The total amount of magnesium in the body is about 1000mmol (22.66g), which is the fourth most abundant cation in the body besides sodium, potassium and calcium. 50% to 60% exists in bones, and only 1% exists in extracellular fluid. The concentration of magnesium [Mg2] in serum is 0.75 to 0.95mmol/L (1.7 to 2.2mg/dl, or 1.5 to 1.9mEq/L). In addition to bones, muscles contain more magnesium. Blood magnesium concentration cannot represent the changes in the total amount of magnesium in the body. Magnesium is widely distributed in animal and plant cells. Magnesium ion is one of the most important ions in the human body. It ranks second only to potassium in the content of intracellular fluid cations, but its concentration in extracellular fluid is very low. The physiological function of magnesium is quite complex. Magnesium ion disorder is a relatively common electrolyte disorder, but its clinical manifestations lack specificity and are easy to be ignored.