Without going into too much detail, a question has come up in a Non-Adventurer's League game that I am running where a player has asked me whether healing their character would restore blood loss from self-inflicted harm.
The difference, as I understand it, between hit points (HP) is that it is the amount of wounds or damage a person can sustain before falling to the dying state where their body works to heal themselves, as represented by the death saving throw rolls, before they die.
So if a person is purposefully bloodletting from their body, would simply healing them restore the immediate blood that they lost and allow them to reopen the wound and drain more blood as if they had never lost any to begin with?
I am pretty sure there is no definitive rules or rulings on this so I am trying to understand more of what healing represents in this context.
Hit points are a poor abstraction for blood loss
Here's the full description of hit points in the Basic Rules, with the most relevant phrases highlighted in bold face:
The first highlighted phrase indicates that hit points don't just encapsulate physical durability, which makes sense, since things like psychic damage don't necessarily inflict physical harm on a target. More to the point, many damage types, such as fire, cold, lightning, poison, radiant, thunder, and (as mentioned) psychic, may not cause the kind of damage that results in any significant blood loss, despite still inflicting damage on the target.
The second highlighted phrase is potentially important, because it indicates that 0 hit points represents an important threshold: prior to that any injuries or other damage received are not sufficient to impair the creature's ability to act in combat.
With all this in mind, we can come to a conclusion: hit points are a poor abstraction for representing blood loss. Ultimately, the purpose of hit points is to track combat damage, and that doesn't match the situation of a person purposefully letting their own blood. Hence, I would not recommend using loss and gain of hit points to model loss and restoration of blood. Instead, let's look at the symptoms of anemia and try to find a D&D mechanic that comes close to modeling them.
Recommendation: Model anemia with exhaustion
Wikipedia gives the following as common symptoms of anemia (the condition that would result from excess blood loss):
If we take a look at the consequences of exhaustion in D&D, we'll find a pretty good match for these symptoms:
Hence, I would recommend that if a character intentionally loses enough blood to cause adverse effects, they should start suffering progressive levels of exhaustion, which are in turn removable by the usual means, such as finishing a long rest and ingesting some food and drink or casting Greater Restoration. (Conveniently, the common means of removing exhaustion just happen to be things that could plausibly heal anemia that is caused by blood loss.)
In addition, I would probably rule that a spell like Regenerate would literally regenerate all your blood cells and thus cure the anemia after some time. This is based on the ability of Regenerate to cure other severe forms of injury not represented by mere loss of hit points, such as those in the Lingering Injuries table of the DMG.
While I originally came up with the idea to use exhaustion to model blood loss from scratch by comparing symptoms to mechanics as described above, it turns out that there is at least one instance in the rules as written where exhaustion is explicitly used to model blood loss: the Living Armor magic item from Eberron: Rising from the Last War, pg. 278:
Of note, this item also suggests an alternative means to model blood loss: spending hit dice.