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Structure and function of the liver, biliary tract, and pancreas 

Structure and function of the liver, biliary tract, and pancreas

Chapter:
Structure and function of the liver, biliary tract, and pancreas
Author(s):

Alexander Gimson

and Simon M. Rushbrook

DOI:
10.1093/med/9780199204854.003.1519
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date: 24 March 2017

Liver and biliary tract

The liver, sited in the right upper quadrant of the abdomen, comprises eight segments, each of which is a complete functional unit with a single portal pedicle and a hepatic vein. Within the functional segments, the structural unit is the hepatic lobule, a polyhedron surrounded by four to six portal tracts containing hepatic arterial and portal venous branches from which blood perfuses through sinusoids, surrounded by walls of hepatocytes that are a single cell thick and lined by specialized endothelial cells with ‘windows’ (fenestrae), to the centrilobular region and the central hepatic veins. Bile secreted through the canalicular membrane of the hepatocyte collects in biliary canaliculi, from which it passes through the biliary tract into the gut.

The liver secretes bile, which aids digestion by emulsifying lipids, and has a central role in metabolism of (1) bilirubin, from haem; (2) bile salts, the principal mechanism for clearance of cholesterol; (3) carbohydrates, e.g. maintenance of blood glucose within a narrow range; (4) amino acids and ammonia, e.g. control of the plasma concentration of amino acids, and clearing portal venous ammonia generated within the gut lumen; (5) proteins, most circulating plasma proteins being produced by hepatocytes; and (6) lipid and lipoproteins.

Pancreas

The pancreas lies in the retroperitoneum and is composed of (1) an exocrine portion centred on acini, producing an alkaline secretion containing digestive enzymes including serine proteases (e.g. trypsinogen, chymotrypsin, which act at various cleavage points), exopeptidases (e.g. carboxypeptidases A and B, which cleave C-terminal amino acids) and lipolytic enzymes, draining through a ductal system into the duodenum; and (2) the islets of Langerhans, which secrete insulin (also glucagon, somatostatin, and pancreatic polypeptide).

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