“When you find yourself breathing hard, is this due to a lack of oxygen or an excess or carbon dioxide?”
When you run really fast, you end up breathing heavily. This isn’t because you have too much oxygen in your lungs. When you inhale, you’re inhaling oxygen, and since there’s such a large supply of it all around us, it’s unlikely that breathing hard is due too much oxygen. This is due to too much carbon dioxide in the blood. Not enough air can reach the parts of your body it needs to, and you start to hyperventilate. However, you can force yourself to breathe normally when this occurs. Taking deep breaths in through your nose and out through your mouth also helps.