Minerals comprise only ~4% of the human body. We only need small amounts of them, but they play critical roles in a large number of functions, including maintenance of pH balance, facilitation the transfer of nutrients across cell membranes, normal communication among neurons, both contraction and relaxation on muscles, and support of the body‘s structure, most notably the bones comprising our skeletons. Multiple minerals, particularly calcium, magnesium, phosphorus (in the form of phosphate [PO4]), magnesium, sodium, and potassium act together in many of these processes and their proper physiologic concentrations in the blood as well as cells of the body is referred to as mineral homeostasis. Maintenance of mineral homeostasis involves complex interactions among multiple organs. The complex processes that maintain mineral homeostasis are well illustrated by those involved in maintaining correct calcium levels in blood, bone, and other organs. Four organs play key roles in maintaining normal levels of calcium in the blood: the skeleton, the intestines, the kidneys, and a small organ in the neck called the parathyroid gland. We gain calcium from our diet and it is absorbed through the intestine. Calcium is stored in bone and excreted by the kidney. Specific receptors (calcium-sensing receptors) on cells in the parathyroid gland sense calcium levels in the blood. When levels are too low, this stimulates the release of parathyroid hormone (PTH) which has multiple actions. In the kidney, PTH inhibits calcium excretion and stimulates the production of active vitamin D metabolites, which facilitate absorption of calcium from the intestine. In bone, PTH promotes turnover and the release of calcium by increasing the activity of both osteoblasts and osteoclasts. All of these actions raise calcium levels in the blood and restore homeostasis. If this feedback loop does not operate properly (e.g., as a result of genetic mutations that result in abnormal calcium sensing receptor function or damage to the parathyroid gland that may occur during neck surgery), homeostasis may be lost, and chronic low levels of calcium (hypocalcemia) can lead to numbness (resulting from abnormal nerve function), muscle spasms, seizures, confusion, and even cardiac arrest. Hypocalcemia can also contribute to loss of normal levels of other minerals, including magnesium and phosphate. Once lost, homeostasis for calcium and other minerals may be difficult to restore. In individuals with damage to glands, such as the parathyroid, that produce hormones essential for mineral homeostasis, recombinant DNA technology has provided replacement hormones that can help to regain homeostasis.
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