More than one half of the body's magnesium is stored in the bones. Most of the rest resides within the cells, with less than 1 per cent located in the extracellular fluid. Although the total plasma magnesium concentration is about 1.8 mEq/L, more than one half of this is bound to plasma proteins. Therefore, the free ionized concentration of magnesium is only about 0.8 mEq/L.
The normal daily intake of magnesium is about 250 to 300 mg/day, but only about one half of this intake is absorbed by the gastrointestinal tract. To maintain magnesium balance, the kidneys must excrete this absorbed magnesium, about one half the daily intake of magnesium, or 125 to 150 mg/day. The kidneys normally excrete about 10 to 15 per cent of the magnesium in the glomerular filtrate.
Renal excretion of magnesium can increase markedly during magnesium excess or can decrease to almost nil during magnesium depletion. Because magnesium is involved in many biochemical processes in the body, including activation of many enzymes, its concentration must be closely regulated.
Regulation of magnesium excretion is achieved mainly by changing tubular reabsorption. The proximal tubule usually reabsorbs only about 25 per cent of the filtered magnesium. The primary site of reabsorption is the loop of Henle, where about 65 per cent of the filtered load of magnesium is reabsorbed. Only a small amount (usually less than 5 per cent) of the filtered magnesium is reabsorbed in the distal and collecting tubules.
The mechanisms that regulate magnesium excretion are not well understood, but the following disturbances lead to increased magnesium excretion: (1) increased extracellular fluid magnesium concentration, (2) extracellular volume expansion, and (3) increased extracellular fluid calcium concentration.
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