magnesium (Mg) /magnē″sē·əm, magnē″zhəm/ [Gr, magnesia, geographic area in Greece] , a silver-white mineral element. Its atomic number is 12; its atomic mass is 24.32. Magnesium occurs abundantly in nature, always in combination with other elements. It is obtained chiefly by the electrolysis of fused salts containing magnesium chloride or by the thermal reduction of magnesia and is used in photography, metallurgy, and various pharmaceuticals such as magnesium sulfate. Magnesium is the second most abundant cation of the intracellular fluids in the body. It is essential for many enzyme activities and the interaction of intracellular particles and binding of macromolecules to subcellular organelles, such as the binding of messenger ribonucleic acid to ribosomes. It also is important to neurochemical transmissions and muscular excitability. The body of the average 145-pound adult contains about 2000 mEq of magnesium, about 50% of which is in the bones, 45% existing as intracellular cations, and about 5% in the extracellular fluid. Absorption of magnesium occurs in the upper small bowel by means of an active process closely related to the transport system for calcium. Magnesium is excreted mainly by the kidney. Renal excretion of magnesium increases during diuresis induced by ammonium chloride, glucose, and organic mercurials. Magnesium affects the central nervous, neuromuscular, and cardiovascular systems. Insufficient magnesium (hypomagnesemia) in extracellular fluid increases the release of acetylcholine and can cause changes in cardiac and skeletal muscle. Some of the conditions that can produce hypomagnesemia are diarrhea, steatorrhea, chronic alcoholism, and diabetes mellitus. Hypomagnesemia may occur in newborns and infants who are fed cow’s milk or artificial formulas, apparently because of the high phosphate/magnesium ratio in such diets. Hypomagnesemia is often treated with parenteral fluids containing magnesium sulfate or magnesium chloride. Excess magnesium (hypermagnesemia) in the body can slow the heartbeat, and concentrations greater than 15 mEq/L can produce cardiac arrest in diastole. Excess magnesium also causes vasodilation by direct effects on the blood vessels and by ganglionic blockade. Hypermagnesemia is usually caused by renal insufficiency and is manifested by hypotension, electrocardiographic changes, muscle weakness, sedation, and a confused mental state.
