Lung

The lung is an organ in many vertebrates (animals having a spine, or backbone). It takes blood oxygen from the air, and expels carbon dioxide. Most vertebrates with lungs have two of them.

In animals, the lungs are the area where gas exchange takes place. Without gas exchange, oxygen would not pass into the blood from the lungs so the body cells would not be able to receive the oxygen needed for respiration.

Gas exchange happens in the alveoli. The alveoli are moist to allow oxygen to move from the lung through the alveoli into blood vessels and red blood cells. Carbon dioxide passes from the blood into the alveoli. The oxygen-filled blood goes back to the heart and the carbon dioxide in the alveoli is pushed out of the lungs and into the air we breathe out.