We inhale. We exchange 500 air every single time we breathe. Now, only a portion of this air actually ends up in the alveoli of the lungs, where gas exchange actually takes place. The rest of that air is trapped within the passageways of our lungs, and that includes the trachea, that includes the bronchi as well as the bronchioles. And because gas exchange only takes place in the alveoli, no gas exchange takes place in the passageways. We call the air inside the passageways the anatomic death space because it's the air in the passageways that cannot be used for gas exchange because gas exchange only takes place inside the alveoli of our lungs.