The stomach is where food is initially broken down for the purpose of digestion, after which it travels into the first part of the small intestine, commonly referred to as the duodenum. In the duodenum, digestive juices (secretions) are added that further process the food into usable energy for the body. These secretions come via the biliary tree.
The biliary tree is a system of vessels that directs these secretions from the liver, gallbladder and pancreas through a series of ducts into the duodenum. The exit hole into the duodenum is called the papilla of Vater. (Because of the structure of the connections to each organ, the biliary tree, or tract, bears some resemblance to a tree and its branches, hence the name.)
Breaking down fats —also known as lipids, fats are very concentrated forms of energy essential for proper growth and maintenance of the body— is an important part of the digestive process. However, lipids that are still present in foods that have been passed from the stomach to the duodenum are quite difficult to dissolve.
The liver, the second-largest organ in the body, creates bile to help dissolve lipids. Bile is a yellow, bitter liquid —mostly water, but also containing bile salts— that dissolves fats so that the its energy can be absorbed by the body. Bile also reduces the acidity of the contents of the stomach once it enters into the duodenum.
The gallbladder is a small organ attached to the liver that stores the bile for use when it's needed. Unfortunately, there is a form of cholesterol in bile that can create abnormal, stone-like masses called gallstones. Gallstones can then get caught in the curvature of the cystic duct, a channel that connects the gallbladder to the biliary tree. This obstruction can cause a great amount of discomfort for patients.
The pancreas, an organ situated near the stomach that is approximately the same size and shape as a small banana, uses the pancreatic duct to supply pancreatic juices that also assist with digestion. These juices, or secretions, are referred to as exocrine juices. The biliary tree can also be obstructed by gallstones to the extent that the pancreas is unable to supply exocrine juices to the duodenum. This can result in a very dangerous condition called pancreatitis.