The Hepatic Microvascular System

The liver receives as much as 25 percent of the cardiac output, and much of this blood has already passed through the intestinal tract. In conjunction with handling such a large quantity of incoming blood, it also responds to potential infectious agents that have entered the blood. To do all this, the liver has evolved to maximize vascular spaces and developed mechanisms to fight foreign entities.

The hepatic microvascular system consists of a portal circulation including portal venules, sinusoids, and central venules and an arterial circulation (hepatic arterioles). Blood leaving the gut via the portal venules enters the sinusoids via the sphincter-like cells. Arteriole blood enters the sinusoids through branches of the hepatic arterioles. Both portal and arterial vessels can contract and therefore regulate blood flow within the sinusoids. The sinusoids take blood through the lobule from the portal tract to the central

(terminal) vein. Close to the portal tract the sinusoids form an interconnecting network that becomes organized into parallel vessels. As blood leaves the sinusoids it flows through outlet sphincters. These sphincters control the rate of blood flow entering the central veins.

The portal blood represents a route through which infectious gut-derived organisms can enter the liver. Specific mechanisms have evolved to instigate and regulate inflammatory responses against invading pathogens. The close proximity and cross-talk between the different cells within the liver dictates that many of the hepatic cells are involved in an inflammatory response. Unfortunately the development of an inflammatory response, if uncontrolled, can manifest in permanent liver injury. Indeed, there is irrefutable evidence for the involvement of inflammatory cells in a large number of known liver diseases, including alcoholic hepatitis and cirrhosis, viral hepatitis, ischemia-reperfusion injury (transplantation, tumor resection, shock), sepsis- or endotoxin-induced liver injury, acute and chronic rejection, primary biliary cirrhosis, and primary sclerosing cholangi-tis. Following is an overview of the inflammatory cells, molecules and mechanisms used by the liver to combat and regulate liver injury and dysfunction. As this review is brief, we strongly encourage the readers to turn to our list of suggested reading, and elsewhere, for more information.

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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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