Anatomy of the Lung
The lungs are separated by the mediastinum. This area contains the heart, trachea, esophagus, and many lymph nodes. The lungs are covered by a protective membrane known as the pleura and are separated from the abdominal cavity by the muscular diaphragm.
With each inhalation, air is pulled through the windpipe (trachea) and the branching passageways of the lungs (the bronchi), filling thousands of tiny air sacs (alveoli) at the ends of the bronchi. These sacs, which resemble bunches of grapes, are surrounded by small blood vessels (capillaries). Oxygen passes through the thin membranes of the alveoli and into the bloodstream. The red blood cells pick up the oxygen and carry it to the body's organs and tissues. As the blood cells release the oxygen they pick up carbon dioxide, a waste product of metabolism. The carbon dioxide is then carried back to the lungs and released into the alveoli. With each exhalation, carbon dioxide is expelled from the bronchi out through the trachea.