Which part of the nephron is located after Bowman's capsule and is responsible for most reabsorption of water and solutes?

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

Answer: (A)

Bulk reabsorption in the nephron happens primarily in the part right after Bowman's capsule. The proximal convoluted tubule reclaims most of the filtered water and solutes back into the bloodstream. Its cells have a dense brush border that greatly increases surface area, and Na+/K+-ATPase pumps on the basolateral side create the driving force for reabsorbing sodium and other solutes. As sodium is reabsorbed, water follows by osmosis through aquaporin-1 channels, so a large amount of water reabsorption occurs here. This segment also reclaims nearly all filtered glucose and amino acids and a large portion of bicarbonate, chloride, and other ions, helping maintain fluid and electrolyte balance and acid-base status. Although other nephron parts also reabsorb water and solutes, they do so to a lesser extent or under hormonal regulation, making the proximal tubule the major site of reabsorption after Bowman's capsule.