Where in the nephron does reabsorption occur?

• A. Collecting Duct
• B. Loop of Henle
• C. Proximal Convoluted Tubule
• D. Bowman's Capsule

Answer: (C)

Reabsorption is the process of reclaiming substances from filtrate back into the bloodstream. The proximal convoluted tubule is where most of this reabsorption happens. It has a large surface area due to its brush border of microvilli and many transport proteins, including Na+/K+ ATPase on the basolateral side. Sodium reabsorption creates a driving force that pulls in glucose, amino acids, bicarbonate, and other solutes, and water follows osmotically. Because of this setup, about the majority of filtered Na+, water, glucose, and amino acids are reabsorbed here, so the filtrate leaving this segment is already greatly depleted of useful substances and still contains waste that needs further processing downstream.

Other nephron segments do contribute to reabsorption as well: the loop of Henle reabsorbs water in the descending limb and salts in the ascending limb, the distal convoluted tubule fine-tunes ion reabsorption, and the collecting duct adjusts water reabsorption under hormonal control. But the bulk of reabsorption occurs in the proximal tubule, making it the most important site for reclaiming filtered solutes and water.