Final answer:
The duodenum releases hormones like intestinal gastrin, secretin, CCK, and gastric inhibitory peptide, which are part of the biologically active proteins involved in digestion. The pyloric sphincter is the structure that regulates the movement of stomach contents into the duodenum.
Step-by-step explanation:
The duodenum releases a variety of biologically active proteins, including hormones like intestinal (enteric) gastrin, secretin, cholecystokinin (CCK), and gastric inhibitory peptide. These hormones are crucial in regulating digestive processes such as gastric juice secretion, pancreatic juice secretion, and bile release.
The sphincter that opens at appropriate intervals to allow stomach contents to pass into the duodenum is the pyloric sphincter. This sphincter acts as a gateway, controlling the passage of chyme from the stomach into the duodenum.