The liver vasculature is arranged into subunits that allow the arterial and portal blood to mix and provide nutrition for the liver cells. Each subunit, called an acinus, is about 300 to 350 |xm long and wide. In humans, usually three acini occur together. The core of each acinus is supplied by a single terminal portal venule, sinusoidal capillaries originate from this venule (Fig. 17.4). The endothelial cells of the capillaries have fenestrated regions with discrete openings that facilitate exchange between the plasma and interstitial spaces. The capillaries do not have a basement membrane, which partially contributes to their high permeability.
The terminal hepatic arteriole to each acinus is paired with the terminal portal venule at the acinus core, and blood from the arteriole and blood from the venule jointly perfuse the capillaries. The intermixing of the arterial and portal blood tends to be intermittent because the vascular smooth muscle of the small arteriole alternately constricts and relaxes. This prevents arteriolar pressure from causing a sustained reversed flow in the sinusoidal capillaries, where pressures are 7 to 10 mm Hg. The best evidence is that hepatic artery and portal venous blood first mix at the level of the capillaries in each acinus. The sinusoidal capillaries are drained by the terminal hepatic venules at the outer margins of each acinus, usually at least two hepatic venules drain each acinus.
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