Dear Dr. Grasshopper,
How much blood can a character lose before they die?
Well, it kinda depends on the size of the character, to be honest. If a very large man loses a liter of blood, it’ll have a completely different effect than if a very small woman loses a liter. So the specific amount that a character can lose varies with the character. But I’ll try to give some general guidelines so you can figure it out for your character.
First of all, let’s figure out how much blood is actually in your character.
Blood is made of plasma, and blood cells, and all kinds of good stuff. (Remind me to write a post about blood composition.) Regardless, here’s a reasonable way to figure out how much blood your character is starting out with.
WARNING! MATH AHEAD! DON’T RUN AWAY; YOU WILL JUST HAVE TO DO THE CALCULATIONS TIRED! (PS: I suck royally at math, so if I screw up, please let me know.)
(Important terminology – “Hematocrit” = the percentage of blood volume occupied by red blood cells. Typically, around 47% in men and around 42% in women. Plus or minus a few percent.)
Okay, now that that’s out of the way, we can do some calculating!
Plasma volume in L = (Body weight in kg) x 0.05
Total blood volume in L = (Plasma Volume in L ) x (100/[100-hematocrit])
(That’s a little hard to read. So let me clarify. Take 100 minus the hematocrit. Divide 100 by the number that results from that operation. Then take the number you have now, and multiply it by the plasma volume in liters.)
And, here’s an example!
Our hero, Toughguy Manley, is a 70 kg man, with a hematocrit around 47. Let’s see how much blood he has.
Plasma volume in L = 70 x 0.05 = 3.5 L
Total blood volume in L = 3.5L x (100/[100-47]) = 6.6 L
(Note: I’m using 47 because that’s the usual hematocrit for a man. If we were calculating a woman, I’d use 42.)
So, Toughguy Manley will have about 6.6 liters of blood in his body, or 6600 mL.
Now that you know how it’s done, you can plug in your own character’s weight in kilograms. (Here’s a handy calculator for people who use non-metric systems of measurement.)
I’ll wait while you do some calculations.
Now that we know how much blood your character has, we can figure out how much she can afford to lose.
The pretentious medical term for bleeding is “hemorrhage”. (“hemo-“ = blood, “-rrhea” = flow). The way doctors classify hemorrhage is by percentage of blood volume loss.
Here’s a quick rundown of the hemorrhage classes:
Class I Hemorrhage – Loss of up to 15% of blood volume. In a healthy character, this won’t cause too much trouble. At the very most, her heart will start beating slightly faster. But that’s about it. Her blood pressure won’t change appreciably, and she probably won’t have too many symptoms in general.
Class II Hemorrhage – Loss of 15-30% of blood volume. Your character’s heart will beat faster to circulate the remaining blood faster, to make up for the fact that there’s less of it. Her blood pressure will undergo a mild to moderate decrease, and she may start to have some symptoms related to the drop in blood pressure. For example, her skin will get cold and clammy as her peripheral blood vessels narrow down, squeezing the blood out of the skin so it can supply more vital organs. She may feel light-headed, and have some changes in her mental status. If she tries to sit or stand up quickly, she may get dizzy. She won’t make as much urine, since there’s less volume passing through her kidneys for processing.
Class III Hemorrhage – More than 30% blood volume loss. The symptoms that started to show up in Class II will get worse. Her heart will beat very quickly, but it won’t be enough to keep her blood pressure up. The blood pressure will fall even more dramatically, resulting in even less perfusion of the skin and the extremities (which will make them cold to the touch), and her mental status will decline significantly.
Class IV Hemorrhage – More than 40% blood volume loss. Your character’s body will not be able to compensate for a blood loss this severe. She must either get an emergency transfusion, or she will die from hemodynamic decompensation.
So with this information, now we can figure out how much blood, to the milliliter, will leave your character’s body during your dramatic wounding-and-possibly-killing-of-character scene!
Let’s have an entertaining example!
Remember, Toughguy Manley is a 70 kg male, and he has about 6600 mL of blood. He’s just found the Seekrit Hiding Bunkur that houses the Doomsday Device of Doom (Of! DOOOOOOOM!). Suddenly, during the dramatic climax of the story, he’s shot by Best Friend McTurncoat, who was actually working for the Evil Bad Dudes all along! Now, he can’t die from this, because he has to tell Gorgeous Genius just how badly he’s fallen for her, and he has to be just loopy enough that he’ll tell Gorgeous Genius just how badly he’s fallen for her! So, let’s put him at an early Class III hemorrhage. Say, 32% blood loss.
6600 mL x 0.32 = 2112 mL, or 2.1 L
Toughguy Manley has lost a bit more than 2 liters of blood. He’s giddy, his heart is beating like crazy, his blood pressure is low, and his hands and feet and skin are cold and clammy. And he’s still losing blood. And Best Friend McTurncoat is standing over him, expressions of triumph and deep anguish fighting for dominance on his face.
It’s not looking good for Toughguy Manley.
Luckily, at that moment, Gorgeous Genius activates the Doomsday Device of Doom (Of! DOOOOOOOM!), after re-jiggering it to use only a small percentage of its capacity, and to affect only Evil Bad Dudes! (Plus, now it writes novels! And assembles bicycles! And makes three kinds of french toast!) All the Evil Bad Dudes fall over unconscious, and the day is saved! Hip, hip, hooray!
And as Gorgeous Genius saves Toughguy Manley’s life using the blood-transfusion setting on the rejiggered Doomsday Device of Doom (Of! DOOOOOOOM!), he professes his undying love for her and for her absolute bad-assery. And for physiology. Because physiology rocks.
Hey! If you want me to answer a question about using medicine in fiction, shoot me an email! doctorgrasshopper (at) gmail (dot) com. (Remember, though, that I don’t have a lot of time to research answers, so please be patient with me. Thanks!)
Ganong, William F. Review of Medical Physiology. 21st edition. McGraw-Hill, 2003.
Manning, James E. Fluid and Blood Resuscitation. Tintalli’s Emergency Medicine, Chapter 31. McGraw-Hill, 2004.
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