What is Bile?

The liver is one of the largest and most important organs. It is responsible for many of the metabolic functions that keep the body operational, including breaking down and converting substances, extracting energy, and removing toxins. Hepatocytes, special liver cells that filter blood and direct many of these metabolic functions, are also in charge of the production and secretion of bile. An average adult liver creates between 400 and 800 milliliters of bile each day.

Bile is a highly complex and variable secretion that consists of many organic and inorganic solutes. At formation, water makes up around 94 to 97 percent of bile. Around one percent of the remaining products are bile salts, and around 0.5 percent are fats such as cholesterol, lecithin, and fatty acids. The remainder is mostly the waste compound bilirubin and inorganic salts. At various moments, bile may also contain hepatocyte proteins, peptides, amino acids, vitamins, heavy metals, and electrolytes.

Bile performs two critically important functions: it aids digestion and excretes waste products. Bile acids are derivatives of hepatocyte functions. Cholesterol enters the body and undergoes oxidation to become bile acids. Hepatocytes then temporarily merge these acids with amino acids to create conjugated bile acids or bile salts. Bile acids are responsible for breaking down fat globules into microscopic droplets as part of emulsification. This makes the fats more easily digestible. Bile acids also carry lipids and fat-soluble vitamins.

Though it is not one of its main functions, bile also helps the body absorb vitamins. It carries fat-soluble A, D, E, and K, which the body absorbs using the same mechanisms that it uses to absorb lipids. By breaking down fat globules and making them digestible, bile has the additional effect of increasing the number of fat-soluble vitamins the body can absorb.

One of the main waste compounds that bile helps the body eliminate is bilirubin. Red blood cells contain the red pigment hemoglobin, which in turn contains heme. When the body begins to break down red blood cells, it strips heme molecules from the hemoglobin. Then, through various processes, the heme becomes biliverdin. Though biliverdin can have many effects, the body usually breaks it down further into bilirubin. The yellow pigment in bilirubin is responsible for the yellow coloration of bruises and jaundice.

The path that bile takes from the liver to the small intestine is the biliary tract. At the beginning of bile transport, hepatocytes secrete bile into small ducts known as the bile canaliculi. These ducts merge and form bile ductules which eventually become the common hepatic duct. The common hepatic duct leaves the liver and connects to the cystic duct of the gallbladder. The cystic duct and the common hepatic duct meld together to create the common bile duct. Eventually, the common bile duct connects to the pancreatic duct. These ducts pass through the ampulla of Vater and enter the duodenum of the small intestine.

During the transport process, the body stores bile in the gallbladder. Sitting just below the liver, the gallbladder is a small, hollow organ with a pear-like shape. Bile enters the gallbladder through the common hepatic duct and remains there until the body digests food. At any moment, the gallbladder contains 30 to 60 milliliters of bile. When food enters the digestive tract, the duodenum secretes the hormone cholecystokinin. In response to this hormone, the gallbladder contracts and releases bile through the common bile duct.

As the body transports it through the biliary tract, the bile composition changes. Hepatic bile from the liver is primarily water with varying amounts of other substances. Bile in the gallbladder changes as the organ removes water and electrolytes, which makes the bile three to ten times more concentrated. At this point, the substance is primarily cholesterol and waste substances such as bilirubin. Without water, bile takes on a sludge-like texture.

Rarely, the bile ducts of the biliary tract can be obstructed, causing a blockage of bile and an accumulation of bilirubin in the bloodstream. The most common side effect of this is jaundice, skin or eyes with yellow coloration. There are many ways a bile duct can become obstructed, including cancer, physical trauma, cysts, and benign conditions that cause the bile ducts to narrow. The most common causes of bile duct blockages are gallstones.

Sometimes, the lack of water within the bile of the gallbladder causes the bile to form stone-like objects or gallstones. Around 80 percent of people with gallstones never experience any symptoms. However, if a gallstone leaves the gallbladder, it can block a bile duct. This can have various side effects such as cholecystitis, pancreatitis, or bile duct infections. Gallstones that are primarily cholesterol are cholesterol stones, while gallstones that are primarily bilirubin are pigment stones. Factors that increase the likelihood of gallstones include obesity, diabetes, liver disease, weight loss, pregnancy, and birth control pills.