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The enterohepatic circulation involves the passage of bile, and compounds within it, from the liver to the small intestines. These compounds typically help digest molecules of fat in food, for example, are normally reabsorbed by the intestine, and are then transported back to the liver through the blood stream. Bile acids are usually absorbed by the lining of the small intestine, a process which is aided by bacteria that normally live inside it. Such organisms can also produce enzymes to break down substances such as sulfates and glycine into particles that can pass through the intestinal wall.
Cells in the liver can produce bile acids from cholesterol-based compounds to form molecules that dissolve in water. Called bile salts, these are usually passed on to the gall bladder and stored there until food begins to move into the intestine. A duct can allow bile to pass through structures called papilla in the lining of the duodenum, the beginning of the small intestine.
Further down the intestine, the bacteria can trigger molecular changes in the bile, forming types of it that can be dissolved in fat and reabsorbed. Enterohepatic circulation is completed by the passage of these and various nutrients through the portal vein up to the liver. Cells in the liver typically remove the bile acids so that almost none of them pass into the rest of the circulatory system. Bile molecules can be recycled through the enterohepatic circulation up to 20 times throughout digestion.
In addition to bile acids, cholesterol, estrogen, and substances metabolized from vitamin D are typically passed through enterohepatic circulation. Drugs such as morphine, which often get processed by the liver, can become part of the circulation as well. When antibiotics are taken, sometimes these reduce the number of intestinal bacteria, causing the enzymes produced by the organisms to decline. Drugs that enter into enterohepatic circulation are often less effective because the antibiotics don’t allow them to be absorbed.
Generally, enterohepatic circulation is regulated by the formation of bile salts from proteins and interactions with other substances such as nuclear receptors. These molecular receptors can protect other cells from bile salts that could be damaging. Other compounds can activate genes that code for the production of bile and how much is made. This process can affect whether bile levels, or those of other molecules or drugs, become toxic to the liver and the rest of the body.