Q. A number of very thin, irregular-walled and vascularised bag-like structures are called as

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Solution:

Each lung, performing gaseous exchange, consists of a small balloon-like unit called an alveolus. The alveolus is lined by simple squamous epithelium. There are around 480 millions of very thin, irregular-walled, vascularised bag-like structures called alveoli in the human lungs.