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.
Examples of such systems include the hepatic portal system, the hypophyseal portal system and (in non-mammals) the renal portal system. The functional significance of such a system is that it transports products of one region directly to another region in relatively high concentrations.

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