Where does your breath go when you breathe?

Your breath doesn't "go" anywhere. When you breathe in, air enters your lungs and fills up the alveoli, which are tiny air sacs. Oxygen from the air diffuses across the walls of the alveoli and into your bloodstream. At the same time, carbon dioxide, a waste product of your metabolism, diffuses out of your bloodstream and into the alveoli. When you breathe out, the carbon dioxide-rich air is expelled from your lungs.