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Jaundice is not a disease but a symptom of an underlying disease or condition. It is caused by too much bilirubin in the blood stream, and is characterized by yellowness of skin, sclera (white of eyes), mucous membranes, and of body fluids such urine and blood plasma. The resulting yellow color (jaune means yellow in French) is also described by the Latin term icterus.

Most bilirubin, which is a reddish pigment, is a byproduct of red blood cells. When a red blood cell, which has a lifespan of about 120 days, is no longer functional. It is "recycled" by organs such as the spleen and liver. Hemoglobin, the red pigment in a red blood cell, is broken down, or catabolized, into several substances that are either used to make new blood cells, proteins, or other pigments. After the bilirubin is liberated from the red blood cell, it is bound to albumin and transported via blood plasma to the liver, where it is conjugated or joined to glucuronic acid, and excreted into the bile ducts. Bilirubin is also the prominent pigment of bile, a digestive substance secreted by the liver into the gallbladder and small intestine (jejunum), and it is also responsible for the brown color of feces.

There are several ways in which a person can have too much bilirubin, or become jaundiced. Hemolytic jaundice occurs when the liver is overloaded with bilirubin. In conditions and diseases causing hemolysis (the separation of hemoglobin from the red blood cells), including hemolytic anemias, incompatible blood transfusion, and extreme heat or cold, the liver is unable to remove enough bilirubin from the blood stream. Liver cells which are damaged by hepatocellular disease (hepatitis, cirrhosis), toxins, tumors, or inflammatory conditions, are unable to conjugate bilirubin, thus preventing normal excretion. Neonatal jaundice, especially in premature infants, is fairly common. This type of jaundice is caused by the fact that the liver is immature and lacks specific enzymes to conjugate bilirubin, and a large amount of bilirubin is consequently excreted into the blood stream instead of being incorporated into bile.

The most common cause of jaundice is obstruction of bile flow through the biliary system. For example, a gallstone, a liver tumor, or a pancreatic tumor can block a biliary duct. The treatment of jaundice depends on the cause.

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