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Overview of Calcium

By James L. Lewis, III, MD

About 99% of the body’s calcium is stored in the bones, but cells (particularly muscle cells) and blood also contain calcium. Calcium is essential for the following:

  • Formation of bone and teeth

  • Muscle contraction

  • Normal functioning of many enzymes

  • Blood clotting

  • Normal heart rhythm

The body precisely controls the amount of calcium in cells and blood. The body moves calcium out of bones into blood as needed to maintain a steady level of calcium in the blood. If people do not consume enough calcium, too much calcium is mobilized from the bones, weakening them (hypocalcemia—see see Hypocalcemia (Low Level of Calcium in the Blood)). Osteoporosis can result. To maintain a normal level of calcium in blood without weakening the bones, people need to consume at least 1,000 to 1,500 milligrams of calcium a day. The level of calcium in blood is regulated primarily by two hormones: parathyroid hormone and calcitonin .

Parathyroid hormone is produced by the four parathyroid glands, located around the thyroid gland in the neck. When the calcium level in blood decreases, the parathyroid glands produce more parathyroid hormone. When the calcium level in blood increases (hypercalcemia— see see Hypercalcemia (High Level of Calcium in the Blood)), the parathyroid glands produce less hormone. Parathyroid hormone does the following:

  • Stimulates bones to release calcium into blood

  • Causes the kidneys to excrete less calcium in urine

  • Stimulates the digestive tract to absorb more calcium

  • Causes the kidneys to activate vitamin D , which enables the digestive tract to absorb more calcium

Calcitonin is produced by cells of the thyroid gland. It lowers the calcium level in blood by slowing the breakdown of bone, but only slightly.

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