With the help of the red blood cells in your bloodstream.

Your red blood cells are like box cars on train tracks.

They show up at the sacs at just the right time, ready to

trade in old carbon dioxide that your body's cells have

made for some new oxygen you've just breathed in. In the

process, these red blood cells turn from purple to that

beautiful red color as they start carrying the oxygen to

all the cells in your body.