= Hypophyseal portal vein
A <hypophyseal portal vein> is a <vein> connecting the first and second <capillary> networks of the <hypophyseal portal system>. The main route carries <blood> from the <median eminence> region of the <hypothalamus>, down the pituitary stalk, to the <anterior pituitary>. It delivers hypothalamic <hormones> before their dilution in the wider circulation.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section17.3, Anterior Pituitary and Figure17.9: primary plexus, portal veins and secondary plexus]{ref}
The word “portal” describes the route: <blood> leaving one capillary bed reaches another capillary bed before returning to the <heart>. Here the first bed receives a regulatory message and the second delivers it to responding <biological cells>. A <hypophyseal portal vein> is therefore an interconnecting segment, rather than the entire <hypophyseal portal system>.
A useful example is a pulse of <gonadotropin-releasing hormone>. In conscious <sheep>, simultaneous sampling of portal and peripheral <blood plasma> linked portal <GnRH> pulses with peripheral <luteinizing hormone> episodes. Sampling the <hypophyseal portal vein> route accesses the incoming hypothalamic signal; peripheral sampling detects the resulting pituitary output. That distinction is why the sampling location is part of the experiment, not just a convenient place to draw <blood>.<The temporal relationship between gonadotropin releasing hormone (GnRH) and luteinizing hormone (LH) secretion in ovariectomized ewes by Iain J. Clarke and James T. Cummins>[Abstract: synchronized portal GnRH and peripheral LH secretion in conscious ovariectomized ewes]{ref}
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