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Bilirubin (BR) (from the Latin for "red bile") is a red-orange compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates.This catabolism is a necessary process in the body's clearance of waste products that arise from the destruction of aged or abnormal red blood cells. [3]
Bilirubin derives from hemoglobin by glucuronidation. Bile tends to be alkaline on average. The pH of common duct bile (7.50 to 8.05) is higher than that of the corresponding gallbladder bile (6.80 to 7.65). Bile in the gallbladder becomes more acidic the longer a person goes without eating, though resting slows this fall in pH. [12]
Bile is secreted by the liver into small ducts that join to form the common hepatic duct. [4] Between meals, secreted bile is stored in the gallbladder. [5] During a meal, the bile is secreted into the duodenum (part of the small intestine) to rid the body of waste stored in the bile as well as aid in the absorption of dietary fats and oils. [5]
Bilirubin is conjugated with glucuronic acid in the liver by the enzyme glucuronyltransferase, making it soluble in water. Much of it goes into the bile and thus out into the small intestine. Although 20% of the secreted bilirubinoid bile is reabsorbed by the small intestine, [2] conjugated
Bile acids have other functions, including eliminating cholesterol from the body, driving the flow of bile to eliminate certain catabolites (including bilirubin), emulsifying fat-soluble vitamins to enable their absorption, and aiding in motility and the reduction of the bacteria flora found in the small intestine and biliary tract.
The bile components that form gallstones include cholesterol, bile salts, and bilirubin. [2] Gallstones formed mainly from cholesterol are termed cholesterol stones, and those formed mainly from bilirubin are termed pigment stones. [2] [3] Gallstones may be suspected based on symptoms. [4] Diagnosis is then typically confirmed by ultrasound. [2]
Bilirubin metabolism. Haem is converted into unconjugated bilirubin then conjugated bilirubin. Conjugated bilirubin is then secreted along with bile into the intestine and is either excreted in faeces as urobilinogen or reabsorbed into blood and transported back to the liver.
Urobilin is generated from the degradation of heme, which is first degraded through biliverdin to bilirubin. Bilirubin is then excreted as bile, which is further degraded by microbes present in the large intestine to urobilinogen. The enzyme responsible for the degradation is bilirubin reductase, which was identified in 2024.