Why Do Vampires Need Blood?

I need suggestions for why the vampires in my story need/crave blood. Part of the reason behind the craving is psychological, but I want their to be a physical/biological reason why they need blood to survive. I haven't decided yet if animal blood will also sustain them, so I would also appreciate reasons why only human blood would work.

Please note that there is no magic in my story and my vampires are not undead, they are just another species very similar to humans but with enhanced abilities (strength, speed, senses, etc.) so the reason can't be anything to do with magic or curses.

Thanks in advance for any help you can give me :)

I'm not sure how accurate CSI is, but in one episode there was a woman who drank blood because it kept her healthy because of this disease:

http://en.wikipedia.org/wiki/Porphyria

She was offered the standard medication but was a bit of a whackjob so decided to drink human blood instead.

My vampires are created by a capillary parasite that essentially devours their blood and requires new blood to be introduced to the system in order for the parasite to survive. So it's the parasite's need that drives the host to crave it.

If your vampires are natural in origin (no magic or supernatural whatevers) it could be that there is something they need that is missing from their biological makeup that's in blood. Whatever it is that they need is some component that they can't make themselves and have to "steal" from others, like replenishing their red blood cells or something - I'm making this up as I go, so no eye rolling plz XD

It could be possible that their bodies can't make blood at all, and so they have to keep renewing theirs with human or animal blood in order to survive.

Or maybe drinking blood is the catalyst to start cell division, so without it they just dry up and die. Or decompose or something. Drinking the blood keeps their cells alive and growing/dividing so they live and heal and do all the things living beings do.

Uhhh...that was it, I'm tapped.

In 'Let the Right One In' being bitten by a vampire and surviving meant that a parasite went into your bloodstream and formed a secondary brain behind your heart - the parasitic brain fed on the blood in the host's system and caused the host to need to drink human blood in order to survive.

ive read a lot of vampires book and seen films im sure one thing said they needed the blood for scientific reasons, but i cant rmebered all the science
something to do with oxygen in the blood sorry i cant rmeber why did i post

In my MC's pre vamp days she drinks the blood for the iron to keep her energy levels up and when she becomes a vamp tries to sustain herself as much as possible with animal blood but because the genetic make up of animals is different to that of humans. Sort of like the difference in energy a meat eater would get from a steak as compared to a block of tofu. The human blood gives more energy to my MC.

I have been doing alot of research into that myself i need a reason. My vamps were made as well and will have some a bilities depending on the vamp. For me I am looking at the brain and ho it interacts with the blood, how blood dies and is reproduced. I have been reading medicle journals and talking to people to understand more... but this is nano so i am fudging alot of it to just get the story written and Iwill come back and change it up on my re -write after I have completed the challenge.

Vampires need blood because they are vampires: The Law of Narrative Causality says they need to, so they do. If they didn't want blood, they'd be just normal undead, or maybe just ghouls.

I made it that their metabolism required a heightened intake of iron in order to fuel their physical immortality (don't ask me how...) and that they are unable to process food and oxygen in the normal way because their human digestive systems have atrophied in favour of a more efficient, but slightly problematic, handling of digestion within most of the body's tissues.. In blood the amino acid strings are already pre-reduced, and the vampiric agent in their blood (somewhere between WBC a virus) can break down the haemoglobin in order for it to be used.

That said, in principle, I agree with Quinpilix. I never tell my readers any of this. I expect them to accept that vampires need blood. I have all this stuff because I need to know it works in at least a hock-scientific fashion.

I've heard from modern vampires that a vampire is actually someone who survives off the energy of others. In most eastern cultures, the life force or energy was represented by the breath. (Not sure how using another person's breath would work, but there are stories.) In our western culture (sorry, I'm assuming here), blood is the life force, and so in our legends, it is blood that vampires take to survive. However, some modern vampires are psychological vampires. What I mean is this: they will spend time around an energetic person to steal their energy. You can only tell a person is a vampire (physiologically) by feeling drained after spending time with the person. It's a mental rather than a physical transaction. I hope this gives you ideas and insight. =)

I think of it as an exaggeration of a human disease, like anemia or hemophilia. The former would mean the vampire has very little energy and can't breathe well if he hasn't fed in a while, the latter would mean that he would bruise and bleed really easily. I'm leaning towards the former, but I haven't worked out all of the details. One thing I insist, though, is that my vampires still need to eat regular food as well. Magical vampires can get away with just blood, but mine are staying grounded in science. From that perspective, there simply aren't enough calories in blood alone to power a body. It just seems more believable to me.

There are real people, called 'sanguinarians', who crave blood not because they literally think they are undead immortals, but simply because they like it and feel a craving for it the same as a pregnant woman might feel a craving for chocolate ice cream with italian dressing.

Maybe they just like the taste. Or maybe, since they're higher up on the food chain (so to speak), they just look at humans like we look at cattle, pigs, or chickens. They don't hate us, they don't technically need us to survive. But we sure are awful tasty! :D

Because Blood is like food for them