The carry air to microscopic air sacs called alveoli

"The bronchi then divide into smaller and smaller branches (bronchioles), finally becoming microscopic. The bronchioles eventually end in clusters of microscopic air sacs called alveoli. In the alveoli, oxygen from the air is absorbed into the blood."(

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The bronchi split into smaller and smaller tubes called bronchioles, which then end in microscopic alveoli (air sacs).