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.
The majority of bony fish has swim bladder that keeps them buoyant. The organ is also called gas bladder, fish maw, or air sac.
Amniotic sac
Amniotic sac
gallbladder
The biological term for an air sac is "pneumatic sac," which refers to a structure in the respiratory system of certain animals that aids in breathing by storing or regulating air flow. These structures are commonly found in birds and some reptiles.
An alveolus is a single air-sac, and alveoli are multiple air-sacs.