Final answer:
The functional units of the lungs that are air sacs are called alveoli, which provide the surface area for gas exchange between the lungs and blood due to their thin-walled structure and direct contact with capillaries.
Step-by-step explanation:
The functional units of the lungs that are air sacs are called alveoli. The terminal bronchioles split into respiratory bronchioles, which further subdivide into several alveolar ducts. Surrounding these ducts are numerous alveoli and alveolar sacs, providing a vast surface area for gas exchange.
Gas exchange between the lungs and blood takes place in the alveoli, where oxygen diffuses into the blood, and carbon dioxide is expelled from the bloodstream to be exhaled. Alveoli are in direct contact with the capillaries, ensuring efficient gas exchange due to the thin-walled structure and extensive surface area (approximately 75 m²).