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lungs

lungs The name ‘lungs’ is derived from their lightness in weight, since they contain air, and the butcher refers to them as ‘lights’ for this reason. Adult lungs will float in water, but lungs from a fetus who has not breathed will sink. ‘Pulmonary’, from the Latin, refers to the lungs and is used in medical terminology, as in pulmonary function tests or pulmonary disease.

When the lungs are taken out of the chest they partially collapse. This is because they contain elastic fibres in the walls of the airways and alveoli (air sacs); open a festive balloon and it will empty itself. In addition, a thin layer of liquid lining the alveoli exerts surface tension, tending to collapse the lungs, although this surface tension is greatly decreased by the presence of surfactant. A soap bubble will collapse when pricked, although it has no elastic material; the surface tension of the film provides force enough.

The function of the lungs is to provide an enormous surface for gas exchange, with oxygen entering the body and carbon dioxide leaving it. The surface has to be protected against physical and environmental assault, in this respect resembling the internal gills of fishes and differing from the external gills of, for example, tadpoles. Thus the lungs are encased in structures — the chest wall and diaphragm — which will also provide the means of its ventilation. Amphibia inflate their lungs by pumping air in from the mouth; mammals, far more active and usually much larger, suck air into the lungs by muscular effort which creates a negative pressure around them.

Our knowledge of the structure and func-tion of the lungs has depended on two major technological advances over the past three centuries: the light microscope and, later, the electron microscope. For almost two thousand years it was thought that the lungs were generally similar in structure to the liver, spleen, and pancreas, the only important difference being that air could enter the lungs through the trachea to mix physically with blood, cooling it by passing into the blood vessels. The superb anatomical dissections of Leonardo da Vinci (1452–539) and Versalius (1514–64) were regarded as consistent with this view. It was the use of the microscope by Malpighi (1628–94) that demonstrated for the first time the air-filled alveoli, the blind ends of the air passages into the lungs. He described them as ‘an almost infinite number of orbicular bladders’. Malpighi also showed that the blood capillaries in the lungs were vessels with walls that separated blood from gas, and allowed the passage of blood through the lungs, as deduced but not demonstrated by William Harvey. However, Malpighi knew nothing of gas exchange and thought the function of alveolar ventilation (the flow of air in and out of the alveoli) was to stir and mix the blood in the capillaries. From the 1950s onwards, the electron microscope displayed in detail the structures of the walls of the alveoli and of the tracheobronchial tree, extending to ‘ultrastructure’ within cells, knowledge of which had advanced meanwhile as light microscopy improved.

The airways below the larynx consist of (i) the trachea, a tube that extends almost to the middle of the chest; (ii) the bronchi (bronchial tree), formed by the trachea splitting into two and then each branch dividing again; (iii) the bronchioles — thin and short distensible airways that again divide many times to form (iv) the alveolar ducts from which (v) the alveoli arise. This multiple division results in about 23–5 generations of airway, with geometrically increasing numbers and total cross-sectional areas, and decreasing diameters. For example, from one trachea with a diameter of about 180 mm in an adult, by the tenth generation we have 1000 bronchi, each with a diameter of about 1.3 mm; by the twentieth generation we have 1 million bronchioles, each with a diameter of about 0.5 mm; and right at the end there are about 300 million alveoli. The diameter of the alveoli, like that of the bronchi and bronchioles, varies with the degree of lung inflation, in the range 0.1–0.3 mm. The alveolar surface area may be 30–100 m2 — often described as the size of a tennis court.

In the human embryo the lungs first begin to develop at about 3 weeks after fertilization. The alimentary tract develops earlier, and a ‘bud’ from its ventral surface progressively extends into the chest to form the airways and lungs. This common embryological source of the lungs and gut is reflected in adult structure and function; thus there are similarities in that both have smooth muscle in their walls, glandular secretion of mucus from their linings, and a nerve supply from the autonomic nervous system, although of course other differences in structure and function are considerable. At about 16 weeks alveoli begin to appear, looking rather like glands with thick walls. Only at about 20–25 weeks does the lung begin to resemble the adult tissue. Even at full-term birth the lungs, although functionally adequate, are not fully developed anatomically, having fewer bronchial branches and a far smaller alveolar surface area than they eventually acquire.

The airways: trachea, bronchi, and bronchioles

These are the conducting airways, and do not take part in gas exchange. Their function is to condition the air we breathe in and to conduct it to the alveoli. If inspired air reached the alveoli directly, if it were cold it would cool the tissue, if hot it would heat it, and if dry it would parch and destroy the alveolar walls. Only if we breathed air at 37°C and 100% humidity — a rare occurrence — would we avoid tissue damage. When we breathe through the mouth, as in exercise or with nasal blockage, we have eliminated the air conditioning role of the nose, and the mouth is much less efficient for this purpose. If the inspired air is cold and dry it will be raised to body temperature and full humidity by the first few generations of bronchi. This makes the airway lining itself (the mucosa) cold and dry, but protects the alveoli. On breathing out the mucosa will take up heat and water vapour from the expired air, restoring it to normal. Not only are the alveoli protected, but loss of heat and water from the body as a whole is minimized.

The walls of the trachea and bronchi have several layers. On the inner lining surface, the ‘luminal’ side, there is a layer of epithelium as a kind of skin. Most of the epithelial cells are ciliated, with microscopic ‘hairs’ (cilia) that continuously sweep any surface material towards the larynx, where it is coughed up or swallowed. This is the ‘ciliary escalator’. Other cells secrete mucus, the slimy liquid that constitutes phlegm and lies on the cilia. Just under the epithelium is a dense blood capillary network that provides nutrition for the epithelium and glands, and may be the site of uptake of inhaled pollutants and drugs. Deeper in the wall are the submucosal glands, the main source of the mucus that lines the airways. The glands are stimulated to secrete by many factors, the most important being pollutants, including cigarette smoke, and viral or bacterial infections of the airways. Smoker's cough brings up the mucus thus secreted, and in chronic bronchitis there is the overproduction of mucus that characterizes the disease and is due to local pathological changes. The secreted mucus normally has several important defensive effects. It will create a barrier and take up soluble pollutants and smoke particles, slowing down their entry into the body and protecting the epithelium from their harmful effects, and eliminating them via the ciliary escalator. It will stimulate cough as an even more rapid means of their removal. In health the mucus sheet is very thin and difficult to measure; it is probably about 0.02–0.05 mm thick. Even in disease when the output of mucus is greatly increased, it remains too thin to block the airways, unless there is associated inflammation.

