I recently finished the gripping and disturbing anime series ‘Shiki’, which features vampire-like beings. I’d recommend it to those who like their entertainment bloody and thought provoking (and full of really bizarre hairdos).
One thought it provoked in me was whether it is feasible to live off blood alone. A few months ago I was reading the Q&A section on the BBC Science Focus site, where people can ask weird and wonderful things for some science boffin to answer, and this particular question was asked. The basic answer is that it isn’t possible, since we wouldn’t be able to extract water from it – because it has the same water and salt content as our own blood, there is no osmotic gradient to cause the water to move into the bloodstream – and it’s also too salty for us. Dehydration and kidney failure would quickly dispatch any wannabe vampire. The rich iron content could also prove toxic, although the author of the article states that 26 litres of blood would have to be imbibed per day to bring about iron poisoning. So obligatory hematophagy, or just eating blood, is not a possibility for humans.
What would it take to be able to live off blood then, and what adaptations would vampires need if they were a real life species? The best way to consider this is to look at existing examples.