Video narration
After you breathe in, where does the air go? Take one deeper breath, like after climbing stairs, and follow it. It enters through your nose or mouth, travels down the windpipe, and reaches your lungs. As your chest expands, the diaphragm moves down, helping draw the breath in. There, air reaches tiny sacs called alveoli. But your lungs don't send the whole breath into your blood. At the sacs, oxygen crosses into the blood; carbon dioxide crosses out of it and into the air. The breath has now changed: it carries carbon dioxide your body needs to remove. As you breathe out, that air travels back through your airways and leaves through your nose or mouth. So where did your breath go? It reached tiny air sacs, where oxygen entered your blood and carbon dioxide left to be breathed out.