Deeper in the airway wall there is cartilage and smooth muscle. The cartilage stabilizes the airways and prevents their collapse during vigorous acts of breathing, such as coughing. The smooth muscle has not been shown to have a physiological role, unlike that in the intestines, which is responsible for the squeezing movement of peristalsis, but possibly it adjusts the diameter of the airways to make them optimally efficient for conducting gas to the alveoli.

The trachea and bronchi contain many sensory nerves, in general of two types. In the smooth muscle are receptors that signal the degree of stretch and therefore of inflation of the airways and lungs, and control the pattern of breathing — its rate and depth, probably to make it as efficient as possible. If the vagus nerves that carry sensory information from the bronchi are cut, in most animals the breathing becomes slow, deep, and mechanically inefficient. Secondly, in the epithelium there is a network of fine nerve fibres, with finger-like projections reaching almost to the airway lumen, that respond to inhaled pollutants and inflammatory mediators and set up a range of reflex responses. The most striking is the cough, but there is also reflex mucus secretion and smooth muscle contraction. The nerves look like, and act as, tripwires and sensing rods just under the surface, ready to respond to any adverse intruder.

The smallest air-conducting vessels, the bronchioles, are distinguished by having no cartilage in their walls, no mucus cells in their epithelium, and few or no submucosal glands. When the lungs inflate they probably distend equally with the alveoli, but there is little gas exchange in them. If they are inflamed, as in bronchiolitis, the alveoli they supply collapse, with a stiffening of the lungs and a failure of gas exchange.

Because the airways take no part in gas exchange, they are sometimes referred to as the ‘anatomical deadspace’. At rest their volume is about 150 ml in a healthy adult. If an average tidal volume of 500 ml is inhaled, at the end of inspiration only 350 ml will have entered the alveoli, and 150 ml will remain in the airways. The ventilation used for gas exchange will be only 350/500ths — or 70% — of the total ventilation. The rest could be called wasted ventilation but, as described earlier, it has an essential function in conditioning the inspired air. When we breathe out, the first 150 ml is unchanged ‘fresh’ air, followed by 350 ml of air from the alveoli, rich in carbon dioxide and partly depleted of oxygen.

Asthma

In asthma the contraction of bronchial smooth muscle can have a profound effect by narrowing the airways; a greater muscular effort is then required to inflate the lungs. The smooth muscle contracts in response to two main stimuli, chemical and nervous, and either can cause the wheezing associated with asthma. Most types of asthma involve inflammation, with release of chemical mediators like histamine, bradykinin, and substance P, which diffuse to the smooth muscle and make it contract. In addition, nervous signals can come down from the brain via the vagus nerves, when asthma and wheezing are induced by emotional factors in susceptible subjects.

It used to be thought that asthma was solely due to smooth muscle contraction narrowing the airways. This view was supported by the effectiveness of treatment by smooth muscle relaxants such as salbutamol. But asthma is now considered to be an inflammation of the airways, with multiple effects that all narrow the airways: smooth muscle contraction, thickening of the mucosa by oedema because of leaking blood vessels, and secretion of mucus into the airway lumen. The use of anti-inflammatory drugs has become general.

The alveolar ducts and alveoli

Here gas exchange takes place. There are about 15 million alveolar ducts, and each gives rise to about 20 alveolar air sacs. Each of these alveoli is surrounded by a network of blood capillaries — a bit like a balloon in a close-fitting string bag except that the alveoli are not spherical, and they share the ‘string’ with the adjacent alveoli all around (see figure for shape in cross-section). The entire output of the right heart goes through the alveolar blood vessels and then into the left heart. The enormous alveolar surface, up to 300 m2, promotes gas exchange between blood and air, since the rate of diffusion of a gas depends on the surface area, the thinness of the diffusion barrier, and the solubility of the gas (Fick's Law). The alveolar wall is extremely thin, from 0.2–0.5 μm, depending on the degree of inflation of the lungs. The barrier to diffusion has three components. On the surface of the alveoli is a thin layer of secretion, containing surfactant, the detergent phospholipid that lowers the surface tension of the lungs and allows them to be stretched by relatively low pressures. The surfactant layer is about 0.15 μm thick. Then there is the epithelial cell layer of the alveoli. This consists of two types of cells, those which mainly provide a mechanical sheet (type I) and those that secrete surfactant (type II). Together they constitute the lining ‘skin’ of the alveoli. The capillary endothelium is the third component of the barrier. Cells of a different type, the alveolar macrophages, are found within the cavities of the alveoli; their function is to ingest and remove solid particles, such as those of smoke.

Carbon dioxide is over twenty times more soluble than oxygen in body liquids, and diffuses twenty times more quickly out of the body than oxygen enters. In any diseases where alveolar gas exchange is decreased, for example when the alveolar wall is thickened by alveolitis, the first changes in blood gas transfer will be with oxygen, and the patient may develop quite severe hypoxia before the blood carbon dioxide begins to increase.

For many years at the beginning of the twentieth century there was intense scientific dispute as to whether the alveoli of the human lungs could secrete oxygen into the bloodstream. The argument was that at high altitudes the oxygen pressure was so low that it could not maintain blood oxygen pressure without active transport through the epithelium. Perhaps the indirect methods to test the problem were not sensitive enough for a clear solution — and it was known that some fishes could actively secrete oxygen into their swim bladders, taking advantage of the properties of their haemoglobin that did not seem to apply to human haemoglobin. The problem was finally solved when more sensitive analysis showed that human lungs, and presumably those of other mammals, could not secrete oxygen and that all gas exchanges could be explained by passive diffusion. Even in the absence of oxygen secretion, some climbers can just get to the top of Mount Everest without added oxygen.

John Widdicombe


See respiratory system.See also breathing; carbon dioxide; development and growth: birth and infancy; oxygen.

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Lung Abscess

Lung Abscess

Definition

Lung abscess is an acute or chronic infection of the lung, marked by a localized collection of pus, inflammation, and destruction of tissue.

Description

Lung abscess is the end result of a number of different disease processes ranging from fungal and bacterial infections to cancer. It can affect anyone at any age. Patients who are most vulnerable include those weakened by cancer and other chronic diseases; patients with a history of substance abuse, diabetes, epilepsy, or poor dental hygiene; patients who have recently had operations under anesthesia; and stroke patients. In children, the most vulnerable patients are those with weakened immune systems, malnutrition, or blunt injuries to the chest.

