Bile Contains Phospholipids and Cholesterol Maintained in Aqueous Solution by the Detergent Action of Bile Acids

Hepatocytes form bile acids from cholesterol. The chemical changes necessary to convert cholesterol to cholic acid, a representative bile acid, are shown in Figure 29-6. Cholesterol is almost totally insoluble in water, but the chemical changes involved in the conversion of cholesterol to bile acids result in a molecule with a water-soluble (hydrophilic, or “water-loving”) side and a lipid-soluble (hydrophobic, or “water-hating”) side.

Bile acids are produced in the smooth endoplasmic retic­ulum of the hepatocytes. As they are secreted from the cells into the lumen of the canaliculi, bile acids “dissolve away” some of the cell membrane components: phospholipids and cholesterol. These constituents—phospholipids, cholesterol, and bile acids—are the major functional components of bile and are important for the digestion and absorption of fats. The mechanism by which bile aids in fat digestion is discussed in Chapter 30.

Bile acids are secreted into the canaliculi as their sodium salts. The presence of bile acid salts and sodium in the cana­liculi draws water, by osmosis, from the cells into bile. The electrolyte composition of canalicular bile usually resembles that of plasma but may be somewhat lower in chloride. As bile flows through the bile ducts, water and electrolytes are added. Bicarbonate may be secreted by the duct cells, so the bicarbonate concentration of bile is often higher than that in blood serum.

In addition to bile acids, phospholipids, and cholesterol, bile contains other lipid-soluble organic substances. Of these, the bile pigments are present in the highest concentration. Bile pigments are breakdown products of heme porphyrin, a portion of the hemoglobin molecule. The principal bile pigment is bilirubin, which is produced during the normal process of red blood cell turnover. Bilirubin gives bile its characteristic green color. In the lumen of the gut, bilirubin is converted by bacterial action to other compounds. These secondary compounds are responsible for the characteristic brown color of the feces of nonherbivorous animals. Bile pigments serve no useful digestive function: the body simply uses bile, and ultimately feces, as a route for the excretion of these waste products.

The liver serves as an excretory organ for many lipid- soluble substances in addition to bilirubin. The detergent action of the bile acids makes the liver an ideal excretory organ, in comparison with the kidney, for these types of compounds. Substances metabolized and secreted by the liver include many important drugs and toxins. This is important clinically because the actions of these agents can be potentiated by impaired liver function.