Basic Aspects of the Hepatic Microvascular System
Robert S. McCuskey
The hepatic microvascular system comprises all blood and lymphatic vessels immediately involved in the delivery and removal of fluids to and from the hepatic parenchyma. The majority of blood enters the sinusoids (exchange vessels) from portal venules, the remainder from hepatic arteri- oles. Blood leaves the sinusoids by flowing into central venules. Arterial blood enters some of the sinusoids through branches of the hepatic arterioles.
Occasional direct connections also have been observed with the portal venules and post-sinusoidal vessels. Because all of these vessels are inde- pendently contractile, the sinusoids receive a varying mixture of portal venous and arterial blood.
The sinusoid network exhibits structural and dimensional heterogeneity.
Near portal venules, sinusoids are arranged in interconnecting polygonal net- works; further away from the portal venules, the sinusoids become organized as parallel vessels that terminate in the central venules. In addition, the diam- eter of periportal sinusoids is less than those in the centrilobular region.
The ultrastructural fenestration of the sinusoids also exhibits regional heterogeneity.
The functional unit is thought to be a conical “hepatic microvascular subunit” of the classic lobule consisting of a group of sinusoids supplied by a single inlet venule and its associated termination of a branch of the hepatic arteriole and cholangiole.
There are several potential morphological sites for regulating blood flow through the sinusoids. These include the various segments of the afferent portal venules and hepatic arterioles, the sinusoids themselves, and the central and hepatic venules. The principal site of regulation of blood flow
135 Department of Cell Biology and Anatomy, College of Medicine, University of Arizona Tucson, AZ, USA
through the sinusoids, however, is thought to reside in the sinusoid itself, where the major blood pressure drop occurs in the liver. Sinusoidal lining cells produce, and are responsive to, a variety of substances. Sinusoidal endothe- lial cells may act like sphincters by swelling or contracting, thereby narrow- ing the sinusoidal lumen and limiting blood flow. More recently, attention has focused on the perisinusoidal stellate cell as the cell responsible for control- ling sinusoidal diameter and blood flow.
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