|Human Physiology 6/e Fox|
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Disorders of the Respiratory System
Bronchitis (brong-ki'tis) is an inflammation of the bronchi caused by irritants, such as cigarette smoke, air pollution, or infections. The inflammation results in swelling of the mucous membrane lining the bronchi, increased mucus production, and decreased movement of mucus by cilia. Consequently, the diameter of the bronchi is decreased, and ventilation is impaired. Bronchitis can progress to emphysema.
Emphysema (em-fi-se'mah) results in the destruction of the alveolar walls. Many individuals have both bronchitis and emphysema, which are often referred to as chronic obstructive pulmonary disease (COPD). Chronic inflammation of the bronchioles, usually caused by cigarette smoke or air pollution, probably initiates emphysema. Narrowing of the bronchioles restricts air movement, and air tends to be retained in the lungs. Coughing to remove accumulated mucus increases pressure in the alveoli, resulting in rupture and destruction of alveolar walls. Loss of alveolar walls has two important consequences. The respiratory membrane has a decrease surface area, which decreases gas exchange, and loss of elastic fibers decrease the ability of the lungs to recoil and expel air. Symptoms of emphysema include shortness of breath and enlargement of the thoracic cavity. Treatment involves removing sources of irritants (for example, stopping smoking), promoting the removal of bronchial secretions, retraining people to breathe so that expiration of air is maximized, and using antibiotics to prevent infections. The progress of emphysema can be slowed, but there is no cure.
Cystic fibrosis is an inherited disease that affects the secretory cells lining the lungs, pancreas, sweat glands, and salivary glands. The defect produces an abnormal transport protein, resulting in decreased chloride ion secretion out of cells. Normally, the diffusion of chloride and sodium ions out of the cells causes water to follow by osmosis. In the lungs, the water forms a thin fluid layer over which mucus is moved by ciliated cells. In cystic fibrosis, the decreased chloride ion diffusion results in dehydrated respiratory secretions. The mucus is more viscous, resisting movement by cilia, and it accumulates in the lungs. For reasons not completely understood, the mucus accumulation increases the likelihood of infections. Chronic airflow obstruction causes difficulty in breathing, and coughing in an attempt to remove the mucus can result in pneumothorax and bleeding within the lungs. Once fatal during early childhood, many victims of cystic fibrosis are now surviving into young adulthood. Future treatments could include the development of drugs that correct or assist the normal ion transport mechanism. Alternatively, cystic fibrosis may some day be cured through genetic engineering by inserting a functional copy of the defective gene into a person with the disease. Research on this exciting possibility is currently underway.
Asthma (az'mah) is a disorder in which there are periodic episodes of contractions of bronchial smooth muscle, which restricts air movement. Many cases of asthma result from allergic responses to pollen, dust, animal dander, or other substances.
Treatment includes the use of drugs that relax the bronchiole smooth muscles and reduce inflammation. Sometimes injections are given to reduce the sensitivity of the immune system to the substances that stimulate an asthma attack.
Pulmonary fibrosis is the replacement of lung tissue with fibrous connective tissue, making the lungs less elastic and breathing more difficult. Exposure to asbestos, silica, or coal dust is the most common cause.
Lung cancer arises from the epithelium of the respiratory tract. Cancers arising from tissues other than respiratory epithelium are not called lung cancer, even though they occur in the lungs. Lung cancer is the most common cause of cancer death in males and females in the United States, and almost all cases occur in smokers. Because of the rich lymph and blood supply in the lungs, cancer in the lung can readily spread to other parts of the lung or body. In addition, the disease is often advanced before symptoms become severe enough for the victim to seek medical aid. Typical symptoms include coughing, sputum production, and blockage of the airways. Treatments include removal of part or all of the lung, chemotherapy, and radiation.
Sudden infant death syndrome (SIDS), or crib death, is the most frequent cause of death of infants between 2 weeks and 1 year of age. Death results when the infant stops breathing during sleep. Although the cause of SIDS remains controversial, there is evidence that damage to the respiratory center during development is a factor. There is no treatment, but at-risk babies can be placed on monitors that sound an alarm if the baby stops breathing.
