Five-year-old Carly had what her parents at first thought was just a bug that was passing through the family. but after 12 hours of flu-like symptoms, Carly's temperature shot up to 105°F, her chest began to hurt, and her breathing became rapid and shallow. Later that day, a chest X ray confirmed what the doctor suspected--Carly had pneumonia. Apparently, the bacteria that had caused a mild upper respiratory infection in her parents and sisters had taken a detour in her body, infecting her lower respiratory structures instead.

Carly's bacterial pneumonia was successfully treated with antibiotics. Pneumonia can also be caused by a viral infection, or, as is often the case in people with AIDS, by Pneumocystis carinii infection. Whatever the cause, the events within the infected lung are similar: alveolar linings swell with edema and become abnormally permeable, allowing fluids and white blood cells to accumulate in the air sacs. As the alveoli fill, the surface area available for gas exchange diminishes. Breathing becomes difficult. Untreated, pneumonia can kill.

Tuberculosis is a different type of lung infection, caused by the bacterium Mycobacterium tuberculosis. In this condition, fibrous connective tissue develops around the sites of infection, forming structures called tubercles. By walling off the bacteria, the tubercles help inhibit their spread. Sometimes this protective mechanism fails, and the bacteria flourish throughout the lungs, and may even spread to other organs. In the later stages of infection, other types of bacteria may cause secondary infections. As lung tissue is destroyed, the surface area for gas exchange decreases. In addition, the widespread fibrous tissue increases the thickness of the respiratory membrane, further restricting gas exchange. A variety of drugs can treat tuberculosis, but in recent years, strains resistant to drugs have arisen, and these can be swiftly deadly.

Another type of condition that impairs gas exchange is atelectasis. This is the collapse of a lung, or some part of it, together with the collapse of the blood vessels that supply the affected region. Obstruction of a respiratory tube, such as by an inhaled foreign object or excess mucus secretion, may cause atelectasis. The air in the alveoli beyond the obstruction is absorbed, and as the air pressure in the alveoli decreases, their elastic walls collapse, and they can no longer function. Fortunately, after a portion of a lung collapses, the functional regions that remain are often able to carry on enough gas exchange to sustain the body cells.

Adult respiratory distress syndrome (ARDS) is a special form of atelectasis in which alveoli collapse. It has a variety of causes, all of which damage lung tissues. These include pneumonia and other infections, near drowning, aspiration of stomach acid into the respiratory system, or physical trauma to the lungs from an injury or surgical procedure. This damage disrupts the respiratory membrane that separates the air in the alveoli from the blood in the pulmonary capillaries, allowing protein-rich fluid to escape from the capillaries and flood the alveoli. They collapse in response, and surfactant is nonfunctional. Blood vessels and airways narrow, greatly elevating blood pressure in the lungs. Delivery of oxygen to tissues is seriously impaired. ARDS is fatal about 60% of the time.

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