Body fluids are distributed into two major compartments: intracellular fluid (ICF) and extracellular fluid (ECF). These compartments maintain the internal environment of the body and play a vital role in physiological processes such as transport of nutrients, waste removal, maintenance of cell shape, regulation of pH, electrolyte balance, and normal functioning of organs. Intracellular fluid (ICF) is the fluid present inside the cells of the body. It constitutes approximately two-thirds of the total body water, accounting for nearly 40% of the total body weight in a healthy adult. Every cell contains intracellular fluid enclosed by the plasma membrane, which regulates the movement of substances into and out of the cell. The intracellular fluid provides the medium for numerous metabolic activities, including protein synthesis, enzyme reactions, energy production, and cellular growth and repair. The intracellular fluid contains high concentrations of potassium (K+), magnesium (Mg2+), phosphate ions, and proteins. In contrast, sodium (Na+) and chloride (Cl−) concentrations are relatively low. The unique composition of intracellular fluid is maintained by active transport mechanisms, especially the sodium-potassium pump, which continuously exchanges sodium and potassium ions across the cell membrane.