Your bodily fluids are intended to symbolize yourself, they are part of your essence and are traditionally used in magick. Instead of having the negative energies hitting you, they hit your "representative" in the Witch-bottle, the part of your essence.
Many of my posts -- and many of our most primal fears -- are about avoiding evil in our homes. The essence of much good horror is that you are never safe, not even where you feel most comfortable. The walls, the thresholds, and the basement crawlspace all hold secrets. Don't ask what's in them. You don't want to know.