Causes and symptoms

The immediate cause of most lung abscesses is infection caused by bacteria. About 65% of these infections are produced by anaerobes, which are bacteria that do not need air or oxygen to live. The remaining cases are caused by a mixture of anaerobic and aerobic (air breathing) bacteria. When the bacteria arrive in the lung, they are engulfed or eaten by special cells called phagocytes. The phagocytes release chemicals that contribute to inflammation and eventual necrosis, or death, of a part of the lung tissue. There are several different ways that bacteria can get into the lung.

Aspiration

Aspiration refers to the accidental inhalation of material from the mouth or throat into the airway and lungs. It is responsible for about 50% of cases of lung abscess. The human mouth and gums contain large numbers of anaerobic bacteria; patients with periodontal disease or poor oral hygiene have higher concentrations of these organisms. Aspiration is most likely to occur in patients who are unconscious or semiconscious due to anesthesia, seizures, alcohol and drug abuse, or stroke. Patients who have problems swallowing or coughing, or who have nasogastric tubes in place are also at risk of aspiration.

Bronchial obstruction

The bronchi are the two branches of the windpipe that lead into the lungs. If they are blocked by tissue swelling, cancerous tumors, or foreign objects, a lung abscess may form from infection trapped behind the blockage.

Spread of infection

About 20% of cases of pneumonia that cause the death of lung tissue (necrotizing pneumonia) will develop into lung abscess. Lung abscess can also be caused by the spread of other infections from the liver, abdominal cavity, or open chest wounds. Rarely, AIDS patients can develop lung abscess from Pneumocystis carinii and other organisms that take advantage of a weakened immune system.

Lung abscess is usually slow to develop. It may take about two weeks after aspiration or bronchial obstruction for an abscess to produce noticeable symptoms. The patient may be acutely ill for two weeks to three months. In the beginning, the symptoms of lung abscess are difficult to distinguish from those of severe pneumonia. Adults will usually have moderate fever (101-102 °F/38-39 °C), chills, chest pain, and general weakness. Children may or may not have chest pain, but usually suffer weight loss and high fevers. As the illness progresses, about 75% of patients will cough up foul or musty-smelling sputum; some also cough up blood.

Lung abscess can lead to serious complications, including emphysema, spread of the abscess to other parts of the lung, hemorrhage, adult respiratory distress syndrome, rupture of the abscess, inflammation of the membrane surrounding the heart, or chronic inflammation of the lung.

Diagnosis

The diagnosis is made on the basis of the patient's medical history (especially recent operations under general anesthesia ) and general health as well as imaging studies. Smears and cultures taken from the patient's sputum are not usually very helpful because they will be contaminated with bacteria from the mouth. The doctor will first use a bronchoscope (lighted tube inserted into the windpipe) to rule out the possibility of lung cancer. In some cases of serious infection, the doctor can use a fiberoptic bronchoscope with a protected specimen brush to take material directly from the patient's lungs, for identification of the organism. This technique is time-consuming and expensive, and requires the patient to be taken off antibiotics for 48 hours. It is usually used only to evaluate severely ill patients with weakened immune systems.

In most cases, the doctor will use the results of a chest x ray to help distinguish lung abscess from empyema, cancer, tuberculosis, or cysts. In patients with lung abscess, the x ray will show a thick-walled unified clear space or cavity surrounded by solid tissue. There is often a visible air-fluid level. The doctor may also order a CT scan of the chest, in order to have a clearer picture of the exact location of the abscess.

Blood tests cannot be used to make a diagnosis of lung abscess, but they can be useful in ruling out other conditions. Patients with lung abscess usually have abnormally high white blood cell counts (leukocytosis ) when their blood is tested, but this condition is not unique to lung abscess.

Treatment

Lung abscess is treated with a combination of antibiotic drugs, oxygen therapy, and surgery. The antibiotics that are usually given for lung abscess are penicillin G, penicillin V, and clindamycin. They are given intravenously until the patient shows signs of improvement, and then continued in oral form. The patient may need to take antibiotics for a month or longer, until the chest x ray indicates that the abscess is healing. Oxygen may be given to patients who are having trouble breathing.

Surgical treatment

Most patients with lung abscess will not need surgery. About 5% of patients-usually those who do not respond to antibiotics or are coughing up large amounts of blood (500 mL or more)may have emergency surgery for removal of the diseased part of the lung or for insertion of a tube to drain the abscess. Antibiotic treatment is considered to have failed if fever and other symptoms continue after 10-14 days of treatment; if chest x rays indicate that the abscess is not shrinking; or if the patient has pneumonia that is spreading to other parts of the lung.

Supportive care

Because lung abscess is a serious condition, patients need quiet and bed rest. Hospital care usually includes increasing the patient's fluid intake to loosen up the secretions in the lungs, and physical therapy to strengthen the patient's breathing muscles.

Follow-up

Patients with lung abscess need careful follow-up care after the acute infection subsides. Follow-up usually includes a series of chest x rays to make sure that the infection has cleared up. Treatment with antibiotics may continue for as long as four months, to prevent recurrence.

Prognosis

About 95% of lung abscess patients can be treated successfully with antibiotics alone. Patients who need surgical treatment have a mortality rate of 10-15%.

Prevention

Some of the conditions that make people more vulnerable to lung abscess concern long-term lifestyle behaviors, such as substance abuse and lack of dental care. Others, however, are connected with chronic illness and hospitalization. Aspiration can be prevented with proper care of unconscious patients, which includes suctioning of throat secretions and positioning patients to promote drainage. Patients who are conscious can be given physical therapy to help them cough up material in their lungs and airways. Patients with weakened immune systems can be isolated from patients with pneumonia or fungal infections.

Resources

BOOKS

Stauffer, John L. "Lung." In Current Medical Diagnosis and Treatment, 1998, edited by Stephen McPhee, et al., 37th ed. Stamford: Appleton & Lange, 1997.

KEY TERMS

Abscess An area of injured body tissue that fills with pus, as in lung abscess.

Anaerobe A type of bacterium that does not require air or oxygen to live. Anaerobic bacteria are frequent causes of lung abscess.

Aspiration Inhalation of fluid or foreign bodies into the airway or lungs. Aspiration often happens after vomiting.

Bronchoscope A lighted, flexible tube inserted into the windpipe to view the bronchi or withdraw fluid samples for testing. Bronchoscopy with a protected brush can be used in the diagnosis of lung abscess in severely ill patients.

