I've often thought about what makes a vampire... a vampire. I can describe what my vampire characters look like on the outside, but what about inside?
I've written a little bit about what I think happens inside the body when a human becomes a vampire. This is my opinion only based on nothing but my imagination and thought. All of my characters (that have been 'turned') lose organs that will not be necessary for their life as an 'undead' creature. The reproductive organs are the most visible thing to 'go'. (i.e, my vampires notice this ;). Do they care? No. Why not? Because the human sexual urges are gone. They are now controlled by blood alone. Having said that, it is very evident in my books that the blood (and lust for it) is all consuming. The act of drinking blood is indeed sexual to them. The way it makes them feel physically is ten times beyond what physical sex was for them (or so they have exclaimed in some of my writing.)
Of course there are people today who have studied vampire lore. They analyze why so many (of us) are attracted to vampires. They publish papers about the sexuality of vampires, what its supposed to mean, how Freud would see it, etc. But I say, balderdash! Unless you are a vampire you have no clue what it feels like or what it means ;)
But you can imagine.
I often think that the circulatory system of a vampire has grown into this enormous tangled mess of arteries and veins. I think the heart of a vampire grows, and I have sometimes entertained the idea that the vampire might have more than one heart. I think the bone marrow becomes super charged and pumps out stem cells at an alarming rate, thus enabling the vampire to regenerate quickly, never get sick, never die (unless beheaded, burned, and spread). In essence I think the inside of a vampire is one huge circulatory system that keeps the body animated and keeps the brain nourished.
Think on that. Or not. :)