Calcium Homeostasis

A normal adult body contains approximately 1000 g of calcium, of which around 99% is extracellular, most of which is stored in bones and teeth (around 990 g). In bone, calcium is important for preserving the structural integrity of the skeleton, as well as a reservoir of calcium ion for other physiological functions. This compartment represents a dynamic storage of calcium, since the ion is continually and freely exchanged under the influence of several calciotropic hormones (Figure 6.1). The remaining 1% of total body calcium (900mg/g) is

DIETARY CALCIUM

DIETARY HABITS, SUPPLEMENTS

1000 mg

Extracellular Space 900 mg

DIETARY HABITS, SUPPLEMENTS

1000 mg

Extracellular Space 900 mg

500 mg 10 000 mg

SMALL INTESTINE

825 mg

175 mg

Figure 6.l Calcium homeostasis

500 mg

500 mg 10 000 mg

BONE

990 g

1-g hydroxilase I

1-g hydroxilase I

SMALL INTESTINE

825 mg

175 mg

Figure 6.l Calcium homeostasis in the extracellular space including plasma, lymph, extracellular water and other tissues (Figure 6.1). Because calcium is so important to so many physiological processes, its concentration in the extracellular and intracellular milieu is rigorously controlled,1,4 although intracellular calcium content may tend to increase with aging and certain diseases.5 The elevated gradient between intra- and extracellular compartments (approximately 100nmol/l and 1mmol/l respectively) is necessary to maintain the cellular responsiveness to diverse extracellular stimuli. This difference in calcium concentration is maintained by the concerted effects of a variety of transport mechanisms operating at various subcellular levels.4,5 Calcium enters the cell via voltage-dependent and hormone-dependent channels; and leaves the cell through the operation of a magnesium-dependent calcium-ATPase and a sodium-calcium exchanger. Uptake of calcium by intracellular organelles such as the mitochondria and the endoplasmic reticulum helps to maintain a low cytoplasmatic calcium level.4,6

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