18. Calcitriol increases blood calcium levels by _____________.

Question

BlackTom AI · Accepted Answer

Calcitriol (1,25‑dihydroxyvitamin D3) increases serum calcium primarily by enhancing intestinal absorption of dietary Ca2+.

Relevant knowledge points: vitamin D is synthesized from cholecalciferol (skin/liver) to 25‑hydroxyvitamin D in the liver and then to the active form calcitriol in the kidney via 1α‑hydroxylase (stimulated by PTH). Calcitriol acts on intestinal enterocytes, bone, and kidney to raise blood Ca2+. In the intestine it upregulates calcium channels (TRPV6), calbindin (intracellular Ca2+ shuttle), and the Ca2+‑ATPase on the basolateral membrane, increasing net transcellular Ca2+ uptake. In bone it promotes osteoclastogenesis indirectly (via osteoblast RANKL), increasing bone resorption and Ca2+ release. In the kidney it modestly increases distal tubular Ca2+ reabsorption.

Step‑by‑step reasoning:
1. Low serum Ca2+ or PTH stimulates renal 1α‑hydroxylase → increased calcitriol production.
2. Calcitriol binds nuclear vitamin D receptors (VDR) in enterocytes, increasing transcription of TRPV6, calbindin, and PMCA1b.
3. Increased expression of these proteins raises transcellular Ca2+ transport from the gut lumen into the blood, therefore increasing serum Ca2+.
4. Simultaneous effects on bone (increased resorption) and kidney (increased reabsorption) further support the rise in blood Ca2+.

No numerical calculations are required.

Why A is correct: it directly describes the principal mechanism by which calcitriol elevates blood calcium—stimulating dietary Ca2+ absorption in the intestine via upregulation of transport proteins.

Why other plausible options would be incorrect: statements claiming calcitriol decreases Ca2+ absorption, increases urinary excretion, or solely acts by depositing Ca2+ into bone contradict its known actions (it increases, not decreases, intestinal uptake; it reduces urinary loss rather than increasing it; and it promotes bone resorption rather than deposition when increasing serum Ca2+).

Thus the correct answer is A: stimulating absorption of Ca2+ from the diet.

18. Calcitriol increases blood calcium levels by _____________.单项选择题

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Calcium absorption is likely to be highest in

10. Which of the following decreases calcium absorption in the small intestine?

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Question at position 18 The calcitriol-vitamin D receptor complex travels to the nucleus of intestinal cells where it promotes calcium absorption by increasing the synthesis of which of the following proteins?CholecalciferolCalcitoninErgocalciferolCalbindin

Vitamin D is used to enhance absorption of

Comprehension-Level Multiple-Choice Items A person’s absorption of calcium varies based on:

What is the primary function of Vitamin D?

For which of the following functions is Vitamin D is important?

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18. Calcitriol increases blood calcium levels by _____________.单项选择题

类似问题

Calcium absorption is likely to be highest in

10. Which of the following decreases calcium absorption in the small intestine?

Calcium absorption is likely to be highest in

Question at position 18 The calcitriol-vitamin D receptor complex travels to the nucleus of intestinal cells where it promotes calcium absorption by increasing the synthesis of which of the following proteins?CholecalciferolCalcitoninErgocalciferolCalbindin

Vitamin D is used to enhance absorption of

​Comprehension-Level Multiple-Choice Items A person’s absorption of calcium varies based on:​

What is the primary function of Vitamin D?

For which of the following functions is Vitamin D is important?

更多留学生实用工具

希望你的学习变得更简单

Comprehension-Level Multiple-Choice Items A person’s absorption of calcium varies based on: