Asbestos Pleural Disease

In gaining a better understanding of what asbestos pleural disease is and how it effects respiratory health, it is helpful to know something about the anatomy of the respiratory system and the thoracic or chest cavity.

How We Breathe

In mammals, the respiratory system has two sections. The upper tract, also called theconducting zone, is that through which air passes, or is "conducted." The lower tract is known as the respiratory zone proper, and is where oxygen from the atmosphere is exchanged for carbon dioxide from the blood.

The conducting zone consists of the nose, nasal cavity, pharynx, larynx, glottis and trachea. As one breathes, air is filtered by mucus membranes in the nose and nasal cavity, ornasopharynx, and warmed and humidified (this is the reason that peoples whose ancestors lived in extreme climates tend to have large noses). Passing through the oropharynx (the opening behind the mouth), air travels past the larynx, or vocal muscles and through theglottis, which is the space between these muscles, or "folds." The air ultimately enters the lower respiratory tract by way of the trachea, or "windpipe."

Once inside the thoracic (chest) cavity, the trachea divides into two more passages, called the "left" and "right"bronchi, or bronchial tubes. As they penetrate the fleshy sacs that make up the lungs, these bronchi subdivide sixteen more times into increasingly smaller bronchial passages, or bronchioles.

The 4,096 bronchioles in each lung lead to the respiratory zone and terminate in small tubes and air sacs known as the alveolar ducts and alveoli, respectively. (The word alveoli, incidentally is from the Latin term for "grapes," which gives one a good idea of what they look like.)

It is within these alveoli that oxygen molecules enter the bloodstream as they are exchanged for carbon dioxide from the bloodstream. Each time a mammal or other land vertebrate breathes out, this CO2 is expelled. During inhalation, fresh air is taken in, and the cycle begins all over again.

This process, which happens about 15-18 times per minute for the average human at rest, is controlled by a muscle located below the lungs called the diaphragm. As the diaphragm contracts, air is pulled into the lungs; when it relaxes, waste gases are allowed to escape. Other muscles may assist in breathing if necessary, such as the muscles between the ribs, and in the neck and back.

The Pleural Lining

As you might imagine, with the beating heart, breathing lungs, moving diaphragm in abdominal cavity all located in the thoracic cavity, it's a little crowded in there. Everything is touching and moving against everything else. In order to keep this rubbing of parts from creating irritation, everything is coated with a thin, moist, lubricating membrane called themesothelium. Inside the body, the fluid surrounding the mesothelium acts in the body in the same way as motor oil inside an internal combustion engine; it lubricates all the surfaces in order to allow them to move and function properly.

The pleural lining is that part of the mesothelium that covers the outer surfaces of the lungs. Pleural disease is any one of a number of illnesses that attack this layer, such as the asbestos cancer most commonly diagnosed,pleural mesothelioma. When one contracts a disease of the pleural lining, it interferes with the lungs ability to expand.

How Asbestos Attacks

Amphibole fibers - the strongest and most deadly type of asbestos - resemble miniature needles when examined under a microscope. When inhaled, they literally impale the tissues of the alveolar sacs and start to burrow through toward the outer layers of the lung. (Those who have suffered from asbestos disease and describe the pain as being stabbed by "thousands of needles" with every breath aren't terribly wrong.) As these fibers travel through the lung tissue and emerge into the pleural lining, they appear to interact with the DNA of living cells, causing mutations that medical science is only now beginning to understand.

The medical profession classifies asbestos-caused pleural disease as either benign ormalignant. If the disease remains confined to the pleural lining and is non-cancerous, it is considered benign. Cancer of the pleural lining - i.e., mesothelioma - that is prone to spread is labeled as malignant.

Benign Pleural Plaques

An example of a "benign" asbestos pleural disease is pleural plaque. According to a 2003 article, pleural plaques are the "most common manifestation of asbestos exposure," and appear as "white or yellow thickening on the parietal pleura". Essentially, the asbestos fibers cause plaque (similar to that which builds up on teeth) to build up on the pleural lining, causing it to stiffen and inhibiting the lungs' ability to expand. Like other asbestos diseases, pleural plaques may take as long as 30 years to manifest.

Benign Pleural Effusion

Under normal conditions, there is no more than 15 ml. of fluid between the surface of the lung and the inner pleural lining. Normally, this fluid is circulated by the lymphatic system, which can absorb up to 300 ml. of fluid if necessary. However, when the system breaks down and fluid builds up beyond that point, it causes pleural effusion.

Benign asbestos-related pleural effusions can appear in as little as ten years following exposure, and can be caused by minimal exposure. These effusions can reduce movement of the chest, diminishing lung capacity.

Diffuse Pleural Thickening

This is a type of fibrosis that develops on the pleural lining, causing it to stiffen. As the asbestos fibers penetrate living tissues, bodily defenses - antibodies known asmicrophages - go into action, attacking the fibers as if they were a virus or bacterial infection. The problem is that asbestos fibers are made from inorganic silicon, therefore, the digestive enzymes that would destroy a virus has no effect.

Instead, the asbestos fibers tear open the microphages, causing their digestive molecules to eat away at the inner pleural lining and surface of the lungs. As these heal, they cause a build-up of scar tissue. Eventually, the fluid-filled space between the lung surface and the pleural lining disappears. Diffuse pleural thickening is the result.

Rounded Atelectasis

This condition is also called "folded lung" and Bleskovsky Syndrome. It happens when the pleural lining develops scar tissue and literally shrinks (in the way the scar from a severe skin wound "puckers"), rolling in on itself. This constricts the lungs, preventing them from taking in air, resulting in a small area of collasped lung. They are often mistaken for tumors.

Mesothelioma

Unlike the conditions described above, malignant mesothelioma is a highly aggressive, malignant cancer that starts out by attacking the pleural lining. From there, it has been known to spread to all parts of the body.

Unlike most tumors that create lumps, mesothelioma spreads in a sheet-like manner across the lung surface, making it difficult to detect in the early stages. So far, asbestos exposure is the only known cause of mesothelioma; medical research indicates that it is triggered by mutations caused by the asbestos fibers as they interact with cellular DNA. However, recent findings have made some tenuous connections to a virus identified as SV-40, which is known to have contaminated some batches of polio vaccines during the late 1950s. In addition, tobacco use has been known to greatly exacerbate the chances of receiving a mesothelioma diagnosis.

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