Vaalingrade
Legend
They feel it the same way we feel that we're not okay. We know there's degrees of being not okay and what they indicate: are you tired, woozy, physically injured, anxious, like you're losing the fight. You don't know discreet numbers and percentages, but you know enough that you can describe it usefully in words.But does the character say that? Do the people in your setting have a concept of hit points, do the people in the setting talk using game mechanical terms? Also, how does the character know that they have 23 hit points? How does it feel to them to have 23 hit points? How does that feel different from having 40 hit points?
Of course, of HP were meat, you could weight the viscera scattered around you whenever you lose an HP as a percentage vs your starting body weight too.