Physiology

Calcium is essential for neuromuscular excitability, cardiac automaticity, mitotic division, coagulation, muscle contraction, neurotransmitter and hormone secretion and action, and the activity of many enzymes. Parathyroid hormone (PTH) and vitamin D maintain the extracellular calcium concentration within a narrow range. PTH increases intestinal calcium absorption, increases osteoclastic release of calcium and phosphorus from bone, decreases renal clearance of calcium, and enhances formation of 1,25-dihydroxyvitamin D by the kidney. Levels of ionized calcium and magnesium determine the secretion of PTH. Vitamin D augments the effects of PTH and is necessary for calcium absorption from the GI tract. Calcitonin from thyroid parafollicular cells lowers serum calcium in two ways: by inhibiting osteoclast activity and by inhibiting renal reabsorption of calcium and phosphorus.