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Our teacher said that these two hormones are normally never present in the blood stream, and to be carfeul of it if it comes as a question. I'm not conviced to be honest, is this true?

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I think your teacher means that these hormones only go from the hypothlamus to the anterior pituitary gland, where they stimulate hormone secretion. Like, for example, of the FSH or LH, who are stimulated by GnRH. GnRH is not present in the systemic sirculation, but it's definately present in the one between the hypothalamus and the APG. FSH and LH are though. I don't remember the name of the small blood-system between the hypothalamus and the APG, but I don't think it's connected to the systemic circulatory system.

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  • $\begingroup$ Yes that was what he meant, he said that if one did a blood test and took blood samples (from places like the viens of the arms , like normal) you wouldnt find these hormones. $\endgroup$ – Dahen Apr 30 '18 at 14:58
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    $\begingroup$ I would think it's true, but if you just google whether the different blood systems (systemic VS hypothalamus-APG) are connected you have your answer. $\endgroup$ – Thale Mathiassen Apr 30 '18 at 15:15
  • $\begingroup$ I see. Thanks, I didn't know the name of that system. I'll google it and see what I get. $\endgroup$ – Dahen Apr 30 '18 at 15:18
  • $\begingroup$ en.wikipedia.org/wiki/Hypophyseal_portal_system got it! $\endgroup$ – Thale Mathiassen Apr 30 '18 at 17:23
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    $\begingroup$ This isn't strictly true. The hypophyseal portal system, like other portal systems (e.g., the hepatic portal system), is part of the systemic circulation, but has a second capillary bed. Releasing hormones act at very low concentrations. They are released in a pulsatile manner and are inactivated by enzymes in the blood, providing tight control of their activity in space and time. $\endgroup$ – De Novo Nov 13 '18 at 16:59

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