The pool of calcium in bone is flexible and changes to some small extent throughout life. The concentration of calcium in blood cannot vary without severe illness and even death. All these functions are vital for normal living. Vitamin D is a fat-soluble vitamin added to many dairy products. This vitamin is also activated in the body when the skin is exposed to sunlight.
Vitamin D increases the intestinal absorption of calcium therefore increasing calcium blood levels.
Parathyroid hormone is produced by the parathyroid gland located behind the thyroid gland. As the calcium level in the blood increases, there is a feedback mechanism to slow absorption and osteoclast activity. Increased blood calcium will shut off parathyroid hormone secretion. At the same time the forth factor, calcitonin, is released from the thyroid gland. Calcitonin has the opposite effect of parathyroid hormone, it decreases calcium absorption.