The lungs bring air in and out, taking in oxygen and getting rid of carbon dioxide gas, a waste product of the body. The lining, which surrounds the lungs and helps to protect them and to facilitate the sliding motion during respiration, is called the pleura. The chest cavity is called the pleural cavity. It divides into tubes called the bronchi, which divide into smaller branches called the bronchioles. At the end of the bronchioles are tiny air sacs known as alveoli.
Most lung cancers start in the lining of the bronchi.
But cancer can also begin in the trachea, bronchioles or alveoli. Lung cancers are thought to develop over a period of many years. First, there may be areas of precancerous changes in the lung. These changes do not form a mass or tumor. They cannot be seen on an x-ray, and they do not cause symptoms.