It says that these bottles often contain bodily fluids to represent you, such as blood and urine. I don't really understand what real affect these substances can have because they aren't really specific to you.
With witch bottles, we're talking about
magickal properties and linksg, not scientific.
The urine and blood are
off you, thus in a magickal sense linked to you. The organic matter of the witch bottle maker in the bottle represents the person and acts as a target - diversion to the ill effects sent to her, which would then hit the bottle, not the person.
That said, urine does have DNA, as has blood and other traditional "personal bits" put in witch bottles, like hair and pubic hair. (
Here is a small paper on using DNA from urine as a potential source of identification, it's a one-page pdf-file).
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