Why can blockage of the common bile duct enlarge the gallbladder?

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An anatomical illustration of the human pancreas shows its location near the stomach and intestines, a detailed cross-section highlighting ducts and the islet of Langerhans, and an inset of pancreatic tissue histology with labeled alpha, beta, and delta cells. — pancreatic cancer The human pancreas is composed of two main types of tissue: acinar cells and islets of Langerhans. The acinar cells produce digestive enzymes, which are secreted into tiny ducts that feed into the pancreatic duct. Islets of Langerhans are clusters of cells that secrete hormones such as insulin and glucagon directly into a capillary network, which also joins the pancreatic duct.

When a tumor in the head of the pancreas causes a blockage of the common bile duct, it prevents bile from flowing normally into the duodenum.12

Under normal conditions, bile produced by the liver travels through the hepatic and common bile ducts. When the small intestine is empty, the sphincter of Oddi closes, forcing bile to back up into the cystic duct and into the gallbladder for storage.1

When the common bile duct is obstructed by a pancreatic mass, bile cannot exit into the intestine and instead accumulates within the biliary system. This persistent backup of bile increases pressure within the ducts and the gallbladder, leading to its distension or enlargement. This obstruction often manifests clinically as obstructive jaundice.12