Alveoli: Most adults have over 300 million of these in their lungs, and this is where most gas exchange occurs. Oxygen molecules inhaled wait here for an available hemoglobin molecule.  

Red Blood Cells (RBC):  These look like little red boats or donuts.  In the middle is a protein called hemoglobin.  RBCs ride single file down the capillary beds in the lungs where they come into contact with the alveoli.

Capillary:  These tiny vessels transports RBCs that have carbon dioxide (a cellular waste product) attached to the hemoglobin instead of oxygen.  These RBCs are purplish-blue in color.

As you watch, you can see that once one of these RBCs comes into contact with the alveoli, the carbon dioxide molecule hops off the hemoglobin and an oxygen molecule hops on.  The RBC then turns red.  Oxygen now takes a ride to a tissue somewhere in the body.  

Millions of these reactions happen with every breath you take, and this is how your body is oxygenated.  (watch videos here and here)

You suddenly feel a draft, and are sucked back up the respiratory tract and (A-chooooo) out the mouth and into the air.  

Its' cooler out here, and smells better too.  You are now free to return to your normal size.

Thanks for traveling with us.