흡수된 음식물의 흡수와 enterohepatic circulation(장간 순환)

[문형철]

인체에서 탄수화물, 단백질은 소장에서 흡수되어(장), 혈액내에서 사용되거나 

간으로 들어간다. 

지방은 소장에서 흡수되어 

소장과 연결된 림프로 들어가, 흉관을 경유하여 정맥혈관계로 들어간다. 혈액내에서 세포를 만나 지방산과 글리세롤로 분해되어 지방산은 에너지로 사용된다. 

여기서 중요한 개념 enterohepatic circulation개념에 대해서 알아보자.

콜레스테롤은 장간순환과정에서 하루에 0.5g만 대변으로 빠져나감. 

Enterohepatic circulation refers to the circulation of biliary acids, bilirubin, 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.

The circuit[edit]

Hepatocytes metabolize cholesterol to cholic acid and chenodeoxycholic acid. These lipid-soluble bile acids are conjugated (reversibly attached) 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, 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 sinusoids. 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.

Function[edit]

The presence of biliary acids in the intestines helps in digestion of fats and other substances.^[1]

Bilirubin[edit]

Bilirubin 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 and stercobilin. Urobilin, stercobilin and their degradation products give feces its brown color. ^[2] 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.

Drugs[edit]

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

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