Liver and intrahepatic bile ducts - nontumor
Normal physiology

Topic Completed: 1 March 2012

Revised: 16 March 2020

Copyright: 2002-2020,, Inc.

PubMed Search: Liver[title] physiology[title] normal

Komal Arora, M.D.
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Table of Contents
Definition / general
Cite this page: Arora K. Normal physiology. website. Accessed March 31st, 2020.
Definition / general
  • Liver produces about 500 ml/day of bile
  • Promotes dietary fat absorption via detergent action of bile salts; eliminates waste products (bilirubin, excess cholesterol, xenobiotics) that are insufficiently water soluble to be excreted into urine

Bile acids:
  • Carboxylate steroid molecules, derived from cholesterol, that promote bile flow and secretion of phospholipid and cholesterol
  • Primary bile acids are cholic acid and chenodeoxycholic acid, secreted as taurine and glycine conjugates; two secondary ones (deoxycholic and lithocholic acid) are formed in the colon by bacterial action

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Bile acids

Bile acid circulation:
  • All bile acids are reabsorbed via sodium bile acid cotransporter in apical membrane of ileal enterocytes and transported back to liver
  • Enterohepatic circulation of bile acids maintains a large endogenous pool of bile acids for digestive and excretory purposes

  • Also called icterus
  • Discoloration of skin and sclera due to disruption of bile formation by either retention of pigmented bilirubin or block in bilirubin secretion (cholestasis)

  • End product of heme degradation
  • 200 mg produced daily from old red blood cells broken down via monocyte phagocytic system in spleen, liver, marrow
  • Also from turnover of P450 cytochromes and premature destruction of marrow red blood cells (with ineffective erthyropoiesis)
  • Red blood cells are broken down and produce hemoglobin
  • Globin proteins are removed, leaving heme molecule
  • Heme is converted to biliverdin via heme oxygenase
  • Biliverdin is converted to bilirubin via biliverdin reductase
  • Bilirubin is bound to serum albumin since it is insoluble in blood at physiologic pH; the % unbound increases in severe hemolytic disease or if protein binding drugs displace bilirubin
  • Hepatocytes take in bilirubin at sinusoidal membrane, conjugate it with glucuronic acid using bilirubin uridine diphosphate glucuronosyltransferase (UGT) in endoplasmic reticulum, then bilirubin is excreted into bile
  • Bacteria have beta glucuronidases which deconjugate and degrade bilirubin to colorless urobilinogens, excreted in feces as stercobilin, a brown pigment that colors stools
  • 20% of urobilinogens are reabsorbed in ileum / colon, returned to liver, reexcreted into bile
  • Some urobilinogen is reabsorbed and excreted by the kidney, oxidized to urobilin - a pigment that gives amber color to urine
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