User:DayWalsh/Background - What is COPD?/alveoli

The smallest airways end in tiny air sacs called alveoli. Here the oxygen from the air is absorbed into tiny blood vessles called capillaries which criss cross the walls of the alveoli (29). At the same time a waste product, called carbon dioxide, comes out of the capillaries and back into the alveoli ready to be breathed out (29).