Key Points

• The concentration of ionized calcium in blood is regulated within narrow limits.

• Maintenance of the concentration of calcium in extracellular fluid depends on the rates of calcium absorption from the intestine, excretion in the urine, and exchange with bone.

• Parathyroid hormone (PTH) increases blood calcium by stimulating calcium mobilization from bone and calcium reabsorption from the glomerular filtrate. It also indirectly stimulates calcium absorption from the gut by increasing the synthesis of the active form of vitamin D.

• Parathyroid hormone lowers blood phosphate by decreasing the reabsorption of phosphate in the proximal tubules of the kidney.

• Secretory cells of the parathyroid glands directly monitor blood calcium concentrations and increase their rates of PTH secretion when calcium levels decline. Conversely, high concentrations of blood calcium inhibit PTH secretion.

• Calcitonin is secreted by the C cells of the thyroid gland in response to increasing concentrations of blood calcium. Its principal physiologic effect is to inhibit the activity of osteoclasts in bone.

• Ultraviolet light catalyzes the conversion of 7-dehydrocholesterol to vitamin D3 in the skin. Successive hydroxylations in the liver at carbon 25 and in the kidney at carbon 1 result in the active form, 1,25(OH)2D3.

Essential Medical Physiology, Third Edition

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