Interrelationships of Vitamin D and Parathyroid Hormone in Calcium Homeostasis.

IN man and most mammals the concentration of calcium in the blood and body fluids remains almost constant under normal conditions, with only slight fluctuations. For the various cellular functions which are dependent on calcium, for example neuromuscular transmission, it is the ionized fraction of calcium (approximately 50 per cent of the total serum calcium level) which is important, and it is this fraction which is so carefully maintained at a constant level in the body fluids. In order to stabilize this level, an elaborate regulating mechanism is necessary, since there are many routes of entry into and exit from the blood available to calcium. Thus, calcium is added to the blood by absorption from the intestine and by resorption from bone, and leaves via formation of new bone, and by excretion into urine, fweces, sweat and milk. Changes in other components of the body fluids, protein, citrate and phosphate, affect the concentration of ionized calcium. The magnitude of all these factors will vary in response to growth, dietary intake, changes in activity, or pregnancy, and yet the net effect on concentration of ionized calcium is almost undetectable. Two factors of major importance in the regulation of serum levels of calcium are the parathyroid glands and vitamin D, and both of these are necessary to maintain normal levels.