Paralysis of the respiratory muscles can result from transection of the spinal cord in the cervical or thoracic regions. The damage interrupts nerve tracts that transmit action potentials to the muscles of respiration. Transection of the spinal cord can result from trauma such as automobile accidents or diving into water that is too shallow. Another cause of paralysis is poliomyelitis, a viral infection that damages neurons of the respiratory center or motor neurons that stimulate the muscles of respiration. Finally, anesthetics or central nervous system depressants can depress the function of the respiratory center if they are taken or administered in large enough doses.
Diseases of the Upper Respiratory Tract
Strep throat is caused by a streptococcal bacteria and is characterized by inflammation of the pharynx and by fever. Frequently, inflammation of the tonsils and middle ear are involved. Without a throat analysis, the infection cannot be distinguished from viral causes of pharyngeal inflammation. Current techniques allow rapid diagnosis within minutes to hours, and antibiotics are effective in treating strep throat.
Diphtheria (dif-the're-ah) was once a major cause of death among children. It is caused by a bacterium. A grayish membrane forms in the throat and can block the respiratory passages totally. A vaccine against diphtheria is part of the normal immunization program for children in the United States.
The common cold is the result of a viral infection. Symptoms include sneezing, excessive nasal secretions, and congestion. The infection easily can spread to sinus cavities, lower respiratory passages, and the middle ear. Laryngitis and middle ear infections are common complications. The common cold usually runs its course to recovery in about 1 week.
Diseases of the Lower Respiratory Tract
Laryngitis (lar-in-ji'tis) is an inflammation of the larynx, especially the vocal cords, and bronchitis (brong-ki'tis) is an inflammation of the bronchi. Bacterial or viral infection can move from the upper respiratory tract to cause laryngitis or bronchitis. Bronchitis is also often caused by continually breathing air containing harmful chemicals, such as those found in cigarette smoke.
Whooping cough (pertussis) is a bacterial infection. The infection causes a loss of cilia of the respiratory epithelium. Mucus accumulates, and the infected person attempts to cough up the mucous accumulations. The coughing can be severe. A vaccine for whooping cough is part of the normal vaccination procedure for children in the United States.
Tuberculosis (tu-ber'ku-lo'sis) is caused by a tuberculosis bacterium. In the lung, the tuberculosis bacteria forms lesions called tubercles. The small lumps contain degenerating macrophages and tuberculosis bacteria. An immune reaction is directed against the tubercles, which causes the formation of larger lesions and inflammation. The tubercles can rupture, releasing bacteria that infect other parts of the lung or body. Recently, a strain of the tuberculosis bacteria has developed that is resistant to treatment, and there is concern that tuberculosis will again become a widespread infectious disease.
Pneumonia (nu-mo'ne-ah) refers to many infections of the lung. Most pneumonias are caused by bacteria, but some result from viral, fungal, or protozoan infections. Symptoms include fever, difficulty in breathing, and chest pain. Inflammation of the lungs results in the accumulation of fluid within alveoli (pulmonary edema) and poor inflation of the lungs with air. A protozoal infection that results in pneumocystosis pneumonia is rare, except in persons who have a compromised immune system. This type of pneumonia has become one of the infections commonly suffered by persons who have AIDS.
Flu (influenza) is a viral infection of the respiratory system and does not affect the digestive system as is commonly assumed. Flu is characterized by chills, fever, headache, and muscular aches, in addition to respiratory symptoms. There are several strains of flu viruses. The mortality rate from flu is approximately 1%, and most of those deaths are among the very old and very young. During a flu epidemic the infection rate is so rapid and the disease is so widespread that the the total number of deaths is substantial, even though the percentage of deaths is relatively low. Flu vaccines can provide protection against the flu.
A number of fungal diseases, such as histoplasmosis (his'to-plaz-mo'sis) and coccidioidomycosis (kok-sid-e-oy'do-mi-ko'sis), affect the respiratory system. The fungal spores usually enter the respiratory system through dust particles. Spores in soil and feces of certain animals make the rate of infection higher in farm workers and in gardeners. The infections usually result in minor respiratory infections, but in some cases they can cause infections throughout the body.
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