It's all blood! Or rather every hit is some blood, some endurance, some everything else. If you take 8 points of damage maybe only half or a third is actual injury, but some of it always is. Perhaps your hp for class is endurance and your con modifier is injury. It doesn't really matter because each hit brings me closer to death! I don't think, how that hit make me tired. I think, a couple of more hits like that and I'm dead!