Calcium metabolism
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Calcium metabolism or calcium homeostasis is the mechanism by which the body maintains adequate calcium levels. Derangements of this mechanism lead to hypercalcemia or hypocalcemia, which both can have important consequences for health.
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Normal ranges
The serum level of calcium is closely regulated with a normal total calcium of 2.2-2.6 mmol/L (9-10.5 mg/dL) and a normal ionized calcium of 1.1-1.4 mmol/L (4.5-5.6 mg/dL). The amount of total calcium varies with the level of albumin, a protein to which calcium is bound. The biologic effect of calcium is determined by the amount of ionized calcium, rather than the total calcium. Ionized calcium does not vary with the albumin level, and therefore it is useful to measure the ionized calcium level when the serum albumin is not within normal ranges, or when a calcium disorder is suspected despite a normal total calcium level.
Corrected calcium level
One can derive a corrected calcium level when the albumin is abnormal. This is to correct for the change in total calcium due to the change in albumin-bound calcium, and gives an estimate of what the calcium level would be if the albumin were within normal ranges.
- Corrected calcium (mg/dL) = measured total Ca (mg/dL) + 0.8 (4.4 - serum albumin [g/dL]), where 4.4 represents the average albumin level.
When there is hypoalbuminemia (a lower than normal albumin), the corrected calcium level is higher than the total calcium.
Effector organs
Sources
About 25 mmol of calcium enters the body in a normal diet. It can be lower if the diet is deficient in milk or other calcium-containing substances. Of this, about 40% (10 mmol) is absorbed in gut, and 5 mmol leaves the body in feces, netting 5 mmol of calcium a day. Vitamin D is an important co-factor in the intestinal absorption of calcium, as it increases the number of calcium binding proteins, involved in calcium absorbtion through the apical membrane of enterocytes in small intestine.
Excretion
The kidney excretes 250 mmol a day in pro-urine, and resorbs 245 mmol, leading to a net loss in the urine of 5 mmol/l. In addition to this, the kidney processes Vitamin D into calcitriol, the active form that is most effective in assisting intestinal absorption. Both processes are stimulated by parathyroid hormone (PTH).
The role of bone
Although calcium flow to and from the bone is neutral, about 5 mmol is turned over a day. Bone serves as an important storage point for calcium, as it contains 99% of the total body calcium. Potassium is released from bone by parathyroid hormone. Calcitonin stimulates incorporation of calcium in bone, although this process is largely independent of calcitonin.
Regulatory organs
The only real regulatory organ is the parathyroid gland. The parathyroid glands are located behind the thyroid, and produce parathyroid hormone in response to low calcium levels.
The parafollicular cells of the thyroid produce calcitonin in response to high calcium levels, but its significance is much smaller than that of PTH.
Calcium problems
Hypocalcemia and hypercalcemia are both serious medical disorders. Renal osteodystrophy is a consequence of chronic renal failure related to the calcium metabolism. Osteoporosis and osteomalacia have been linked to calcium metabolism disorders.