When you inhale the oxygen enters in your nose and mouth,travels going to trachea,bronchi,bronchioles,and then to your alveoli or your air sacs,from your lungs it travels going to your left part of your heart and the heart pumps blood which blood driven the oxygenated blood going to your cells inside your body,when the cell reaches it released energy,and it travels again going to your capillary and which happen the gas exchange and the carbon dioxide travels to your right part of your heart by de oxygenated blood going to your lungs and when you exhale the carbon dioxide released going to out.