What regulates the flow of bile and pancreatic juice into the duodenum?

• A. The sphincter of Oddi; relaxes in response to cholecystokinin and bile release.
• B. The pyloric sphincter.
• C. The ileocecal valve.
• D. Esophageal sphincter.

Answer: (A)

The action hinges on the Sphincter of Oddi, a smooth muscle ring that gates the entry of bile and pancreatic juice from the common bile duct and pancreatic duct into the duodenum at the ampulla of Vater. It stays constricted most of the time to prevent backward flow, and it relaxes when digestion is ready to proceed. When chyme rich in fats and proteins enters the duodenum, intestinal cells release cholecystokinin. This hormone, often in concert with vagal stimulation, causes relaxation of the Sphincter of Oddi and also triggers gallbladder contraction, enabling bile and pancreatic juice to flow into the duodenum to aid digestion.

This setup is specific to moving digestive secretions into the small intestine. The other structures mentioned control different parts of the GI tract: the pyloric sphincter regulates emptying of stomach contents into the duodenum, the ileocecal valve controls passage between the ileum and the colon, and the esophageal sphincter prevents reflux from the stomach into the esophagus.