Which component of the nephron is primarily responsible for reabsorption of most solutes and water?

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

Answer: (B)

Most solutes and water are reabsorbed in the proximal convoluted tubule because its lining is specialized for bulk reabsorption. The cells have a tall apical brush border that greatly increases surface area and abundant mitochondria to power active transport. Na+/K+-ATPase pumps on the basolateral membrane maintain a low intracellular Na+ concentration, which drives the reabsorption of many substances through apical transporters. Glucose, amino acids, bicarbonate, and various electrolytes are reabsorbed via carrier proteins and exchangers that use the Na+ gradient, while water moves with these solutes through aquaporins, so a large volume of filtrate is reclaimed in an almost isotonic fashion. Typically, about two-thirds to three-quarters of filtered Na+ and water—and many organic solutes—are reabsorbed here, making it the main site of reabsorption. Other parts of the nephron contribute to selective adjustments later on, but the proximal tubule handles the bulk of reabsorption.