In real life, death through blood loss and suffocation do not happen immediately. Death occurs when the brain is unable to get enough oxygen to keep the body functioning. Similarly, muscle fatigue is caused by the muscles being unable to get enough oxygen.
In order to make the game more realistic, fatigue, consciousness, and death should be tied together and represent points on a sliding scale tied to oxygen in the blood. I propose the following system:
There should be a single counter representing the body's total oxygen level. As long as the creature is able to breathe, is not losing blood, and is not performing high-exertion activities (fighting or running), it remains at maximum. It may decrease due to the following circumstances:
- Being unable to breathe (due to lung damage, choking, or drowning) causes it to drop at a fixed rate. (There should probably be a tag for the specific rate in the creature's raws - the amount of oxygen the creature's body uses when remaining still - but this is a minor detail.)
- Performing exerting activities causes it to drop as well. Being able to breathe lets it fill up again.
- When the creature loses blood, they lose a percentage of oxygen equal to the percentage of blood they lost. Their total oxygen level is capped at their blood percentage as well, so until their blood is restored they will have to function at diminished oxygen levels.
- When a CIRCULATION body part (the heart) is damaged, the oxygen counter does not change. However, the effective oxygen level will be divided by the amount of circulation damage when calculating the effects on the body.
As oxygen level lowers, the following effects will occur.
- The creature receives a penalty to all skill rolls which gets more severe as the oxygen counter lowers further.
- AI creatures will generally opt to slow down if running even from a small amount of exertion, unless in extreme danger. Player characters will not be forced to at first, but will receive a warning that they are becoming tired.
- The creature's top speed will lower.
- The creature gains the 'winded' effect and will occasionally stop to catch their breath. This becomes more frequent as the oxygen counter lowers.
- The creature will become unable to stand.
- The creature will become unable to move at all.
- When the counter hits 0, the creature will fall unconscious.
Once the oxygen counter hits 0, another counter will start to drop, this one representing the brain's health. When this drops to 0, the creature dies. If oxygen is restored, the brain counter will begin to recover. It takes several minutes for actual brain death to occur. Enemies may opt to ignore unconscious enemies or deliver a killing blow to make sure. (This may also introduce a mechanism where one can pretend to be dead, or where an unconscious creature survives a battle by being overlooked by enemies.)
When a creature is being strangled (due to a blood choke) they drop unconscious immediately and their brain starts taking damage, but they do not die until the brain counter falls to zero. They also regain consciousness if the hold is released.
How this interacts with unconventional or magical creatures:
NOBREATHE means that the oxygen counter always rises as if they had fully functioning lungs. It can still lower due to other things though.
NOEXERT means that exerting activities do not lower the oxygen counter, but they can still suffer from the effects of exertion if the counter drops due to other reasons.
NOSTUN prevents unconsciousness due to brain damage, but brain damage still happens. Also, if their oxygen lowers enough, they can still be rendered unable to move, so while vampires in this system can technically survive and maybe talk for a little while after bleeding out (unless they are killed with a decisive finishing blow), they won't be able to do anything. Vampires can technically be strangled, but they will be conscious the whole time until their brain stops working.
Creatures without blood function as if they always had max blood.
Actual brains have very little to do with this; brain damage just means 'whatever part of them that controls their motion is being damaged'.
Only zombies and inorganic creatures, who have no breath, exertion, or blood, will go from functional to properly dead instantly.
In addition to realism, this system adds some extra possibilities to plot generation: Instances where a dying veteran tells his last requests to a fellow soldier before bleeding out, and surviving a battle by falling unconscious (as in The Hobbit).