Chemistry

The Chemistry of Calcium and its Relation to Biological Function



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The importance of calcium to the health of our bones and teeth is well known, but it also plays an important role in other parts of the body.

Over ninety-nine per cent of a body's calcium is stored in our bones. This storage system is in a constant state of change or equilibrium. Bone is made by osteoclasts, and dissolved by osteoblasts. Over time the osteoblast activity becomes more dominant and this is what leads to brittle bones or osteoporosis.

The remaining free calcium in the body is vital for its function. Calcium ions are the messengers of cell physiology and their movement in and out of cells acts as the signal for many processes in the cells. Calcium circulates in the bloodstream and between the cells either as the free ions (Ca++) or bound to albumin. The exact concentration in the blood at any one time is controlled by the parathyroid gland in the neck.

The action of calcium on the cells depends on the type of cells it is entering, or leaving.

The contraction of muscles is caused by entry of the calcium to the cells via the calcium channels. This includes all muscles, both smooth and skeletal. Because of this calcium has a profound effect on blood pressure. One group of anti hypertensive drugs acts by blocking these channels. The calcium channel blockers (including amongst them amlodipine, diltiazem and nifedipine) act by blocking these channels and so relaxing to varying degrees the musculature of blood vessels. This lowers blood pressure. Some act more specifically on heart muscle and so are used to regulate heartbeat in arrhythmias (irregular heart beat).

Other cells react to calcium by secreting something, typically hormones and enzymes. The juxtaglomerular cells in the kidney are a specialized form of cell that reacts to calcium by decreasing secretion of renin. Renin controls and kidney function, and plays a role in blood pressure control.

Calcium plays an important part in nerve function, playing a part in transmitter release.

Calcium is pumped back out of the cells in exchange for sodium in a process known as the calcium sodium pump.

Levels of calcium in the blood are critical for a number of our normal functions and any upset in these levels can be fatal. In hyperparathyroidism the parathyroid produces too much hormone and too much bone is reabsorbed, raising calcium levels in the blood. This rise can be beneficial if calcium levels are low such as in secondary hyperparathyroidism, but in primary hyperparathyroidism the rise is independent of calcium levels and increases them with adverse effects.

The early symptoms of a high calcium level are remembered as "moans, groan, stones, bones and psychiatric overtones." This refers to the symptoms of generally feeling unwell, abdominal pain, kidney stones, bone pain and mental symptoms such as depression, tiredness and fatigue. Additional symptoms include thirst and excessive urination, which is the body's attempt to rid itself of the excess calcium.

If undiagnosed and untreated this will progress to atrial fibrillation, muscle spasm and possibly death. Long term the increased levels of parathyroid hormone will also cause osteoporosis. Hyperparathyroidism can be a sign of chronic renal failure.

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