In the hypothalamo-pituitary portal system, which statement best describes the functional significance of the short portal vessels connecting the posterior pituitary to the anterior pituitary?
- A They allow retrograde blood flow that can deliver anterior pituitary hormones to the hypothalamus ✓
- B They carry ADH from posterior to anterior pituitary to regulate ACTH release
- C They carry oxytocin to regulate FSH and LH surges
- D They have no known functional significance in humans
Correct answer: A. They allow retrograde blood flow that can deliver anterior pituitary hormones to the hypothalamus
Explanation
The short portal vessels create a retrograde route by which anterior pituitary hormones (e.g., prolactin, GH) can reach the hypothalamus at high concentrations to modulate releasing hormone secretion — a form of ultrashort-loop feedback. ADH and oxytocin act on the anterior pituitary via the long portal vessels, not exclusively through short portal vessels. This retrograde flow mechanism explains how the pituitary can modulate its own hypothalamic drive.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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