Pset+5+Rationales

Pset+5+Rationales - Pset 5 Rationales PART 1 1 B...

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Pset 5 Rationales PART 1 1) B Calcitonin (released by the thyroid gland) and PTH (released by the parathyroid gland) work in opposition to each other to maintain homeostatic blood Calcium levels. While Calcitonin stimulates bone growth by inhibiting osteoclasts, thereby decreasing blood calcium levels, PTH stimulates bone breakdown by stimulating osteoblasts, which in turn stimulate osteoclasts through the release of cytokines. The osteoclasts then destroy the bone more quickly than the activated osteoblasts lay it down, thereby increasing blood calcium levels. Vitamin D works with PTH by increasing Ca++ uptake in the gut and inhibiting Ca++ excretion in the kidney. Homeostasis, by definition cannot be maintained by positive feedback. Remember: positive feedback takes us away from the setpoint while negative feedback takes us back to it. 2) B If the blood level of the hormone drops after exposing it to a gland, it is showing negative feedback on that gland. From the data given, we can determine that the hormone is negatively feeding back on the hypothalamus and anterior pituitary, but not on the posterior pituitary. Since the hormone is involved in a negative feedback loop within the hypothalamic-anterior pituitary axis, it is released from the anterior pituitary and not the posterior pituitary, so it cannot be oxytocin or vasopressin. The fact that exposing the hypothalamus to the hormone leads to a drop in plasma concentration suggests that it can exhibit negative feedback on the hypothalamus, and thus is not GnRH because a releasing hormone would never directly inhibit the gland that secreted it (this would be very inefficient). Because the hormone feeds back negatively on the anterior pituitary, the
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Pset+5+Rationales - Pset 5 Rationales PART 1 1 B...

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