Physiology of Calcium, Phosphate, Magnesium and Vitamin D.

The physiology of calcium and the other minerals involved in its metabolism is complex and intimately linked to the physiology of bone. Five principal humoral factors are involved in maintaining plasma concentrations of calcium, magnesium and phosphate and in coordinating the balance between their content in bone. The transmembrane transport of these elements is dependent on a series of complex mechanisms that are partly controlled by these hormones. The plasma concentration of calcium is initially sensed by a calcium-sensing receptor, which then sets up a cascade of events that initially determines parathyroid hormone secretion and eventually results in a specific action within the target organs, mainly bone and kidney. This chapter describes the physiology of these humoral factors and relates them to the pathological processes that give rise to disorders of calcium, phosphate and magnesium metabolism as well as of bone metabolism. This chapter also details the stages in the calcium cascade, describes the effects of calcium on the various target organs, gives details of the processes by which phosphate and magnesium are controlled and summarises the metabolism of vitamin D. The pathology of disorders of bone and calcium metabolism is described in detail in the relevant chapters.