Which of the following describes a function of the proximal tubule?

• A. Reabsorb 75% of salt and water
• B. Concentrate salts
• C. Form the renal threshold
• D. Reabsorb urea

Answer: (A)

Bulk reabsorption of the filtered load happens mainly in the proximal tubule, where a large portion of sodium and water are reclaimed as the filtrate moves along. This segment reabsorbs roughly two-thirds to three-quarters of the filtered Na+ and water, along with significant amounts of bicarbonate, glucose, and amino acids. Because water follows solutes, the filtrate leaving the proximal tubule remains about isosmotic with plasma, which is why saying about 75% of salt and water reabsorption occurs here is a common, rounded approximation. This makes the statement the best description of a proximal tubule function.

Concentrating salts isn’t a defining role of the proximal tubule—osmolality changes that concentrate or dilute urine mainly involve the loop of Henle and the collecting ducts, not this segment. The renal threshold concept relates to transporter capacity and when a substance begins to appear in urine, not a function the proximal tubule specifically “forms.” And while some urea is reabsorbed in the proximal tubule, the primary function to emphasize is bulk Na+ and water reabsorption, which is why the chosen statement best captures its main job.