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That sucks: Keep this up and there'll be no-one left - human or vampire! (Source: Getty Images)

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How long would it take a vampire to drain you of blood?

Tuesday 10th May 2016 12:08 pm

If you’re a sucker for a good vampire movie, be warned … Dr Karl takes two big bites out of the legend.

When movie makers want to guarantee their investment, they’ll pick an old favourite for the lead role. That’s why Santa Claus, Jesus Christ, Sherlock Holmes and Dracula the vampire appear so frequently.

I love movies, but I also love accuracy. And that’s why I have to tell you that as far as vampires are concerned, the movies tell you two big lies about the bite.

First, it would take more than just a few seconds for the victim to go unconscious. And second, if biting the victim turns them into a vampire — well, by now we should all be vampires.

For a vampire to survive, we’re told they have to feed on the life essence of a living creature — usually their blood. Part of the unnatural process of the vampire getting their nourishing meal of blood is the victim falling unconscious — and being set on the pathway of becoming a vampire themselves.

Legends of supernatural creatures that survived on the blood or flesh of the living have been around for thousands of years.

The ancient Persians, Babylonians, Hebrews, Assyrians, Greeks and Romans each had their own folklore about blood-guzzling demons and spirits.

Similar legends exist across Africa, both North and South America, and Asia. But it was the 1897 novel, Dracula, by Bram Stoker, that seems to have started the modern European vampire character and subsequent vampire culture.

In 2016, a group of physics students from the University of Leicester in the UK worked out how long it would take an actual vampire to make a real person unconscious from a bite on the neck.

They assumed you would fall unconscious once you lost about 15 per cent of your blood. Now a typical and worthy blood donation is around 10 per cent of your blood volume. But the physics students probably figured that a bit more blood loss (15 per cent) combined with the shock of somebody else’s canine teeth stabbing into the carotid artery in your neck would make you swoon.

The students also assumed that the vampire’s fangs made two tiny puncture holes each 0.5 millimetres across, and that the victim’s blood would come out under a pressure of around 100 mm Hg.

Using these numbers, they came up with a time of 6.4 minutes to drain 0.75 litres of blood from the carotid artery. (By the way, the reason that it takes longer at the blood bank to drain about half a litre of blood is that it’s seeping from a vein at low pressure, rather than squirting from an artery at high pressure.)

But in the land of movies, the victim faints within a few seconds. That’s brief enough for the vampire to make a quick getaway before the hero arrives with a crucifix and garlic. Sadly, it might not enough time for the vampire to get a proper feed.

Vampires are a dying breed

But there’s another problem — what if there are no more humans left to bite!

Let’s start with just one vampire. They plunge their fangs into the neck of a victim, who shortly joins them in the vampire club. Now we have two vampires.

Now let’s make a very conservative assumption — let’s assume that vampires need a feed only once every month. In the first month we start with one vampire — but in the second month we have two.

In the third and fourth months, we double and double again to four and then eight vampires.

By 10 months, there are 512 vampires. Twenty months gets us to half-a-million vampires, while 30 months has half-a-billion vampires roaming the planet looking for their next meal from a human.

Thirty-three months after the first vampire had their first bite, there are now four billion vampires — and somewhat fewer than four billion rather nervous humans remaining.

And bingo, 34 months, or less than three years after the first vampire had their hot first liquid meal, every human on the planet has been turned into a vampire.

Suddenly, there is no food remaining and the vampires are left with the combination of immortality and an unquenchable thirst.

But one way to guarantee their remaining human stock would be to kill all other vampires by driving a wooden stake through the heart of each newly created vampire.

Yes, the vampire cycle of life would continue, but then they would have no vampire friends.

So now we have the astonishing stats on vampires. But don’t be afraid — it’s only a movie …

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This blog first appeared on Dr Karl's Great Moments in Science

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