Enterohepatic Circulation

A number of substances that are absorbed into blood or lymph do not originate from foods but are released into the intestinal lumen endogenously. For instance, sloughed-off mucosa cells are digested and their components thereby "recycled." The same applies to most intestinal secretions. Enterohepatic circulation is a prominent example of the body's substance-sparing tendency towards recycling. Bile acids, which are essential for the digestion of fat-soluble compounds, reach the duodenum (A) as mixed micelles (see pp. 88-90). After resorption of the micellar lipids, bile acids are left behind in the small intestine. They are absorbed actively and passively later during the intestinal passage. Bacterial deconjugation and dehydroxyla-tion increase their lipid solubility, thereby facilitating passive diffusion. More than 50 % of them, however, are reabsorbed actively in the terminal ileum. This recycling is so effective that only about 0.8 g out of the total 20-30 g secreted per day is lost with fecal matter and has to be replaced by neosyn-thesis in the liver. The body's entire bile acids pool is just about 3 g. Since a critical minimum concentration is required for micelle formation, this pool is insufficient for a single high-fat meal. Digestion of larger amounts of fat is possible solely because of the long, controlled retention of fatty foods in the stomach and the effective recycling of bile acids.

Bilirubin from erythrocyte breakdown also circulates enterohepatically, but is not recycled (B). Per day, ~250-300 mg bilirubin is excreted with bile. Bilirubin is conjugated inside the hepatocytes' endoplasmic reticulum (ER) and released as glucuronide into the intestine, where it is only partially re-absorbed. In the terminal ileum and colon, bacteria convert bilirubin to urobilinogen and other compounds. About 15-20% of those are reabsorbed into the portal vein, then reach the liver and are added back to bile. A small portion of the urobilinogen reaches the systemic circulation, where it becomes subject to renal excretion. The bilirubin excreted through the stools in the form of uro-bilinogen and other compounds is responsible for the brown color of feces.

If this enterohepatic circle is interrupted, e. g., through damage to liver cells, or obstruction of biliary ducts, discoloration of stools ensues. Also, bilirubin, which is yellow, accumulates in the blood, causing "icterus," the yellow coloration of jaundice. Unconju-gated bilirubin is bound to albumin in plasma and is unable to pass through membranes in this form. At higher concentrations, however, or in case of albumin deficiency, unbound, unconjugated bilirubin may occur. This may cross the blood-brain barrier and have a toxic effect on the brain.

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