Which statement about proximal tubule reabsorption is supported by the material?

• A. Two-thirds of glomerular ultrafiltrate is reabsorbed in the proximal convoluted tubule
• B. Two-thirds of glomerular ultrafiltrate is reabsorbed in the distal tubule
• C. Two-thirds of glomerular ultrafiltrate is reabsorbed in the loop of Henle
• D. Two-thirds of glomerular ultrafiltrate is reabsorbed in the collecting duct

Answer: (A)

Bulk reabsorption occurs mainly in the proximal tubule. This segment reclaiming about two-thirds of the filtered fluid makes sense because it transports Na+ and a large array of solutes (glucose, amino acids, bicarbonate, etc.) back into the bloodstream, and water follows this reabsorption by osmosis. The process is isosmotic, so the fluid leaving the proximal tubule remains roughly in balance with plasma osmolality. Mechanistically, many transporters on the apical side (like Na+/H+ exchangers and SGLTs) bring solutes into cells, while Na+/K+-ATPase on the basolateral side pumps Na+ out, driving reabsorption of water and solutes. Bicarbonate reclamation also occurs here via carbonic anhydrase–mediated reactions, contributing to bulk recovery of filtered solutes.

Because the proximal tubule handles the majority of filtrate reabsorption, the statement about two-thirds reabsorbed there is the best reflection of physiology. The distal tubule, loop of Henle, and collecting duct reabsorb additional fluid and solutes but in much smaller, hormonally regulated amounts, not nearly two-thirds overall. This is why bulk reabsorption being in the proximal tubule is the supported concept.