Liver Lobules

The hepatic plates are arranged into functional units called liver lobules (figs. 18.20 and 18.21). In the middle of each lobule is a central vein, and at the periphery of each lobule are branches of the hepatic portal vein and of the hepatic artery, which open into the sinusoids between hepatic plates. Arterial blood and portal venous blood, containing molecules absorbed in the GI tract, thus mix as the blood flows within the sinusoids from the periphery of the lobule to the central vein. The central veins of different liver lobules converge to form the hepatic vein, which carries blood from the liver to the inferior vena cava.

Bile is produced by the hepatocytes and secreted into thin channels called bile canaliculi, located within each hepatic plate

Branch of portal vein

Bile ductule

Bile canaliculus Sinusoids

Bile canaliculus Sinusoids

Branch of hepatic artery Hepatic plate Central vein

■ Figure 18.21 The flow of blood and bile in a liver lobule. Blood flows within sinusoids from a portal vein to the central vein (from the periphery to the center of a lobule). Bile flows within hepatic plates from the center to bile ductules at the periphery of a lobule.

(fig. 18.21). These bile canaliculi are drained at the periphery of each lobule by bile ducts, which in turn drain into hepatic ducts that carry bile away from the liver. Since blood travels in the sinusoids and bile travels in the opposite direction within the hepatic plates, blood and bile do not mix in the liver lobules.

In cirrhosis, large numbers of liver lobules are destroyed and replaced with permanent connective tissue and "regenerative nodules" of hepatocytes. These regenerative nodules do not have the platelike structure of normal liver tissue, and are therefore less functional. One indication of this decreased function is the entry of ammonia (produced by intestinal bacteria) from the hepatic portal blood into the general circulation. Cirrhosis may be caused by chronic alcohol abuse, biliary obstruction, viral hepatitis, or by various chemicals that attack liver cells.

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