What percentage of filtered sodium is normally reabsorbed in the kidneys?

What percentage of filtered sodium is normally reabsorbed in the kidneys?

• A. 99%
• B. 50%
• C. 0%
• D. 75%

Answer: (A)

Sodium reabsorption in the kidneys is extremely efficient, with about 99% of filtered sodium being reabsorbed under normal conditions. Only roughly 1% is excreted in the urine. The bulk of reabsorption happens in the proximal tubule (around two-thirds), driven by the basolateral Na+/K+ ATPase that creates a gradient for sodium to enter the cells from the tubular lumen. Water follows this reabsorption osmotically, so a large amount of sodium reabsorption is coupled with water reabsorption. The remaining reabsorption occurs along the loop of Henle, distal tubule, and collecting ducts, where hormonal regulation (like aldosterone increasing sodium reabsorption in the collecting duct and ANP decreasing it) can fine-tune the final amount excreted. This high reabsorption is why only a small percentage of filtered sodium is normally lost in urine.

Sodium reabsorption in the kidneys is extremely efficient, with about 99% of filtered sodium being reabsorbed under normal conditions. Only roughly 1% is excreted in the urine. The bulk of reabsorption happens in the proximal tubule (around two-thirds), driven by the basolateral Na+/K+ ATPase that creates a gradient for sodium to enter the cells from the tubular lumen. Water follows this reabsorption osmotically, so a large amount of sodium reabsorption is coupled with water reabsorption. The remaining reabsorption occurs along the loop of Henle, distal tubule, and collecting ducts, where hormonal regulation (like aldosterone increasing sodium reabsorption in the collecting duct and ANP decreasing it) can fine-tune the final amount excreted. This high reabsorption is why only a small percentage of filtered sodium is normally lost in urine.