Figure 178

Diagram of the flow 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.

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 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 as well as 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.

Hepatic sinusoids differ from other sinusoids in that a second cell type, the stellate sinusoidal macrophage, or Kupffer cell (Fig. 17.9), is a regular part of the vessel lining.

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