The liver, the largest internal organ in the body, is situated in the right hypochondrium. Its upper border lies between the fifth and sixth ribs and its lower border can sometimes be palpated below the right costal margin on inspiration. The liver is divided into two main lobesright and left. The right is larger and also contains the quadrate and caudate lobes. Riedel’s lobe is an extension of the lateral portion of the right lobe and it can occasionally be palpated in a normal abdomen. The blood supply to the liver, constituting 25% of the resting cardiac output, is via two main vessels:
1 The hepatic artery, which is a branch of the coeliac axis, supplies 25% of the total blood flow and 50% of the oxygen.
2 The portal vein drains most of the gastrointestinal tract and the spleen. It supplies 75% of the blood flow but only 50% of the total oxygen supply.
Both vessels enter the liver at the porta hepatis and the blood is distributed via the portal tracts into the sinusoids throughout the liver. The functional component of the liver is the acinus. This consists of parenchyma supplied by the smallest portal tracts containing portal vein radicles, hepatic arterioles and bile ductules . The hepatocytes near this triad (zone 1) are well supplied with oxygenated blood and are more resistant to damage than the cells nearer the central veins (zone 3). Blood passes from the portal tract via the sinusoids to the terminal hepatic vein (central vein). These sinusoids are lined by specialized endothelial cells, Kupffer’s cells (phagocytic cells) and fatstorage cells (Ito cells) and are separated by plates ofliver cells (hepatocytes). The potential space that lies between the sinusoids and hepatocytes is the space of Disses.
Bile canaliculi form a network between the hepatocytes. These join to form thin bile ductules near the portal tract, which in turn enter the bile ducts in the portal tracts. These then combine to form the right and left hepatic ducts that leave each liver lobe. The hepatic ducts join at the porta hepatis to form the common hepatic duct. The cystic duct connects the gallbladder to the lower end of the common hepatic duct. The gallbladder lies under the right lobe of the liver and stores and concentrates hepatic bile; it has a capacity of approximately 50 ml, The common bile duct is formed by the combination of the cystic and hepatic ducts and is approximately 8 mm in diameter, narrowing at its distal end to pass into the duodenum. The common bile duct and pancreatic duct open into the second part of the duodenum through a common channel at the ampulla of Vater. The lower end of the common bile duct contains the muscular sphincter of Oddi, which contracts rhythmically and prevents bile entering the duodenum in the fasting state.