By breathing we inhale air that provides oxygen. Without this gas, no life can exist. When we breathe out or exhale air, it has changed. As the air made its way through our system, some of the oxygen was used up, while the amounts of carbon dioxide and water were increased.
Nature keeps a balanced supply of oxygen for our breathing. Over a period of time there is little change in the amounts of oxygen and carbon dioxide found in the air. This is because the carbon dioxide we breathe out into the air is taken up by the plants which in turn breathe out oxygen which we need.