every four months you have a fresh army of red blood cells
A hundred million new cells are born every minute
and a hundred million old cells are destroyed
It's actually the breakdown products of these red blood cells that turn your bruises and urine yellow
Every 10 years, you've got a new skeleton
a special team of cells breaks down old bone
and another builds new bone
Every 15 years your muscles are refreshed
You might think you gain and lose fat cells when you gain and lose weight
but the actually just get bigger and smaller
Over the course of 25 years though, most of them turn over
But there are a few things that stick around for your entire life
About half of your heart stays with you from birth to death because those cells
are replaced very slowly
Certain parts of your brain add a few new neurons over the course of your life
but the vast majority of your neurons developed before you were born
It's the connections between those neurons -- the circuits that store memories --
that are constantly changing
And there's one more part of you that lasts your whole life
Months before you were born,
a little cluster of cells stretched and filled themselves with transparent protein
As you grew, even after birth, more and more fibers were added, but that center endured
This is your lens the window through which you are watching this video right now
and its core has remained the same since the moment you first opened your eyes
So how old is your body really?
Some of it is brand-new and some of it is as old as you are
To learn how scientists figured all this out, check out our post at skunkbear.tumblr.com
And listen to Invisibilia -- the latest episode is about whether or not personality persists throughout your entire life
If you have science questions send them our way
and please subscribe to our channel!