Which of the following regulates the level of calcium in the blood?

Question

BlackTom AI · Accepted Answer

Parathyroid hormone (PTH) and calcitonin are the principal hormonal regulators that maintain blood calcium within a narrow physiological range by altering bone resorption, renal handling, and intestinal absorption.

1. Relevant knowledge points and concepts
- Blood calcium homeostasis is hormonally regulated, primarily by PTH (from parathyroid glands) and calcitonin (from thyroid parafollicular or C cells).
- PTH raises serum Ca2+ by: stimulating osteoclast-mediated bone resorption (releasing Ca2+ and PO4^3-), increasing renal Ca2+ reabsorption in the distal tubule, decreasing renal phosphate reabsorption (phosphaturic effect), and stimulating 1α-hydroxylase in the kidney to activate vitamin D (calcitriol), which increases intestinal Ca2+ absorption.
- Calcitonin lowers serum Ca2+ by inhibiting osteoclast activity and promoting Ca2+ deposition in bone; its role is more modulatory and short-term compared with PTH.

2. Step-by-step reasoning process
- If serum Ca2+ falls, chemoreceptors in the parathyroid glands detect this and secrete PTH.
- PTH acts on bone to increase osteoclast-mediated resorption → immediate release of Ca2+ into blood.
- PTH acts on kidney to both increase Ca2+ reabsorption (conserving Ca2+) and activate vitamin D → enhanced intestinal absorption of dietary Ca2+.
- If serum Ca2+ rises, thyroid C cells secrete calcitonin → inhibits osteoclasts → reduces bone resorption and lowers serum Ca2+.

3. Calculations/intermediate steps
- No numerical calculation is required; mechanism steps above constitute the intermediate physiologic steps that change serum Ca2+.

4. Why the correct answer is correct
- The two hormones directly sense and correct deviations in serum calcium: PTH for hypocalcemia and calcitonin for hypercalcemia. Together they provide rapid and longer-term control of blood calcium through bone, kidney, and gut actions.

5. Why the other options are incorrect
- Dietary intake of calcium affects total calcium available but does not actively regulate rapid homeostasis; hormones do.
- Dietary phosphorus influences mineral balance but is not the primary regulator of serum Ca2+ levels.
- Glucagon and epinephrine regulate glucose and acute stress responses, not steady-state calcium homeostasis.

6. Conclusion
- Therefore the best answer is parathyroid hormone and calcitonin, which matches the provided correct answer.

Which of the following regulates the level of calcium in the blood?单项选择题

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