If you any active chemical in your system that alters the homoestasis, why does this chemical eventually leave the body? What prevents it from staying in the body forever?
In: Biology
You have a few systems in the body to get rid of everything and then reabsorb things you want to keep.
Your kidneys dump everything possible in your blood into your urine. There is a very fine filter so only small things can get out into the urine and things like red blood cells and large-ish proteins cannot. Then your kidneys reabsorb the good stuff like sugar and electrolytes.
Not everything can be dumped into the urine right away though, lots of things get stuck to the proteins in your blood. And lots of things don’t dissolve in water.
Your liver helps out and has a super advanced system that is designed to try to break down any molecule possible. (It is not perfect though, and sometimes it breaks an outside molecule down into poison.) The liver also helps overcome the water soluable problem by dumping things into your bile which has more fat solubility.
So all in all your bodies strategy is to break down and dump everything and take back only a few specific things. You keep making new molecules that you need from your food, like proteins and blood cells etc. (Even your bones are constantly being replaced at a microscopic level.)
Tl;dr throw everything out, build it from scratch
Ps some drugs *can and do* last for a very long time. Super rat poisons can last in your body for months to years and you have to take superhuman doses of vitamin k, if you even survive at all.
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