Bile Secretion 

Bile

The digestive function of the liver is to produce bile. Hepatocytes secrete 500-1000mL of bile every day. Bile is a yellow/green fluid formed from water, bile salts and acids, cholesterol, bile pigments, phospholipids and ions. The rate of bile release is controlled through the hormone Secretin which is released when chyme arrives in the duodenum. Secretin causes an increase in the rate of release of bile.

Bile is used in digestion as the bile salts it contains emulsify fats by breaking down lipid globules into smaller droplets which are more easily absorbed by the body as they present a larger surface area. The principal pigment of bile is bilirubin which is formed from the breakdown of the heme group in old red blood cells. Bilirubin is secreted into bile and is broken down into urobilinogen by bacteria in the intestine.

Intrahepatic Bilary Tree

Bile is secreted by the hepatocytes and enters bile canaculi which are narrow intercelluar channels that empty into small interlobular bililary ducts. The bile progresses along these bile ductules passing into the bile ducts at the periphery of the lobules in the hepatic triads. Bile flows in the opposite direction to blood. The bile ducts merge forming the left and right hepatic ducts, which merge and exit the liver as the common hepatic duct. The common hepatic duct joins the the cystic duct from the gallbladder to form the common bile duct. Bile enters the cystic duct and is stored in the gall bladder.

The bile duct can be 5-15cm long and travels down in the lesser omentum to the duodeum. Near the duodenum the bile duct joins with the pancreatic duct which creates the hepatopancreatic ampulla. The hepatopancreatic ampulla ends in the major duodenal papilla in the duodenal wall. The papilla contains a circular muscle called the sphincter of Oddi which controls release of digestive juices and bile. When contracted bile cannot enter the ampulla and duodenum, it backs up along the cystic duct and overflows into the gallbladder.

Gallbladder

The Gallbladder is a small tear drop shaped sac roughly 7-10cm long , found in the gallbladder fossa on the posterior of the liver. It concentrates bile up to ten times and stores it, untill it is needed in the small intestine. The gallbladder has a capacity of up to 50mL of bile. It has 3 parts to it:

• The fundus is the wide end which projects down from the inferior edge of the liver.
• The body is on the visceral surface of the liver and contacts the transverse colon and the superior aspect of the duodenum.
• The neck is narrow and is directed towards the porta hepatis, it has a spirally folded lining which allows bile to easily be diverted into it.

Concentration occurs by the absorption of water and ions by the gallbladder muscosa which consists of a simple comlumnar epithelium arranged in rugae, with a middle muscular coat formed from smooth muscle fibres. When these smooth muscle fibres contract causes the ejection of bile from the gallbladder into the cystic duct. Gallstones can form due to crystallization of cholesterol, causing minimal or complete blockage of the flow of bile. These can easily be treated with drugs, surgery or shock wave therapy.

Jaundice

Jaundice is recognised by the yellowing of the skin and whites of the eye. This is caused by a build up of bilirubin which is after it is formed is transported to the liver where it is made into bile and excreted. Prehepatic jaundice is caused by excess bilirubin production. Hepatic jaundice is due to diseases of the liver such as cirrhosis and extrahepatic jaundice is caused when there is a blockage in the drainage of bile by gall stones or diseases such as cancer.

Figure: A male patient with Jaundice

Image courtesy of https://upload.wikimedia.org/wikipedia/commons/a/ad/Jaundice2008.jpg This image is in the public domain and thus free of any  copyright restrictions.