Actually, I'd say the figure is closer to 1.5 to 2 litres (or approx 3 to 4 pints) based on what I've come across. The theory is that the body (of a healthy person) should be capable of compensating up to a loss of 30% of blood volume; with an average person having 5 to 6 litres of blood. Before this point, a person would typically experience symptoms like a fast heart rate or a narrowed blood pressure but is otherwise, for the most part, alright. Moreover, the tipping point is a loss of around 40% (2 to 2.4 litres of blood), which is when the body's compensatory limits are reached and urgent treatment must be given to prevent death.
As such, what you have here is a range of between 1.5 to 2.5 litres to play with where a person is sick/weak, possibly unstable but not in immediate danger of death.
edited 14th Jan '13 2:45:39 PM by peasant