The respiratory system is the system the body uses to get oxygen from the air. 
The oxygen gives cells the ability to produce the energy they need to live. 
Air enters the respiratory system through the mouth and nose and it travels down the trachea and bronchial tubes to the lungs. Inside the lungs, the air is sent to millions of air sacs called alveoli. These alveoli pass the oxygen from the air into the blood stream and also remove the carbon dioxide from the blood stream to be exhaled back into the air.