Okay, so from what I understand, Superman’s invulnerability comes from a somewhat inpenitrable force field that eminates a few milimeters from his body, right? So, how does that make his insides invulnerable, too?
I don’t read much DC, but I do recall a couple of times where Supes has inhaled large amounts of poisonous gas in order to prevent it from killing others (like in the Death in the Family story, where he inhales a room full of Joker gas to save the UN). And in Superman vs. Aliens*, he’s got a little critter inside him, manages to get in our solarsystem with enough time to get powered up, and the little alien kills itself trying to rip through his ribcage. Does his forcefield go inward as well? If not, how can he survive inhaling nerve gas without any consequences? And when the alien bleeds out, how does his body handle the acidic blood coating his internal organs?