The main reason I am writing this is that I'm an author too and in my pursuit to find this information, I have stumbled across so many other writers looking for the same thing. I have never seen this type of information collected into one place, so I decided to create this page in the hopes it might help a few people. Note that this could also be used for any general fiction where a victim has substantial bloodloss, such as a gunshot wound, etc.
Now, I'm not attesting to the medical accuracy of this information. I've done quite a bit of research, reading a lot of contradictory stuff. It was hard to nail down some specifics, but I feel this information is pretty darn close. Certainly close enough for a fiction situation where you can fudge things a bit. Since most authors are going to take some level of artistic license, I think this is a good overall summary and should get you going. But, if you need a bit more, there are few links in the article that may help.
Before I go on, I have to do all the legal, cover-my-butt stuff and tell you all that I'm not a doctor. This information is not meant for medical evaluation or treatment. It's totally for entertainment purposes. If you really have a medical situation and need some advice about blood loss, treatment, etc., consult a medical professional. I'm just a writer, okay? Now that that's over, let's get on with it.
Vampires feed off of humans. The human body holds a finite amount of blood and if a person loses too much, they will die. But the questions remain: How much blood can a vampire victim lose before dying? How long can a person survive being regularly drained? How much blood does a vampire really need in order to survive? While I can't answer those questions directly, here are information nuggets that can help you come to your own conclusions.
When a human donates blood (pure blood, not just platelets, etc), they typically donate 1 pint. That's 2 cups, or about .50 litres. The American Red Cross allows blood donation every 56 days--so about every 2 months. By their standards, humans can safely donate about 6 pints a year.
When a human donates blood, it takes the body 24-48 hours to replace the plasma portion. Plasma is about 80% water. However, it takes the body 3-5 weeks to replace the red blood cells. Ingestion of iron-rich foods, particularly red meat, helps aid the production of red blood cells. Someone who is on a high-iron diet or taking supplements would recover those cells quicker than someone who may be on a regular or less iron-intense regimen.
So, conceivably, someone could have a normal blood volume days after an attack/feeding, but with the lack of red blood cells, could still experience serious medical issues such as anemia. Anemia can be mild and result in fatigue, light-headedness and feeling cold. Severe anemia can cause dizziness, rapid heart beat, passing out, and chest pains, including heart attack.
What does all this mean? If a person loses a fair amount of blood, while the liquid part may replenish rather quickly, the fact that they are depleted of red blood cells would cause medical problems. With a severe enough loss, that may even lead to death. So, it's unlikely that a vampire is feeding heavily off a victim night-after-night (or even just a few times a week) for an extended period without causing any major impact on the victim's health.
Though, it is interesting to note (and keep in mind) that men have a slightly higher red blood cell count than women. Much of this has to do with the fact that women menstruate and lose blood every month. So, chances are a man would fare better from the same amount of blood loss than a woman would. Hey, no sexism here, people, this is biology.
An average human adult has 10 pints of blood in their body. Of course, children would have less. While it's true that a person in compromised health can die from losing a mere pint of blood, most healthy adults fare a bit better. A loss of 40% or more is considered grave and in the fatal range.
Now, it should be noted that the fatality risk is gauged by several factors, including age and frailty of the person losing blood. That means that the elderly, sickly, or very young would be far more at risk than an average healthy human. Other factors include how fast the body is losing blood and if there is compromised blood flow to other parts of the body. So, if you have a victim whose throat is torn out or has had their jugular punctured, you're going to have rapid blood loss and the person would be at a much higher risk of dying should they reach that 40% range. A vampire victim who is being drained through normal drinking would be losing blood at a much slower rate and may fare better. This is assuming that the vampire doesn't have some sort of vacuum cleaner suck that can pull all the blood out of the veins rapidly.
Symptoms and Blood Replacement:
It's often a big deal in vampire stories when a victim is drained and needs a transfusion in order to survive. The loss of large amounts of blood and the effects on the human body are classified as "Hypovolemic Shock." There are 4 stages of Hypovolemic Shock, each showing more pronounced symptoms. Since this is for fiction, we're looking at this more for overall symptoms and need for transfusion. My pint approximations in the data below are off a bit, I know, but they are there to give the author an idea of how much blood loss would be involved:
* Stage 1 - 0-15% blood loss (approx 1 pint)
Victims may show minor signs of anxiety and discomfort, but typically there are no symptoms at this point of loss. In most cases, no immediate treatment is needed other than making sure that any wounds are treated and bleeding has stopped. As with blood donation, the victim would be advised to rest, take in fluids and eat iron-rich foods.
* Stage 2 - 15-30% blood loss (approx 2 pints)
Victims may show mild to moderate symptoms. Some confusion, a little light-headed, shortness of breath. There are medical treatments that may be administered to help, such as IV fluids, but blood transfusions are typically not given at this point of loss.
*Stage 3 - 30-40% blood loss (approx 3 pints)
More prominent symptoms will develop, including palor, chills, anxiety. Here is where you will see the more classic signs associated with shock. At this stage of loss, victims will typically be given a transfusion. However, not in all cases.
*Stage 4 - 40% or more blood loss (4 pints or more)
Severe symptoms, including sweating, pale complexion, cold skin and shivering. Victim will typically show more pronounced signs of classic shock. Loss of consciouness at this point is highly likely. Victims at this stage will need a transfusion in order to survive. However, even if they receive blood, they may not survive depending on the damage to the body as a result of the blood loss.
