Given a serum calcium value that falls from 9.8 mg/dL to 7.6 mg/dL, predict the coordinated endocrine responses and resulting effects on bone, kidney, and intestine.

• A. Parathyroid glands increase PTH secretion causing bone resorption to release calcium, kidneys increase calcium reabsorption and activate vitamin D, and activated vitamin D increases intestinal calcium absorption
• B. Adrenal medulla secretes catecholamines to directly increase intestinal absorption and deposit calcium into bone as hydroxyapatite
• C. Pancreatic insulin secretion increases to stimulate osteoblasts and promote net bone calcium uptake until serum calcium normalizes
• D. Thyroid releases calcitonin causing immediate bone mineralization, kidneys to excrete more calcium, and intestines to decrease calcium uptake

Answer: (A)