>>716084328For things like brain injury, you could establish that the major organs have a separate "health" bar that decreases as you age, and should it reach 0 you also die, even if you still possess all of your blood.
As for injuries not bleeding exact amounts, that is due to most games not going for realistic models, in a realistic setting most injuries would inflict various statuses upon the individual, such as bleeding, poison, burns, internal bleeding, bacterial infection and so on, all of which would decrease one or both bars over time and thus lead to your death.