Bronchiectasis refers to chronic, abnormal dilation of the bronchi. Often the cause is an infection, such as severe pneumonia or tuberculosis. But other irritations of the bronchi, such as might occur in cystic fibrosis, may be responsible.
Dilation of bronchi normally tends to be prevented by smooth muscle and surrounding connective tissue. But as components of a bronchial wall are destroyed by inflammation, negative intrapleural pressure lead to the pathogical expansion. On the other hand, it can also occur in lungs in restrictive lung disease due to the excessive fibrous connective tissue.
The patient tends to have a persistent, productive cough. Moreover, recurring infections of the airway are common.
Optional: CT scan of patient with tuberculosis and bronchiectasis.
Cystic fibrosis illustrates how improperly functioning ion channels may disrupt epithelial transport. This genetic disorder is one of the most common. It is characterized by abnormally viscous secretions from all exocrine glands. This is often most obvious in the lungs, where thick mucus tends to clog the airways. The abnormal environment in the airways also tends to promote serious lung infections.
The primary abnormality in cystic fibrosis is defective chloride channels, which do not open in response to regulatory signals. As a result, chloride cannot move from the cells into the mucus secreted into the airways. Since the movement of chloride and other ions drives the subsequent movement of water by osmosis, the defective chloride channels lead to less movement of water by osmosis and thus abnormally concentrated secretions.
The resulting viscous mucus secretions clog the ducts of the airways and glands. But many of the biggest difficulties are due to opportunistic infections that arise in the abnormal airway environment. The term "fibrosis" refers to refers to structural changes in the degenerating glands. Damage to the small airways in the lung cause then to thicken, producing bronchiectasis.
The importance of surfactant is most apparent in premature infants, whose underdeveloped lungs often do not produce enough of this substance. As the afflicted babies exhale, some alveoli tend to collapse due to their low compliance (greater stiffness).
QUESTION: How does surfactant affect surface tension in small alveoli in comparison to large alveoli?
QUESTION: With less surfactant, will the surface tension in small alveoli become closer to that in large alveoli?
QUESTION: If the surface tension in small and large alveoli were about the same, would smaller alveoli have a higher pressure or lower pressure than larger alveoli? (Think: "Law of Laplace".)
Thus, since lack of enough surfactant causes the pressure in the smaller alveoli to be higher than in the large, gas would tend to flow from the small to the large alveoli. As small alveoli collapse, the blood flowing through their pulmonary capillaries necessarily does not become oxygenated. And therefore blood leaving the lungs in the pulmonary veins contains less than normal levels of oxygen. This tends to cause cyanosis.
QUESTION: When there is not enough surfactant, is the compliance of the lungs larger or smaller? And how does this affect ventilation?
Thus, the poor compliance of the lungs also causes the infants to struggle in ventilation; they breathe rapidly and produce little grunts on exhalation.
The term for this disorder is infant respiratory distress syndrome (hyaline membrane disease). Initially, a medical solution is to keep the alveoli open by applying positive pressure to the lungs with a respirator. But this can't be continued very long. The problem can be addressed for a longer time by infusing surfactant from animals into the baby's lungs. This helps until the type II cells start synthesizing enough surfactant.
Pneumonia is a general term that refers to infections of the lung with subsequent "solidification" of the tissue of the lung. Sometimes the patient develops the disorder without any underlying immunological problems. But immunocompromised patients or weakened patients in a hospital setting are particularly susceptible.
Bacteria are usually the cause of pneumonia in adults. Streptococcus pneumonia is the most common pathogen, although there are many other possibilities. Usually the bacteria are normal residents of the body. As the infection progresses, alveoli fill with edema, followed by massive accumulation of neutrophils and macrophages.
Pneumonia often begins after a viral respiratory infection, which provides the bacteria with an opportunity to begin the disorder. But there are many other predisposing factors, such as heart disease, alcoholism, chronic obstructive pulmonary disease and cigarette smoking. In children viruses are usually the underlying pathogen.
When a virus such as cytomegalovirus causes pneumonia, the damage tends to be interstitial rather than alveolar.
Sometimes, pneumonia causes damage to small airways that causes them to thicken, producing bronchiectasis.
Optional: Chest X-ray of 76 year old. While in the hospital for tumor and clots, the patient developed fever and leukocytosis. Note densities in lower lobe.
In the fall, we discussed tuberculosis, which is one of the most important bacterial diseases and is found throughout the world. In the last century in Europe and North America it was a devastating disease and among the most common causes of death. Effective treatments nearly eliminated tuberculosis in developed countries, although its incidence is rising again. Particularly troubling are the drug-resistant strains generated by world-wide misuse of antibiotics. A person contracts tuberculosis by breathing in bacteria coughed out by an infected person. However, the chances of developing the disease are much higher in malnourished persons living in crowded conditions. AIDS patients are particularly suspectible.
Once inhaled, the bacteria are engulfed by macrophages. But somehow the bacteria may prevent lysosomes from fusing with the phagocytic vesicles, in which the bacteria multiply and from which they are subsequently released. Eventually, the cellular immune system responds and T cells release cytokines (e.g.; interferon-gamma) that activate macrophages. These macrophages now have the capacity to destroy the bacteria.
At a site where macrophages are destroying the bacteria, activated macrophages surround and isolate the bacteria, producing a local accumulation of macrophages called a granuloma. But in this type of inflammation, the battle usually leads to significant necrosis (death of cells) and fibrosis. The tuberculosis bacteria themselves do not release any known substance that directly damages tissues.
Optional: CT scan of tuberculosis.
In a person with a strong immune response, the granulomas usually successfully contain the bacteria. Typically, the necrosis gives the interior of the granulomas a cheeselike (caseous) consistency (unlike in sarcoidosis). But if the cellular immune response is weak (as in a newborn), the granulomas form poorly and the infection spreads in the lungs and often elsewhere.
Secondary tuberculosis develops when the bacteria, which for a period had been successfully contained, emerge from the granulomas. Even decades may elapse. Causes include immunosuppressive therapy, cancer, old age, and AIDS.