Which nephron segment is primarily responsible for reabsorbing the majority of filtered solutes from the filtrate?

Which nephron segment is primarily responsible for reabsorbing the majority of filtered solutes from the filtrate?

• A. Proximal convoluted tubule
• B. Loop of Henle
• C. Distal convoluted tubule
• D. Collecting duct

Answer: (A)

Bulk reabsorption of filtered solutes occurs primarily in the proximal convoluted tubule. This segment has a large brush border and abundant mitochondria, giving it very high surface area and an energetic pump system that drive reabsorption. Sodium is actively pumped out of the tubule cells on the basolateral side by Na+/K+-ATPase, creating a strong gradient that pulls sodium and many solutes from the lumen into the cells. Glucose and amino acids are reabsorbed via sodium-dependent transporters, bicarbonate is reabsorbed through a carbonic anhydrase–based mechanism, and many ions and other solutes follow along with water. Because water follows solute reabsorption, the proximal tubule is also highly permeable to water, so the filtrate becomes isosmotic with blood as it moves along. This combination—high surface area, numerous transporters, and water permeability—makes the proximal convoluted tubule the main site reclaiming the majority of filtered solutes. The Loop of Henle, distal convoluted tubule, and collecting duct handle substantial reabsorption too, but to a much smaller, more fine-tuned extent, often regulated by hormones.

Bulk reabsorption of filtered solutes occurs primarily in the proximal convoluted tubule. This segment has a large brush border and abundant mitochondria, giving it very high surface area and an energetic pump system that drive reabsorption. Sodium is actively pumped out of the tubule cells on the basolateral side by Na+/K+-ATPase, creating a strong gradient that pulls sodium and many solutes from the lumen into the cells. Glucose and amino acids are reabsorbed via sodium-dependent transporters, bicarbonate is reabsorbed through a carbonic anhydrase–based mechanism, and many ions and other solutes follow along with water.

Because water follows solute reabsorption, the proximal tubule is also highly permeable to water, so the filtrate becomes isosmotic with blood as it moves along. This combination—high surface area, numerous transporters, and water permeability—makes the proximal convoluted tubule the main site reclaiming the majority of filtered solutes.

The Loop of Henle, distal convoluted tubule, and collecting duct handle substantial reabsorption too, but to a much smaller, more fine-tuned extent, often regulated by hormones.