Here's a schematic of the internal operation.  When you inhale, two separate muscular actions are involved.  The diaphragm tightens up, so that its center pushes down on the abdominal innards and pulls up on the ribs.  At the same time, the muscles between the ribs contract at an angle, pivoting the ribs upward and outward.   The downward push on the innards forces them to expand the belly, unless you're also tensing your abdominal muscles.

My python page
My ShareCG freebies