Needs a Better Title
, Needs a Better Description
The human body contains about 8-9 pints of blood. When donating blood, you drop 1 pint--even that much makes you tired and kind of weak for a while, and you must wait 56 days before doing it again. Blood does not regenerate all that fast; that's why he have transfusions.
But who wants to deal with all that when you're writing action?
Blood loss doesn't affect fictional characters so much. They can get into a battle with several wounds, but only the flesh damage is shown to be a problem. See various MMORPGS, in which you can fight for a long time without worrying about collapsing if you have enough Hit Points