- Water content of blood is controlled by the balance of electrolytes, primarily sodium in the extracellular fluid. - Potassium is a crucial intracellular electrolyte involved in cell volume and membrane function. - The kidneys, regulated by hormones, control electrolyte and water balance. - Among the options provided, Potassium is the only major electrolyte with a significant role in overall fluid and electrolyte balance impacting blood water content, especially through its influence on kidney function and cellular osmotic balance.