In the nephron, where is most of the filtered load reabsorbed and what solutes are commonly reabsorbed there?

• A. Distal tubule; Na+ and water
• B. Loop of Henle; Na+, K+, Cl−
• C. Collecting duct; bicarbonate and water
• D. Proximal tubule; Na+, glucose, amino acids, bicarbonate, and water

Answer: (D)

Most of the filtered load is reabsorbed in the proximal tubule. This section is specialized for bulk, isoosmotic reabsorption because it has a large, highly active transport system driven by Na+-K+-ATPase on the basolateral surface, which keeps intracellular Na+ low. That gradient powers many Na+-coupled reabsorptive processes: glucose and amino acids are reabsorbed together with Na+ via specific transporters (SGLT for glucose and various Na+-amino acid cotransporters for amino acids). Bicarbonate reabsorption also occurs here through mechanisms involving Na+/H+ exchange and the carbonic anhydrase–catalyzed reactions that convert bicarbonate to CO2 and water in the lumen and back to bicarbonate inside the cell. Water follows the solutes osmotically through aquaporins, so a large amount of filtered water is reclaimed along with these solutes. Other nephron segments do reabsorb important ions and water, but they handle smaller portions and more selective adjustments, making the proximal tubule the primary site for reclaiming Na+, glucose, amino acids, bicarbonate, and water.