calcium homeostasis

Meaning: Blood Ca²⁺ is held in a narrow range because it is needed for muscle contraction, neurotransmitter release, clotting and bone mineral. Bone is the large reservoir. When blood Ca²⁺ falls, the parathyroid glands release PTH. PTH stimulates osteoclasts to resorb bone, increases Ca²⁺ reabsorption in the kidney, and promotes activation of vitamin D (calcitriol), which increases Ca²⁺ absorption in the intestine. When blood Ca²⁺ rises, parafollicular (C) cells of the thyroid release calcitonin, which inhibits osteoclasts and so favors keeping Ca²⁺ in bone. In adult humans PTH is the dominant regulator, but the MCAT presents calcitonin as its opposing hormone.

On the MCAT: Expect a feedback-loop diagram or a passage about removing the parathyroid glands, vitamin D deficiency, or excess PTH. You predict the direction of change in blood Ca²⁺ and name the target tissue. Low Ca²⁺ can cause muscle spasms and tetany because excitable cells become more easily depolarized.

Common confusion: Students mix up the two glands. PTH comes from the parathyroids, calcitonin from the thyroid. Students also assume PTH raises phosphate along with Ca²⁺ because bone releases both. PTH actually decreases phosphate reabsorption in the kidney, so blood phosphate tends to fall while Ca²⁺ rises.