You just got done with a long run. You are gasping for air. Why does your body react this way? What is the purpose of breathing? All the cells of your body need oxygen to work properly. Your bodys circulatory system works with the respiratory system to deliver the oxygen. Your blood carries red blood cells. The main job of red blood cells is to carry oxygen throughout your body. The red blood cells get oxygen in the lungs. The lungs are the main organs of the respiratory system. The respiratory system is the body system that takes in oxygen. It then releases carbon dioxide back to the atmosphere. The carbon dioxide is the waste material from the cells. 
What waste material do our cells release into the atmosphere?
Oxygen