Video narration
After climbing the stairs, what is your next breath really doing? You feel air come in through your nose or mouth, and your chest rises as your body works harder. That air travels down your windpipe, then through smaller branching tubes inside your lungs, toward tiny air sacs. But your lungs do more than hold air. In those tiny sacs, oxygen moves into your blood, while carbon dioxide moves from the blood into the air. Your blood carries oxygen onward to your body; the carbon dioxide is now ready to leave. As you catch your breath on the stairs, a broad muscle beneath your lungs, the diaphragm, contracts and moves down to help draw air in. When it relaxes, air flows back out. Each breath is an exchange, not simply a filling. That stair-climbing breath brings oxygen into your blood and carries carbon dioxide out with your exhale.