What percentage of the bile acid pool is reabsorbed in the ileum through the enterohepatic circulation?a. 25%
Bile is mainly composed of water, electrolytes, bile salts, proteins, lipids, and bile pigments. Sodium, potassium, calcium, and chlorine have the same concentration in bile as in plasma or extracellular fluid. The pH of hepatic bile is usually neutral or slightly alkaline, but varies with diet; an increase in protein shifts the bile to a more acidic pH. The primary bile salts, cholate and chenodeoxycholate, are synthesized in the liver from cholesterol. They are conjugated there with taurine and glycine and act within the bile as anions (bile acids) that are balanced by sodium. Bile salts are excreted into the bile by the hepatocyte and aid in the digestion and absorption of fats in the intestines. In the intestines, about 80% of the conjugated bile acids are absorbed in the terminal ileum. The remainder is de-hydroxylated (de-conjugated) by gut bacteria, forming secondary bile acids deoxycholate and lithocholate. These are absorbed in the colon, transported to the liver, conjugated, and secreted into the bile. Eventually, about 95% of the bile acid pool is reabsorbed and returned via the portal venous system to the liver, the so-called enterohepatic circulation. Five percent is excreted in the stool, leaving the relatively small amount of bile acids to have maximum effect.