Final answer:
The circular opening in the small intestine where bile and pancreatic enzymes enter is through the major duodenal papilla linked to the hepatopancreatic ampulla, controlled by the hepatopancreatic sphincter (sphincter of Oddi).
Step-by-step explanation:
The circular opening in the small intestine where bile and pancreatic enzymes enter is the major duodenal papilla, which is connected to the hepatopancreatic ampulla (ampulla of Vater). The hepatopancreatic ampulla is where the bile duct and the main pancreatic duct converge and it opens into the duodenum. It's regulated by the hepatopancreatic sphincter, also known as the sphincter of Oddi. This complex arrangement ensures that digestive juices, consisting of both bile and pancreatic enzymes, which are crucial for digesting fats, carbohydrates, and proteins, are secreted into the duodenum for further digestion.