Think About It

While raising blood calcium levels, PTH lowers blood phosphate levels by promoting urinary excretion of phosphate. Explain why this is important for achieving the purpose of PTH.

Saladin: Anatomy & 7. Bone Tissue Text © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

234 Part Two Support and Movement

^ W Blood calcium m T concentration

Correction for hypercalcemia

Less bone _ resorption

More bone deposition "

Reduced ■ osteoclast ■ activity

Increased osteoblast activity

Calcitonin __*

^ A Blood calcium W concentration T

Correction for hypocalcemia

More bone resorption

Less bone deposition t

Prevention of " hydroxyapatite " formation

Conservation of calcium

Increased osteoclast activity

Reduced ■ osteoblast activity

More urinary phosphate excretion

_ Less urinary calcium excretion

Parathyroid hormone

Figure 7.16 Negative Feedback Loops in Calcium Homeostasis. (a) The correction of hypercalcemia by calcitonin. (b) The correction of hypocalcemia by parathyroid hormone.

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