What is the definition or description of: lung?

Exchange CO2 for O2: Deoxygenated blood enters the lungs from the right side of the heart and travels to the lungs. When you inspire, oxygen flows into the lungs, transverses the capilliares and attaches to hemoglobin down a gradient. At the same time, co2 diffuses into the capilaries and is expelled with exhalation. Oxygen rich blood then flows to the left side of the heart and into the circulation for cellular use.