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.[ref]
Anatomy and Physiology 2e by J. Gordon Betts and others. Section17.3, Anterior Pituitary and Figure17.9: primary plexus, portal veins and secondary plexus
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.[ref]
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
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