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The proximal convoluted tubule is the key structure in the nephron responsible for selective reabsorption. This segment is highly specialized for the reabsorption of essential substances from the filtrate back into the bloodstream. Approximately 65-70% of the filtered water, electrolytes (like sodium and chloride), and nutrients (such as glucose and amino acids) are reabsorbed in this region.

The proximal convoluted tubule is lined with microvilli, which significantly increase the surface area and enhance its ability to absorb substances effectively. Active transport mechanisms and co-transport systems facilitate the reabsorption of these vital molecules. As a result, the composition of the filtrate changes considerably as it passes through this segment, reflecting the nephron's role in maintaining homeostasis.

In contrast, other structures in the nephron serve different functions. Bowman's capsule primarily serves as the site for the initial filtration of blood, while the loop of Henle is involved in creating a concentration gradient for water reabsorption but does not significantly contribute to selective reabsorption of solutes. The collecting duct is crucial for the final adjustments to water and salt balance but is not the primary site for selective reabsorption.