Which of the following ion is present in greatest amount to determine volume of extracellular fluid?
Na+
K+
Cl-
Ca2+
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.