Half a character's hit points is Vigor. This represents fighting prowess, spirit, hardiness, and generally the ability to persevere. Losing Vigor represents exhaustion, parries, near-misses, etc.
All Vigor returns if characters have a moment to catch their breath at the end of a battle or hazardous situation. Vigor cannot be restored by cure wounds spells.
The other half of a character's hit points is Health. This represents actual capacity to withstand wounds. A character recovers 10% of their max health per day of rest, doubled if they do nothing but rest and have access to medical care.
Damage that a character receives is first subtracted from Vigor. When Vigor is exhausted, damage comes off of Health. At 0 Health, a character is mortally wounded and will die in 1d6 rounds unless medical aid is rendered. Furthermore, a character cannot have more Vigor points than he has Health points, so a character who has lost all but 3 of his Health cannot have more than 3 Vigor unless he heals up.
Option: In combat, any attack that scores a natural 20 is subtracted directly from Health, bypassing Vigor.
Option 2: Missile weapons subtract directly from Health.
Option 3: Energy drain and other life-draining spells and effects come directly from Health.