This is a theoretical exercise that I've never tested out, though I think True20 does something similar.
I was reading this paper on handgun wounding effectiveness factors and was struck by the parsimony of the descriptions of injury. Either someone is incapacitated or they are not. Adrenaline and endorphins make pain responses much less likely, to the point where falling down and clutching the wound when getting shot in combat is a learnt behaviour according to the author. Incapacitation results either from blood loss (specifically loss of blood pressure and the capability to efficiently transfer oxygen to muscular tissues) or from damage to the central nervous system.
One of the interesting things the author discusses is how temporary cavities do not generally contribute to incapacitation. That is, when a bullet hits you, the force temporarily pushes your flesh apart, but as it penetrates or stops moving, the flesh flaps back closed behind it, with only a smaller, more permanent cavity being important. Handgun bullets and other projectiles tend to leave only small permanent cavities, which must penetrate into the central nervous system, key blood vessels or other organs where a small loss of mass is sufficient to incapacitate the person. The end result of this is many people, when injured but not incapacitated by a weapon, just keep on doing whatever they're doing (trying to kill you or run away from you, mainly).
I've been considering whether this kind of knowledge could be usefully transferred into damage systems in games in such a way that it is not simply a binary system with "alive and fully functional" and "incapacitated" as the two categories. Specifically, I want a way to track when someone is maimed but not incapacitated (for fantasy games where swords can cleave off a hand or arrows can put out an eye) and when someone is not currently incapacitated, but is bleeding out and will be eventually. I think it might also be useful to cover when someone is specifically dead, and not just incapacitated. I'm not sure a track is the most effective way to handle this.
One possibility I've been considering is making damage rolls a sort of reverse saving throw with gradations, where the attacker's roll must exceed certain values in order to inflict each one of the four injury possibilities. I favour these values ascending with level, rather than being based on Constitution or another ability score, so that there is something like, but not quite the same as, ascending HP.
For example, a 1st level fighter might have injury scores of 8/12/14/16, where the first covers bleeding, the second maiming, the third incapacitation and the fourth instant death. An attacker rolls a d20 to attack and gets a 13. The fighter is bleeding and maimed, but not incapacitated or dead. If the attacker rolls a 14, the fighter is bleeding, maimed, incapacitated and dead.
As a possible variation to consider, I might stagger the results so that it goes bleeding, incapacitated, maimed, dead, which would order them in terms of the permanency of the result rather than the severity of the effect on combat capability.
The effects of each status:
Bleeding: Attacking a bleeding opponent adds +2 to your attack roll (thus making further damage more severe). People who are bleeding out may make a saving throw at the end of combat to stop bleeding.
Incapacitated: The person is unable to perform strenuous activity and is either unconscious, in shock or writhing in pain. Attackers get +4
Maimed: The person has a piece of themselves permanently destroyed or detached. Attackers get a +2 to all attack rolls.
1 Right Leg
2 Left Leg
3 Right Arm
4 Left Arm
5 Torso injury (spinal, respiratory system, circulatory system)
6 Facial injury (eye, ear)
Death: The character must immediately make a saving throw or die.
This is all very tentative, so I'd appreciate feedback and / or offers to playtest it.