When you breathe in, fresh air goes into your lungs.
Then the air goes from the lungs into your blood.
When you breathe out, stale air, called carbon dioxide, comes out of the lungs.
The respiratory or (breathing) system is made up of the parts of your body that take oxygen into the body, and carbon dioxide out of it.
When you breathe in through your mouth or nose, air or oxygen travels down your windpipe or trachea (say tray-kee-uh) and into your lungs. Near its end, the trachea divides into two tubes called bronchi, one into each of the lungs.
The lungs are covered with millions of tiny air sacs called alveoli (say al-vee-oh-lee) These air sacs fill with air and the lungs get bigger.
Each air sac is covered with tiny blood vessels called capillaries (say cuh-pill-ar-ees). Blood which has travelled around the body and has had all the oxygen taken from it, comes into the lungs from the heart through the blood vessels. The blood is carrying carbon dioxide which the body doesn't want. The blood leaves the carbon dioxide in the lungs and picks up fresh oxygen from the lungs. When you breathe out, the carbon dioxide leaves your body. The fresh oxygen is carried around the body in the blood.
The muscle that works the the system is called the diaphram (say di-uh-fram)