Notes on Jaundice

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Bilirubin Metabolism Diagram:

bilirubin metabolism diagram

1. Catabolism of hemoglobin: What are the steps in the breakdown of hemoglobin? What is the final product?

• Macrophages in liver, spleen or red bone marrow capture old, worn-out RBCs.

• Heme and globin portions are split apart.

• Globin broken down into amino acids, with can be reused.

• Iron is removed from the heme portion, transferred to ferritin, then sent to RBC precursor cells for reuse.

• The iron-less heme is called biliverdin which is converted to bilirubin.

• Bilirubin enters the blood stream and is transported to the liver.

• Within liver, bilirubin is converted to bile which is passed to the intestines.

2. Transport of bilirubin to the liver:

Bilirubin is formed outside the liver and released bound to serum albumin because bilirubin is insoluble in aqueous solutions, hence, impossible to transport in blood. Then occurs carrier-mediated hepatic uptake of bilirubin.

3. Liver uptake and conjugation of bilirubin: What is meant by "conjugation"? What is bilirubin conjugated with What facilitates ("catalyzes") this reaction? Why is conjugation of bilirubin necessary?

• Conjugation is the coupling of two or more biomolecules.

• Bilirubin is conjugated with one or two molecules of glucuronic acid with the action of bilirubin UDP-glucuronyltransferase.

• Liver is conjugated to make it water soluble inside the liver.

4. Bile secretion and entry of bilirubin into the small intestine: What is the relationship between bilirubin and bile? Are they the same thing?

Once in the liver, bilirubin is secreted by liver cells into bile.

5. Intestinal bacterial action on conjugated bilirubin:

Intestinal bacteria convert conjugated bilirubin back into unconjugated bilirubin, which is then excreted in feces.

6. Enterohepatic circulation:

Liver receives blood from two sources: hepatic artery (oxygenated blood without nutrients) & hepatic portal (deoxygenated blood with nutrients) vein. Deoxygenated blood from the liver flows into the hepatic vein. NOTE: Hepatic vein and hepatic portal vein are not the same thing!

7. Excretory pathways for the metabolites of bilirubin

• Bilirubin enters the blood and is transported into the liver.

• Within liver, bilirubin is converted into bile, and then secreted into the intestines.

• In the large intestine, bacteria convert bilirubin into urobilinogen.

• Some urobilinogen is absorbed back into blood and converted into a yellow pigment called urobilin and excreted in urine.

• Most urobilinogen is excreted in feces in the form of a brown pigment called stercobilin.

8. Hyperbilirubinemia and jaundice, the possible causes, and the mechanism by which jaundice results from each of the causes:

Jaundice:

Bile formation requires healthy hepatocytes. If hepatocytes aren't fully functional, a yellow discoloration becomes evident in the skin and sclerae due to retention of pigmented bilirubin.

Increased red blood cell destruction (What does this lead to? What kind of anaemia do we call this?)

Hemolytic Anemia:

This type of anemia is called hemolytic anemia caused due to intravascular hemolysis. This leads to:

1. A shortened RBC lifespan.

2. Elevated erythropoietin levels and increased erythropoiesis in the marrow and other sites to compensate for the blood loss.

3. Accumulation of the products of hemoglobin catabolism.

Diminished hepatic uptake of bilirubin:

Bilirubin slowly accumulates in blood, resulting in a lower percentage of RBCs in blood. A diminished supply of RBCs causes anemia.

Defective hepatic conjugation of bilirubin:

Same as above.

Biliary tract obstruction: intra-hepatic or extrahepatic obstruction:

Same as above.

9. Fecal and urinary manifestations of hyperbilirubinemia, and their physiological basis.

• Urine turns dark or brownish in color due to presence of bilirubin.

• Stool turns pale due to absence of urobilinogen.

Additional Reading:

Basic Gastroenterology

1. Basic Gastrointestinal Physiology
2. Digestion FAQ, Defecation reflex, etc.
3. Digestion
4. Notes on Functions of the Liver
5. Notes on Jaundice
6. Types of Jaundice
7. Diagram of Gastric Blood Supply
8. FAQ on Gastric Digestion
9. Usage of the D-xylose Absorption Test

Gastroenterology Videos

1. Video of Abdominal Examination in a Clinical Setting

Related Topics

1. Gastrointestinal Disorders
2. Hepatobiliary and Pancreatic Disorders
3. Histology of the Digestive Tract I: Oral Cavity
4. Histology of the Digestive Tract II: Esophagus through Intestines
5. Histology of the Liver, Pancreas, and Gall Bladder
6. Abdominal Examination for Internal Medicine

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Useful Medical Images & Diagrams (link opens in a new window)

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