Factors Affecting Utilization of Absorbed Calcium and Phosphorus

Factors Affecting Utilization of Absorbed Calcium and Phosphorus 

The inclusion of factors under this heading has been made with some reservation, as it is obvious that conditions which induce profound disturbance of any vital metabolism may have indirect influence upon others. We have, therefore, included only those factors which appear to have a well-defined effect, either direct or indirect, upon calcium and phosphorus metabolism. 

This hormone is secreted by the parathyroid glands, which are four in number and lie on the posterior surface of the thyroid gland. The exact manner in which the hormone exerts its action upon calcium and phosphorus metabolism has not been proved satisfactorily, but the most immediate effect of administration of parathyroid extract which has been generally observed is an increased excretion of phosphorus in the urine, which is accompanied by a lowering of the serum inorganic phosphate. The increase in the urinary phosphate excretion has been ascribed by Harrison and Har- 

This initial effect of the hormone upon the urinary and serum phosphate is followed by an increase in the concentration of calcium in the serum and an increased urinary excretion of calcium, although the latter is not of sufficient magnitude to prevent hypercalcemia. Continued administration of the hormone brings about impairment of renal function so that urinary excretion of phosphate decreases and serum inorganic phosphate becomes increased above normal, with the result that calcium phosphate is precipitated in the form of renal calculi. 

The effect of parathyroid hormone upon bone is considered by Albright to be secondary to the increase in urinary phosphate excretion, the depletion of serum phosphate causing accelerated resorption of phosphate from bone which is inevitably accompanied by increased calcium resorption. This author distinguishes between hyperparathyroidism with bone disease and a like condition without bone disease. In the former there is an increase in the resorption of bone with a compensatory increase in bone formation, the osteoblasts and osteoclasts being numerically increased and the phosphatase activity of the serum raised. The latter condition shows normal bone metabolism, and the increased calcium excretion is postulated to come entirely from increased calcium intake and absorption, whereas serum phosphatase activity is normal. 

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VI. Factors Affecting Utilization of Absorbed Calcium and Phosphorus. . 418... 

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