Calcium, phosphorus and magnesium turnover.

The body content of calcium, phosphorus and magnesium is important, not only from the structural or supporting aspect, but also from the aspect of cellular function. Membrane integrity, nervous tissue conduction, neuromuscular excitability, muscle contraction, hormone secretion, intermediary metabolism and synthesis, to mention but a few activities, are all influenced by these elements. The normal body content and turnover of calcium, phosphorus and magnesium are outlined, together with a brief description of their physiological roles, emphasizing intracellular functions rather than their predominance within the skeleton. Regulatory mechanisms, particularly for calcium, involving vitamin D and its metabolites, parathormone, and possibly calcitonin, are discussed within the context of maintaining normal body content and turnover. Homeostatic mechanisms maintaining plasma concentrations in health and disease, as far as they have been identified, are also outlined. Lastly, disorders of calcium, phosphorus and magnesium turnover are considered from the aspect of altered plasma concentrations of these substances. Throughout emphasis is given to common problems and their practical management. Metabolic bone disease and uncommon disorders of bone and bone mineral are not discussed specifically.