Answer to Question 1
Blood calcium above normal results in calcium rigor: the muscles contract and cannot relax. Similarly, blood calcium below normal causes calcium tetanyalso characterized by uncontrolled muscle contraction. These conditions do not reflect a dietary excess or lack of calcium; they are caused by a lack of vitamin D or by abnormal secretion of the regulatory hormones. A chronic dietary deficiency of calcium, or a chronic deficiency due to poor absorption over the years, depletes the bones. Again: the bones, not the blood, are robbed by a calcium deficiency.
Answer to Question 2
Calcium homeostasis involves a system of hormones and vitamin D. Whenever blood calcium falls too low or rises too high, three organ systems respond: the intestines, bones, and kidneys, and vitamin D and two hormonesparathyroid hormone and calcitoninreturn blood calcium to normal.
Parathyroid hormone
Stimulates the activation of vitamin D
Stimulates calcium reabsorption from the kidneys into the blood
Stimulates osteoclast cells to break down bone, releasing calcium into the blood
Vitamin D
Stimulates calcium reabsorption from the kidneys into the blood.
Enhances calcium absorption in the intestines
Stimulates osteoclast cells to break down bone, releasing calcium into the blood
Calcitonin
Inhibits the activation of vitamin D
Prevents calcium reabsorption in the kidneys
Limits calcium absorption in the intestines
Inhibits osteoclast cells from breaking down bone, preventing the release of calcium
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