Calcium is vitally important, not just for strong bones, but for the proper functioning of nearly every cell in the human body. Within our cells, calcium levels must be maintained at an optimal range, and problems arise when there is too much or too little. Calcium homeostasis, also called calcium metabolism, is the process by which our bodies maintain calcium levels within this optimal range. Two primary hormones drive this process, working together to regulate the absorption and release of calcium from our diets and from our bones.
Most people are aware that calcium gives strength and rigidity to bones, and the vast majority of the calcium in our bodies is stored in our skeletal system. Calcium also plays an important role in regulating muscle contractions, where it acts as a signal messenger. In the brain, calcium is necessary for nerve conduction and helps control the pathway used by the brain's chemical messengers, called neurotransmitters. Inside our cells, calcium acts again as a messenger, carrying instructions from one part of the cell to another.
Calcium homeostasis is maintained by the actions of two hormones; parathyroid hormone and calcitonin. Parathyroid hormone is made in the parathyroid glands in response to low levels of calcium in the cells or blood. It stimulates the release of calcium from the bones into the blood, where it can be used by the cells. Calcitonin has the opposite effect; when calcium concentration is too high, calcitonin slows the release of calcium from the bones.
Hypercalcemia is a condition in which calcium homeostasis is upset by too much calcium. Overactive parathyroid glands can create this condition; by producing too much parathyroid hormone, too much calcium is released into the blood, and it overwhelms the body's ability to produce enough calcitonin to maintain calcium homeostasis. Sometimes, surgical removal of the defective gland is necessary to correct the imbalance of hormones. Certain cancers may also cause calcium levels to become too high. Hypercalcemia can cause kidney stones and organ damage; it can also contribute to osteoporosis because too much calcium is being released from the bones.
Calcium homeostasis can also be upset in the opposite direction; hypocalcemia is caused by not enough calcium in the blood and cells. Hypocalcemia can be caused by inadequate dietary intake of calcium, and supplements may be necessary to correct this deficiency. Even when dietary intake is adequate, the body may have problems absorbing the available calcium because of a deficiency in Vitamin D. Over time, hypocalcemia can contribute to osteoporosis, osteomalacia, muscle spasms, and in severe cases, cardiac dysfunction.