refers to the circulation of biliary acid
, drugs, or other substances from the liver
to the bile
, followed by entry into the small intestine
, absorption by the enterocyte
and transport back to the liver.
s metabolize cholesterol
to cholic acid
and chenodeoxycholic acid
. These lipid
- soluble bile acid
s are conjugated (reversibly attached) mainly to glycine
molecules to form water soluble
primary conjugated bile acids, sometimes called "bile salts". These bile acids travel to the gall bladder during the interdigestive phase for storage and to the descending part of the duodenum
via the common bile duct
through the major duodenal papilla during digestion. 95% of the bile acids which are delivered to the duodenum will be recycled by the enterohepatic circulation.
Due to the pH of the small intestine, most of the bile acids are ionized and mostly occur as their sodium salts which are then called “primary conjugated bile salts.” In the lower small intestine and colon
, bacteria dehydroxylate some of the primary bile salts to form secondary conjugated bile salts (which are still water-soluble). Along the proximal and distal ileum
, these conjugated primary bile salts are reabsorbed actively into hepatic portal circulation. Bacteria deconjugate some of the primary and secondary conjugated bile salts back to lipid-soluble bile acids, which are passively absorbed into hepatic portal circulation. Finally, the conjugated bile acids which remained un-ionized conjugated bile acids are passively absorbed.
Venous blood from the ileum goes straight into the portal vein and then into the liver sinusoid
s. There, hepatocytes extract bile acids very efficiently, and little escapes the healthy liver into systemic circulation.
The net effect of enterohepatic recirculation is that each bile salt molecule is reused about 20 times, often multiple times during a single digestive phase.
The presence of biliary acids in the intestines helps in digestion
of fats and other substances. Lipoproteins: Lipid Digestion & Transport
by Joyce J. Diwan. Rensselaer Polytechnic Institute. Retrieved June 2012
is conjugated with glucuronic acid
in the liver by the enzyme glucuronyltransferase
, making it soluble in water. Much of it goes into the bile and thus out into the small intestine. Although 95% of the secreted bile is reabsorbed by the small intestine, conjugated bilirubin is not reabsorbed in small intestine. All conjugated bilirubin in the large intestine is metabolised by colonic bacteria to urobilinogen
, which is then further oxidized to urobilin
. Urobilin, stercobilin and their degradation products give feces its brown color. However, just like bile, some of the urobilinogen reabsorbed is resecreted in the bile which is also part of enterohepatic circulation. The rest of the reabsorbed urobilinogen is excreted in the urine where it is converted to an oxidized form, urobilin
, which gives urine its characteristic yellow color.
Enterohepatic circulation also means that some molecules which would not otherwise be very toxic can become so because of this recycling process.