Bronchus One of the two large tubes connecting the windpipe and the lungs.

Leukocytosis An increased level of white cells in the blood. Leukocytosis is a common reaction to infections, including lung abscess.

Necrotizing pneumonia Pneumonia that causes the death of lung tissue. It often precedes the development of lung abscess.

Sputum The substance that is brought up from the lungs and airway when a person coughs or spits. It is usually a mixture of saliva and mucus, but may contain blood or pus in patients with lung abscess or other diseases of the lungs.

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Lung Surgery

Lung Surgery

Definition

Lung surgery includes a variety of procedures used to diagnose or treat diseases of the lungs. Biopsies are performed to extract a small amount of tissue for diagnosis, resections remove a portion of lung tissue, and other surgeries are aimed at reducing the volume of the lungs, removing cancerous tumors, or improving lung function.

Purpose

The type of lung surgery performed will depend upon the underlying disease or condition, as well as other factors.

  • Pneumonectomy usually refers to the removal of a lung, or sometimes one or more lobes (sections containing lung tissue, air sacs, ducts, and respiratory bronchiole). It is most commonly indicated in certain forms and stages of lung cancer.
  • Thoracotomy, or surgical incision of the chest wall, is used primarily as a diagnostic tool when other procedures have failed to provide adequate diagnostic information.
  • Lobectomy is the term used to describe removal of one lobe of a lung. It is most commonly indicated for lung cancer, but may also be used for cystic fibrosis patients if other treatments have failed.
  • Other surgical procedures include segmental resection or wedge resection. A resection is the removal of a part of the lung, often in order to remove a tumor. Wedge resection is removal of a wedge-shaped portion of lung tissue.
  • Volume reduction surgery is a newer surgery used to help relieve shortness of breath and increase tolerance for exercise in patients with chronic obstructive pulmonary disease, such as emphysema.
  • Other surgeries are continuously improved upon to make biopsy less invasive and surgery more effective, such as video-assisted lobectomy. Other purposes for lung surgery may include severe abscess, areas of long-term infection, or permanently enlarged or collapsed lung tissue.

Precautions

Thoracotomy should not be performed on patients whose general health status will not tolerate major surgery. Any surgery carries with it risks associated with general anesthesia and possibility of infection. Patients whose risk for these complications outweighs benefit may not be considered candidates for lung surgery. Each individual patient's condition will be reviewed prior to the treatment decision.

Description

Lung surgery procedures will vary depending on the underlying cause of the surgical test or intervention. A patient will be placed under general anesthesia during the surgery. An incision is made to examine the lungs. Diseased tissue is removed and may be sent for biopsy. Following the surgery, drainage tubes may be placed in the chest to drain fluids, blood, and air from the chest cavity. Tubes will most likely remain in place for one to two days, depending on the surgery and the patient's condition. The chest cavity, ribs, and skin are closed and the incision will be sutured. Hospital stay averages from three to 10 days.

Pneumonectomy consists of removal of all of one lung. It may often be indicated only when a lobectomy does not successfully remove the cancerous or damaged tissue. Thoracotomy consists of reaching the lung tissue through incision and obtaining tissue for a biopsy. The biopsy is used to diagnose or stage cancer, and thoracotomy may be avoided until other less invasive methods have failed. Volume reduction surgery involves incision and removal of those parts of the lung or lungs which are the most destroyed, in order to allow for full function of the remaining lung structure. This procedure is still being studied.

Lobectomy is performed in the same general manner as other lung surgeries, but will involve removal of an entire lobe of the lung. Most patients with Stage I or II non-small cell lung cancer will receive this treatment for their disease, or a less extensive resection. Lobectomy may only be performed if a wedge or segmental resection is ineffective, but is generally preferred as treatment for primary lung cancer in any patient who can tolerate the procedure. Wedge and segmental resections are still major surgery, but remove less tissue and may be the first choice for some patients, such as those with Stage I and Stage II non-small cell lung cancer. Patients who do not have enough pulmonary function to undergo a lobectomy will receive a wedge or segmental resection instead. This may lead to a higher recurrence rate of cancer. In general, the surgery method chosen will depend on specific circumstances and consideration of benefit versus risk.

Preparation

Preparation for lung surgery is much like that for any major surgery. Patients will receive instructions from a physician concerning limit of food or water intake prior to the surgery, as well as risks and expected recovery. Patients should continue to follow treatment for the underlying condition, unless instructed otherwise by the physician, and should discuss medications and changes in condition with their physician prior to the surgery.

Aftercare

The chest tube inserted at the end of surgery will remain in place until the lung has fully expanded. Patients will be carefully monitored in the hospital for complications and infection. Deep breathing is recommended to help lessen the risk of pneumonia and infection. Breathing exercises will also help expand the lung. After discharge from the hospital, the patient may still receive some pain or infection-fighting medications and should recover within one to three months of the operation.

Risks

Risks of lung surgery follows those of any major surgery involving general anesthesia. These risks include reactions to anesthetics or medications, bleeding, infection, and problems restoring breathing. Lung surgery, in particular, offers the risk of pneumonia and blood clots. Thoracotomy, as a biopsy procedure, offers greater risk than most biopsy procedures.

Normal results

Outcome for any lung surgery depends on many factors and the severity of disease. In general, the predicted benefits, which justified the surgery, are normal expected results. Thoracotomy results in a definitive diagnosis in more than 90% of patients. Volume reduction surgery has been shown to result in relief of some symptoms and improvement in quality of life for selected patients with severe emphysema and have shown short-term promise.

Mortality from lung surgery improves as procedures move from the more complete pneumonectomy to lobectomy, and the lowest rate for segmental resection.

Resources

ORGANIZATIONS

American Cancer Society. 1599 Clifton Rd., NE, Atlanta, GA 30329-4251. (800) 227-2345. http://www.cancer.org.

American Lung Association. 1740 Broadway, New York, NY 10019. (800) 586-4872. http://www.lungusa.org.

National Heart, Lung and Blood Institute. P.O. Box 30105, Bethesda, MD 20824-0105. (301) 251-1222. http://www.nhlbi.nih.gov.

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Lung Diseases Due to Gas or Chemical Exposure

Lung Diseases Due to Gas or Chemical Exposure

Definition

Lung diseases due to gas or chemical exposure are conditions that can be acquired from indoor and outdoor air pollution and from ingesting tobacco smoke.

