The common bile duct is formed by the union of the common hepatic duct and the cystic duct.
The CBD is later joined by the pancreatic duct to form the ampulla of Vater.
There, the two ducts are surrounded by the muscular sphincter of Oddi.
When the sphincter of Oddi is closed, newly synthesized bile from the liver is stored in the gallbladder.
When the sphincter of Oddi is open, the stored and concentrated bile exits into the duodenum.
This conduction and carrying of bile is the main function of the common bile duct.
Cholecystokinin hormone is stimulated
by a fatty meal, and promotes bile secretion by increased production of hepatic bile, contraction of the gallbladder, and relaxation of the sphincter of Oddi.
A diameter of the CBD of more than 8 mm is regarded as abnormal dilatation and is a sign of cholestasis
diameter in adults:
Normal ≤ 8 mm
Mild dilatation 8 – 12 mm
Moderate dilatation 12 – 16 mm
Severe dilatation 16 – 20 mm
Extremely severe dilatation >20 mm
Following cholecystectomy the CBD normally gets slightly dilated after cholecystectomy, with upper limit being about 10 mm after a few months.
On abdominal ultrasonography, the common bile duct is generally seen best in the border area between the common hepatic duct and the CBD, by the hilum of the liver?
The absence of a Doppler signal distinguishes it from the portal vein and hepatic artery.
When obstructed by a gallstone, choledocholithiasis can result.
An obstructed CBD is especially vulnerable to ascending cholangitis.