Bile secretion and enterohepatic circulation

Bile is a greenish liquid that’s made by the liver and is stored in the gallbladder.

Bile is a bit like an alkaline soup and it’s ingredients include a variety of organic molecules.

Bile does a number of things including helping with digestion, absorption of fats, and excretion of various molecules.

Normally, lipids are insoluble in water, so that’s why bile is needed to help emulsify and solubilize them.

The organic composition of bile is mainly made up of bile salts and phospholipids, with cholesterol, and bile pigments called bilirubin, making up only a small percentage.

Bile is first manufactured in the liver by cells called hepatocytes.

Hepatocytes use an enzyme called 7-alpha-hydroxylase to convert cholesterol into two primary bile acids, called cholic acid and chenodeoxycholic acid.

In the intestines, some of these primary bile acids get dehydroxylated, giving rise to secondary bile acids - deoxycholic acid and lithocholic acid.

The liver can conjugate, or attach the amino acids glycine or taurine to all four of these bile acids and this ultimately gives rise to 8 different forms of bile salts. And it’s these bile salts that are the main component of bile.

So bile is made in the liver and flows into the intestines.

The journey starts when bile flows into the left and right hepatic ducts which eventually merge to form the common hepatic duct.

The common hepatic duct then leads to the cystic duct which brings the bile to the gallbladder.

The gallbladder is a small pear-shaped hollow organ located beneath the liver, and this is where bile is stored and becomes more concentrated.

Approximately 30 minutes after consuming a meal, the food is broken down into a slurry called chyme, and that chyme begins to enter the first part of the small intestine - the duodenum. When that happens, I-cells, which are in the mucosal lining of the intestine secrete a hormone called cholecystokinin, or CCK, into the bloodstream.

Cholecystokinin travels through the blood and reaches the gallbladder, causing it to contract really strongly. Those strong contractions cause the stored bile to be get squirted out of the gallbladder and into the common bile duct.

The bile ducts are lined by cells called cholangiocytes, which secrete bicarbonate-rich fluid into the biliary tree.