Description

The lungs are susceptible to many airborne poisons and irritants. Mucus present in the airways blocks foreign particles of a certain size, however it is unable to filter all airborne particulates. There are hundreds of substances that can pollute air and harm lungs. Harmful gases and chemicals are just one type of airborne pollutant that can adversely affect the lungs. They include:

  • Vehicle exhaust
  • Localized pollutants such as arsenic, asbestos, lead, and mercury
  • Outdoor pollutants caused by industry and intensified by weather conditions
  • Household heating, such as wood-burning stoves
  • Household chemical products
  • Tobacco smoke.

Lungs respond to irritants in four ways, each of which can occur separately or, more often, trigger other responses.

  • Asthma occurs when irritation causes the smooth muscles surrounding the airways to constrict.
  • Increased mucus comes from irritated mucus glands lining the airway. Excess mucus clogs the airway and prevents air from circulating.
  • Constriction of the lungs results from scarring when the supporting tissues are damaged.
  • Cancer is caused by certain irritants, such as asbestos and tobacco smoke.

The major categories that airborne irritants fall into are allergic, organic, inorganic, and poisonous, with many agents occupying more than one category.

  • Allergic irritants bother only people who are sensitive to them. Cat hair, insect parts, and pollen are common allergens. Chemicals called sulfites, which are widely used as food preservatives, also cause asthma.
  • There are many organic dusts that irritate the lungs. Most of them occur on the job and cause occupational lung disease. Grain dust causes silo filler's disease. Cotton and other textile dusts cause byssinosis. Mold spores in hay cause farmer's lung.
  • Inorganic dusts and aerosolized chemicals also are found mostly on the job. Classic among them are asbestos and coal dust. Many metals (cadmium, arsenic, chromium, and phosphorus), various other fine particles (cement, mica, rock), acid fumes, ammonia, ozone, and automobile and industrial emissions are part of a very long list.
  • While tobacco smoke is a culprit in many smokers, a 2003 report found that those who work in the tobacco industry experience higher incidence of lung disease from tobacco dust in their work environment.
  • Most intentional poisons (cyanide, nerve gas) that enter through the lungs pass through and damage other parts of the body. Mustard gas, used during World War I and banned since, directly and immediately destroys lungs.
  • Tobacco use scars the lungs and causes emphysema and lung cancer.

Causes and symptoms

Lung disease generates three major symptomscoughing, wheezing, and shortness of breath. It also predisposes the lungs to infections such as bronchitis and pneumonia. Cancer is a late effect, requiring prolonged exposure to an irritant. In the case of tobacco, an average of a pack of cigarettes a day for forty years, or two packs a day for twenty years, will greatly increase the risk of lung cancer.

Diagnosis

A history of exposure combined with a chest x ray and lung function studies completes the diagnostic evaluation in most cases. Lung function measures the amount of air breathed in and out, the speed it moves, and the effectiveness of oxygen exchange with the blood. If the cause still is unclear, a lung biopsy aids diagnosis.

Treatment

Eliminating the offending irritant and early antibiotics for infection are primary. There are many techniques available to remove excess mucus from the lungs. Respiratory therapists are trained in these methods. Finally, there are several machines available to enrich the oxygen content of breathed air.

A surgical treatment called lung reduction volume surgery is emerging as a treatment for certain people over age 65 with severe emphysema. It promises substantial return of lung function for selected patients by cutting away diseased parts of the lungs so that healthy tissue functions better. In the fall of 2003, Medicare announced that it would begin paying for the surgery.

Prognosis

Many of these diseases are progressive, because the irritants stay in the lungs forever. Others remain stable after the offensive agents are removed from the environment. Lungs do not heal from destructive damage, but they can clean out infection and excess mucus, and function better.

Prevention

Industrial air filters, adequate ventilation, and respirators in polluted work sites now are mandatory. Tobacco smoke is the world's leading cause of lung disease and many other afflictions. Smoking cessation programs are widely available.

Resources

PERIODICALS

"Medicare Will Cover Lung Volume Reduction Surgery for Certain Patients." Health & Medicine Week October 20, 2003: 245.

Mustajbegovic, Jadranka, et al. "Respiratory Findings in Tobacco Workers." Chest May 2003: 1740-1749.

ORGANIZATIONS

American Lung Association. 1740 Broadway, New York, NY 10019. (800) 586-4872. http://www.lungusa.org.

KEY TERMS

Allergen A substance that causes an allergic reaction in those who are sensitive to it.

Asthma Temporary airway narrowing that causes wheezing and shortness of breath due to allergies.

Bronchitis Infection in the bronchi (breathing tubes).

Pneumonia Infection or inflammation in the lung itself.

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Lung Perfusion and Ventilation Scan

Lung Perfusion and Ventilation Scan

Definition

A lung perfusion scan is a nuclear medicine test that produces a picture of blood flow to the lungs. A lung ventilation scan measures the ability of the lungs to take in air and uses radiopharmaceuticals to produce a picture of how air is distributed in the lungs.

Purpose

Lung perfusion scans and lung ventilation scans are usually performed in the same session. They are done to detect pulmonary embolisms, determine how much blood is flowing to lungs, determine which areas of the lungs are capable of ventilation, and assess how well the lungs are functioning after surgery. These tests are called by different names, including perfusion lung scan, aerosol lung scan, radionucleotide ventilation lung scan, ventilation lung scan, xenon lung scan, ventilation/perfusion scanning (VPS), pulmonary scintiphotography, or, most commonly, V/Q scan.

Precautions

The amount of radioactivity a person is exposed to during these tests is very low and is not harmful. However, if the patient has had other recent radio-nuclear tests, it may be necessary to wait until other radiopharmaceuticals have been cleared from the body so that they do not interfere with these tests.

Description

In a lung perfusion scan, a small amount of the protein labeled with a radioisotope is injected into the patient's hand or arm vein. The patient is positioned under a special camera that can detect radioactive material, and a series of photographs are made of the chest. When these images are projected onto a screen (oscilloscope), they show how the radioactive protein has been distributed by the blood vessels running through the lungs.

In a lung ventilation scan, a mask is placed over the nose and mouth, and the patient is asked to inhale and exhale a combination of air and radioactive gas. Pictures are then taken that show the distribution of the gas in the lungs. Each test takes 15-30 minutes.

Preparation

There is little preparation needed for these tests. The patient may eat and drink normally before the procedure. Tests to check for pulmonary embolism are often performed on an emergency basis.

Aftercare

No special aftercare is needed. The patient may resume normal activities immediately.

Risks

There are practically no risks associated with these tests.

