The concentration of sodium ion is least in the –
**Core Concept**
The question is testing the student's knowledge of the concentration of sodium ions in different body fluids. The concentration of sodium ions is crucial for maintaining proper fluid balance, nerve function, and muscle contraction.
**Why the Correct Answer is Right**
Sodium ions are primarily found in extracellular fluids (ECF), with the highest concentration in the serum. However, the concentration of sodium ions is least in the intracellular fluids (ICF). This is because the cell membrane is selectively permeable, allowing potassium ions to enter the cell while sodium ions are actively pumped out by the sodium-potassium pump (Na+/K+ ATPase). This pump maintains a low concentration of sodium ions inside the cell, while high concentrations are maintained in the ECF.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the cerebrospinal fluid (CSF) has a lower concentration of sodium ions compared to the serum, but it is not the lowest concentration in the body.
**Option B:** This option is incorrect because the synovial fluid, which lubricates joints, has a higher concentration of sodium ions compared to the intracellular fluids.
**Option C:** This option is incorrect because the interstitial fluid, which surrounds cells, has a higher concentration of sodium ions compared to the intracellular fluids.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the concentration of sodium ions is crucial for maintaining proper fluid balance. Hyponatremia (low sodium levels) can lead to swelling of brain cells, while hypernatremia (high sodium levels) can cause dehydration and seizures.
**Correct Answer: C. Intracellular fluids.