About Blood

magic — the voodoo doll, may be the only instance of it besides the other paranormal things that can't be attributed to much anything.
limited supplies — not very strong example, but you certainly can't shoot brazenly about like you can in most other shooters. Finding secret stashes helps a lot, but these are secrets.
healing stations — life essences can't be carried about and therefore function like automatic single-use health dispensers.
immortals — the zombies raise again an unidentifiable number of times, though you can put an end to it if you put some effort in it.