Which substance is responsible for maintaining fluid balance in the body?

Maintaining fluid balance in the body is primarily the responsibility of electrolytes. These are minerals in the body fluids that carry an electric charge and help regulate a variety of physiological processes, including hydration, nerve function, and muscle function. Electrolytes such as sodium, potassium, calcium, and magnesium play critical roles in osmotic pressure, which is essential for fluid movement between cells and their surrounding environment.

When levels of electrolytes are balanced, they help keep the body's fluids in the right proportion and distribution, ensuring that cells function properly. An imbalance of these substances can lead to conditions such as dehydration or fluid overload, affecting overall health.

While water is essential for life and contributes to fluid balance, it is the presence and concentration of electrolytes that directs how fluids are distributed and retained within the body. Creatinine and urea, on the other hand, are waste products that are primarily concerned with renal function and do not play a direct role in fluid balance.