View source for Enterohepatic circulation

[[Image:Enterohepatic.JPG|thumb|right| Enterohepatic Circulation.]]
{{confused2|the [[hepatic portal system]] which directs nutrient rich blood from the intestines to the liver}}

'''Enterohepatic circulation''' refers to the circulation of [[biliary acid]]s from the [[liver]], where they are produced and secreted in the [[bile]], to the [[small intestine]], where it aids in [[digestion]] of fats and other substances, back to the liver.<ref name="urlSect. 6, Ch. 5: Enterohepatic Circulation of Bile Acids">{{cite web |url=http://www.lib.mcg.edu/edu/eshuphysio/program/section6/6ch5/s6ch5_17.htm |title=Sect. 6, Ch. 5: Enterohepatic Circulation of Bile Acids |format= |work= |accessdate=}}</ref> 

[[Endogenous]] bacteria play an important role in enterohepatic circulation.<ref name="urlMetabolic Activities of the Microflora">{{cite web |url=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?indexed=google&rid=mmed.section.5106 |title=Metabolic Activities of the Microflora |format= |work= |accessdate=}}</ref>

Hepatocytes [[metabolize]] [[cholesterol]] to [[cholic acid]] and [[chenodeoxycholic acid]]. These [[lipid]]-[[soluble]] [[bile acid]]s are conjugated mainly to glycine or taurine 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 (anatomy)|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, [[hepatocyte]]s extract bile acids very efficiently, and little escapes the healthy liver into systemic circulation.  If bile does escape, [[jaundice]] may be observed.

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.

==Drugs==
Enterohepatic circulation also means that some molecules which would not otherwise be very toxic can become extremely [[Hepatotoxicity|hepatotoxic]] as they reach unexpectedly high hepatic concentrations. [[medication|Drugs]] may remain in the enterohepatic circulation for a prolonged period of time as a result of this recycling process.

==See also==
*[[Enteroenteric circulation]]

==References==
{{reflist}}

==External links==
* {{GPnotebook|1040580666}}

{{Gastrointestinal physiology}}
[[Category:Hepatology]]
[[Category:Cardiovascular system]]
[[Category:Digestive system]]

[[de:Enterohepatischer Kreislauf]]
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[[ja:腸肝循環]]
[[ru:Кишечно-печёночная циркуляция желчных кислот]]
[[fi:Enterohepaattinen kierto]]
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