Abstract
The hepatic microcirculation, the result of the total merging of hepatic arterial and portal stream in the sinusoidal delta, is organized in well-defined microcirculatory hepatic units. The unit consists of a terminal portal venule (TPV) and a glomus of sinusoids branching off from it. The associated hepatic arterioles form a periductular plexus wherefrom efferent arterial capillaries join the sinusoids close to TPV; other arterioles bypass the periductular plexus and empty directly into the sinusoids. The microcirculatory unit provides the vascular framework for the structural and functional hepatic unit, the liver acinus. Simple and complex acini are microscopic masses of parenchyma surrounding like berries their supplying terminal and preterminal vessels, respectively, along with the associated ductules, lymph vessels, and nerve fibres. The structure and dimensions of the hepatic microvasculature and their hemodynamic relevance is discussed. Hydrostatic pressure in the TPV is low. The impressive drop of pressure in the sinusoidal bed down to hepatic venular level is tentatively explained. In the sinusoids, the site of A-V shunting, the energy of arteriolar pressure is transformed into velocity; the fast flow of arterial blood exerts a symphoning effect on interconnected sinusoids. Intermittent flow in the microcirculatory units is regulated by the smooth muscles in the arteriolar wall and by precapillary sphincters responding to nervous stimuli, hormones, metabolites, bile salts, and vasoactive substances. Portal flow is controlled by the mesenteric and splenic microvessels. The definition of the microcirculatory unit within the acinus offers a hemodynamic approach to the understanding of pathologic circulatory and structural changes in the liver.
Published Version
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