Figure 1.6 • Electrolyte Concentration in Extracellular and Intracellular Fluid – The primary extracellular fluid (ECF) cation is sodium, and the primary interstitial fluid (ICF) cation is potassium. This difference is maintained by the basolateral Na⁺/K⁺ ATPases, which transport three Na⁺ molecules out of the cell in exchange for two K⁺ molecules transported into the cell. A balance of positive and negative charges is maintained in each compartment, but by different ions. (Values are approximate.)