Renal threshold for sodium excretion is typically between which values?

• A. 100-120
• B. 110-130
• C. 120-140
• D. 90-110

Answer: (B)

Renal threshold for excretion is the plasma sodium concentration at which the kidneys begin to excrete sodium in the urine because reabsorption cannot keep up with the filtered load. Sodium reabsorption is highly efficient and tightly regulated along the nephron, especially by aldosterone in the distal parts. Only when the filtered load pushes reabsorption toward its limit does natriuresis start. This onset of sodium excretion typically occurs when plasma sodium concentration is in the 110–130 mEq/L range. That is why the 110–130 mEq/L option is the best choice. The other ranges don’t align with the level at which reabsorptive capacity is exceeded to produce detectable Na+ in urine.