The secretion of bile into the duodenum varies throughout the day. During fasting, the smooth muscle of the gallbladder and biliary tree exhibits activity similar to the interdigestive MMC seen in the GI tract. Approximately every 2 H a contraction of the gallbladder expresses about 10 ml bile into the duodenum in the fasted individual.
Gallbladder contraction within a few minutes of starting eating and bile output reaches a maximum at about 1 H after ingestion of food. Thereafter it reaches a plateau and remains constant until gastric emptying has been completed. The initial rise is due to gallbladder contraction that empties about two-thirds of the bile stored. Thereafter, during the plateau phase there is a constant secretion of bile by the liver that bypasses the gallbladder and enters the small bowel through a relaxed sphincter of Oddi. Owing to the enterohepatic circulation, it is estimated that the total pool of bile salts is re-circulated two-to-three times every meal.
Figure GI.12 Physiology of bile
The initial gallbladder contraction in response to a meal is vagally mediated. There are also enteric nervous system reflexes that stimulate gallbladder contraction in response to gastric distension. Biliary secretion is affected by the following factors:
• Chlolecystokinin is the main hormone controlling biliary secretion. The presence of intraluminal fat in the duodenum stimulates the release of cholescystokinin which causes relaxation of the sphincter of Oddi and gallbladder contraction
• Biliary secretion is also stimulated by increasing concentrations of bile salts returning to the liver in the portal circulation which occurs in the post prandial phase due to the enter-hepatic circulation
• Secretin released in response to acid in the duodenum increases the production of watery alkaline bile and probably augments the action of cholecystokinin
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