Sometimes, it’s hard to know when writing a scene when to pull the big guns out. If you have a character that is bleeding a lot (by whatever mechanism) when should you think about giving them blood? Or, better yet, when will the lack of blood begin to hamper their ability to function.
Fairly consistent among resources, hemorrhagic shock (shock related specifically to blood loss) is a life-threatening condition that results when you lose more than 20% or 1/5th of your blood supply. Patients will feel lightheaded, dizzy. Their respiratory rate and heart rate will be elevated. Their blood pressure might be low. They’ll look pale, pasty. Their skin might feel cool, clammy, dough-like.
But exactly how much blood does that translate to? I actually found this nifty little calculator that will give you a person’s estimated blood volume based on their age, sex, and weight. For instance, a man weighing 100 kg has an estimated blood volume of 7,500 ml. So losing 20% of his blood volume would be 1,500 ml of blood or approximately 3 pints of blood. A pint of whole blood (what you would donate) is approx 500 ml. A woman of the same weight has only 6,500 ml of circulating blood. An infant weighing 10 kg has only 800 ml of blood. You can see how that 20% translates much differently depending on the characters age, sex and weight.