What happens to the body when you overdose ? What happens physically?

When someone takes an overdose, what exactly does it do to the body. I have overdosed in the past, and I know that people may get kidney o rliver damage.. luckily for me, that never happened. However, I did develop cardiac problems due to all the overdoses.. But how does the body react to being pretty much poisoned by a whole bunch of pills ? I never once threw up. Never. I was always found unresponsive and rushed to the hospital and spent days in the Intensive Care Unit, on a heart monitor, iv, everything imagineable.. So how does the body react and what happens if you take a pill that is XR ( extended release ).. when does it go into the body