If you concentrate, you can control your breathing muscles. They’re controlled automatically when your brain senses carbon dioxide in your blood. If this builds up, you yawn or take a deep breath. You breathe hard after running because your lungs can’t take in oxygen quickly enough. Mountain climbers need to carry oxygen tanks because the higher you climb the thinner the air gets. We don’t run out of oxygen because plants release it in a process called photosynthesis. Plants also help by taking in and using carbon dioxide from the air.