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