Hepatic Microvascular System

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, namely, portal venules, hepatic arterioles, sinusoids, central venules, and lymphatics. Figure 2 is a diagram illustrating the afferent and efferent microvascular connections to the sinusoids within a single hepatic lobule.

Most blood enters the sinusoids from portal venules. These inlets are reported to be guarded by sphincters composed of sinusoidal lining cells termed the afferent or inlet sphincters. Arterial blood enters some of the sinusoids, principally through branches of the hepatic arterioles. These vessels, arterio-sinus twigs, terminate in sinusoids near their origins from portal venules. In addition, occasional direct connections (arterio-portal anastomoses, APA) have been observed with the terminal portal venules. The frequency of these APAs appears to be species-dependent. Because all of these structures are independently contractile, the sinusoids receive a varying mixture of portal venous and hepatic arterial blood. Finally, some evidence suggests that the fraction of blood delivered to the sinusoids by the hepatic artery differs between the hilus and periphery of hepatic lobes. Within the network of sinusoids, blood flow is reportedly regulated by contractile sinusoidal lining cells that control not only the velocity of flow but also its distribution within the network.

Figure 2 Hepatic microvasculature as determined by in vivo microscopic studies. PV, portal venule; HA, hepatic arteriole; L, lymphatic; BD, bile ductule; N, nerve; CV, central venule; SLV, sublobular hepatic vein. Arrows indicate direction of flow. (see color insert)

Blood leaves the sinusoids by flowing into central (terminal hepatic) venules, reportedly by passing through outlet or efferent sphincters composed of sinusoidal lining cells.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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