Alveoli: The Tiny Air Sacs Essential for Gas Exchange

The average adult has about 300 million alveoli—microscopic air sacs in the lungs where oxygen and carbon dioxide are exchanged between the air we breathe and the bloodstream.

Each alveolus is surrounded by capillaries. Oxygen diffuses into the blood, while carbon dioxide moves from the blood into the alveoli to be exhaled. This vast surface area (about 70 square meters) makes efficient gas exchange possible.

Damage to alveoli from smoking, pollution, or diseases like emphysema impairs breathing and oxygen delivery.