since there is no answer to this, im just going to go ahead and guess :
the lungs .
The bladder.
Amniotic sac
Amniotic sac
The bladder is a sac-like organ in the body where urine is collected before being excreted through the urethra. The bladder expands as it fills with urine and contracts to release the urine when necessary.
A sac-like organ is a structure in the body that is shaped like a pouch or bag. Examples include the gallbladder, which stores bile; the urinary bladder, which collects urine; and the intestines, which have sac-like segments for digestion and absorption. These organs often have elastic walls that can stretch to accommodate and store various substances.
It's an Air Sac.
I Think You Thinking Of The Alveoli/Alveoli's
the air sac itself.
Marsupial sac.
The bladder.
The urinary bladder acts as a storage sac for urine in the body.
Amniotic sac
Amniotic sac
The majority of bony fish has swim bladder that keeps them buoyant. The organ is also called gas bladder, fish maw, or air sac.
gallbladder
An alveolus is a single air-sac, and alveoli are multiple air-sacs.
It is easy for air to pass across the wall of an air sac because the wall is thin.