Final answer:
Air sacs are structures in the respiratory system that help with efficient gas exchange and make certain animals, like birds, lighter.
Step-by-step explanation:
The term 'air sacs' refers to structures found in the respiratory system of certain animals, which serve various functions. One example of an animal with air sacs is a bird. Birds have a unique respiratory system that includes air sacs connected to their lungs. These air sacs help to facilitate efficient gas exchange and make birds lighter compared to other animals of similar size.
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