Hey OP, I can offer a little insight to your question as I used to be a paramedic and sadly have witnessed first hand exactly what happens with opioid overdose deaths. I guess the simple answer to your overall question is that your body basically shuts down and goes to sleep....forever. Specifically though, in a nutshell, your brain's communication pathways to your body is whats know as your Central Nervous System [CNS], drugs such as opiates and opioids work as CNS depressants on your body which basically means it suppresses most of the functions of one's CNS, the goal here being to decrease the ability of your CNS to signal to your brain that some part of your body is in pain, the downside however is that if its suppressed TOO much for TOO long basically your brain is sitting there saying "umm..ok body..wtf...I'm not getting any feedback, are you there? something wrong? ...oh shit something is wrong!" Sooo then your body starts its purging / saving itself functions such as reducing bloodflow to certain organs and somewhat shuts them down, thats one thing that starts the death process. Then usually what happens is your blood pressure bottoms out and your breathing slows way way down because of the brain trying to slow everything down while it tries to figure out whats going on, thats number two. Then basically your brain tells your body to purge itself which causes vomiting, and when you vomit when youre unconscious you aspirate on the vomit, meaning that it flows back down in to your lungs, literally "drowning" you, thats number three. Now when you combine all of those things, low pulse, slow breathing, vomit in lungs...you will eventually succumb to a mixture of all of those things if enough time elapses and no emergency antagonist like naloxone is administered in time...eventually your brain says "ok...Ive done all I know to do....I'm out..." .....Then you're dead. Sorry for the hyperbole but I tried explaining it in a not-so-scientific way for the sake of time. Hope that helped you question