What are the primary electrolytes of crystalloid fluids?

• A. Potassium and phosphate
• B. Sodium and chloride
• C. Calcium and magnesium
• D. Sodium and potassium

Answer: (B)

Crystalloid fluids are water-based IV solutions designed to replace extracellular fluid and maintain osmotic balance. The ions present in the largest amounts in these fluids are sodium and chloride, which mirror the composition of the extracellular space. Normal saline, a common crystalloid, has sodium and chloride in high, equal amounts, so these two ions set the fluid’s primary electrolyte profile. Balanced crystalloids aim to imitate extracellular fluid by including sodium and chloride as the dominant electrolytes, with smaller amounts of other ions. While potassium, calcium, magnesium, or phosphate can be present in some formulations, they are not the defining electrolytes of crystalloids. Therefore, sodium and chloride are the primary electrolytes.