The Enterohepatic Circulation Recycles Bile Salts Between the Small Intestine and the Liver

The enterohepatic circulation of bile salts is the recycling of bile salts between the small intestine and the liver. The total amount of bile acids in the body, primary or secondary, conjugated or free, at any time is defined as the total bile acid pool. In healthy people, the bile acid pool ranges from 2 to 4 g. The enterohepatic circulation of bile acids in this pool is physiologically extremely important. By cycling several times during a meal, a relatively small bile acid pool can provide the body with sufficient amounts of bile salts to promote lipid absorption. In a light eater, the bile acid pool may circulate 3 to 5 times a day,- in a heavy eater, it may circulate 14 to 16 times a day. The intestine is normally extremely efficient in absorbing the bile salts by carriers located in the distal ileum. Inflammation of the ileum can lead to their malabsorption and result in the loss of large quantities of bile salts in the feces. Depending on the severity of illness, malabsorption of fat may result.

Bile salts in the intestinal lumen are absorbed via four pathways (Fig. 27.16). First, they are absorbed throughout the entire small intestine by passive diffusion, but only a small fraction of the total amount of bile salts is absorbed in this manner. Second, and most important, bile salts are absorbed in the terminal ileum by an active carrier-mediated process, an extremely efficient process in which usually less than 5% of the bile salts escape into the colon. Third, bacteria in the terminal ileum and colon deconjugate the bile salts to form bile acids, which are much more lipophilic than bile salts and, thus, can be absorbed passively. Fourth, these same bacteria are responsible for transforming the primary bile acids to secondary bile acids (deoxycholic and lithocholic acids) by dehydroxylation.. Deoxycholic acid may be absorbed, but lithocholic acid is poorly absorbed.

Liver

Liver

Cholesterol Enterohepatic Circulation

~500 mg bile acids lost daily in feces

Colon

~500 mg bile acids lost daily in feces

Conjugated bile salts

Small intestine

Colon

Deoxycholic Free acid bile Lithocholic acids acid

Terminal ileum

Cecum

The enterohepatic circulation of bile salts.

Bile salts are recycled out of the small intestine in four ways: (1) passive diffusion along the small intestine (plays a relatively minor role); (2) carrier-mediated active absorption in the terminal ileum (the most important absorption route); (3) de-conjugation to primary bile acids before being absorbed either passively or actively; (4) conversion of primary bile acids to secondary bile acids with subsequent absorption of deoxycholic acid.

Although bile salt and bile acid absorption is extremely efficient, some salts and acids are nonetheless lost with every cycle of the enterohepatic circulation. About 500 mg of bile acids are lost daily. They are replenished by the synthesis of new bile acids from cholesterol. The loss of bile acid in feces is, therefore, an efficient way to excrete cholesterol.

Absorbed bile salts are transported in the portal blood bound to albumin or high-density lipoproteins (HDLs). The uptake of bile salts by hepatocytes is extremely efficient. In just one pass through the liver, more than 80% of the bile salts in the portal blood is removed. Once taken up by hepatocytes, bile salts are secreted into bile. The uptake of bile salts is a primary determinant of bile salt secretion by the liver.

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Responses

  • Caden
    How are bile acids cycled through the enterohepatic ciculation?
    5 years ago
  • michele
    What is the blood flow between large intestine and the liver that bile to recycled?
    5 years ago
  • adolfa
    Why are salts important for skeletal muscle?
    5 years ago
  • Caio
    Why Is Enterohepatic Circulation Important?
    4 years ago
  • kia hasti
    How much distal ileum is needed for enterohepatic circulation?
    3 years ago
  • tommi koponen
    What is enterohepatic circulation and its significance?
    9 months ago
  • grant
    What happens during the entero hepatic circulation?
    3 months ago

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