Disturbances in Calcium and Phosphate

Calcium homeostasis is a delicate balance among a number of organ systems and functions. These include kidneys, thyroid, parathyroid, bone, adrenal glands, gastrointestinal tract, nutrition, infectious disease, and medication. Malfunction in any of these modalities can result in hypercalcemia or hypocalcemia with the potential for serious morbidity and mortality. Total body calcium is balanced between plasma and the bony skeleton in a state of dynamic equilibrium. Approximately 1% of total calcium is in circulation, and the remaining 99% is stored in bone. In plasma, circulating calcium is approximately 40% protein (albumin) bound; 45% exists in an ionized state (Ca++); and about 15% is found as various salts (calcium citrate, calcium lactate, calcium phosphate, calcium sulfate). Bony calcium exists in an active state with constant deposition and resorption under the influence of parathyroid hormone (PTH, parathormone), calcitonin, osteoclastic and osteoblastic activity, and neoplastic disease.

The primary factor driving increases in circulating calcium is PTH, which increases bone resorption and converts vitamin D3 (cholecalciferol) into 1,25-dihydroxycholecalciferol, the active form of vitamin D3. Cholecalciferol is primarily formed in the skin from solar irradiation, and some evidence suggests that ultraviolet (UV) radiation exposure of tanning beds can raise vitamin D3 (Tangpricha et al., 2004). Dietary sources are also important and can be obtained from fortified milk, fruit juices, fish oil, and other sources. The active form of vitamin D3 is required to facilitate calcium absorption from the gut. Calcium homeostasis is further maintained by circulating levels of ionized calcium and calcitonin's negative effect on osteoclastic bone resorption (Hall, 2011). Figure 35-9 shows the pathway for conversion of vitamin D3 into its active form and vitamin D's role in controlling plasma calcium concentration.

Normal levels of total circulating calcium, with normal albumin levels, range between 8.5 and 10.5 mg/dL (^ 2.4 mmol/L). Ionized levels, which are not albumin dependent, will range between 1.17 and 1.33 mmol/L (^ 4.7 mg/dL) (Bringhurst and Leder, 2006).

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