The Sac-Like Structures Inside Your Lungs: Alveoli Explained

The microscopic, balloon-shaped air sacs lining the inner walls of the lungs are called alveoli. These tiny structures are the primary site of gas exchange in the respiratory system.

Oxygen from inhaled air diffuses across the ultra-thin alveolar walls into the surrounding capillaries and enters the bloodstream. Simultaneously, carbon dioxide—a metabolic waste product—moves from the blood into the alveoli to be exhaled. The combined surface area of millions of alveoli creates an expansive interface that enables rapid, efficient gas exchange with every breath.

For more detailed information, see the Mayo Clinic's overview of lung anatomy.