= Portal system
{disambiguate=circulation}
A <portal system (circulation)> carries <blood> through two <capillary beds> in sequence before returning it to the <heart>. The connecting vessel sends material collected from the first bed to the second without first mixing it through the whole systemic circulation. In the hepatic example, the <portal vein> takes blood from digestive organs to the <liver>; the <hypophyseal portal system> connects the <hypothalamus> with the <anterior pituitary>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[sections17.3 and20.5, hypothalamic–pituitary connection and hepatic portal circulation]{ref}
A simple transport calculation explains the advantage for chemical signaling. If a source adds a substance at rate $q$ to a steady <blood flow> $Q$, with negligible removal along the connection, its added <concentration> is $\Delta C=q/Q$. In a hypothetical example, $q=1\,\mathrm{pmol\,min^{-1}}$ and $Q=0.5\,\mathrm{mL\,min^{-1}}$ give $2\,\mathrm{pmol\,mL^{-1}}$. Mixing the same input into $500\,\mathrm{mL\,min^{-1}}$ instead gives $0.002\,\mathrm{pmol\,mL^{-1}}$. These are explanatory flow rates, not measured human values.
Direct sampling makes the portal route experimentally accessible. In a 1982 study, <Iain J. Clarke> and <James T. Cummins> measured <GnRH> in portal plasma and <LH> in peripheral plasma of conscious ewes while maintaining pituitary function. The paired pulses connected a local hypothalamic signal with the downstream pituitary response.<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, paired portal and jugular sampling]{ref}
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