What is a portal vein?
A portal vein arises from capillary bed of one organ and ends in capillary bed of anther organ, neither of the organ being heart/lung. Thus portal vein carries deoxygenated blood.
There are three different types of portal veins reported from different vertebrates. Only two are there in mammals: hepatic portal vein and hypothalamo-hypophyseal portal vein .
In lower vertebrates there is an additional renal portal system. Portal veins are unusual as these are not returning to heart, rather entering another organ. Hepatic portal vein brings in absorbed nutrients from gut wall and delivers it in metabolically active liver.
To know in detail about hepatic portal vein please read the following (a featured answer):