Normal results

Normal results in both tests show an even distribution of radioactive material in all parts of the lungs.

Abnormal results

In the lung perfusion scan, an absence of radioactive marker material suggests decreased blood flow to that part of the lung, and possibly a pulmonary embolism. However, pneumonia, emphysema, or lung tumors can create readings on the lung perfusion scan that falsely suggest a pulmonary embolism is present.

In the lung ventilation scan, absence of marker material when the lung perfusion scan for the area is normal suggests lung disease.

Certain combinations of abnormalities in lung perfusion and ventilation scans suggest pulmonary embolism.

Resources

BOOKS

Pagana, Kathleen Deska. Mosby's Manual of Diagnostic and Laboratory Tests. St. Louis: Mosby, Inc., 1998.

KEY TERMS

Pulmonary embolism A blood clot in the arteries going to the lungs.

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lungs

lungs, elastic organs used for breathing in vertebrate animals, excluding most fish, which use gills, and a few amphibian species that respire through the skin. The word is sometimes applied to the respiratory apparatus of lower animals.

The human lungs are paired organs, located on either side of the heart and occupying a large portion of the chest cavity from the collarbone to the diaphragm. Air enters the body through a series of passages, beginning with the nose or mouth. It travels to the chest cavity through the trachea, which divides into two bronchi, each of which enters a lung. The bronchi divide and subdivide into a network of countless tubules. The smallest tubules, or bronchioles, enter cup-shaped air sacs known as alveoli, which number about 700 million in both lungs. Each alveolus is surrounded by a net of capillaries. As blood flows through these vessels, carbon dioxide passes into the alveoli, and oxygen diffuses into the bloodstream. The capillaries are part of a vast network of pulmonary blood vessels that connect the lungs directly to the heart via the large pulmonary arteries and veins. The alveoli are clustered in groups, or lobules, and the lobules are clustered into lobes.

In humans, the left lung has two lobes; the right lung three. The lungs are covered by a thin membrane called the pleura. They are expanded and contracted (thereby inhaling and exhaling air) by the combined movement of the diaphragm and the rib cage, which is alternately raised (expansion) and lowered (contraction) by the chest muscles. In recent years, smoking has been found to cause severe and sometimes fatal diseases of the lung, such as cancer and emphysema. Pneumonia is an inflammation of the lung tissue caused by various agents or organisms such as viruses. Asthma, a hypersensitivity or allergic response to some stimuli, covers a range of severity and is characterized by bronchial spasms and difficult breathing. See respiration.

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lungs

lungs Organs of the respiratory system of vertebrates, in which the exchange of gases between air and blood takes place. The lungs lie in the pleural cavity within the ribcage. Two sheets of tissue (the pleura) line this cavity: one coats the lungs, and the other lines the walls of the thorax. Between the pleura is a fluid that cushions the lungs and prevents friction. Light and spongy, lung tissue consists of tiny air sacs, called alveoli, which are served by networks of fine capillaries. See also alveolus; gas exchange; ventilation

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lung book

lung book The respiratory organ of some arachnids. Lung books occur in pairs on the ventral side of the abdomen. They consist of leaflike folds of ectoderm sunk into pockets having slitlike openings at the abdominal surface. Gaseous exchange occurs by diffusion through the ectodermal folds.

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Lung Perfusion and Ventilation Scan

Lung Perfusion and Ventilation Scan

Definition

A lung perfusion scan and ventilation study are two diagnostic imaging studies. A lung perfusion scan assesses blood flow to the lungs. A lung ventilation study reveals the distribution of air space within the lungs. These are two separate studies that are often performed sequentially. The tests are called by different names, including perfusion lung scan, aerosol lung scan, ventilation lung scan, xenon lung scan, ventilation/perfusion scanning (VPS), pulmonary scintiphotography, or most commonly, V/Q scan.

Purpose

Lung scans may be performed for patients with chest pain, for those coughing up blood (hemoptysis), or for those having difficulty breathing (dyspnea). A perfusion scan alone or both tests are frequently performed for patients with a suspected pulmonary embolism (blood clot in the lung) or for follow-up in patients with known pulmonary embolism. Lung scans are a sensitive method for demonstrating the presence of pulmonary disease but are not often specific for a certain disease. For example, an abnormal scan may also be caused by chronic obstructive pulmonary disease (COPD), asthma, pneumonia, venous hypertension, pleural effusion, and cardiomegaly.

Precautions

The amount of radioactivity a person is exposed to during these tests is very low and is not harmful. However, if the patient has had other recent nuclear medicine tests, it may be necessary to wait until other radiopharmaceuticals have been cleared from the body so that they do not interfere with these tests.

Description

These tests are typically done in a hospital nuclear medicine department or out-patient radiology facility. Scans to diagnose pulmonary embolism are often done on an emergency basis. Most often, both studies are needed. Sometimes a perfusion scan is done without a ventilation scan. Rarely, a ventilation scan is done alone.

For a lung perfusion scan, the patient is injected intravenously with radioactive particles, known as Tc 99m MAA (macroaggregated albumin). The particles pass through the larger blood vessels and become temporarily trapped in small blood vessels. The images thus reflect blood perfusion in the lungs. Images are obtained anteriorly, posteriorly, laterally, and obliquely.

For a lung ventilation scan, the patient inhales a radioactive gas through a mask placed over the nose and mouth. Images of the ventilation lung scan show the distribution of the gas in the lungs. The test typically consists of three phases. The first stage is the initial, or ventilation stage, which reflects the rate of ventilation of the different lung segments. Second is the equilibrium stage, which represents gas volume of the lungs. The third stage is the wash-out phase, which demonstrates any gas trapping that may occur in obstructive diseases. Images are typically obtained posteriorly, although additional views may also be performed. Each test takes approximately 15 to 30 minutes. If possible, the patient usually sits up while the images are taken.

Preparation

To accompany the lung scan, the patient should have a chest x ray within 12 to 24 hours of the study. Otherwise, there is no special preparation needed for these tests. The patient may eat and drink normally before the procedure.

Aftercare

No special aftercare is needed. The patient may resume normal activities immediately.

Complications

There are no complications associated with these tests.

KEY TERMS

Pulmonary embolism— A blood clot or other blockage in the arteries leading to the lungs.

Results

Normally, there is a physiological relationship between the perfusion of the pulmonary blood vessels and their regional alveolar ventilation. An imbalance of this relationship as demonstrated by these studies reflect various respiratory diseases. Other diagnostic tests are often required to confirm a diagnosis.

