Respiratory System

V

Diseases and Disorders

The diseases and disorders of the respiratory system can affect any part of the respiratory tract and range from trivial to life-threatening. The nasal passages and pharynx, for example, are targets for the viruses that cause colds. These viruses infiltrate and destroy the cells of the nasal passage membranes. The immune system fights back by increasing blood flow to the area, bringing numerous virus-attacking white blood cells to the scene; this causes the membranes to swell, resulting in the stuffy nose associated with colds. Mucous secretions increase in response to the viral attack, creating the runny nose typical of colds. The infection can spread to the sinuses, the membrane-lined cavities in the head, as well as the lower respiratory tract and the middle ear.

The respiratory system is also subject to allergic reactions such as hay fever and asthma, brought about when the immune system is stimulated by pollen, dust, or other irritants. Hay fever is characterized by a runny nose, watery eyes, and sneezing. It usually occurs seasonally in response to abundant pollen in the air. In asthma, a person has difficulty breathing because the bronchi and bronchioles are temporarily constricted and inflamed. An asthma attack is typically mild, but can be severe enough to be life threatening.

Laryngitis, an inflammation of the larynx, is caused by a viral infection, irritants such as cigarette smoke, or by overuse of the voice. Laryngitis may cause hoarseness, or the voice may be reduced to a whisper until the inflammation subsides. Bronchitis is an inflammation of the membranes that line the bronchi or bronchioles. Bronchitis results from viral or bacterial infection or from irritating chemicals. Infections caused by bacteria or viruses can lead to pneumonia, a potentially serious condition of the lungs in which fluid and inflammation builds up in the alveoli, impeding the flow of oxygen and carbon dioxide between the capillaries and the alveoli.

Tuberculosis is caused by a bacterium that attacks the lungs and sometimes other body tissues as well. If infections in the lungs are left untreated, the disease destroys lung tissue. In the past, antibiotics have controlled tuberculosis, but recently, new antibiotic-resistant strains of the tuberculosis bacterium have evolved. These new strains now pose a significant public health problem.

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In emphysema the alveolar tissue is partially destroyed and the remaining alveoli are weakened and enlarge. The bronchioles collapse on exhalation, trapping air in the alveoli. Over time this process impairs the ability to exchange oxygen and carbon dioxide with the circulatory system, leading to breathing difficulties. A noncontagious disease, emphysema results from multiple factors, including a genetic predisposition to the condition, smog, cigarette smoke, and infection.

Lung cancer develops in individuals with a genetic predisposition to the disease who are exposed to cancer-causing agents, such as tobacco smoke, asbestos, and uranium. Cancerous tumors may start in the bronchi, bronchioles, or in the alveolar lung tissue. If lung cancer is detected before the cancer has spread to other parts of the body, treatments are more effective, and the prognosis for full recovery is good. Unfortunately, 85 percent of lung cancer cases are diagnosed after the cancer has spread, and for these cases, the prognosis is very poor.

Respiratory Distress Syndrome (RDS) is the name for a cluster of symptoms that indicate severe malfunctioning of the lungs. In infants, RDS is termed Infant Respiratory Distress Syndrome (IRDS). Commonly found in premature infants, IRDS results when the alveoli fail to fully expand during inhalation. Expansion of the alveoli requires a chemical called surfactant, but in many premature infants, the alveoli are not developed enough to produce this vital substance. IRDS is treated by administering air and surfactant through a breathing tube until the alveoli begin producing surfactant on their own. Adult Respiratory Distress Syndrome (ARDS) results when lungs are severely injured, for example, in an automobile accident, by poisonous gases, or as a response to inflammation in the lungs. ARDS is a life-threatening condition with a survival rate of about 50 percent.

VI

Respiratory Systems in Other Organisms

The need to take in oxygen and expel carbon dioxide is almost universal among organisms. The movement of these gases between an organism and its environment, called gas exchange, is accomplished in a variety of ways by different organisms. In one-celled aquatic organisms, such as protozoans, and in seaweeds, sponges, jellyfish, and other aquatic organisms that are only a few cell layers thick, oxygen and carbon dioxide diffuse directly between the water and cells. Diffusion works for these simple organisms because all cells of the organism are within a few millimeters of an oxygen source.

Animals with many cell layers cannot rely on diffusion because cells several layers deep in the body would die before oxygen reached them. As a result, for gas exchange, more-complex animals require special respiratory organs, such as gills or lungs, in combination with circulatory structures, such as blood, blood vessels, and a heart. The earliest development of these gas exchange structures is seen in roundworms, microscopic invertebrates abundant in water and moist soil. In roundworms, oxygen diffuses through the skin into a fluid that fills an internal cavity. As the worm moves, the fluid sloshes around in the cavity, bringing oxygen into contact with the digestive system, reproductive organs, and other structures in the cavity. This primitive circulatory system is called an open circulatory system because the fluid is not contained within vessels. In clams an open circulatory system is combined with a heart that pumps fluid around the internal cavity. Clams also use gills, thin-walled filaments that are extensions of the body surface. Gills provide a more extensive surface area for gas exchange than the body surface alone, enabling clams and larger organisms to obtain the amount of oxygen they need. Fish have gills, a heart, and a closed circulatory system, one in which blood is transported in vessels by the pumping action of the heart.

Relatively simple land-dwelling organisms, including some plants, fungi, and animals such as flatworms, accomplish gas exchange by diffusion. More-complex organisms, however, rely on specialized respiratory structures. Instead of gills, whose delicate filaments collapse unless supported by water, land animals use lungs. Located inside the body, lungs are formed by the infolding of membranes. The folds form a single balloon-like sac, as in amphibians; they may be arranged in stacks, as in the book lungs of spiders; or they are composed of millions of tiny air sacs, such as the lungs of most mammals. In virtually all vertebrates, a heart and a closed circulatory system work with the lungs to deliver oxygen and to remove carbon dioxide from cells.

Insects have a unique respiratory system made up of small tubes called tracheae. The tracheae connect all parts of the body to small openings on the surface of the insect. Oxygen and carbon dioxide are transported through the tracheae, and from the tracheae to the blood of the insect by diffusion. The blood of most insects is contained in an open circulatory system and is moved around the internal organs by a heart.

The respiratory system of birds, adapted for flight, is very different from that of land-bound animals. The lungs have two openings, one for taking in oxygen-filled air; the other for expelling carbon dioxide-laden air. Rather than ending up in alveoli, the air loops through the lungs so that the oxygen flow through the lungs is continuous. This design enables birds to obtain the amount of oxygen they need to power the extremely high energy demands of flight.

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