What is the normal renal threshold of sodium (measured in millimoles per liter)?

• A. 80-85
• B. 90-110
• C. 110-130
• D. 135-148

Answer: (C)

The main idea is that the renal threshold for a solute is the plasma concentration at which the kidneys begin to excrete it into urine because reabsorptive transport becomes saturated. For sodium, reabsorption is highly efficient and tightly regulated, so Na+ starts to spill into urine only when filtered Na+ load exceeds what the tubules can handle. That threshold for sodium is about 110–130 mmol/L. Below this range, nearly all filtered sodium is reabsorbed and urine sodium is minimal; above it, more sodium is excreted to help maintain balance. The other ranges don’t reflect where spillover begins: they are either below the threshold or correspond to typical plasma sodium levels rather than the point at which excretion starts.