How much blood can someone lose before fainting?

One pint: Dizzness and headache for a day Two pints: Lasting dizziness and headache for a couple days. If I'm wrong, let me know. The funny thing is I was eager to find an answer for this question too. Not just someone's guess but from a medical student or professional. ------------------------------------------------------ The question is asked in a very open way, which makes it hard to answer with brevity. FOr instance, the primary symptom of bleeding is, well, blood -- someplace where it shouldn't be. Ans this symptom can be confirmed instantly, far before bloodloss would endanger the usual patient. The second difficulty relates to the condition of the patient before the bloodloss occurs, as well as the rapidity of the loss. A large patient (say 180lbs) can, depending on the person and the circumstances, part with as little as a quarter unit or as much as about 1 unit before some symptoms are visible. I should also add that the patient's activities post blood loss are important as well. I healthy blood donor can donate a unit, then lie down with a cookie and some OJ, and feel no apparentl ill effects. The same man, running a marathon at the time, will be in extremis. Then there's shock. An otherwise minor wound that shows a lot fo bleeding can induce shock in the patient -- and even in other patients (and observers!) who see it. Scalp wounds are famous for this. Then there's the different between blood volume (or how much blood and serum are in the circulatory system) and hemoglobin/hematocrit (how much O2 can be carried and how much space there is between red corpuscles). Bleeding causes problems with all three, and which becomes the most dangerous depends on the patient's condition before the event -- both immediately and chronically. In short, there's no real way to answer this question concisely without taking a myriad of other factors into account. That said, for a healthy human, you'll usually see effects after a unit, stronger effects after 2-3 units, and after that a need to resanguinate.