[Chooriwrocafn]

It happens somewhat like the awareness of the breath, I think ... our breathing keeps on happening and whilst functioning can become under conscious control and the mind can be trained to be more aware of it, nature ensures we breathe to survive.

Thanks Andy. This is a very good observation.
The body is mere nature and it breathes on its own whether the mind attends to it or not. Body is one thing, awareness is another. This is vividly visible during the first stages of anapanasati that it is almost impossible to deny it.