When I first started researching how to write fantasy novels, I read that if you’re going to have witches or vampires or elves or hobbits that you should create a new world, like JKR did with Harry Potter, or how JRR Tolkien did with LOTR and The Hobbit.  If you are going to put them in our real, scientific world, you need to go along with how our real, scientific world.  You can’t have sex if you can’t get hard, and if you have no blood in your veins, then you CAN’T.  Period.  Even if you DRINK blood, it won’t go into your veins.  Do you have YOUR diet go through your veins all the time?  No.  If your fantasy novel is set, not only in a real country, but a real TOWN, you need to go along with it.  Not just hope that no one notices your plot holes.   Because they will.  Always.