The liver is the largest organ in the abdominal cavity and is located under the right and central portions of the diaphragm. It performs over two hundred functions including digestive, metabolic, storage, and other functions. This reddish-brown organ consists of two major lobes, the right lobe and the left lobe, and two smaller lobes, the caudate lobe and the quadrate lobe.
Lying under the right lobe is the gallbladder, a muscular sac that is anatomically and physiologically associated with the liver. Emerging from the gallbladder is the cystic duct.
The lobes contain liver cells (hepatocytes), which secrete bile, an alkaline , yellow-green liquid that is composed of water, bile salts, and several other substances. Bile is delivered to the duodenum, the first portion of the small intestine, where the bile salts emulsify lipids ; that is, break down large lipid globules into small droplets, in order to increase the efficiency of lipid digestion and absorption by the small intestine.
The hepatocytes secrete bile into numerous tiny ducts, which merge to form progressively wider ducts. These ducts ultimately merge to form the common hepatic duct, which descends from the liver. This duct merges with the gallbladder's cystic duct to form the bile duct, which opens into the duodenum. The opening is guarded by a sphincter , a circular muscle that is usually closed. Since the sphincter is usually closed, bile flowing down from the liver is prevented from entering the duodenum and, consequently, backs up via the cystic duct into the gallbladder.
Within the gallbladder, bile is stored and concentrated until it is expelled, when needed, via the cystic and bile ducts into the duodenum. Expulsion of bile occurs due to the simultaneous contraction of the gallbladder walls and relaxation of the sphincter guarding the entrance to the duodenum.
In addition to producing bile for the emulsification of dietary lipids, the liver also plays an important role in the maintenance of normal blood glucose concentration, inactivation of toxins, synthesis of plasma proteins , and the metabolism of carbohydrates , fats, and proteins.
Saladin, Kenneth S. Anatomy & Physiology: The Unity of Form and Function, 2nd ed. New York: McGraw-Hill, 2001.