FIGURE F18.2.1 • Photomicrograph of centrilobular necrosis in human liver. This photomicrograph shows a routine H&E liver biopsy specimen from an individual with congestive heart failure. Pathologic changes (referred to as ischemic necrosis) are most severe in hepatocytes in zone 3. This zone surrounds the terminal hepatic venule (central vein). This type of necrosis is referred to as centrilobular necrosis. Note the presence of multiple round vacuoles, which indicates extensive lipid accumulation. No noticeable changes are seen in the periphery of the lobule, that is zone 1 and much of zone 2. 320.
Digestive System III: Liver, Gallbladder, and Pancreas  LIVER
Blood Vessels of the Parenchyma
The blood vessels that occupy the portal canals are called interlobular vessels. Only the interlobular vessels that form the smallest portal triads send blood into the sinusoids. The larger interlobular vessels branch into distributing vessels that are located at the periphery of the lobule. These distributing vessels send inlet vessels to the sinusoids (Fig. 18.7). In the sinusoids, the blood flows centripetally toward the central vein. The central vein courses through the central axis of the classic liver lobule, becoming larger as it progresses through the lobule and empties into a sublobular vein. Several sublobular veins converge to form larger hepatic veins that empty into the inferior vena cava.
The structure of the portal vein and its branches within the liver is typical of veins in general. The lumen is much larger than that of the artery associated with it. The structure of the hepatic artery is like that of other arteries (i.e., it has a thick muscular wall). In addition to providing arterial blood directly to the sinusoids, the hepatic artery provides arterial blood to the connective tissue and other structures in the larger portal canals. Capillaries in these larger portal canals return the blood to the interlobular veins before they empty into the sinusoid.
The central vein is a thin-walled vessel receiving blood from the hepatic sinusoids. The endothelial lining of the central vein is surrounded by small amounts of spirally arranged connective tissue fibers. The central vein, so named because of its central position in the classic lobule, is actually the terminal venule of the system of hepatic veins and, thus, is more properly called the terminal hepatic venule. The sublobular
FIGURE 18.7 • Diagram of the fow of blood and bile in the liver. This schematic diagram of a part of a classic lobule shows the components of the portal triads, hepatic sinuses, terminal hepatic venule (central vein), and associated plates of hepatocytes. Red arrows indicate the direction of the blood flow in the sinusoids. Note that the direction of bile flow (green arrows) is opposite that of the blood flow.
vein, the vessel that receives blood from the terminal hepatic venules, has a distinct layer of connective tissue fibers, both collagenous and elastic, just external to the endothelium. The sublobular veins and the hepatic veins, into which they drain, travel alone. Because they are solitary vessels, they can be readily distinguished in a histologic section from the portal veins that are members of a triad. There are no valves in hepatic veins.
Hepatic sinusoids are lined with a thin discontinuous endothelium.
The discontinuous sinusoidal endothelium has a discontinuous basal lamina that is absent over large areas. The discontinuity of the endothelium is evident in two ways:  Large fenestrae, without diaphragms, are present within the endothelial cells.  Large gaps are present between neighboring endothelial cells.