A bit of interpretation on ghost biology:
-Physics don't work
normally with ghost particles. That's how they manage their superhuman feats of strength and
magic.
-There's a far greater emphasis on feelings, emotions and beliefs.
My take on it is that organs needed for simple bodily survival are spiritually more complex and more tied to the integrity of your make-up as a ghost. You can still survive fine without an arm, it's just (very) inconvenient. Without lungs or a stomach, Nemo couldn't breathe or would starve to death, so they're harder to reconstruct.
So using this bit of (head)canon, we can simply assume that there's a simplified model of human body, that a normal... soul-person
believes to be true.
After all, breaking off a piece of a Hollow's mask removes their
hunger. This has nothing to do with suddenly gaining an infinite source of calories. They just feel more
human, and as such they are untouched by the primal need for sustenance.
This does lead to a fascinating question: is the spiritual importance of heart greater than physical one?
While in real life all it does is pump blood, in common belief it's a source of emotions, if not a seat of the soul.
This would make one's heart a
very powerful reagent, as it contain's most of said person's essence.