Of course, all of that may still be a bit more complicated than you like. Here's a summary I heard someone give once and it's pretty good:
2 pints and you feel dizzy
3 pints and you can't walk
4 pints and you fall asleep
4+ pints and you die
All of that said, we have to go back to the issue of the rate of blood loss. If a victim is losing blood volume quickly, they are far more likely to die than someone who is losing it at a slow rate. So, what happens if the vampire is taking very small amounts from a victim over a period of time? Here's what the Merck Manual has to say:
"When the blood loss is rapid—over several hours or less—loss of just one third of the blood volume can be fatal. Dizziness upon sitting or standing after a period of lying down (orthostatic hypotension) is common when blood loss is rapid. When the blood loss is slower—over several weeks or longer—loss of up to two thirds of the blood volume may cause only fatigue and weakness or no symptoms at all, if the person drinks enough fluids.
Note that the above is best and worst case scenarios. The general guides for Hypovolemic Shock are your best bet. On the other hand, what Merck says does give a lot of wiggle room for vampire stories where you need the victim to survive for an extended period--or die rather quickly from an attack.
Here's a good page for more information about Hypovolemic Shock. Lots of short and easy information. Click here.
My own musings: One thing I find funny is that in most vampire stories, there is the dramatic scene where the vampire is feeding and can hear the victim's heartbeat start to slow and fade...and then stops. However, in real-life, the heart would first go into intense and rapid beating, progressively getting weaker as the heart tries to get blood circulating through the veins. In fact, the likelihood would be that the person would go into cardiac arrest (heart attack) first rather than a slow, peaceful death.
Odd "Drinking" Trivia:
In the movies, you often see a vampire biting their victim and a mere 30 seconds later the person is dead. Could a vampire really drink that fast? Particularly since they need to suck the blood out of the veins? To get an idea of how many
"Real" vampires admit that they drink about the equivalent of a shotglass of blood per feeding. That is the equivalent of about 1 1/2 ounces. I couldn't find any sound data on this, but typically when a person is drinking, they take in about 2 ounces per swallow. Double that if "gulping". So, using the threshold of 4 pints of blood loss being fatal, here's what it would take:
2oz = 32 swallows
4oz = 16 gulps
How fast can someone swallow? It would stand to reason that if you're ravenous or starving, you'll feed quickly. However, as the hunger is somewhat satiated, your feeding would slow. This would assume more rapid feeding in the beginning and then slower as the meal ends. Not to mention that vampires often "come up for air" and take a break once or twice during their feeding. So, from a realistic point of view, it would probably take at least a minute, more likely 2-3 minutes, for a victim to die. And that's assuming that the vampire is being hasty in its feeding. With a slow feeding, it would take much longer.
How Much Can A Vampire Drink?
We all know that most fictional vampires are not human. They are dead...or undead...or somewhere in-between. They don't necessarily fall under the guidelines of normal human eating/nourishment. However, since many of them were once human and have human bodies, we can use that as a guide to help us get an idea of how much blood they might be able to drink--or need to drink nightly.
Typically, the human stomach can hold about 1 litre (4 cups) of liquid comfortably. However, the stomach is distendable, meaning it can stretch. According to Wikipedia, a human stomach can hold 2-3 liters (8-12 cups) of liquid overall. Add food to that and a human can hold up to 4 litres (16 cups) of food and drink in their stomachs. Wow, now you know why you have to unbuckle your pants after a big meal.
Okay, so those numbers makes it very likely that a vampire could drink enough blood to kill a person in one feeding. But, would a vampire feed to the point where they feel like they are gonna burst? A comfortable amount in a feeding would be 2 pints, which would leave the victim weak and a bit disoriented, but not in danger of immediate death, right? Well, that's if we're gauging them against a human who usually feeds 2-3 times a day. If you're a vampire who feeds once a night, you might very well double that and put your victim in the fatal range of blood loss to get your nutrition. This could go both ways!
Which begs the question: Is blood nutritious for a vampire? Now, I'm not sure if vampires actually need calories, since they're dead. But assuming they do, let's look at it from that viewpoint. Now, I've seen conflicting reports, but the general consensus is that the calorie count for a pint of blood is approximately 450 calories. A typical human* needs 1800-2600 calories a day, depending on if they are a man or a woman and their body size. Based on those numbers, a vampire would need about 4-6 pints of blood a day to survive. Off of 1 victim, that would probably be fatal. 2-3 victims and you're looking at humans with sore necks, but still alive to see another day.
*I am aware athletes and other heavy laborers would need more. We're looking at averages here.
So, there you have it. A lot of information that may or may not help you in your vampire story. If nothing else, you have to admit it's interesting. Again, PLEASE don't flame me and say I don't know anything about vampires or that you have the "facts" on what a vampire does and doesn't need. I'm a writer and I have written vampire stories for over 20 years. I grew up on old-school vampires like Dracula, and modern incarnations like Lost Boys. I don't get into Twilight, but I respect the fact that the series fabricates its own legends and gives vampires a spin. Most modern writers create their own unique vampire mythology anyway. This is just giving you some information so your stories can sound somewhat plausible.
Have fun writing!