I have a creature that has a humanoid torso and abdomen except for it being larger to accommodate air sacs. It has a flexible spine; not rigid like birds. The pelvic bones are like a human's(except for it being longer front to back to accommodate larger torso/abdomen, and slightly wider), not a bird's. Human pelvis takes up more space in the abdomen than a bird's does. So the creature is basically an upright bird(ish) body shape with a human pelvis and back and lacking the big bird sternum.
The structure of the posterior air sacs is that it wraps around the organs; this means a good portion of it is in the front and sides of the midsection, and in front of the intestines. Unlike bird and human organs displacing outwards due to breathing, the air sacs on this would surround the organs in a way that there's no place for that outward displacement of other organs. If it wraps around the organs, would it squish them too much when it inflates/deflates?
Even with this differing structure, could it properly breathe with a bird respiratory system? More specifically, since the humanoid pelvis takes up more space in the abdomen than bird pelvis, can I have the air sacs wrapped around the other organs in the front and sides of the midsection plus in front of intestines without that causing issue?