The respiratory system allows air to enter the body through the nose and mouth, then travel through the windpipe (trachea) into the lungs. In the lungs, oxygen is exchanged for carbon dioxide through small blood vessels. The oxygen then enters the bloodstream to be circulated to cells throughout the body, while carbon dioxide is exhaled. The respiratory system is made up of the trachea, lungs, and rib cage which protects the lungs.