Normal results for both tests show an even distribution of radioactive material in all parts of the lungs. For the lung perfusion scan, diminished or absent perfusion suggests decreased blood flow to that part of the lung, and possibly a pulmonary embolism. However, pneumonia, emphysema, or lung tumors can create readings on the lung perfusion scan that falsely suggest a pulmonary embolism is present. For the ventilation study, areas that show an increased accumulation of radioactive gas, particularly after the wash-out phase, suggests obstructive lung disease. Areas where there is decreased or absent radioactive gas flow suggests mechanical obstruction of air flow, such as an embolus. Certain combinations of abnormalities in lung perfusion and ventilation scans suggest pulmonary embolism.

Health care team roles

Both the lung perfusion and ventilation scans are performed by a nuclear medicine technologist. The technologist is trained to handle radioactive materials, operate the equipment, and process the data. The tests are interpreted by a radiologist who may specialize in nuclear medicine. Patients receive the results from their personal physician or the doctor who ordered the test.

Resources

BOOKS

Klingensmith III, M.D., Wm. C., Dennis Eshima, Ph.D., John Goddard, Ph.D. Nuclear Medicine Procedure Manual 2000–2001.

Pagana, Kathleen, and James Pagana. "Lung Scan." In Mosby's Diagnostic and Laboratory Test Reference, 2nd ed. St. Louis: Mosby, 1995, pp. 533-34.

"Scanning Tests." In Illustrated Guide to Diagnostic Tests. Springhouse: Springhouse Corp., 1996, pp. 679-82.

Zaret, Barry, ed. "Lung Scan." In The Patient's Guide to Medical Tests. New York: Houghton Mifflin, 1997, pp. 38-40.

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Lungs

Lungs

Definition

The two lungs are spongy and highly elastic organs of respiration in the pulmonary cavities of the thorax, where the aeration of blood occurs.

Description

Each lung has an irregular conical shape with a blunt top, called the apex, extending into the root of the neck. They have concave bottoms resting on the arc of the diaphragm, a mostly concave inner mediastinal surface that follows the lines of the pericardium, and a convex outer (costal) surface. The right lung is larger than the left, and consists of three lobes (upper, middle, and basal or lower). The left lung consists of two lobes, an upper and a basal, or lower, lobe.

Each lung consists of an exterior plasma coat comprised of an organ coat which folds back to make an interior lining for the chest cavity. The inner lung contains sub-serous areolar tissue with elastic fibers interspersed over the surface of the organ. The parenchyma, or functional part of the organ, is composed of secondary lobules (alveolar ducts) that differentiate into primary lobules (alveoli) consisting of blood vessels, lymphatics, nerves, and an alveolar duct that connects with air space.

The lung, as it relates to inspiration and expiration, has two distinct zones in which the lung passages convey air to the alveolar sacs. The zones relate to the two functions of these passages. One is for conducting air, and the other is for respiration. The parts of the conducting zone do not participate in gas transfer, rather they convey air to and from the respiratory zone. All of the parts of the respiratory zone can take part in gas transfer. However, the uppermost branches, such as the respiratory bronchioles, participate in respiration only in times of exertion.

The conducting zone starts at the trachea and branches out to the bronchi. The bronchi differentiate into bronchioles and then into terminal bronchioles. The respiratory zone starts after the terminal bronchioles at the respiratory bronchioles. These differentiate into the alveolar ducts, which terminate at the alveolar sacs. The lungs consist mainly of the tiny air containing alveolar sacs.

Function

The lung is the sole means of gas exchange in respiration. Air is brought into the body through the mouth or nose and trachea to the lung. There oxygen diffuses from the airspace of the alveoli into the blood stream and carbon dioxide diffuses from the blood into the alveoli's airspace.

The alveoli are small hollow sacs. Their ends connect to the lumens of the airways. The air adjacent to surfaces of the alveolar wall are lined by a single cell layer of flat epithelial cells called type I alveolar cells. In between type I cells are type II cells. They are thicker, and secrete a fluid called surfactant. In the alveolar walls this fluid and connective tissue fills the interstitial space and is interspersed with capillaries. In some places the interstitial space is nonexistent and the epithelial cell membranes are in direct contact with the capillaries. The blood in the capillaries is separated from the air by a single layer of flat epithelial cells. The surface area in a single alveoli is roughly the size of a small basketball court due to the undulating terrain of the type I and II epithelial cells. There are around 300 million alveoli in the adult male. Thus, there is a large surface area where the air and the blood stream are in close proximity. This large surface area is necessary for gas exchange to easily occur. The respiratory system also needs a continual supply of fresh air, which is supplied by the process of breathing.

The process of breathing is aided by the position of the lungs in the thorax (chest). The thorax is a closed chamber that extends from the neck muscles to the diaphragm. The diaphragm is a dome shaped sheet of skeletal muscle that separates the thorax from the abdomen. The sides of the thorax are bounded by connective tissue around the spine, ribs, intercostal muscles, and sternum.

A completely enclosed sac consisting of a thin sheet of cells, called the pleura, surround each lung. Between the pleura and the lung is interstitial fluid. As the diaphragm expands and contracts the intrapleural pressure placed on the lungs causes the lung to inflate and deflate. Breathing allows a fresh supply of air and oxygen to enter the lung upon inflation and carbon dioxide to exit the lung upon deflation. It also causes a change in the pressure of the lung.

The epithelial surface from the conducting zone to the respiratory bronchioles is lined with cilia that continually beat in the direction of the pharynx. There are epithelial cells and glands on this surface that secrete mucus. This mucus catches particulate and bacterial matter, and the material (and mucus) is slowly moved by the cilia toward the pharynx. There it is either swallowed or coughed up as sputum. The epithelial layer also secretes another viscous fluid that allows the cilia to move mucus easily out of the lung.

Toxic substances can inhibit ciliary action. Agents like cigarette smoke can paralyze the cilia for extended periods of time. This inhibits the movement of mucus and particles out of the lungs. The suspension of this process can inhibit gas exchange and eventually cause prolonged oxygen deficiency.

