The term AIR SAC is used to describe a series of structures in the respiratory system. The 8-letter solution, ALVEOLUS, refers to the small sacs in the lungs where oxygen and carbon dioxide are exchanged. The 7-letter solution, ALVEOLI, is the plural form of alveolus. The 4-letter solution, CYST, is a fluid-filled sac that can form in the body. The 6-letter solution, AMNION, is a thin membrane that surrounds the fetus in the uterus. Finally, the 5-letter solution, LEMAY, is a type of air sac found in birds. Together, these solutions provide an overview of the various structures that make up the air sac system.