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why do vampires need to drink blood

The short idea is this - vampires are parasitic creatures with a trade-off: in exchange for the abilities they possess, their ability to produce blood on their own is hampered. The key word being
hampered. They can/do still produce blood on their own, but at a rate that is slightly less then they require. Of course, you can adjust the rate they produce blood yourself to suit your story - anywhere from needing a top-up only rarely to every day. Rather than "drinking" the blood and extracting nutrients from their stomachs, the blood they steal from their victims via their fangs goes directly into their own bloodstream. One pesky problem with this approach is that if they need to drink 5-6 pints to kill a person they themselves would need to be down at least 5-6 pints. and they would be dead by then. And thus the "blood-bladder" is introduced; an elasticized internal vein that can swell to hold blood. This direct transfusion process explains why vampires cannot drink animal blood.


They literally use the blood they consume and (like people) are not compatible with other animals. This mechanism has the added potential story benefit/issue of allowing vampires to neatly deal with (and possibly contract) disease, if you want to go that route. Have a bad flu? Go chomp on someone who recently got a flu vaccine and put their white blood cells to work. Tough luck if you eat someone with the flu though. The blood-borne disease issue can also be sidestepped by simply noting that vampires produce excellent quantities of highly effective white blood cells that take care of pretty much any blood-borne entities that are not beneficial to the vampire. Perhaps their self-produced white blood cells learn from every white blood cell they injest, making older vampires much more resistant to disease than newer vampires. Remember how we said that vampires do not produce ENOUGH blood on their own? Well, "blood oranges" contain particular enzymes that stimulate increased blood production.


I would suggest that unlike human blood, blood oranges are probably consumed in the traditional fashion. Vampire lore is a mess of contradictions and it makes sense to identify how a vampire 'works' in order to provide a more detailed answer to this question. For instance, the fact that they're (in a word) dead, cold and either immortal or extremely long lived indicates a very slow metabolism. On the other hand, super healing capabilities would indicate a very fast metabolism. In a I pointed out that excess oxidisation is actually likely to cause faster ageing than not; free radicals are the enemy to a prolonged life. 2) Super healing is either a myth they promote or a result of excess handwavium in their systems. With that in mind, vampires would digest blood, meaning they break it down for nutrients, making the blood type irrelevant. The reason that they need blood (and blood only) is that the haemoglobin contains just enough oxygen to metabolise the nutrition in the blood, and no more.


There's probably no other food type that contains sufficient oxygen to allow the food to be absorbed directly without additional O2 being supplied, which is why; What all this means is that vampires don't breathe. When you stop and think about it, that actually makes sense. They're dead and they can survive in coffins or underground for extended periods. Some lore also has them able to survive underwater, but if you go to the Dracula book by Bram Stoker, they can't cross running water at all which would be another contradiction. Suffice it to say, that our vampires don't breathe so need oxygen in their food to metabolise it (hence fresh blood). If we disregard Bram Stoker again (who tells us vampires can survive daylight but with curtailed abilities) and go with the regular lore that vampires are destroyed by sunlight, this is also consistent. Low energy creatures such as vampires would find the relative heat of the day to be overpowering by comparison to their normal very low body temperatures.


Assuming they're still warm blooded in nature (in this case meaning that they have to maintain a differential between the outside temp and their internally consistent body temp) then standing in daylight when their normal core temp is so low would be like us standing in the middle of a forest fire and trying to survive. They probably wouldn't burst into flame, but they would die of heat stress pretty quickly. With all this in mind, I'm going to say that if my understanding of vampire physiology is correct, blood type wouldn't matter to a vampire, but relative freshness would. That means that artificial blood, or even blood stored in a blood bank, would be useless to a vampire. Ideally, they'd want blood fresh from a living artery (going between the lungs and heart of a live animal) so as to be able to metabolise the blood through digestion-like processes that combine the oxidisation step in situ.

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