In the circulatory system of animals, a portal venous system occurs when a capillary bed drains into another capillary bed through veins. Both capillary beds and the blood vessels that connect them are considered part of the portal venous system.
They are relatively uncommon as the majority of capillary beds drain into veins which then drain into the heart, not into another capillary bed. Portal venous systems are considered venous because the blood vessels that join the two capillary beds are either veins or venules.
Examples of such systems include the hepatic portal system and the hypophyseal portal system. Unqualified, "portal venous system" often refers to the hepatic portal system. For this reason, "portal vein" most commonly refers to the hepatic portal vein.
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