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Speaker 1: Welcome to brain Stuff from How Stuff Works. Hey, brain Stuff,

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Speaker 1: it's Christian Seger. Today's question, does your body really replace

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Speaker 1: itself every seven years? The short answer is no, But

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Speaker 1: don't worry. This isn't a case of chicanerous researchers pulling

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Speaker 1: the wool of shoddy science over your eyes. Your body

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Speaker 1: mostly replaces itself every seven to fifteen years. Some bits

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Speaker 1: are never replaced. Others, like the lining of your stomach

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Speaker 1: and intestines, are renewed much faster due to constant wear

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Speaker 1: and tear from the process of digestion. These cells have

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Speaker 1: an average lifespan of just five days. Yes, the organs

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Speaker 1: that work the hardest have the fastest change over. You

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Speaker 1: get a whole new skin every two to four weeks,

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Speaker 1: and your red blood cells last less than half a year.

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Speaker 1: That's not bad considering that they're route through your circulatory

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Speaker 1: system is about a thousand miles. The average American car

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Speaker 1: doesn't even travel that far, and your liver renews itself

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Speaker 1: at least once every couple of years. As the human

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Speaker 1: bodies detoxifier, it goes through a lot. Other tissues take

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Speaker 1: longer to completely replenish themselves, like your bones, for instance.

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Speaker 1: Skeletal cells die and new ones grow constantly, but the

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Speaker 1: complete process takes about ten years, and the process slows

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Speaker 1: down as we get older, which is why our bones

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Speaker 1: tend to get weaker as we age. And like I said,

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Speaker 1: some parts of your body stay with you for life.

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Speaker 1: The cells of the inner lens of your eye formed

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Speaker 1: when you were just an embryo. Your tooth enamel wears

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Speaker 1: down with use, never to return, and evidence indicates that

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Speaker 1: you can't regrow the neurons of your cerebral cortex. It's

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Speaker 1: loss can lead to diseases like dementia. Luckily, other parts

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Speaker 1: of your brain do regenerate, like the hippocampus, which helps

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Speaker 1: us create memories, and the olive factory bulb, which helps

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Speaker 1: us smell. So how do we know all this? Well,

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Speaker 1: it turns out it's thanks to our old pal nuclear

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Speaker 1: weapons testing high fives for radioactive stuff being released into

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Speaker 1: the atmosphere no really above ground nuclear detonations during World

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Speaker 1: War Two and the Cold War spiked Earth's air supply

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Speaker 1: with extra carbon fourteen. It's been declining back toward the

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Speaker 1: norm at a predictable rate since the nineteen sixties, which

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Speaker 1: means that you can use the amount of it present

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Speaker 1: in any given tissue sample to determine when those cells

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Speaker 1: were born. More carbon fourteen means older cells. Check out

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Speaker 1: the brain stuff channel on YouTube, and for more on

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Speaker 1: this and thousands of other topics, visit how staff works

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Speaker 1: dot com