8th February 2024

Biliary system

The liver produces bile, which is typically stored in the gallbladder. It comprises bile salts, bile pigments (bilirubin and biliverdin), phospholipids, cholesterol and protein. We will look at the metabolism of both bilirubin and bile salts. 

Bilirubin metabolism

Bilirubin is produced from the degradation of haemoglobin by the reticuloendothelial system. Red blood cells are broken down within the spleen, and haemoglobin is broken down into haem and globin. Haem is catabolised into bilirubin, while the globin is broken down into amino acids and recycled. Iron released from haem is also recycled. 

Bilirubin is bound to albumin and is transported to the liver. The liver conjugates bilirubin with glucuronic acid, forming conjugated bilirubin. This is done by glucuronyl transferase. The urine will directly excrete a portion of this, while some will enter the intestinal system via bile and will be deconjugated by the intestinal bacteria to form urobilinogen. 

At the terminal ileum, the enterohepatic circulation reabsorbs a portion of urobilinogen and will return to the liver. Some reabsorbed urobilinogen will re-enter bile while the kidneys will excrete the remainder.

Intestinal bacteria will break down the remainder of the urobilinogen within the large intestine into urobilin and stercobilin before being excreted by faeces.

Image Source: Degruyter (2021). almed-2021-0047, Licensed under a creative commons licence. Available for free at: https://www.degruyter.com/document/doi/10.1515/almed-2021-0047/html#APA   (Accessed: 9 January 2024).

Bile salts

Bile salts are constantly produced by the liver and are excreted by the large intestine. This consists of primary and secondary bile salts. Primary bile salts consist of cholic acid and chenodeoxycholic acid. They are synthesised from cholesterol by hepatocytes. Intestinal bacterial enzymes convert secondary bile salts from primary bile salts, consisting of deoxycholic acid and lithocholic acids.

Bile salts aim to solubilise dietary lipids by emulsifying fats into phospholipids and fatty acids. It also forms micelles to help solubilise cholesterol and fatty acids. This helps with the absorption of fatty acids, cholesterol and fat-soluble vitamins. 

CCK, gastrin and secretin stimulate the secretion of bile. CCK stimulates the contraction of the sphincter of Oddi, while gastrin and secretin stimulate the production of bile and HCO3-. Increased parasympathetic stimulation also increases the secretion of bile. 

References

Tortora, G. J., & Derrickson, B. (2017). Tortora’s principles of anatomy and physiology. John Wiley and Sons.

Kumar, P. and Clark, M., (2012). Kumar & Clark’s clinical medicine. 8th ed.

Fairweather, J., Findlay, M., & Isles, C. (2020). Clinical companion in nephrology. Springer International Publishing.