The products of digestion that are absorbed into blood capillaries in the intestine do not directly enter the general circulation. Instead, this blood is delivered first to the liver. Capillaries in the digestive tract drain into the hepatic portal vein, which carries this blood to capillaries in the liver. It is not until the blood has passed through this second capillary bed that it enters the general circulation through the hepatic vein that drains the liver. The term portal system is used to describe this unique pattern of circulation: capillaries ^ vein ^ capillaries ^ vein. In addition to receiving venous blood from the intestine, the liver also receives arterial blood via the hepatic artery.
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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.