The abstract hit point system of D&D is not easily understood since its lack of detail makes the realism within it hard to see. But I have already written at length about such matters so I shall not reproduce that work here. If you wish to delve into the realism of the hit point system, you can start by following this link: The Justification Of The Hit Point System(Are Hit Points Realistic Or What?) I will, however, address another aspect of this system more closely, since that is where many of the problems arise. What problem? Healing.