Final answer:
The functional gas exchanging units in the lung are called alveoli. Alveoli are tiny air sacs where gas exchange takes place.
Step-by-step explanation:
The functional gas exchanging units in the lung are called alveoli. Alveoli are tiny air sacs that provide a huge total surface area for gas exchange to take place. Gas exchange occurs in the alveoli, where oxygen from the air inside the alveoli is absorbed by the blood in the capillaries surrounding the alveoli, and carbon dioxide is released into the air inside the alveoli, to eventually be exhaled.