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    Calcium serves as an extracellular cation that regulates nerve transmission, muscle contraction, bone metabolism, and blood pressure regulation and is necessary for blood clotting. Calcium is regulated by parathyroid hormone (PTH), calcitonin, vitamin D, and phosphorus. Through a complex system of regulation among multiple organs, including the kidney, GI tract, and bone, calcium absorption can be enhanced to increase calcium reabsorption to maintain homeostasis. When serum calcium levels are low, PTH causes release of calcium from the bones and stimulates increased absorption from the GI tract. Calcitonin works in the opposite direction, while phosphorus inhibits calcium absorption in the GI tract. Oral calcium supplements are most often the first-line therapy in the absence of symptoms. Because other hormones, electrolytes, and vitamins are involved in calcium regulation, these are assessed in the setting of true hypocalcemia. Low phosphorus and magnesium levels must be restored before calcium levels can be corrected.

   Sodium (Na+) is the major cation of extracellular fluid. Secretions such as bile and pancreatic juice contain substantial amounts of sodium. Gastric secretions and diarrhea also contain sodium, but contrary to common belief sweat is hypotonic and contains a relatively small amount of sodium. Sodium is also important in neuromuscular function and maintenance of acid-base balance. Exportation of sodium from the cell is the driving force for facilitated transporters, which import glucose, amino acids, and other nutrients into the cells.

   Magnesium is the second most prevalent intracellular cation. Approximately half of the body’s magnesium is located in bone, whereas another 45% resides in soft tissue; only 1% of the body’s magnesium content is in the extracellular fluids. Magnesium (Mg 2+) is an important cofactor in many enzymatic reactions in the body and is also important in bone metabolism as well as central nervous system and cardiovascular function. Many of the enzyme systems regulated by magnesium are involved in nutrient metabolism and nucleic acid synthesis, leading to the body’s need to carefully regulate magnesium status.

   Phosphorus is the primary intracellular anion and its role in adenosine triphosphate (ATP) is vital in energy metabolism. In addition, phosphorus is important in bone metabolism. About 80% of the body’s phosphorus is found in bones. Large amounts of free energy are released when the phosphate bonds in ATP are split. In addition to this role, phosphorus is vital for cellular function in phosphorylation and dephosphorylation reactions, as a buffer in acid-base balance, and in cellular structure as part of the phospholipid membrane. Because of the vital role that phosphorus plays in energy production, severe hypophosphatemia can be a life-threatening event.

   Approximately 98% of Potassium (K+) is in the intracellular space, K+ is the major cation of intracellular fluid. Potassium functions with sodium, it is involved in maintaining a normal water balance, osmotic equilibrium, and acid-base balance. In addition to calcium, K+ is important in the regulation of neuromuscular activity. Concentrations of sodium and potassium determine membrane potentials in nerves and muscle. Potassium also promotes cellular growth. The potassium content of muscle is related to muscle mass and glycogen storage; therefore, if muscle is being formed, an adequate supply of potassium is essential. Potassium has an integral role in the Na/K-ATPase pump.

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