You usually don’t have to think much about your breathing because your brain controls it automatically. When you have a lot of carbon dioxide—the waste gas produced by body processes—in your blood, your brain gets the message and tells your lungs to exhale and dispose of it. This action then causes you to inhale, drawing in air that eventually delivers oxygen to every cell in your body. This carefully regulated exhaling and inhaling takes place about 10 to 14 times each minute when you are breathing calmly.
When you need more oxygen than usual, your brain takes care of that too. When you are exercising or working hard, your brain tells you to breathe more quickly, taking in 15 to 20 times more air. If that still doesn’t deliver all the oxygen that your muscles need, you may “run out of breath,” which forces you to rest. You will still breathe hard at that point—every second or so—until your muscles are able to work again.