The lungs are the largest organ of the respiratory system and located within the chest cavity. Lung diseases include more than just the well known lung cancer, but also alveolar disorders and obstructive disorders, such as COPD. First though, to understand the debilitating effects of lung disease, one must understand the structure and function of the lungs.
On a gross scale, the lungs are divided into lobes, three on the right side of the chest cavity and two on the left. The lack of a third lobe on the left side leaves room for the heart, which pumps blood directly to the lungs for oxygenation. The lungs receive air through the bronchi. The bronchial tree branches off the trachea, which is a fork off the pharynx, the cavity behind the mouth. The other fork off the pharynx is the esophagus, which carries food to the stomach. In the trachea is mucus that catches foreign particles and infectious agents before they can enter the lungs. Any irritation of this tube or alteration in the mucus secretion can lead to infection or obstruction in the airway, including aspiration of food from the esophagus. The trachea branches left and right into the lungs (left and right bronchus), which then branch further into increasingly smaller bronchioles that terminate in the alveoli.
In general, during inhalation, air enters the mouth or nose, passes through the pharynx, past the larynx (voicebox), through the trachea, and into the bronchial tree where it is disseminated to the lung tissue. Exhalation is the passive process of the pressure in the lungs readjusting to force out the air, which is less oxygenated than when it entered and containing carbon dioxide from the blood. The respiratory centers of the brain usually control this process involuntarily, and there is always some air reserve in the lungs to ensure efficient gas exchange, which is the basic function of the lungs. Lung diseases cause deviations in this process due to damage, infection, or genetic mutations.
The structure of the lung tissue accommodates the exchange of gases between the air breathed into the lungs from the external environment and the blood. This exchange occurs through the thin-walls of the lung tissue, the alveoli. The lung tissue is actually made of air sacs that expand when filled with air to increase the surface area across which gas can be exchanged. Among these air sacs are capillaries that carry blood close to the surface. Restricted expansion of the lung tissue, restricted blood flow to the alveolar sacs, or obstruction of air flow into the alveoli result in compromised gas exchange, and ultimately a lack of oxygen in the blood.
Alveoli Disorders
-Hyaline membrane disease, also known as respiratory distress syndrome of the newborn, is a lack of surfactant, the lipoprotein secreted by alveolar cells to assist gas exchange. Some premature births have not yet begun producing pulmonary surfactant and are unable to take their first breaths outside the womb; the lungs cannot inflate without the surfactant, which prevents the alveoli from collapsing between breaths. A synthetic surfactant or respiratory assistance can be given in the delivery room. This disorder often disappears with time as the baby's lungs finish developing.
-Pneumothorax is the presence of air in the potential pleural space. Breathing is dependent on maintaining a negative pressure (less than atmospheric) between the lungs and the chest wall. This space between membranes is referred to as potential because it isn't a space at all. Tension pneumothorax is the entry of air into the space due to trauma, such as a puncture wound. A spontaneous pneumothorax is when there is no sign of trauma, and may be due to alveolar rupture, which may occur as the result of other lung disease. Both types result in lung collapse.
-Pulmonary edema is the accumulation of fluid in alveoli. This disorder is often the result of congestive heart failure as the blood backs up in the capillaries.
-Pneumonia is a bacterial infection of the lungs, most often caused by Streptococcus pneumonia, which is a transient inhabitant of the upper respiratory tract. When the bacteria make their way into the lower respiratory tract, they infect the alveoli, resulting in fluid secretion during the immune response. This fluid then results in a blockage of efficient gas exchange, excess mucus, and expected symptoms of infection (fever, exhaustion, etc). Pneumonia is sometimes a secondary infection of the flu, which is caused by the virus influenza and also infects the respiratory tract. Hospitalization is often necessary.
-Emphysema is a degenerative disease, part of chronic obstructive pulmonary disease (COPD). The breakdown of elastin in the alveoli results in the loss of elasticity in the tissue. Exhalation becomes an active process. This disorder can be the result of chronic asthma or due to exposure to pollutants and lung irritants, such as cigarette smoke, severe air pollution, or industrial dust.
Obstructive Disorders
-COPD is a progressive disease often credited to cigarette smoking or exposure to toxic pollutants. The airways become obstructed, inflamed, and damaged, leading to their dysfunction over time. This then results in low blood oxygen levels and heart disease. As mentioned above, COPD includes emphysema, but it also includes chronic obstructive bronchitis. COPD is the 4th leading cause of death in the United States.
-Asthma is due to the constriction of the smooth muscle in the bronchioles and excess mucus secretion as an inflammatory reaction to an allergen or infection. The constriction restricts airflow and results in wheezing and coughing. Severe attacks require treatment with steroids or IgE blockers, depending on the cause. Over time, the lack of ventilation to the alveoli results in the breakdown of lung tissue.
-Bronchitis is the irritation and inflammation of the bronchials due to infection, or in chronic bronchitis (as mentioned above for COPD) long-term exposure to irritants. An inability to pass air through the bronchials is due to constriction and excess mucus.
Lung Cancer
Cancer is a disease that can occur in any tissue of the body. It occurs when cells lose the ability to control their replication and growth, forming tumors. Tumors can cause obstruction, compete for blood supply, and damage nearby tissue by invasion. Cancer cells can also metastasize to other tissues, causing systemic, malignant disease. Lung tumors have varying effects depending on where they occur. A tumor in the bronchus would be obstructive to air flow; a tumor in the lobes would limit gas exchange locally, and potentially lung expansion as it grew. Cigarette smoking is thought to be a leading cause of lung cancer, as well as a cause of many of the diseases listed above.
In 2009, more than 150,000 Americans died of lung cancer. Two types of lung cancer are differentiated: small cell (fast growing) and non-small cell. Men have a higher incidence of lung cancer, and the median age of diagnosis is 71 years (most cases occur over the age of 45). See the National Cancer Institute for more statistics. See the American Cancer Society for information on treatment, symptoms, and prognosis.