The lungs are organs which are responsible for transporting oxygen from the surrounding air into the bloodstream for use round the body, whilst removing carbon dioxide.
The structure of the respiratory (breathing) system can be compared to branches of a tree; the main air pipe (trachea) is the trunk of the tree, which divides to from two small branches (right and left main bronchi), each supplying air to the right and left lung, respectively. From there, the airways get progressively smaller (bronchioles) until they reach very small airway spaces (alveoli and alveoli sacs), which resemble bunches of grapes. Within the alveoli, the oxygen moves across the alveoli wall into a blood vessel on the other side. Simultaneously, carbon dioxide leaves the blood stream and enters the alveoli to be exhaled.