Circulating quantities of sodium, potassium, calcium, inorganic phosphorus, and magnesium in serum of healthy newborn infants.

The main serum ions were determined in healthy, full-term newborn infants having different plasma volumes. The serum sodium, potassium, and calcium concentrations were found to be constant, thus their circulating quantities were dependent on the actual plasma expansion. The serum inorganic phosphorus and magnesium concentrations were found to be inversely related to the plasma volume and therefore the circulating total quantities of these ions remained constant in different expansion of the plasma volume.