Well, does the knife itself last forever or does the knife become a new knife each time we can ask the same question about our bodies, because, like this knife bits of you are always breaking down and being thrown out even being replaced, i mean even right now Parts of you are dying, some of your cells even died before you were born and some will never come back. These are all very strange, mildly, uncomfortable things to think about, but the fact is, if parts of us werent constantly dying, we couldnt be alive and whats. Even stranger than that is that a big fraction of your body is was, and will never be alive, so how much of you is dead? How much of you has been replaced like the knife and has any of you always been you Music, hey smart people, joe here im gon na put this away now, because its getting a little weird versions of that thought. Experiment actually go back millennia. A greek philosopher, plutarch, is credited with one of the first versions and his starts with a ship. It basically goes like this. This guy, named theseus, who may or may not have actually existed, sailed his ship to athens, after defeating the minotaur that mythical bull man who lived in the labyrinth to honor theseus. The people of athens preserved his ship for hundreds of years whenever part of the ship needed to be fixed. They replaced it over and over plank by plank until eventually no original pieces remained.
So is it still theseuss ship? I mean take this stack of blocks. I can replace it brick by brick until its a new version of itself. I mean it looks the same. So is it what defines the essence of the stack its stackiness? Is it the number or the pattern of the colors? Well then, perhaps its the same stack or maybe the atoms in the blocks or the stack itself, then you might say its a different stack or maybe its something else in japan. People have been doing something like this since the 7th century, this shinto shrine gets rebuilt piece by piece every 20 years, theres, basically a new shrine or is there it aligns with shinto beliefs around the impermanence of nature, but also this idea that the only way something Can last is if its eternally rebuilt, some of you might say, theres always been just one ship of these or one stack of blocks or one shrine, a thing bigger than its pieces. Others might argue that when a thing or any part of it is replaced, then that thing becomes new. The greek philosopher, heraclitus once said its impossible to step in the same river twice. The outlines may be the same, but the water before is long gone, and so it is with our own bodies, roughly 330 billion of your bodys 30 to 40 trillion. Total cells are replaced every day. Of course not all of your cells get replaced, but this means every 100 days in total cell numbers at least you regenerate a new use worth of you.
Are you the same you 100 days from now. Clearly, all of this cell regeneration must come with an equivalent amount of cell death. Otherwise, youd be like a huge walking, tumor blob or something, and this story of how your body manages its own death piece by piece gets interesting, because different cells kick the bucket faster or slower than others, and some not at all and a bunch of this cell Death happens before youre even born, for instance, the cells that help grow the tiny bones in your middle ear. Well, they did their job in the womb and died right before you were born and after about eight weeks of pregnancy. If your body didnt direct some very particular tissue to die at precisely the right time, youd have webbed fingers today, which would make it easier to swim, but your ring collection would be completely obsolete. Broadly speaking, cells die in two ways: cell death, thats programmed by your body, is called apoptosis or apoptosis its greek. The second p, should be silent, but i know that feels weird. So, however, you want to say it is fine. Language evolves anyway. Thats greek, for falling off when the time comes for a cell to stop existing or its broken beyond repair. It starts to shrink and form little blebs on its surface thats, really what theyre called dna and cellular machinery are chopped up and carted off for recycling by your bodys, molecular salvage crew and theres another kind of cell death if cells are say, damaged beyond repair by Toxins or environmental extremes they basically just spill their guts cleaning.
That up is a bit messier, but your bodys janitors are good at what they do. Think of your body like a car, you have to do maintenance to keep it running. If you couldnt ever get rid of old parts. I mean, instead of changing your oil just kept pouring more in. You need new brake pads, stack them on top of the old ones. I mean pretty soon. Luckily that doesnt happen, because you are dying inside right now in a carefully programmed and beautifully orchestrated way. So what part of you is dying the fastest? Well, you have around 200 different types of cells, all in all, acting as the building blocks for everything that you are from bone to blood intestines to eyeballs and everything in between on the most immortal end. Are your neurons networks of these cells hold your thoughts, your memories, your very consciousness, so obviously, killing and trying to replace neurons could really mess. All that up. Neurons do make new connections and trim others as you learn or forget, but after your brain is done, growing. Youre pretty much stuck with the neurons youve got. This is why brain or spinal injuries are often so permanent. Interestingly, scientists have found that in one part of your brain, the hippocampus neurons can regrow. This brain region is important in learning and memory and about a third of it gets pruned and replaced throughout your life. They proved this in a really amazing way, so massive amounts of radioactive carbon were released into the environment by nuclear bomb tests, the 1950s and 60s and people alive during this time had radioactive carbon inside neurons in their hippocampus.
