since there is no answer to this, im just going to go ahead and guess :
the lungs .
diaphragm
The bladder.
Bladder
Amniotic sac
Amniotic sac
The urinary bladder
It's an Air Sac.
I Think You Thinking Of The Alveoli/Alveoli's
the air sac itself.
Marsupial sac.
The bladder.
the stomach
The majority of bony fish has swim bladder that keeps them buoyant. The organ is also called gas bladder, fish maw, or air sac.
Bladder
Amniotic sac
Amniotic sac
The biological term for an air sac is alveolus. The plural is alveoli.
gallbladder