Fluid & Electrolyte Q 68

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Which electrolyte would the nurse identify as the major electrolyte responsible for determining the concentration of the extracellular fluid?

  

    A. Potassium

    B. Phosphate

    C. Chloride

    D. Sodium

    

    

Correct Answer: D. Sodium

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Sodium is the electrolyte whose level is the primary determinant of the extracellular fluid concentration. Sodium a cation (e.g., positively charged ion), is the major electrolyte in extracellular fluid. Sodium, which is an osmotically active anion, is one of the most important electrolytes in the extracellular fluid. It is responsible for maintaining the extracellular fluid volume, and also for regulation of the membrane potential of cells. Sodium is exchanged along with potassium across cell membranes as part of active transport.

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Option A: Potassium (a cation) is a major electrolyte in the intracellular fluid. Potassium is mainly an intracellular ion. The sodium-potassium adenosine triphosphatase pump has the primary responsibility for regulating the homeostasis between sodium and potassium, which pumps out sodium in exchange for potassium, which moves into the cells.

Option B: Phosphate (an anion) is a major electrolyte in the intracellular fluid. Phosphate is an essential electrolyte in the human body as it constitutes about 1% of the total body weight. In an adult, the normal serum phosphate level ranges between 2.5 to 4.5 mg/d L. The normal serum levels of phosphate tend to decrease with age and its highest levels i.e., 4.5 to 8.3 mg/dL are seen in infants, about 50% higher than adults; this is because infants and children need more phosphate for their growth and development.

Option C: Chloride, an anion (e.g., negatively charged ion), is also present in extracellular fluid, but to a lesser extent. Chloride is an anion found predominantly in the extracellular fluid. The kidneys predominantly regulate serum chloride levels. Most of the chloride, which is filtered by the glomerulus, is reabsorbed by both proximal and distal tubules (majorly by proximal tubule) by both active and passive transport.

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