It seems fairly arbitrary. is it meant to be similar to the plasma concentration in human blood?
'10% FBS' is what it says. It's how much FBS was put into the medium. But you will never know what's in the FBS, because this supplement is not chemically defined.
This has been so since tissue culture was invented. Everyone knows FBS is not defined, but it 'works' to keep cells happy. People use what works, and then the next generation of researchers repeats it.
Alternatively, we have synthetic media in which identity and concentration of every component is known. So each lab has a choice, what is best for your experiment. Defined media would seem more logical, but so many experiments have been done in FBS, it will probably be used forever.
In addition to what Elizabeth said, with each new batch of FBS that you receive (as in - completely different batch and lot numbers) you would normally do a test to ensure that it will work appropriately to keep your cells alive.
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