in obstructive jaundice ,how the bile duct get obstucted ?plz give me a reasons

Bile is the exocrine secretion of the liver. It is secreted by hepatocytes. The bile produces flows directly from the liver into the duodenum through a system of ducts, ultimately draining into the common bile duct. Bile duct obstruction is a blockage that occurs in the tubes that carry bile from the liver to the gall bladder and the small intestine. This condition can occur at various levels in the biliary system. The failure of bile to reach its proper destination may be because of mechanical means or by metabolic factors.  Gallstone disease is the most common cause of biliary obstruction. The bile ducts must have become narrow because of formation of scars, or inflammation in the bile ducts, Tumours in bile ducts or pancreas, enlarged lymph’s  in the porta hepatitis are also the possible reasons for the obstruction of bile duct.