= Hypophyseal portal system
{wiki}
The <hypophyseal portal system> is the vascular route carrying regulatory <hormones> from the <hypothalamus> directly to the <anterior pituitary>. Hypothalamic <neurons> release their chemical signals into a first network of <capillaries>. <Hypophyseal portal veins> then carry that <blood> to a second capillary network alongside the secretory <biological cells> of the <anterior pituitary>. This is a <portal system (circulation)>: two capillary beds connected in sequence.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section 17.3, Anterior Pituitary and Figure 17.9]{ref}
The route makes a small local secretion an effective input to another gland. In an ideal steady <mass balance>, let a <hormone> enter <blood plasma> at molar rate $r$, with plasma <volumetric flow rate> $Q$. Ignoring removal and assuming complete mixing, the downstream <molar concentration> is
$$
\boxed{C_{\mathrm{out}}=C_{\mathrm{in}}+\frac{r}{Q}.}
$$
This is a delivery model, not a universal physiological constant: changing secretion, flow, extraction or mixing changes the resulting signal. It explains the benefit of reaching the target before whole-body dilution.
A 1982 experiment by <Iain J. Clarke> and <James T. Cummins> sampled this upstream communication directly in conscious ovariectomized <sheep>, using a procedure that maintained pituitary function. They measured <gonadotropin-releasing hormone> (<GnRH>) in portal <blood plasma> and <luteinizing hormone> (<LH>) in peripheral jugular <blood plasma>. The secretory pulses were synchronous: each observed <LH> episode was linked to a pulsatile <GnRH> discharge through the portal network.<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: portal and peripheral plasma measurements and pulse synchrony]{ref}
Sampling the two locations matters. The portal measurement detects the hypothalamic message before it reaches the <anterior pituitary>; the peripheral measurement detects the pituitary's output. A single downstream <hormone> trace would not reveal both sides of that timing relationship.
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