Portal venous system
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Blood flow to the liver is unique in that it receives both oxygenated and deoxygenated blood. As a result, the partial pressure of oxygen (pO2) and perfusion pressure of portal blood are lower than in other organs of the body. Blood passes from branches of the portal vein through cavities between hepatocellular sinusoids. Blood also flows from branches of the hepatic artery and mixes in the sinusoids to supply the hepatocytes with oxygen. This mixture percolates through the sinusoids and collects in a central vein which drains into the hepatic veins. The hepatic veins subsequently drain into the inferior vena cava.
A portal venous system is one in which veins connect two capillary beds; or in other words drain one organ/organ system and pass into another organ/organ system rather than being directly returned to the heart. The hepatic portal vein is the only portal system many are aware of. However, there is also a hypophyseal portal system which passes blood from the hypothalamus to the anterior pituitary 1. Therefore, strictly therefore the system draining the GI tract and passing to the liver should be termed the hepatic portal venous system to distinguish it from the less well-known hypothalamic-pituitary portal system.