Respiration

Respiration is the process by which the body takes in oxygen and emits carbon dioxide. The following is a summary of the steps of respiration:

  • ventilation
  • interchange of CO2 and O2 between alveolar air and blood in lung capillaries
  • transport of CO2 and O2 through the bloodstream
  • interchange of CO2 and O2 between blood in lung capillaries and alveolar air by diffusion
  • use of O2 and production of CO2 by cells in metabolism

Ventilation is the interchange of air between the atmosphere and the alveoli by bulk flow. Bulk flow is the movement of air from a region of high pressure to one of low pressure. Bulk flow may be thought of as occurring between the outside air, the air in most of the lung, and the air in the alveolar sacs. Flow of some gases (especially oxygen and carbon dioxide) also occurs between the alveolar air and the blood. It is important to note that the pressure of individual gases is different in different types of air. For example, air going into the lungs is rich in oxygen and low in carbon dioxide. Air leaving the lungs is rich in carbon dioxide and low in oxygen. The different concentrations (or pressures) of individual gases are known as the partial pressures, and the partial pressure of each individual gas adds up to the total pressure of the gas.

When air is inspired (taken in), it has a higher partial pressure of oxygen than the air already in the lung, and a lower partial pressure of carbon dioxide. Therefore, inspired air allows oxygen to flow from the area of highest pressure (inspired air) to the alveolar sacs (that have a lower partial pressure of oxygen), and into the bloodstream. The same inspired air has a low partial pressure of carbon dioxide, so carbon dioxide leaves the bloodstream (where it has a high partial pressure), enters the alveolar air (where the pressure is lower), and is passed onto the inspired air (where the partial pressure is even lower). Thus, carbon dioxide gas and oxygen gas both move from areas of highest pressure to lowest pressure in an attempt to reach a pressure (or concentration) equilibrium. This process is called gas exchange. After gas exchange has taken place, the air is expired, or expelled to rid the body of air that has a high concentration (partial pressure) of carbon dioxide gas. Then the process begins again.

Lung expansion and contraction

The concept of bulk flow (explained above) and Boyle's law explain the expansion and contraction of the lung. Boyle's law states that, at constant temperature, an increase in the volume of a container (lung) lowers the pressure of a gas, and a decrease in the container (lung) volume raises the pressure. Thus, when the volume of the lung expands, the pressure inside the lung is lowered, and when the volume of the lung contracts, the pressure inside the lung rises.

Inspiration occurs when the muscles of inspiration increase the volume of the thoracic cavity. The decrease in pressure in the cavity causes the lungs to expand to fill the cavity, which lowers the pressure inside the lung. Since air flows from areas of high pressure to low pressure, air fills the lungs to equalize the air pressure inside the lungs with the outside air, and inspiration occurs. The difference between the internal pressure in the lung and the pressure of the outside air is called the transpulmonary pressure.

During expiration, the muscles of inspiration relax, and the lung contracts. The decreased volume causes increased pressure inside the lungs, which results in air being expired, or expelled. In normal adults, expiration does not require any effort.

Role in human health

The lungs ability to extract oxygen from the atmosphere and supply it to the body's tissues is essential for metabolism and therefore for life. Disease and disorder can interfere with the body's normal function and slow a normally healthy person. Serious interference with the lung's function can cause hypoxia and even death.

Common diseases and disorders

Asthma is an intermittent disease characterized by a chronic inflammation of the airways, causing smooth muscle contraction in the airway. The causes vary from person to person and can include allergies, viral infections, environmental pollutants, mold, dust, dander, cigarette smoke, overexertion, and naturally released bronchiorestrictors. Ingested items such as food coloring, preservatives, and medications can trigger an attack.

Chronic obstructive pulmonary disease (COPD) refers to emphysema, chronic bronchitis, or a combination of the two. This category of disease is one of the major causes of death and disability in the world. These diseases restrict ventilation and the oxygenation of the blood.

Chronic bronchitis is characterized by excessive mucus production in the bronchi and chronic inflammatory changes in the small airways. The accumulation of mucus and thickening of inflamed airways obstruct the flow of air. It is primarily a result of cigarette smoking, although pollution may also play a role.

Emphysema is a major cause of hypoxia and is characterized by the destruction of the alveolar walls, and the atrophy and collapse of the lower airways. The lungs self-destruct through the secretion of proteolytic enzymes by white blood cells. Cigarette smoke stimulates the release of harmful enzymes and destroys the enzymes that normally protect against proteolysis. The proteolytic enzymes cause the breakdown of the alveolar walls. The damaged alveoli fuse and a gradual decrease in the surface area available for gas exchange results. Emphysema increases the work of breathing and, when severe enough, causes hypoventilation (inadequate ventilation). The obstruction caused by the collapse of the lower airways is accompanied with destruction of the lung's elastic tissues and the eventual collapse of the airways.

Pneumonia is normally caused by bacterial or viral infection. It can be triggered by the inhalation of toxic chemicals, chest trauma, yeast, rickettsiae, and fungi. It is the inflammation and compaction of the lung parenchyma. The alveolar spaces fill with mucus, inflammatory cells, and fibrin.

Tuberculosis is caused by the infection of Mycobacterium tuberculosis. It can affect most organs but is most commonly found in the lungs. The bacteria cause lesions to be formed on the lungs and spread to other tissues. Pulmonary tissue in motion will be chronically affected and may eventually be destroyed, if left untreated. The erosion of lung tissue into the blood vessels can result in life-threatening hemorrhages.

Other less common diseases of the lung include Legionnaire's disease, cystic fibrosis, histoplasmosis, coccidiomycosis, and Mycobacterium avium complex.

KEY TERMS

Interstitial space— The spaces found within organs and tissues.

Metabolism— A series of chemical and physiological changes in the body that either build larger molecules out of smaller molecules (anabolism) or break down larger molecules into smaller ones (catabolism).

Parenchyma— The active portion of an organ that fulfills its function (as opposed to purely structural portions of the organ).

Proteolysis— The breaking down of proteins by cleaving or hydrolyzing peptide bonds (the bonds connecting amino acids within the protein).

Resources

BOOKS

Bullock, John, et. al. National Medical Series for Independent Study—Physiology. Third ed. Williams & Wilkins, 1995.

Vander, Arthur et. al. Human Physiology—the Mechanisms of Body Function. Eighth ed. McGraw-Hill, 2001.

ORGANIZATIONS

The American Lung Association. 1740 Broadway, New York, NY, 10019. 212-315-8700. 〈http://www.lungusa.org/〉.

OTHER

Thompson, B. H., W. J. Lee, J.R. Galvin, and J. S. Wilson. "Lung Anatomy." Virtual Hospital. University of Iowa Health Care. 〈http://www.vh.org/Providers/Textbooks/LungAnatomy/LungAnatomy.html〉.

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