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Jan 3
Bilirubin is a yellow pigment that is produced during the breakdown of red blood cells in the body. It is a waste product that needs to be eliminated from the body to maintain normal functioning. The mechanism of bilirubin involves several steps:
1. Hemoglobin breakdown: Red blood cells have a lifespan of about 120 days, after which they are broken down in the spleen and liver. The heme component of hemoglobin, which carries oxygen in red blood cells, is converted into biliverdin by the enzyme heme oxygenase.
2. Conversion to bilirubin: Biliverdin is then converted into bilirubin by the enzyme biliverdin reductase. This conversion occurs in the reticuloendothelial cells of the liver, spleen, and bone marrow.
3. Binding to albumin: Bilirubin is not very soluble in water, so it binds to a protein called albumin for transport in the bloodstream. This bound form of bilirubin is called unconjugated bilirubin.
4. Uptake by liver cells: Bilirubin-albumin complex is taken up by liver cells through receptor-mediated endocytosis. Once inside the liver cells, bilirubin is released from albumin.
5. Conjugation: Inside the liver cells, bilirubin is conjugated with glucuronic acid by the enzyme UDP-glucuronosyltransferase (UGT). This conjugation makes bilirubin more water-soluble and allows it to be excreted from the body.
6. Secretion into bile: Conjugated bilirubin is transported into the bile canaliculi, small ducts within the liver, by the transporter protein called multidrug resistance-associated protein 2 (MRP2). It is then released into the bile.
7. Excretion: Bile containing conjugated bilirubin is stored in the gallbladder and released into the small intestine when needed. In the intestine, bilirubin is further metabolized by bacteria to form urobilinogen. Some urobilinogen is reabsorbed into the bloodstream and excreted in the urine, giving it a yellow color. The remaining urobilinogen is converted into stercobilin, which gives feces its brown color.
In summary, bilirubin is produced from the breakdown of hemoglobin, converted into a water-soluble form in the liver, and excreted from the body through bile and urine. Any disruption in this mechanism can lead to an accumulation of bilirubin in the body, resulting in jaundice and other liver-related disorders.
1. Hemoglobin breakdown: Red blood cells have a lifespan of about 120 days, after which they are broken down in the spleen and liver. The heme component of hemoglobin, which carries oxygen in red blood cells, is converted into biliverdin by the enzyme heme oxygenase.
2. Conversion to bilirubin: Biliverdin is then converted into bilirubin by the enzyme biliverdin reductase. This conversion occurs in the reticuloendothelial cells of the liver, spleen, and bone marrow.
3. Binding to albumin: Bilirubin is not very soluble in water, so it binds to a protein called albumin for transport in the bloodstream. This bound form of bilirubin is called unconjugated bilirubin.
4. Uptake by liver cells: Bilirubin-albumin complex is taken up by liver cells through receptor-mediated endocytosis. Once inside the liver cells, bilirubin is released from albumin.
5. Conjugation: Inside the liver cells, bilirubin is conjugated with glucuronic acid by the enzyme UDP-glucuronosyltransferase (UGT). This conjugation makes bilirubin more water-soluble and allows it to be excreted from the body.
6. Secretion into bile: Conjugated bilirubin is transported into the bile canaliculi, small ducts within the liver, by the transporter protein called multidrug resistance-associated protein 2 (MRP2). It is then released into the bile.
7. Excretion: Bile containing conjugated bilirubin is stored in the gallbladder and released into the small intestine when needed. In the intestine, bilirubin is further metabolized by bacteria to form urobilinogen. Some urobilinogen is reabsorbed into the bloodstream and excreted in the urine, giving it a yellow color. The remaining urobilinogen is converted into stercobilin, which gives feces its brown color.
In summary, bilirubin is produced from the breakdown of hemoglobin, converted into a water-soluble form in the liver, and excreted from the body through bile and urine. Any disruption in this mechanism can lead to an accumulation of bilirubin in the body, resulting in jaundice and other liver-related disorders.
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