It is important to recognize that the liver has a dual blood supply. Approximately 80 percent of the blood entering the liver is poorly oxygenated blood supplied by the portal vein containing venous blood flowing from the intestines, pancreas, spleen, and gallbladder. The remaining 20 percent of the blood supply is well oxygenated and delivered by the hepatic artery. Both the hepatic portal vein and hepatic artery enter the liver at the hilus, where efferent bile ducts as well as lymphatics also exit the organ.
Branches of the hepatic artery, hepatic portal vein, bile duct, and lymphatic vessels travel together in portal tracts through the liver parenchyma. After repeated branching, terminal branches of the blood vessels (portal venules and hepatic arterioles) supply blood to the sinusoids, which are the exchange vessels in the liver. Branches of hepatic arteri-oles also supply the peribiliary plexus of capillaries nourishing the bile ducts and then drain into sinusoids or occasionally into portal venules. After flowing through the sinusoids, blood is collected in small branches of hepatic veins termed central venules (central veins, terminal hepatic venules). These course independently of the portal tracts and drain via hepatic veins, which leave the liver on the dorsal surface and join the inferior vena cava. Occasionally, branches of hepatic arterioles bypass the sinusoidal bed to supply the walls of hepatic veins.
Lymphatic vessels originate as blind-ending capillaries in the connective tissue spaces of portal tracts. The fluid contained in these lymphatics flows toward the hepatic hilus and eventually into the cisternae chyli.
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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.