Enterohepatic Circulation

In addition to the normal constituents of bile, a wide variety of exogenous compounds (drugs) are secreted by the liver into the bile ducts (table 18.2). The liver can thus "clear" the blood of particular compounds by removing them from the blood and excreting them into the intestine with the bile. Molecules that are cleared from the blood by secretion into the bile are eliminated in the feces; this is analogous to renal clearance of blood through excretion in the urine (chapter 17).

Many compounds that are released with the bile into the intestine are not eliminated with the feces, however. Some of these can be absorbed through the small intestine and enter the hepatic portal blood. These molecules are thus carried back to the liver, where they can be again secreted by hepatocytes into the bile ducts. Compounds that recirculate between the liver and intes-

Table 18.2 Compounds Excreted by the Liver into the Bile Ducts

Category Compound Comments


Bile salts,

High percentage



reabsorbed and has an



enterohepatic circulation*


Small percentage reabsorbed

and has an enterohepatic



No enterohepatic circulation


Ampicillin, streptomycin,

High percentage reabsorbed



and has an enterohepatic


Sulfonamides, penicillin

Small percentage reabsorbed

and has an enterohepatic


*Compounds with an enterohepatic circulation are absorbed to some degree by the intestine and are returned to the liver in the hepatic portal vein.

*Compounds with an enterohepatic circulation are absorbed to some degree by the intestine and are returned to the liver in the hepatic portal vein.



■ Figure 18.22 The enterohepatic circulation. Substances secreted in the bile may be absorbed by the intestinal epithelium and recycled to the liver via the hepatic portal vein.

tine in this way are said to have an enterohepatic circulation (fig. 18.22). For example, a few grams of bile salts (discussed shortly) released into the intestine recirculate six to ten times a day, with only about 0.5 g of bile salts per day excreted in the feces.

Clinical Investigation Clue

Remember that Alan had normal blood levels of free bilirubin, ammonia, and urea.

What do these measurements suggest about the health of Alan's liver?

1 Fox: Human Physiology, Eighth Edition

1 18. The Digestive System 1 Text 1 1 © The McGraw-Hill 1

Companies, 2003

The Digestive System


Table 18.3 Major Categories of Liver Function

Functional Category Actions

Detoxication of Blood

Phagocytosis by Kupffer cells

Chemical alteration of biologically active molecules (hormones and drugs)

Production of urea, uric acid, and other molecules that are less toxic than parent compounds

Excretion of molecules in bile

Carbohydrate Metabolism

Conversion of blood glucose to glycogen and fat

Production of glucose from liver glycogen and from other molecules (amino acids, lactic acid) by gluconeogenesis Secretion of glucose into the blood

Lipid Metabolism

Synthesis of triglycerides and cholesterol Excretion of cholesterol in bile Production of ketone bodies from fatty acids

Protein Synthesis

Production of albumin

Production of plasma transport proteins

Production of clotting factors (fibrinogen, prothrombin, and others)

Secretion of Bile

Synthesis of bile salts

Conjugation and excretion of bile pigment (bilirubin)

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