In the nephron, where is the majority of filtered water and solutes reabsorbed?

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Multiple Choice

In the nephron, where is the majority of filtered water and solutes reabsorbed?

The major portion of filtered water and solutes is reabsorbed in the proximal convoluted tubule. This segment is highly permeable and actively reabsorbs about two-thirds to three-quarters of what gets filtered. The reabsorption here is driven by active Na+ reuptake from the tubular lumen: Na+/K+ ATPase on the basolateral side creates a gradient that powers multiple cotransporters and exchangers (for example, Na+-glucose and Na+-amino acid transporters) to move solutes back into the blood. Water follows these solutes osmotically through aquaporin channels, so the filtrate leaving the proximal tubule is still isosmotic with plasma yet much more concentrated with nutrients that have been reclaimed. The remaining segments do reabsorb water and solutes, but not to the same extent. The loop of Henle, with its descending limb reabsorbing water and ascending limb reabsorbing salts, contributes substantially to concentrating urine and establishing medullary gradients, but the overall bulk reabsorption of water and solutes occurs earlier, in the proximal tubule. The distal tubule and collecting ducts adjust reabsorption under hormonal control, but they reclaim a much smaller fraction under normal conditions. The glomerulus is the filtration site, not a reabsorption site.

The major portion of filtered water and solutes is reabsorbed in the proximal convoluted tubule. This segment is highly permeable and actively reabsorbs about two-thirds to three-quarters of what gets filtered. The reabsorption here is driven by active Na+ reuptake from the tubular lumen: Na+/K+ ATPase on the basolateral side creates a gradient that powers multiple cotransporters and exchangers (for example, Na+-glucose and Na+-amino acid transporters) to move solutes back into the blood. Water follows these solutes osmotically through aquaporin channels, so the filtrate leaving the proximal tubule is still isosmotic with plasma yet much more concentrated with nutrients that have been reclaimed.

The remaining segments do reabsorb water and solutes, but not to the same extent. The loop of Henle, with its descending limb reabsorbing water and ascending limb reabsorbing salts, contributes substantially to concentrating urine and establishing medullary gradients, but the overall bulk reabsorption of water and solutes occurs earlier, in the proximal tubule. The distal tubule and collecting ducts adjust reabsorption under hormonal control, but they reclaim a much smaller fraction under normal conditions. The glomerulus is the filtration site, not a reabsorption site.

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