Calcium homeostasis

Calcium metabolism refers to all the movements (and how they are regulated) of calcium atoms and ions into and out of various body compartments, such as the gut, the blood plasma, the interstitial fluids which bathe the cells in the body, the intracellular fluids, and bone. An important aspect, or component, of calcium metabolism is plasma calcium homeostasis, which describes the mechanisms whereby the concentration of calcium ions in the blood plasma is kept within very narrow limits. Derangements of this mechanism lead to hypercalcemia or hypocalcemia, both of which can have important consequences for health. In humans, when the blood plasma ionized calcium level rises above its set point, the thyroid gland releases calcitonin, causing the plasma ionized calcium level to return to normal. When it falls below that set point, the parathyroid glands release parathyroid hormone (PTH), causing the plasma calcium level to rise.

Calcium is the most abundant mineral in the human body. The average adult body contains in total approximately 1 kg, 99% in the skeleton in the form of calcium phosphate salts. The extracellular fluid (ECF) contains approximately 22 mmol, of which about 9 mmol is in the plasma. Approximately 10 mmol of calcium is exchanged between bone and the ECF over a period of twenty-four hours. The concentration of calcium ions inside the cells (in the intracellular fluid) is more than 7,000 times lower than in the blood plasma (i.e. at <0.0002 mmol/L, compared with 1.4 mmol/L in the plasma)

Calcium has several main functions in the body. It readily binds to proteins, particularly those with amino acids whose side chains terminate in carboxyl (-COOH) groups (e.g. glutamate residues). When such binding occurs the electrical charges on the protein chain change, causing the protein's tertiary structure (i.e. 3-dimensional form) to change. Good examples of this are several of the clotting factors in the blood plasma, which are functionless in the absence of calcium ions, but become fully functional on the addition of the correct concentration of calcium salts. The voltage gated sodium ion channels in the cell membranes of nerves and muscle are particularly sensitive to the calcium ion concentration in the plasma. Relatively small decreases in the plasma ionized calcium levels (hypocalcemia) cause these channels to leak sodium into the nerve cells or axons, making them hyper-excitable (positive bathmotropic effect), thus causing spontaneous muscle spasms (tetany) and paraesthesia (the sensation of "pins and needles") of the extremities and round the mouth. When the plasma ionized calcium rises above normal (hypercalcemia) more calcium is bound to these sodium channels having a negative bathmotropic effect on them, causing lethargy, muscle weakness, anorexia, constipation and labile emotions.

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