The digestive system's accessory organs include the liver and gallbladder. The liver, a solid organ that sits just below the diaphragm in the upper right side of the abdomen, is derived from the endoderm germ layer formed during fetal development. One of the liver's functions is to make bile, a greenish digestive fluid that emulsifies fats in the small intestine. Bile is primarily produced by the chief functional cells of the liver, called hepatocytes. Bile's three main components are cholesterol, bilirubin, and bile salts.
Once the liver makes bile, about half is stored and concentrated in the gallbladder. The hormone cholecystokinin (CCK) triggers the gallbladder to contract and push bile into the small intestine when food is present. Bile salts mechanically digest lipids through a process called emulsification. The liver connects to a series of ducts called the biliary tree, which includes the bile ducts in and out of the liver and the common bile duct that leads to the exocrine pancreas.
<ul> <li>Liver and Bile <ul> <li>A digestive function of the liver is bile production</li><ul> <li>Three main components of bile: cholesterol, bilirubin, and bile salts</li></ul> <li>Hepatocytes: chief functional cells of the liver</li> </ul> </li> <li>Gallbladder <ul> <li>Function: storing and concentrating bile</li><ul> <li>Cholecystokinin (CCK) triggers gallbladder contraction and bile release</li></ul> </ul> </li> <li>Emulsification and digestion of lipids <ul> <li>Bile salts aid in mechanical digestion</li> <li>Emulsification increases surface area of lipids</li> </ul> </li> <li>Biliary tree <ul> <li>Series of ducts connecting liver and pancreas</li> <li>Includes bile ducts and common bile duct</li> </ul> </li> </ul>
The liver is responsible for producing bile, an alkaline compound that aids in the digestion and absorption of fats and fat-soluble vitamins. Hepatocytes, the main functional cells in the liver, synthesize bile constituents, including bile salts, bilirubin, and cholesterol. Bile is then stored and concentrated in the gallbladder until it is needed for digestion. When fatty food enters the small intestine, the hormone cholecystokinin is released, causing the gallbladder to contract and release bile into the biliary tree, eventually reaching the duodenum for emulsification of fats.
Bile salts are formed through the breakdown of cholesterol in the liver. They are essential components of bile that play a crucial role in the digestion and absorption of dietary fats. Bile salts function as biological surfactants, reducing the surface tension at the oil-water interface of fat droplets in the small intestine, which leads to the emulsification of fats. This emulsification process increases the surface area of fat droplets, enabling lipase enzymes to efficiently break down fats into absorbable molecules like fatty acids and monoglycerides.
Bilirubin is a waste product formed from the breakdown of heme, a component of hemoglobin in red blood cells. The liver plays a crucial role in processing bilirubin. In the liver, bilirubin is conjugated with glucuronic acid to make it more water-soluble. This conjugated bilirubin is then excreted from hepatocytes into bile and eventually transported to the small intestine. In the intestine, some of the bilirubin is converted into urobilinogen by gut bacteria and excreted in feces, giving stool its characteristic brown color, while a small amount is reabsorbed and excreted through the kidneys in urine.
The biliary tree is a network of small ducts that transport bile from the liver cells to the gallbladder and eventually the small intestine. The bile produced by hepatocytes is first secreted into structures called bile canaliculi, which merge to form small bile ducts. These small bile ducts eventually join to form the larger hepatic ducts that carry bile from the liver. The hepatic ducts join to create the common bile duct, which connects to the gallbladder's cystic duct. Here, bile is stored and concentrated until it is needed for digestion. When the gallbladder contracts, bile travels through the cystic duct, merging with the common bile duct, and enters the small intestine via the ampulla of Vater for fat emulsification.
Cholecystokinin (CCK) is a hormone produced by the cells lining the small intestine, mainly in response to the presence of fats and protein-rich food in the stomach. CCK plays a critical role in gallbladder function by stimulating the gallbladder to contract and release bile into the biliary tree. Furthermore, CCK promotes the relaxation of the sphincter of Oddi, a muscular valve that controls the flow of bile and pancreatic secretions into the small intestine. These actions help ensure that bile is available for the digestion and absorption of fats in the small intestine.