When I think of my bowels (which is sort of frequently, actually), I imagine their empty state as being hollow tubes that are about the diameter of a piece of poop. It was only today that I realized their empty state is probably collapsed, with the opposite sides touching each other. Otherwise, there's no explanation for farts. Why would your bowels push out some air if they are constantly filled with air?
I'm not sure why it took me this long to figure this out.
NB: Remember, the "math" label is the "science" label.