How does the death concept work on Bleach?

I've only watched episode 22 to 39 so I'm a little confused. I get that the death reapers find souls of the living and send them to the soul society. But what happens when death gods are killed? and can the souls be killed once they are in the soul society? and do the souls remember their past lives in the soul society? I just wondered because the soul reapers live in the soul society so if they died they would just be able to reappear there.... heh