Definition for Portal venous system
From Biology Forums Dictionary
In the circulatory system of animals, a portal venous system occurs when a capillary bed pools into another capillary bed through veins, without first going through the heart. 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, the hypophyseal portal system and (in non-mammals) the renal 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.
The functional significance of such a system is that it transports products of one region directly to another region in relatively high concentrations. If the heart was involved in the blood circulation between those two regions, those products would be spread around the rest of the body.
