Your lungs are two large, spongy organs shaped something like an upside-down butterfly. One lung is located on each side of your chest. They're separated by the mediastinum — the tissues and organs of your mid chest, which include your heart, esophagus and windpipe (trachea) as well as lymph nodes and major blood vessels such as the aorta. Each lung is divided into upper and lower sections called lobes. Your left lung has two lobes, and your right lung, which is larger and heavier, has three lobes.
Every time you inhale, air is carried through the windpipe to your lungs in two major airways called bronchi. Inside your lungs, the bronchi subdivide nearly 20 times into a million smaller airways (bronchioles), which finally end in clusters of tiny air sacs called alveoli. Within the air sacs, oxygen is absorbed into your bloodstream and carbon dioxide — a waste product of metabolism — is released.
How cancer forms
The lining of the airways and windpipe is made up of rectangular-shaped surface cells (columnar epithelium) and glands that produce mucus and other fluids. In healthy lungs, these cells divide in a controlled and orderly way. But sometimes this growth gets out of control — cells continue reproducing even when new cells aren't needed.
These abnormal cells primarily originate when the lungs are exposed to cancer-causing substances (carcinogens), such as those found in cigarette smoke, radon and asbestos.
At first only a small number of abnormal cells (precancerous lesions) may appear, but with repeated exposure to carcinogens over a number of years, these cells may increase and eventually become cancerous (malignant). And because cells in your lungs have easy access to a large number of blood and lymph vessels, tumors may be carried to other sites within your body, even before you have any symptoms.
Cigarette smoking accounts for the great majority of all lung cancers. Tobacco smoke contains more than 3,500 chemicals, at least 40 of which are known carcinogens. Cigarettes also contain at least 30 toxic metals, including nickel and cadmium, as well as radioactive compounds such as polonium 210.
Other risk factors for lung cancer include exposure to secondhand smoke, to asbestos and other industrial carcinogens, and to high concentrations of radon — an odorless gas that's released into the air from the breakdown of uranium in the soil and water. Smokers exposed to asbestos and radon are far more likely to develop cancer than are nonsmokers.
Primary lung cancer is uncommon in nonsmokers, but cancer of the breast, colon, prostate, testicle, kidney, thyroid, bone or other organs may spread to the lungs. In that case, however, the cancer is still referred to by the name of the organ in which it originated, rather than being called lung cancer.
Types of lung cancer
Lung cancer is commonly divided into two groups: small cell and non-small cell. Each grows and spreads in different ways and is treated differently. Small cell lung cancer, for example, spreads aggressively and responds best to chemotherapy and radiation. It occurs almost exclusively in smokers and accounts for about 20 percent of lung cancers.
Non-small cell lung cancer, which is more common, accounts for almost 75 percent of lung cancers. If caught early when it's confined to a small area, it often can be removed surgically. There are three major categories of non-small cell lung cancer:
Squamous cell carcinoma. This cancer forms in cells lining your airways. It's the most common type of lung cancer in men.
Adenocarcinoma. This type of cancer usually begins in the mucous-producing cells of the lung. It's the most common type of lung cancer in women and in people who have never smoked.
Large cell carcinoma. This type of cancer originates in the peripheral part of the lungs.
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