Bile Formation and Composition
The liver produces bile continuously and excretes it into the bile canaliculi. The normal adult consuming an average diet produces within the liver 500 to 1000 mL of bile a day. The secretion of bile is responsive to neurogenic, humoral, and chemical stimuli. Vagal stimulation increases secretion of bile, whereas splanchnic nerve stimulation results in decreased bile flow. Hydrochloric acid, partly digested proteins, and fatty acids in the duodenum stimulate the release of secretin from the duodenum that, in turn, increases bile production and bile flow. Bile flows from the liver through to the hepatic ducts, into the common hepatic duct, through the common bile duct, and finally into the duodenum. With an intact sphincter of Oddi, bile flow is directed into the gallbladder.
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. In the intestines, about 80% of the conjugated bile acids are absorbed in the terminal ileum. The remainder is dehydroxylated (deconjugated) 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.
Cholesterol and phospholipids synthesized in the liver are the principal lipids found in bile. The synthesis of phospholipids and cholesterol by the liver is, in part, regulated by bile acids. The color of the bile is due to the presence of the pigment bilirubin diglucuronide, which is the metabolic product from the breakdown of hemoglobin, and is present in bile in concentrations 100 times greater than in plasma. Once in the intestine, bacteria convert it into urobilinogen, a small fraction of which is absorbed and secreted into the bile.
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Motor Activity
Gallbladder filling is facilitated by tonic contraction of the sphincter of Oddi, which creates a pressure gradient between the bile ducts and the gallbladder. During fasting, the gallbladder does not simply fill passively. In association with phase II of the interdigestive migrating myenteric motor complex in the gut, the gallbladder repeatedly empties small volumes of bile into the duodenum. This process is mediated at least in part by the hormone motilin. In response to a meal, the gallbladder empties by a coordinated motor response of gallbladder contraction and sphincter of Oddi relaxation. One of the main stimuli to gallbladder emptying is the hormone cholecystokinin (CCK). CCK is released endogenously from the duodenal mucosa in response to a meal.7 When stimulated by eating, the gallbladder empties 50 to 70% of its contents within 30 to 40 minutes. Over the following 60 to 90 minutes, the gallbladder gradually refills. This is correlated with a reduced CCK level. Other hormonal and neural pathways also are involved in the coordinated action of the gallbladder and the sphincter of Oddi. Defects in the motor activity of the gallbladder are thought to play a role in cholesterol nucleation and gallstone formation.
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