Which of the following ion is present in greatest amount to determine volume of extracellular fluid?

The volume of the ECF is determined primarily by the total amount of osmotically active solute in the ECF. Because Na+ and Cl- are by far the most abundant osmotically active solutes in ECF, and because changes in Cl- are to a great extent secondary to changes in Na+, the amount of Na+ in the ECF is the most impoant determinant of ECF volume. Therefore, the mechanisms that control Na+ balance are the major mechanisms defending ECF volume. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 38. Regulation of Extracellular Fluid Composition & Volume. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.