Question

vitamin d enhances ca²+ absorption into the blood by the intestines
into the blood by the kidneys
pth stimulates activation of vitamin d
based on the diagram, which of the following most likely occurs when a persons blood ca²+ level is 8 mg/100 ml?
choose 1 answer:
the amount of calcium removed from the body via the kidneys increases.
the secretion of pth from the parathyroid glands decreases.
the release of ca²+ from bones into the bloodstream increases.
the amount of active vitamin d in the body decreases.

Explanation:

When blood \(Ca^{2+}\) level is low (\(8\ mg/100\ mL\) is lower than normal), parathyroid hormone (PTH) is secreted. PTH has multiple effects: it stimulates the release of \(Ca^{2+}\) from bones into the bloodstream, stimulates the activation of vitamin D (which enhances \(Ca^{2+}\) absorption in intestines), and reduces calcium excretion by kidneys.

• Option A: PTH reduces calcium removal via kidneys, so this is incorrect.
• Option B: Low \(Ca^{2+}\) increases PTH secretion, so this is incorrect.
• Option C: PTH stimulates release of \(Ca^{2+}\) from bones. When \(Ca^{2+}\) is low, this process increases. This is correct.
• Option D: PTH stimulates activation of vitamin D, so active vitamin D amount increases, not decreases. This is incorrect.

Answer:

C. The release of \(Ca^{2+}\) from bones into the bloodstream increases.