32 - Bilirubin,urobilinogen, jaundice explained

KYJ32- The Yellow Family

Meet Billy and Rubin, these guys are responsible for the yellow discolouration of skin (Jaundice and bruising) the colour of urine, bile and faeces.

Of course I am talking about Bilirubin.  To understand bilirubin, we need to go back to its origins.

Your red blood cells live for about 120 days, then disintegrate. A large part of the red blood cell is haemoglobin the oxygen carrying protein in all RBCs.

Haemoglobin when broken down yields a protein (Globin) that is further recycled into it's basic amino acids.
The haeme pigment is turned in to Biliverdin by white blood cells gobbling up the fragments of red blood cell.

An enzyme in the liver, spleen and other body tissues, converts the biliverdin into Bilirubin.  About 4 mg /kg is produced daily.  It is insoluble at this point so the liver conjugates bilirubin into a soluble form and releases it to the gut via the gall bladder as Bile.  It is a strongly yellow green pigment which stains bile and subsequently faeces.

Gut bacteria break down the bilirubin as bile into a deep yellow pigment called  urobilinogen. Some of this is reabsorbed into the blood stream and excreted via kidneys giving the urine it's characteristic straw yellow colour.  Much of the urobilinogen in the gut is further broken down to stercobilin and makes poo yellowy-brown.

This is all normal physiology. But when things go wrong in the liver, the insoluble bilirubin can't be processed, and accumulates in the blood stream.  The higher the level of bilirubin, the more yellow will be the skin and eyeballs (sclera) appear.  This is called Jaundice.
It is not a disease but a symptom of hepatic dysfunction.
In new born babies, their blood brain barrier is immature and poorly formed.  Bilirubin can cross the BBB and cause lethargy, seizures and coma in the extreme.

Jaundice we have described here is an accumulation of unprocessed bilirubin.
A second type is Cholestatic Jaundice.  In this type, bilirubin is conjugated to water soluble bilirubin, released as bile, but a blockage in the biliary system (eg gall stones) causes this bile pigments to back up into
The liver and spill into systemic blood. These bile pigments stain blood yellow causing jaundice, but also irritates the skin causing itching. Incessant scratching can damage the skin in these people.  The unbearable itching is called pruritis.

There we have it more jargon:
Haemoglobin into biliverdin into unconjugated (insoluble) bilirubin.

Bilirubin into bile into urobilinogen into urine (urobilin) and faeces (stercobilin)

Accumulation of unconjugated bilirubin causing yellowing (jaundice)

Accumulation of bile in blood causing Cholestatic jaundice and itching (pruritis).

Happy new Year... Don't drink too much or it just might be ..... "All yellow"