The only way it could have gotten there is if they were making new neurons during their life in that little part of their brain. The majority of your bodys, bulk and mass is made up of fat and muscle cells, and these last for decades, of all those muscle cells, the ones in your heart. Stick around the longest. I mean to keep your blood pressure. Steady keep feeding your brain enough oxygen, its basically too risky to regenerate those the cells that help build your bones. They last about 10 years. So a lot of your basic scaffolding and structural support that sticks around a while about 86 of the 30 trillion cells that you lose every few months are blood cells, specifically red blood cells, whose only job is to carry oxygen from your lungs to your tissues and Bring carbon dioxide back these cells make up about half of your blood by volume and they only last about 120 days, some white blood cells, which only make up about one percent of your blood. They only last hours, 12 of your cellular recycling happens in your gut. I mean digestion is a dirty job. The cells that line your colon, for example, they last less than a week before being replaced and then theres the stuff on the outside. You lose about 500 million skin cells a day which means in a lifetime. You lose enough. Skin to cover about a thousand bodies which is pretty horrifying, so where does all this dead? You go well everywhere.
Every time you bathe, cough sneeze, poop, spit youre, sending bits of dead you out into the world, all in all about two thirds of your body. Mass is made up of stuff that will, at some point die and regenerate, which brings up a couple interesting questions. If you fall in love with someone – and maybe you spend decades of your life together, are they literally a different person than who you first met and how much of you will never be alive in the first place? Let me explain that second question see almost a third of your mass is stuff thats, well, its not made of cells, its made by cells and so its not really alive. For instance, the fluid outside of our cellular membranes things like the plasma in your blood tears. Mucus lymph spinal fluid the vitreous humor in your eyes, and even milk in some bodies, the liquid outside your pipes, makes up about 25 percent of your mass and another seven percent is hard stuff. Like the calcium in our bones the minerals in our teeth, then youve got unliving stuff. You carry with you like the dead protein keratin. That makes up your hair and fingernails, and you remember all those trillions of red blood cells, youre, constantly losing and making more of well. They have no nucleus, they have no mitochondria, they carry no genes, they cant make more of themselves. They dont metabolize, i mean. Are they actually even alive? Does that mean all of us have trillions of squishy zombie cells flowing through our veins keeping us alive thats? The kind of thought when you have the shower can mess up your whole day or your connective tissue things like ligaments, tendons, cartilage, theyre, actually made of these stretchy or spongy webs of protein and other molecules again made by cells but not made of cells.
Three fourths of your connective tissue and all your joints and muscles is just this dead molecular scaffolding all in all a sizable fraction of what you probably consider you, the thinking living being watching this video is actually not alive. Of course, every persons body is unique and all kinds of things can influence your cellular turnover and the number of living cells that you possess. I mean a healthy liver has 240 ish billion cells in it, but a disease liver could have as few as 172 billion. The person that that belongs to are they less alive than a healthy person or not, and some cancer cells are cancer because theyve lost the genetic software that told them when to die is a person with cancer more alive on a cellular level. The answer is less clear than you probably thought before you watch this. We can go even further, much of the life that you carry within that quasi living dermal sac. That holds you together, its not you. The human body is home to about as many bacteria and other microbes as it is human cells, as you digest and poop your way through your time on earth, the balance of life within you shifts back and forth. We think of dead and alive as two sides of a coin, but its more like a spectrum, a gradient. I mean it has beginnings and ends, but any point between them is a mix of both and maintaining the right balance of dying and regeneration turns out to be essential to health and complex life itself.
In a way were a lot like that shinto temple arent. We, if your body had a philosophy, perhaps that would be it. Nothing can truly. Last as it is, life is a balance of death and regeneration and rebuilding your body is a temple blah blah blah blah. You get it anyway, the next time someone asks who are you youve got a much more interesting answer to give him stay. Curious: hey. Have you checked out out of our elements yet its a new pbs tara series that explores the molecular stories underpinning our everyday natural world from the first molecule in the universe to the water that we drink out of our elements shows that if you look closely enough, What may seem familiar can actually be kind of extraordinary check it out down the link in the description and be sure to tell them. I sent you as always wed like to thank everyone who supports the show on patreon and, if youd like to join them, we would really like your support. The today you not the 100 days from now you, whatever cells, that person is made of. But if future you would like to support the show too, you can sign up for an annual membership, now just check out the link down in the description to learn more, oh wait. A second quick fact check. My muscle cells are particularly immortal were recording all this. I keep forgetting stop talking when youre, not its you, the right to remain silent.