Calcium metabolism
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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2 Regulatory organs 3 Calcium problems |
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.
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.