Video narration
Where does the air go after you breathe it in? With one ordinary breath, air enters through your nose or mouth, travels down your windpipe, then through smaller and smaller branching airways into your lungs. That's the route; your respiratory system does more than move air. The lungs do more than fill: alveoli, tiny air sacs, swap gases with your blood. Oxygen passes into the blood; carbon dioxide moves from it into the air. Your blood carries oxygen on to the rest of your body, then brings carbon dioxide back to the lungs. As you inhale, the diaphragm, a muscle beneath your lungs, contracts and moves down, helping draw air in and make room for the lungs to expand. As it relaxes, air moves out. Air reaches tiny air sacs: oxygen enters your blood, and carbon dioxide leaves to be breathed out.