When we breathe, oxygen is inhaled by our nose or mouth. 

Oxygen then goes down our trachea (windpipe), through our bronchiole tubes (airways), and to tiny air sacs called alveoli.

These thin air sacs contain capillaries, or blood vessels, going through them. 

Once oxygen reaches the alveoli, it passes to the blood in the capillaries. 

The oxygen rich blood circulates through the body, nourishing cells. 

When we breathe out, carbon dioxide -- the waste product of our cells’ metabolism -- moves from the blood to the alveoli, and back up the airways to be released from the body.

<img src="http://www.nycsleep.com/wp-content/uploads/2011/02/Illu_bronchi_lungs-225x142.jpg" alt="" title="Illu_bronchi_lungs" width="225" height="142" class="alignnone size-medium wp-image-201" />