The most abundant cation in extracellular fluid is sodium. Sodium plays a critical role in maintaining extracellular fluid volume and osmotic balance, which is essential for normal cell function and blood pressure regulation. The high concentration of sodium outside cells drives the resting membrane potential and facilitates nerve impulse transmission and muscle contraction. In contrast, potassium is predominantly an intracellular cation. Understanding sodium’s role is crucial for managing fluid imbalances and electrolyte disturbances in surgical patients.

Reference: Guyton and Hall Textbook of Medical Physiology, John E. Hall, 14th Edition.