What are tiny air filled sacks in lungs called?

The tiny air-filled sacks in the lungs are called alveoli. They are responsible for gas exchange, which is the process by which oxygen is taken up into the bloodstream and carbon dioxide is released. The alveoli are lined with capillaries, which are tiny blood vessels. Oxygen diffuses from the alveoli into the capillaries, and carbon dioxide diffuses from the capillaries into the alveoli.