The liver acinus. The liver acinus is a functional interpretation of liver organization. It consists of adjacent sectors of neighboring hexagonal fields of classic lobules partially separated by distributing blood vessels. The zones, marked 1, 2, and 3, are supplied with blood that is most oxygenated and richest in nutrients in zone 1 and least so in zone 3. The terminal hepatic venules (central veins) in this interpretation are at the edges of the acinus instead of in the center, as in the classic lobule. The vessels of the portal canals, namely, terminal branches of the portal vein and hepatic artery that, along with the smallest bile ducts, make up the portal triad, are shown at the corners of the hexagon that outlines the cross-sectioned profile of the classic lobule.
Liver injury may be triggered by hemodynamic changes in the circulatory system. In congestive heart failure, the heart is unable to provide sufficient oxygenated blood to meet the metabolic requirements of many tissues and organs, including the liver, which is readily affected by hypoperfusion and hypoxia (low blood oxygen content). Zone 3 of the liver acinus is the first to be affected by this condition. The hepatocytes in this zone are the last to receive blood as it passes along the sinusoids; as a result, these cells receive a blood supply already depleted in oxygen. Examination of a liver biopsy specimen from an individual with congestive heart failure shows a distinct pattern of liver necrosis. Hepatocytes in zone 3, which is located around the central vein, undergo ischemic necrosis. Typically, no noticeable changes are seen in zones 1 and 2, representing the periphery of a classic lobule. Necrosis of this type is referred to as centrilobular necrosis. Figure 17.7 shows the centrilobu-lar portion of a classic lobule. The multiple round vacuoles indicate lipid accumulation, and the atrophic changes are the result of dying hepatocytes undergoing autophagocytosis. Centrilobular necrosis due to hypoxia is referred as cardiac cirrhosis; however, unlike true cirrhosis, nodular regeneration of hepatocytes is minimal.
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