Sodium, Potassium, Chloride
The total body water (TBW) in a 70 kg man is 60% of the body weight i.e. about 40 litres. Two thirds of this resides inside the cells, i.e. the intracellular fluid (ICF), while one third is in the extracellular compartment (ECF) that bathes the cells. A minor portion about 1 litter is present in the intestines and anterior chambers of the eyes.
The most important electrolytes in the ECF are sodium and chloride. The concentration of potassium in the ECF is very low. However, potassium is the predominant action (k+) in the ICF whereas sodium and chloride in the ICF are negligible.