The main intracellular cation in the human body is Potassium. This ion plays a critical role in maintaining the resting membrane potential and normal cellular function, especially in nerve and muscle cells. Potassium's high intracellular concentration is essential for processes such as nerve impulse transmission, muscle contraction, and regulation of heart rhythm. Clinically, disturbances in potassium levels can lead to serious cardiac arrhythmias and muscle weakness, highlighting its importance in surgical patients and perioperative management.

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