- The main positive ion in the extracellular fluid compartment is sodium (Na⁺).
- Sodium plays a critical role in maintaining the osmotic balance and volume of the extracellular fluid.
- Potassium (K⁺) predominantly acts as the main positive ion inside cells (intracellular fluid), where it is essential for cell function, especially in nerve and muscle cells.
- Calcium (Ca²⁺) and magnesium (Mg²⁺) are important electrolytes involved in various physiological functions such as muscle contraction, nerve transmission, and enzymatic reactions, but they are not the principal cations in extracellular fluid.
The high concentration of sodium in the extracellular fluid compartment helps regulate fluid movement between compartments and is vital for processes like nerve impulse transmission and muscle contraction.

Important points:
- Sodium (Na⁺): Major extracellular cation, regulates fluid balance
- Potassium (K⁺): Major intracellular cation
- Calcium (Ca²⁺) and Magnesium (Mg²⁺): Minor extracellular cations, involved in specific physiological functions

Reference: Ganong's Review of Medical Physiology, 26th Edition, Chapter 5: Cellular Physiology and Membrane Transport