9.6: The Biliary System
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Production and transport of bile
The main function of the biliary system is the transportation of bile from the liver to the gut. Bile, produced by hepatocytes, is composed of water, cholesterol, bile pigments (bilirubin), electrolytes (sodium, potassium, chloride, bicarbonate, etc.), and bile salts. Bile salts (bile acids) serve as emulsifying agents useful for the digestion and absorption of lipids and lipid soluble vitamins (e.g. vimatin A and vitamin K) from the intestinal tract.
In contrast to blood flow, the direction of bile flow is from centrilobular (nearer the central vein) towards the portal triads. Bile is produced by hepatocytes and secreted into bile canaliculi, narrow channels formed between the cellular membranes of adjacent hepatocytes. These bile canaliculi are extensive and connect to form a highway for bile transport, all flowing towards the portal triads. As the bile canaliculi join near the periphery of the lobules, they form into larger canals (also known as the Canals of Hering). The Canals of Hering empty into bile ductules, located within portal triads. Each portal triad contains one or several bile ductules.
Bile ductules have a central lumen lined by a single layer of cuboidal epithelial cells. Bile ductules from portal triads course through the liver and connect to form increasingly larger caliber structures, bile ducts, that are lined by short to tall columnar epithelium. These bile ducts will eventually empty into the hepatic duct, which empties into the cystic duct and gallbladder (dogs, cats, ruminants) or common bile duct (species that lack gallbladders, such as horses and alpacas).
The gallbladder is a blind-ended, sac-like organ attached to the visceral surface of the liver. The gallbladder receives, stores and concentrates bile entering from the liver. In response to a fatty meal, the enteroendocrine cells of the duodenum secrete cholecystokinin, a hormone that stimulates smooth muscle contraction and release of bile from the gall bladder.
The structure of the gallbladder wall is multilayered, similar to the intestinal tract. The luminal mucosal surface is lined by simple columnar epithelium. Beneath the epithelium lies a vascular-rich lamina propria. External to the lamina propria is the muscularis externa, composed of randomly oriented smooth muscle bundles. Occasionally, diverticula or invaginations of the mucosa can extend in the muscularis externa. External to the muscularis externa lies the adventitia.
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