Think of it less biologically. Maybe he was dead, or dying - and his heart was replaced by a corpse's heart, and then with magic "Juiced" so it now lives. Maybe he's a ghost trapped in a living body, OR a living soul trapped in a dead body (or just mind and conscience, for that matter).Does a half-undead heart beat? Or only the left ventriculos? Maybe he has some rotting limbs but not the whole body...