Video narration
Why do you breathe faster after running or climbing stairs? It feels like your lungs are doing all the work-but the real story is happening inside your muscle cells. Breathing brings oxygen in and carries carbon dioxide out. Cellular respiration is different: it is the chemical process cells use to break down glucose and release usable energy. That energy is packaged as ATP, the ready-to-use supply for cell activities. In aerobic respiration, glucose plus oxygen produces carbon dioxide, water, and energy. Carbon dioxide travels back to your lungs and leaves when you breathe out. Run harder, and your muscles need more ATP, so they use glucose and oxygen faster. If oxygen cannot arrive quickly enough, they temporarily switch to anaerobic respiration, making less energy and lactic acid. So why the breathlessness? Your running muscles demand more energy and use oxygen faster. You breathe faster to supply it.