Being eaten is obviously not a process most organisms can survive. But assuming one is swallowed whole, and isn't injured before reaching the stomach, what actually ends up killing them? Acid? Heat? Pressure? Lack of oxygen? Would it necessarily be painful, or could you peacefully fade away into soup?
Edit: If it makes the question less off-topic, I'm particularly interested in fictional carnivorous humanoid mammals somewhat resembling anthropomorphic wolverines swallowing rat or weasel sized mammals. They digest the creatures they ingest wholesale, bones included.