I get that we loose some through skin shedding, hair falling out, body fluids, etc… But do we keep most of the atoms we are born with? Or do they all get replaced through the passage of time?
Most of the atoms you were born with have been lost. However, some of them in the central nervous system and the lens of your eye are probably still there.
Of course, since you were born your mass has increased by 30-40 times or more so that most of your atoms would be new anyway.
Most cell types have a life span of only a few days to months. When they die, they are replaced by new cells probably built up in part from new atoms derived from your food (although atoms recycled from cells that have died could also be incorporated).
Nerve cells, lens cells, and oocytes (egg cells) last a lifetime. (Of course, any new cells that were not present at birth are made of new atoms.) While there will be some cell repair that will replace atoms, they probably contain many of their original atoms.
Great answer. Great cite.
ETA: Great site at cite. A keeper.
How about bones?
Even if you have some of your original cells, each cell is also continually exchanging material with its environment: Taking in glucose, oxygen, and amino acids, and releasing water, carbon dioxide, and proteins. And some of those processes involve swapping atoms, not just releasing the same atoms that were brought in for the raw materials.
Bones are constantly being torn down and rebuilt. Probably safe to presume that most of the atoms that were present in your bones when you were born have long since been replaced.
I’m sure I’ve read stories about radioactive isotopes accumulating in bodies. Is that just alarmism, then? Or is it like, this particular atom stops being in Cell #XXXX but after that cell dies and gets torn down, Cell #YYYYY grabs it up to reuse it?
Depends on the isotope. One of the radioactive isotopes of greatest concern is Strontium-90. It mimics calcium and so is deposited in bone. Because bone is constantly being torn down and rebuilt is exactly why it gets deposited. But once bone is deposited it may be a while before that particular part is recycled. The average half-life of Strontium-90 in the body is estimated at 18 years, in which time it can do a lot of damage.
The cite in my first posts mentions that the life span of various cell types has been estimated by tracking the C14 deposited in cells by atmospheric nuclear testing before the Test-ban Treaty.
Interesting, thank you.
Now I remember that my father had some test that involved a radioactive isotope – related to a heart problem, I don’t remember/never knew the details – and for a while afterwards his, uh, excreta was collected for disposal outside of the normal plumbing system. I guess it only took a few days for his body to eliminate all of it.
Or maybe whatever was used had a really short half-life.
There are two half-lives talked about for medical isotopes: The physical half-life and the biological one. The physical one is how long it takes for the element to undergo radioactive decay, while the biological one is how long it takes for it to leave the body. Sometimes one is much shorter than the other, in which case that one is much more relevant, and sometimes they’re comparable, in which case you get a total effective half-life that’s shorter than either.
78% of your body was water when you were born. This water is going in and out of your cells through diffusion all the time, passing into the blood stream where it is excreted and replaced with new fluids as you drink, so I doubt that there are very many of your original water atoms left.
That gum you swallowed as a kid might still be in there.
That brings up another question–once the water molecules leave your body, they reenter the hydrological cycle. So around how many of those molecules (or atoms) can you expect to consume again later in life?
Randall Munroe had a “What If” column that touched on this a little … ah, here it is, the “Soda Planet” column. His conclusion was that most of the water molecules we drink haven’t ever been inside another person, but they’ve all been through many dinosaurs.
ETA: I forgot to mention that he also says “On the other hand, it’s just about guaranteed that some of the water molecules in any mouthful have been drunk by someone else.”